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1

On the mid-Pleistocene transition to 100-kyr glacial cycles and the asymmetry between glaciation and deglaciation times  

Microsoft Academic Search

A mechanism is proposed for the mid-Pleistocene transition from a dominant periodicity of 41 kyr to 100 kyr in glacial oscillations. The same mechanism is shown to also explain the asymmetry between the long glaciation and short deglaciation phases of each cycle since that transition versus the symmetry of the 41-kyr oscillations prior to the transition. These features arise naturally

Eli Tziperman; Hezi Gildor

2003-01-01

2

Obliquity pacing of the late Pleistocene glacial terminations.  

PubMed

The 100,000-year timescale in the glacial/interglacial cycles of the late Pleistocene epoch (the past approximately 700,000 years) is commonly attributed to control by variations in the Earth's orbit. This hypothesis has inspired models that depend on the Earth's obliquity (approximately 40,000 yr; approximately 40 kyr), orbital eccentricity (approximately 100 kyr) and precessional (approximately 20 kyr) fluctuations, with the emphasis usually on eccentricity and precessional forcing. According to a contrasting hypothesis, the glacial cycles arise primarily because of random internal climate variability. Taking these two perspectives together, there are currently more than thirty different models of the seven late-Pleistocene glacial cycles. Here we present a statistical test of the orbital forcing hypothesis, focusing on the rapid deglaciation events known as terminations. According to our analysis, the null hypothesis that glacial terminations are independent of obliquity can be rejected at the 5% significance level, whereas the corresponding null hypotheses for eccentricity and precession cannot be rejected. The simplest inference consistent with the test results is that the ice sheets terminated every second or third obliquity cycle at times of high obliquity, similar to the original proposal by Milankovitch. We also present simple stochastic and deterministic models that describe the timing of the late-Pleistocene glacial terminations purely in terms of obliquity forcing. PMID:15791252

Huybers, Peter; Wunsch, Carl

2005-03-24

3

Early Pleistocene Glacial Cycles and the Integrated Summer Insolation Forcing  

Microsoft Academic Search

Long-term variations in Northern Hemisphere summer insolation are generally thought to control glaciation. But the intensity of summer insolation is primarily controlled by 20,000-year cycles in the precession of the equinoxes, whereas early Pleistocene glacial cycles occur at 40,000-year intervals, matching the period of changes in Earth's obliquity. The resolution of this 40,000-year problem is that glaciers are sensitive to

Peter Huybers

2006-01-01

4

Intensified deep Pacific inflow and ventilation in Pleistocene glacial times.  

PubMed

The production of cold, deep waters in the Southern Ocean is an important factor in the Earth's heat budget. The supply of deep water to the Pacific Ocean is presently dominated by a single source, the deep western boundary current east of New Zealand. Here we use sediment records deposited under the influence of this deep western boundary current to reconstruct deep-water properties and speed changes during the Pleistocene epoch. In physical and isotope proxies we find evidence for intensified deep Pacific Ocean inflow and ventilation during the glacial periods of the past 1.2 million years. The changes in throughflow may be directly related to an increased production of Antarctic Bottom Water during glacial times. Possible causes for such an increased bottom-water production include increasing wind strengths in the Southern Ocean or an increase in annual sea-ice formation, leaving dense water after brine rejection and thereby enhancing deep convection. We infer also that the global thermohaline circulation was perturbed significantly during the mid-Pleistocene climate transition between 0.86 and 0.45 million years ago. PMID:11518963

Hall, I R; McCave, I N; Shackleton, N J; Weedon, G P; Harris, S E

2001-08-23

5

"Pleistocene Park" - A Glacial Ecosystem in a Warming World  

NASA Astrophysics Data System (ADS)

Most people if asked what association they have to the phrase - ice age, will answer - "Mammoth". But mammoths are not only big wooly elephants which went extinct in the beginning of Holocene. They were also part of a great ecosystem, the Northern Steppe or Mammoth Ecosystem, which was the world's largest ecosystem for hundreds thousand of years. This ecosystem, with extremely high rates of biocycling, could maintain animal densities which can be hardly found anywhere in the modern world. Northern steppe played an important role in shaping the glacial climate of the planet. High albedo grasslands reflected a much bigger portion of sun heat back to the atmosphere. Cold soils and permafrost served as sinks of carbon, helping to keep greenhouse gas concentration in the atmosphere at low levels. In the beginning of Holocene, simultaneously with wave of human expansion, an extinction wave took place. Tens of megafauna species became extinct at that time worldwide, while ones that resisted the extinction substantially dropped in numbers. The Northern Steppe ecosystem became imbalanced. Without large numbers of herbivores grazing and trampling the pasture, trees, shrubs and moss invaded grasslands. Within just a few hundreds years the mammoth ecosystem was gone, replaced by much lower productivity ecosystems. Already 14 thousand year ago, by simply increasing hunting pressure, humans managed to dramatically change Earth's appearance. We propose that by artificially maintaining a high animal density and diversity on a limited territory for extended period of time, it will be possible to reverse the shift, reestablishing the productive Northern Steppe ecosystem. Moss, shrubs and tree sprouts are not able to resist grazing pressure so they will be quickly replaced by grasses and herbs. Animals digesting all aboveground biomass would accelerate nutrition cycling and consequently increase bioproductivity. Higher bioproductivity would increase evapotranspiration, keeping soils dry and runoff low. This would further increase nutrient availability in the soil. Water limitation would force roots grow deeper to cold soil horizons where these roots (carbon) will be sequestered for a long period of time. After high productivity and high diversity of animals in the ecosystem is reached, this ecosystem will once again be able to compete and to expand. To test this hypothesis, we have started the experiment named "Pleistocene Park". For over 15 years we have brought different herbivore species to the fenced area in the Kolyma river lowland, keep them at high density and see the ecosystem transformation. Now Pleistocene Park is size of 20 km2 and home for 7 big herbivores species. It is a small version of how the Mammoth Steppe ecosystem looked in the past and may look in the future. Pleistocene Park is a place where scientists can conduct in situ research and see how restoration of the ice age ecosystem may help mitigate future climatic changes. Arctic is a weakly populated region with no possibilities for agriculture. Modern civilization treats bigger part of the Arctic as wastelands. So why don't turn this "wasteland" into something that can strongly benefit our civilization in the future?

Zimov, N.; Zimov, S. A.

2011-12-01

6

Early Pleistocene Glacial Lake Lesley, West Branch Susquehanna River valley, central Pennsylvania  

Microsoft Academic Search

Laurentide glaciers extended into north central Pennsylvania repeatedly during at least the last 2 million years. Early Pleistocene glaciation extended farther south into central Pennsylvania than any subsequent glaciation, reaching the West Branch Susquehanna River (WBSR) valley. Early Pleistocene ice dammed the northeast-flowing West Branch Susquehanna River at Williamsport, forming Glacial Lake Lesley, a 100-km-long proglacial lake. In this paper,

Joan M. Ramage; Thomas W. Gardner; Ira D. Sasowsky

1998-01-01

7

Middle Pleistocene cold stage climates in the Mediterranean: New evidence from the glacial record  

Microsoft Academic Search

A major reduction in summer temperatures during a Middle Pleistocene glacial cycle caused the most extensive glaciation recorded in the Mediterranean region. Glaciers in the mountains of Greece formed during marine isotope stage (MIS) 12 (474 000 427 000 years BP) under climatic conditions characterised by mean summer temperatures at least 11 °C cooler than today and annual precipitation of

P. D. Hughes; J. C. Woodward; P. L. Gibbard

2007-01-01

8

Multiple instabilities and modes of glacial rhythmicity in the Plio-Pleistocene: A general theory of late Cenozoic climatic change  

SciTech Connect

Several distinct modes of glacial oscillation have existed during the past few million years, ranging from low-amplitude, high-frequency oscillations in the early Pliocene, through relatively high amplitude, predominantly near 40 ky period, oscillations in the late Pliocene and early Pleistocene, to the major near 100 ky period oscillations of the late Pleistocene. In addition to other plausible mechanisms, this study illustrates another possible contributor based on the hypothesis that the slow-response climatic system is bistable and that two kinds of internal instability may be operative along with externally imposed forcing due to earth-orbital (Milankovitch) radiation changes and slow, tectonically-induced changes in atmospheric carbon dioxide. Within the framework of a dynamical model containing the possibility for these two instabilities, as well as for stable modes, the study shows (1) how Milankovitch radiative changes or stochastic forcing influencing ice sheets can induce aperiodic (chaotic) transitions between the possible stable and unstable modes, and (2) how progressive, long-term, tectonically-induced, changes in carbon dioxide, acting in concert with earth-orbital radiative variations in high Northern Hemisphere latitudes, can force systematic transitions between the modes. This is a minimum dynamical model of the late Cenozoic climatic changes, containing the main physical factors determining these changes: ice mass, bedrock depression, atmospheric carbon dioxide concentration, deep ocean thermohaline state, Milankovitch radiation forcing, and slow tectonically-induced carbon dioxide forcing. 34 refs., 13 figs., 2 tabs.

Saltzman, B.; Verbitsky, M.Ya. [Yale Univ., New Haven, CT (United States)

1993-10-01

9

Oscillators and relaxation phenomena in Pleistocene climate theory  

Microsoft Academic Search

Ice sheets appeared in the northern hemisphere around 3 million years ago and glacial-interglacial cycles have paced Earth's climate since then. Superimposed on these long glacial cycles comes an intricate pattern of millennial and sub-millennial variability, including Dansgaard-Oeschger and Heinrich events. There are numerous theories about theses oscillations. Here, we review a number of them in order draw a parallel

Michel Crucifix

2011-01-01

10

Late Pleistocene glacial advances in the western Tibet interior  

NASA Astrophysics Data System (ADS)

It has long been observed that the timing of glacial advances is asynchronous across the Himalaya-Karakoram-Tibet Plateau (HKTP) but the climatic implications, if any, remain unclear. Resolving this question requires additional glacial chronologies from unique spatial and climatic regimes as well as an analysis of how glaciers within different regimes are likely to have responded to past climate changes. This study presents a 10Be-21Ne chronology from the Mawang Kangri range of western Tibet (˜34°N, 80°E); an arid high-elevation site. We identify advances at ˜123, 83, and 56 kyr, which agree reasonably well with sites in the immediate vicinity, but are asynchronous relative to sites across the entire HKTP, and relative to sites in the western HKTP. To evaluate HKTP-wide asynchroneity, we compile dated glacial chronologies and classify them by the approximate timing of their maximum recent advance. This result shows a strong spatial clustering of young (MIS 1-2) relative to older (MIS 3-5) maximum advances. Further comparison with modern precipitation, temperature, and topographic data show that the pattern of HKTP-wide asynchroneity is broadly independent of topography and can potentially be explained by local responses to changes in temperature at either very warm-wet or cold-dry sites. Sites that receive intermediate amounts of precipitation are more ambiguous, although spatial clustering of MIS 1-2 vs. MIS 3-5 advances is suggestive of past variations in precipitation at these sites. In western Tibet, no spatial or climatic correlation is observed with the timing of maximum glacial advances. We suggest this could arise from mis-interpretation of disparate boulder ages generated by a prolonged MIS-3/4 glacial advance in the western HKTP.

Amidon, William H.; Bookhagen, Bodo; Avouac, Jean-Philippe; Smith, Taylor; Rood, Dylan

2013-11-01

11

Case for periodic, colossal jokulhlaups from Pleistocene glacial Lake Missoula.  

USGS Publications Warehouse

Lake Missoula (2500 km3) remained sealed as long as any segment of the glacial dam remained grounded; when the lake rose to a critical level c.600 m in depth, the glacier bed at the seal became buoyant, initiating underflow from the lake. Subglacial tunnels then grew exponentially, leading to catastrophic discharge. Calculations of the water budget for the lake basin (including input from the Cordilleran ice sheet) suggest that the lake filled every three to seven decades. -from Author

Waitt, Jr, R. B.

1985-01-01

12

Geochemical Variations in Late Cenozoic Glacial Sediments Deposited by the Laurentide Ice Sheet: Implications for the Middle Pleistocene Transition  

Microsoft Academic Search

We use geochemical indices derived from midcontinent U.S. glacial deposits to evaluate the hypothesis that the middle Pleistocene transition (MPT) was caused by erosion of a regolith and subsequent exposure of underlying fresh bedrock by the Laurentide Ice Sheet. Late Pliocene-Pleistocene glacial sedimentary sequences were divided into three age groups: two older groups of reverse-polarity tills (Matuyama Chron) and one

M. Roy; P. U. Clark

2002-01-01

13

Pleistocene sea-surface temperature evolution: Early cooling, delayed glacial intensification, and implications for the mid-Pleistocene climate transition  

NASA Astrophysics Data System (ADS)

The mid-Pleistocene climate transition (MPT) is defined by the emergence of high amplitude, quasi-100 ka glacial-interglacial cycles from a prior regime of more subtle 41 ka cycles. This change in periodicity and amplitude cannot be explained by a change in 'external' astronomical forcing. Here, we review and integrate published records of sea-surface temperatures (SSTs) to assess whether a common global expression of the MPT in the surface ocean can be recognized, and examine our findings in light of mechanisms proposed to explain climate system reorganization across the MPT. We show that glacial-interglacial variability in SSTs is superimposed upon a longer-term cooling trend in oceanographic systems spanning the low- to high-latitudes. Regional variability exists in the timing of the onset and magnitude of cooling but, in most cases, a long-term cooling trend begins or intensifies from ~ 1.2 Ma (Marine Isotope Stage, MIS, 35-34). The SST cooling accompanies a long-term trend towards higher global ice volume as recorded in benthic foraminifera ?18O, but predates a step-like increase in ?18O at ~ 0.9 Ma (MIS 24-22) that is argued to reflect expansion of continental ice-sheets. The strongest expression of Pleistocene cooling is found during glacial stages, whereas minor or negligible trends in interglacial temperatures are identified. However, pronounced cooling during both glacial and interglacial maxima is evident at 0.9 Ma. Alongside the long-term SST cooling trends, quasi-100 ka cycles begin to emerge in both the SST and ?18O records at 1.2 Ma, and become dominant with the expansion of the ice-sheets at 0.9 Ma. We show that the intensified glacial-stage cooling is accompanied by evolving pCO2, abyssal ocean ventilation, atmospheric circulation and/or dust inputs to the Southern Ocean. These changes in diverse environmental parameters suggest that glacial climate boundary conditions evolved across the MPT. In turn, these modified boundary conditions may have altered climate sensitivity to orbital forcing by placing pre-existing ice-sheets closer to some threshold of climate-ice sheet response.

McClymont, Erin L.; Sosdian, Sindia M.; Rosell-Melé, Antoni; Rosenthal, Yair

2013-08-01

14

Middle Pleistocene cold stage climates in the Mediterranean: New evidence from the glacial record  

Microsoft Academic Search

A major reduction in summer temperatures during a Middle Pleistocene glacial cycle caused the most extensive glaciation recorded in the Mediterranean region. Glaciers in the mountains of Greece formed during marine isotope stage (MIS) 12 (474000–427000 years BP) under climatic conditions characterised by mean summer temperatures at least 11 °C cooler than today and annual precipitation of ?2300 mm at the equilibrium line

P. D. Hughes; J. C. Woodward; P. L. Gibbard

2007-01-01

15

The permafrost glacial hypothesis - how permafrost carbon dynamics controlled atmospheric CO2 and Pleistocene climate  

NASA Astrophysics Data System (ADS)

Revised carbon storage estimates highlight that the soil organic carbon pools in northern permafrost regions have long been extremely underestimated and may exceed 1670 Pg - more than twice the atmospheric carbon pool. The reason for that is that permafrost conditions can be very favorable for preservation of soil organic matter, which more than compensates for low biomass productivity. Although recent findings of increasing CO2 (and methane) emissions from warming and thawing permafrost ecosystems have fueled concerns about strong positive climate feedbacks, the role of permafrost carbon dynamics for atmospheric CO2 levels and global climate on glacial-interglacial timescales has largely escaped scientific interests. The still prevailing ‘ocean hypothesis’ is that CO2 was trapped in the glacial deep ocean leading to lower atmospheric concentrations, even though to date no deep ocean carbon pool during the glacial times has been found. We investigated a 15 m high, ~240 ka old permafrost loess profile in NE-Siberia using inter alia compound-specific deuterium measurements (?D) on plant-derived long-chain n-alkanes. Our results show that the organic-rich horizons accumulated during cold (glacial) periods as indicated by more negative ?D values, whereas soil organic material degraded and mineralized more intensively during warm (interglacial) periods (more positive ?D values). These findings illustrate the long-term carbon dynamics of permafrost soils, and spatial extrapolation of these dynamics to the vast, non-glaciated Siberian plains indicates that more than 1000 Pg soil organic carbon might have accumulated slowly during each glacial. Similar amounts of carbon could have been released rapidly during terminations, an equivalent to ~500 ppm CO2 in the atmosphere, which means that the oceans would have to act as sinks rather than sources as suggested by the ‘ocean hypothesis’. An external forcing for the permafrost carbon dynamics must be sought in integrated annual insolation, affecting mean annual temperatures and permafrost conditions. The ~40 ka periodicity of ice ages during the early Pleistocene can readily be explained by obliquity, which controls high-latitude integrated annual insolation. With the Mid-Pleistocene Transition, the overall Pleistocene cooling trend reached a threshold: permafrost reached mid-latitudes (~45°N), where integrated annual insolation is no longer controlled by obliquity, but eccentricity. As a consequence, obliquity cycles (glacial terminations) were skipped, unless they coincided with increasing eccentricity, resulting in ~100 ka glacial cycles.

Zech, R.; Huang, Y.; Zech, M.; Tarozo, R.

2010-12-01

16

Glacially-influenced late Pleistocene stratigraphy of a passive margin: New Jersey's Record of the North American ice sheet  

USGS Publications Warehouse

Glacial isostasy and the sediment supply changes associated with the waxing and waning of ice sheets have dramatic effects on the stratigraphy of adjacent continental shelves. In ancient stratigraphic records, the glacial influences on such deposits could be difficult to recognize because of the removal of coeval terrestrial glacial deposits by erosion. This study illustrates the effects of the Laurentide Ice Sheet on a basin near its maximum limit, the New Jersey continental shelf. Analysis of 1600 km of Geopulse???, Uniboom???, Minisparker??? and airgun profiles reveals four depositional sequences that have a maximum thickness of ???75 m near the shelf edge. Sequences I and IV correspond to the major glacial-interglacial sea level changes at Marine Isotope Chron (MIC) 6/5e and 2/1, whereas sequences II and III reflect smaller-scale sea-level fluctuations during chrons 4/3c and 3b/3a, respectively. Sequences I and IV are characterized by relatively thick low stand to early transgressive deposits near the shelf edge formed during times of increased sediment supply, but are thin and discontinuous across much of the shelf. Reflection horizons in these units deepen northward in the northern half of the study area due to collapse of a peripheral bulge that formed at the margin of the Laurentide Ice Sheet. The Hudson River moved from a more southerly drainage pattern to the modern Hudson Shelf Valley position, possibly under the influence of the advancing peripheral bulge. Sequences II and III are largely preserved within a broad mid-shelf swale likely created by the migration of an ancestral Hudson River, and their thickness implies much higher sedimentation rates during chrons 4 and 3 than seen today. If the terrestrial glacial record was eroded, the increased rates of sedimentation during the Pleistocene, dominance of sediments derived from northern New England, and northward tilting of strata could be interpreted as a result of uplift of a northern source area. The unusually high frequency of the relative sea-level oscillations (20 kyr), the concentration of sediment supply during low stands and early transgressions, and the correspondence of sea-level change with climatic change could be used to infer their relationship to a nearby ice sheet. Geologists studying deposits formed during times of widespread continental glaciation should consider possible glacial influences on the stratigraphy of mid-latitude deposits, even in the absence of sediments directly deposited by ice. ?? 2005 Elsevier B.V. All rights reserved.

Carey, J. S.; Sheridan, R. E.; Ashley, G. M.; Uptegrove, J.

2005-01-01

17

Agulhas salt-leakage oscillations during abrupt climate changes of the Late Pleistocene  

NASA Astrophysics Data System (ADS)

An ensemble of new, high-resolution records of surface ocean hydrography from the Indian-Atlantic oceanic gateway, south of Africa, demonstrates recurrent and high-amplitude salinity oscillations in the Agulhas Leakage area during the penultimate glacial-interglacial cycle. A series of millennial-scale salinification events, indicating strengthened salt leakage into the South Atlantic, appear to correlate with abrupt changes in the North Atlantic climate and Atlantic Meridional Overturning Circulation (AMOC). This interhemispheric coupling, which plausibly involved changes in the Hadley Cell and midlatitude westerlies that impacted the interocean transport at the tip of Africa, suggests that the Agulhas Leakage acted as a source of negative buoyancy for the perturbed AMOC, possibly aiding its return to full strength. Our finding points to the Indian-to-Atlantic salt transport as a potentially important modulator of the AMOC during the abrupt climate changes of the Late Pleistocene.

Marino, Gianluca; Zahn, Rainer; Ziegler, Martin; Purcell, Conor; Knorr, Gregor; Hall, Ian R.; Ziveri, Patrizia; Elderfield, Henry

2013-09-01

18

Meltwater pathways and grain size transformation in a Pleistocene Mediterranean glacial-fluvial system  

NASA Astrophysics Data System (ADS)

The Pleistocene sedimentary records of Mount Orjen, western Montenegro, have been used to investigate changes in grain size characteristics of fine sediments transported from the glaciated mountains to the fluvial systems downstream. Understanding the particle size characteristics of the fine sediments transported by these cold stage river systems is important for several reasons. The braided rivers draining the glaciated mountains of the western Balkans may have been an important source of loess for example. It is also important to establish the grain size signature of suspended sediment delivered to the marine environment to aid land-marine correlations. The fine-grained component of the tills is dominated by glacially-comminuted limestone particles. Detailed particle size analysis of the fine sediment matrix component (<63 ?m) of glacial till and alluvial deposits has been undertaken using multiple samples at 12 sites surrounding the Orjen massif. This limestone karst terrain includes a range of meltwater pathways and depositional contexts, including: river valleys, alluvial fans, poljes, and ice marginal settings. 35 U-series ages and soil development indices have been used to develop a robust geochronology for the Pleistocene records Two dominant surface meltwater and sediment pathways have been identified around Mount Orjen. The particle size distributions reveal that these transportation routes can have distinctive sedimentological signatures. Type 1 pathways deliver meltwater and sediments downstream via bedrock gorges. In these settings, the fine grained alluvial matrix presents a largely bimodal particle size distribution (PSD). Type 2 pathways represent meltwater channels draining directly from the ice margin. Alluvial sediments within these environments more closely resemble the normally distributed PSD of the glacial tills. The transition to bimodal PSDs, downstream of Type 1 meltwater routes, suggests that the glacially-comminuted sediments are modified in the fluvial environment. Significantly, the carbonate component is preferentially depleted or removed from the fine silt size fraction. Non-carbonate sediments are instead concentrated into this particle size window. This is thought to be a product of physical and chemical weathering as well as the mechanical sorting of glacially-derived limestone sediments. This has important implications for our understanding of sediment transfer processes within glaciated catchments before these sediments are transported offshore.

Adamson, Kathryn; Woodward, Jamie; Hughes, Philip

2013-04-01

19

Climatic implications of correlated upper Pleistocene glacial and fluvial deposits on the Cinca and Gallego rivers, NE Spain  

SciTech Connect

We correlate Upper Pleistocene glacial and fluvial deposits of the Cinca and Gallego River valleys (south central Pyrenees and Ebro basin, Spain) using geomorphic position, luminescence dates, and time-related trends in soil development. The ages obtained from glacial deposits indicate glacial periods at 85 {+-} 5 ka, 64 {+-} 11 ka, and 36 {+-} 3 ka (from glacial till) and 20 {+-} 3 ka (from loess). The fluvial drainage system, fed by glaciers in the headwaters, developed extensive terrace systems in the Cinca River valley at 178 {+-} 21 ka, 97 {+-} 16 ka, 61 {+-} 4 ka, 47 {+-} 4 ka, and 11 {+-} 1 ka, and in the Gallego River valley at 151 {+-} 11 ka, 68 {+-} 7 ka, and 45 {+-} 3 ka. The times of maximum geomorphic activity related to cold phases coincide with Late Pleistocene marine isotope stages and heinrich events. The maximum extent of glaciers during the last glacial occurred at 64 {+-} 11 ka, and the terraces correlated with this glacial phase are the most extensive in both the Cinca (61 {+-} 4 ka) and Gallego (68 {+-} 7 ka) valleys, indicating a strong increase in fluvial discharge and availability of sediments related to the transition to deglaciation. The global Last Glacial Maximum is scarcely represented in the south central Pyrenees owing to dominantly dry conditions at that time. Precipitation must be controlled by the position of the Iberian Peninsula with respect to the North Atlantic atmospheric circulation system. The glacial systems and the associated fluvial dynamic seem sensitive to (1) global climate changes controlled by insolation, (2) North Atlantic thermohaline circulation influenced by freshwater pulses into the North Atlantic, and (3) anomalies in atmospheric circulation in the North Atlantic controlling precipitation on the Iberian peninsula. The model of glacial and fluvial evolution during the Late Pleistocene in northern Spain could be extrapolated to other glaciated mountainous areas in southern Europe.

Lewis, Claudia J [Los Alamos National Laboratory; Mcdonald, Eric [NON LANL; Sancho, Carlos [NON LANL; Pena, Jose- Luis [NON LANL

2008-01-01

20

Oscillators and relaxation phenomena in Pleistocene climate theory  

PubMed Central

Ice sheets appeared in the northern hemisphere around 3?Ma (million years) ago and glacial–interglacial cycles have paced Earth's climate since then. Superimposed on these long glacial cycles comes an intricate pattern of millennial and sub-millennial variability, including Dansgaard–Oeschger and Heinrich events. There are numerous theories about these oscillations. Here, we review a number of them in order to draw a parallel between climatic concepts and dynamical system concepts, including, in particular, the relaxation oscillator, excitability, slow–fast dynamics and homoclinic orbits. Namely, almost all theories of ice ages reviewed here feature a phenomenon of synchronization between internal climate dynamics and astronomical forcing. However, these theories differ in their bifurcation structure and this has an effect on the way the ice age phenomenon could grow 3 Ma ago. All theories on rapid events reviewed here rely on the concept of a limit cycle excited by changes in the surface freshwater balance of the ocean. The article also reviews basic effects of stochastic fluctuations on these models, including the phenomenon of phase dispersion, shortening of the limit cycle and stochastic resonance. It concludes with a more personal statement about the potential for inference with simple stochastic dynamical systems in palaeoclimate science.

Crucifix, Michel

2012-01-01

21

Correlation of main climatic glacial-interglacial and loess-palaeosol cycles in the Pleistocene of Poland and Ukraine  

Microsoft Academic Search

LINDNER, L., BOGUTSKY, A., GOZHIK, P., MARCINIAK, B., MARKS, L., ¸ANCZONT, M. & WOJTANOWICZ, J. 2002. Correlation of main climatic glacial-interglacial and loess-palaeosol cycles in the Pleistocene of Poland and Ukraine. Acta Geologica Polonica, 52 (4), 459-469. Warszawa. An integrated analysis of climatic rhythms in the territory between the Baltic Sea and the Black Sea is presented. There is precise

LESZEK LINDNER; ANDREY BOGUTSKY; PETRO GOZHIK; BARBARA MARCINIAK; LESZEK MARKS; JÓZEF WOJTANOWICZ

22

Seismic characteristics of Pleistocene glacial cycles near shelf edge, offshore Louisiana, Gulf of Mexico  

SciTech Connect

Seismic stratigraphic studies of the shelf edge and the upper slope basins in the southern parts of the South Marsh Island, Eugene Island, Ship Shoal, and Green Canyon areas of the Louisiana outer continental shelf reveal at least four Pleistocene seismic stratigraphic cycles. These apparently reflect cyclic depositional patterns associated with glacially driven highstands and lowstands of sea level during this time. In the upper slope basins, a strong continuous reflector probably of turbiditic origin marks the base of each cycle. This reflector is thought caused by initial slumping occurring as sea level begins to fall. Overlying this reflector is a zone of chaotic-to-hummocky reflectors thought caused by slumping associated with knick-point erosion and channel-cutting during falling sea level. The upper portion of the cycle is largely reflectorless or weakly reflective punctuated with occasional strong, continuous turbidite reflectors. The reflectorless portion of the cycles is thought to represent homogeneous hemipelagic sedimentation during highstands. Shelf reflectors are usually moderately strong and continuous. A strong reflection(s), identified in some instances with gas sands, marks several sea level lowstands. Erosion is locally evident during lowstands. Otherwise, shelf reflectors are relatively uniform and show few characteristics associated with rising, falling, or highstanding parts of the sea level cycle.

Watkins, J.S.; Schneider, L.; Hilterman, F.

1987-05-01

23

Polyploid evolution and Pleistocene glacial cycles: A case study from the alpine primrose Primula marginata (Primulaceae)  

PubMed Central

Background Recent studies highlighted the role of Pleistocene climatic cycles in polyploid speciation and of southern Alpine refugia as reservoirs of diversity during glacial maxima. The polyploid Primula marginata, endemic to the southwestern Alps, includes both hexaploid and dodecaploid cytotypes that show no ecological or morphological differences. We used flow cytometry to determine variation and geographic distribution of cytotypes within and between populations and analyses of chloroplast (cp) and nuclear ribosomal (nr) DNA sequences from the Internal Transcribed Spacer (ITS) region to infer the evolutionary history of the two cytotypes and the auto- vs. allopolyploid origin of dodecaploid populations. Results We did not detect any intermediate cytotypes or variation of ploidy levels within populations. Hexaploids occur in the western and dodecaploids in the eastern part of the distributional range, respectively. The cpDNA and nrDNA topologies are in conflict, for the former supports shared ancestry between P. marginata and P. latifolia, while the latter implies common origins between at least some ITS clones of P. marginata and P. allionii. Conclusions Our results suggest an initial episode of chloroplast capture involving ancestral lineages of P. latifolia and P. marginata, followed by polyploidization between P. marginata-like and P. allionii-like lineages in a southern refugium of the Maritime Alps. The higher proportion of ITS polymorphisms in dodecaploid than in hexaploid accessions of P. marginata and higher total nucleotide diversity of ITS clones in dodecaploid vs. hexaploid individuals sequences are congruent with the allopolyploid hypothesis of dodecaploid origin.

2012-01-01

24

Provenance of Palouse Loess and Relation to Late Pleistocene Glacial Outburst Flooding, Washington State  

NASA Astrophysics Data System (ADS)

The eolian system of the Pacific Northwest is a product of long-term deflation of expansive sedimentary units by prevailing winds throughout the Quaternary. The Palouse loess is a deposit of wind-blown silt that covers approximately 10,000 sqare km up to 75 m thick. Late Quaternary units of the loess become finer texturally and thinner to the northeast, suggesting that they were derived from sedimentary basins south and west. The source of the loess has been inferred and hypothesized but never directly determined. A geochemical study of the late-Pleistocene to Holocene L1 unit of the Palouse loess and its possible sources was conducted to determine its provenance. There are two sedimentary units that lie upwind of the loess that may have contributed sediment via eolian deflation: 1) sand- and silt-rich slackwater sediment derived from late-Pleistocene outburst flooding of glacial Lake Missoula, and 2) sand- and silt-rich sediment from the Miocene-Pliocene Ringold Formation. Both are very similar in mineral composition, being derived from plutonic, metamorphic, and volcanic rocks of the western United States and southern British Columbia. Major and trace element data determined by x-ray fluorescence (XRF) of silt to very fine sand from loess and potential source sediments was used to pinpoint the exact source of the loess. A one-to-one relationship of major and trace elements exists between eolian and flood sediments, whereas Ringold Formation sediments have elevated Ti, P, Mg, and Ca oxides and lower K oxide values as well as scattered trace element values relative to Palouse loess. These trends may be due to the presence of basalt lithic grains in flood sediment that have been broken down and distributed throughout the loess. The Ringold Formation lacks appreciable amounts of basalt. The geochemical data from this study demonstrates that flood sediment is the dominant source of eolian material for the Palouse loess. The spatial distribution of the possible source sediments also suggests that flood slackwater sediment is the dominant source. Slackwater flood sediments rest in basins upwind of loess, where deflation is documented today. The Ringold Formation has limited exposure near the present day Columbia River and was eroded by outburst floods or covered by slackwater sediments. Its limited extent and exposure makes the Ringold Formation an less likely candidate for a source of the L1 loess. Study of the oldest units of Palouse loess will show whether or not the Ringold contributed a larger proportion of eolian material prior to the onset of glacial outburst flooding. This new provenance data will allow estimates to be made of the volume of dust ejected into the atmosphere from the Palouse eolian system since the last glacial maximum, which is essential to modeling of atmospheric dust fluxes that force climate fluctuations.

Sweeney, M. R.; Busacca, A. J.; Gaylord, D. R.; Zender, C. S.

2002-12-01

25

All together now? Sensitivity, dynamics, and predictability of planktonic foraminiferal species abundance versus community structure across Plio-Pleistocene glacial-interglacial cycles  

NASA Astrophysics Data System (ADS)

Most studies to date of biospheric sensitivity to global change have focused on understanding the sensitivity of modern species and communities to recent or experimental environmental change. However, it is unclear how to scale these results towards predicting the response of the biosphere to ongoing global change given that i) similar species often respond individualistically the same perturbation, ii) biotic response often scales nonlinearly with the size and/or duration of environmental change, and iii) many terrestrial and marine community types known from the recent past lack modern analogs. In this context, marine microfossils from deep sea sediments hold enormous promise for furthering our understanding of biotic sensitivity as they capture temporally expanded records of paleoceanographic and biotic response across a range climatic regimes (e.g., icehouse versus greenhouse climates), disturbance types (e.g., from background climate oscillations to mass extinctions), and habitats (e.g., low vs. high latitudes, upwelling vs. gyre ecosystems, etc). Here we use the repeated glacial-interglacial cycles and longer term trend of intensifying Northern Hemisphere glaciation from the Pliocene-Pleistocene to examine issues related to the sensitivity of planktonic foraminiferal species and communities to global change in an icehouse world. More specifically, we quantify the sensitivity and predictability of changes in planktonic foraminiferal species abundance (species specific mass accumulation rates) and community structure (dissimilarity indices and community classification) to glacial-interglacial cycles in the Plio-Pleistocene in two Atlantic sites (ODP Sites 999 and 662). We first examine whether the sensitivity of species and communities to glacial-interglacial cycles in the early Pliocene (~5-3 million years ago) is predictive of i) their sensitivity to the intensification of Northern Hemisphere glaciation (~3-2 million years ago), or ii) their sensitivity to glacial-interglacial cycles following the transition (< 2 million years ago). We then test the predictability of species and community change before, during, and after intensification, and the relative timing of biotic and environmental change. Our results build on existing faunal studies of nannoplankton and foraminiferal species dynamics (both shown to exhibit individualistic dynamics controlled, in part, by species ecology) to examine change at the community level and with regards to biosphere sensitivity.

Hull, P. M.; Norris, R. D.; Sexton, P.

2012-12-01

26

Dating Plio-Pleistocene glacial sediments using the cosmic-ray-produced radionuclides 10Be and 26Al  

USGS Publications Warehouse

We use the cosmic-ray-produced radionuclides 26Al and 10Be to date Plio-Pleistocene glacial sediment sequences. These two nuclides are produced in quartz at a fixed ratio, but have different decay constants. If a sample is exposed at the surface for a time and then buried by overburden and thus removed from the cosmic-ray flux, the 26Al/10Be ratio is related to the duration of burial. We first attempted to date pre-Wisconsinan tills by measuring 26Al and 10Be in fluvial sediments beneath them and applying the method of "burial dating," which previous authors have used to date river sediment carried into caves. This method, however, requires simplifying assumptions about the 26Al and 10Be concentrations in the sediment at the time of burial. We show that these assumptions are not valid for river sediment in glaciated regions. 26Al and 10Be analyses of such sediment do not provide accurate ages for these tills, although they do yield limiting ages in some cases. We overcome this difficulty by instead measuring 26Al and 10Be in quartz from paleosols that are buried by tills. We use a more general mathematical approach to determine the initial nuclide concentrations in the paleosol at the time it was buried, as well as the duration of burial. This technique provides a widely applicable improvement on other means of dating Plio-Pleistocene terrestrial glacial sediments, as well as a framework for applying cosmogenic-nuclide dating techniques in complicated stratigraphic settings. We apply it to pre-Wisconsinan glacial sediment sequences in southwest Minnesota and eastern South Dakota. Pre-Wisconsinan tills underlying the Minnesota River Valley were deposited 0.5 to 1.5 Ma, and tills beneath the Prairie Coteau in eastern South Dakota and adjacent Minnesota were deposited 1 to 2 Ma.

Balco, G.; Stone, J. O. H.; Jennings, C.

2005-01-01

27

New data on the Late Pleistocene and Holocene glacial, climate and relative sea-level changes at Fildes Peninsula, King George Island (South Shetlands Islands, West Antarctica)  

Microsoft Academic Search

New data on the Late Pleistocene and Holocene glacial, climate and relative sea-level changes at Fildes Peninsula, King George Island (South Shetlands Islands, West Antarctica) Verkulich S. R. (Arctic and Antarctic Research Institute, St. Petersburg, Russia) Pushina Z.V. (Arctic and Antarctic Research Institute, St. Petersburg, Russia) Tatur A. (Department of Antarctic Biology, Polish Academy of Sciences, Warsaw, Poland) During the

Sergey Verkulich; Zina Pushina; Andrej Tatur

2010-01-01

28

Millennial-Scale Climate Variability During a mid-Pleistocene Glacial (MIS 12) from a Terrestrial Lacustrine Record in the Valles Caldera, New Mexico  

NASA Astrophysics Data System (ADS)

We present a high-resolution terrestrial climate record from the Valles Caldera, New Mexico which spans some 200,000 years from mid MIS 14 to early MIS 10. The glacial periods represented in the record exhibit millennial-scale Dansgaard-Oeschger like variability, especially in MIS 12, one of the coldest glacials in the Pleistocene. High resolution proxies from core VC-3 including scanning XRF data, sediment density, color, and magnetic susceptibility show approximately 23 millennial-scale oscillations in MIS 12 with an average duration of 2,300 years. Many of these oscillations are characterized by relatively slow coolings followed by abrupt warmings, similar to D-O events in the Greenland ice core record. MBT/CBT MAT estimates in the MIS 12 portion of the core show stadial to interstadial warmings of up to 6 °C. The VC-3 stadials correlate with high percentages of boreal taxa pollen ( Picea, Abies ) (up to 25%) while interstadials have lower boreal pollen percentages (~5%) and many correlate with local maxima in Juniperus> and Quercus> . Significant changes in the hydrologic cycle also occur at these millennial timescales. Oxygen isotope data from diatom silica record changes of up to 10 per mil from stadial to interstadial, probably reflecting a combination of changes in moisture source (Pacific vs. Gulf of Mexico), moisture transport pathway, and the seasonality of precipitation. Several interstadials correlate with increases in Cyperaceae (sedge) pollen suggesting a shallower lake with a broad marshy zone around its margin. This zone was minimized during stadials when the lake was deeper. Interstadial shallowing probably resulted from higher evaporation rates and/or a reduction in winter precipitation. This combination of proxies from the Valles Caldera suggests that glacial stage millennial-scale climate variability in the American southwest was strongly driven by changes in the strength and location of the winter polar jet, which in turn affected the local hydrologic cycle and isotopic composition of precipitation, regional temperature change, watershed vegetation, the amount of fluvial runoff vs. atmospheric dust loading in the Valles Caldera lake, and contributed to the abrupt warmings ending the D-O like cycles.

Fawcett, P. J.; Brown, E. T.; Werne, J. P.; Contreras, S.; Anderson, R. S.; Dodd, J. P.; Sharp, Z. D.; Heikoop, J. M.; Allen, C. D.

2011-12-01

29

Stratigraphic constraints on late Pleistocene glacial erosion and deglaciation of the Chukchi margin, Arctic Ocean  

Microsoft Academic Search

At least two episodes of glacial erosion of the Chukchi margin at water depths to ?450 m and 750 m have been indicated by geophysical seafloor data. We examine sediment stratigraphy in these areas to verify the inferred erosion and to understand its nature and timing. Our data within the eroded areas show the presence of glaciogenic diamictons composed mostly of reworked

Leonid Polyak; Dennis A. Darby; Jens F. Bischof; Martin Jakobsson

2007-01-01

30

Effects of glacial isostatic adjustment since the late Pleistocene on the uplift of the Tibetan Plateau  

Microsoft Academic Search

We simulate the seven components of the geophysical signatures arising from the glacial isostatic adjustment in the Tibetan Plateau for three different ice models in order to investigate the implications for the plateau's uplift. Particular attention is devoted to the effects of the presence of asthenosphere and its uncertainty with regard to viscosity on the computational results. For this purpose,

Hansheng Wang

2001-01-01

31

Physiological and growth responses of C3 and C4 plants at the Pleistocene glacial maximum  

SciTech Connect

A C3 plant (Abutilon theophrasti) and a C4 plant (Amaranthus retroflexus) were grown from seed in the Duke University Phytotron under four CO2 concentrations (15 Pa, below the Pleistocene minimum), 27 Pa (pre-industrial), 35 Pa (current), and 70 Pa (future) to examine photosynthetic, growth and reproduction responses of annual plants to historic and future levels of CO2. Net photosynthesis and growth were greatly inhibited at 15 Pa and greatly stimulated at 70 Pa. in the C3 Abutilon but only slightly affected in the C4 Amaranthus. Flower bud initiation was not affected by CO2 treatment in either species but all flower buds in 15 Pa CO2 aborted in the C3 within two days of appearance while no inhibition of reproduction was observed at low CO2 in the C4. Differences in physiology, growth and reproduction to the low levels of atmospheric CO2 of the Pleistocene suggest that competitive interactions of C3 and C4 annuals have changed through geologic time. A major question concerning the survival and evolution of obligate C3 annuals during the CO2 minima of the Pleistocene is raised by the results of this study.

Strain, B.R. [Duke Univ., Durham, NC (United States)

1995-06-01

32

The glacial geomorphology and Pleistocene history of South America between 38°S and 56°S  

NASA Astrophysics Data System (ADS)

This paper presents new mapping of the glacial geomorphology of southern South America between latitudes 38°S and 56°S, approximately the area covered by the former Patagonian Ice Sheets. Glacial geomorphological features, including glacial lineations, moraines, meltwater channels, trimlines, sandur and cirques, were mapped from remotely sensed images (Landsat 7 ETM+, pan-sharpened Landsat 7 and ASTER). The landform record indicates that the Patagonian Ice Sheets consisted of 66 main outlet glaciers, together with numerous local cirque glaciers and independent ice domes in the surrounding mountains. In the northern part of the mapped area, in the Chilean Lake District (38-42°S), large piedmont glaciers developed on the western side of the Andes and the maximum positions of these outlet glaciers are, in general, marked by arcuate terminal moraines. To the east of the Andes between 38°S and 42°S, outlet glaciers were more restricted in extent and formed "alpine-style" valley glaciers. Along the eastern flank of the Andes south of ˜45°S a series of large fast-flowing outlet glaciers drained the ice sheet. The location of these outlet glaciers was topographically controlled and there was limited scope for interactions between individual lobes. West of the Andes at this latitude, there is geomorphological evidence for an independent ice cap close to sea level on the Taitao Peninsula. The age of this ice cap is unclear but it may represent evidence of glacier growth during the Antarctic Cold Reversal and/or Younger Dryas Chronozone. Maximum glacier positions are difficult to determine along much of the western side of the Andes south of 42°S because of the limited land there, and it is assumed that most of these glaciers had marine termini. In the south-east of the mapped area, in the Fuegan Andes (Cordillera Darwin), the landform record provides evidence of ice-sheet initiation. By adding published dates for glacier advances from the literature we present maps of pre-Last Glacial Maximum (LGM) glacier extent, LGM extent and the positions of other large mapped moraines younger than LGM in age. A number of large moraines occur within the known LGM limits. The age of these moraines is unknown but, since many of them lie well outside the established maximum Neoglacial positions, the possibility that they reflect a return to glacial climates during the Younger Dryas Chronozone or Antarctic Cold Reversal cannot be discounted.

Glasser, Neil F.; Jansson, Krister N.; Harrison, Stephan; Kleman, Johan

2008-02-01

33

Early onset and tropical forcing of 100,000-year Pleistocene glacial cycles.  

PubMed

Between 1.5 and 0.6 Myr ago, the period of the Earth's glacial cycles changed from 41 kyr, the period of the Earth's obliquity cycles, to 100 kyr, the period of the Earth's orbital eccentricity, which has a much smaller effect on global insolation. The timing of this transition and its causes pose one of the most perplexing problems in palaeoclimate research. Here we use complex demodulation to examine the phase evolution of precession and semiprecession cycles--the latter of which are phase-coupled to both precession and eccentricity--in the tropical and extratropical Atlantic Ocean. We find that about 1.5 Myr ago, tropical semiprecession cycles (with periods of about 11.5 kyr) started to propagate to higher latitudes, coincident with a growing amplitude envelope of the 100-kyr cycles. Evidence from numerical models suggests that cycles of about 10 kyr in length may be required to explain the high amplitude of the 100-kyr cycles. Combining our results with consideration of a modern analogue, we conclude that increased heat flow across the equator or from the tropics to higher latitudes around 1.5 Myr ago strengthened the semiprecession cycle in the Northern Hemisphere, and triggered the transition to sustained 100-kyr glacial cycles. PMID:11081508

Rutherford, S; D'Hondt, S

2000-11-01

34

Multiple genetic divergences and population expansions of a Mediterranean sandfly, Phlebotomus ariasi, in Europe during the Pleistocene glacial cycles  

PubMed Central

Phlebotomus ariasi is one of the two sandflies transmitting the causative agent of zoonotic leishmaniasis, Leishmania infantum, in France and Iberia, and provides a rare case study of the postglacial re-colonization of France by a Mediterranean species. Four DNA sequences were analysed—mitochondrial cytochrome b (cyt b), nuclear elongation factor-1? (EF-1?) and two anonymous nuclear loci—for 14–15 French populations and single populations from northeast Spain, northwest Spain, Portugal and Morocco. The presence of cryptic sibling species was not revealed by phylogenetic analyses and testing for reproductive isolation between sympatric populations defined by the two most divergent cyt b haplogroups. No locus was shown to be under positive directional or balancing selection and, therefore, molecular variation was explained demographically. Each nuclear locus showed shallow isolation by distance from Portugal to the French Pyrenees, but for both cyt b and EF-1? there was then a step change to the upland Massif Central, where leading-edge populations showed low diversity at all loci. Multiple genetic divergences and population expansions were detected by analyses of cyt b and dated to the Pleistocene. Endemicity of one cyt b sub-lineage suggested the presence of a refuge north of the Pyrenees during the last glacial period. Monopolization of the Massif Central by genetically differentiated populations of P. ariasi might possibly hinder the northwards spread of leishmaniasis.

Mahamdallie, S S; Pesson, B; Ready, P D

2011-01-01

35

Climatic impact of glacial cycle polar motion: Coupled oscillations of ice sheet mass and rotation pole position  

USGS Publications Warehouse

Precessional motion of Earth's rotation axis relative to its orbit is a well-known source of long-period climatic variation. It is less well appreciated that growth and decay of polar ice sheets perturb the symmetry of the global mass distribution enough that the geographic location of the rotation axis will change by at least 15 km and possibly as much as 100 km during a single glacial cycle. This motion of the pole will change the seasonal and latitudinal pattern of temperatures. We present calculations, based on a diurnal average energy balance, which compare the summer and winter temperature anomalies due to a 1° decrease in obliquity with those due to a 1° motion of the rotation pole toward Hudson Bay. Both effects result in peak temperature perturbations of about 1° Celsius. The obliquity change primarily influences the amplitude of the seasonal cycle, while the polar motion primarily changes the annual mean temperatures. The polar motion induced temperature anomaly is such that it will act as a powerful negative feedback on ice sheet growth. We also explore the evolution of the coupled system composed of ice sheet mass and pole position. Oscillatory solutions result from the conflicting constraints of rotational and thermal stability. A positive mass anomaly on an otherwise featureless Earth is in rotational equilibrium only at the poles or the equator. The two polar equilibria are rotationally unstable, and the equatorial equilibrium, though rotationally stable, is thermally unstable. We find that with a plausible choice for the strength of coupling between the thermal and rotational systems, relatively modest external forcing can produce significant response at periods of 104–106 years, but it strongly attenuates polar motion at longer periods. We suggest that these coupled oscillations may contribute to the observed dominance of 100 kyr glacial cycles since the mid-Pleistocene and will tend to stabilize geographic patterns that are suitable to glaciations.

Bills, Bruce G.; James, Thomas S.; Mengel, John G.

1999-01-01

36

Speciation of two desert poplar species triggered by Pleistocene climatic oscillations.  

PubMed

Despite the evidence that the Pleistocene climatic fluctuations have seriously affected the distribution of intraspecific diversity, less is known on its impact on interspecific divergence. In this study, we aimed to test the hypothesis that the divergence of two desert poplar species Populus euphratica Oliv. and P. pruinosa Schrenk. occurred during the Pleistocene. We sequenced 11 nuclear loci in 60 individuals from the two species to estimate the divergence time between them and to test whether gene flow occurred after species separation. Divergence time between the two species was estimated to be 0.66-1.37 million years ago (Ma), a time at which glaciation was at its maximum in China and deserts developed widely in central Asia. Isolation-with-Migration model also indicated that the two species had diverged in the presence of gene flow. We also detected evidence of selection at GO in P. euphratica and to a lesser extent at PhyB2. Together, these results underscore the importance of Pleistocene climate oscillations in triggering plant speciation as a result of habitats divergence. PMID:24065180

Wang, J; Källman, T; Liu, J; Guo, Q; Wu, Y; Lin, K; Lascoux, M

2014-02-01

37

Middle Pleistocene (?) buried glacial ice on Bylot Island, Canadian Arctic Archipleago  

NASA Astrophysics Data System (ADS)

Bylot Island is located north of Baffin Island (73°N, 80°W). More than the half of the island is covered by an ice cap and its outlet glaciers flowing towards the arctic lowland of the Lancaster formation. The study site comprises four main stratigraphic units. Overlying the shales (Tertiary) of the Lancaster Formation (500 m a.s.l.), a diamicton (unit 1) is covered by a “fossil forest-tundra” sequence (unit 2) containing abundant remains of trees and plants (Allard et al., submitted). Paleontological correlation of extinct species and reverse to normal palomagnetism polarities suggest a Late Pliocene to Early Pleistocene age for this unit. A sequence (unit 3) of ice-contact proximal to distal glacio-fluvial sediments overlies the organic beds. Paleomagnetic analysis showed that the upper glacio-fluvial sediments were likely deposited during the Brunhes polarity chron (younger than 0.73 Ma). The uppermost unit (unit 4) consists in a lodgement till containing clasts of Paleozoic limestone erratics. Based on amino acid ratios of shells fragments in the drift, Klassen (1993) suggested that this “foreign drift” was probably deposited during an "old" Quaternary glaciation named “Baffin glaciation” During July 2009 several active-layer detachment slides at the head of large gullies exposed large massive ice bodies located at the junction between units 3 and 4. A preliminary analysis of the ice facies and ice crystals revealed the presence of two distinct types of massive ice: 1) clear-ice bodies with very few sediments and no organic inclusions. The ice crystals were large (cm) and air bubbles were observed at the junction of crystals. These characteristics could potentially indicate an englacial origin for these clear ice bodies. In some places, the ice was stratified with undulating layers of sands and gravels. These micro-structures are very similar to basal ice facies we observed at the Matanuska Glacier in Alaska. The exposed massive ice sections were a few tens of meter wide and about 2 to 4 m deep but the real width and thickness of these ice masses are unknown. The upper part of the clear ice and stratified massive ice bodies were always in contact with various types of glacio-fluvial sediments which suggest that their preservation were likely related to rapid burial of the ice and refreezing of the overlying sediments following permafrost aggradation. 2) large, white to milky, epigenetic ice wedges with a typical sub-vertical foliated structure. The ice wedges were formed in unit 4 and, in some places, penetrated into the clear massive ice bodies described above which created a sharp visual contrast between the two types of ice. This also indicates that ice wedge development post-date the massive ice burial. Based on the chrono-stratigraphic context and on the similarities between 1) the clear ice masses and the contemporary englacial ice facies (e.g. on Bylot Island); and 2) the cryostructures of the stratified massive ice at the study site and the contemporary basal ice cryostructures observed at the Matanuska glaciers, we propose that the massive ice bodies exposed on Bylot Island are related to a Middle Pleistocene glaciation.

Fortier, D.; Godin, E.; Kanevskiy, M. Z.; Allard, M.

2009-12-01

38

Late-glacial environmental oscillations as recorded in the soil archives of Gasserplatz (Vorarlberg, Austria).  

NASA Astrophysics Data System (ADS)

Gasserplatz is a shallow basin in the rather flat, glacially eroded confluence area of the former Rhine glacier and the Ill glacier. It became ice-free during the Feldkirch stadium ( ? 15.500 calBP) and transferred in a tiny lake. During the Late-glacial lacustrine carbonate (calcareous gyttja) was deposited, in the Holocene peat accumulated. Gasserplatz deposits are valuable soil archives for palaeo-environmental research. The Late-glacial environmental fluctuations have been recorded in the gyttja deposits. The combined results of pollen, macro-remains and stable isotope analyses that have been put into an independent time frame demonstrate that these fluctuations are associated with large scale temperature oscillations as registered in Greenland ice cores. The results show also a slight delay in environmental response on temperature change. The isotope stratigraphy points to higher temperatures during the whole Bølling while Betula trees arrived in Gasserplatz area later. During the Allerød there are frequent short-lived oscillations in temperature, but the palynological expression of these changes resulted in not more than three Betula peaks. Comparison with the research results of similar Alpine lake deposits makes clear that the fluctuations in the Betula curve are not a local but a regional phenomenon.

van Mourik, Jan; Slotboom, Ruud; van der Plicht, Hans; Streurman, Harm Jan; Kuijper, Wim; Hoek, Wim; de Graaff, Leo

2013-04-01

39

The hierarchical structure of glacial climatic oscillations: Interactions between ice-sheet dynamics and climate  

SciTech Connect

Abrupt climatic oscillations around the North Atlantic have been identified recently in Greenland ice cores as well as in North Atlantic marine sediment cores. The good correlation between the {open_quote}Dansgaard Oeschger events{close_quote} in the ice and the {open_quote}Heinrich events{close_quote} in the ocean suggests climate, in the North Atlantic region, underwent several massive reorganizations in the last glacial period. A characteristic feature seems to be their hierarchical structure. Every 7 to 10-thousand years, when the temperature is close to its minimum, the ice-sheet undergoes a massive iceberg discharge. This Heinrich event is followed by an abrupt warming. then by other oscillations, each lasting between one and two thousand years. These secondary oscillations do not have a clear signature in marine sediments but constitute most of the{open_quote} Dansgaard-Oeschger events{close_quote} found in the ice. A simplified model coupling an ice-sheet and an ocean basin, to illustrate how the interactions between these two components can lead to such a hierarchical structure. The ice-sheet model exhibits internal oscillations composed of growing phases and basal ice melting phases that induce massive iceberg discharges. These fresh water inputs in the ocean stop for a moment the thermohaline circulation, enhancing the temperature contrast between low- and high-latitudes. Just after this event, the thermohaline circulation restarts and an abrupt warming of high-latitude regions is observed. For some parameter values, these warmer temperatures have some influence on the ice-sheet, inducing secondary oscillations similar to those found in paleoclimatic records. Although the mechanism presented here may be too grossly simplified. it nevertheless underlines the potential importance of the coupling between ice-sheet dynamics and oceanic thermohaline circulation on the structure of the climatic records during the last glacial period. 33 refs., 14 figs., 1 tab.

Paillard, D. [Centre d`Etudes de Saclay, Gif sur Yvette (France)] [Centre d`Etudes de Saclay, Gif sur Yvette (France)

1995-04-01

40

Pliocene-Pleistocene record of glacial Interglacial Cyclicity in the McMurdo Sound from the ANDRILL-MIS carbonate record  

NASA Astrophysics Data System (ADS)

Carbonates are long known to contain paleo-climatic and paleo-environmental records. We have examined the carbonate record of the upper 200 m of the ANDRILL-MIS core and found distinct signals of glacial/interglacial cyclicity. Indicators for glacial-interglacial cyclicity are the isotopic composition of carbonates as well as carbonate mineral composition. d18O isotope excursions usually correspond with an opposite shift in d13C. This shift is likely caused by a shift in carbon source and/or porewater chemistry leading to the carbonate precipitation, indicating a climatic or environmental change. Several positive isotope excursions, indicator for interglacial conditions, were found in the Pleistocene record. one of them at 25 mbsf possibly indicating a collapse of the West Antarctic Ice Sheet during Marine Isotope Stage 11 or 5e (e.g. Scherer et al. 1998). Further detailed work is currently under way to further define individual Pliocene and Pleistocene interglacial periods and number of interglacials recorded in the carbonate record.

Vogel, S. W.; Helling, D.; Brown, R. E.; Kuhn, G.; Tollstrup, D.

2009-04-01

41

Multiple cosmogenic nuclides document complex Pleistocene exposure history of glacial drifts in Terra Nova Bay (northern Victoria Land, Antarctica)  

Microsoft Academic Search

Geomorphological and glacial geological surveys and multiple cosmogenic nuclide analyses (10Be, 26Al, and 21Ne) allowed us to reconstruct the chronology of variations prior to the last glacial maximum of the East Antarctic Ice Sheet (EAIS) and valley glaciers in the Terra Nova Bay region. Glacially scoured coastal piedmonts with round-topped mountains occur below the highest local erosional trimline. They represent

Luigia Di Nicola; Stefan Strasky; Christian Schlüchter; Maria Cristina Salvatore; Naki Akçar; Peter W. Kubik; Marcus Christl; Haino Uwe Kasper; Rainer Wieler; Carlo Baroni

2009-01-01

42

Phylogeographic analysis of the red seaweed Palmaria palmata reveals a Pleistocene marine glacial refugium in the English Channel  

Microsoft Academic Search

Phylogeography has provided a new approach to the analysis of the postglacial history of a wide range of taxa but, to date, little is known about the effect of glacial periods on the marine biota of Europe. We have utilized a combination of nuclear, plastid and mitochondrial genetic markers to study the biogeographic history of the red seaweed Palmaria palmata

JIM PROVAN; REMI A. W ATTIER; CHRISTINE A. M AGGS

43

Low but structured chloroplast diversity in Atherosperma moschatum (Atherospermataceae) suggests bottlenecks in response to the Pleistocene glacials  

PubMed Central

Background and Aims The cool temperate rainforests of Australia were much reduced in range during the cold and dry glacial periods, although genetic evidence indicates that two key rainforest species, Nothofagus cunninghamii and Tasmannia lanceolata, survived within multiple locations and underwent only local range expansions at the end of the Last Glacial. To better understand the glacial response of a co-occurring but wind-dispersed and less cold-tolerant rainforest tree species, Atherosperma moschatum, a chloroplast phylogeographic study was undertaken. Methods A total of 3294 bp of chloroplast DNA sequence was obtained for 155 samples collected from across the species' range. Key Results The distribution of six haplotypes observed in A. moschatum was geographically structured with an inferred ancestral haplotype restricted to Tasmania, while three non-overlapping and endemic haplotypes were found on the mainland of south-eastern Australia. Last glacial refugia for A. moschatum are likely to have occurred in at least one location in western Tasmania and in Victoria and within at least two locations in the Great Dividing Range of New South Wales. Nucleotide diversity of A. moschatum was lower (? = 0·00021) than either N. cunninghamii (0·00101) or T. lanceolata (0·00073), and was amongst the lowest recorded for any tree species. Conclusions This study provides evidence for past bottlenecks having impacted the chloroplast diversity of A. moschatum as a result of the species narrower climatic niche during glacials. This hypothesis is supported by the star-like haplotype network and similar estimated rates of chloroplast DNA substitution for A. moschatum and the two more cold tolerant and co-occurring species that have higher chloroplast diversity, N. cunninghamii and T. lanceolata.

Worth, James R. P.; Marthick, James R.; Jordan, Gregory J.; Vaillancourt, Rene E.

2011-01-01

44

Phylogeography of the Alcippe morrisonia (Aves: Timaliidae): long population history beyond late Pleistocene glaciations  

Microsoft Academic Search

BACKGROUND: The role of Pleistocene glacial oscillations in current biodiversity and distribution patterns varies with latitude, physical topology and population life history and has long been a topic of discussion. However, there had been little phylogeographical research in south China, where the geophysical complexity is associated with great biodiversity. A bird endemic in Southeast Asia, the Grey-cheeked Fulvetta, Alcippe morrisonia,

Gang Song; Yanhua Qu; Zuohua Yin; Shouhsien Li; Naifa Liu; Fumin Lei

2009-01-01

45

Soil-Geomorphic Analysis of Late-Pleistocene Glacial Sequences in the McGee, Pine, and Bishop Creek Drainages, East-Central Sierra Nevada, California  

NASA Astrophysics Data System (ADS)

Late-Pleistocene glacial sequences at McGee, Pine, and Bishop creeks, in the east-central Sierra Nevada, are resolvable into distinct relative-age groups on the basis of soil development, the weathering of surface and subsurface clasts, and geomorphic criteria. The data differentiate moraines from Tioga and Tahoe glaciations at McGee Creek, and Tioga, Tahoe, and pre-Tahoe/post-Sherwin glaciations at Pine and Bishop creeks; no moraines from a Tenaya glaciation are differentiated by the data, perhaps because the soil-geomorphic methods are not sensitive enough to resolve small age differences between moraines. The lack of fine age resolution is probably due to a combination of factors, including (1) slow rates of soil development, (2) low amounts of atmospheric dust added to the soils, (3) the susceptibility of crest soils to erosion, and (4) the susceptibility of footslope soils to burial by colluvium. Age resolution is improved by evaluating soils at both the moraine crests and the relatively wetter footslope sites, and by basing age assignments on a combination of macro- and micromorphologic soil properties, the disintegration of subsurface clasts, and parameters of surface-clast weathering.

Berry, Margaret E.

1994-03-01

46

Sedimentological evidence for a deforming bed in a late Pleistocene glacial sequence from ANDRILL AND-1B, Ross Sea, Antarctica  

NASA Astrophysics Data System (ADS)

A 1,284.87m-long sediment core (AND-1B) was drilled from beneath the McMurdo Ice Shelf sector of the Ross Ice Shelf as part of the Antarctic geological drilling program, ANDRILL. Snapshots of diamictite depositional processes and paleoenvironmental conditions have been interpreted from a nested set of samples collected at overlapping scales of observation. Data used for detailed sedimentological analyses include cm-scale core logging based on x-radiographs of the archive halves in addition to the original core description, bulk samples, and oriented 45 x 70mm thin sections of diamictites for micromorphology analysis. The 5.8m-thick interval studied contains a complete glacial advance-retreat sequence that is bracketed by glacial surfaces of erosion (GSE) at 41.9 and 47.7mbsf recording glacial advance over the core site. 4.6m of subglacial till is deposited above the lower GSE represented by a sequence of thin muddy conglomerate with diverse pebble lithologies, massive clast-rich muddy diamicite, and stratified diamictite with clast-rich and clast-free beds. The sand size fraction of bulk samples and thin sections from the till are dominated by aggregate grains, termed till pellets following terminology used by sedimentologists in the Ross Sea. The core of the pellet may be a lithic grain or stiff till with additional clay plastered on the outside forming rounded grains from angular ones. Till pellets are rounded, spherical to prolate in form and are associated with turbate structures and aligned grains in till thin sections - evidence of rotational deformation. The area beneath an ice shelf in front of a grounding line is recorded by a thin bed of granular particles that transitions to silty claystone stratified with granules. Granular layers are thought to be from periodic winnowing by strong currents focused near the grounding line. The sub-ice shelf transition from proximal grounding line to distal is recorded by a gradational contact between stratified silty claystone and massive silty claystone. The subglacial diamictites within this interval appear to have formed within a deforming bed. Clay content and high porewater pressures are key to development of till pellets because flow within the dilated till layer rounds intraclasts without crushing grains. Upon a drop in porewater pressure and draining of the till, accretion occurs as the grains solidify. Continuous deforming beds are likely to occur in ice-marginal locations where soft sediment is abundant and where subglacial water is spread diffusely rather than focused in channels.

Cowan, E. A.; Powell, R. D.

2009-12-01

47

Water versus ice: The competing roles of modern climate and Pleistocene glacial erosion in the Central Alps of Switzerland  

NASA Astrophysics Data System (ADS)

Recent studies have identified relationships between landscape form, erosion and climate in regions of landscape rejuvenation, associated with increased denudation. Most of these landscapes are located in non-glaciated mountain ranges and are characterized by transient geomorphic features. The landscapes of the Swiss Alps are likewise in a transient geomorphic state as seen by multiple knickzones. In this mountain belt, the transient state has been related to erosional effects during the Late Glacial Maximum (LGM). Here, we focus on the catchment scale and categorize hillslopes based on erosional mechanisms, landscape form and landcover. We then explore relationships of these variables to precipitation and extent of LGM glaciers to disentangle modern versus palaeo controls on the modern shape of the Alpine landscape. We find that in grasslands, the downslope flux of material mainly involves unconsolidated material through hillslope creep, testifying a transport-limited erosional regime. Alternatively, strength-limited hillslopes, where erosion is driven by bedrock failure, are covered by forests and/or expose bedrock, and they display oversteepened hillslopes and channels. There, hillslope gradients and relief are more closely correlated with LGM ice occurrence than with precipitation or the erodibility of the underlying bedrock. We relate the spatial occurrence of the transport- and strength-limited process domains to the erosive effects of LGM glaciers. In particular, strength-limited, rock dominated basins are situated above the equilibrium line altitude (ELA) of the LGM, reflecting the ability of glaciers to scour the landscape beyond threshold slope conditions. In contrast, transport-limited, soil-mantled landscapes are common below the ELA. Hillslopes covered by forests occupy the elevations around the ELA and are constrained by the tree line. We conclude that the current erosional forces at work in the Central Alps are still responding to LGM glaciation, and that the modern climate has not yet impacted on the modern landscape.

Schlunegger, Fritz; Norton, Kevin P.

2013-08-01

48

Lithologic expressions of glacial/interglacial and millennial-scale variability in the Pacific sub-Arctic record during the Pleistocene (Bering Sea, IODP Exp. 323)  

NASA Astrophysics Data System (ADS)

Diatom-rich, Pleistocene sediments collected in the Bering Sea during IODP Exp. 323 in the Bering Slope (Sites U1339 and U1344) and at the Bowers Ridge (Site U1340) show prominent variability of physical properties (e.g. bulk density), lithology and in the preservation of diatom valves. Using the shipboard physical property data in combination with newly generated smear slide counts and laser particle size (LPS) analyses we were able to quantify the sedimentary components for statistical analysis. Our data confirm that bulk density is negatively correlated with mean grain size (~30%) and abundance of diatoms (~40%) while it positively correlates with clay size particles (~20%) and silt-size siliciclastic particles (~20%). However, clay size particles and silt-size siliciclastic show no significant correlation, suggesting independent sources. We also found that diatom valve integrity is correlated with the abundance of diatoms (~40%) suggesting that diatom preservation increases with increasing opal fluxes. Finally, we found a surprisingly low correlation (only ~30%) between abundance of clay minerals (from smear slide counts) and percent clay-sized particles (LPS); SEM analysis supports the interpretation that a significant portion of clay-sized particles could derive from the dissolution/fragmentation of diatom biosilica. In conclusion, more than 40% of lithologic variability in the Bering Sea sediments reflects changes in the abundance of diatoms and siliciclastic particles: glacial/stadial (interglacial/interstadial) conditions were characterized by lower (higher) primary productivity, higher (lower) terrigenous input, and diatom valve dissolution and formation of clay-size biosilica particles (higher diatom valve preservation). Our approach offers new insights on the links between changes in sedimentation and oceanography at different scales of climate variability in the Bering Sea and potentially in other similar high latitude basins.

Drake, M.; Aiello, I. W.

2012-12-01

49

Post-Last Glacial Maximum (Latest Pleistocene to Holocene) geology of the Santa Barbara shelf, southern California  

NASA Astrophysics Data System (ADS)

High-resolution bathymetric and seismic-reflection data collected for the California Seafloor Mapping Program (http://walrus.wr.usgs.gov/mapping/csmp/) provide new insights for understanding the post-Last Glacial Maximum (LGM) evolution of the Santa Barbara shelf, highlighting relationships between tectonics, eustasy, and sediment supply. The west-trending shelf extends offshore for 5 to 7 km and is bounded on the south by the deep Santa Barbara basin and on the north by a narrow coastal zone and the steep, rapidly uplifting Santa Ynez Mountains. The active, west-trending, north-dipping Ventura-Pitas Point-North Channel and Red Mountain fault systems form the structural boundary between two distinct shelf domains. The smooth, gently sloping, southern shelf is flooded by thick (35 to 40 m), prograding Santa Clara and Ventura River deltaic deposits. These thick strata drape the shelfbreak and fill the accommodation space created by rising sea level, largely masking the influence of active tectonics. In contrast, the northern shelf has complex bathymetry and a well-defined, sharp shelfbreak at ~90 m water depth. The northern shelf is relatively sediment starved (mean sediment thickness is 3 to 4 m), with thickest accumulations (up to ~18 m) forming shallow (< 30 m), discontinuous to laterally coalescing, inner-shelf bars that are best developed at the mouths of steep coastal watersheds. These watersheds also feed several distinct, coarse-grained sediment lobes (as large as ~1.5 km2, extending to 3 km offshore and depths of 70 m) that probably formed during massive flood events. The relative lack of offshore deposits on the northern shelf suggests sediment transport is dominated by easterly nearshore drift. Faulting and folding on the northern shelf are significant controls on sediment distribution and thickness, the occurrence of bedrock uplifts, and common hydrocarbon-associated seeps, pockmarks, and mounds. Bedrock, typically "soft" Neogene strata, is especially common on the mid- to-outer shelf, forming low-relief ribbed outcrops. Bedrock on the flat outer shelf contains nearshore clam (pholad) borings and is interpreted as the ~20 ka lowstand (Stage 2) wave-cut platform; its depth (< 90 m) indicates post-LGM uplift of about 40 m (rate of ~2 mm/yr) that is tied to slip on the underlying North Channel fault. Three or more(?) distinct submerged strandlines and wave-cut platforms occur within the northern shelf at shallower depths, and are inferred to record relative post-LGM stillstands associated with either pulses of slower sea-level rise or periods when sea level rise was matched by tectonic uplift.

Johnson, S. Y.; Ritchie, A. C.; Conrad, J. E.; Dartnell, P.; Phillips, E.; Sliter, R. W.

2011-12-01

50

A stochastic nonlinear oscillator model for glacial millennial-scale climate transitions derived from ice-core data  

NASA Astrophysics Data System (ADS)

A stochastic Duffing-type oscillator model, i.e noise-driven motion with inertia in a potential landscape, is considered for glacial millennial-scale climate transitions. The potential and noise parameters are estimated from a Greenland ice-core record using a nonlinear Kalman filter. For the period from 60 to 20 ky before present, a bistable potential with a deep well corresponding to a cold stadial state and a shallow well corresponding to a warm interstadial state is found. The system is in the strongly dissipative regime and can be very well approximated by an effective one-dimensional Langevin equation.

Kwasniok, F.; Lohmann, G.

2012-11-01

51

Tempo of genetic diversification in southern African rodents: The role of Plio-Pleistocene climatic oscillations as drivers for speciation  

NASA Astrophysics Data System (ADS)

The evolution of the southern African faunal assemblages is thought to have been largely influenced by climatic oscillations of the Plio-Pleistocene. These fluctuations presumably had a major impact in the form of vicariant diversification of taxa by causing simultaneous speciation/cladogenetic events due to habitat fragmentation. We aimed to test this hypothesis by comparing the timing of diversification observed for several rodent lineages with three peaks of aridification described at approximately 2.8, 1.7 and 1.0 Mya. Our study included nine rodent taxa (Nannomys, Aethomys, Otomys, Myotomys, Rhabdomys and Mastomys for the Muridae, Saccostomus for the Nesomyidae, Cryptomys for the Bathyergidae, and Xerus for the Sciuridae) that showed intrageneric mitochondrial cytochrome b cladogenesis during the last 5 Ma. Phylogenetic analysis performed with maximum likelihood and Bayesian methods supported the monophyly of all subgenera and genera. Most diversifications are also well supported and in agreement with previously published studies. Divergence dates between lineages were estimated using a Bayesian relaxed molecular clock and the 7 Myr split between different Apodemus species as well as the divergence between Tatera and Gerbillurus at 6.3 Myr were used as calibration points. Our results did not provide any convincing evidence of a correspondence between rodent diversification events and peaks in aridity during the Plio-Pleistocene. The nearly perfect linear correlation between cladogenesis and time, during the last 5 Myr, strongly suggests that the diversification of southern African rodent lineages is driven by complex interactions between different factors, including life history, climatic changes, and topographic barriers.

Montgelard, Claudine; Matthee, Conrad A.

2012-07-01

52

Correlating tidewater glacial meltwater discharge with El Niño-Southern Oscillation in Southern Alaska  

NASA Astrophysics Data System (ADS)

Over the past two decades coincident with Arctic climate warming, Alaskan glaciers have experienced significant volume loss with the resulting discharge substantially contributing to rising sea level. This thinning has occurred during a period of pronounced variability in regional climatology and oceanography. However, the meteorological parameters forcing melting have not been well established, as there are few annual records of meltwater discharge directly from large cliff-calving glaciers. To test how meltwater production varies in response to climate forcing, a multi-decadal proxy record of meltwater discharge was established for tidewater glaciers in Disenchantment Bay and in Muir Inlet, Glacier Bay National Park, southern coastal Alaska. 33 piston cores and 8 multicores were collected from 24 stations to measure annual sediment deposition rates over the last several decades. Sedimentary facies, physical properties, and short-lived radioisotopes were used to delineate summertime sediment deposition rates on fortnightly to annual time scales, as tidal and seasonal forcing create distinctly laminated (cyclopels) and annually varved deposits. Magnetic susceptibility, sortable silt, and sediment floc characteristics were used as proxies of meltwater plume competence. Complete varves were identified from cores collected in Disenchantment Bay for up to 22 years (1981-2003) and the period of highest and most variable summer layer thickness at Hubbard Glacier coincides with El Ni ñ o-dominated conditions in the North Pacific from 1990-1995 and a warm phase of the Pacific Decadal Oscillation (PDO). The velocity and competence of the meltwater plume increased in the period 1987-1992, likely associated with the numerous ENSO events that resulted in higher winter precipitation. Hubbard Glacier summer sediment thickness data follow a similar temporal trend as the Alsek River discharge hydrograph, the only currently glacierized basin gaged by the USGS in the region, suggesting that the two drainage basins respond similarly to regional climate forcing. Cross-correlation time-series of Alsek River discharge with temperature and precipitation data from the weather station at Yakutat, AK show that summers with higher mean temperatures correlate with increased discharge and to a lesser extent with the amount of precipitation accumulated during the preceding winter. Two 17-m-long jumbo cores from Muir Inlet contain up to 75 years of complete melt-season deposits and associated spring-neap packages. Multi-taper method and singular spectrum analysis spectral estimation techniques were used to calculate statistically significant periodicity in both the melt season deposit thicknesses and spring-neap package thicknesses for each core. Periodicity in melt season deposit thickness shows that both cores contain significant ENSO signals and that one core (EW0408 60JC) contains significant PDO signals. This research has shown that high-resolution records of fjord sedimentation from tidewater glaciers may be a reliable proxy for past magnitude, duration, and temporal distribution of annual meltwater discharge into Alaskan glacial fjords, thus adding to the climate change record in a sensitive area.

Jaeger, J. M.; Cowan, E.; Jackolski, C.; Powell, R. D.; Chapman, G.

2007-12-01

53

Inter-basin Sea Surface Salinity Contrasts and the Fate of Ocean Thermohaline Circulation: Application to 8.2 ka Outburst Floods from Glacial Lake Agassiz  

Microsoft Academic Search

For decades, substantial modeling effort has been directed to understanding freshwater impacts on the thermohaline ocean circulation (THC) in order to decipher the glacial cycles of the Pleistocene. Longer-term glacial-interglacial oscillations are paced by variations in Earth's orbital parameters, but shorter-term fluctuations of climate, especially in the North Atlantic region, are clearly driven by internal climate dynamics on millennial and

D. Seidov; B. J. Haupt; G. K. Clarke

2004-01-01

54

Spatial and temporal variations of glacial erosion in the Rhône valley (Swiss Alps): insights from numerical modeling  

NASA Astrophysics Data System (ADS)

The present-day topography of the European Alps shows evidence of intense glacial reshaping. However, significant questions regarding Alpine landscape evolution during glaciations still persist. For example, large-scale topographic analyses suggest that glacial erosion is maximized at and above the glaciers' long-term Equilibrium Line Altitude. In contrast, measurements of long-term denudation rates from low temperature thermochronology suggest high erosion towards low altitudes, leading to an increase of local relief in response to glacial erosion. Based on sediment record, low-temperature thermochronology and burial cosmogenic nuclide dating, it has also been proposed that the mid-Pleistocene climatic transition from symmetric, 40kyr to asymmetric, 100kyr glacial/interglacial oscillations sets the onset of intense glacial erosion within the Alps. However, this climate threshold in glacial erosion has not been showed in other orogens, and positive feedbacks between climate periodicity and glacial erosion efficiency still remain to be proven. We focus on the Rhône valley (Swiss Alps), and use a numerical model to estimate patterns and magnitudes of glacial erosion. Comparing modeling results on an advanced reconstruction of the pre-glacial topography (Sternai et al., 2012) and the present-day landforms, we found that erosion propagates headward as the landscape evolves from a fluvial to a glacial state, leading to an initial increase of local relief in the major valley trunk followed by subsequent erosion at high elevations. We also test the mid-Pleistocene transition hypothesis by running a 2Myr numerical experiment including a shift from symmetric, 40kyr to asymmetric, 100kyr glacial/interglacial oscillations at 1Myr. Although the change of climate periodicity may have produced an intensification of glacial erosion, our results suggest that other factors such as an increase of rock uplift and/or progressive climate cooling are required to explain enhanced valley carving at approximately 1Myr.

Sternai, Pietro; Herman, Frédéric; Valla, Pierre; Champagnac, Jean-Daniel; Willett, Sean

2013-04-01

55

Record of Late Pleistocene Glaciation and Deglaciation in the Southern Cascade Range. II. Flux of Glacial Flour in a Sediment Core from Upper Klamath Lake, Oregon  

Microsoft Academic Search

During the late Wisconsin, glacial flour from alpine glaciers along the east side of the Cascade Range in southern Oregon was deposited in Upper Klamath Lake. Quantitative interpretation of magnetic properties and grain-size data of cored sediments from Caledonia Marsh on the west side of the lake provides a continuous record of the flux of glacial flour spanning the last

Joseph G. Rosenbaum; Richard L. Reynolds

2004-01-01

56

Influence of Glacial Oscillations on Deformation in the Himalayas of Central Nepal  

NASA Astrophysics Data System (ADS)

Recent studies indicate that variations in surface loads associated with the evolution of ice caps or lakes can modulate the stress pattern inside the crust [e.g. Hampel et al., 2007; Luttrell et al., 2007; Turpeinen et al., 2008]. In particular these studies point out that such variations may be large enough to change the stress acting on faults and modify the timing of the seismic cycle and long-term slip rates. Glacial loads can impact the stress regime of mountain ranges in different ways, by simultaneously (1) loading with ice masses and (2) unloading it by bedrock erosion promoted by glacial processes. Furthermore, as a response to climate changes, major glacier retreats and advances occur with durations of a few kyrs, inducing fast rates of variation for the crustal stress field. An ongoing debate in Himalayan geodynamics concerns the deformation distribution inside the range and how the ~20 mm/yr of convergence that is accommodated by the orogen is partitioned between the different structures. In central Nepal, previous studies show that, over the Holocene, the MFT has accommodated ~20 mm/yr, i.e., the entire far-field convergence [Lavé and Avouac, 2001].These data support deformation models where the whole Himalayan range is overthrust along the MHT/MFT system. On the other hand, recent studies point to a Quaternary reactivation of the MCT, suggesting that the mode of deformation can substantially change at a time scale of 10-100 kyrs [e.g. Hodges et al., 2004]. The mechanisms that may lead to a shift from one behavior to another, however, are still poorly understood. We hypothesize that variations in crustal loads associated with changes in the glacial cover may induce variations in the stress pattern that are high enough to significantly modify the deformation regime of the Himalayas and the slip rate on the MCT. We first assess the range of variations in surface loads, associated with both ice loading and bedrock erosion. Then we develop thermomechanical models of the Himalayan range that incorporate the main structural and geodynamical features of the present orogen. They include an explicit description of both the MFT/MHT and MCT as discrete frictional interfaces, in order to assess the activity of both fault systems as a response to various loading and unloading scenarios. We test the implications of several hypotheses concerning the evolution and distribution of glacial cover in the high range in term of (1) timing of the loading and unloading events, (2) magnitude of the loads and (3) position of the load with respect to the main structures. Our results illustrate that loading/unloading in the high range significantly impact the stress pattern on the MCT and could modulate the slip rate by several mm/yr. An important controlling parameter on the response of the MCT is the unloading velocity, with rapid unloading tending to promote a short-lived slip acceleration on the MCT, in relation with a decrease in the normal stress on the fault plane. Slower evolution of the loads leads to more subdued and gradual responses. Those preliminary results suggest that, in cases of rapid unloading and sediment transport during deglaciations, the MCT could accommodate a non-negligible fraction of the long-term shortening.

Godard, V.; Burbank, D. W.

2009-12-01

57

Climatic oscillations triggered post-Messinian speciation of Western Palearctic brown frogs (Amphibia, Ranidae)  

Microsoft Academic Search

Oscillating glacial cycles over the past 2.4 million years are proposed to have had a major impact on the diversity of contemporary species communities. We used mitochondrial and nuclear DNA sequence data to infer phylogenetic relationships within Western Palearctic brown frogs and to test the influence of Pliocene and Pleistocene climatic changes on their evolution. We sequenced 1976bp of the

M. Veith; J. Kosuch; M. Vencesb

2003-01-01

58

Spatial and temporal variations of glacial erosion in the Rhône valley (Swiss Alps): Insights from numerical modeling  

NASA Astrophysics Data System (ADS)

The present-day topography of the European Alps shows evidence of intense glacial reshaping. However, significant questions regarding Alpine landscape evolution during glaciations still persist. In this study, we focus on the Rhône valley (Swiss Alps), and use a numerical model to estimate patterns and magnitudes of glacial erosion. Comparing modeling results on a reconstructed pre-glacial topography and the present-day landforms, we find that the landscape response to glaciation is more complex than a simple "buzzsaw" mechanism (by which glacial erosion sets the height of mountain ranges) or increase of relief due to localized valley incision. Instead, glacial erosion propagates headward as the landforms evolve from a fluvial to a glacial state, leading to an initial increase of local relief followed by subsequent erosion at high elevations. It has also been proposed that the mid-Pleistocene climatic transition of glacial/interglacial oscillations from periods of 40 kyr (with symmetric shapes) to periods of 100 kyr (with asymmetric shapes) promoted glacial erosion within the Alps. Although this change of climate periodicity may have contributed to enhance glacial erosion, our results suggest that other factors such as an increase in rock uplift and/or progressive climate cooling are required to explain enhanced glacial carving at ˜1Ma.

Sternai, Pietro; Herman, Frédéric; Valla, Pierre G.; Champagnac, Jean-Daniel

2013-04-01

59

Facies and ground-penetrating radar characteristics of coarse-grained beach deposits of the uppermost Pleistocene glacial Lake Algonquin, Ontario, Canada  

Microsoft Academic Search

The lithofacies of the uppermost Pleistocene (ca 11 800 to 10 400 14 Cy rbp), cold-temperate, coarse-grained beach deposits of Lake Algonquin, the precursor of the present Lake Huron of North America, have been studied and interpreted based on analogous features of modern beaches from the same region. Ice foot and ice-cementation develop during winter but, unlike Arctic beaches, ice-related

VINCENZO PASCUCCI; I. PETER; ANTHONY L. ENDRES

2008-01-01

60

Carbon cycle instability as a cause of the late Pleistocene ice age oscillations - Modeling the asymmetric response  

NASA Technical Reports Server (NTRS)

A dynamical model of the Pleistocene ice ages is presented, which incorporates many of the qualitative ideas advanced recently regarding the possible role of ocean circulation, chemistry, temperature, and productivity in regulating long-term atmospheric carbon dioxide variations. This model involves one additional term (and free parameter) beyond that included in a previous model (Saltzman and Sutera, 1987), providing the capacity for an asymmetric response. It is shown that many of the main features exhibited by the delta(O-18)-derived ice record and the Vostok core/delta(C-13)-derived carbon dioxide record in the late Pleistocene can be deduced as a free oscillatory solution of the model.

Saltzman, Barry; Maasch, Kirk A.

1988-01-01

61

Carbon cycle instability as a cause of the late Pleistocene ice age oscillations: Modeling the asymmetric response  

Microsoft Academic Search

A dynamical model of the Pleistocene ice ages is presented, which incorporates many of the qualitative ideas advanced recently regarding the possible role of ocean circulation, chemistry, temperature, and productivity in regulating long-term atmospheric carbon dioxide variations. This model involves one additional term (and free parameter) beyond that included in a previous model (Saltzman and Sutera, 1987), providing the capacity

Barry Saltzman; Kirk A. Maasch

1988-01-01

62

Spatial and temporal variations of glacial erosion in the European Alps: numerical models and implications for slope stability (Invited)  

NASA Astrophysics Data System (ADS)

Glacial erosion in alpine landscapes can be highly variable in space and time and lead to significant morphologic modification and mass redistribution at virtually all scales. Because they affect the near-surface stress and strain distribution by producing cyclic variations of the surface load, removing and abrading rocks, storing/releasing sediments and affecting the surface and subsurface hydrology, glaciations have multiple effects on slope stability. Understanding how glacial erosion evolves in space and time is thus important for investigating potential feedbacks between glacial erosion and deep-seated gravitational slope deformation (DSGSD). The present-day topography of the European Alps shows evidence of intense glacial erosion. However, significant questions regarding Alpine landscape evolution during glaciations still persist. For example, large-scale topographic analyses suggest that glacial erosion is maximized at and above the glaciers' long-term Equilibrium Line Altitude. In contrast, measurements of long-term denudation rates from low-temperature thermochronology and reconstructions of the pre-glacial Alpine topography suggest high erosion towards low altitudes and formation of overdeepnenings, in turn indicating an increase of local relief in response to glacial processes. Based on sediment record, low-temperature thermochronology and burial cosmogenic nuclide dating, it has also been proposed that the mid-Pleistocene climatic transition from symmetric, 40kyr to asymmetric, 100kyr glacial/interglacial oscillations sets the onset of intense glacial erosion within the Alps. However, this climate threshold in glacial erosion has not been showed in other orogens, and positive feedbacks between climate periodicity and glacial erosion efficiency still remain to be proven. Numerical modeling provides estimates of the patterns and magnitudes of glacial erosion through time. Modeling results on an advanced reconstruction of the pre-glacial topography and the present-day landform suggest that glacial erosion propagates headward as the landscape evolves from a fluvial to a glacial state. This evolution of glacial erosion leads to an initial increase of the topographic relief, followed by subsequent erosion at high elevations, in turn reconciling previous, apparently discordant findings. Numerical predictions also suggest that the mid-Pleistocene climate transition may have produced an intensification of glacial erosion. However, other factors such as an increase of rock uplift and/or progressive climate cooling are required to explain enhanced valley carving at approximately 1Myr, as suggested from the Alpine record.

Sternai, P.; Herman, F.; Willett, S.; Champagnac, J.; Fox, M.; Valla, P.; Salcher, B.

2013-12-01

63

Climatic Oscillations 10,000-155,000 yr B.P. at Owens Lake, California Reflected in Glacial Rock Flour Abundance and Lake Salinity in Core OL-92  

USGS Publications Warehouse

Chemical analyses of the acid-soluble and clay-size fractions of sediment samples (1500-yr resolution) reveal oscillations of lake salinity and of glacial advances in core OL-92 back to 155,000 yr B.P. Relatively saline conditions are indicated by the abundance of carbonate and smectite (both pedogenic and authigenic), reflected by Ca, Sr, and Mg in the acid-soluble suite, and by Cs2O, excess MgO, and LOI (loss on ignition) in the clay-size fraction. Rock flour produced during glacial advances is represented by the abundance of detrital plagioclase and biotite in the clay-size fraction, the ratio of which remains essentially constant over the entire time span. These phases are quantitatively represented by Na2O, TiO2, Ba, and Mn in the clay fraction. The rock-flour record indicates two major ice-advances during the penultimate glacial cycle corresponding to marine isotope stage (MIS) 6, no major advances during the last interglaciation (entire MIS 5), and three major advances during the last glacial cycle (MIS 2, 3, and 4). The ages of the latter three correspond rather well to 36Cl dates reported for Sierra Nevada moraines. The onset of the last interglaciation is shown by abrupt increases in authigenic CaCO3 and an abrupt decrease in rock flour, at about 118,000 yr B.P. according to our time scale. In contrast, the boundary appears to be gradual in the ??18O record in which the change from light to heavy values begins at about 140,000 yrs B.P. The exact position of the termination, therefore, may be proxy-dependent. Conditions of high carbonate and low rock flour prevailed during the entire period from 118,000 yr B.P. until the glacial advance at 53,000 yr B.P. signaled the end of this long interglaciation. ?? 1997 University of Washington.

Bischoff, J. L.; Menking, K. M.; Fitts, J. P.; Fitzpatrick, J. A.

1997-01-01

64

North American Pleistocene Stages Reconsidered in Light of Probable Pliocene-Pleistocene Continental Glaciation  

Microsoft Academic Search

Confusion due to the conceptual usage of classic North American Pleistocene stage terms indicates the need for a new or revised terminology. The Pliocene-Pleistocene boundary has tentatively been dated about 2.5 ± 0.1 × 106 years. Glacial deposits in the central United States older than about 2.2 × 106 years may span that boundary.

John Boellstorff

1978-01-01

65

Correlation of Marine and Continental Glacial and Interglacial Events, Arctic Ocean and Banks Island.  

National Technical Information Service (NTIS)

Correlation of Pleistocene glacial and interglacial sediments deposited on land with sediments deposited synchronously in the ocean basins is rarely accomplished. In the Arctic Ocean, the definition of a partial magnetic stratigraphy for a Pleistocene seq...

D. L. Clark J. S. Vincent G. A. Jones W. A. Morris

1984-01-01

66

Morphology and palaeoenvironmental interpretation of deformed soft-sediment clasts: examples from within Late Pleistocene glacial outwash, Tempo Valley, Northern Ireland  

NASA Astrophysics Data System (ADS)

Glacial outwash, deposited during deglaciation of the late Devensian ice sheet, is present as a flat-topped valley fill in the Tempo Valley on the southern flanks of the Fintona Hills, Northern Ireland. Sedimentologically, the outwash comprises well-sorted and interbedded rippled to massive sands which record distal deposition within a proglacial water body. Beds of ripple-drift cross-laminated sands contain deformed (folded and contorted) soft-sediment clasts which are composed mainly of silt and clay. The soft-sediment clasts were deformed prior to final deposition because clast a- b planes lie conformable to sand laminae which are undeformed. Morphological characteristics of the soft-sediment clasts, and their facies context, provide evidence for transport mechanisms, depositional environment, and processes of clast deformation. The soft-sediment clasts were transported into a proglacial water body by unidirectional water currents (˜1.5-2.5 m s -1). Sediment transport processes include sediment bypassing within the water column, a low bedload component, and grain flow activity during waning flow stages. The overall morphology of soft-sediment clasts records between 1 and 3 distinct phases of hydroplastic deformation prior to emplacement. The deformation phases are recognised on the basis of morphologically `unrolling' the superimposed folds of the soft-sediment clasts. Deformation structures (i.e. fold style) and direction of the principal stress axis relative to clast axes suggest that clasts were reoriented with respect to water flow direction following each deformation phase. Processes of deformation include folding-over of the clast along its b axis into two or more components, crumpling and abrasion of the outer margins of the b plane, and squashing of the clast c axis (some of which may be post-depositional deformation). The presence of silt- and clay-rich soft-sediment clasts within the outwash succession suggests that they were ripped-up from shallow and irregular pools on the glacier forefield, into which fine sediments accumulated after flood or meltwater events, and transported distally into a proglacial water body. These inferences based on facies evidence and styles of hydroplastic deformation impact on reconstructions of local palaeogeography, and the wider interpretation of similar soft-sediment clasts in the geological record.

Knight, Jasper

1999-10-01

67

Eucampia Index as an indicator of the Late Pleistocene oscillations of the winter sea-ice extent at the ODP Leg 119 Site 745B at the Kerguelen Plateau  

Microsoft Academic Search

A new paleoenvironmental proxy, Eucampia Index, was used to trace the Late Pleistocene oscillations of winter ice extent at ODP Leg 119, Site 745B (59° 35.71' S and 85° 51.60' E) on the Kerguelen Plateau. The index is calculated as the ratio of winter terminal to intercalary valves of the diatom Eucampia antarctica sensu lato. During the early Brunhes the

I. Kaczmarska; N. E. Barbrick; J. M. Ehrman; G. P. Cant

1993-01-01

68

Correlation of Late-Pleistocene Lake-Level Oscillations in Mono Lake, California, with North Atlantic Climate Events  

USGS Publications Warehouse

Oxygen-18 (18O) values of sediment from the Wilson Creek Formation, Mono Basin, California, indicate three scales of temporal variation (Dansgaard-Oeschger, Heinrich, and Milankovitch) in the hydrologic balance of Mono Lake between 35,400 and 12,900 14C yr B.P. During this interval, Mono Lake experienced four lowstands each lasting from 1000 to 2000 yr. The youngest low-stand, which occurred between 15,500 and 14,000 14C yr B.P., was nearly synchronous with a desiccation of Owens Lake, California. Paleomagnetic secular variation (PSV) data indicate that three of four persistent lowstands occurred at the same times as Heinrich events H1, H2, and H4. 18O data indicate the two highest lake levels occurred ???18,000 and ???13,100 14C yr B.P., corresponding to passages of the mean position of the polar jet stream over the Mono Basin. Extremely low values of total inorganic carbon between 26,000 and 14,000 14C yr B.P. indicate glacial activity, corresponding to a time when summer insolation was much reduced. ?? 1998 University of Washington.

Benson, L. V.; Lund, S. P.; Burdett, J. W.; Kashgarian, M.; Rose, T. P.; Smoot, J. P.; Schwartz, M.

1998-01-01

69

Glacial Isostatic Adjustment  

NSDL National Science Digital Library

To prepare for this activity students should be familiar with basic vocabulary associated with ice age glaciations. In the case of this course, they should have read the chapter on glaciation in Tarbuck & Lutgens, Earth, 8th edition. Their task begins with designing a hands-on lab to investigate the relationship between depressions in a surface caused by adding mass to a compressible medium such as a sponge. Students then use ArcGIS Explorer to investigate the Antarctic ice sheet and the Pleistocene ice sheet. The activity wraps up with students collecting data related to glacial isostatic adjustment in rebound centers around Canada over the past 6,000 years.

Awad, Aida

70

Glacial Landforms  

NSDL National Science Digital Library

The students have been lectured to about glacial processes, but the homework was given prior to a lecture about glacial landforms. A field trip surveying the glacial landforms of SE South Dakota was just completed. The students must draw upon their knowledge and utlize other sources to interpret the landforms they see in the imagery. Designed for a geomorphology course

Sweeney, Mark

71

Late Pleistocene Glaciations in the Northwestern Sierra Nevada, California  

Microsoft Academic Search

Pleistocene fluvial landforms and riparian ecosystems in central California responded to climate changes in the Sierra Nevada, yet the glacial history of the western Sierra remains largely unknown. Three glacial stages in the northwestern Sierra Nevada are documented by field mapping and cosmogenic radionuclide surface-exposure (CRSE) ages. Two CRSE ages of erratic boulders on an isolated till above Bear Valley

L. Allan James; Jon Harbor; Derek Fabel; Dennis Dahms; David Elmore

2002-01-01

72

Age of the crowfoot advance in the Canadian Rocky Mountains. A glacial event coeval with the Younger Dryas oscillation  

SciTech Connect

A suite of sediment core samples was recovered from two lakes, Crowfoot and Bow lakes, that are adjacent to the Crowfoot moraine type locality, to identify and radiocarbon date sediments related to the Crowfoot advance. The Crowfoot moraine system, widely recognized throughout northwestern North America, represents a glacial advance that is post-Wisconsin and pre-Mazama tephra in age. An interval of inorganic sediments bracketed by accelerator mass spectrometry radiocarbon ages of ca. 11,330 and 10,100 [sup 14]C yr B.P. is associated with the Crowfoot moraine. The Crowfoot advance is therefore approximately synchronous with the European Younger Dryas cold event (ca. 11,000-10,000 [sup 14]C yr B.P.). Furthermore, the termination of the Crowfoot advance also appears to have been abrupt. These findings illustrate that the climatic change responsible for the European Younger Dryas event extended beyond the northern Atlantic basin and western Europe. Equilibrium-line altitude (ELA) depressions associated with the Crowfoot advance are similar to those determined for the Little Ice Age advance, whereas Younger Dryas ELA depressions in Europe significantly exceed Little Ice Age ELA depressions. 26 refs., 3 figs., 1 tab.

Reasoner, M.A.; Rutter, N.W. (Univ. of Alberta, Edmonton, Alberta (Canada)); Osborn, G. (Univ. of Calgary, Alberta (Canada))

1994-05-01

73

Guatemalan forest synthesis after Pleistocene aridity  

PubMed Central

Sediments from two lakes in the Peten Department, Guatemala, provide palynological evidence from Central America of late Pleistocene aridity and subsequent synthesis of mesic forests. Late Glacial vegetation consisted of marsh, savanna, and juniper scrub. An early Holocene temperate forest preceded a mesic tropical forest with Brosimum (ramon). Thus “primeval” rain forests of Guatemala are no older than 10,000 to 11,000 years and are considerably younger in the Peten due to Mayan disturbances. Among dated Neotropical sites, the Peten has the most mesic vegetation yet shown to have supplanted xeric vegetation present during the Pleistocene. The arid late Glacial-humid early Holocene transition appears to have been pantropical in the lowlands. The Peten was not a Pleistocene refugium for mesophytic taxa, as has been suggested. Thus genesis of extant rain forests in northern Central America and southern Mexico remains unexplained. Images

Leyden, Barbara W.

1984-01-01

74

Miocene and Pliocene dominated diversification of the lichen-forming fungal genus Melanohalea (Parmeliaceae, Ascomycota) and Pleistocene population expansions  

PubMed Central

Background Factors promoting diversification in lichen symbioses remain largely unexplored. While Pleistocene events have been important for driving diversification and affecting distributions in many groups, recent estimates suggest that major radiations within some genera in the largest clade of macrolichens (Parmeliaceae, Ascomycota) vastly predate the Pleistocene. To better understand the temporal placement and sequence of diversification events in lichens, we estimated divergence times in a common lichen-forming fungal genus, Melanohalea, in the Northern Hemisphere. Divergence times were estimated using both concatenated gene tree and coalescent-based multilocus species tree approaches to assess the temporal context of major radiation events within Melanohalea. In order to complement our understanding of processes impacting genetic differentiation, we also evaluated the effects of Pleistocene glacial cycles on population demographics of distinct Melanohalea lineages, differing in reproductive strategies. Results We found that divergence estimates, from both concatenated gene tree and coalescent-based multilocus species tree approaches, suggest that diversification within Melanohalea occurred predominantly during the Miocene and Pliocene, although estimated of divergence times differed by up to 8.3 million years between the two methods. These results indicate that, in some cases, taxonomically diagnostic characters may be maintained among divergent lineages for millions of years. In other cases, similar phenotypic characters among non-sister taxa, including reproductive strategies, suggest the potential for convergent evolution due to similar selective pressures among distinct lineages. Our analyses provide evidence of population expansions predating the last glacial maximum in the sampled lineages. These results suggest that Pleistocene glaciations were not inherently unfavorable or restrictive for some Melanohalea species, albeit with apparently different demographic histories between sexually and vegetatively reproducing lineages. Conclusions Our results contribute to the understanding of how major changes during the Miocene and Pliocene have been important in promoting diversification within common lichen-forming fungi in the northern Hemisphere. Additionally, we provide evidence that glacial oscillations have influenced current population structure of broadly distributed lichenized fungal species throughout the Holarctic.

2012-01-01

75

Investigating the effects of Pleistocene events on genetic divergence within Richardsonius balteatus, a widely distributed western North American minnow  

PubMed Central

Background Biogeographers seek to understand the influences of global climate shifts and geologic changes to the landscape on the ecology and evolution of organisms. Across both longer and shorter timeframes, the western North American landscape has experienced dynamic transformations related to various geologic processes and climatic oscillations, including events as recently as the Last Glacial Maximum (LGM; ~20 Ka) that have impacted the evolution of the North American biota. Redside shiner is a cyprinid species that is widely distributed throughout western North America. The species’ native range includes several well-documented Pleistocene refugia. Here we use mitochondrial DNA sequence data to assess phylogeography, and to test two biogeographic hypotheses regarding post-glacial colonization by redside shiner: 1) Redside shiner entered the Bonneville Basin at the time of the Bonneville Flood (Late Pleistocene; 14.5 Ka), and 2) redside shiner colonized British Columbia post-glacially from a single refugium in the Upper Columbia River drainage. Results Genetic diversification in redside shiner began in the mid to late Pleistocene, but was not associated with LGM. Different clades of redside shiner were distributed in multiple glacial age refugia, and each clade retains a signature of population expansion, with clades having secondary contact in some areas. Conclusions Divergence times between redside shiner populations in the Bonneville Basin and the Upper Snake/Columbia River drainage precedes the Bonneville Flood, thus it is unlikely that redside shiner invaded the Bonneville Basin during this flooding event. All but one British Columbia population of redside shiner are associated with the Upper Columbia River drainage with the lone exception being a population near the coast, suggesting that the province as a whole was colonized from multiple refugia, but the inland British Columbia redside shiner populations are affiliated with a refugium in the Upper Columbia River drainage.

2014-01-01

76

Glacial cycles and ice-sheet modelling  

Microsoft Academic Search

An attempt is made to simulate the Pleistocene glacial cycles with a numerical model of the Northern Hemisphere ice sheets. This model treats the vertically-integrated ice flow along a meridian, including computation of bedrock adjustment and temperature distribution in the ice. Basal melt water is traced and controls ice-mass discharge.

J. Oerlemans

1982-01-01

77

The first glacial maximum in North America.  

PubMed

Despite marine evidence for at least 50 Pliocene-Pleistocene ice sheet advances, only the most recent one has been accurately reconstructed from terrestrial evidence, because there are few techniques for dating older glacial deposits. Here we show that the cosmic ray-produced nuclides beryllium-10 and aluminum-26 can be used to date tills that overlie paleosols. PMID:15653495

Balco, Greg; Rovey, Charles W; Stone, John O H

2005-01-14

78

Evolution of salt diapir and karst morphology during the last glacial cycle: Effects of sea-level oscillation, diapir and regional uplift, and erosion (Persian Gulf, Iran)  

NASA Astrophysics Data System (ADS)

Marine, fluvial and cave sediments, and karst phenomena were studied and dated by 14C, U-series, and OSL methods to determine the evolution of the Namakdan diapir and the world's longest salt cave (3N Cave) during the Holocene and the Last Glacial. Sea-level oscillations, the uplift rate of the diapir and its surroundings, and erosion are the main factors influencing the diapir morphology. Although the diapir uplift rate has been constant for the last 50 kyr (˜ 4 mm/yr at a distance 600 m from the diapir edge), the uplift rate decreases with the distance from the diapir center. Drag-induced host rock deformation extends for ˜ 300 m from the outside edge of the diapir, and host rocks in this zone have an uplift rate of 0.4-0.6 mm/yr, which is 2-3 times greater than the regional uplift rate. Based on known sea-level oscillations, radiometric dating, and geological evidence, the Namakdan diapir was repeatedly flooded by sea water between 130 and 80 kyr BP. Submarine residuum composed mainly of gypsum and dolomite formed cap rock on the diapir. After ˜ 80 kyr BP, surficial drainage network and karst development started. Blind valleys and their corresponding cave systems evolved continuously for ˜ 20-30 kyr. Between 9 and 6 cal kyr BP the rate of sea-level rise exceeded the Namakdan diapir uplift rate by the factor of 3. As a consequence upward incision of cave streams (paragenetic trend) occurred, and blind valleys near the seashore were filled with gravels. Cave passages now accessible on the Namakdan and Hormoz diapirs started to form 3-6 cal kyr BP when sea level stabilized and downward stream incision began. Older cave levels are still preserved but are filled with sediments and salt precipitates. A comparison of the Namakdan diapir evolution with data from the Hormoz and Larak diapirs shows that the evolution of diapir morphology is strongly affected by the differences in uplift rates and geological settings. The general scheme of the evolution of the Namakdan diapir is believed to be partly applicable to many other diapirs in coastal settings.

Bruthans, Ji?í; Filippi, Michal; Zare, Mohammad; Churá?ková, Zdenka; Asadi, Naser; Fuchs, Markus; Adamovi?, Ji?í

2010-09-01

79

Style and timing of glacial and paraglacial sedimentation in a monsoon-influenced high Himalayan environment, the upper Bhagirathi Valley, Garhwal Himalaya  

NASA Astrophysics Data System (ADS)

The Gangotri Glacier, at the source of the Ganges River, has fluctuated greatly throughout the late Quaternary in response to climatic oscillations. This has resulted in impressive moraines, paraglacial debris flow fans and terraces along the upper stretches of the Bhagirathi Valley. Cosmogenic radionuclide (CRN) dating of glacial and paraglacial landforms shows that fans, terraces and associated moraines formed approximately synchronously as the landscape readjusted to changing environmental conditions. This synchronicity suggests that fan and terrace formation is intimately related to glaciation, and that fluctuations in glacial and associated environments during times of climatic instability cause rapid sediment transfer and resedimentation of glacial landforms. The CRN dates show that all existing glacial and paraglacial landforms in the upper Bhagirathi Valley formed during the late Pleistocene and Holocene. This demonstrates that, in this high mountain environment, paraglacial and glacial landforms are eroded and resedimented within about 20,000 years. Furthermore, this testifies to the dynamic nature of glacial environments in monsoon-influenced high mountain regions.

Barnard, Patrick L.; Owen, Lewis A.; Finkel, Robert C.

2004-03-01

80

Middle to Late Pleistocene ice extents, tephrochronology and paleoenvironments of the White River area, southwest Yukon  

NASA Astrophysics Data System (ADS)

Sedimentary deposits from two Middle to Late Pleistocene glaciations and intervening non-glacial intervals exposed along the White River in southwest Yukon, Canada, provide a record of environmental change for much of the past 200 000 years. The study sites are beyond the Marine Isotope stage (MIS) 2 glacial limit, near the maximum regional extent of Pleistocene glaciation. Non-glacial deposits include up to 25 m of loess, peat and gravel with paleosols, pollen, plant and insect macrofossils, large mammal fossils and tephra beds. Finite and non-finite radiocarbon dates, and twelve different tephra beds constrain the chronology of these deposits. Tills correlated to MIS 4 and 6 represent the penultimate and maximum Pleistocene glacial limits, respectively. The proximity of these glacial limits to each other, compared to limits in central Yukon, suggests precipitation conditions were more consistent in southwest Yukon than in central Yukon during the Pleistocene. Conditions in MIS 5e and 5a are recorded by two boreal forest beds, separated by a shrub birch tundra, that indicate environments as warm or warmer than present. A dry, treeless steppe-tundra, dominated by Artemisia frigida, upland grasses and forbs existed during the transition from late MIS 3 to early MIS 2. These glacial and non-glacial deposits constrain the glacial limits and paleoenvironments during the Middle to Late Pleistocene in southwest Yukon.

Turner, Derek G.; Ward, Brent C.; Bond, Jeffrey D.; Jensen, Britta J. L.; Froese, Duane G.; Telka, Alice M.; Zazula, Grant D.; Bigelow, Nancy H.

2013-09-01

81

Pleistocene plants from North Carolina  

USGS Publications Warehouse

The field work upon which this report is based was done in 1906 and 1907 as a part of the cooperative study of the Atlantic Coastal Plain, under the direction of the late William Bullock Clark. Associated with the writer in this work were L. W. Stephenson, B. L. Miller, Jr., and J. E. Pogue. Preliminary accounts of the plants collected were published in 1907 and 1909. As has been frequently emphasized, the study of the Pleistocene floras in this country is in an exceedingly backward state as measured by the volume and precision of our knowledge of Pleistocene floras in Europe. Researches in Pleistocene geology in North America have been confined almost entirely to glaciology, and the problem of the correlation of the glacial deposits with those outside the glaciated area has not been solved, nor is there any general agreement regarding the genesis of the Pleistocene deposits south of the terminal moraines. The present account of what is known of the Pleistocene flora of North Carolina and the conclusions that may be legitimately derived from it is offered in the hope that it may stimulate an interest in a neglected field of research and form a small part of the evidence upon which to base future more comprehensive conclusions and generalizations. A word of explanation regarding the illustrations is required. Nearly all of them have been made from leaves preserved as carbonaceous films in the peaty clays. These specimens were carefully washed out, and blue prints were made directly from them. Outlines and as much of the venation as could be seen were inked on the blue prints, which were then bleached. This procedure made it possible to handle a much larger amount of material and prevented any possible damage to the exceedingly fragile specimens, which were mounted on cards or between glass. The accompanying drawings were made from tracings of the original nature prints.

Berry, Edward Wilber

1926-01-01

82

Hydrogeologic and Geotechnical Properties of Pleistocene Materials in North-Central Wisconsin.  

National Technical Information Service (NTIS)

Glacial materials cover most of Wisconsin and play an important role in ground-water investigations and geotechnical projects. A lithostratigraphic framework for the Pleistocene units in Wisconsin outlines the extent and physical characteristics of the Pl...

M. A. Muldoon K. R. Bradbury D. M. Mickelson J. W. Attig

1988-01-01

83

Modeling past abrupt climate changes: driven oscillators and synchronization phenomena in Paleoclimate theory  

NASA Astrophysics Data System (ADS)

According to Milankovitch theory of ice ages, summer insolation at high northern latitudes drives the glacial cycles, i.e. the growth and reduction of Northern Hemisphere ice sheets, and there is evidence that astronomical forcing controls indeed the timing of Pleistocene glacial-interglacial cycles. However, the ?18Otime series (the ?18O is a proxy for global ice volume) available for the last few million years reveal a non-linear response of the climate to the external forcing: transitions from the glacial to the interglacial states occur more rapidly than the transitions from the interglacials to the glacials, resulting in the so-called saw-tooth shape of the signal. These terminations were very abrupt compared to the smooth changes in insolation. Moreover, insolation alone cannot explain the Mid-Pleistocene transition. During this event, occurred about one million years ago, the dominant 41 kyr glacial cycles, were replaced by longer saw-tooth shaped cycles with a time scale around 100 kyr. The asymmetry in the oscillations indicates a non-linear response to the orbital forcing, expressed through a bifurcation, or tipping point. As an introduction to the problem, we studied simple driven oscillators that can exhibit asymmetric oscillations between the glacial and interglacial states under the effect of the astronomical forcing, such as the Van der Pool and the Duffing oscillators. In order to understand how these simple low-dimensional models enter theories of ice ages and rapid events, we studied synchronization phenomena between simple driven oscillators and astronomical forcing, focusing on distinguishing between the so-called resonance scenario and the so-called phase locking scenario. We next examined the possible mechanisms for the Mid-Pleistocene transition. Here we show that the transition could be explained as a result of frequency-locking to the external forcing. This change can be interpreted as a result of an internal change in climate response (that might correspond, for example, to the decrease of global CO2), since it does not correspond to any changes in the orbital forcing.

Marchionne, Arianna

2014-05-01

84

Geological controls on Pleistocene glaciation and cirque form in Greece  

NASA Astrophysics Data System (ADS)

Limestone and ophiolite rocks are common across the eastern Mediterranean and many of the highest mountains are formed in these rock types. In northwest Greece, Pleistocene glacial erosion was much more effective on limestone terrain where pronounced glacial incision and subglacial glacio-karst processes produced locally-complex topography. This enabled Pleistocene glaciers to form on a range of slope orientations in contrast to ophiolite terrains, where glaciers were strongly controlled by aspect. On limestone terrains, the largest ice masses formed on south-facing slopes, whereas in neighbouring higher mountains formed in ophiolite, glaciers were much more restricted and predominantly formed on north- and east-facing slopes.

Hughes, P. D.; Gibbard, P. L.; Woodward, J. C.

2007-08-01

85

Pleistocene Speciation in the Genus Populus (Salicaceae)  

PubMed Central

The macroevolutionary consequences of recent climate change remain controversial, and there is little paleobotanical or morphological evidence that Pleistocene (1.8–0.12 Ma) glacial cycles acted as drivers of speciation, especially among lineages with long generation times, such as trees. We combined genetic and ecogeographic data from 2 closely related North American tree species, Populus balsamifera and P. trichocarpa (Salicacaeae), to determine if their divergence coincided with and was possibly caused by Pleistocene climatic events. We analyzed 32 nuclear loci from individuals of P. balsamifera and P. trichocarpa to produce coalescent-based estimates of the divergence time between the 2 species. We coupled the coalescent analyses with paleodistribution models to assess the influence of climate change on species' range. Furthermore, measures of niche overlap were used to investigate patterns of ecological differentiation between species. We estimated the divergence date of P. balsamifera and P. trichocarpa at approximately 75 Ka, which corresponds closely with the onset of Marine Isotope Stage 4 (?76 Ka) and a rapid increase in global ice volume. Significance tests of niche overlap, in conjunction with genetic estimates of migration, suggested that speciation occurred in allopatry, possibly resulting from the environmental effects of Pleistocene glacial cycles. Our results indicate that the divergence of keystone tree species, which have shaped community diversity in northern North American ecosystems, was recent and may have been a consequence of Pleistocene-era glaciation and climate change.

Levsen, Nicholas D.; Tiffin, Peter; Olson, Matthew S.

2012-01-01

86

Cenozoic Glacial History Revisited  

NASA Astrophysics Data System (ADS)

Recent geological discoveries have shaken the long-standing view of Earth's Cenozoic glacial history, which traditionally calls for the first continental-scale glaciation of East Antarctica in the earliest Oligocene (~33.6 Ma), followed by the onset of major Northern Hemispheric glacial cycles in the late Pliocene about 30 million years later. For example, new evidence from Arctic and North Atlantic oceans suggests Northern Hemispheric sea ice and glaciers have existed intermittently through much of the Cenozoic, not just the last few million years. In terms of the early glacial history of Antarctica, it has recently been suggested that significant glacial ice might have formed at various times during the overall greenhouse warmth of the Cretaceous and Eocene, and when more permanent, major glaciation began in the earliest Oligocene, a proto-West Antarctic Ice Sheet (WAIS) might have grown in concert with the East Antarctic Ice Sheet, rather than forming much later in the Neogene as is usually assumed. These data hint at previously unconsidered ice accommodation during the Oligocene and Miocene that could help to explain the discrepancy between large variations in global ice volume implied by deep-sea-core records, and the much smaller amplitude variations predicted by numerical climate-ice sheet models of East Antarctica alone. In the more recent Pliocene and Pleistocene, recent sedimentary drilling by ANDRILL has shown that the Antarctic ice shelves and WAIS have waxed and waned with far greater frequency than previously suspected. Here, we review these recent geological findings from the polar regions of both hemispheres, while considering them in the context of globally distributed proxy records from the deep sea and new model results using the latest generation of coupled atmosphere-ocean-cryosphere-isotope models. We offer a revised view of Earth's cryospheric evolution through the Cenozoic, and note important discrepancies between traditional interpretations of proxy ice volume records, based mainly on oxygen isotope and Mg/Ca records from the deep sea, and numerical models simulations that consider the long-term evolution of Cenozoic paleogeography and atmospheric carbon dioxide.

Deconto, R.; Pollard, D.; Wilson, P.; Pagani, M.

2009-04-01

87

Pleistocene fluctuations of the Antarctic Ice Sheet in the Ross Embayment  

NASA Astrophysics Data System (ADS)

Past fluctuations of the marine-based Antarctic Ice Sheet in the Ross Embayment are reconstructed for the Pleistocene by developing a model for the glacimarine depositional sequences documented from the ANDRILL McMurdo Ice Shelf project drill core AND-1B. This model reveals glacial to interglacial fluctuations of the AIS in the Western Ross Embayment responding at orbital frequencies. Chronology is constrained by an age model based on 40Ar/39Ar dating of volcanic ashes and magnetostratigraphy. The glacimarine sequences in AND-1B appear to correlate one-to-one with cycles in the benthic ^18O record for the past ~0.8 Myr (Marine Isotope Stages 20-2). Five sequences between ~1.7 and 1.0 Myr can also be matched with specific intervals in the ^18O record, and indicate oscillations of the AIS grounding line operating at a 40-kyr frequency. This record provides new insight into the response of the AIS in the Ross Embayment across the Mid-Pleistocene Transition. Prior to 1.0 Myr, glacimarine sequences have 40-kyr duration, whereas subsequently 100-kyr glacimarine cycles can be clearly recognised in the core. During this "100-kyr world", subglacial to grounding-zone sedimentation dominates at the AND-1B site, with thin intervals of ice-shelf deposition during interglacials also preserved in the AND-1B sedimentary record. An unconformity in AND-1B that spans most (~200 kyr) of the Mid-Pleistocene Transition is inferred to represent large scale expansion of AIS in the Ross Embayment at ~0.8 Myr. Prior to the Mid-Pleistocene Transition, interglacial periods are characterised by open-water conditions with high abundances of volcanoclastic deposits and occasional diatomaceous sediments, indicating that the marine based ice sheet was more dynamic during this interval.

McKay, R. M.; Naish, T. R.; Powell, R. D.

2009-04-01

88

Glacial earthquakes  

Microsoft Academic Search

We have identified a new class of moderate earthquakes (seismic magnitude around 5) that occur beneath glaciers. The previously unknown glacial earthquakes generate long-period (20--60~sec) seismic surface waves that are well recorded on globally distributed seismic stations, but which have previously gone undetected because they do not generate the high-frequency seismic waves on which traditional earthquake detection and location methodologies

G. Ekström; M. Nettles; G. A. Abers

2003-01-01

89

Late Pleistocene glaciation of the Diancang and Gongwang Mountains, southeast margin of the Tibetan plateau  

Microsoft Academic Search

Late Pleistocene glaciations on Mt. Diancang and Mt. Gongwang were investigated and the chronostratigraphy of the glacial advances were developed based on the geomorphic features and numerical ages of thermoluminescence (TL) and AMS radiocarbon dating. Glaciers disappeared ?1300 years ago on Mt. Diancang and several earlier glacial advances, occurred at 3–5, 10, 15–22ka and one during MIS 3. The glacial

Jianqiang Yang; Wei Zhang; Zhijiu Cui; Chaolu Yi; Kexin Liu; Yuanjiang Ju; Xiaoyong Zhang

2006-01-01

90

Glacial earthquakes  

NASA Astrophysics Data System (ADS)

We have identified a new class of moderate earthquakes (seismic magnitude around 5) that occur beneath glaciers. The previously unknown glacial earthquakes generate long-period (20--60~sec) seismic surface waves that are well recorded on globally distributed seismic stations, but which have previously gone undetected because they do not generate the high-frequency seismic waves on which traditional earthquake detection and location methodologies are based. A glacial earthquake on September 4, 1999, beneath the Dall glacier (Denali range, Alaska) was well recorded by the BEAAR temporary broad-band seismic network. Inverse modeling of the regional long-period seismograms shows that the seismic waves are inconsistent with tectonic faulting beneath the glacier, but instead consistent with sudden and massive glacial sliding. Using the centroid-single-force formalism developed by Kawakatsu (1989) for analysis of landslide-generated seismic waves, we obtain an estimate of the product of sliding mass and sliding distance. The value of this parameter for the Dall glacier event is 1.3 x 1014~kg-m, consistent with the displacement of 10~km3 of ice by 13 meters, though we cannot constrain the mass and distance independently. The duration of sliding was approximately 40 seconds. By analysis of data from the Global Seismographic Network for 1999--2001, we have detected and located forty-two glacial earthquakes of similar magnitude beneath the Greenland ice sheet. Preliminary analysis of the five best-recorded earthquakes indicates that these, too, are consistent with glacial sliding. We speculate that the dynamics of these earthquakes are controlled by stick-slip motion of the glacier along its basal surface, and that the phenomenon involves the displacement of a large mass over a relatively short distance. Microearthquakes occur in association with glaciers, and some have been linked to sliding motion at the base, and in particular to motion on so-called sticky spots. Alternatively, rapid variations in pore pressure at the glacier base or the non-linear weakening of a deforming till may generate conditions for rapid lowering of the effective friction, growth of a slipping patch, and sudden large-scale motion of the glacier.

Ekström, G.; Nettles, M.; Abers, G. A.

2003-12-01

91

ROCK WEATHERING CHARACTERISTICS AS RELATIVE-AGE INDICATORS FOR GLACIAL AND POSTGLACIAL LANDFORMS ON MARION ISLAND  

Microsoft Academic Search

Rock weathering characteristics as relative-age indicators are tested on rock surfaces of generally known chronological sequence on Marion Island in the sub-Antarctic. Relict Late Pleistocene glacial bedrock surfaces and glacial deposit rock surfaces provide both Schmidt hammer rebound values and weathering rind thickness indicative of the longest exposure to weathering. While the weathering characteristics could not establish any chronosequence in

P. SUMNER; W. NEL; S. HOLNESS; J. BOELHOUWERS

2002-01-01

92

Large inert carbon pool in the terrestrial biosphere during the Last Glacial Maximum  

Microsoft Academic Search

During each of the late Pleistocene glacial–interglacial transitions, atmospheric carbon dioxide concentrations rose by almost 100 ppm. The sources of this carbon are unclear, and efforts to identify them are hampered by uncertainties in the magnitude of carbon reservoirs and fluxes under glacial conditions. Here we use oxygen isotope measurements from air trapped in ice cores and ocean carbon-cycle modelling

M. Cuntz; L. Bopp; M. Scholze; G. Hoffmann; A. Lourantou; S. P. Harrison; I. C. Prentice; D. I. Kelley; C. Koven; S. L. Piao; P. Ciais; A. Tagliabue

2011-01-01

93

Plio-pleistocene African climate  

SciTech Connect

Marine records of African climate variability document a shift toward more arid conditions after 2.8 million years ago (Ma), evidently resulting from remote forcing by cold North Atlantic sea-surface temperatures associated with the onset of Northern Hemisphere glacial cycles. African climate before 2.8 Ma was regulated by low-latitude insolation forcing of monsoonal climate due to Earth orbital precession. Major steps in the evolution of African hominids and other vertebrates are coincident with shifts to more arid, open conditions near 2.8 Ma, 1.7 Ma, and 1.0 Ma, suggesting that some Pliocene (Plio)-Pleistocene speciation events may have been climatically mediated. 65 refs., 6 figs.

deMenocal, P.B. [Lamont-Doherty Earth Observatory, Palisades, NY (United States)

1995-10-06

94

Late Pleistocene dune activity in the central Great Plains, USA  

NASA Astrophysics Data System (ADS)

Stabilized dunes of the central Great Plains, especially the megabarchans and large barchanoid ridges of the Nebraska Sand Hills, provide dramatic evidence of late Quaternary environmental change. Episodic Holocene dune activity in this region is now well-documented, but Late Pleistocene dune mobility has remained poorly documented, despite early interpretations of the Sand Hills dunes as Pleistocene relicts. New optically stimulated luminescence (OSL) ages from drill cores and outcrops provide evidence of Late Pleistocene dune activity at sites distributed across the central Great Plains. In addition, Late Pleistocene eolian sands deposited at 20-25 ka are interbedded with loess south of the Sand Hills. Several of the large dunes sampled in the Sand Hills clearly contain a substantial core of Late Pleistocene sand; thus, they had developed by the Late Pleistocene and were fully mobile at that time, although substantial sand deposition and extensive longitudinal dune construction occurred during the Holocene. Many of the Late Pleistocene OSL ages fall between 17 and 14 ka, but it is likely that these ages represent only the later part of a longer period of dune construction and migration. At several sites, significant Late Pleistocene or Holocene large-dune migration also probably occurred after the time represented by the Pleistocene OSL ages. Sedimentary structures in Late Pleistocene eolian sand and the forms of large dunes potentially constructed in the Late Pleistocene both indicate sand transport dominated by northerly to westerly winds, consistent with Late Pleistocene loess transport directions. Numerical modeling of the climate of the Last Glacial Maximum has often yielded mean monthly surface winds southwest of the Laurentide Ice Sheet that are consistent with this geologic evidence, despite strengthened anticyclonic circulation over the ice sheet. Mobility of large dunes during the Late Pleistocene on the central Great Plains may have been the result of cold, short growing seasons with relatively low precipitation and low atmospheric CO 2 that increased plant moisture stress, limiting the ability of vegetation to stabilize active dune sand. The apparent coexistence of large mobile dunes with boreal forest taxa suggests a Late Pleistocene environment with few modern analogs.

Mason, J. A.; Swinehart, J. B.; Hanson, P. R.; Loope, D. B.; Goble, R. J.; Miao, X.; Schmeisser, R. L.

2011-12-01

95

Phylogeography of the Cape velvet worm (Onychophora: Peripatopsis capensis) reveals the impact of Pliocene/Pleistocene climatic oscillations on Afromontane forest in the Western Cape, South Africa.  

PubMed

Habitat specialists such as soft-bodied invertebrates characterized by low dispersal capability and sensitivity to dehydration can be employed to examine biome histories. In this study, the Cape velvet worm (Peripatopsis capensis) was used to examine the impacts of climatic oscillations on historical Afromontane forest in the Western Cape, South Africa. Divergence time estimates suggest that the P. capensis species complex diverged during the Pliocene epoch. This period was characterized by dramatic climatic and topographical change. Subsequently, forest expansion and contraction cycles led to diversification within P. capensis. Increased levels of genetic differentiation were observed along a west-to-south-easterly trajectory because the south-eastern parts of the Cape Fold Mountain chain harbour larger, more stable fragments of forest patches, have more pronounced habitat heterogeneity and have historically received higher levels of rainfall. These results suggest the presence of three putative species within P. capensis, which are geographically discreet and genetically distinct. PMID:22409213

McDonald, D E; Daniels, S R

2012-05-01

96

Hybridization among Arctic white-headed gulls (Larus spp.) obscures the genetic legacy of the Pleistocene  

USGS Publications Warehouse

We studied the influence of glacial oscillations on the genetic structure of seven species of white-headed gull that breed at high latitudes (Larus argentatus, L. canus, L. glaucescens, L. glaucoides, L. hyperboreus, L. schistisagus, and L. thayeri). We evaluated localities hypothesized as ice-free areas or glacial refugia in other Arctic vertebrates using molecular data from 11 microsatellite loci, mitochondrial DNA (mtDNA) control region, and six nuclear introns for 32 populations across the Holarctic. Moderate levels of genetic structure were observed for microsatellites (FST= 0.129), introns (?ST= 0.185), and mtDNA control region (?ST= 0.461), with among-group variation maximized when populations were grouped based on subspecific classification. Two haplotype and at least two allele groups were observed across all loci. However, no haplotype/allele group was composed solely of individuals of a single species, a pattern consistent with recent divergence. Furthermore, northernmost populations were not well differentiated and among-group variation was maximized when L. argentatus and L. hyberboreus populations were grouped by locality rather than species, indicating recent hybridization. Four populations are located in putative Pleistocene glacial refugia and had larger t estimates than the other 28 populations. However, we were unable to substantiate these putative refugia using coalescent theory, as all populations had genetic signatures of stability based on mtDNA. The extent of haplotype and allele sharing among Arctic white-headed gull species is noteworthy. Studies of other Arctic taxa have generally revealed species-specific clusters as well as genetic structure within species, usually correlated with geography. Aspects of white-headed gull behavioral biology, such as colonization ability and propensity to hybridize, as well as their recent evolutionary history, have likely played a large role in the limited genetic structure observed.

Sonsthagen, Sarah A.; Chesser, R. Terry; Bell, Douglas A.; Dove, Carla J.

2012-01-01

97

Hybridization among Arctic white-headed gulls (Larus spp.) obscures the genetic legacy of the Pleistocene  

PubMed Central

We studied the influence of glacial oscillations on the genetic structure of seven species of white-headed gull that breed at high latitudes (Larus argentatus, L. canus, L. glaucescens, L. glaucoides, L. hyperboreus, L. schistisagus, and L. thayeri). We evaluated localities hypothesized as ice-free areas or glacial refugia in other Arctic vertebrates using molecular data from 11 microsatellite loci, mitochondrial DNA (mtDNA) control region, and six nuclear introns for 32 populations across the Holarctic. Moderate levels of genetic structure were observed for microsatellites (FST= 0.129), introns (?ST= 0.185), and mtDNA control region (?ST= 0.461), with among-group variation maximized when populations were grouped based on subspecific classification. Two haplotype and at least two allele groups were observed across all loci. However, no haplotype/allele group was composed solely of individuals of a single species, a pattern consistent with recent divergence. Furthermore, northernmost populations were not well differentiated and among-group variation was maximized when L. argentatus and L. hyberboreus populations were grouped by locality rather than species, indicating recent hybridization. Four populations are located in putative Pleistocene glacial refugia and had larger ? estimates than the other 28 populations. However, we were unable to substantiate these putative refugia using coalescent theory, as all populations had genetic signatures of stability based on mtDNA. The extent of haplotype and allele sharing among Arctic white-headed gull species is noteworthy. Studies of other Arctic taxa have generally revealed species-specific clusters as well as genetic structure within species, usually correlated with geography. Aspects of white-headed gull behavioral biology, such as colonization ability and propensity to hybridize, as well as their recent evolutionary history, have likely played a large role in the limited genetic structure observed.

Sonsthagen, Sarah A; Chesser, R Terry; Bell, Douglas A; Dove, Carla J

2012-01-01

98

The Pleistocene Boundary and the Beginning of the Quaternary  

NASA Astrophysics Data System (ADS)

The Pleistocene Boundary and the Beginning of the Quaternary presents the results of a forty-year international program to establish a precise definition for the most important boundary in geologic time - the beginning of the modern age of glacial climates. This book, the final report of International Geological Correlation Project 41, describes the selection of a fixed reference point in the sequence of deep-water marine strata exposed at Vrica, Italy. This point, dated to 1.81 million years before the present, coincides with the onset of the first of four major glacial phases. The opening chapters of the book document the geology and palaeontology of the agreed physical reference point, and the successful proposal of this point as the boundary-stratotype of the Pleistocene Epoch. The book then describes the correlation of the boundary throughout the ocean basins and continents of the world in terms of detailed stratigraphic criteria such as planktonic biostratigraphy, magnetostratigraphy and radiometric age determinations.

van Couvering, John A.

2004-12-01

99

Quaternary evolution of glaciated gneiss terrains: pre-glacial weathering vs. glacial erosion  

NASA Astrophysics Data System (ADS)

Vast areas previously covered by Pleistocene ice sheets consist of rugged bedrock-dominated terrain of innumerable knolls and lake-filled rock basins - the ‘cnoc-and-lochan' landscape or ‘landscape of areal scour'. These landscapes typically form on gneissose or granitic lithologies and are interpreted (1) either to be the result of strong and widespread glacial erosion over numerous glacial cycles; or (2) formed by stripping of a saprolitic weathering mantle from an older, deeply weathered landscape. We analyse bedrock structure, erosional landforms and weathering remnants and within the ‘cnoc-and-lochan' gneiss terrain of a rough peneplain in NW Scotland and compare this with a geomorphologically similar gneiss terrain in a non-glacial, arid setting (Namaqualand, South Africa). We find that the topography of the gneiss landscapes in NW Scotland and Namaqualand closely follows the old bedrock-saprolite contact (weathering front). The roughness of the weathering front is caused by deep fracture zones providing a highly irregular surface area for weathering to proceed. The weathering front represents a significant change in bedrock physical properties. Glacial erosion (and aeolian erosion in Namaqualand) is an efficient way of stripping saprolite, but is far less effective in eroding hard, unweathered bedrock. Significant glacial erosion of hard gneiss probably only occurs beneath palaeo-ice streams. We conclude that the rough topography of glaciated ‘cnoc-and-lochan' gneiss terrains is formed by a multistage process: 1) Long-term, pre-glacial chemical weathering, forming deep saprolite with an irregular weathering front; 2) Stripping of weak saprolite by glacial erosion during the first glaciation(s), resulting in a rough land surface, broadly conforming to the pre-existing weathering front (‘etch surface'); 3) Further modification of exposed hard bedrock by glacial erosion. In most areas, glacial erosion is limited, but can be significant beneath palaeo-ice stream. The roughness of glaciated gneiss terrains is crucial for modelling of the glacial dynamics of present-day ice sheets. This roughness is shown here to depend on the intensity of pre-glacial weathering as well as glacial erosion during successive glaciations.

Krabbendam, Maarten; Bradwell, Tom

2014-07-01

100

Pleistocene environmental dynamics recorded in the loess of the middle and lower Danube basin  

NASA Astrophysics Data System (ADS)

The substantial loess deposits of the middle and lower Danube basin in southeastern Europe represent one of the thickest and most comprehensive terrestrial palaeoenvironmental records on the continent, yet are also the least well understood. Environmental conditions over the last million years have resulted in relatively continuous deposits uninterrupted by glaciation and tundra conditions, which nevertheless reflect oscillations between relatively warm-humid ("interglacial") and cold-dry ("glacial") intervals. This relative environmental stability may have proven important for hominins migrating into and through the region. The loess stratigraphy comprises distinct loess-paleosol sequences, reflecting glacial-interglacial phases which can be quantified for intensity using environmental magnetism and geochemistry. These phases are emphasised by variations in vegetation and malacofauna which respond to climatic change. The loess deposits demonstrate broadly similar sedimentological characteristics across the basin. Danubian loess deposits initiated in response to the tectonic formation of the Pannonian basin, retreat of the large palaeolakes, and increased sediment supply from the Danube. The period from ˜1 Ma-500 ka (MIS 27-13) was characterised by alternating loess deposition and pedogenesis during glacial and interglacial periods respectively, in response to relatively humid, forested conditions. This period represents the opening of the Danube corridor and provides the backdrop for initial hominin arrival into Europe. After ˜500 ka, and particularly after MIS 9, loess accumulation rates increased in response to relatively more steppic, arid, environments. MIS 9 and 13-15 were the most humid phases of the last ˜600 ky. The MIS 5 interglacial period was the warmest, and relatively most humid, period preceding the Holocene, and was followed by substantially increased loess accumulation during MIS 4, which may be linked to North Atlantic circulation. The complexity of the MIS 3 interstadial paleosol suggests that conditions were not uniformly warm and wet during this time. MIS 3 corresponds with the first arrival of anatomically modern humans to Europe. The last glacial maximum and Younger Dryas of MIS 2 were characterised by substantially increased loess accumulation indicating cold steppe environments most likely influenced by the North Atlantic, although conditions were sufficiently mild that the region acted as a refugium for thermophilic biota, as may also have been the case for most of the Pleistocene glacial cycles. The Holocene soil represents relatively wamer and more humid conditions corresponding to the current interglacial.

Fitzsimmons, Kathryn E.; Markovi?, Slobodan B.; Hambach, Ulrich

2012-05-01

101

Dating megafaunal extinction on the Pleistocene Darling Downs, eastern Australia: the promise and pitfalls of dating as a test of extinction hypotheses  

Microsoft Academic Search

A key to understanding Late Pleistocene megafaunal extinction dynamics is knowledge of megafaunal ecological response(s) to long-term environmental perturbations. Strategically, that requires targeting fossil deposits that accumulated during glacial and interglacial intervals both before and after human arrival, with subsequent palaeoecological models underpinned by robust and reliable chronologies. Late Pleistocene vertebrate fossil localities from the Darling Downs, eastern Australia, provide

Gilbert J. Price; Gregory E. Webb; Jian-xin Zhao; Yue-xing Feng; Andrew S. Murray; Bernard N. Cooke; Scott A. Hocknull; Ian H. Sobbe

2011-01-01

102

Late Glacial to Holocene coastal changes of SE Rügen Island (Baltic Sea, NE Germany)  

Microsoft Academic Search

The aim of this study was to investigate the long-term coastal evolution of Mönchgut peninsula (Rügen Island) on the non-tidal SW Baltic Sea coast. The geological setting of the barriers was determined by coring Late Pleistocene to Holocene deposits. The sediment succession starts with a Late Glacial till overlain by Late Pleistocene\\/Early Holocene sandy freshwater deposits of fluvial origin. There

Gösta Hoffmann; Jens Barnasch

2005-01-01

103

Glacial Erosion\\/Sedimentation of the Baltic Region and the Effect on the Postglacial Uplift  

Microsoft Academic Search

\\u000a Plio-Pleistocene erosion and sedimentation significantly impact postglacial uplift. We estimate in the last glacial cycle\\u000a sedimentation could produce up to 155 m of subsidence and erosion 32 m of uplift. To show this we determine the changes in\\u000a surface load caused by glacial and postglacial erosion and sedimentation over 1,000 year time intervals (coarser intervals\\u000a before 50,000 years) utilizing a

Aleksey Amantov; Willy Fjeldskaar; Lawrence Cathles

104

Siberia at the Last Glacial Maximum: Environment and Archaeology  

Microsoft Academic Search

This article focuses on the presence of humans in Siberia and the Russian Far East at the coldest time of the Late Pleistocene,\\u000a called the Last Glacial Maximum (LGM) and dated to c. 20,000–18,000 rcbp. Reconstruction of the LGM environment of Siberia,\\u000a based on the latest models and compilations, provides a background for human existence in this region. Most of Siberia

Yaroslav V. Kuzmin

2008-01-01

105

Examining the effects of glacial erosion on the extent of glaciation  

NASA Astrophysics Data System (ADS)

Landscapes modified by warm-based glacial activity in alpine settings show a distinct distribution of surface area with elevation (hypsometry), with a maximum in surface area just below the local snowline altitude. The emergence of this distinct hypsometric signature seems to be a consequence of effective glacial and periglacial erosion above the local snowline. Here we examine the response of mountain range glaciations to this distinct topographic distribution, and investigate how its formation influences patterns of glacial extent, and therefore also patterns of glacial erosion, over several glacial cycles. We use numerical modeling experiments, and show first how the hypsometry of characteristic natural landscapes affects glaciations for a simple climate forcing. The results suggest that glacial extent is highly sensitive to the hypsometry of glacially modified landscapes in addition to the climate forcing. Secondly, we show, using a synthetic landscape, how the gradual development of the distinct glacial hypsometric maximum influences the extent of glaciations on a timescale comparable to the Quaternary period. A Quaternary-like climate forcing results in two different phases of glacial erosion, suggesting a first phase of cirque formation followed by a phase of main valley deepening after the mid-Pleistocene transition. The numerical modeling experiments therefore suggest a significant increase in glacial extent and glacial erosion across the mid-Pleistocene transition. The results are obtained using iSOSIA, a higher-order ice sheet model approach, for simulating the flow of ice. Glacial erosion, represented by abrasion and quarrying processes, is approximated as functions of both sliding velocity, the amount of entrained sediment in the ice, and the bed slope in the direction of sliding. Temperature is linked to elevation through a constant lapse rate, ablation is a linear function of positive temperatures, and accumulation is a linear function of negative temperatures up to a maximum value.

Pedersen, V. K.; Egholm, D.

2012-12-01

106

Upper Pleistocene facies sequences and relative sea-level trends along the south coast of Ireland  

Microsoft Academic Search

Upper Pleistocene sequences, deposited around 20 ka provide a record of sedimentation during the last glacial\\/deglacial cycle along the south coast of Ireland. A stratigraphy based on eight lithofacies associations is recognized. Typically, the facies sequences overlie a glaciated shore platform furrowed by subglacial meltwaters. Elements within the stratigraphy comprise: (1) ice advance southwards onto the continental shelf; (2) stagnation-zone

A. M. McCabe

1996-01-01

107

The Pleistocene of Denmark: A review of stratigraphy and glaciation history  

Microsoft Academic Search

Climatically dependent environmental changes and the advance and retreat of glaciers have controlled the development of sedimentary successions in Denmark during the Middle and Late Pleistocene. Over the last four glacial-interglacial cycles radical alternations of marine and terrestrial environments, combined with eustatic and isostatic response to glaciation, has lead to the following geological history. The first Quaternary glaciation reached the

Michael Houmark-Nielsen

2004-01-01

108

Genetic consequences of Pleistocene glaciations for the tundra vole ( Microtus oeconomus ) in Beringia  

Microsoft Academic Search

Repeated glacial events during the Pleistocene fragmented and displaced populations throughout the northern continents. Different models of the effects of these climate-driven events predict distinct phylogeographic and population genetic outcomes for high-latitude faunas. The role of glaciations in (i) promoting intraspecific genetic differentiation and (ii) influencing genetic diversity was tested within a phylogeographic framework using the rodent Microtus oeconomus .

KURT E. G ALBREATH; JOSEPH A. C OOK

2004-01-01

109

Small mammal diversity loss in response to late-Pleistocene climatic change  

Microsoft Academic Search

Communities have been shaped in numerous ways by past climatic change; this process continues today. At the end of the Pleistocene epoch about 11,700years ago, North American communities were substantially altered by the interplay of two events. The climate shifted from the cold, arid Last Glacial Maximum to the warm, mesic Holocene interglacial, causing many mammal species to shift their

Jessica L. Blois; Jenny L. McGuire; Elizabeth A. Hadly

2010-01-01

110

Glaciers and rivers: Pleistocene uncoupling in a Mediterranean mountain karst  

NASA Astrophysics Data System (ADS)

Large-scale coupling between headwater catchments and downstream depocentres is a critical influence on long-term fluvial system behaviour and on the creation of the fluvial sedimentary record. However, it is often difficult to examine this control over multiple Quaternary glacial cycles and it has not been fully explored in karst basins. By investigating the Pleistocene glacial and fluvial records on and around Mount Orjen (1894 m) in Montenegro, we show how the changing connectivity between glaciated mountain headwater source zones and downstream alluvial basins is a key feature of long-term karst system behaviour - especially in relation to the creation and preservation of the surface sedimentary record. Middle and Late Pleistocene glacial deposits are well preserved on Mount Orjen. Uranium-series dating of 27 carbonate cements in fluvial sediments shows that many alluvial depocentres were completely filled with coarse glacial outwash before 350 ka during the largest recorded glaciation. This major glaciation is correlated with the Skamnellian Stage in Greece and Marine Isotope Stage 12 (MIS 12, c 480-420 ka). This was a period of profound landscape change in many glaciated catchments on the Balkan Peninsula. Later glaciations were much less extensive and sediment supply to fluvial systems was much diminished. The extreme base level falls of the Late Miocene produced the world's deepest karst networks around the Mediterranean. After MIS 12, the subterranean karst of Mount Orjen formed the dominant pathway for meltwater and sediment transfer so that the depositional basins below 1000 m became disconnected (uncoupled) from the glaciated headwaters. There is little evidence of post-MIS 12 aggradation or incision in these basins. This absence of later Pleistocene and Holocene fluvial activity means these basins contain some of the thickest and best-preserved outwash deposits in the Mediterranean.

Adamson, K. R.; Woodward, J. C.; Hughes, P. D.

2014-06-01

111

Upper Pleistocene facies sequences and relative sea-level trends along the south coast of Ireland  

SciTech Connect

Upper Pleistocene sequences, deposited around 20 ka provide a record of sedimentation during the last glacial/deglacial cycle along the south coast of Ireland. A stratigraphy based on eight lithofacies associations is recognized. Typically, the facies sequences overlie a glaciated shore platform furrowed by subglacial meltwaters. Elements within the stratigraphy comprise: (1) ice advance southwards onto the continental shelf; (2) stagnation-zone retreat triggered by rising sea level related to isostatic depression coupled with subglacial meltwater events that furrowed the platform; (3) progressive rise in relative sea level recorded by a submergent facies sequence on an isostatically depressed slope (beach gravels {yields} subaqueous jet efflux sediments {yields} wave-influenced sands {yields} glaciomarine mud drape); ice-marginal oscillation is recorded by glaciotectonically deformed gravels, sands, and foliated diamict; (4) terrestrial emergence is marked by angular breccias derived from local slopes by periglacial weathering. There is a clear facies transition between the breccias and underlying wave-influenced sands. Facies sequences suggest that the local deglacial cycle was out of phase with the global eustatic cycle along the south coast of Ireland. Stagnation-zone retreat was largely dependent on magnitudes of isostatic depression, high relative sea level, and meltwater events, and not on climatic forcing.

McCabe, A.M. [Univ. of Ulster, Londonderry (Ireland). Dept. of Environmental Studies; O`Cofaigh, C. [Univ. of Alberta, Edmonton, Alberta (Canada). Dept. of Geography

1996-03-01

112

Modern and last local glacial maximum snowlines in the Central Andes of Peru, Bolivia, and Northern Chile  

Microsoft Academic Search

Late Pleistocene snowlines in the central Andes were 500–1200m lower than at present. Radiocarbon dates imply that the late-Pleistocene glacial maximum in the region occurred prior to 20 14Cka, but lack of maximum limiting ages adds considerable uncertainty to the exact timing. Snowline modeling demonstrates that snowlines in the eastern and western cordilleras of the central Andes respond differently to

Andrew G. Klein; Geoffrey O. Seltzer; Bryan L. Isacks

1999-01-01

113

Glacial oceanographic contrasts explain phylogeography of Australian bull kelp.  

PubMed

The evolutionary effects of Southern Hemisphere Pleistocene oceanographic conditions - marked by fluctuations in sea levels and water temperatures, and redirected currents - are poorly understood. The southeastern tip of Australia presents an intriguing model system for studying the biological impacts of palaeoceanography. In particular, contrasting oceanographic conditions that existed on eastern vs. western sides of the Bassian Isthmus during Pleistocene glacial periods allow for natural comparisons between putative refugial vs. re-invading populations. Whereas many western Tasmanian marine taxa were likely eliminated by cold subantarctic water during the last glacial period, eastern Tasmanian populations would have persisted in relatively warm temperatures mediated by the ongoing influence of the East Australian Current (EAC). Here we test for the effects of contrasting palaeoceanographic conditions on endemic bull kelp, Durvillaea potatorum, using DNA sequence analysis (COI; rbcL) of more than 100 individuals from 14 localities in southeastern Australia. Phylogenetic reconstructions reveal a deep (maximum divergence 4.7%) genetic split within D. potatorum, corresponding to the 'eastern' and 'western' geographical regions delimited by the Bassian Isthmus, a vicariant barrier during low Pleistocene sea levels. Concordant with the western region's cold glacial conditions, samples from western Tasmania and western Victoria are genetically monomorphic, suggesting postglacial expansion from a mainland refugium. Eastern samples, in contrast, comprise distinct regional haplogroups, suggesting the species persisted in eastern Tasmania throughout recent glacial periods. The deep east-west divergence seems consistent with earlier reports of morphological differences between 'western' and 'eastern' D. potatorum, and it seems likely that these forms represent reproductively isolated species. PMID:19389161

Fraser, Ceridwen I; Spencer, Hamish G; Waters, Jonathan M

2009-05-01

114

Late Pleistocene vegetation of Kings Canyon, Sierra Nevada, California  

NASA Astrophysics Data System (ADS)

Seven packrat midden samples make possible a comparison between the modern and late Pleistocene vegetation in Kings Canyon on the western side of the southern Sierra Nevada. One modern sample contains macrofossils and pollen derived from the present-day oak-chaparral vegetation. Macrofossils from the six late Pleistocene samples record a mixed coniferous forest dominated by the xerophytic conifers Juniperus occidentalis, Pinus cf. ponderosa, and P. monophylla. The pollen spectra of these Pleistocene middens are dominated by Pinus sp., Taxodiaceae-Cupressaceae-Taxaceae (TCT), and Artemisia sp. Mesophytic conifers are represented by low macrofossil concentrations. Sequoiadendron giganteum is represented by a few pollen grains in the full glacial. Edaphic control and snow dispersal are the most likely causes of these mixed assemblages. The dominant macrofossils record a more xeric plant community than those that now occur on similar substrates at higher elevations or latitudes in the Sierra Nevada. These assemblages suggest that late Wisconsin climates were cold with mean annual precipitation not necessarily greater than modern values. This conclusion supports a model of low summer ablation allowing for the persistence of the glaciers at higher elevations during the late Wisconsin. The records in these middens also suggest that S. giganteum grew at lower elevations along the western side of the range and that P. monophylla was more widely distributed in cismontane California during the Pleistocene.

Cole, Kenneth

1983-01-01

115

Global deep-sea extinctions during the Pleistocene ice ages  

NASA Astrophysics Data System (ADS)

The dark, near-freezing environment of the deep oceans is regarded as one of the most stable habitats on Earth, and this stability is generally reflected in the slow turnover rates (extinctions and appearances) of the organisms that live there. By far the best fossil record of deep-sea organisms is provided by the shells of benthic foraminifera (Protista). A little-known global extinction of deep-sea benthic foraminifera occurred during the Pleistocene ice ages. In the southwest Pacific, it caused the disappearance of at least two families, 15 genera, and 48 species (˜15% 25% of the fauna) of dominantly uniserial, elongate foraminifera with distinctive apertural modifications. These forms progressively died back and became extinct during glacial periods in the late Pliocene to middle Pleistocene (ca. 2.5 0.6 Ma); most extinctions occurred between 1.0 and 0.6 Ma, at the time of the middle Pleistocene climatic revolution. This first high-resolution study of this extinction event indicates that it was far more significant for deep-sea diversity loss than previously reported (10 species). The middle Pleistocene extinction was the most dramatic last phase of a worldwide decline in the abundance of these elongate forms, a phase that began during cooling near the Eocene-Oligocene boundary and continued during the middle Miocene. Clearly these taxa declined when the world cooled, but the reason is yet to be resolved.

Hayward, Bruce W.

2001-07-01

116

Ecological change, range fluctuations and population dynamics during the Pleistocene.  

PubMed

Apart from the current human-induced climate change, the Holocene is notable for its stable climate. In contrast, the preceding age, the Pleistocene, was a time of intensive climatic fluctuations, with temperature changes of up to 15 degrees C occurring within a few decades. These climatic changes have substantially influenced both animal and plant populations. Until recently, the prevailing opinion about the effect of these climatic fluctuations on species in Europe was that populations survived glacial maxima in southern refugia and that populations died out outside these refugia. However, some of the latest studies of modern population genetics, the fossil record and especially ancient DNA reveal a more complex picture. There is now strong evidence for additional local northern refugia for a large number of species, including both plants and animals. Furthermore, population genetic analyses using ancient DNA have shown that genetic diversity and its geographical structure changed more often and in more unpredictable ways during the Pleistocene than had been inferred. Taken together, the Pleistocene is now seen as an extremely dynamic era, with rapid and large climatic fluctuations and correspondingly variable ecology. These changes were accompanied by similarly fast and sometimes dramatic changes in population size and extensive gene flow mediated by population movements. Thus, the Pleistocene is an excellent model case for the effects of rapid climate change, as we experience at the moment, on the ecology of plants and animals. PMID:19640497

Hofreiter, Michael; Stewart, John

2009-07-28

117

Phylogeographic heterogeneity of the brown macroalga Sargassum horneri (Fucaceae) in the northwestern Pacific in relation to late Pleistocene glaciation and tectonic configurations.  

PubMed

Pleistocene glacial oscillations and associated tectonic processes are believed to have influenced the historical abundances and distribution of organisms in the Asia Northwest Pacific (ANP). Accumulating evidence indicates that factors shaping tempospatial population dynamics and distribution patterns of marine taxa vary with biogeographical latitude, pelagic behaviour and oceanographic regimes. To detect what kinds of historical and contemporary factors affected genetic connectivity, phylogeographic profiles of littoral macroalga Sargassum horneri in the ANP were analysed based on mitochondrial (Cox3) and chloroplast (rbcL) data sets. Five distinct clades were recovered. A strong signature of biogeographical structure was revealed (?(CT) = 0.487, P < 0.0001) derived from remarkable differentiation in clade distribution, as clade I is restricted to Chinese marginal seas (Yellow-Bohai Sea, East China Sea and South China Sea), whereas clades II-V are discontinuously scattered around the main Islands of Japan. Furthermore, two secondary contact regions were identified along the south Japan-Pacific coastline. This significant differentiation between the two basins may reflect historical glacial isolation in the northwestern Pacific, which is congruent with the estimates of clade divergence and demographic expansion during the late Quaternary low sea levels. Analysis of molecular variance and the population-pair statistic F(ST) also revealed significant genetic structural differences between Chinese marginal seas and the Japanese basin. This exceptional phylogeographic architecture in S. horneri, initially shaped by historical geographic isolation during the late Pleistocene ice age and physical biogeographical barriers, can be complicated by oceanographic regimes (ocean surface currents) and relocating behaviour such as oceanic drifting. PMID:21851438

Hu, Zi-Min; Uwai, Shinya; Yu, Shen-Hui; Komatsu, Teruhisa; Ajisaka, Tetsuro; Duan, De-Lin

2011-09-01

118

Earth's glacial record and its tectonic setting  

NASA Astrophysics Data System (ADS)

Glaciations have occurred episodically at different time intervals and for different durations in Earth's history. Ice covers have formed in a wide range of plate tectonic and structural settings but the bulk of Earth's glacial record can be shown to have been deposited and preserved in basins within extensional settings. In such basins, source area uplift and basin subsidence fulfill the tectonic preconditions for the initiation of glaciation and the accomodation and preservation of glaciclastic sediments. Tectonic setting, in particular subsidence rates, also dictates the type of glaciclastic facies and facies successions that are deposited. Many pre-Pleistocene glaciated basins commonly contain well-defined tectonostratigraphic successions recording the interplay of tectonics and sedimentation; traditional climatostratigraphic approaches involving interpretation in terms of either ice advance/retreat cycles or glacio-eustatic sea-level change require revision. The direct record of continental glaciation in Earth history, in the form of classically-recognised continental glacial landforms and "tillites", is meagre; it is probable that more than 95% of the volume of preserved "glacial" strata are glacially-influenced marine deposits that record delivery of large amounts of glaciclastic sediment to offshore basins. This flux has been partially or completely reworked by "normal" sedimentary processes such that the record of glaciation and climate change is recorded in marine successions and is difficult to decipher. The dominant "glacial" facies in the rock record are subaqueous debris flow diamictites and turbidites recording the selective preservation of poorly-sorted glaciclastic sediment deposited in deep water basins by sediment gravity flows. However, these facies are also typical of many non-glacial settings, especially volcanically-influenced environments; numerous Archean and Proterozoic diamictites, described in the older literature as tillites, have no clearly established glacial parentage. The same remarks apply to many successions of laminated and thin-bedded facies interpreted as "varvites". Despite suggestions of much lower values of solar luminosity (the weak young sun hypothesis), the stratigraphic record of Archean glaciations is not extensive and may be the result of non-preservation. However, the effects of very different Archean global tectonic regimes and much higher geothermal heat flows, combined with a Venus-like atmosphere warmed by elevated levels of CO 2, cannot be ruled out. The oldest unambiguous glacial succession in Earth history appears to be the Early Proterozoic Gowganda Formation of the Huronian Supergroup in Ontario; the age of this event is not well-constrained but glaciation coincided with regional rifting, and may be causally related to, oxygenation of Earth's atmosphere just after 2300 Ma. New evidence that oxygenation is tectonically, not biologically driven, stresses the intimate relationship between plate tectonics, evolution of the atmosphere and glaciation. Global geochemical controls, such as elevated atmospheric CO 2 levels, may be responsible for a long mid-Proterozoic non-glacial interval after 2000 Ma that was terminated by the Late Proterozoic glaciations just after 800 Ma. A persistent theme in both Late Proterozoic and Phanerozoic glaciations is the adiabatic effect of tectonic uplift, either along collisional margins or as a result of passive margin uplifts in areas of extended crust, as the trigger for glaciation; the process is reinforced by global geochemical feedback, principally the drawdown of atmospheric CO 2 and Milankovitch "astronomical" forcing but these are unlikely, by themselves, to inititiate glaciation. The same remarks apply to late Cenozoic glaciations. Late Proterozoic glacially-influenced strata occur on all seven continents and fall into two tectonostratigraphic types. In the first category are thick sucessions of turbidites and mass flows deposited along active, compressional plate margi

Eyles, N.

1993-09-01

119

Glacial lake hides bacteria  

NSDL National Science Digital Library

This article highlights the published work of a geomicrobiology research team led by Eric Gaidos from the University of Hawaii and Brian Lanoil, from the University of California, Riverside. This group reports the identification of bacteria from an Icelandic sub-glacial lake, and how the collection and description of these microorganisms immured within glacial ice and sub-surface water serve as a model in the search for extra-terrestrial life.

Peplow, Mark; Online, Bioed

120

Subsurface warming in the subpolar North Atlantic during rapid climate events in the Early and Mid-Pleistocene  

NASA Astrophysics Data System (ADS)

A new high-resolution reconstruction of the temperature and salinity of the subsurface waters using paired Mg/Ca-?18O measurements on the planktonic foraminifera Neogloboquadrina pachyderma sinistrorsa (sin.) was conducted on a deep-sea sediment core in the subpolar North Atlantic (Site U1314). This study aims to reconstruct millennial-scale subsurface hydrography variations during the Early and Mid-Pleistocene (MIS 31-19). These rapid climate events are characterized by abrupt shifts between warm/cold conditions, and ice-sheet oscillations, as evidenced by major ice rafting events recorded in the North Atlantic sediments (Hernández-Almeida et al., 2012), similar to those found during the Last Glacial period (Marcott et al, 2011). The Mg/Ca derived paleotemperature and salinity oscillations prior and during IRD discharges at Site U1314 are related to changes in intermediate circulation. The increases in Mg/Ca paleotemperatures and salinities during the IRD event are preceded by short episodes of cooling and freshening of subsurface waters. The response of the AMOC to this perturbation is an increased of warm and salty water coming from the south, transported to high latitudes in the North Atlantic beneath the thermocline. This process is accompanied by a southward shift in the convection cell from the Nordic Seas to the subpolar North Atlantic and better ventilation of the North Atlantic at mid-depths. Poleward transport of warm and salty subsurface subtropical waters causes intense basal melting and thinning of marine ice-shelves, that culminates in large-scale instability of the ice sheets, retreat of the grounding line and iceberg discharge. The mechanism proposed involves the coupling of the AMOC with ice-sheet dynamics, and would explain the presence of these fluctuations before the establishment of high-amplitude 100-kyr glacial cycles. Hernández-Almeida, I., Sierro, F.J., Cacho, I., Flores, J.A., 2012. Impact of suborbital climate changes in the North Atlantic on ice sheet dynamics at the Mid-Pleistocene Transition. Paleoceanography 27, PA3214. Marcott, S.A., Clark, P.U., Padman, L., Klinkhammer, G.P., Springer, S.R., Liu, Z., Otto-Bliesner, B.L., Carlson, A.E., Ungerer, A., Padman, J., He, F., Cheng, J., Schmittner, A., 2011. Ice-shelf collapse from subsurface warming as a trigger for Heinrich events. Proceedings of the National Academy of Sciences 108, 13415-13419

Hernández-Almeida, Iván; Sierro, Francisco; Cacho, Isabel; Abel Flores, José

2014-05-01

121

New exposure ages for the Last Glacial Cycle in the Sanabria Lake region (northwestern Spain)  

NASA Astrophysics Data System (ADS)

The Sanabria Lake region is located in the Trevinca Massif, a mid-latitude mountain area up to 2128 m asl in the northwest corner of the Iberian Peninsula (42oN 6oW). An ice cap glaciation took place during the Last Glacial Cycle in this massif, with an equilibrium line altitude of 1687 m for the Tera glacial outlet at its local maximum (Cowton et al., 2009). A well preserved glacial sequence occurs on an area of 45 km2 around the present Sanabria Lake (1000 m asl) and is composed by lateral and end moraines in close relationship with glaciolacustrine deposits. This sequence shows the ice snout oscillations of the former Tera glacier during the Last Glacial Cycle and offers a good opportunity to compare radiocarbon and OSL- based chronological models with new cosmogenic isotope dates. The new dataset of 10Be exposure ages presented here for the Sanabria Lake moraines is based on measurements conducted on 23 boulders and is compared with previous radiocarbon and OSL data conducted on ice related deposits (Pérez-Alberti et al., 2011; Rodríguez-Rodríguez et al., 2011). Our results are coherent with the available deglaciation radiocarbon chronology, and support a last deglaciation origin for the whole set of end moraines that are downstream the Sanabria Lake (19.2 - 15.7 10Be ka). Discrepancies between results of the different dating methods concern the timing of the local glacial maximum, with the cosmogenic exposure method always yielding the youngest minimum ages. As proposed to explain similar observations made elsewhere (Palacios et al., 2012), reconciling the ages from different dating methods would imply the occurrence of two glacial advances close enough in extent to generate an overlapping polygenic moraine. Cowton, T., Hughes, P.D., Gibbard, P.L., 2009. Palaeoglaciation of Parque Natural Lago de Sanabria, northwest Spain. Geomorphology 108, 282-291. Rodríguez-Rodríguez, L., Jiménez-Sánchez, M., Domínguez-Cuesta, M.J., Rico, M.T., Valero-Garcés, B., 2011. Last deglaciation in northwestern Spain: New chronological and geomorphologic evidence from the Sanabria region. Geomorphology 135, 48-65. Palacios, D., Andrés, N., Úbeda, J., Alcalá, J., Marcos, J., Vázquez-Selem, L., 2012. The importance of poligenic moraines in the paleoclimatic interpretation from cosmogenic dating. Geophysical Research Abstracts 14, EGU2012-3759-1. Pérez-Alberti, A., Valcárcel-Díaz, M., Martini, I.P., Pascucci, V., Andrucci, S., 2011. Upper Pleistocene glacial valley-junction sediments at Pias, Trevinca Mountains, NW Spain. In: Martini, I.P., French, H.M., Pérez-Alberti, A. (Eds.), Ice-Marginal and Periglacial Processes and Sediments. Geological Society (London) Special Publication 354, pp. 93-110. Research funded by the projects LIMNOCLIBER (REN2003-09130-C02-02), IBERLIMNO (CGL2005-20236-E/CLI), LIMNOCAL (CGL2006-13327-C04-01) and GRACCIE (CSD2007-00067) of the Spanish Inter-Ministry Commission of Science and Technology (CICYT). Additional funding was provided by the Fundación Patrimonio Natural de Castilla y León through the project "La investigacion en el Lago de Sanabria dentro del proyecto CALIBRE: perspectivas y posibilidades", and by the projects Consolider Ingenio 2006 (CSD2006-0041, Topo-Iberia), 2003 PIRA 00256, HF02.4, and RISKNAT (2009SGR520). L. Rodríguez-Rodríguez has developed her research under a Severo Ochoa Programme fellowship (FICYT- Asturias).

Rodríguez-Rodríguez, Laura; Jiménez-Sánchez, Montserrat; Domínguez-Cuesta, María Jose; Rinterknecht, Vincent; Pallàs, Raimon; Braucher, Régis; Bourlès, Didier; Valero-Garcés, Blas

2013-04-01

122

A northern glacial refugium for bank voles (Clethrionomys glareolus)  

PubMed Central

There is controversy and uncertainty on how far north there were glacial refugia for temperate species during the Pleistocene glaciations and in the extent of the contribution of such refugia to present-day populations. We examined these issues using phylogeographic analysis of a European woodland mammal, the bank vole (Clethrionomys glareolus). A Bayesian coalescence analysis indicates that a bank vole population survived the height of the last glaciation (?25,000–10,000 years B.P.) in the vicinity of the Carpathians, a major central European mountain chain well north of the Mediterranean areas typically regarded as glacial refugia for temperate species. Parameter estimates from the fitted isolation with migration model show that the divergence of the Carpathian population started at least 22,000 years ago, and it was likely followed by only negligible immigration from adjacent regions, suggesting the persistence of bank voles in the Carpathians through the height of the last glaciation. On the contrary, there is clear evidence for gene flow out of the Carpathians, demonstrating the contribution of the Carpathian population to the colonization of Europe after the Pleistocene. These findings are consistent with data from animal and plant fossils recovered in the Carpathians and provide the clearest phylogeographic evidence to date of a northern glacial refugium for temperate species in Europe.

Kotlik, Petr; Deffontaine, Valerie; Mascheretti, Silvia; Zima, Jan; Michaux, Johan R.; Searle, Jeremy B.

2006-01-01

123

Spores of the dung fungus Sporormiella: Increased abundance in historic sediments and before Pleistocene megafaunal extinction  

NASA Astrophysics Data System (ADS)

Spores of the dung fungus Sporormiella become abundant following the historic introduction of grazing herbivores at seven sites in the western United States. During the Holocene they are generally rare, but at six sites Sporormiella spores are abundant before the extinction of Pleistocene megaherbivores ca. 11,000 yr B.P. Sporormiella spores are directly linked to extinct megaherbivores by their presence in mammoth dung preserved in Bechan Cave, Southern Utah. Their abundance in late-glacial sediments may reflect the abundance of megaherbivores during Quaternary, thereby indicating the age of Pleistocene extinctions where other indicators are absent.

Davis, Owen K.

1987-09-01

124

Glacial modification of granite tors in the Cairngorms, Scotland  

USGS Publications Warehouse

A range of evidence indicates that many granite tors in the Cairngorms have been modified by the flow of glacier ice during the Pleistocene. Comparisons with SW England and the use of a space-time transformation across 38 tor groups in the Cairngorms allow a model to be developed for progressive glacial modification. Tors with deeply etched surfaces and no, or limited, block removal imply an absence of significant glacial modification. The removal of superstructure and blocks, locally forming boulder trains, and the progressive reduction of tors to stumps and basal slabs represent the more advanced stages of modification. Recognition of some slabs as tor stumps from which glacial erosion has removed all superstructure allows the original distribution of tors to be reconstructed for large areas of the Cairngorms. Unmodified tors require covers of non-erosive, cold-based ice during all of the cold stages of the Middle and Late Pleistocene. Deformation beneath cold-based glacier ice is capable of the removal of blocks but advanced glacial modification requires former wet-based glacier ice. The depth of glacial erosion at former tor sites remains limited largely to the partial or total elimination of the upstanding tor form. Cosmogenic nuclide exposure ages (Phillips et al., 2006) together with data from weathering pit depths (Hall and Phillips, 2006), from the surfaces of tors and large erratic blocks require that the glacial entrainment of blocks from tors occurred in Marine Isotope Stages (MIS) 4-2, 6 and, probably, at least one earlier phase. The occurrence of glacially modified tors on or close to, the main summits of the Cairngorms requires full ice cover over the mountains during these Stages. Evidence from the Cairngorms indicates that tor morphology can be regarded as an important indicator of former ice cover in many formerly glaciated areas, particularly where other evidence of ice cover is sparse. Recognition of the glacial modification of tors is important for debates about the former existence of nunataks and refugia. Copyright ?? 2006 John Wiley & Sons, Ltd.

Hall, A. M.; Phillips, W. M.

2006-01-01

125

A first 10Be cosmogenic glacial chronology from the High Atlas, Morocco, during the last glacial cycle.  

NASA Astrophysics Data System (ADS)

Glacial geomorphological mapping, 10Be cosmogenic exposure ages of 21 erratics from cirque-valley systems and paleo-glacier climate modelling in the High Atlas Mountains, Morocco (31.1° N, 7.9° W), provides new and novel insights as to the history and evolution of the largest desert region on Earth. The Atlas Mountains display evidence of extensive and multiple Late Pleistocene glaciations whose extent is significantly larger than that recognised by previous workers. The largest glaciers formed in the Toubkal massif where we find 3 distinct phases of glacial advances within the last glacial cycle. The oldest moraines occurring at the lowest elevations have yielded eight 10Be ages ranging from 30 to 88 ka. Six of eight samples from moraines at intermediate elevations gave ages of 19 to 25 ka (2 outliers) which correlates well with the global Last Glacial Maximum (ca. 26-21 ka) and the last termination during marine isotope stage 2. Five erratics from the youngest and most elevated moraines yielded a suite of normally distributed exposure ages from 11 to 13 ka which supports a correlation with the northern hemisphere Younger Dryas (12.9-11.7 ka). The glacial record of the High Atlas effectively reflects moisture supply to the north-western Sahara Desert and can provide an indication of shifts between arid and pluvial conditions. The plaeo equilibrium line altitudes (ELA) of these three glacier phases was more than 1000 m lower than the predicted ELA based on today's temperatures. Glacier-climate modelling indicates that for each of these glacier phases climate was not only significantly cooler than today, but also much wetter. The new evidence on the extent, timing and palaeoclimatic significance of glaciations in this region has major implications for understanding moisture transfer between the North Atlantic Ocean and the Sahara Desert during Pleistocene cold stages.

Fink, David; Hughes, Philip; Fenton, Cassie

2014-05-01

126

Comparison of Late Pleistocene and Modern Glacier Extents in Central Nepal Based on Digital Elevation Data and Satellite Imagery  

Microsoft Academic Search

Late Pleistocene and modern ice extents in central Nepal are compared to estimate equilibrium line altitude (ELA) depressions. New techniques are used for determining the former extent of glaciers based on quantitative, objective geomorphic analyses of a ?90-m resolution digital elevation model (DEM). For every link of the drainage network, valley form is classified as glacial or fluvial based on

Christopher C. Duncan; Andrew J. Klein; Jeffrey G. Masek; Bryan L. Isacks

1998-01-01

127

Mammoths in the insular Nearctic? Some constraints on the existence of a Pleistocene megafaunal refugium in the Canadian Arctic Archipelago  

Microsoft Academic Search

Mammoth fossils recovered on Banks and Melville Islands in the western part of the Canadian Arctic Archipelago have been used as a basis for inferring that these islands functioned as a Late Pleistocene refugium. However, only two fossils have actually been found in these locations, and both date to the period immediately before the Last Glacial Maximum (LGM). This paper

R. D. E. MacPhee

2007-01-01

128

Pleistocene permafrost in Western Transbaikalia  

Microsoft Academic Search

Progressive cooling of the Pleistocene climate led to the formation of permafrost in the southern part of Eastern Siberia, in particular, in low mountains and valley floors of the Transbaikal area. It was thought that in Western Transbaikalia the permafrost first appeared in the Middle Pleistocene and was related to the maximum — Riss (Samarovo) — glaciation (Ravsky et al.,

Nadezhda V Alexeeva; Margarita A Erbajeva

2000-01-01

129

Vegetation context and climatic limits of the Early Pleistocene hominin dispersal in Europe  

NASA Astrophysics Data System (ADS)

The vegetation and the climatic context in which the first hominins entered and dispersed in Europe during the Early Pleistocene are reconstructed, using literature review and a new climatic simulation. Both in situ fauna and in situ pollen at the twelve early hominin sites under consideration indicate the occurrence of open landscapes: grasslands or forested steppes. The presence of ancient hominins ( Homo of the erectus group) in Europe is only possible at the transition from glacial to interglacial periods, the full glacial being too cold for them and the transition interglacial to glacial too forested. Glacial-interglacial cycles forced by obliquity showed paralleled vegetation successions, which repeated c. 42 times during the course of the Early Pleistocene (2.58-0.78 Ma), providing 42 narrow windows of opportunity for hominins to disperse into Europe. The climatic conditions of this Early Pleistocene vegetation at glacial-interglacial transitions are compared with a climatic simulation for 9 ka ago without ice sheet, as this time period is so far the best analogue available. The climate at the beginning of the present interglacial displayed a stronger seasonality than now. Forest cover would not have been hampered though, clearly indicating that other factors linked to refugial location and soils leave this period relatively free of forests. Similar situations with an offset between climate and vegetation at the beginning of interglacials repeated themselves throughout the Quaternary and benefitted the early hominins when colonising Europe. The duration of this open phase of vegetation at the glacial-interglacial transition was long enough to allow colonisation from the Levant to the Atlantic. The twelve sites fall within rather narrow ranges of summer precipitation and temperature of the coldest month, suggesting the hominins had only a very low tolerance to climate variability.

Leroy, S. A. G.; Arpe, K.; Mikolajewicz, U.

2011-06-01

130

Glacial integrative modelling.  

PubMed

Understanding the mechanisms of past climate changes requires modelling of the complex interaction between all major components of the Earth system: atmosphere, ocean, cryosphere, lithosphere and biosphere. This paper reviews attempts at such an integrative approach to modelling climate changes during the glacial age. In particular, the roles of different factors in shaping glacial climate are compared based on the results of simulations with an Earth-system model of intermediate complexity, CLIMBER-2. It is shown that ice sheets, changes in atmospheric compositions, vegetation cover, and reorganization of the ocean thermohaline circulation play important roles in glacial climate changes. Another example of this approach is the modelling of two major types of abrupt glacial climate changes: Dansgaard-Oeschger and Heinrich events. Our results corroborate some of the early proposed mechanisms, which relate abrupt climate changes to the internal instability of the ocean thermohaline circulation and ice sheets. At the same time, it is shown that realistic representation of the temporal evolution of the palaeoclimatic background is crucial to simulate observed features of the glacial abrupt climate changes. PMID:14558899

Ganopolski, Andrey

2003-09-15

131

Mid-Pleistocene Orbital and Millennial Scale Climate Change in a 200 ky lacustrine sediment core from SW North America  

NASA Astrophysics Data System (ADS)

How anthropogenic climate change will affect hydroclimate of the arid regions of SW North America over the next century is a concern. Model projections suggest permanent “dust bowl-like” conditions; however, any anthropogenic change will be superimposed on long-term natural climate variability. We use the paleoclimatic record from an 82-m deep lacustrine sediment core (VC-3) from the Valles Caldera, New Mexico to examine continental climate variations spanning two glacial cycles through the middle Pleistocene from MIS 14 to MIS 10 (552 ka to ~360 ka). Both orbital and millennial-scale variations are evident in multiple proxies, and a strong relationship occurs between the warmest temperatures in the record and periods of extended aridity. We suggest that these periods of aridity are characterized by decreased winter as well as summer precipitation amounts. A new group of organic geochemical proxies (MBT and CBT) allow us to reconstruct the annual mean air temperature (MAT) of the Valles Caldera watershed as well as the watershed soil pH down the length of the core. We compare these proxies to climatically sensitive pollen taxa and other core properties. The MAT record of VC-3 shows considerable glacial-interglacial variation and significant variability within individual glacial and interglacial periods. The warmest interglacial MATs (5 to 7°C) compare favorably with modern MATs of ~5°C in the Valle Grande. MIS 11 has three warm substages, based on MAT estimates (2°C warmer than the cool substages), warm (Juniperus, Quercus, Rosaceae) vs. cool (Abies, Picea, Artemisia) pollen taxa and variation in aquatic productivity proxies (TOC, Si:Ti). The three warm substages of MIS 11 appear to correspond to the three precessional peaks that occur during this interval. Glacial MATs range from -5 to +2°C, with multiple millennial-scale temperature oscillations evident. Several of the interstadials show a distinct pattern of relatively slower temperature increases and progressive declines in cold boreal taxa pollen percentages (Picea, Abies), while others are characterized by abrupt warmings and decreases in boreal taxa pollen. Maximum interstadial temperatures are followed by abrupt coolings of as much as 6 to 7°C, and rapid increases in Picea and Abies pollen. These results show that the continental climate of SW North America had a strong response to millennial-scale climate change as well as to orbital forcing, even during a time of muted precessional cycles (MIS 11).

Fawcett, P. J.; Werne, J. P.; Anderson, R. S.; Heikoop, J. M.; Brown, E. T.; Berke, M. A.; Smith, S.; Goff, F. E.; Hurley, L. L.; Cisneros Dozal, L. M.; Schouten, S.; Sinninghe Damsté, J. S.; Huang, Y.; Toney, J. L.; Fessenden, J. E.; Woldegabriel, G. W.; Geissman, J. W.; Allen, C. D.

2009-12-01

132

A Lower Threshold for North Atlantic ice Rafting in the Late Pliocene Than the Late Pleistocene in Response to Low-slung Slippery ice Sheets?  

Microsoft Academic Search

Suborbital variability in late Pleistocene records of IRD and SST in the North Atlantic Ocean appears most extreme during times of enlarged ice-sheets with a well-constrained benthic oxygen isotope-defined 'ice- volume threshold' (delta18OT) for the '100 ka (inter)glacial' world. Information on North Atlantic climate instability for the earlier Pleistocene and late Pliocene is more fragmentary and\\/or of much lower temporal

I. Bailey; C. T. Bolton; R. Schiebel; R. M. Deconto; D. Pollard; P. A. Wilson

2008-01-01

133

On the Interpretation of Late Pleistocene 100-kyr Phases  

NASA Astrophysics Data System (ADS)

Phase calculations are often used to measure leads and lags in paleoclimate records and then make inferences about the causal mechanisms associated with orbitally forced glacial cycles. Fourier and wavelet spectral analyses work well for near-sinusoidal cycles, but Late Pleistocene 100-kyr glacial cycles are typically sawtooth shaped (e.g., 10 kyr of warming followed by 90 kyr of cooling) and are thus extremely non-sinusoidal. Here I present phase calculations for a variety of 100-kyr sawtooth shapes similar to those observed in paleoclimate records. These calculations demonstrate that variations in cycle shape can produce apparent differences in 100-kyr phase even when records experience synchronous warming and cooling. For example, changing the amplitude of MIS 5e in otherwise identical records can shift the 100-kyr wavelet phase by 13° (3.6 kyr). Therefore, spectral phase calculations are not well-suited for characterizing leads, lags, or response times of 100-kyr cyclicity in paleoclimate records. Direct comparison of the timing of distinctive features (e.g., termination onset or the onset of cooling at the end of an interglacial) is more appropriate for evaluating possible causal sequences in Late Pleistocene 100-kyr cycles.

Lisiecki, L. E.

2013-12-01

134

Clast fabric in a Stony Brook campus moraine: Testing models for the process of glacial lobe dynamics  

Microsoft Academic Search

A layer of glacial till found along the north shore of Long Island and throughout the Stony Brook University campus, located in Stony Brook, New York, was deposited around twenty-two thousand years ago late in the Wisconsinan stage of the Pleistocene Epoch. The Stony Brook University campus is located on the Stony Brook moraine, which is part of the Harbor

Jennifer DePaoli; Charles Regulinski; Josh Timlin; Dan Davis; Elliot Klein

135

Glacial geomorphic evidence for a late climatic change on Mars  

NASA Technical Reports Server (NTRS)

In a series of preliminary reports, we documented evidence of former glacial epochs on Mars. Apparent glacial landforms seemed to be concentrated primarily at middle to high southern latitudes. We now have additional evidence supporting the view that Martian glaciation appears to have been more extensive than previously recognized. The growth and collapse of ice sheets on Mars seems closely analogous to the growth and decline of Earth's great Pleistocene ice sheets. This implies that climate change was probably somewhat comparable on the two planets, although in the case of Mars the entire planet seems to have changed rapidly to a cold, dry present-day environment after the collapse of the ice sheets.

Kargel, J. S.; Strom, R. G.

1992-01-01

136

Glacial Legacies of New York State  

NSDL National Science Digital Library

Glacial Legacies of New York State is a presentation of digital images illustrating the origins of the glacial record in New York. Designed for classroom use, Glacial Legacies has a complementing narrative for each slide, images of glacial features in New York, classic examples of glacial features from around the country, and diagrams of glacial processes.

2001-04-09

137

Plio-Pleistocene history of Ferrar Glacier, Antarctica: Implications for climate and ice sheet stability  

Microsoft Academic Search

The areal distribution and elevation of glacial drifts in Vernier Valley, southern Victoria Land, are used to reconstruct the Plio-Pleistocene history of upper Ferrar Glacier. 21Ne cosmogenic-nuclide analyses of surface cobbles on four moraines, Ferrar 1, 2, 3, and 4, provide age control. A minimum-age estimate for Ferrar Drifts calculated by assuming zero surface erosion indicates that the oldest moraine,

J. W. Staiger; D. R. Marchant; J. M. Schaefer; P. Oberholzer; J. V. Johnson; A. R. Lewis; K. M. Swanger

2006-01-01

138

Alaska PaleoGlacier Atlas: A Geospatial Compilation of Pleistocene Glacier Extents  

NSDL National Science Digital Library

The Alaska PaleoGlacier (APG) Atlas is a recently released, web-based summary of Pleistocene glaciation across Alaska. Students can access a gallery of maps depicting the extent of glaciers during the late Wisconsin glaciation in Alaska as well as the maximum extent reached during the last 3 million years by valley glaciers, ice caps, and the northwestern Cordilleran Ice Sheet. a set of links is also provided to sites on galcial geology and glacial geospatial data.

Manley, William

139

Climatic variability and plant food distribution in Pleistocene Europe: Implications for Neanderthal diet and subsistence  

Microsoft Academic Search

Contrary to their cold-adapted image, Neanderthals inhabited Pleistocene Europe during a time of great climatic fluctuation with temperatures ranging from as warm as present-day during the last interglacial to as cold as those of the last glacial maximum. Cold-adapted Neanderthals are similarly most often associated with the exploitation of large mammals who are themselves cold-adapted (mammoth, bison, reindeer, etc.). Cold,

Bruce L. Hardy

2010-01-01

140

Glacial-Interglacial Cycle  

NSDL National Science Digital Library

This online lab exercise focuses on the causes, characteristics and effects of the glacial-interglacial cycle. The sixth in a 10-part lab series on weather and climate, this lab exercise is designed for first and second year college geoscience students (majors and non-majors) as well as pre-service STEM teachers.

141

What Drives Glacial Cycles?  

Microsoft Academic Search

The Milankovitch theory advocates that the glacial cycles have three components: the tilt of the earth's spin axis; the shape of the earth's orbit; and the interaction between the tilt and the eccentricity effects. These three factors work together to vary the amount of sunshine reaching the high northern latitudes in summer and allow the great ice sheets to grow

Wallace S. Broecker; George H. Denton

1990-01-01

142

Early Pleistocene climate changes in the central Mediterranean region as inferred from integrated pollen and planktonic foraminiferal stable isotope analyses  

NASA Astrophysics Data System (ADS)

Vegetation inherited from a Pliocene subtropical climate evolved through obliquity oscillations and global cooling leading to modern conditions. An integrated, highly time-resolved record of pollen and stable isotopes ( ?18O and ?13C of Globigerina bulloides) was obtained to understand vegetation responses to Early Pleistocene climate changes. Continental and marine responses are compared in the Central Mediterranean region with a particular consideration of environmental changes during anoxic events. Pollen data illustrate vegetation dynamics as follows: [1] development of mesothermic elements (warm and humid conditions); [2] expansion of mid- and high-altitude elements (cooler but still humid conditions); and [3] strengthening of steppe and herb elements (cooler and dry conditions). These successions correlate with precession. ?18O variations recorded by Globigerina bulloides define two cycles (MIS 43-40) related to obliquity. At northern low- to mid-latitudes, the pollen signal records temperature and wetness changes related to precession even during global climate changes induced by obliquity. This may result in unexpected increasing wetness during glacial periods, which has to be considered specific to the Central and Eastern Mediterranean region. Lastly, an analysis of anoxic events reveals that enhanced runoff is indicated by increasing frequency of the riparian trees Liquidambar and Zelkova.

Joannin, Sébastien; Quillévéré, Frédéric; Suc, Jean-Pierre; Lécuyer, Christophe; Martineau, François

2007-03-01

143

Giant glacial cirques of non-mountainous terrains  

NASA Astrophysics Data System (ADS)

Cirques are usually considered as specific landforms of hill and mountain terrains produced by alpine glaciers, and/or slope failures (landslides). However, glacial cirques seem to be present also in non-mountainous terrains that underwent extensive Pleistocene ice-sheet glaciations and strong glacial and fluvio-glacial erosion. The largest form in the Baltic region is Severoladozhsky (North Lake Ladoga) cirque, probably the world's largest representative, with the length and width close to 100 km. Another example is the deepest Landsort basin of the Baltic Sea. In those cases Meso-Neoproterozoic sediments were subject to selected erosion, with evident overdeepening of the bedrock surface in comparison with surrounding crystalline frame. The bowl headwall shape of the cirque-like landforms was determined by the outline of the margin of exhumed basin. The origin of the major basins of margins of the Baltic and Canadian shields are similar. However, direct analogues of giant cirques are not well developed in this part of North America due to geological deviations and dominant directions of ice movement. Comparable landforms (like the South Chippewa basin of the Lake Michigan) are therefore less mature. We define glacial cirque as an amphitheatre-shape depression with comparable values of length and width, steep headwall with adjacent side slopes and gentle lip with commonly increased glacial accumulation. They are usually located within an ice stream that created typical relief profile with normal horseshoe overdeepening, and in areas predefined by geological and geomorphological peculiarities. This definition likely fits both classic mountain cirques, and giant ones created in favorable conditions in domains that underwent extensive glaciations and relevant selective glacial erosion. Length/width ratio typical for giant cirques group is close to 1:1, being comparable with classical alpine ones. Major differences (like length/height ratio of other order and possible larger internal landscape complications) are related to the extreme size of typical representatives of giant cirques of non-mountainous terrains.

Amantov, A.; Amantova, M.

2012-04-01

144

Ecological niche conservatism and Pleistocene refugia in the Thrush-like Mourner, Schiffornis sp., in the neotropics.  

PubMed

Recent studies have increasingly implicated deep (pre-Pleistocene) events as key in the vertebrate speciation, downplaying the importance of more recent (Pleistocene) climatic shifts. This work, however, has been based almost exclusively on evidence from molecular clock inferences of splitting dates. We present an independent perspective on this question, using ecological niche model reconstructions of Pleistocene Last Glacial Maximum (LGM) potential distributions for the Thrush-like Mourner (Schiffornis turdina) complex in the neotropics. LGM distributional patterns reconstructed from the niche models relate significantly to phylogroups identified in previous molecular systematic analyses. As such, patterns of differentiation and speciation in this complex are consistent with Pleistocene climate and geography, although further testing will be necessary to establish dates of origin firmly and unambiguously. PMID:18005155

Peterson, A Townsend; Nyári, Arpád S

2008-01-01

145

Quaternary glacial landforms and evolution in the Cantabrian Mountains (Northern Spain): a synthesis from current data  

NASA Astrophysics Data System (ADS)

In Northern Iberian Peninsula are located the Cantabrian Mountains, a mountain system of 450 km length, reaching 2648 m in the Picos de Europa. It is an Atlantic mountain in the North slope, with a Atlantic Mediterranean transitional climate in the South slope.More than thirty-five massifs developed glaciers during the Pleistocene. Studies on glacial morphology are known from the XIX century and they have focused mainly on the maximum extent of glaciers. Nowadays there are detailed geomorphological maps, morphostratigraphic surveys and estimation of Equilibrium Line Altitude in different massifs and on different stages. During the last decade studies on glacial evolution and glaciation phases have been made, and the first chronological data have been published. In this work we presents the reconstruction of the glacial evolution in the Cantabrian Mountains during the Pleistocene and Holocene, based on recent chronological data (30 dates made using OSL, AMS and C14) and morphostratigraphic correlations obtained by several research groups. The number of reconstructed glacial stages varies among the different massifs, form one to four different stages. The highest massifs located in the central portion of the Cantabrian Mountains have the most complex glacial features, with at least four different moraine complexes stepped between the 400 m a.s.l in the Northern slope and 800 m a.s.l. in the Southern slope for the lowest moraine complexes, and the highest and youngest, located above 2100 m a.s.l. An ancient glacial phase has been pointed to MIS 12 -more than 400 ka-, disconnected from the present day glacial morphology. During Upper Pleistocene three main stages have been identified. The first one, the local glacial maximum, could be prior to the LGM, as all dates refer to chronologies prior to 28-38 ka. Some authors locate this stage prior to 45 and 65 ka, during the 50-70 ka cold stage. It could be a wet stage, when the main fronts reached the Iberian Peninsula from the SW. The second stage is located to around 30 ka, and point to a dryer stage when glaciers was shorter but thicker. The third stage is located at 20-18 ka, contemporary from the LGM. Glaciers are located inside of glacier-shaped mountain valleys. A few moraine complexes located in the highest massif have been related to Lateglacial, coinciding with cold phases (Dryas) recorded in the Picos de Europa lakes and paleolakes. Finally, during the Holocene only small glaciers developed in the Picos de Europa, which have been assigned to LIA. Nowadays there are still glacial ice remains in four glacial cirques of Picos de Europa, close to the LIA moraines.

Serrano, Enrique; José González-Trueba, Juan; Pellitero, Ramón; González-García, María; Gómez-Lende, Manuel

2014-05-01

146

Ecological structure of recent and last glacial mammalian faunas in northern Eurasia: the case of Altai-Sayan refugium.  

PubMed

Pleistocene mammalian communities display unique features which differ from present-day faunas. The paleocommunities were characterized by the extraordinarily large body size of herbivores and predators and by their unique structure consisting of species now inhabiting geographically and ecologically distinct natural zones. These features were probably the result of the unique environmental conditions of ice age ecosystems. To analyze the ecological structure of Last Glacial and Recent mammal communities we classified the species into biome and trophic-size categories, using Principal Component analysis. We found a marked similarity in ecological structure between Recent eastern Altai-Sayan mammalian assemblages and comparable Pleistocene faunas. The composition of Last Glacial and Recent eastern Altai-Sayan assemblages were characterized by the occurrence of large herbivore and predator species associated with steppe, desert and alpine biomes. These three modern biomes harbor most of the surviving Pleistocene mammals. None of the analyzed Palearctic Last Glacial faunas showed affinity to the temperate forest, taiga, or tundra biome. The Eastern part of the Altai-Sayan region could be considered a refugium of the Last Glacial-like mammalian assemblages. Glacial fauna seems to persist up to present in those areas where the forest belt does not separate alpine vegetation from the steppes and deserts. PMID:24454791

Pavelková ?i?ánková, V?ra; Robovský, Jan; Riegert, Jan

2014-01-01

147

Ecological Structure of Recent and Last Glacial Mammalian Faunas in Northern Eurasia: The Case of Altai-Sayan Refugium  

PubMed Central

Pleistocene mammalian communities display unique features which differ from present-day faunas. The paleocommunities were characterized by the extraordinarily large body size of herbivores and predators and by their unique structure consisting of species now inhabiting geographically and ecologically distinct natural zones. These features were probably the result of the unique environmental conditions of ice age ecosystems. To analyze the ecological structure of Last Glacial and Recent mammal communities we classified the species into biome and trophic-size categories, using Principal Component analysis. We found a marked similarity in ecological structure between Recent eastern Altai-Sayan mammalian assemblages and comparable Pleistocene faunas. The composition of Last Glacial and Recent eastern Altai-Sayan assemblages were characterized by the occurrence of large herbivore and predator species associated with steppe, desert and alpine biomes. These three modern biomes harbor most of the surviving Pleistocene mammals. None of the analyzed Palearctic Last Glacial faunas showed affinity to the temperate forest, taiga, or tundra biome. The Eastern part of the Altai-Sayan region could be considered a refugium of the Last Glacial-like mammalian assemblages. Glacial fauna seems to persist up to present in those areas where the forest belt does not separate alpine vegetation from the steppes and deserts.

Pavelkova Ricankova, Vera; Robovsky, Jan; Riegert, Jan

2014-01-01

148

A high-resolution Late Glacial to Holocene record of environmental change in the Mediterranean from Lake Ohrid (Macedonia/Albania)  

NASA Astrophysics Data System (ADS)

Lake Ohrid (Macedonia/Albania) is the oldest extant lake in Europe and exhibits an outstanding degree of endemic biodiversity. Here, we provide new high-resolution stable isotope and geochemical data from a 10 m core (Co1262) through the Late Glacial to Holocene and discuss past climate and lake hydrology (TIC, ?13Ccalcite, ?18Ocalcite) as well as the terrestrial and aquatic vegetation response to climate (TOC, TOC/N, ?13Corganic, Rock Eval pyrolysis). The data identifies 3 main zones: (1) the Late Glacial-Holocene transition represented by low TIC and TOC contents, (2) the early to mid-Holocene characterised by high TOC and increasing TOC/N and (3) the Late Holocene-Present which shows a marked decrease in TIC and TOC. In general, an overall trend of increasing ?18Ocalcite from 9 ka to present suggests progressive aridification through the Holocene, consistent with previous records from Lake Ohrid and the wider Mediterranean region. Several proxies show commensurate excursions that imply the impact of short-term climate oscillations, such as the 8.2 ka event and the Little Ice Age. This is the best-dated and highest resolution archive of past Late Glacial and Holocene climate from Lake Ohrid and confirms the overriding influence of the North Atlantic in the north-eastern Mediterranean. The data presented set the context for the International Continental scientific Drilling Program Scientific Collaboration On Past Speciation Conditions in Lake Ohrid project cores recovered in spring-summer 2013, potentially dating back into the Lower Pleistocene, and will act as a recent calibration to reconstruct climate and hydrology over the entire lake history.

Lacey, Jack H.; Francke, Alexander; Leng, Melanie J.; Vane, Christopher H.; Wagner, Bernd

2014-05-01

149

Late Pleistocene carbonate dissolution in the Venezuela Basin, Caribbean Sea  

SciTech Connect

Piston cores from water depths greater than 4000 m in the Venezuela Basin (Caribbean Sea) provide continuous late Pleistocene records of carbonate dissolution and accumulation. The authors examination of multiple dissolution indices indicate that, at least for the last 150,000 years, dissolution of carbonate in the Venezuela Basin has been more intense during interglacial than glacial periods, a pattern opposite to more general observations from the deep Atlantic and Gulf of Mexico. By virtue of its shallow sill depth (1815 m), the Venezuela Basin is relatively isolated from the mainstream of Atlantic thermohaline circulation, and presently is filled with homogeneous, relatively warm (3.8/sup 0/C) waters primarily derived from Upper North Atlantic Deep Water. During the last glacial, the enhanced preservation of carbonate in the Venezuela Basin suggests the presence of a less corrosive, more oxygenated water mass in the Atlantic near sill depth. However, this simple interpretations is potentially complicated by past changes in the rain of biogenic materials from surface waters to the deep basin in what must be an essentially closed system below sill depth. Their observations of increased interglacial dissolution may help to explain previously noted discrepancies in the local glacial to interglacial amplitude of delta/sup 18/O variations recorded by coccoliths and planktonic foraminifera.

Cofer-Shabica, N.B.; Peterson, L.C.

1985-01-01

150

Glacial history of sub-Antarctic South Georgia based on the submarine geomorphology of its fjords  

NASA Astrophysics Data System (ADS)

We present multibeam swath bathymetric surveys of the major fjords surrounding the sub-Antarctic island of South Georgia to characterise the glacial geomorphology and to identify the relative timings and extent of past glacial advance and retreat. Bathymetry data revealed a range of glacial features including terminal, retreat and truncated moraines, deep (distal) outer and shallow (proximal) inner basins and cross shelf troughs. These provide evidence of glacial advance and retreat through several glacial cycles. A near consistent pattern of large scale submarine geomorphological features was observed in the different fjords suggesting a similar response of margins of the island ice cap to past climate forcing. A relative chronology based on the relationships between the submarine features with their radiocarbon and cosmogenic isotope dated terrestrial counterparts suggests that widely observed inner basin moraines date from the last major glacial advance or Last Glacial Maximum, while deep basin moraines may date from an earlier (pre-LGM) more extensive glaciation, which we speculate corresponds to MIS6. On the sides of the deep basins a series of truncated moraines show ice advance positions from preceding glacial periods. The cross shelf troughs, and mid-trough moraines are interpreted as the product of much more extensive glaciations that predate the fjord geomorphology mapped here, thus possibly older than MIS6. This hypothesis would suggest that South Georgia followed a glacial history similar to that of central Patagonia (46deg S)where a series of Pleistocene glaciations (of MIS 20 and younger) extended beyond LGM limits, with the most extensive glacial advance occurring at c. 1.1 Ma.

Hodgson, Dominic A.; Graham, Alastair G. C.; Griffiths, Huw J.; Roberts, Stephen J.; Cofaigh, Colm O.; Bentley, Michael J.; Evans, David J. A.

2014-05-01

151

A 2 million year glacial chronology of the Hatherton Glacier, Antarctica and implications for the size of the East Antarctic Ice Sheet at the Last Glacial Maximum  

NASA Astrophysics Data System (ADS)

A series of distinct glacial deposits flanking the margins of the upper Hatherton Glacier, an outlet glacier in the central Transantarctic Mountains, are used to constrain the behaviour of the Antarctic ice-sheets. Cosmogenic exposure ages of 18 erratics from four glacial drifts covering the ice free Dubris and Bibra valleys, range in age from 5 to 1997 ka. Our results document four glacial advance and retreat events superimposed on an overall long-term ice thickness reduction of about 500 m since the mid-Pleistocene. The lack of field evidence and absence of LGM exposure ages in the glacial deposits of the Hatherton Glacier supports our conclusion that at the LGM the East Antarctic Ice Sheet was of similar size, or may have been slightly smaller, than present. Minimum exposure ages from the oldest two glacial events, represented by the Isca and Danum drifts, are ˜1-2 Ma and ˜0.5 Ma respectively. The Britannia-II Drift, previously assumed to mark the maximum extent of the Last Glacial Maximum advance, has a mean 10Be age of 126 ± 3.2 ka (n = 5). Ages from the younger Britannia-I Drift suggest that since the mid-Holocene (6.5 ± 1.2 ka, n = 5), approximately 200 m of additional ice has been lost.

Joy, Kurt; Fink, David; Storey, Bryan; Atkins, Cliff

2014-01-01

152

Palaeo-ice Streams along the Western Margin of the Scandinavian Ice Sheet during the Last Glacial Maximum. Evidence from Regional and Detailed Bathymetry and Seismic Profiles  

Microsoft Academic Search

The large-scale form of the mid-Norwegian shelf (62°N-68°N) is due to the thick late Pliocene\\/Pleistocene prograding wedges comprising up to 1500 m of sediments at the shelf break. These units are interpreted mainly to be of glacial origin. Above these sequences, separated by an angular unconformity, the sediments represent the last few interglacial\\/glacial cycles. A regional bathymetric dataset of the

D. Ottesen; L. Rise; L. Olsen; J. Dowdeswell; K. Berg; P. Bryn

2003-01-01

153

Correlating Pleistocene sequences across the New Jersey margin  

NASA Astrophysics Data System (ADS)

The biostratigraphic ranges of palynomorphs, as well as climatostratigraphic changes recorded by both marine and terrestrial palynomorphs, provide one of the few means of dating and correlating neritic environments where calcareous microfossils are rare. The transport of terrestrial palynomorphs to marine environments also allows for direct land-sea correlation. In addition, palynological assemblages provide insights into sediment transport and sea levels. Over 500 m of upper Pleistocene sediments were recovered in ODP Hole 1073A on the upper New Jersey slope. Palynomorphs in this Hole record rapid progradation during the late Pleistocene. A prominent unconformity, pp3(s), forms the upper boundary of this thick progradational sequence. Palynological analysis from ODP Sites 1072 and 1073 allows this surface to be correlated from the outer shelf to the upper slope, and also suggests that it was generated during a sea level lowstand, probably during Oxygen Isotope Stage 12 (˜450-425 ka). Both the Atlantic Coastal Plain and ODP Site 1072 on the outer New Jersey shelf appear to have become erosional during this progradational phase which began around the time that the magnitude of Northern Hemisphere ice accumulation increased. This suggests that sediments prograded across the New Jersey margin as accommodation in inner neritic and coastal environments decreased in response to amplified glacioeustatic fluctuation and substantially increased sediment availability (supplied by glacial erosion). Erosion associated with the generation of unconformity pp3(s) appears to have increased accommodation on the New Jersey margin, allowing sediments to aggrade on the continental shelf and even to accumulate on the Atlantic Coastal Plain during interglacial highstands. The upper Pleistocene architecture of the New Jersey margin appears to have been controlled by glacioeustatic fluctuations and the accompanying increase in sediment supply. In contrast, unconformable surface pp4(s) is more complex and separates upper Miocene sediments deposited between 7.4 and 5.9 Ma from lower Pleistocene sediments deposited prior to approximately 1.4 Ma in Hole 1072A, whereas in Hole 1073A, pp4(s) is in sediments of early Pleistocene age, between 1.6 and 1.3 Ma. These data suggest that unconformity pp4(s) was generated during the early Pleistocene, but there is no clear palynological evidence that glacioeustasy was responsible for the generation of this unconformity.

McCarthy, F. M. G.; Gostlin, K. E.

2000-07-01

154

Late Pleistocene/Holocene radiolarian and pollen records from sediments in the Sea of Okhotsk  

NASA Astrophysics Data System (ADS)

In two cores with oxygen isotope stratigraphy from the southern Okhotsk Sea, marine pollen and siliceous microfauna record concurrent late glacial through Holocene variations in regional terrestrial and marine environments. Glacial vegetation around the southern Okhotsk basin, which resembles the present tundra/steppe of the northwest coast of this marginal sea, yields to spruce-dominated boreal forests during the glacial/interglacial transition. Temperate forest components, such as oak, peak during the mid-Holocene. Decreasing oak accompanied by increasing spruce reflects the effect of global cooling on local vegetation during the last 4 kyr. Although the radiolarian fauna in the Okhotsk Sea samples is similar to that present in the northwest Pacific, the dominant species in both regions differ. Concentrations of radiolarians are low in latest glacial samples, with higher concentrations occurring above and below this interval. Cycladophora davisiana, the dominant radiolarian species in the majority of Holocene Okhotsk Sea sediments, is present at lower percentages in late glacial samples from our two sites. Thus, this species' Holocene/latest Pleistocene abundance pattern in Sea of Okhotsk sediments is the reverse of that recorded in high-latitude open ocean sites. The combined marine pollen and radiolarian records indicate changes in the Sea of Okhotsk's physical oceanographic conditions and surrounding vegetation during the late glacial which were associated with this region's response to global climate change.

Morley, Joseph J.; Heusser, Linda E.; Shackleton, Nicholas J.

1991-02-01

155

Geochronology and paleoenvironments of the glacial-age Tahoka Formation, Texas and New Mexico High Plains  

Microsoft Academic Search

Radiocarbon ages on organic matter from the late Pleistocene Tahoka Formation at White Lake, Bailey County, Texas, provide a new chronology for playa lakes on the southern High Plains. Lacustrine muds at White Lake accumulated at least 20,000-17,000 14 C yrs B.P. during the last glacial maximum. The basin at White Lake also contained standing water ca 37,000 14 C

Salt Lake; Coyote Lake

2001-01-01

156

A glacial chronology for the Fish Creek drainage of Boulder Mountain, Utah, USA  

Microsoft Academic Search

Boulder Mountain, located in South Central Utah, is one of several mountain ranges on the Colorado Plateau that was glaciated during the late Pleistocene. Using 3He exposure-age dating (corrected for non-cosmogenic 3He with shielded samples), we determined 3He exposure-ages for boulders from the most well-preserved moraines in the Fish Creek drainage of Boulder Mountain. 3He exposure-ages indicate a last glacial

David W. Marchetti; Thure E. Cerling; Elliott W. Lips

2005-01-01

157

Sea ice as the glacial cycles' climate switch: Role of seasonal and orbital forcing  

Microsoft Academic Search

A box model of the coupled ocean, atmosphere, sea ice, and land ice climate system is used to study glacial-interglacial oscillations under seasonally and orbitally varying solar forcing. The dominant 100 kyr oscillation in land ice volume has the familiar sawtooth shape of climate proxy records, and to zeroth order, it does not depend on the seasonal and Milankovitch forcing.

Hezi Gildor; Eli Tziperman

2000-01-01

158

Is Gene Flow Promoting the Reversal of Pleistocene Divergence in the Mountain Chickadee (Poecile gambeli)?  

PubMed Central

The Pleistocene glacial cycles left a genetic legacy on taxa throughout the world; however, the persistence of genetic lineages that diverged during these cycles is dependent upon levels of gene flow and introgression. The consequences of secondary contact among taxa may reveal new insights into the history of the Pleistocene’s genetic legacy. Here, we use phylogeographic methods, using 20 nuclear loci from regional populations, to infer the consequences of secondary contact following divergence in the Mountain Chickadee (Poecile gambeli). Analysis of nuclear data identified two geographically-structured genetic groups, largely concordant with results from a previous mitochondrial DNA (mtDNA) study. Additionally, the estimated multilocus divergence times indicate a Pleistocene divergence, and are highly concordant with mtDNA. The previous mtDNA study showed a paucity of sympatry between clades, while nuclear patterns of gene flow show highly varied patterns between populations. The observed pattern of gene flow, from coalescent-based analyses, indicates southern populations in both clades exhibit little gene flow within or between clades, while northern populations are experiencing higher gene flow within and between clades. If this pattern were to persist, it is possible the historical legacy of Pleistocene divergence may be preserved in the southern populations only, and the northern populations would become a genetically diverse hybrid species.

Manthey, Joseph D.; Klicka, John; Spellman, Garth M.

2012-01-01

159

Pleistocene glaciation history of the Northern North Sea and Norwegian Channel documented by basin-scale 3D seismic analysis  

NASA Astrophysics Data System (ADS)

A regionally merged (c. 30,000 km2) 'megasurvey' 3D seismic dataset and an extensive set of 2D lines, tied to the Troll (89-03) core and wireline logs, was used to investigate the glacial and inter-glacial evolution of the northernmost North Sea through the Plio-Pleistocene. An extensive regional unconformity (URU) exists throughout the study area truncating the Naust Formation, a Plio-Pleistocene glacially-influenced progradational delta system, and older strata. This major erosion surface forms the base of the Norwegian Channel, a large (800 km long) cross-shelf trough located along the southern Norwegian coast. The evolution and exact erosion mechanism of this enigmatic feature is still debated. The stratigraphic succession above the URU consists of relatively flat-lying, alternating glacial and glacio-marine units of mid Pleistocene-Holocene age. This study is the first to present fully 3D seismic-constrained maps of the URU, the Naust clinoforms and all major glacial erosion surfaces within the Norwegian Channel infill. Furthermore it documents the geometries and sedimentary facies characteristics of the till and marine units preserved within the Norwegian Channel and the Norwegian sector of the Northern North Sea. Mapped erosional surfaces reveal a diverse assemblage of glacial morpologies interpreted as mega-scale glacial lineations, tunnel valleys, glaciotectonic thrust complexes, terminal moraines and meltwater conduits demarcating the terminus of successive grounded palaeo-ice sheets. Ice berg ploughing was common along the margin between 2.6 and 1.1 Ma with ice streaming commencing prior to 1.1 Ma. Repeated occupation of the NC by fast flowing ice streams, during the Elsterian, Saalian, and Weichselian (MIS 12, 10, 8, 6, 2), led to a progressively westward erosion of the channel margin, migrating approximately 60 km between 1.1 Ma and the LGM. Although well imaged by seismic data, the prolific record of glaciations and interglacials in the Northern North Sea require better age constraints to further fine tune the record of Pleistocene environmental changes. Whilst a large number of wells exist in the North Sea, giving basic lithological information, only very few have sufficiently detailed stratigraphic data in the Pleistocene section. Further research should thus include coring tied to high-resolution seismic data that can be linked to the basin-scale 3D seismic observations presented herein. As this study provides a unique insight into the spatial and temporal dynamics of shelf-edge glaciation in the northern North Sea and its Atlantic margin throughout the late Cenozoic, the plethora of features documented within the Northern North Sea may serve as a template for interpreting other less well imaged glaciated margins.

Huuse, J.; Huuse, M.

2012-04-01

160

Last glacial aeolian dynamics at the Titel loess plateau (Vojvodina, Serbia)  

NASA Astrophysics Data System (ADS)

The Titel loess plateau (Vojvodina, Serbia) is situated at the confluence of the rivers Danube and Tisa, in the southeastern part of the Ba?ka subregion. Various phases of fluvial erosion have shaped the ellipsoid form of the plateau, which is characterized by steep slopes on the margins. The Titel loess plateau is a unique geomorphologic feature, further emphasising the wide diversity of the loess landforms. The plateau is an island of loess with a maximum length of about 16 km and a maximum width of 7.2 km. Thick loess deposits of between 35 and 55 m are intercalated by 5 main pedocomplexes likely deposited thought the last 5 glacial/interglacial cycles. Steep loess cliffs expose several important sections for understanding climatic and environmental change during the middle and late Pleistocene in the region. The succession of palaeosols through the sequence strongly suggests a transition from humid interglacial climates in the middle Pleistocene, to drier interglacial climates in the late Pleistocene. Past aeolian dynamics have been reconstructed using magnetic susceptibility, grain size, geochemical and malacological investigations by depth in the thick last glacial unit. Luminescence dating and magnetic susceptibility inter-profile correlation provide the chronological framework. Lower last glacial loess unit V-L1L2 is loosely cemented porous sandy loess, with occasional fine laminations and thin, fine sand beds. Identified malacofauna indicates very dry climatic conditions and poor steppic vegetation. It is hypothesized that while the last glacial vegetation cover is extremely sparse, significant sedimentation rates during the lower last glacial can be explained by the presence of a cyanobacterial crust. Protection of loess sediments from deflation by the presence of a cyanobacterial crust is observed at present in loess quarries (Ruma, Crvenka, Petrovaradin). The middle glacial was warmer and relatively moist, as indicated by an increase in clay content and magnetic susceptibility values in a weakly developed pedocomplex, V-L1S1. Loess sub-layers intercalated into V-L1S1 preserve evidence of episodes of abrupt cooling and aridification. In contrast with other European loess sites, the middle glacial pedocomplex is weakly developed at exposures on the Titel loess plateau. The uppermost late glacial loess stratum V-L1L1 shows low values of magnetic susceptibility and clay content, plus high values of carbonate content and the presence of a few frigophilous and cold resistant snails, preserved in sediments laid down during the coldest palaeoclimatic interval of the last glacial period. Composite mollusc associations in loess unit V-L1L1 suggest a higher diversity of environments in comparison to those preserved in the V-L1L2 and V-L1S1 units. In addition to climatic changes over interglacial-glacial and interstadial-stadial timescales, climate proxies (especially grain size) in the last glacial loess exhibit many abrupt fluctuations. Evidence of similar abrupt high frequency fluctuations during the last glacial period appear in loess through out much of Eurasia. Without a detailed and precise chronological framework, provided by independent dating, as yet it is not possible to determine whether the variations recorded in the Titel plateau loess are related to widely documented events in the North Atlantic or independent regional or local depositional/environmental variations. The intensity of deposition of coarser material during the relatively cold early last glacial exceeds that during the coldest last glacial maximum. This may be explainable via changes in general atmospheric circulation, as well as changes in the transportation and depositio regime of the Danube fluvial system. During the last glacial maximum, extension of the ice sheets in northern Europe was greatest and may have redirected the penetration of Atlantic air masses to the east (e.g. Dodonov and Baiguzina, 1995). Model results presented in van Huissteden and Pollard (2003) indicate strong anticyclonal circulation over the Fennoscandian ice sheet

Markovi?, S. B.; Bokhorst, M. P.; Machalett, B.; Štrbac, D.; Hambach, U.; Basarin, B.; Svir?ev, Z.; Stevens, T.; Frechen, M.; Vandenberghe, J.

2009-04-01

161

Millennial climate oscillation spied  

SciTech Connect

Although evaluating the effects of greenhouse gases on climatic warming has been a major growth industry, greenhouse gases are not the only effect on the global climate. Analysing climate records stored in sediments and glacial ice, researchers have detected a slow climate oscillation that has alternately warmed and cooled the world very couple of thousand years for the past hundred thousand years, perhaps millions of years. This article gives an overview of the evidence.

Kerr, R.A.

1996-01-12

162

Coalescent-based hypothesis testing supports multiple Pleistocene refugia in the Pacific Northwest for the Pacific giant salamander (Dicamptodon tenebrosus).  

PubMed

Phylogeographic patterns of many taxa are explained by Pleistocene glaciation. The temperate rainforests within the Pacific Northwest of North America provide an excellent example of this phenomenon, and competing phylogenetic hypotheses exist regarding the number of Pleistocene refugia influencing genetic variation of endemic organisms. One such endemic is the Pacific giant salamander, Dicamptodon tenebrosus. In this study, we estimate this species' phylogeny and use a coalescent modeling approach to test five hypotheses concerning the number, location and divergence times of purported Pleistocene refugia. Single refugium hypotheses include: a northern refugium in the Columbia River Valley and a southern refugium in the Klamath-Siskiyou Mountains. Dual refugia hypotheses include these same refugia but separated at varying times: last glacial maximum (20,000 years ago), mid-Pleistocene (800,000 years ago) and early Pleistocene (1.7 million years ago). Phylogenetic analyses and inferences from nested clade analysis reveal distinct northern and southern lineages expanding from the Columbia River Valley and the Klamath-Siskiyou Mountains, respectively. Results of coalescent simulations reject both single refugium hypotheses and the hypothesis of dual refugia with a separation date in the late Pleistocene but not hypotheses predicting dual refugia with separation in early or mid-Pleistocene. Estimates of time since divergence between northern and southern lineages also indicate separation since early to mid-Pleistocene. Tests for expanding populations using mismatch distributions and 'g' distributions reveal demographic growth in the northern and southern lineages. The combination of these results provides strong evidence that this species was restricted into, and subsequently expanded from, at least two Pleistocene refugia in the Pacific Northwest. PMID:16842421

Steele, Craig A; Storfer, Andrew

2006-08-01

163

Marine ice sheets of Pleistocene age on the East Siberian Continental Margin (Invited)  

NASA Astrophysics Data System (ADS)

Based on swath bathymetry, sediment echosounding, seismic profiling and sediment coring we present results of the RV "Polarstern' cruise ARK-XIII/3 (2008) and RV "Araon" cruise ARA03B (2012), which investigated an area between the Chukchi Borderland and the East Siberian Sea between 165°W and 170°E. At the southern end of the Mendeleev Ridge, close to the Chukchi and East Siberian shelves, evidence is found for the existence of Pleistocene ice sheets/ice shelves, which have grounded several times in up to 1200 m present water depth. We found mega-scale glacial lineations associated with deposition of glaciogenic wedges and debris-flow deposits indicative of sub-glacial erosion and deposition close to the former grounding lines. Glacially lineated areas are associated with large-scale erosion, accentuated by a conspicuous truncation of pre-glacial strata typically capped with mostly thin layers of diamicton draped by pelagic sediments. Our tentative age model suggests that the youngest and shallowest grounding event of an ice sheet should be within Marine Isotope Stage (MIS) 3. The oldest and deepest event predates MIS 6. According to our results, ice sheets of more than one km in thickness continued onto, and likely centered over, the East Siberian Shelf. They were possibly linked to previously suggested ice sheets on the Chukchi Borderland and the New Siberian Islands. We propose that the ice sheets extended northward as thick ice shelves, which grounded on the Mendeleev Ridge to an area up to 78°N within MIS 5 and/or earlier. These results have important implication for the former distribution of thick ice masses in the Arctic Ocean during the Pleistocene. They are relevant for global sea-level variations, albedo, ocean-atmosphere heat exchange, freshwater export from the Arctic Ocean at glacial terminations and the formation of submarine permafrost. The existence of km-thick Pleistocene ice sheets in the western Arctic Ocean during glacial times predating that of the Last Glacial Maximum (LGM) also implies significantly different atmospheric circulation patterns, in particular availability and distribution of moisture during pre-LGM glaciations.

Niessen, F.; Hong, J.; Hegewald, A.; Matthiessen, J. J.; Stein, R. H.; Kim, H.; Kim, S.; Jensen, L.; Jokat, W.; Nam, S.; Kang, S.

2013-12-01

164

15 k.y. paleoclimatic and glacial record from northern New Mexico  

NASA Astrophysics Data System (ADS)

The southern Sangre de Cristo Mountains, New Mexico, contain evidence of glacial activity from the late Pleistocene to late Holocene. Sediment cores recovered from an alpine bog (3100 m) trapped behind a Pinedale age moraine, ˜2 km downvalley from a high-elevation cirque, reached glacial-age debris and recovered ˜6 m of lake clays overlain by gyttja. Accelerator mass spectrometry dating, sedimentology, variations in magnetic properties, and organic carbon data reveal six distinct periods of glacial and/or periglacial activity. These include a late Pleistocene Pinedale glacial termination just before 12120 14C yr B.P., a Younger Dryas chron cirque glaciation, an early Neoglacial periglacial event (ca. 4900 14C yr B.P.), a late Holocene cirque glaciation (3700 14C yr B.P.), as well as late Holocene periglacial events at 2800 14C yr B.P. and the Little Ice Age (ca. 120 14C yr B.P.). Cold events in the middle to late Holocene correlate with subtle ice-rafting events in the North Atlantic and records of cold events in North America and Europe and were probably hemispheric in extent.

Armour, Jake; Fawcett, Peter J.; Geissman, John W.

2002-08-01

165

Geomorphic controls on Pleistocene knickpoint migration in Alpine valleys  

NASA Astrophysics Data System (ADS)

Recent insights into sub-glacial bedrock stress conditions suggest that the erosional efficiency of glaciers may reduce markedly following a major erosional cycle [Leith et al., 2013]. This implies that the formation of large glacial valleys within the Alps is likely to have occurred shortly after the onset of 100 ky glacial-interglacial cycles (at the mid-Pleistocene Revolution (MPR)). The majority of landscape change since this time may have therefore been driven by sub-aerial processes. This hypothesis is supported by observations of hillslope and channel morphology within Canton Valais (Switzerland), where major tributary valleys display a common morphology along their length, hinting at a shared geomorphic history. Glaciers currently occupy the headwaters of many catchments, while the upper reaches of rivers flow across extensive alluvial planes before abruptly transitioning to steep channels consisting of mixed bedrock and talus fan deposits. The rivers then converge to flow out over the alluvial plane of the Rhone Valley. Characteristically rough topographies within the region are suggested to mark the progressive transition from a glacial to fluvially-dominated landscape, and correlate well with steepened river channel sections determined from a 2.5 m resolution LiDAR DEM. We envisage a landscape in which ongoing tectonic uplift drives the emergence of Alpine bedrock through massive sedimentary valley infills (currently concentrated in the Rhone Valley), whose elevation is fixed by the consistent fluvial baselevel at Lake Geneva. As fluvial incision ceases at the onset of glaciation, continued uplift causes the formation of knickpoints at the former transition from bedrock to sedimentary infill. These knickpoints will then propagate upstream during subsequent interglacial periods. By investigating channel morphologies using an approach based on the steady-state form of the stream power equation, we can correlate steepened channel reaches (degraded knickpoints) across most major tributaries south of the Rhone River. The timing of apparent uplift events correlates well with that of cool Marine Isotope Stages derived from global oxygen isotope data up to the beginning of MIS 12. A weak correlation up to the beginning of MIS 18 suggests initial glacial incision may have occurred some time during MIS 14 - 20, and valley development has since been driven by fluvial processes. Leith, K., J. R. Moore, F. Amann, and S. Loew (2013), Sub-glacial extensional fracture development and implications for Alpine valley evolution, J. Geophys. Res. Earth Surf., doi:10.1002/2012JF002691.

Leith, Kerry; Fox, Matt; Moore, Jeffrey R.; Brosda, Julian; Krautblatter, Michael; Loew, Simon

2014-05-01

166

Glacial isostasy and plate motion  

Microsoft Academic Search

The influence of glacial-isostatic adjustment (GIA) on the motion of tectonic plates is usually neglected. Employing a recently developed numerical approach, we examine the effect of glacial loading on the motion of the Earth’s tectonic plates where we consider an elastic lithosphere of laterally variable strength and the plates losely connected by low viscous zones. The aim of this paper

Volker Klemann; Zdenek Martinec; Erik R. Ivins

2008-01-01

167

The aftermath of megafaunal extinction: ecosystem transformation in Pleistocene Australia.  

PubMed

Giant vertebrates dominated many Pleistocene ecosystems. Many were herbivores, and their sudden extinction in prehistory could have had large ecological impacts. We used a high-resolution 130,000-year environmental record to help resolve the cause and reconstruct the ecological consequences of extinction of Australia's megafauna. Our results suggest that human arrival rather than climate caused megafaunal extinction, which then triggered replacement of mixed rainforest by sclerophyll vegetation through a combination of direct effects on vegetation of relaxed herbivore pressure and increased fire in the landscape. This ecosystem shift was as large as any effect of climate change over the last glacial cycle, and indicates the magnitude of changes that may have followed megafaunal extinction elsewhere in the world. PMID:22442481

Rule, Susan; Brook, Barry W; Haberle, Simon G; Turney, Chris S M; Kershaw, A Peter; Johnson, Christopher N

2012-03-23

168

The consequences of pleistocene climate change on lowland neotropical vegetation  

SciTech Connect

Palynological reconstructions indicate that lowland tropical America was subject to intense cooling during the last ice-age. The descent of presently montane taxa into the lowlands of Amazonia and Minas Gerais indicate temperature depressions ranging from 5[degrees]C to 9[degrees]C cooler-than-present. The strengthened incursion of southerly airmasses caused a reassortment of vegetation throughout Amazonia. Presently allopatric species are found to have been sympatric as novel forest assemblages and formed and dissolved. Modest drying, perhaps a 20% reduction in precipitation, accounts for all the records that show a Pleistocene expansion of savanna. No evidence is found to support the fragmentation of Amazonian forests during glacial times, and the hypothesis of forest refuges as an explanation of tropical speciation is rejected on empirical grounds.

De Oliveira, P.E.; Colinvaux, P.A. (Smithsonian Tropical Research Institute, Panama City (Panama))

1994-06-01

169

Somma-Vesuvius ground deformation over the last glacial cycle  

NASA Astrophysics Data System (ADS)

Vertical ground movements at Somma-Vesuvius during the last glacial cycle have been inferred from micropalaeontological and petrochemical analyses of rock samples from boreholes drilled at the archaeological sites of Herculaneum and Pompeii as well as on the apron of the volcano and the adjacent Sebeto and Sarno Valleys. Opposing movements occurred during the periods preceding and following the Last Glacial Maximum (LGM). The uplift began 20 ka ago with marine deposits rising several tens of metres up to 25 m a.s.l., recovering previous subsidence which occurred during the Late glacial period, suggesting a strict connection between volcano-tectonic and glacial cycles. Here we present the analysis of deposits predating the LGM, which confirms subsidence of the Campanian Plain where Mt. Somma-Vesuvius is located, shows variable surface loading effects and highlights the volcano-tectonic stages experienced by the volcano. The self-balancing mechanism of the volcanic system, evolving towards an explosive, subaerial activity 60 ka ago, is testified to by a large ground oscillation in phase with sea level change during the last glacial cycle.

Marturano, Aldo; Aiello, Giuseppe; Barra, Diana

2013-04-01

170

Polymorphism in pleistocene land snails.  

PubMed

Under suitable conditions the colors and patterns of the shells of land snails may be preserved for thousands of years. In a late Pleistocene population of Limicolaria martensiana all the major color forms that occur in modern living snails may be distinguished, and the basic polymorphism is at least 8,000 to 10,000 year old. PMID:17830234

Owen, D F

1966-04-01

171

Pleistocene marine ice sheets and ice shelves at the East Siberian continental margin  

NASA Astrophysics Data System (ADS)

RV "Polarstern" cruise ARK-XIII/3 (2008) and RV "Araon" cruise ARA03B (2012) investigated an area in the Arctic Ocean located between the Chukchi Borderland and the East Siberian Sea (between 165°W and 170°E). Based on swath bathymetry, sediment echosounding, seismic profiling and sediment coring we present evidence that the western Arctic Ocean had a glaciated continental margin during several glacial periods of the Pleistocene (Niessen et al. 2013). At the southern end of the Mendeleev Ridge and on the Chukchi and East Siberian continental slopes ice sheets and ice shelves grounded in up to 1200 m present water depth. We found mega-scale glacial lineations (MSGL) associated with deposition of glaciogenic wedges and debris-flow deposits indicative of sub-glacial erosion and deposition close to the former grounding lines. Glacially lineated areas are associated with large-scale erosion, capped with diamicton and draped by, in places, several metres of pelagic sediments. On the Arlis Plateau, a detailed bathymetric map exhibits several generations of MSGL, which we interpret as relicts of different Pleistocene glaciations. Traces of former grounding line positions suggest that an ice shelf of approximately 900 m in thickness has spread across the Southern Mendeleev Ridge in a north-easterly direction. According to our results, ice sheets of more than one km in thickness continued onto, and likely centered over, the East Siberian Shelf. A preliminary age model suggests that the youngest and shallowest grounding event of an ice sheet should be within Marine Isotope Stage (MIS) 3 and clearly predates the Last Glacial Maximum. The oldest and deepest event predates MIS 6. The youngest grounding event on the Arlis Plateau is tentatively dated to have occurred during MIS 4. These results have important implication for the former distribution of thick ice masses in the Arctic Ocean during the Pleistocene. They are relevant for albedo, ocean-atmosphere heat exchange, moisture supply to and freshwater export from the Arctic Ocean and the formation of submarine permafrost on the East Siberian Shelf. Niessen, F., Hong, J. K. , Hegewald, A. , Matthiessen, J. , Stein, R. , Kim, H. , Kim, S. , Jensen, L. , Jokat, W. , Nam, S. I. and Kang, S. H. (2013) Repeated Pleistocene glaciation of the East Siberian continental margin, Nature Geoscience, 6 (10), pp. 842-846.

Niessen, Frank; Kuk Hong, Jong; Hegewald, Anne; Matthiessen, Jens; Stein, Rüdiger; Kim, Sookwan; Jensen, Laura; Jokat, Wilfried; Nam, Seung Il

2014-05-01

172

Pleistocene pollen stratigraphy from borehole 81/34, devil's hole area, central north sea  

NASA Astrophysics Data System (ADS)

Twelve pollen assemblage zones are identified in a 229 m deep borehole (BH 81/34) from the Devil's Hole area in the central North Sea (British sector). The sediment from this borehole is Early to Late Pleistocene in age and the observation of massulae from Azolla filiculoides in sediment with reversed polarity indicates an age younger than the Olduvai geomagnetic event for the entire sequence. The Early Pleistocene sediments were at least partly deposited in the vicinity of a river outlet and can be correlated either with the Eburonian or the Menapian cold stage and with the Bavel interglacial and the Linge glacial within the Bavelian stage in the Dutch stratigraphy. The Middle Pleistocene sequence contains an interval rich in Abies, Picea and Pinus, probably deposited during the end of either Cromerian Complex interglacial IV (Noordbergum) or possibly the Holsteinian. The uppermost 80 m of the core contains high frequencies of pre-Quaternary and deteriorated palynomorphs indicating extensive glacial or glaciofluvially reworked sediment.

Ekman, Sten R.

1998-09-01

173

Pleistocene cycles and marine records of West Antarctic Ice Sheet expansion and retreat  

NASA Astrophysics Data System (ADS)

The Pleistocene is very poorly represented in Antarctic continental shelf settings. Most piston cores do not penetrate last-glacial age diamictons. Moreover, nearly all sedimentary records have been subjected to the erosional effects of multiple expansions of the WAIS. No existing sediment core recovered from the continental shelf preserves a complete record of the last 1.2 Ma. The ANDRILL-MIS drillcore (AND-1B) from the southwestern Ross Sea may contain the best continental shelf record of the Late Pleistocene, but it is not well dated and probably incomplete. The upper 55 m of AND-1B contains sediments devoid of diatoms, including Pleistocene fragments or robust reworked Tertiary forms, suggestive of strong glacial shearing (Scherer et al., 2004, Geology). It is characterized by thick diamictites with very rare, thin mudstone units. The absence of diatoms, even in the mudstones, may result from (1) continuously expanded mid-Pleistocene ice (grounded or grounding-line proximal), (2) glacial erosion and removal of interglacial diatomaceous sediments, or (3) interglacial non-deposition of diatomaceous sediments, despite episodic open water with marine productivity. The southeastern Ross Sea, which has a non-winnowed relict glacial surface, despite high primary productivity, provides an analog for scenario 3 (Dunbar et al., 1985). McKay et al. (GSA Bull., 2009) interpret this interval as reflecting a nearly continuous Pleistocene record reflecting a predominance of grounded ice throughout much of the late Pleistocene, with the thin mudstones reflecting exclusively sub-ice shelf interglacial deposits. However, there is evidence of warmer than present interglacial conditions in Southern Ocean sediment cores (numerous references), and in East Antarctic ice cores (Sime et al., 2009, Nature). Furthermore, recent modeling of the WAIS suggests late Pleistocene instability, including episodic collapse of the Ross Ice Shelf and interior grounded ice (Pollard & Deconto, 2009, Nature). The enigmatic AND-1B record limits interpretation of Late Pleistocene interglacials and WAIS history in the Ross Sea. Although the Southern Ocean has a rich record of Pleistocene interglacial events, interpretation of WAIS collapse from these records is equivocal, in part due to lower latitude influences and their distance from the WAIS. The best sediment core available to date that is proximal to the WAIS is the Polarstern core PS58/254 from the Amundsen Sea (69°190S, 108°270W, water depth 4014 m), offshore from Pine Island Bay (Hillenbrand et al., 2009, QSR). The core is well-dated and contains a nearly continuous record of the last ca. 1.2 Ma. It's proximity to Pine Island Bay, the "weak underbelly" of the WAIS (Hughes, 1981), suggests that stratigraphic changes recognized in the core will have been strongly influenced by past WAIS collapse. We will present new diatom results from this core that bear on past WAIS fluctuations via the Pine Island Bay corridor.

Scherer, Reed; Konfirst, Matthew; Hillenbrand, Claus-Dieter; Kuhn, Gerhard

2010-05-01

174

Dynamics of Pleistocene population extinctions in Beringian brown bears.  

PubMed

The climatic and environmental changes associated with the last glaciation (90,000 to 10,000 years before the present; 90 to 10 ka B.P.) are an important example of the effects of global climate change on biological diversity. These effects were particularly marked in Beringia (northeastern Siberia, northwestern North America, and the exposed Bering Strait) during the late Pleistocene. To investigate the evolutionary impact of these events, we studied genetic change in the brown bear, Ursus arctos, in eastern Beringia over the past 60,000 years using DNA preserved in permafrost remains. A marked degree of genetic structure is observed in populations throughout this period despite local extinctions, reinvasions, and potential interspecies competition with the short-faced bear, Arctodus simus. The major phylogeographic changes occurred 35 to 21 ka B.P., before the glacial maximum, and little change is observed after this time. Late Pleistocene histories of mammalian taxa may be more complex than those that might be inferred from the fossil record or contemporary DNA sequences alone. PMID:11910112

Barnes, I; Matheus, P; Shapiro, B; Jensen, D; Cooper, A

2002-03-22

175

Late Pliocene-Pleistocene changes in mass accumulation rates of eolian deposits on the central Chinese Loess Plateau  

Microsoft Academic Search

Late Pliocene-Pleistocene changes in the mass accumulation rates (MARs) of eolian deposits on the central Chinese Loess Plateau (CLP) are reconstructed from measured bulk sediment densities combined with sedimentation rates calculated for two loess\\/red clay sequences. The reconstructed MARs demonstrate that over the past 3600 kyr, distinct aridity-humidity fluctuations occurred over glacial-interglacial timescales, and these were superimposed on a gradual

Youbin Sun; Zhisheng An

2005-01-01

176

Analysis of glacial earthquakes  

NASA Astrophysics Data System (ADS)

In 2003, Ekström et al. reported on the detection of a new class of earthquakes that occur in glaciated regions, with the vast majority being in Greenland. The events have a characteristic radiation pattern and lack the high-frequency content typical of tectonic earthquakes. It was proposed that the events correspond to large and sudden sliding motion of glaciers. Here we present an analysis of all 184 such events detected in Greenland between 1993 and 2005. Fitting the teleseismic long-period surface waves to a landslide model of the source, we obtain improved locations, timing, force amplitudes, and force directions. After relocation, the events cluster into seven regions, all of which correspond to regions of very high ice flow and most of which are named outlet glaciers. These regions are Daugaard Jensen Glacier, Kangerdlugssuaq Glacier, Helheim Glacier, the southeast Greenland glaciers, the northwest Greenland glaciers, Rinks Isbrae, and Jakobshavn Isbrae. Event amplitudes range from 0.1 to 2.0 × 1014 kg m. Force directions are consistent with sliding in the direction of glacial flow over a period of about 50 s. Each region has a different temporal distribution of events. All glaciers are more productive in the summer, but have their peak activity in different months. Over the study period, Kangerdlugssuaq has had a constant number of events each year, whereas Jakobshavn had most events in 1998-1999, and the number of events in Helheim and the northwest Greenland glaciers has increased substantially between 1993 and 2005. The size distribution of events in Kangerdlugssuaq is peaked above the detection threshold, suggesting that glacial earthquakes have a characteristic size.

Tsai, Victor C.; EkströM, GöRan

2007-09-01

177

Co-operation between Gda?sk and Vilnius Universities in Pleistocene geochronology investigations  

NASA Astrophysics Data System (ADS)

The thermoluminescence (TL) dating of aquatic sand'y sediments, carried out as a co-operation between Gda?sk and Vilnius Universities, provided a more accurate chronology of the Middle-Upper Pleistocene in Lithuania. Based on TL dating, Middle and Upper Pleistocene fine-grained sands of aquatic origin have been attributed to the But?nai (Holsteinian) Interglacial (Tartokai outcrop), Snaigup?l? (Drenthe-Wartha) Interglacial (Tartokai and Valakampiai (Valakupiai) outcrops), Merkin? (Eemian) Interglacial (Tartokai and Netiesos outcrops) and Nemunas (Vistulian) Glacial (Tartokai, Netiesos and Rokai outcrops). The dating of samples from the outcrops studied show the age of the But?nai Interglacial to be 430.2 to 280.3 ka years BP, of the Snaigup?l? Interglacial 239.4 to 179.3 ka years BP and the Merkin? Interglacial 135.9 to 103.2 ka years BP. The Early Nemunas and the Middle Nemunas non-glacial sediments accumulated between 67.2-30.6 ka years BP. Tills in the upper part of the Tartokai and Rokai outcrops are younger than 30,000 BP and belong to the Late Nemunas glacial maximum in Lithuania. Different dosimetric (TL, OSL) ages of granular fractions of the same sample indicate different parametres predetermined by the distribution of grain size fractions during aquatic sedimentation of quartz sand. The granulometry of sand or the grain size distribution of quartz particles in samples reflect the state of the hydrodynamic sedimentation space.

Gaigalas, Algirdas; Fedorowicz, Stanis?aw

2009-10-01

178

Glacial-Holocene Deep Atlantic Variability  

NASA Astrophysics Data System (ADS)

Despite decades of research on deep ocean circulation during the Last Glacial Maximum (LGM) and deglaciation, many uncertainties remain. Even first order questions such as whether Antarctic Intermediate Water (AAIW) influenced the North Atlantic in the past are unresolved. Here, we update the glacial western Atlantic benthic ?13C transect of Curry and Oppo (2005) including new data from four cores recovered between 450 and 1100 m water depth, at AAIW depths in the western tropical North Atlantic. Low glacial values are consistent with the presence of AAIW. However, in the modern ocean, remineralization of organic matter drives ?13C values at these water depths lower than expected from their end-member composition. As this may have also been the case in the past, insights from more conservative tracers like ?18O of calcite, the air-sea exchange ?13C signature (?13Cas), and neodymium isotopes (?Nd) are important. We evaluate new and published relevant data and present a new ?13Cas transect for the LGM (updated from Marchitto and Broecker, 2006). A preliminary inversion of LGM data using an ocean pathways model (Gebbie and Huybers, 2010) will be presented. ?13C values in these same four western tropical North Atlantic cores during the Heinrich Event are also consistent with, but may not require, a contribution of AAIW. ?13C values decrease further following the Heinrich event and remain low throughout the deglaciation, during which the records exhibit coherent millennial-scale oscillations. For much of the deglaciation, ?13C values in these cores appear to be lower than values at other sites from similar depths in the western North and South Atlantic, suggestive of non-conservative behavior. The benthic records exhibit high amplitude ?18O variability, which may reflect vertical movement of isopynals, in association with variations in geostrophic flow (e.g. Lynch-Stieglitz et al., 2011). Our new deglacial data will be discussed in the broader context of published multi-proxy records.

Oppo, D.; Curry, W. B.; Huang, K.; Gebbie, G.; Keigwin, L. D.

2012-12-01

179

Comparative phylogeography of two North American 'glacial relict' crustaceans.  

PubMed

The Pleistocene glaciations represent the most recent and dramatic series of habitat changes since the Cretaceous. The impact of these events was particularly acute for aquatic taxa with poor powers of dispersal, but few organisms have evolutionary histories more intimately entwined with the advance and retreat of ice than the 'glacial relicts'. In this study, we used a mitochondrial gene, cytochrome c oxidase subunit I (COI), to examine and compare the phylogeographical structure of two glacial relict crustaceans (Limnocalanus macrurus and members of the Mysis relicta species group) across North America. In both cases, we found a sharp phylogenetic division between populations from inland lakes formed during glacial retreat, and arctic lakes isolated from polar seas via isostatic rebound. However, the depth of this phylogenetic partition varied between taxa. In L. macrurus, nucleotide sequence divergence of 2.2% between these zones is consistent with its current status as a single morphologically variable species, but in Mysis the split occurred among recently described, morphologically conserved species, at a divergence of 8.2%. The disparity in the depth of divergence indicates a history of recurrent freshwater invasions from the arctic seas, in concordance with previous studies of Eurasian glacial relicts. However, we suggest further consideration of a largely overlooked explanation that could account for some of the discrepancies between molecular divergences and glaciation events. Many cladogenetic events could have occurred in arctic seas prior to the transition to inland waters, a possibility supported both by the complex physical and ionic history of arctic seas and by high marine and estuarine lineage diversity in the north. PMID:17107476

Dooh, R T; Adamowicz, S J; Hebert, P D N

2006-12-01

180

Pliocene-Pleistocene paleo-productivity changes in the Bering Sea: results from IODP Expedition 323 (Invited)  

NASA Astrophysics Data System (ADS)

IODP Exp 323 drilled 7 sites to study Bering Sea paleoceanography, and recovered 5741 m of high quality sediments with high sedimentation rates covering the last 5 myrs. The sites included hemipelagic Bowers Ridge and a N-S transect of slope sites proximal to the Bering shelf, covering water depths from 818 to 3174 m. The bulk of the cores were obtained with APC and thus the disturbance was minimal so that detailed high resolution paleoceanographic studies are feasible for the first time in this high latitude climate sensitive Pacific marginal sea setting. Objectives include the characterization of changes that occurred in the Bering Sea around the times of (1) onset of the Northern Hemisphere Glaciation (NHG, 2.7 Ma); (2) Mid Pleistocene Transition (MPT, ca. 1 Ma); and (3) glacial-interglacial cycles (<1 Ma). Changes in biological productivity can be depicted by studying diatom assemblages together with biogenic opal concentrations. Extent of sea-ice cover can be monitored by examining sea-ice related diatoms, dinoflagellates and Cycladophora davisiana, an intermediate water dwelling radiolarian taxon whose %values increase when sea-ice cover becomes extensive. Biogenic opal values typically range from 10 to 60% at Bowers Ridge, whereas those at slope sites typically range from 5 to 25% because of dilution by siliciclastics despite high productivity underneath the Green Belt (with the exception of site U1339 which has ca. 50% biogenic opal due to its isolated location on a topographic high beyond the shelf break). Sea-ice cover, illustrated by sea-ice related Thalassiosira antarctica spores and others, occurred more extensively at the slope sites and further north than Bowers Ridge, reflecting an anticyclonic circulation pattern in both the Pliocene and Pleistocene. Since the main entry points of the relatively warm Pacific waters of the Alanskan Stream were located at the central to the western end of the Aleutians, Bowers Ridge sites are influenced by Pacific water. Thus, higher %values of pelagic taxa (e.g., Neodenticula seminae and Actinocyclus spp.) were observed at Bowers Ridge. At U1341 there were differences in productivity and in the amplitude of %biogenic opal before and after of ca. 2.65 Ma, which coincides with the expansion of NHG as well as the emergence of sea-ice related diatoms. The opal values increased from ca. 40 to 50% and the amplitude increased slightly. Changes are noticeable in the amplitudes of pertinent parameters around the MPT, ca. 1 Ma, including %biogenic opal, T. antarctica spores, and C. davisiana. Clear glacial-interglacial oscillations in biogenic opal content is discernible at Site U1341, reflecting changes in extent of sea-ice cover and productivity; ca. 15-20% during the glacial and ca. 60% during the interglacials.

Takahashi, K.; Ravelo, A. C.; Alvarez Zarikian, C. A.; Nagashima, T.; Kanematsu, Y.; Hioki, Y.; Ikehara, M.; Kim, S.; Khim, B.; Aiello, I. W.; Onodera, J.; Radi, T.; Sakamoto, T.; Stroynowski, Z. N.; Asahi, H.; Chen, M.; Colmenero-Hidalgo, E.; Husum, K.; Ijiri, A.; Kender, S.; Lund, S.; Okada, M.; Okazaki, Y.; Horikawa, K.; Seki, O.; Iodp Expedition 323 Shipboard Scientists

2010-12-01

181

Pleistocene sea-level fluctuations and human evolution on the southern coastal plain of South Africa  

NASA Astrophysics Data System (ADS)

Humans evolved in Africa, but where and how remain unclear. Here it is proposed that the southern coastal plain (SCP) of South Africa may have served as a geographical point of origin through periodic expansion and contraction (isolation) in response to glacial/interglacial changes in sea level and climate. During Pleistocene interglacial highstands when sea level was above -75 m human populations were isolated for periods of 360-3400 25-yr generations on the SCP by the rugged mountains of the Cape Fold Belt, climate and vegetation barriers. The SCP expands five-fold as sea level falls from -75 to -120 m during glacial maxima to form a continuous, unobstructed coastal plain accessible to the interior. An expanded and wet glacial SCP may have served as a refuge to humans and large migratory herds and resulted in the mixing of previously isolated groups. The expansive glacial SCP habitat abruptly contracts, by as much as one-third in 300 yr, during the rapid rise in sea level associated with glacial terminations. Rapid flooding may have increased population density and competition on the SCP to select for humans who expanded their diet to include marine resources or hunted large animals. Successful adaptations developed on an isolated SCP are predicted to widely disperse during glacial terminations when the SCP rapidly contracts or during the initial opening of the SCP in the transition to glacial maxima. The hypothesis that periodic expansion and contraction of the SCP, as well as the coastal plain of North Africa, contributed to the stepwise origin of our species over the last 800 thousand years (kyr) is evaluated by comparing the archeological, DNA and sea-level records. These records generally support the hypothesis, but more complete and well dated records are required to resolve the extent to which sea-level fluctuations influenced the complex history of human evolution.

Compton, John S.

2011-03-01

182

Is gene flow promoting the reversal of pleistocene divergence in the Mountain Chickadee (Poecile gambeli)?  

PubMed

The Pleistocene glacial cycles left a genetic legacy on taxa throughout the world; however, the persistence of genetic lineages that diverged during these cycles is dependent upon levels of gene flow and introgression. The consequences of secondary contact among taxa may reveal new insights into the history of the Pleistocene's genetic legacy. Here, we use phylogeographic methods, using 20 nuclear loci from regional populations, to infer the consequences of secondary contact following divergence in the Mountain Chickadee (Poecile gambeli). Analysis of nuclear data identified two geographically-structured genetic groups, largely concordant with results from a previous mitochondrial DNA (mtDNA) study. Additionally, the estimated multilocus divergence times indicate a Pleistocene divergence, and are highly concordant with mtDNA. The previous mtDNA study showed a paucity of sympatry between clades, while nuclear patterns of gene flow show highly varied patterns between populations. The observed pattern of gene flow, from coalescent-based analyses, indicates southern populations in both clades exhibit little gene flow within or between clades, while northern populations are experiencing higher gene flow within and between clades. If this pattern were to persist, it is possible the historical legacy of Pleistocene divergence may be preserved in the southern populations only, and the northern populations would become a genetically diverse hybrid species. PMID:23152877

Manthey, Joseph D; Klicka, John; Spellman, Garth M

2012-01-01

183

Small mammal diversity loss in response to late-Pleistocene climatic change.  

PubMed

Communities have been shaped in numerous ways by past climatic change; this process continues today. At the end of the Pleistocene epoch about 11,700 years ago, North American communities were substantially altered by the interplay of two events. The climate shifted from the cold, arid Last Glacial Maximum to the warm, mesic Holocene interglacial, causing many mammal species to shift their geographic distributions substantially. Populations were further stressed as humans arrived on the continent. The resulting megafaunal extinction event, in which 70 of the roughly 220 largest mammals in North America (32%) became extinct, has received much attention. However, responses of small mammals to events at the end of the Pleistocene have been much less studied, despite the sensitivity of these animals to current and future environmental change. Here we examine community changes in small mammals in northern California during the last 'natural' global warming event at the Pleistocene-Holocene transition and show that even though no small mammals in the local community became extinct, species losses and gains, combined with changes in abundance, caused declines in both the evenness and richness of communities. Modern mammalian communities are thus depauperate not only as a result of megafaunal extinctions at the end of the Pleistocene but also because of diversity loss among small mammals. Our results suggest that across future landscapes there will be some unanticipated effects of global change on diversity: restructuring of small mammal communities, significant loss of richness, and perhaps the rising dominance of native 'weedy' species. PMID:20495547

Blois, Jessica L; McGuire, Jenny L; Hadly, Elizabeth A

2010-06-10

184

10Be in glacial marine sediment of the Ross Sea, Antarctica, a potential tracer of depositional environment and sediment chronology  

NASA Astrophysics Data System (ADS)

Beryllium isotopes ( 9Be and 10Be) distribution in Holocene and late Pleistocene glacial ice and sediment facies from sub-ice stream and sub-ice shelf settings of the Ross sector of the West Antarctic Ice Sheet (WAIS) and from the open Ross Sea are presented, to assess the temporal and spatial variability of these tracers. Significant variation is detected (>one order of magnitude) among Holocene post-glacial deposits. Late Pleistocene diamictons from bathymetric lows in the Ross Sea show depleted concentrations that are less variable among sites and distinctly lower than bank deposits of similar age. In general, sub-ice shelf and ice stream sediments are the most depleted in 10Be. 10Be with its longer half-life and different pathways compared apparant to 14C is found to be a sensitive marker for evaluating the complex spatial and temporal relationships between texturally similar sediments formed by different depositional processes.

Sjunneskog, C.; Scherer, R.; Aldahan, A.; Possnert, G.

2007-06-01

185

Shorelines of Glacial Lake Hitchcock  

NSDL National Science Digital Library

Bob Newton, Smith College Summary This activity uses GIS methods to subtract the isostatic rebound from a DEM in order to create a map of the shoreline of Glacial Lake Hitchcock. Students are then able to evaluate ...

Newton, Bob

186

Stratigraphic framework for the late Pleistocene in the northwest Pacific Ocean  

NASA Astrophysics Data System (ADS)

Analyses of the relative abundance of Cycladophora davisiana in 10 northwest Pacific deep-sea cores show that the species exhibits abundance variations during the late Pleistocene (?450,000 years) comparable to those in the subantarctic and North Atlantic. Comparison of the C. davisiana curves in the cores with tephra-stratigraphic and biostratigraphic datum levels indicates that the major features of the C. davisiana records are synchronous within the northwest Pacific. Calculated ages for the most recent abundance peak and a distinctively low abundance interval of the C. davisiana curve are similar to the estimated ages for the last glacial and interglacial maxima, respectively. With the development of a detailed stratigraphic framework for late Pleistocene northwest Pacific sediments we have been able to revise the estimated ages of the upper limits of Lychnocanium grande, the Rhizosolenia complex, and Druppatractus acquilonius to 49,000, 276,000, and 329,000 y B.P., respectively.

Morley, Joseph J.; Hays, James D.; Robertson, James H.

1982-12-01

187

Pliocene warmth, polar amplification, and stepped Pleistocene cooling recorded in NE Arctic Russia.  

PubMed

Understanding the evolution of Arctic polar climate from the protracted warmth of the middle Pliocene into the earliest glacial cycles in the Northern Hemisphere has been hindered by the lack of continuous, highly resolved Arctic time series. Evidence from Lake El'gygytgyn, in northeast (NE) Arctic Russia, shows that 3.6 to 3.4 million years ago, summer temperatures were ~8°C warmer than today, when the partial pressure of CO2 was ~400 parts per million. Multiproxy evidence suggests extreme warmth and polar amplification during the middle Pliocene, sudden stepped cooling events during the Pliocene-Pleistocene transition, and warmer than present Arctic summers until ~2.2 million years ago, after the onset of Northern Hemispheric glaciation. Our data are consistent with sea-level records and other proxies indicating that Arctic cooling was insufficient to support large-scale ice sheets until the early Pleistocene. PMID:23661643

Brigham-Grette, Julie; Melles, Martin; Minyuk, Pavel; Andreev, Andrei; Tarasov, Pavel; DeConto, Robert; Koenig, Sebastian; Nowaczyk, Norbert; Wennrich, Volker; Rosén, Peter; Haltia, Eeva; Cook, Tim; Gebhardt, Catalina; Meyer-Jacob, Carsten; Snyder, Jeff; Herzschuh, Ulrike

2013-06-21

188

Glacial Features of North Dakota  

NSDL National Science Digital Library

The glacial landforms of North Dakota are revealed through photographs of an end moraine, glacial erratic, eskers, ice thrust masses, an ice-walled lake plain, a kame, kettle lakes, a meltwater channel, outwash, and till. Each image is accompanied by a brief description of the location, how the landform originated, and its composition. The descriptions point out that some features have commercial value as sources of sand and gravel.

189

Last global extinction in the deep sea during the mid-Pleistocene climate transition  

NASA Astrophysics Data System (ADS)

Twenty percent (19 genera, 95 species) of cosmopolitan, deep-sea (500-4000 m), benthic foraminiferal species became extinct during the late Pliocene-Middle Pleistocene (3-0.12 Ma), with the peak of extinctions (76 species) occurring during the mid-Pleistocene Climate Transition (MPT, 1.2-0.55 Ma). One whole family (Stilostomellidae, 30 species) was wiped out, and a second (Pleurostomellidae, 29 species) was decimated with just one species possibly surviving through to the present. Our studies at 21 deep-sea core sites show widespread pulsed declines in abundance and diversity of the extinction group species during more extreme glacials, with partial interglacial recoveries. These declines started in the late Pliocene in southern sourced deep water masses (Antarctic Bottom Water, Circumpolar Deep Water) and extending into intermediate waters (Antarctic Intermediate Water, North Atlantic Deep Water) in the MPT, with the youngest declines in sites farthest downstream from high-latitude source areas for intermediate waters. We infer that the unusual apertural types that were targeted by this extinction period were adaptations for a specific kind of food source and that it was probably the demise of this microbial food that resulted in the foraminiferal extinctions. We hypothesize that it may have been increased cold and oxygenation of the southern sourced deep water masses that impacted on this deep water microbial food source during major late Pliocene and Early Pleistocene glacials when Antarctic ice was substantially expanded. The food source in intermediate water was not impacted until major glacials in the MPT when there were significant expansion of polar sea ice in both hemispheres and major changes in the source areas, temperature, and oxygenation of global intermediate waters.

Hayward, Bruce W.; Kawagata, Shungo; Grenfell, Hugh R.; Sabaa, Ashwaq T.; O'Neill, Tanya

2007-09-01

190

Matuyama–Brunhes boundary in key sections of the loess–paleosol–glacial formations on the East European Plain  

Microsoft Academic Search

Multidisciplinary studies of two key sections (parastratotypes) of the loess–soil–glacial formation (Strelitsa and Sebryakovo-Mikhailovka sections) located in the central part of the East European Plain placed the Matuyama–Brunhes reversal well below the Lower Pleistocene till. The till attributed to the Don glaciation (maximum glaciation on the East European Plain) is separated from the M\\/B reversal by several units, as follows:

A. A. Velichko; V. V. Semenov; G. A. Pospelova; T. D. Morozova; V. P. Nechaev; Yu. N. Gribchenko; K. G. Dlusskii; N. Rutter; N. Catto; E. Little

2006-01-01

191

Sedimentary architecture of the Amundsen Sea Embayment shelf, West Antarctica, from pre-glacial to glacial processes  

NASA Astrophysics Data System (ADS)

Studies of the sedimentary architecture and characteristics of Antarctic shelves provide clues of past ice sheet advance-retreat cycles and help improve constraints for paleo-ice dynamic models since early glacial periods. A first seismostratigraphic analysis of the Amundsen Sea Embayment shelf and slope of West Antarctica reveals insights into the structural architecture of the continental margin and shows stages of sediment deposition, erosion and transport history from pre-glacial times to early glaciation and to the most recent glacial periods. The shelf geometry consists of a large pre- and syn-rift basin in the middle shelf region between outcropping basement of the inner shelf and basement ridges and highs beneath the outer shelf. A middle shelf sub-basin exists which may have formed as a result of motion along an early West Antarctic Rift System branch. At least 4 km of pre-glacial strata has been eroded from the present inner shelf and coastal hinterland by ice sheet advances since the onset of glaciation. Some of the eroded sediments were deposited as a progradational wedge extending the outer shelf by 25 to 65 km oceanward of the pre-glacial shelf-break. Comparing the observed seismic characteristics with those of other Antarctic shelf sequences, we assign an Early Cretaceous age for bottom sedimentary unit ASS-1, a Late Cretaceous to Oligocene age for unit ASS-2, an Early to Mid-Miocene age for unit ASS-3, a Mid-Miocene age for unit ASS-4, a Late Miocene to Early Pliocene age for unit ASS-5, and a Pliocene to Pleistocene age for the top unit ASS-6. The survival of buried grounding zone wedges in the upper part of unit ASS-5 of the outer shelf is consistent with the onset of a long warming phase and a retreated ice sheet in the early Pliocene as observed for the Ross Sea shelf and reconstructed from paleo-ice sheet models. Our data also reveal that the paleo-ice flow paths of the central Pine Island Trough system have remained stationary across the middle and outer shelf since early glacial advances. This study and its stratigraphic constraints will serve as a basis for future drilling operations required for an improved understanding of processes and mechanisms leading to West Antarctic Ice Sheet retreats, such as the rapid ice retreat presently observed in the Amundsen Sea Embayment.

Gohl, Karsten; Uenzelmann-Neben, Gabriele; Hillenbrand, Claus-Dieter; Larter, Robert; Nitsche, Frank

2013-04-01

192

A Pleistocene coastal alluvial fan complex produced by Middle Pleistocene glacio-fluvial processes  

NASA Astrophysics Data System (ADS)

A coarse-grained alluvial fan sequence at Lipci, Kotor Bay, in western Montenegro, provides a sedimentary record of meltwater streams draining from the Orjen Massif (1,894 m a.s.l.) to the coastal zone. At Lipci sedimentary evidence and U-series ages have been used alongside offshore bathymetric imagery and seismic profiles to establish the size of the fan and constrain the nature and timing of its formation. Establishing the depositional history of such coastal fans is important for our understanding of cold stage sediment flux from glaciated uplands to the offshore zone, and for exploring the impact of sea level change on fan reworking. There is evidence of at least four phases of Pleistocene glaciation on the Orjen massif, which have been U-series dated and correlated to MIS 12, MIS 6, MIS 5d-2 and the Younger Dryas. A series of meltwater channels delivered large volumes of coarse- and fine-grained limestone sediment from the glaciated uplands into the Bay of Kotor. At the southern margin of the Orjen massif, a series of large (>700 m long) alluvial fans has developed. Some of these extend offshore for up to 600 m. Lipci fan lies downstream of end moraines in the valley immediately above, which were formed by an extensive outlet glacier of the Orjen ice cap during MIS 12. The terrestrial deposits are part of the fan apex (50 m a.s.l.) that lies at the foot of a steep bedrock channel, but the majority of the fan is now more than 25 m below sea level. The terrestrial fan sediments are strongly cemented by multiple generations of calcite precipitates: the oldest U-series ages are infinite indicating that the fan is >350 ka in age. These ages are in agreement with alluvial sedimentary evidence and U-series ages from other fluvial units on Mount Orjen. The terrestrial portion of the Lipci fan surface contains several channels. These are well preserved due to cementation with calcium carbonate. Submarine imagery indicates that the now submerged portion of the fan also contains deeply incised (up to 10 m) channels which are similar in morphology to those exposed onshore. It is likely that strong cementation of the fan sediments, and associated channel forms, has protected them from coastal erosion during several regression-transgression cycles. These records provide important opportunities to correlate the Pleistocene terrestrial glacial and fluvial records with the marine archive.

Adamson, Kathryn; Woodward, Jamie; Hughes, Philip; Giglio, Federico; Del Bianco, Fabrizio

2014-05-01

193

Pleistocene glaciation in the blue ridge province, southern appalachian mountains, north Carolina.  

PubMed

Glacial polish, grooves, and striations discovered at an elevation of 1370 meters in the headwaters of Boone Fork on Grandfather Mountain, North Carolina, indicate the former, existence of alpine glaciation at a latitude of 36 degrees 07'N. The Boone Fork glacier was located 890 kilometers south of the previously recognized southern limit of alpine glaciation in the Appalachian Mountains, and 350 kilometers southeast of the nearest point on the Laurentide ice sheet. This find has significant implications for studies of Pleistocene geomorphology, paleobiology, and paleoclimatology in the eastern United States. PMID:17736977

Berkland, J O; Raymond, L A

1973-08-17

194

Glacial landforms of the southern Ungava Bay region (Canada): implications for the late-glacial dynamics and the damming of glacial Lake Naskaupi  

NASA Astrophysics Data System (ADS)

The Laurentide ice sheet played an important role in the late Pleistocene climate, notably through discharges of icebergs and meltwater. In this context, the Ungava Bay region in northern Quebec-Labrador appears particularly important, especially during the last deglaciation when the retreating ice margin dammed major river valleys, creating large proglacial lakes (e.g., McLean, aux Feuilles). The history of these lakes is closely related to the temporal evolution of the Labrador-Quebec ice dome. There are, however, large uncertainties regarding the position of its ice divide system through time, thereby limiting our understanding of the history of these glacial lakes. Here we focus on glacial and deglacial landforms present in the George River valley, south of Ungava Bay, in order to bring additional constraints on the late-glacial ice dynamics of this region, which also comprised glacial Lake Naskaupi. This work is based on surficial mapping using aerial photos and satellite imagery, combined with extensive fieldwork and sediment sampling. Our investigation showed significant differences in the distribution of glacial landforms across the region. The area east of the George River is characterized by well-developed Naskaupi shorelines while the elevated terrains show a succession of geomorphological features indicative of cold-based ice or ice with low basal velocities. In the easternmost part of this sector, ice flow directional data indicate that the ice was flowing towards ENE, against the regional slope. Eskers show paleocurrent directions indicating a general ice retreat from east to west. In the western part of this sector, near the George River valley, eskers are absent and the region is covered by felsenmeer and ground moraine that likely reflect the presence of a residual ice mass that was no longer dynamic. The presence of a stagnant ice represents the best mechanism to explain the formation of glacial lakes in the George River valley and its main tributaries. In contrast, the area west of the George River valley shows very few shorelines, implying that Lake Naskaupi was mostly in contact with the decaying ice margin. The abundance ice-marginal meltwater channels allowed the reconstruction of the general ice retreat pattern. The area is also characterized by abundant WNW-trending drumlins and crag-and-tails indicating an important ice flow towards Ungava Bay. These glacial lineations may be linked with eskers further to west that terminated into the postglacial Iberville Sea, forming large ice-contact deltas. This setting suggests that this landform assemblage likely developed during the deglaciation. Our results thus underlie important differences in the subglacial regime across the ice divide of the Labrador sector during the late-glacial and early deglacial interval. The so-called horseshoe unconformity appear to delineate an inner area characterized by warm-based conditions that allowed a massive deglacial ice flow to developed in Ungava Bay, while the area under and proximal to the divide in the east appears to have evolved towards cold-based ice conditions, resulting with a stagnant ice mass that dammed the major proglacial lakes.

Dube-Loubert, Hugo; Roy, Martin

2014-05-01

195

Early human-plant interactions based on palaeovegetation simulations of Africa over glacial-interglacial cycles  

NASA Astrophysics Data System (ADS)

A greater understanding of African palaeovegetation environments over the Pleistocene (1.6 Mya) is important for evaluating potential catalysts underlying the anatomical, social and demographic changes observed in early human populations. We used a state-of-the-art fully-coupled earth system model (HADLEY-GCM3) to simulate typical glacial and interglacial environments likely encountered by late-Pleistocene humans. Our simulations indicate that tropical broadleaf forests of central Africa were not severely restricted by expanding grasslands during the last glacial maximum, although the carbon content of stem and density of leaf components were substantially reduced. We interpret a natural eastern migration corridor between southern Africa and the Rift Valley based on simulations of a no-analogue vegetation assemblage characterised by a unique combination of grass and low density forest. We postulate that early human populations in southern Africa were isolated from northern groups during warm interglacials, and that trans-African migration was facilitated during glacial cycles via a more openly forested eastern corridor.

Cowling, S. A.; Cox, P. M.; Jones, C. D.; Maslin, M. A.; Spall, S. A.

2003-04-01

196

Thermohaline circulation crisis and impacts during the mid-Pleistocene transition.  

PubMed

The mid-Pleistocene transition (MPT) marked a fundamental change in glacial-interglacial periodicity, when it increased from ~41-thousand-year to 100-thousand-year cycles and developed higher-amplitude climate variability without substantial changes in the Milankovitch forcing. Here, we document, by using Nd isotopes, a major disruption of the ocean thermohaline circulation (THC) system during the MPT between marine isotope stages (MISs) 25 and 21 at ~950 to 860 thousand years ago, which effectively marks the first 100-thousand-year cycle, including an exceptional weakening through a critical interglacial (MIS 23) at ~900 thousand years ago. Its recovery into the post-MPT 100-thousand-year world is characterized by continued weak glacial THC. The MPT ocean circulation crisis facilitated the coeval drawdown of atmospheric CO2 and high-latitude ice sheet growth, generating the conditions that stabilized 100-thousand-year cycles. PMID:24968939

Pena, Leopoldo D; Goldstein, Steven L

2014-07-18

197

Geochemistry of glacial sediments in the area of the Bend massive sulfide deposit, north-central Wisconsin  

USGS Publications Warehouse

Geochemical exploration in northern Wisconsin has been problematic because of thick glacial overburden and complex stratigraphic record of glacial history. To assess till geochemical exploration in an area of thick glacial cover and complex stratigraphy samples of glacial materials were collected from cores from five rotasonic boreholes near a known massive sulfide deposit, the Bend deposit in north-central Wisconsin. Diamond drilling in the Bend area has defined a long, thin zone of mineralization at least partly intersected at the bedrock surface beneath 30-40 m of unconsolidated glacial sediments. The bedrock surface has remnant regolith and saprolite resulting from pre-Pleistocene weathering. Massive sulfide and mineralized rock collected from diamond drill core from the deposit contain high (10s to 10,000s ppm) concentrations of Ag, As, Au, Bi, Cu, Hg, Se, Te, and Tl. Geochemical properties of the glacial stratigraphic units helped clarify the sequence and source areas of several glacial ice advances preserved in the section. At least two till sheets are recognized. Over the zone of mineralization, saprolite and preglacial alluvial and lacustrine samples are preserved on the bedrock surface in a paleoriver valley. The overlying till sheet is a gray, silty carbonate till with a source hundreds of kilometers to the northwest of the study area. This gray till is overlain by red, sandy till with a source to the north in Proterozoic rocks of the Lake Superior area. The complex glacial stratigraphy confounds down-ice geochemical till exploration. The presence of remnant saprolite, preglacial sediment, and far-traveled carbonate till minimized glacial erosion of mineralized material. As a result, little evidence of down-ice glacial dispersion of lithologic or mineralogic indicators of Bend massive sulfide mineralization was found in the samples from the rotasonic cores. This study points out the importance of determining glacial stratigraphy and history, and identifying favorable lithologies required for geochemical exploration. Drift prospecting in Wisconsin and other areas near the outer limits of the Pleistocene ice sheets may not be unsuccessful, in part, because of complex stratigraphic sequences of multiple glaciations where deposition dominates over erosion. ?? 2004 Elsevier B.V. All rights reserved.

Woodruff, L. G.; Attig, J. W.; Cannon, W. F.

2004-01-01

198

Coastal staircase sequences reflecting sea-level oscillations and tectonic uplift during the Quaternary and Neogene  

NASA Astrophysics Data System (ADS)

Many coasts feature sequences of Quaternary and Neogene shorelines that are shaped by a combination of sea-level oscillations and tectonics. We compiled a global synthesis of sea-level changes for the following highstands: MIS 1, MIS 3, MIS 5e and MIS 11. Also, we date the apparent onset of sequences of paleoshorelines either from published data or tentatively extrapolating an age for the uppermost, purported oldest shoreline in each sequence. Including the most documented MIS 5e benchmark, we identify 926 sequences out of which 185 also feature Holocene shorelines. Six areas are identified where elevations of the MIS 3 shorelines are known, and 31 feature elevation data for MIS 11 shorelines. Genetic relationships to regional geodynamics are further explored based on the elevations of the MIS 5e benchmark. Mean apparent uplift rates range from 0.01 ± 0.01 mm/yr (hotspots) to 1.47 ± 0.08 mm/yr (continental collision). Passive margins appear as ubiquitously uplifting, while tectonic segmentation is more important on active margins. From the literature and our extrapolations, we infer ages for the onset of formation for ~ 180 coastal sequences. Sea level fingerprinting on coastal sequences started at least during mid Miocene and locally as early as Eocene. Whether due to the changes in the bulk volume of seawater or to the temporal variations in the shape of ocean basins, estimates of eustasy fail to explain the magnitude of the apparent sea level drop. Thus, vertical ground motion is invoked, and we interpret the long-lasting development of those paleoshore sequences as the imprint of glacial cycles on globally uplifted margins in response to continental compression. The geomorphological expression of the sequences matches the amplitude and frequency of glacial cyclicity. From middle Pleistocene to present-day, moderately fast (100,000 yrs) oscillating sea levels favor the development of well identified strandlines that are distinct from one another. Pliocene and Lower Pleistocene strandlines associated with faster cyclicity (40,000 yrs) are more compact and easily merge into rasas, whereas older Cenozoic low-frequency eustatic changes generally led to widespread flat-lying coastal plains.

Pedoja, Kevin; Husson, Laurent; Johnson, Markes E.; Melnick, Daniel; Witt, Cesar; Pochat, Stéphane; Nexer, Maëlle; Delcaillau, Bernard; Pinegina, Tatiana; Poprawski, Yohann; Authemayou, Christine; Elliot, Mary; Regard, Vincent; Garestier, Franck

2014-05-01

199

Glacial History of a Modern Invader: Phylogeography and Species Distribution Modelling of the Asian Tiger Mosquito Aedes albopictus  

PubMed Central

Background The tiger mosquito, Aedes albopictus, is one of the 100 most invasive species in the world and a vector of human diseases. In the last 30 years, it has spread from its native range in East Asia to Africa, Europe, and the Americas. Although this modern invasion has been the focus of many studies, the history of the species’ native populations remains poorly understood. Here, we aimed to assess the role of Pleistocene climatic changes in shaping the current distribution of the species in its native range. Methodology/Principal Findings We investigated the phylogeography, historical demography, and species distribution of Ae. albopictus native populations at the Last Glacial Maximum (LGM). Individuals from 16 localities from East Asia were analyzed for sequence variation at two mitochondrial genes. No phylogeographic structure was observed across the study area. Demographic analyses showed a signature of population expansion that started roughly 70,000 years BP. The occurrence of a continuous and climatically suitable area comprising Southeast China, Indochinese Peninsula, and Sundaland during LGM was indicated by species distribution modelling. Conclusions/Significance Our results suggest an evolutionary scenario in which, during the last glacial phase, Ae. albopictus did not experience a fragmentation phase but rather persisted in interconnected populations and experienced demographic growth. The wide ecological flexibility of the species probably played a crucial role in its response to glacial-induced environmental changes. Currently, there is little information on the impact of Pleistocene climatic changes on animal species in East Asia. Most of the studies focused on forest-associated species and suggested cycles of glacial fragmentation and post-glacial expansion. The case of Ae. albopictus, which exhibits a pattern not previously observed in the study area, adds an important piece to our understanding of the Pleistocene history of East Asian biota.

Porretta, Daniele; Mastrantonio, Valentina; Bellini, Romeo; Somboon, Pradya; Urbanelli, Sandra

2012-01-01

200

Plio-pleistocene African climate  

Microsoft Academic Search

Marine records of African climate variability document a shift toward more arid conditions after 2.8 million years ago (Ma), evidently resulting from remote forcing by cold North Atlantic sea-surface temperatures associated with the onset of Northern Hemisphere glacial cycles. African climate before 2.8 Ma was regulated by low-latitude insolation forcing of monsoonal climate due to Earth orbital precession. Major steps

P. B. deMenocal

1995-01-01

201

Plio-Pleistocene African Climate  

Microsoft Academic Search

Marine records of African climate variability document a shift toward more arid conditions after 2.8 million years ago (Ma), evidently resulting from remote forcing by cold North Atlantic sea-surface temperatures associated with the onset of Northern Hemisphere glacial cycles. African climate before 2.8 Ma was regulated by low-latitude insolation forcing of monsoonal climate due to Earth orbital precession. Major steps

Peter B. Demenocal

1995-01-01

202

Climatic implications of intermediate sized glacial advances in New Zeland valleys during OIS3.  

NASA Astrophysics Data System (ADS)

Recent work has greatly increased the number of known glacial oscillations during the last (Otiran) glaciation in South Island, New Zealand. Here we present summary stratigraphic and age results from a tectonic basin in the upper Rangitata Valley and a trough fill in the Rakaia Valley in Canterbury, New Zealand. The deposits constrain a series of intermediate scale glacial advances during OIS 3 that are not recorded in terminal moraine sequences in these valleys. These records demonstrate that ice limits oscillated substantially during the last glacial cycle but that very significant advances occurred at times other than the LGM, with glacial extents 80-95% of the local last glacial maximum. The timings of these advances appear to coincide with fragmentary evidence for glaciation in some other settings in New Zealand and SE Australia, indicating that the advances represent regionally significant climatic events. In the talk, I will summarise the evidence for the better constrained advances, consider the climate forcing required to maintain extended ice in these valleys through much of the last glacial cycle and consider the impact of antecedent ice limits on the climatic conditions at the LGM.

Shulmeister, James; Thackray, Glenn; Rittenour, Tammy

2014-05-01

203

Interactions between glacial erosion, the extent of glaciation, and topography investigated using numerical modeling  

NASA Astrophysics Data System (ADS)

Landscapes modified by the glacial erosion processes associated with warm-based glaciers in alpine settings seem to have a distinct distribution of surface area with elevation (hypsometry). The distribution is closely related to the local snowline altitude, demonstrating a significant maximum in the hypsometric distribution just below this level. The emergence of this distinct hypsometric signature can be predicted by surface process models, and is suggested to be due to effective glacial erosion above the local snowline. Using numerical model experiments, we focus here on how the development of this distinct glacial hypsometric maximum influences the extent of glaciation in the landscape on timescales of several 100 kyrs, and how the existence of this distinct topographic configuration will influence the general relation between climate change and the extent of glaciation, even on shorter time scales. We start from a steady-state fluvial landscape, and introduce oscillating temperature and glacial conditions, in order to generate a glacial hypsometry comparable with an observed global hypsometry for glaciated regions. The emergence of a distinct glacial hypsometry enables us to investigate the importance of the topographic distribution on the extent of glaciation both for a constant climatic forcing and for a climatic cooling. The results are obtained using iSOSIA, a higher-order ice sheet model approach, for simulating the flow of ice. Glacial erosion is assumed to be controlled by abrasion and quarrying processes, and is therefore a function of both sliding velocity, the amount of entrained sediment in the ice, and the bed slope in the direction of sliding. Our results suggest that glacial extent is highly sensitive to the hypsometry of the landscape, and that the importance of the resulting non-linear relationship between climate change and glacial extent will increase during glacial modification of the landscape.

Pedersen, V. K.; Egholm, D. L.

2012-04-01

204

Reorganization of ice sheet flow patterns in Arctic Canada and the mid-Pleistocene transition  

NASA Astrophysics Data System (ADS)

Evidence for the evolution of Laurentide Ice Sheet (LIS) basal thermal regime patterns during successive glaciations is poorly preserved in the geologic record. Here we explore a new approach to constrain the distribution of cold-based ice across central Baffin Island in the eastern Canadian Arctic over many glacial-interglacial cycles by combining till geochemistry and cosmogenic radionuclide (CRN) data. Parts of the landscaped with geomorphic evidence for limited glacial erosion are covered by till characterized by high chemical index of alteration (CIA) values and CRN concentrations requiring complicated burial-exposure histories. Till from regions scoured by glacial erosion have CIA values indistinguishable from local bedrock and CRN concentrations that can be explained by simple exposure following deglaciation. CRN modeling results based on these constraints suggest that the weathered tills were deposited by 1.9 to 1.2 Ma, and by that time the fiorded Baffin Island coastline must have developed close to its modern configuration as piracy of ice flow by the most efficient fiord systems resulted in a major shift in the basal thermal regime across the northeastern LIS. The resultant concentration of ice flow in fewer outlet systems may help explain the cause of the mid-Pleistocene transition from 41- to 100-kyr glacial cycles.

Refsnider, Kurt A.; Miller, Gifford H.

2010-07-01

205

Increased sensitivity of the Plio-Pleistocene northwest Pacific to obliquity forcing  

NASA Astrophysics Data System (ADS)

Insolation-driven changes in poleward heat transport and changes in greenhouse gas concentrations can explain aspects of the rise of obliquity-paced climate variability during the Pliocene-Pleistocene transition. Based on an alkenone-based sea surface temperature (SST) reconstruction of the Kuroshio Current Extension (KCE), we propose here that emergence of this region as the primary locus of ocean-atmosphere heat transfer in the Pacific Ocean promoted Northern Hemisphere Glaciation (NHG). Our record shows that with intensification of NHG at 2.7 Ma, the KCE cooled 2-4?°C during glacial intervals, likely in NH winter/spring. These high-amplitude 41-kyr SST cycles slightly lead ?O18 variations, ruling out global ice-volume changes as the primary cause. The lead of SST over ?O18 cycles matches the phasing between these two proxies observed in the tropics, supporting changes in CO2 concentrations as a unifying mechanism of ocean surface temperature change. However, the amplitude of the KCE SST cycle is twice that of the tropical records, pointing to an additional, regional process. We infer that cooling of the North Pacific sea surface by the East Asian winter monsoon and associated westerlies intensified during glacial intervals. This transfer of heat and moisture from the ocean to the atmosphere potentially furthered glacial formation by accelerating snow fall in North America. Therefore, these results might also support a role for tropical-extratropical heat balance in enhancing glacial growth via the obliquity pacing.

Venti, Nicholas L.; Billups, Katharina; Herbert, Timothy D.

2013-12-01

206

Tentative correlation of midcontinent glacial sequence with marine chronology  

SciTech Connect

A tentative glacial-interglacial 3-million-year chronology is synthesized by regional correlation of Midcontinent tills and paleosols to marine paleotemperature/eustatic cycles and oxygen isotope stages. The paleotemperature curves of Beard et al. (1982), based on planktonic foraminiferal abundances, correspond directly with eustatic cycles during the last 3 Ma. These generalized curves are shown to correlate reasonably well with standard oxygen isotope stages at least for the past 900 ka. This indicates that paleotemperature and Vail-type eustatic cycles have been glacially induced during the last 3 Ma. The chronology developed here utilizes both paleotemperature and oxygen isotope stages; however, below the Jaramillo magnetic subchron, isotope curves are more variable and only paleotemperature stages are used. Tills and paleosols at type localities in the Midcontinent area of the US are correlated to the SPECMAP oxygen isotope time scale. Because mid-Brunhes events are poorly constrained by radiometric dates, alternative correlations are possible. The oldest known Midcontinent tills correlate to the first Plio-Pleistocene cold paleotemperature stage and drop in sea level at 2.4 Ma. This Late Pliocene event also corresponds to the first major isotopic enrichment and the onset of late Cenozoic ice-rafting in the North Atlantic region.

Dube, T.E.

1985-01-01

207

Late Pleistocene human femoral diaphyseal curvature  

Microsoft Academic Search

Anterior femoral curvature is a consis- tent characteristic of Pleistocene and recent humans, al- though variation exists in the degree of curvature among individuals and across populations. In particular, one group, the Neandertals, has been characterized for a cen- tury as having marked femoral curvature. To evaluate the degree of anterior femoral curvature in both Neandertals and other Late Pleistocene

Laura L. Shackelford; Erik Trinkaus

2002-01-01

208

Molecular biogeography of Europe: Pleistocene cycles and postglacial trends.  

PubMed

The climatic cycles with subsequent glacial and intergalcial periods have had a great impact on the distribution and evolution of species. Using genetic analytical tools considerably increased our understanding of these processes. In this review I therefore give an overview of the molecular biogeography of Europe. For means of simplification, I distinguish between three major biogeographical entities: (i) "Mediterranean" with Mediterranean differentiation and dispersal centres, (ii) "Continental" with extra-Mediterranean centres and (iii) "Alpine" and/or "Arctic" with recent alpine and/or arctic distribution patterns. These different molecular biogeographical patterns are presented using actual examples. Many "Mediterranean" species are differentiated into three major European genetic lineages, which are due to glacial isolation in the three major Mediterranean peninsulas. Postglacial expansion in this group of species is mostly influenced by the barriers of the Pyrenees and the Alps with four resulting main patterns of postglacial range expansions. However, some cases are known with less than one genetic lineage per Mediterranean peninsula on the one hand, and others with a considerable genetic substructure within each of the Mediterranean peninsulas, Asia Minor and the Maghreb. These structures within the Mediterranean sub-centres are often rather strong and in several cases even predate the Pleistocene. For the "Continental" species, it could be shown that the formerly supposed postglacial spread from eastern Palearctic expansion centres is mostly not applicable. Quite the contrary, most of these species apparently had extra-Mediterranean centres of survival in Europe with special importance of the perialpine regions, the Carpathian Basin and parts of the Balkan Peninsula. In the group of "Alpine" and/or "Arctic" species, several molecular biogeographical patterns have been found, which support and improve the postulates based on distribution patterns and pollen records. Thus, genetic studies support the strong linkage between southwestern Alps and Pyrenees, northeastern Alps and Carpathians as well as southeastern Alps and the Dinaric mountain systems, hereby allowing conclusions on the glacial distribution patterns of these species. Furthermore, genetic analyses of arctic-alpine disjunct species support their broad distribution in the periglacial areas at least during the last glacial period. The detailed understanding of the different phylogeographical structures is essential for the management of the different evolutionary significant units of species and the conservation of their entire genetic diversity. Furthermore, the distribution of genetic diversity due to biogeographical reasons helps understanding the differing regional vulnerabilities of extant populations. PMID:17439649

Schmitt, Thomas

2007-01-01

209

The thermal evolution of the western equatorial Pacific warm pool during the Pleistocene and late Pliocene epochs  

Microsoft Academic Search

The late Pliocene and Pleistocene epochs have been characterized by a series of climate cycles generally referred to as ice ages. The ice ages not only involved high latitude climate oscillations as this designation implies, but also variability in low latitude tropical climate. A number of studies, including the series presented in this document, support a relatively novel perspective of

Martin Andres Medina Elizalde

2007-01-01

210

Vegetation of the Central Beringian Lowlands: Evidence of a Glacial Refugium Found in IODP Expedition 323 Sediment  

NASA Astrophysics Data System (ADS)

The lowlands of central Beringia may have acted as a glacial refugium for boreal vegetation, which expanded into eastern and western Beringia as climate changed and glaciers retreated. Persistence of trees, shrubs and mesic-adapted vegetation in the vicinity of the modern Bering Strait and Bering Sea Shelf could have presented a barrier to migrating fauna during Pleistocene glacial stages. These hypotheses have been difficult to test, because sampling has been restricted to lacustrine sediment and peat deposits accessible in eastern and western Beringia. Pollen analysis of cores from IODP Expedition 323 (Bering Sea Expedition) sites U1339 and U1343, on the edge of the Bering Sea Shelf, permits reconstruction of the terrestrial vegetation of adjacent south-central Beringia. Palynological assemblages extracted from sediment that accumulated during Marine Isotope Stages 2 and 6 are dominated by grass (Poaceae ? 15%) and sedge (Cyperaceae ? 20%). Spruce (Picea ? 5%), birch (Betula ? 10%) and alder (Alnus ? 5%) are also consistently present throughout glacial/interglacial cycles, suggesting that small populations of trees and shrubs remained in central Beringia during glacial maxima. These results support the refugium hypothesis. Although it is possible that some of the boreal plant pollen deposited during glacial stages is derived from interglacial sediment reworked by rivers flowing across the emergent shelf, we postulate that such sources only contribute about 1-5% of the total sediment found at these Bering slope sites. Thus we consider the palynological assemblages from IODP Expedition 323 a robust proxy for the glacial vegetation of central Beringia.

Westbrook, R.; Fowell, S. J.; Bigelow, N. H.; VanLaningham, S.

2011-12-01

211

Magnetostratigraphy of continental glacial deposits in southernmost Patagonia  

NASA Astrophysics Data System (ADS)

Southernmost Patagonia has well preserved records of continental and alpine glaciations. It is the only region in the Southern Hemisphere outside Antarctica where large Pleistocene ice sheets developed, thus it offers a unique opportunity to enhance understanding of global climate variability. Previous research on the glacial history of Patagonia is largely based on geomorphology, with limited study of stratigraphy and relatively few absolute ages. Consequently, the number, ages, and geographic extent of individual glaciations are not well understood. Geomorphic evidence of multiple Quaternary glaciations is widespread in the region and includes moraines, erratics, drumlin fields, and outwash plains. Exposures of glacial sediments are common along the Atlantic coast, the Strait of Magellan, meltwater channels, and in gravel pits. Most of the glacial deposits are beyond the range of radiocarbon dating. K-Ar and Ar-Ar dating have been used to determine the ages of basalt flows interlayered with till and outwash in the southern Andes to the west and in the Rio Gallegos valley near the Atlantic coast. These absolute ages suggest that glaciation in Patagonia began in the late Miocene and that the Pleistocene Patagonian ice sheet may have decreased in size with each successive advance. We conducted a paleomagnetic study of glacial sediments to further constrain the timing of continental glaciations. The sediments are stably magnetized, providing clear directional data from which polarity can be determined. Most sediments in the area are normally magnetized, but some sediments near the outer limit of glaciation, within areas covered by the Patagonian ice sheet during the Great Patagonian Glaciation (GPG), are reversely magnetized. Recent field work indicates that drift of the GPG comprises deposits of several glaciations. Our paleomagnetic results suggest that the first continental glaciation(s) in the region occurred during the Matuyama Reversed Chron (2.6 - 0.78 Ma) and subsequent glaciations occurred during the Bruhnes Normal Chron (<0.78 Ma). Two interpretations of the data are possible. Our preferred interpretation is that reversely magnetized till(s) record the onset of the 100 ka Milankovitch cycles during the latest Matuyama Reversed Chron (MIS 22 - 20), and overlying normally magnetized tills were deposited during the Brunhes Normal Chron. An alternative interpretation is that glaciation in southernmost Patagonia was initiated during one of the cold periods preceding the Jaramillo normal subchron (MIS 36 - 32), depositing reversely magnetized till(s), and that one of the overlying normally magnetized tills from the most extensive glaciation was laid down during the Jaramillo subchron (MIS 30). The suite of normally magnetized tills must record more than one glaciation and thus could not have all been deposited within the relatively short time interval of the Jaramillo subcron (1.08 - 0.99 Ma). Rather, most of the younger tills and perhaps one or more of the GPG drift sheets, must date to glaciations within the Brunhes Chron. Both options indicate that the first continental glaciation(s) to affect the region occurred during the late Early Pleistocene, consistent with the magnetostratigraphy and chronologies reported for the Canadian Prairies, Northwestern Europe, and Russia.

Griffing, C. Y.; Barendregt, R. W.; Clague, J. J.; Roberts, N. J.; Corbella, H.; Ercolano, B.; Rabassa, J.

2012-12-01

212

Middle Pleistocene age of the Nome River glaciation, northwestern Alaska  

NASA Astrophysics Data System (ADS)

During the middle Pleistocene Nome River glaciation of northwestern Alaska, glaciers covered an area an order of magnitude more extensive than during any subsequent glacial intervals. The age of the Nome River glaciation is constrained by laser-fusion {40Ar }/{39Ar} analyses of basaltic lava that overlies Nome River drift at Minnie Creek, central Seward Peninsula, that average 470,000 ± 190,000 yr (±1?). Milligram-size subsamples of the lava were dated to identify and eliminate extraneous 40Ar enrichments that rendered the mean of conventional K?Ar dates on larger bulk samples of the same flow too old (700,000 ± 570,000 yr). While the {40Ar }/{39Ar} analyses provide a minimum limiting age for the Nome River glaciation, maximum ages are provided by a provisional K?Ar date on a basaltic lava flow that underlies the Nome River drift at nearby Lave Creek, by paleomagnetic determinations of the drift itself at and near the type locality, and by amino acid epimerization analysis of molluscan fossils from nearshore sediments of the Anvilian marine transgression that underlie Nome River drift on the coastal plain at Nome. Taken together, the new age data indicate that the glaciation took place between 580,000 and 280,000 yr ago. The altitude of the Anvilian deposits suggests that eustatic sea level during the Anvilian transgression rose at least as high as and probably higher than during the last interglacial transgression; by correlation with the marine oxygen-isotope record, the transgression probably dates to stage 11 at 410,000 yr, and the Nome River glaciation is younger still. Analyses of floor altitudes of presumed Nome River cirques indicate that the Nome River regional snowline depression was at least twice that of the maximum late Wisconsin. The cause of the enhanced snowline lowering appears to be related to greater availability of moisture in northwestern Alaska during the middle Pleistocene.

Kaufman, Darrell S.; Walter, Robert C.; Brigham-Grette, Julie; Hopkins, David M.

1991-11-01

213

Tropical environmental change at the mid-Pleistocene transition  

NASA Astrophysics Data System (ADS)

The Mid-Pleistocene Transition (MPT) is the onset of the Late Pleistocene ice ages with an enlarged mean global ice volume changing in sawtooth-like 100-kyr cycles. Before the MPT, a 41-kyr ice-volume cycle was prevalent. During the MPT the Atlantic thermohaline circulation experienced severe reductions. Using lipid biomarkers we characterise the remote effects of the high-latitude climatic changes on the equatorial Atlantic and tropical Africa. A pronounced long-term warming of the equatorial Atlantic sea-surface occurred during the MPT, most likely caused by the reduced heat export to the higher latitudes. The growth of the ice-sheets thus led to a profound temporary deviation of the tropical climate from the global cooling evolution. With respect to glacial-interglacial cycles, the tropical sea-surface temperature (SST) development corresponds to the changing frequency pattern of the high-latitude ice-volume changes, however, reacting significantly earlier on insolation changes in all orbital cycles. Large-scale African vegetation changes are closely coupled to the tropical SST development, determining the continental aridity in equatorial Africa via the tropical evaporation-precipitation balance. In the precession cycle a strong monsoonal component is superimposed on the SST-forcing of aridity. The growth of additional ice-volume at the MPT also influenced the marine production by suppressing the effect of the precessional insolation forcing and enhancing the wind-driven upwelling by stronger and more zonal trade winds. The compression and intensification of the atmospheric circulation also increased the eolian transport of terrigenous plant wax compounds. During the MPT, the prevailing low-latitude forcing of the tropical environmental changes was thus replaced by a predominant ice-volume forcing via strengthened trade winds.

Schefuß, E.; Jansen, J. H. F.; Sinninghe Damsté, J. S.; Schneider, R. R.

2003-04-01

214

Chronology for fluctuations in late pleistocene Sierra Nevada glaciers and lakes  

SciTech Connect

Mountain glaciers, because of their small size, are usually close to equilibrium with the local climate and thus should provide a test of whether temperature oscillations in Greenland late in the last glacial period are part of global-scale climate variability or are restricted to the North Atlantic region. Correlation of cosmogenic chlorine-36 dates on Sierra Nevada moraines with a continuous radiocarbon-dated sediment record from nearby Owens Lake shows that Sierra Nevada glacial advances were associated with Heinrich events 5, 3, and 1. 27 refs., 2 figs., 1 tab.

Phillips, F.M.; Zreda, M.G.; Plummer, M.A. [New Mexico Institute of Mining and Technology, Socorro, NM (United States)] [and others] [New Mexico Institute of Mining and Technology, Socorro, NM (United States); and others

1996-11-01

215

Ice shelves in the Pleistocene Arctic Ocean inferred from glaciogenic deep-sea bedforms.  

PubMed

It has been proposed that during Pleistocene glaciations, an ice cap of 1 kilometre or greater thickness covered the Arctic Ocean. This notion contrasts with the prevailing view that the Arctic Ocean was covered only by perennial sea ice with scattered icebergs. Detailed mapping of the ocean floor is the best means to resolve this issue. Although sea-floor imagery has been used to reconstruct the glacial history of the Antarctic shelf, little data have been collected in the Arctic Ocean because of operational constraints. The use of a geophysical mapping system during the submarine SCICEX expedition in 1999 provided the opportunity to perform such an investigation over a large portion of the Arctic Ocean. Here we analyse backscatter images and sub-bottom profiler records obtained during this expedition from depths as great as 1 kilometre. These records show multiple bedforms indicative of glacial scouring and moulding of sea floor, combined with large-scale erosion of submarine ridge crests. These distinct glaciogenic features demonstrate that immense, Antarctic-type ice shelves up to 1 kilometre thick and hundreds of kilometres long existed in the Arctic Ocean during Pleistocene glaciations. PMID:11260709

Polyak, L; Edwards, M H; Coakley, B J; Jakobsson, M

2001-03-22

216

The impact of Pleistocene glaciation across the range of a widespread European coastal species.  

PubMed

There is a growing consensus that much of the contemporary phylogeography of northern hemisphere coastal taxa reflects the impact of Pleistocene glaciation, when glaciers covered much of the coastline at higher latitudes and sea levels dropped by as much as 150?m. The genetic signature of postglacial recolonization has been detected in many marine species, but the effects of coastal glaciation are not ubiquitous, leading to suggestions that species may intrinsically differ in their ability to respond to the environmental change associated with glacial cycles. Such variation may indeed have a biological basis, but apparent differences in population structure among taxa may also stem from our heavy reliance on individual mitochondrial loci, which are strongly influenced by stochasticity during coalescence. We investigated the contemporary population genetics of Syngnathus typhle, one of the most widespread European coastal fish species, using a multilocus data set to investigate the influence of Pleistocene glaciation and reduced sea levels on its phylogeography. A strong signal of postglacial recolonization was detected at both the northern and eastern ends of the species' distribution, while southern populations appear to have been relatively unaffected by the last glacial cycle. Patterns of population variation and differentiation at nuclear and mitochondrial loci differ significantly, but simulations indicate that these differences can be explained by the stochastic nature of the coalescent process. These results demonstrate the strength of a multilocus approach to phylogeography and suggest that an overdependence on mitochondrial loci may provide a misleading picture of population-level processes. PMID:20854410

Wilson, Anthony B; Eigenmann Veraguth, Iris

2010-10-01

217

Molecular evidence for Pleistocene refugia at the eastern edge of the Tibetan Plateau.  

PubMed

The role of the Quaternary ice ages in forming the contemporary genetic structure of populations has been well studied in a number of global regions. However, due to the different nature of glaciations and complex topography, their role in shaping eastern Eurasian genetic diversity, particular in areas surrounding the Tibetan Plateau have remained largely unstudied. We aimed to address this question by examining the genetic structure of an alpine forest-associated taxon, the blood pheasant (Ithaginis cruentus) to infer its phylogeographic history. We detected three phylogenetic lineages and four current population groups. By comparing molecular and palaeovegetation data, we found that major glaciations during the Pleistocene have had a major impact upon the current genetic diversity of this species. Coalescent simulations indicate that the populations retreated to different refugia during some glacial periods in the Pleistocene, but persisted through the last glacial maximum (LGM). The most significant recent population expansion was found to have occurred before the LGM, during which palaeoclimatic data indicate that the climate was both warmer and wetter than today. In contrast, during the LGM populations may have adopted an altitudinal shift strategy in order to track changes in alpine glaciers, exemplifying a general response for montane species in the region where alpine glaciations were not large enough to cause qualitative changes in vegetation. Although analysis based on a plumage related gene showed that divergent selection may have contributed to current patterns of intra-specific diversity, demographic isolation is inferred to have played a more dominant role. PMID:21689184

Zhan, Xiangjiang; Zheng, Yifang; Wei, Fuwen; Bruford, Michael W; Jia, Chenxi

2011-07-01

218

Paleoclimatic modeling and phylogeography of least killifish, Heterandria formosa: insights into Pleistocene expansion-contraction dynamics and evolutionary history of North American Coastal Plain freshwater biota  

PubMed Central

Background Climatic and sea-level fluctuations throughout the last Pleistocene glacial cycle (~130-0 ka) profoundly influenced present-day distributions and genetic diversity of Northern Hemisphere biotas by forcing range contractions in many species during the glacial advance and allowing expansion following glacial retreat ('expansion-contraction’ model). Evidence for such range dynamics and refugia in the unglaciated Gulf-Atlantic Coastal Plain stems largely from terrestrial species, and aquatic species Pleistocene responses remain relatively uninvestigated. Heterandria formosa, a wide-ranging regional endemic, presents an ideal system to test the expansion-contraction model within this biota. By integrating ecological niche modeling and phylogeography, we infer the Pleistocene history of this livebearing fish (Poeciliidae) and test for several predicted distributional and genetic effects of the last glaciation. Results Paleoclimatic models predicted range contraction to a single southwest Florida peninsula refugium during the Last Glacial Maximum, followed by northward expansion. We inferred spatial-population subdivision into four groups that reflect genetic barriers outside this refuge. Several other features of the genetic data were consistent with predictions derived from an expansion-contraction model: limited intraspecific divergence (e.g. mean mtDNA p-distance?=?0.66%); a pattern of mtDNA diversity (mean Hd?=?0.934; mean ??=?0.007) consistent with rapid, recent population expansion; a lack of mtDNA isolation-by-distance; and clinal variation in allozyme diversity with higher diversity at lower latitudes near the predicted refugium. Statistical tests of mismatch distributions and coalescent simulations of the gene tree lent greater support to a scenario of post-glacial expansion and diversification from a single refugium than to any other model examined (e.g. multiple-refugia scenarios). Conclusions Congruent results from diverse data indicate H. formosa fits the classic Pleistocene expansion-contraction model, even as the genetic data suggest additional ecological influences on population structure. While evidence for Plio-Pleistocene Gulf Coast vicariance is well described for many freshwater species presently codistributed with H. formosa, this species demography and diversification departs notably from this pattern. Species-specific expansion-contraction dynamics may therefore have figured more prominently in shaping Coastal Plain evolutionary history than previously thought. Our findings bolster growing appreciation for the complexity of phylogeographical structuring within North America’s southern refugia, including responses of Coastal Plain freshwater biota to Pleistocene climatic fluctuations.

2013-01-01

219

Vicariance biogeography in the Pleistocene and speciation in North American wood warblers: a test of Mengel's model.  

PubMed Central

It is widely believed that habitat fragmentation during the Pleistocene initiated speciation events in many songbird genera. One such vicariance model for avian speciation in the Pleistocene was developed by R. M. Mengel for North American birds. This model suggests that the first Pleistocene glacial advance reduced the area of an extensive, eastern North American deciduous forest, forcing adaptation by some species to boreal forest. This, in turn, facilitated the development of transcontinental range expansions during interglacials. Subsequent glacial advances repeatedly fragmented the ranges of these species into eastern and western populations; western isolates speciated to form the multispecies groups observed among various North American birds. We used mtDNA restriction site data to reconstruct the phylogeny of the black-throated green warbler complex-the group that Mengel considered the best fit to his model. Contrary to Mengel's model, the phylogeny indicates that not all western endemics were derived from an eastern ancestor. Instead, our results imply a mix, wherein some western endemics were budded off an eastern source, as Mengel posits, while others probably resulted from intermontane isolations in the west.

Bermingham, E; Rohwer, S; Freeman, S; Wood, C

1992-01-01

220

Did the West Antarctic Ice Sheet collapse during late Pleistocene interglacials: A reassessment  

NASA Astrophysics Data System (ADS)

Ever since John Mercer's provocative paper, "West Antarctic ice sheet and CO2 Greenhouse effect: a threat of disaster" (Nature 271:321-325; 1978), researchers have grappled with questions regarding whether, when, how frequently, and, especially, how quickly the West Antarctic Ice Sheet has disintegrated and reformed during the Pliocene and Pleistocene (and whether and when it may again in the future). Oxygen isotope records, being global averages, are insufficient to answer these questions, because (1) the WAIS signal is relatively small, and (2) antiphased behavior between the poles of the precession cycle will tend to cancel part of the signal, as outlined by Raymo et. al. (Science 313, 492-495, 2006). Previously, Antarctica lacked proximal, well-dated Pleistocene and Pliocene marine geologic records, so eustatic, mostly tropical sea-level records were used to infer past WAIS collapses. The first direct evidence of past Pleistocene WAIS collapse came from diatoms recovered from beneath the WAIS on the Whillans Ice Stream (UpB). Scherer (GPC, 4, 395-412, 1991) and Scherer et al. (Science, 281, 82-85, 1998) interpreted these results as most likely reflecting WAIS retreat during MIS-11, but could not rule out other interglacials, including MIS-5e, the penultimate interglacial discussed by Mercer. More recently, proximal evidence of WAIS retreat (or collapse) during early Pleistocene MIS-31 came from drilling at Cape Roberts (CRP-1) and the ANDRILL McMurdo Ice Shelf project (AND-1B) (Scherer et al., GRL, 35, doi:10.1029/2007GL032254, 2008). These diatom results provide evidence of ice sheet retreat events, but no constraint on the rate of ice sheet "collapse," which is critical to assessing the threat of future collapse. These results provided impetus and constraints for new coupled climate/ice sheet models, which are yielding significant insights (Pollard and DeConto, Nature, in press). The ANDRILL-MIS site contains no clear evidence of WAIS collapse events subsequent to MIS-31, but there is poor age control in the recovered diamictons. Furthermore, evidence of significant interglacials may have been lost in glacial erosion. A reassessment of diatom data from UpB indicates that the Pleistocene diatoms identified from beneath the WAIS are compatible with MIS-31 deposition as well as late Pleistocene marine deposition. This, once again, leaves the question of the configuration of the WAIS during MIS-11, MIS-5e and other late Pleistocene interglacials open for discussion.

Scherer, R. P.

2009-04-01

221

Arsenic Geochemistry and Hydrostratigraphy in Midwestern U.S. Glacial Deposits  

USGS Publications Warehouse

Arsenic concentrations exceeding the U.S. EPA's 10 ??g/L standard are common in glacial aquifers in the midwestern United States. Previous studies have indicated that arsenic occurs naturally in these aquifers in association with metal-(hydr)oxides and is released to groundwater under reducing conditions generated by microbial oxidation of organic matter. Despite this delineation of the arsenic source and mechanism of arsenic mobilization, identification of arsenic-impacted aquifers is hindered by the heterogeneous and discontinuous nature of glacial sediments. In much of the Midwest, the hydrostratigraphy of glacial deposits is not sufficiently characterized to predict where elevated arsenic concentrations are likely to occur. This case study from southeast Wisconsin presents a detailed characterization of local stratigraphy, hydrostratigraphy, and geochemistry of the Pleistocene glacial deposits and underlying Silurian dolomite. Analyses of a single core, water chemistry data, and well construction reports enabled identification of two aquifers separated by an organic-rich aquitard. The upper, unconfined aquifer provides potable water, whereas arsenic generally exceeds 10 ??g/L in the deeper aquifer. Although coring and detailed hydrostratigraphic characterization are often considered impractical, our results demonstrate that a single core improved interpretation of the complex lithology and hydrostratigraphy. This detailed characterization of hydrostratigraphy facilitated development of well construction guidelines and lays the ground work for further studies of the complex interactions among aquifer sediments, hydrogeology, water chemistry, and microbiology that lead to elevated arsenic in groundwater. Copyright ?? 2009 The Author(s). Journal compilation ?? 2009 National Ground Water Association.

Root, T. L.; Gotkowitz, M. B.; Bahr, J. M.; Attig, J. W.

2010-01-01

222

The Kuroshio Extension during the Pliocene-Pleistocene climate transition: orbital-scale temperature reconstructions from ODP Site 1208  

NASA Astrophysics Data System (ADS)

Despite the potential importance of the North Pacific in the Pliocene-Pleistocene climate transition, its role remains poorly understood due to a dearth of uninterrupted, high-resolution proxy records from this section. We present first results from a Globigerinoides ruber ?18O record and alkenone-based sea surface temperatures (SST) from ODP Site 1208 in the Kuroshio Current Extension currently spanning 1.8-2.4 Ma and 1.8-1.9 Ma, respectively. Gs. ruber ?18O values represent late summer/early fall hydrography while the alkenone-based (UK’37) proxy should reflect year-round SSTs. Orbital-scale age control for the site is provided by our Pliocene-Pleistocene benthic foraminiferal ?18O time series tuned to the global stack (Lisiecki and Raymo, 2005). Variability in the Gs. ruber ?18O record is dominated by the 41-kyr obliquity period, where it is in phase with the benthic foraminiferal ?18O record, suggesting the dominant influence of the glacial cycles on upper water column hydrography. Amplitude of this obliquity-scale variability averages ~0.3-0.5‰. Assuming that changes in ice volume were 0.3‰ in amplitude (Sosdian and Rosenthal, 2009), and that no changes in evaporation versus precipitation occurred, the glacial-interglacial SST change is ~0-2°C. Results from two the glacial cycles for which we currently have UK’37 data show that SSTs ranged by ~2-3°C from 17-18°C during glacial intervals to 19-20°C during interglacial intervals. Work in progress extends these sea-surface proxy records into the Pliocene to reveal the nature of the relationship between subtropical northwestern Pacific sea surface hydrography and glacial-interglacial climates as Northern Hemisphere ice sheets grew larger.

Venti, N. L.; Billups, K.; Herbert, T.

2010-12-01

223

The Mid-Pleistocene Transition: Deep Sea Temperature And Global Ice Volume From Mg\\/Ca and delta18O In Benthic Foraminifera  

Microsoft Academic Search

The mid-Pleistocene transition (MPT), which occurred between ~1 to 0.7 Ma, is recorded in benthic foraminiferal oxygen isotope (delta18Ob) records as a shift in the periodicity of northern hemisphere glaciations from low amplitude 41-kyr to large amplitude 100-kyr glacial-interglacial cycles. The MPT has variably been attributed to either global cooling associated with a long-term decreasing trend in greenhouse gases, or

S. M. Sosdian; Y. Rosenthal; M. Raymo

2006-01-01

224

A 10Be Chronology of Late Pleistocene and Holocene Glaciation in the Rwenzori Mountains, Uganda  

NASA Astrophysics Data System (ADS)

Although the retreat of glaciers in East Africa has been monitored over the last century, longer-term records of African glacier fluctuations are scarce. The Rwenzori Mountains, located on the border of Uganda and the Democratic Republic of Congo, host the largest glacial system in Africa and provide an opportunity for extensive investigation of past glaciations. We mapped and applied surface exposure (10Be) dating to glacial moraines deposited since the end of the last ice age in the Rwenzori Mountains to test the feasibility of 10Be dating at this site and to develop a chronology of glacial fluctuations. Our study is the first to use 10Be dating of glacial features in Africa and is possible because the Rwenzori host quartz-rich lithologies. By comparing the timing of Rwenzori glacial advances with other paleoclimate records from East Africa, we also will examine the climatic conditions which influenced tropical glacier fluctuations. Osmaston (1989) mapped moraines in the Rwenzori Mountains, documenting three stages of Pleistocene and Holocene glaciations, the Mahoma, Omurubaho and Lac Gris stages. The Mahoma stage moraines are estimated to be older than 17,980 ± 780 yr BP (D. M. Livingstone, 1962) by basal 14C dating of sediments from Lake Mahoma, situated in large lateral moraine at 2990 m asl. The age of the Omurubaho stage moraine is estimated from a basal 14C age (7,730 ± 150 yr BP) Lower Kitandara Lake (3990 m asl) and dammed by an Omurubaho stage moraine. The Lac Gris moraines are estimated at ~150-700 yr BP (de Heinzelin, 1953; Bergström, 1955) based on rates of lichen growth and plant colonization on moraines about 200 m below current glacial positions on Mt. Stanley. Though considerable uncertainty remains for the ages of these glacier deposits, these three stages most likely represent ages from the LGM to the LIA. We present two new 10Be ages of boulders from two moraines in the Nyamagusani Valley, ~4000 m asl. Sample KOP-2 (4033 m asl) is from the innermost moraine on the valley floor and yielded a 10Be age of 9,750 ± 110 yrs. Sample LA-1 (3870 masl) is from a moraine located ~1.8 km down valley and is 10,590 ± 120 yrs. Although the 10Be production rate is not well known in this region, these preliminary ages are in stratigraphic order and suggest glacial advance in the Rwenzori during late glacial to Early Holocene time. In June 2012, we sampled boulders from multiple valleys in the Rwenzori. We are currently processing fifteen samples from Nyamagusani to test the reproducibility of boulder ages on individual landforms and to test the age of Omurubaho stage moraines.

Baber, M.; Kelly, M. A.; Russell, J. M.; Loomis, S. E.

2012-12-01

225

Eccentricity cycles shown by early Pleistocene planktonic foraminifers of the Omma Formation, Sea of Japan  

NASA Astrophysics Data System (ADS)

There is a continuous record of planktonic foraminifers for oxygen isotope stages 50 to 26 (ca. 1.5-1.0 Ma) in the early Pleistocene Omma Formation near Kanazawa City, Central Japan, on the Sea of Japan coast. The warm-water species Globigerinoides ruber entered the Sea of Japan with the Tsushima Current during all interglacial periods and went locally extinct in the succeeding glacial periods. This implies that the marine climate of the Sea of Japan varied predominantly with the 41,000-year period of Earth's orbital obliquity. However, the relative abundances of G. ruber in marine isotope stages 47, 43 and 31 are significantly higher than those in other interglacial stages. These stages correspond to periods when eccentricity-modulated precession extremes were aligned with obliquity maxima. The Tsushima Current is a branch of the warm Kuroshio Current which is the strong northwestern component of the subtropical North Pacific Ocean gyre. Our data imply that the early Pleistocene climate in the northwestern Pacific was influenced not only by obliquity cycles but also by eccentricity cycles. This study also supports the climate model regarding eccentricity's role in the origin of low-frequency climate changes before the late Pleistocene ice ages.

Kitamura, Akihisa; Kimoto, Katsunori

2007-02-01

226

Nonlinear detection of large-scale transitions in Plio-Pleistocene African climate  

NASA Astrophysics Data System (ADS)

Potential paleoclimatic driving mechanisms acting on human development present an open problem of cross-disciplinary scientific interest. The analysis of paleoclimate archives encoding the environmental variability in East Africa during the last 5 Ma (million years) has triggered an ongoing debate about possible candidate processes and evolutionary mechanisms. In this work, we apply a novel nonlinear statistical technique, recurrence network analysis, to three distinct marine records of terrigenous dust flux. Our method enables us to identify three epochs with transitions between qualitatively different types of environmental variability in North and East Africa during the (i) Mid-Pliocene (3.35-3.15 Ma BP (before present)), (ii) Early Pleistocene (2.25-1.6 Ma BP), and (iii) Mid-Pleistocene (1.1-0.7 Ma BP). A deeper examination of these transition periods reveals potential climatic drivers, including (i) large-scale changes in ocean currents due to a spatial shift of the Indonesian throughflow in combination with an intensification of Northern Hemisphere glaciation, (ii) a global reorganization of the atmospheric Walker circulation induced in the tropical Pacific and Indian Ocean, and (iii) shifts in the dominating temporal variability pattern of glacial activity during the Mid-Pleistocene, respectively. A statistical reexamination of the available fossil record demonstrates a remarkable coincidence between the detected transition periods and major steps in hominin evolution. This suggests that the observed shifts between more regular and more erratic environmental variability have acted as a trigger for rapid change in the development of humankind in Africa.

Donges, J. F.; Donner, R. V.; Trauth, M. H.; Marwan, N.; Schellnhuber, H. J.; Kurths, J.

2011-12-01

227

Late Pleistocene climate change, nutrient cycling, and the megafaunal extinctions in North America  

NASA Astrophysics Data System (ADS)

This study proposes an ecological mechanism for the terminal Pleistocene population collapse and subsequent extinction of North American megafauna. Observations of modern ecosystems indicate that feedback mechanisms between plant nutrient content, nitrogen cycling, and herbivore-plant interactions can vary between a nutrient accelerating mode favoring increased herbivore biomass and a nutrient decelerating mode characterized by reduced herbivore biomass. These alternate modes are determined largely by plant nitrogen content. Plant nitrogen content is known to be influenced by atmospheric CO 2 concentrations, temperature, and precipitation. It is argued that Lateglacial climate change, particularly increases in atmospheric CO 2, shifted herbivore-ecosystem dynamics from a nutrient accelerating mode to a nutrient decelerating mode at the end of the Pleistocene, leading to reduced megafaunal population densities. An examination of Sporormiella records - a proxy for megaherbivore biomass - indicates that megafaunal populations collapsed first in the east and later in the west, possibly reflecting regional differences in precipitation or vegetation structure. The fortuitous intersection of the climatically driven nitrogen sink, followed by any one or combination of subsequent anthropogenic, environmental, or extra-terrestrial mechanisms could explain why extinctions took place at the end of the Pleistocene rather than during previous glacial-interglacial cycles.

Faith, J. Tyler

2011-06-01

228

Range expansions in the flightless longhorn cactus beetles, Moneilema gigas and Moneilema armatum, in response to Pleistocene climate changes.  

PubMed

Pollen cores and plant and animal fossils suggest that global climate changes at the end of the last glacial period caused range expansions in organisms indigenous to the North American desert regions, but this suggestion has rarely been investigated from a population genetic perspective. In order to investigate the impact of Pleistocene climate changes and glacial/interglacial cycling on the distribution and population structure of animals in North American desert communities, biogeographical patterns in the flightless, warm-desert cactus beetles, Moneilema gigas and Moneilema armatum, were examined using mitochondrial DNA (mtDNA) sequence data from the cytochrome oxidase I (COI) gene. Gene tree relationships between haplotypes were inferred using parsimony, maximum-likelihood, and Bayesian analysis. Nested clade analysis and coalescent modelling using the programs mdiv and fluctuate were used to identify demographically independent populations, and to test the hypothesis that Pleistocene climate changes caused recent range expansions in these species. A sign test was used to evaluate the probability of observing concerted population growth across multiple, independent populations. The phylogeographical and nested clade analyses reveal a history of northward expansion in both of these species, as well as a history of past range fragmentation, followed by expansion from refugia. The coalescent analyses provide highly significant evidence for independent range expansions from multiple refugia, but also identify biogeographical patterns that predate the most recent glacial period. The results indicate that widespread desert environments are more ancient than has been suggested in the past. PMID:15773934

Smith, Christopher Irwin; Farrell, Brian D

2005-04-01

229

Environmental changes at the Holocene-Late Pleistocene transition: Sedimentation on Akademicheskii Ridge (Lake Baikal, Russia)  

NASA Astrophysics Data System (ADS)

Akademicheskii Ridge of Lake Baikal represents a 300 m deep underwater rise, which separates the Central Basin (1647 m water depth) and the North Basin (970 m water depth) of 640 km long lake. The large distance to the turbid load of particle-carrying tributaries and coastal areas as well as the absence of slide induced turbidites are responsible for low sedimentation rates. A large number of short cores (approx. 120 cm) was used to study in detail the Holocene-Late Pleistocene transition, using lithological composition, magnetic susceptibility, microfossils, pollen and spores, chemistry, grain size and mineral composition. Holocene sediments show sedimentation rates from 0.015 to 0.25 mm y-1 and are mainly composed of biogenic material with rare admixtures of aeolian and ice-rafted terrigenous particles [1]. The sediments are characterized by abundant microfossils, such as diatoms, spicules of sponges, chrysophyte cysts, pollen and spores. Holocene diatom assemblages are representated by Aulacoseira baicalensis, A. skvortzowii, Cyclotella minuta, C. baicalensis, Synedra acus var.radians, Stephanodiscus meyerii, Crateriportula inconspicuus and Cyclostephanos dubius [2]. Concentrations of Corg.,Ntot.,and Sibiog.indicate clearly higher productivity of the lake during the Holocene [1]. Late Pleistocene sediments are composed of clastic, fine-grained, clayey material, mainly of terrigenous origin. This includes also aeolian particles and rare ice-transported sandy material and rock debris. A peak of the diatom species Stephanodiscus flabellatus, observed within the upper part of clayey sediments, defines the Late Pleistocene-Holocene transition [2]. Very low contents of microfossils (diatoms, spicules of sponges, chrysophyte cysts etc.) within Late Pleistocene deposits indicate lower productivity of Lake Baikal. Glacial melt-water dominated the sediment transport processes within the lake during this time. The main minerals of the sand fraction are quartz, feldspars and mica. The heavy mineral assemblage contains amphiboles, pyroxenes, epidote, sphene, magnetite, garnet and chloritoide. Within the Holocene, contents of chloritoide are low (0.6-1.2 %), but they are distinctly higher within the Late Pleistocene sediments (3.2-14.6 %) [1]. An increase of chloritoide in sediments points towards an intensification of aeolian transport by stronger winds and longer-lasting periods of ice cover during the Late Pleistocene [3]. Results of pollen analyses support these findings. They indicate that mountain slopes of the catchment of Lake Baikal were mostly uncovered by vegetation. A polymineral composition is characteristic for the clay fraction of Late Pleistocene deposits: hydro-mica, kaolinite, smectite and chlorite. This is caused by extensive glaciation of the catchment of the lake during this time [4], generating increased transport of terrigenous material to the lake by glacial melt water [5]. References [1] Vologina, E.G. and Sturm, M. 2009. Types of Holocene deposits and regional pattern of sedimentation in Lake Baikal. Russian Geology and Geophysics 50, 1-6. [2] Bradbury, J.P., Bezrukova, Ye.V., Chernyaeva, G.P. et al. 1994. A synthesis of post-glacial diatom records from Lake Baikal. J. Paleolimnol. 10, 213-252.

Vologina, Elena G.; Sturm, Michael

2010-05-01

230

Gulf coastal Pleistocene units and time stratigraphy; reevaluation and problems of Atlantic correlation  

SciTech Connect

Outdated glacial subdivisions and misinterpretations of alluvial interfluve ridges as marine terraces hampered advances in coastal stratigraphy. One problem involves C.W. Cooke's extension of his Atlantic shorelines along the NE Gulf into the Mississippi Embayment. The mirage of an inter-Wisconsinan interglacial gave way to beliefs in high glacial Wisconsinan sea levels that were assumed to have resulted in barriers and intensive alluvial aggradation on the TX-LA coastal plain. Without vertical definitions, Fisk assigned formation status to alluvial and brackish-marine sediments that directly underlie four coastwise Pleistocene terraces in SW Louisiana. The youngest (Prairie) and associated formations were recently (re)defined and correlated with other coastal areas. Brackish and marine deposits in the subsurface have been correlated with Fisk's second youngest coastwise surface. Detailed facies analyses of cores from hundreds of drillholes indicated that, in sharp contrast with Plio-Pleistocene barriers on the Atlantic coast, only a single, Sangamonian (Sg) barrier shore complex remains on the NE Gulf coastal plain after intensive uplift/erosion. Few isolated remnants of pre-Sg Pleistocene alluvial units occur, including flora elements in peat lenses at one location. An early, low Sg sea level stand near Apalachicola is marked by transgressive deposits at c. [minus]37.5m. Thin NE Gulf Sg sequence includes the fine-grained, open marine-to-estuarine Biloxi, the regressive, shallow subtidal-to-supratidal, mainland Gulfport barrier and the alluvial Prairie Formations. These are correlatable Gulfwide. Contrary to widespread assumption, the Gulfport-Ingleside barriers were not islands but mainland strandplains. The Sg complex correlates with oxygen isotope Stage 5 units of the Mid/South Atlantic coastal plain and shelf. Thick LA-TX shelf/slope intervals display about ten fourth-order cycles within 4 primary ones.

Otvos, E.G. (Gulf Coast Research Lab., Ocean Springs, MS (United States). Geology Section)

1993-03-01

231

Ice-Sheet Dynamics and Millennial-Scale Climate Variability in the North Atlantic across the Middle Pleistocene Transition (Invited)  

NASA Astrophysics Data System (ADS)

During the Middle Pleistocene Transition (MPT), the climate system evolved from a more linear response to insolation forcing in the '41-kyr world' to one that was decidedly non-linear in the '100-kyr world'. Smaller ice sheets in the early Pleistocene gave way to larger ice sheets in the late Pleistocene with an accompanying change in ice sheet dynamics. We studied Sites U1308 (49° 52.7'N, 24° 14.3'W; 3871 m) and U1304 (53° 3.4'N, 33° 31.8'W; 3024 m) in the North Atlantic to determine how ice sheet dynamics and millennial-scale climate variability evolved as glacial boundary conditions changed across the MPT. The frequency of ice-rafted detritus (IRD) in the North Atlantic was greater during glacial stages prior to 650 ka (MIS 16), reflecting more frequent crossing of an ice volume threshold when the climate system spent more time in the 'intermediate ice volume' window, resulting in persistent millennial scale variability. The rarity of Heinrich Events containing detrital carbonate and more frequent occurrence of IRD events prior to 650 ka may indicate the presence of 'low-slung, slippery ice sheets' that flowed more readily than their post-MPT counterparts (Bailey et al., 2010). Ice volume surpassed a critical threshold across the MPT that permitted ice sheets to survive boreal summer insolation maxima, thereby increasing ice volume and thickness, lengthening glacial cycles, and activating the dynamical processes responsible for Laurentide Ice Sheet instability in the region of Hudson Strait (i.e., Heinrich events). The excess ice volume during post-MPT glacial maxima provided a large, unstable reservoir of freshwater to be released to the North Atlantic during glacial terminations with the potential to perturb Atlantic Meridional Overtunring Circulation. We speculate that orbital- and millennial-scale variability co-evolved across the MPT and the interaction of processes on orbital and suborbital time scales gave rise to the changing patterns of glacial-interglacial cycles through the Quaternary. Bailey, I., Bolton, C.T., DeConto, R.M., Pollard, D., Schiebel, R. and Wilson, P.A. (2010) A low threshold for North Atlantic ice rafting from "low-slung slippery" late Pliocene ice sheets. Paleoceanography, 25, PA1212-[14pp]. (doi:10.1029/2009PA001736).

Hodell, D. A.; Nicholl, J.

2013-12-01

232

Biosignatures in Pleistocene Cave Pool Speleothems  

NASA Astrophysics Data System (ADS)

A macro- to microscopic biosignature suite from Pleistocene cave pool speleothems indicates that long term preservation of actual organic matter is possible in cave speleothems and perhaps in the subsurface of other planets.

Melim, L. A.; Spilde, M. N.; Northup, D. E.; Boston, P. J.

2010-04-01

233

The distinct roles of the Antarctic and Subantarctic Zones in ocean productivity and atmospheric CO2 across the Mid-Pleistocene transition  

NASA Astrophysics Data System (ADS)

The emergence of low frequency, high-amplitude, quasi-periodic (100-kyr) glacial variability during the middle Pleistocene in the absence of any significant change in orbital forcing indicates a fundamental change internal to the climate system. The mid-Pleistocene transition (MPT), which occurred between 1.2 and 0.7 Myr, has variably been attributed to either global cooling possibly associated with a long-term decrease in greenhouse gas concentrations or changes in internal ice-sheet dynamics independent of changes in atmospheric pCO2. The available low-resolution pCO2 estimates indicate that atmospheric CO2 concentrations were 30 ppm higher during glacial stages before the MPT, but also that interglacial values were similar to those of the late Pleistocene. This resulted in no significant change in the atmospheric CO2 trend. However, the higher atmospheric CO2 concentrations during glacial stages resulted in an increase in glacial temperatures in the tropics, and a 30% decrease in glacial/interglacial amplitude before 450 kyr. During this period Southern Ocean dust fluxes doubled and reached values that are comparable to those of the LGM. Thus, an increase in Fe availability may have potentially contributed, in combination with other mechanisms to explain part of the 30 ppm decrease in glacial atmospheric CO2 observed across the MPT. This observation is coherent with a progressive increase in glacial carbon sequestration due to Fe fertilization in the Southern Ocean as Northern Hemisphere glaciations intensify. Here, we investigate how the combined changes in Fe supply and in the strength of vertical convection have affected the sequestration of remineralized carbon in the ocean interior over the last 1.6 Myrs. We will show highly-resolved, continuous records from two South Atlantic ODP sedimentary archives located on either side of the Antarctic polar front highlighting the existence of a strong positive feedback mechanism between ice volume, Southern Ocean dust deposition and export production that has gradually strengthened through the Pliocene-Pleistocene.

Jaccard, S.; Martinez-Garcia, A.; Hasenfratz, A.; Sigman, D. M.; Haug, G. H.

2012-12-01

234

Across the southern Andes on fin: glacial refugia, drainage reversals and a secondary contact zone revealed by the phylogeographical signal of Galaxias platei in Patagonia.  

PubMed

We employed DNA sequence variation at two mitochondrial (control region, COI) regions from 212 individuals of Galaxias platei (Pisces, Galaxiidae) collected throughout Patagonia (25 lakes/rivers) to examine how Andean orogeny and the climatic cycles throughout the Quaternary affected the genetic diversity and phylogeography of this species. Phylogenetic analyses revealed four deep genealogical lineages which likely represent the initial division of G. platei into eastern and western lineages by Andean uplift, followed by further subdivision of each lineage into separate glacial refugia by repeated Pleistocene glacial cycles. West of the Andes, refugia were likely restricted to the northern region of Patagonia with small relicts in the south, whereas eastern refugia appear to have been much larger and widespread, consisting of separate northern and southern regions that collectively spanned most of Argentinean Patagonia. The retreat of glacial ice following the last glacial maximum allowed re-colonization of central Chile from nonlocal refugia from the north and east, representing a region of secondary contact between all four glacial lineages. Northwestern glacial relicts likely followed pro-glacial lakes into central Chilean Patagonia, whereas catastrophic changes in drainage direction (Atlantic --> Pacific) for several eastern palaeolakes were the likely avenues for invasions from the east. These mechanisms, combined with evidence for recent, rapid and widespread population growth could explain the extensive contemporary distribution of G. platei throughout Patagonia. PMID:19017262

Zemlak, Tyler S; Habit, Evelyn M; Walde, Sandra J; Battini, Miguel A; Adams, Emily D M; Ruzzante, Daniel E

2008-12-01

235

Holocene glacial discharge fluctuations and recent instability in East Antarctica  

NASA Astrophysics Data System (ADS)

Antarctica holds the largest ice sheet in the world, the East Antarctic Ice Sheet (EAIS), and plays a significant role in both local and global climate through the interactions between ice sheets, ocean, sea ice, and atmosphere. Our understanding of East Antarctica Holocene climate variability relies mainly on ice cores that however do not document glacial discharge history. Here, we present the first high resolution ?18Odiatom record derived from two marine sediment cores retrieved on the East Antarctic continental shelf to reconstruct glacial discharge off Adélie Land and George V Land (AL-GVL) over the last 11,000 years from decadal to centennial resolution. Our results suggest multi-centennial glacier advances and retreats until 2000 cal yr BP, followed by a period of relative instability marked by two major glacial retreats centered at ˜1700 cal yr BP and ˜1980 CE. We suggest that the multi-centennial oscillations during the Early/Mid-Holocene reflect glacier fluctuations in response to long-term local seasonal insolation and short-term solar variability. We also propose that ?18Odiatom variability over the last 2000 years was the result of a recent change in the AL-GVL region to increasing atmospheric influence, linked to ENSO intensification and teleconnections strengthening between low and high latitudes.

Crespin, Julien; Yam, Ruth; Crosta, Xavier; Massé, Guillaume; Schmidt, Sabine; Campagne, Philippine; Shemesh, Aldo

2014-05-01

236

Late Pleistocene Glaciation of the Kosciuszko Massif, Snowy Mountains, Australia  

NASA Astrophysics Data System (ADS)

Late Pleistocene glaciation of the Australian mainland was restricted to a small area of the southeastern highlands. Geomorphic mapping of the area and exposure dating using the in situ produced cosmogenic isotope 10Be provides evidence for at least two distinct glaciations. The Early Kosciuszko glaciation consisted of a single glacier advance before 59,300 ± 5400 years ago (Snowy River Advance). The Late Kosciuszko glaciation comprised three glacier advances 32,000 ± 2500 (Headley Tarn Advance), 19,100 ± 1600 (Blue Lake Advance), and 16,800 ± 1400 years ago (Mt. Twynam Advance). The Early Kosciuszko glaciation was the most extensive and the Late Kosciuszko advances were progressively less extensive. These periods of glaciation in the highlands correspond to episodes of periglacial activity and peaks in lake levels and river discharge at lower elevations in southeastern Australia. Glacier advances on the Kosciuszko Massif correlate with advances in Tasmania, South America, and New Zealand and are broadly representative of hemispheric climate changes during the last glacial cycle.

Barrows, Timothy T.; Stone, John O.; Fifield, L. Keith; Cresswell, Richard G.

2001-03-01

237

Extraterrestrial accretion and glacial cycles  

NASA Technical Reports Server (NTRS)

We propose that the approx. 100-k.y. cycle seen in terrestrial glaciation is due to changes in meteor flux that come from changes in the Earth's orbit. This model can explain a 70-k.y. 'anomalous' period in climate data and the apparent discrepancy between present extraterrestrial fluxes and those in oceanic sediments. It can be tested by measuring Ir densities in sediments and ice during glacials and interglacials.

Muller, R. A.

1994-01-01

238

Late pleistocene, Late-glacial and Holocene glacier advances on the Tibetan Plateau  

Microsoft Academic Search

Many end-moraine sequences in various mountain areas of the Tibetan Plateau have already been investigated in detail, but a homogeneous chronostratigraphy is still lacking. The ice-marginal limits are commonly given local names and often the classification does not seem to be secure. This is one reason for controversial discussions on the ice extent at the LGM. Because most organic material

Frank Lehmkuhl

1997-01-01

239

Provenance of Palouse Loess and Relation to Late Pleistocene Glacial Outburst Flooding, Washington State  

Microsoft Academic Search

The eolian system of the Pacific Northwest is a product of long-term deflation of expansive sedimentary units by prevailing winds throughout the Quaternary. The Palouse loess is a deposit of wind-blown silt that covers approximately 10,000 sqare km up to 75 m thick. Late Quaternary units of the loess become finer texturally and thinner to the northeast, suggesting that they

M. R. Sweeney; A. J. Busacca; D. R. Gaylord; C. S. Zender

2002-01-01

240

36Cl dating of the classic Pleistocene glacial record in the northeastern Cascade Range, Washington  

Microsoft Academic Search

and are more degraded. Seventy-six 36Cl dates for the moraines cluster in groups having mean ages ( 1) of 12,500 500, 13,300 800, 16,100 1100, 19,100 3000, 70,900 1500, 93,100 2600, and 105,400 2200 years; a still-older, highly weathered and eroded moraine is undated, but likely is at least 165,000 years old. The moraine ages and relative extent of the

STEPHEN C. PORTER; TERRY W. SWANSON

2008-01-01

241

Middle Pleistocene (?) buried glacial ice on Bylot Island, Canadian Arctic Archipleago  

Microsoft Academic Search

Bylot Island is located north of Baffin Island (73°N, 80°W). More than the half of the island is covered by an ice cap and its outlet glaciers flowing towards the arctic lowland of the Lancaster formation. The study site comprises four main stratigraphic units. Overlying the shales (Tertiary) of the Lancaster Formation (500 m a.s.l.), a diamicton (unit 1) is

D. Fortier; E. Godin; M. Z. Kanevskiy; M. Allard

2009-01-01

242

Climate change and evolving human diversity in Europe during the last glacial.  

PubMed Central

A link between climate change and human evolution during the Pleistocene has often been assumed but rarely tested. At the macro-evolutionary level Foley showed for hominids that extinction, rather than speciation, correlates with environmental change as recorded in the deep sea record. Our aim is to examine this finding at a smaller scale and with high-resolution environmental and archaeological archives. Our interest is in changing patterns of human dispersal under shifting Pleistocene climates during the last glacial period in Europe. Selecting this time frame and region allows us to observe how two hominid taxa, Neanderthals and Crô-Magnons, adapted to climatic conditions during oxygen isotope stage 3. These taxa are representative of two hominid adaptive radiations, termed terrestrial and aquatic, which exhibited different habitat preferences but similar tolerances to climatic factors. Their response to changing ecological conditions was predicated upon their ability to extend their societies in space and time. We examine this difference further using a database of all available radiocarbon determinations from western Europe in the late glacial. These data act as proxies for population history, and in particular the expansion and contraction of regional populations as climate changed rapidly. Independent assessment of these processes is obtained from the genetic history of Europeans. The results indicate that climate affects population contraction rather than expansion. We discuss the consequences for genetic and cultural diversity which led to the legacy of the Ice Age: a single hominid species, globally distributed.

Gamble, Clive; Davies, William; Pettitt, Paul; Richards, Martin

2004-01-01

243

Last Glacial mammals in South America: a new scenario from the Tarija Basin (Bolivia).  

PubMed

The chronology, sedimentary history, and paleoecology of the Tarija Basin (Bolivia), one of the richest Pleistocene mammalian sites in South America, are revised here based on a multidisciplinary study, including stratigraphy, sedimentology, geomorphology, paleontology, isotope geochemistry, and (14)C geochronology. Previous studies have indicated a Middle Pleistocene age for this classic locality. We have been able to obtain a series of (14)C dates encompassing all the fossil-bearing sequences previously studied in the Tarija Basin. The dated layers range in age from about 44,000 to 21,000 radiocarbon years before present (BP), indicating that the Tarija fauna is much younger than previously thought. Glacial advances correlated to marine isotopic stages (MIS) 4 and 2 (ca. 62 and 20 ka BP, respectively) are also documented at the base and at the very top of the Tarija-Padcaya succession, respectively, indicating that the Bolivian Altiplano was not dry but sustained an ice cap during the Last Glacial Maximum. The results of this multidisciplinary study enable us to redefine the chronological limits of the Tarija sequence and of its faunal assemblage and to shift this paleontological, paleoclimatological, and paleoecological framework to the time interval from MIS 4 to MIS 2. PMID:17180614

Coltorti, M; Abbazzi, L; Ferretti, M P; Iacumin, P; Rios, F Paredes; Pellegrini, M; Pieruccini, P; Rustioni, M; Tito, G; Rook, L

2007-04-01

244

Constraints on the Pleistocene chronology of sediments from the Lomonosov Ridge  

USGS Publications Warehouse

Despite its importance in the global climate system, age-calibrated marine geologic records reflecting the evolultion of glacial cycles through the Pleistocene are largely absent from the central Arctic Ocean. This is especially true for sediments older than 200 ka. Three sites cored during the Integrated Ocean Drilling Program's Expedition 302, the Arctic Coring Expedition (ACEX), provide a 27 m continuous sedimentary section from the Lomonosov Ridge in the central Arctic Ocean. Two key biostratigraphic datums and constraints from the magnetic inclination data are used to anchor the chronology of these sediments back to the base of the Cobb Mountain subchron (1215 ka). Beyond 1215 ka, two best fitting geomagnetic models are used to investigate the nature of cyclostratigraphic change. Within this chronology we show that bulk and mineral magnetic properties of the sediments vary on predicted Milankovitch frequencies. These cyclic variations record "glacial" and "interglacial" modes of sediment deposition on the Lomonosov Ridge as evident in studies of ice-rafted debris and stable isotopic and faunal assemblages for the last two glacial cycles and were used to tune the age model. Potential errors, which largely arise from uncertainties in the nature of downhole paleomagnetic variability, and the choice of a tuning target are handled by defining an error envelope that is based on the best fitting cyclostratigraphic and geomagnetic solutions. Copyright 2008 by the American Geophysical Union.

O'Regan, M.; King, J.; Backman, J.; Jakobsson, M.; Palike, H.; Moran, K.; Heil, C.; Sakamoto, T.; Cronin, T. M.; Jordan, R. W.

2008-01-01

245

Biogeochemical Characteristics of Lacustrine Sediments Reflecting a Changing Alpine Neotropical Ecosystem during the Pleistocene  

NASA Astrophysics Data System (ADS)

Continuous lacustrine deposits of the Funza-II core from the Bogotá basin, Colombia (5°N74°W) record late Pleistocene climatic variations, providing an opportunity to assess the influence of glacial-interglacial climate changes on alpine ecosystems in equatorial South America. Biogeochemical response of this tropical alpine system to climate change was inferred from changes in elemental concentrations and ratios and isotopic signatures in the upper 120 m of the lacustrine Funza core. Values of ? 13C org exhibit eight abrupt, positive shifts that are thought to reflect rapid expansions of C 4 grasses in the tropical Andes and algal blooms. One of these excursions, interpreted to correspond to C 4 vegetation expansion, occurred in sediments accumulated during the last glaciation (˜30,000-50,000 yr B.P.) and implies a downslope shift of the upper Andean treeline, regardless of prevailing temperatures. Sedimentary carbon/sulfur ratios are low and indicate significant sequestering of sulfur. Monosulfides are the dominant constituent of sedimentary sulfur during relatively humid intervals, when increased supply of iron caused by enhanced weathering favored the formation of monosulfide minerals under strongly reducing conditions. In contrast, organosulfur compounds dominate the sedimentary sulfur-species in relatively drier intervals when mildly reducing conditions and limited iron input promoted the diagenetic incorporation of sulfur in organic matter. Dry events inferred from the sulfur record typically correlate with glacial maxima, whereas glacial terminations are usually associated with wet periods.

Mora, Germán; Pratt, Lisa M.; Boom, Arnoud; Hooghiemstra, Henry

2002-09-01

246

Cosmogenic beryllium-10 and neon-21 dating of late Pleistocene glaciations in Nyalam, monsoonal Himalayas  

NASA Astrophysics Data System (ADS)

We present cosmogenic 10Be and 21Ne chronologies from 21 erratic boulders on three moraine sequences in Nyalam county, monsoonal Himalayas, southern margin of the Tibetan Plateau. The surface exposure ages provide evidence for at least two distinct glacial advances during the late stage of the last glacial cycle and for one or more significantly older glaciations. The distribution of cosmogenic ages from the three ridges of the old moraine sequence is inconsistent with their stratigraphic order. Because exposure periods of the erratics prior to deposition on the moraine surface is shown to be small, the chronology-stratigraphy mismatch suggests that the cosmogenic ages do not date moraine deposition but most likely significant moraine ridge denudation and related boulder exhumation after initial deposition of the moraines during the penultimate glacial cycle or earlier. The surface exposure ages based on various currently accepted production rate scaling protocols yield age differences of up to 35% reflecting the poor knowledge of terrestrial cosmogenic production rates at low latitude/high altitude sites. Even within this conservative uncertainty range, our results do not yield evidence for late Pleistocene glaciations in monsoonal Tibet to be asynchronous to those in mid-latitudes on both hemispheres. There is an urgent need to improve the knowledge of terrestrial cosmogenic nuclide production rates and their scaling to low latitude regions to fully exploit the climate information archived in tropical moraine sequences.

Schaefer, Joerg M.; Oberholzer, Peter; Zhao, Zhizhong; Ivy-Ochs, Susan; Wieler, Rainer; Baur, Heinrich; Kubik, Peter W.; Schlüchter, Christian

2008-02-01

247

Integrated summer insolation forcing and 40,000-year glacial cycles: The perspective from an ice-sheet\\/energy-balance model  

Microsoft Academic Search

Although the origins of the 40,000-year glacial cycles during the early Pleistocene are readily attributed to changes in Earth's obliquity (also having a 40,000-year period), the lack of ice-volume variability at precession periods (20,000 years) is difficult to reconcile with most parameterizations of the insolation forcing. It was recently proposed that precession's influence on glaciation is muted because variations in

Peter Huybers; Eli Tziperman

2008-01-01

248

Phylogeographical Analysis of mtDNA Data Indicates Postglacial Expansion from Multiple Glacial Refugia in Woodland Caribou (Rangifer tarandus caribou)  

PubMed Central

Glacial refugia considerably shaped the phylogeographical structure of species and may influence intra-specific morphological, genetic, and adaptive differentiation. However, the impact of the Quaternary ice ages on the phylogeographical structure of North American temperate mammalian species is not well-studied. Here, we surveyed ?1600 individuals of the widely distributed woodland caribou (Rangifer tarandus caribou) using mtDNA control region sequences to investigate if glacial refugia contributed to the phylogeographical structure in this subspecies. Phylogenetic tree reconstruction, a median-joining network, and mismatch distributions supported postglacial expansions of woodland caribou from three glacial refugia dating back to 13544–22005 years. These three lineages consisted almost exclusively of woodland caribou mtDNA haplotypes, indicating that phylogeographical structure was mainly shaped by postglacial expansions. The putative centres of these lineages are geographically separated; indicating disconnected glacial refugia in the Rocky Mountains, east of the Mississippi, and the Appalachian Mountains. This is in congruence with the fossil record that caribou were distributed in these areas during the Pleistocene. Our results suggest that the last glacial maximum substantially shaped the phylogeographical structure of this large mammalian North American species that will be affected by climatic change. Therefore, the presented results will be essential for future conservation planning in woodland caribou.

Klutsch, Cornelya F. C.; Manseau, Micheline; Wilson, Paul J.

2012-01-01

249

Glacial climate in the tropics  

SciTech Connect

New findings have caused ideas about the Earth`s climate during the Pleistocene glaciation to change. A consensus seems to be forming that during times of glaciation, climatic conditions in the tropics were quite different from those today. However still to be explained is why strontium-calcium measurements on corals and moble gas measurements of ground water suggest a tropical cooling of 4-6 C while foraminiferal speciation, oxygen isotope, and alkenone results suggest a cooling of no more than 3 C. This article discusses different aspects of the debate. 9 refs., 1 fig.

Broecker, W. [Columbia Univ., Palisades, NY (United States)

1996-06-28

250

Pleistocene vegetation change in central Africa recorded off the Congo River  

NASA Astrophysics Data System (ADS)

Marine sediments from the Congo Canyon accumulate material and information from one of Africa's large inland basins that is covered, at present, mainly with lowland rain forest and swamp forests. The sediments provide a record of monsoon related hydrological and vegetation change. Multiproxy studies were carried out on ODP Site 1075 located north of the undersea canyon, and GeoB Site 1008 located south of the canyon. Pollen records are compared with other terrestrial signals (iron, clay minerals, biomarkers). The differentiated responses of mangroves, grasslands and swamps, lowland rain forest, and Afromontane forest to environmental fluctuations give insight in several aspects of Pleistocene climate cycles. The record of Rhizophora (mangrove tree) pollen is consolidated by biomarker data and shows maxima during interglacial periods and during periods of rapid sea-level rise. While the latter might be the effect of increased erosion of mangrove peat, the first indicates extension of mangrove swamps during periods with increased run-off. The record of lowland forest pollen indicates extension of the rain forest as a response to increased precipitation in periods of strong monsoons of the past 150 ka which is corroborated by clay mineral fluctuations. During the humid periods, Poaceae (grasses) and Cyperaceae pollen percentages are low indicating a closed canopy in large areas of the basin. During interglacial stages of the early Pleistocene, maxima of tropical forest elements combine with maxima of grass and cyperaceous pollen indicating that the warm periods might have been drier than those of the late Pleistocene. Podocarpus pollen percent maxima register extension of the Afromontane forest during cool periods from 1.05 to 0.6 Ma. Restricted distribution of mountainous forest during the late Pleistocene glacial stages (MIS 6, 4-2) is concurrent with extension of open vegetation types indicating more arid conditions in equatorial areas.

Dupont, L. M.

2003-04-01

251

Caribbean Salinity Variation During the Last Glacial Cycle  

NASA Astrophysics Data System (ADS)

Evaporation exceeds precipitation in the tropical Atlantic, resulting in a net freshwater removal across the Central American Isthmus. Because most of the north Atlantic's subtropical gyre water circulates through the Caribbean before flowing north to sub-polar regions via the Gulf Stream, changes in tropical atmospheric circulation have the potential to affect the salinity and density structure of the entire north Atlantic, thereby influencing glacial-interglacial oscillations in North Atlantic Deep Water (NADW) formation. Here, we combine Mg/Ca measurements (a proxy for the temperature of calcification) and ? 18O analyses of shells from the surface-dwelling foraminifera Globigerinoides ruber s.s. (white var.) from the western Caribbean Colombian Basin at ODP Site 999A (2827m; 4cm/ka sed. rate) and VM28-122 (3623m; 4-10cm/ka sed. rate) to produce the first continuous record of western tropical Atlantic ? 18OSEAWATER (? 18OSW) during the last 130ka. In order to generate a record for sea surface salinity (SSS) due to regional hydrological change, we removed the ? 18OSW signal due to glacial ice volume variation and normalized the residual to the modern ? 18OSW value for the Colombian Basin (0.8‰ ). The resulting ice volume-free (? ? 18OIVF-SW) record shows that Caribbean ? ? 18OIVF-SW increased by ˜0.5‰ during the Last Glacial Maximum and Marine Isotope Stage 4. Using a modern western Caribbean ? 18OSW:SSS relationship, these enriched ? 18OSW values suggest glacial Caribbean salinities were 2.3 - 2.8‰ higher than modern after removing the influence of ice-volume. Our data supports the hypothesis that the tropics might have been in a state more similar to the modern El Nino mode, characterized by a more southerly position of the ITCZ, during cold phases of the last glacial cycle. Within the resolution of our ? ? 18OIVF-SW record from VM28-122, elevated glacial Caribbean salinity decreased to modern levels at the onset of the Bolling-Allerod (B/A) interstadial event (14.6 cal ka) as NADW formation reinitiated and the ITCZ migrated northward. We hypothesize that the transport of salty tropical waters into the North Atlantic during the early deglaciation, in combination with a reorganization of ocean circulation, may have served to increase subpolar surface water density and amplify the overturning circulation that produced the warm B/A. The B/A salinity reduction occurs simultaneously with previous evidence for wetter conditions in the Cariaco Basin, suggesting a northward shift in the ITCZ to a wetter, more La Nina-like state in the tropics.

Schmidt, M. W.; Spero, H. J.; Lea, D. W.

2003-12-01

252

Phylogeography of the mitten crab Eriocheir sensu stricto in East Asia: Pleistocene isolation, population expansion and secondary contact.  

PubMed

We examined the impact of Pleistocene glacial cycles on geographical distribution and genetic structure of the mitten crab Eriocheir sensu stricto in East Asia using sequence variation of mitochondrial cytochrome c oxidase I and cytochrome b gene segments. Phylogenies revealed four distinct but shallow structured lineages in Eriocheir s. s. Three lineages dominated the East China Sea-Yellow Sea, the Sea of Japan and the South China Sea on the margins of the region, and one lineage occurred on Okinawa Island. This geographical distribution represents a general phylogeographic pattern in East Asia, which is closely associated with the fluctuations of marginal seas and islands during the Pleistocene. The four lineages are estimated to have diverged during the mid-Pleistocene. Demographic expansions were observed in each lineage, starting within the second-to-latest interglacial period in the marginal sea lineages ( approximately 70-130ka) and within the last glacial period in the Okinawa lineage ( approximately 25-80ka). Expansions have probably taken place northward along the coast of the East China Sea-Yellow Sea, following the rise of sea levels. Centered on the southern Korean Peninsula, expansions have likely occurred northward along the west coast and eastward along the south coast of the Sea of Japan. Each marginal sea has served as a single refugium during glacial periods. Two secondary contact regions were identified, one of the East China Sea-Yellow Sea and South China Sea lineages, and another of the East China Sea-Yellow Sea and Sea of Japan lineages. Phylogeography of Eriocheir s. s. provides insights into the evolutionary history and mechanism for generating biodiversity in East Asia. PMID:19236929

Xu, Jiawu; Chan, Tin-Yam; Tsang, Ling Ming; Chu, Ka Hou

2009-07-01

253

Evolution of Mustang Graven, Tibet Himalayas, due to Eastward Extrusion of Tibet Plateau in and After the Last Glacial Age  

NASA Astrophysics Data System (ADS)

This study clarifies neotectonics and evolution of Mustang Graven due to eastward extrusion of the Tibet plateau in and after the Last Glacial Age. Mustang Graven is a NS trend depression and is located in Tibet Himalayan zone just behind the Higher Nepal Himalayas. Its average height is 4000 meters. Its width and length are less than ten kilometers and more than fifty kilometers respectively. This study depends on interpretation of aerial photographs in scale of 1/50,000 over the Mustang Graven and field survey carried out in Sept. 2002. A distinct topographic contrast occurs along a mount foot line between the graven and the surrounding mountains, the Tibet Himalayas of 5000-7000 meters asl. Fault scarplets on moraines and fan surfaces, which developed as outwash plains in and after the Last Glacial Age, are traceable along the western foot line. Tectonic deformation of the topographic surfaces are cumulative and five to ten meters in relative height . Sense of the faults is normal downthrowing to the east. Valley side fault in a normal sense is also found in the Thakkola formation, the Plio-Pleistocene sediment, near Dhakmar village. Deformation of the Thakkola Formation is more than fifty meters. Such phenomenon indicate that Mustang Graven has been formed by a tensile stress field of EW direction and evolved also in and after the Last Glacial Age. This implies the extrusion of the Tibet Plateau has been continuing throughout the late Pleistocene and to the Holocene.

Yagi, H.; Maemoku, H.; Dangol, V.; Kumahara, Y.; Nakata, T.

2003-12-01

254

Late Glacial-Holocene climatic transition record at the Argentinian Andean piedmont between 33 and 34° S  

NASA Astrophysics Data System (ADS)

The Arroyo La Estacada (~ 33°28' S, 69°02' W), eastern Andean piedmont of Argentina, cuts through an extensive piedmont aggradational unit composed of a dominant Late Pleistocene-early Holocene (LP-EH) alluvial sequence that includes several paleosols. One of these paleosols developed affecting the topmost part of likely Late Glacial aeolian deposits aggraded into a floodplain environment by the end of the Late Pleistocene. The paleosol shows variable grade of development along the arroyo outcrops. Organic matter humification, carbonate accumulation and redox processes were the dominant processes associated with paleosol formation. By the early Holocene, when the formation of the paleosol ended, renewed alluvial aggradation and high magnitude flooding events affected the arroyo's floodplain environment. Accordignly, a period of relative landscape stability in the Arroyo La Estacada basin is inferred from the paleosol developed by the LP-EH transition in response to the climatic conditions in the Andes cordillera piedmont after the Late Glacial arid conditions. The analyzed Late Glacial-Holocene alluvial record of the Andean piedmont constitutes a suitable record of the LP-EH climatic transition in the extra-Andean region of Argentina. It is in agreement with regional paleoclimatic evidence along the southern tip of the South American continent, where other pedosedimentary sequences record similar late Quaternary paleoenvironmental changes over both fluvial and interfluvial areas.

Mehl, A. E.; Zárate, M. A.

2014-04-01

255

Dynamics of the late Plio-Pleistocene West Antarctic Ice Sheet documented in subglacial diamictites, AND-1B drill core  

NASA Astrophysics Data System (ADS)

Geologic studies of sediment deposited by glaciers can provide crucial insights into the subglacial environment. We studied muddy diamictites in the ANtarctic geological DRILLing (ANDRILL) AND-1B drill core, acquired from beneath the Ross Ice Shelf in McMurdo Sound, with the aim of identifying paleo-ice stream activity in the Plio-Pleistocene. Glacial advances were identified from glacial surfaces of erosion (GSEs) and subglacial diamictites within three complete sequences were investigated using lithofacies associations, micromorphology, and quartz sand grain microtextures. Whereas conditions in the Late Pliocene resemble the modern Greenland Ice Sheet where fast flowing glaciers lubricated by surface meltwater terminate directly in the sea (interval 201-212 mbsl) conditions in the Late Pleistocene are similar to modern West Antarctic Ice Sheet (WAIS) ice streams (38-49 mbsl). We identify the latter from ductile deformation and high pore-water pressure, which resulted in pervasive rotation and formation of till pellets and low relief, rounded sand grains dominated by abrasion. In the transitional period during the Mid-Pleistocene (55-68 mbsf), a slow moving inland ice sheet deposited tills with brittle deformation, producing lineations and bi-masepic and unistrial plasma fabric, along with high relief, conchoidally fractured quartz grains. Changes in the provenance of gravel to cobble-size clasts support a distant source area of Byrd Glacier for fast-flowing paleo-ice streams and a proximal area between Darwin and Skelton Glaciers for the slow-moving inland ice sheet. This difference in till provenance documents a shift in direction of glacial flow at the core site, which indirectly reflects changes in the size and thickness of the WAIS. Hence, we found that fast ice streaming motion is a consequence of a thicker WAIS pushing flow lines to the west and introducing clasts from the Byrd Glacier source area to the drill site. The detailed analysis of diamictites in AND-1B demonstrates that Pliocene glacial intervals were warmer than in the Pleistocene when polar ice sheets grew from local inland ice to regional ice streams.

Cowan, Ellen A.; Christoffersen, Poul; Powell, Ross D.; Talarico, Franco M.

2014-08-01

256

Millennial- to decadal-scale fluctuation in the paleo-Kuroshio Current documented in the Middle Pleistocene shelf succession on the Boso Peninsula, Japan  

NASA Astrophysics Data System (ADS)

Millennial-, century-, and decadal-scale cyclicities were identified in a transgressive shelf succession of the Middle Pleistocene (ca. 0.7 Ma) on the Boso Peninsula, Japan. The shelf succession consists mainly of a sandridge complex and associated outer shelf deposits. The deposition was interpreted to have been controlled by fluctuation in speeds and paths of the paleo-Kuroshio Current. In general, the modern Kuroshio Current approaches the southern coasts of the Boso Peninsula in response to its large meander that is interpreted to respond to the El Niño episodes. Depositional cycles in the Kuroshio-Current-controlled shelf succession are interpreted to document high-frequency paleoclimatic oscillation in the equatorial Pacific region during the Middle Pleistocene time. The finding indicates that high-frequency paleoclimatic instability, that has been strongly expressed in the Upper Pleistocene successions in the North Atlantic region, was pervasive in time and space back to the Middle Pleistocene.

Ito, M.; Horikawa, K.

2000-12-01

257

Glacial geomorphology of the Torres del Paine region (southern Patagonia): Implications for glaciation, deglaciation and paleolake history  

NASA Astrophysics Data System (ADS)

The processes affecting paleoclimate variability and Pleistocene glacial landscape development in the southern mid-latitudes remain poorly understood, in part because of the scarcity of comprehensive, well-studied records. Glacial landforms are invaluable for reconstructing past ice-sheet, climate, and associated environmental changes along the southern Andes, but there are significant spatial and temporal gaps in existing data. In this paper, we present new geomorphic and sedimentologic analyses, including surficial maps, for the Torres del Paine region (51°S, 73°W), southern South America. Our findings provide a new framework for understanding changes in the regional glacier history and Pleistocene landscape development. Glacial extent during the local last glacial maximum (LGM) remains unknown but new chronological data supported by geomorphic evidence afford evidence for a larger ice sheet at Torres del Paine than previously assumed. Deglaciation from the local LGM was underway by 17,400 ± 200 (1?) cal. yr. BP. As opposed to previous suggestions, we have found that most of the moraines fringing the lakes in the Torres del Paine national park were deposited during a late-glacial expansion that occurred between 14,100 and 12,500 cal. yr. BP. Late-glacial advances also have been documented recently for the Última Esperanza and Lago Argentino basins to the south and north of Torres del Paine, respectively, suggesting an overall regional ice response to a climate signal. The Tehuelche paleolake accompanied each of the ice-sheet fluctuations in Torres del Paine. New data document at least three main phases of this paleolake, which drained eastward to the Atlantic Ocean, while the Andes gaps were blocked with ice. During the late phase of glacial lake formation, when water levels reached 125-155 m a.s.l., the lake likely merged with paleolake Consuelo in the Última Esperanza area at the end of the last glaciation. Lake Tehuelche in Torres del Paine had drained into the Pacific Ocean by the late-glacial period, suggesting that ice southwest of Torres del Paine may have retreated back into the mountains by this time.

García, Juan-Luis; Hall, Brenda L.; Kaplan, Michael R.; Vega, Rodrigo M.; Strelin, Jorge A.

2014-01-01

258

Orbital- to Sub-Orbital-Scale Cyclicity in Seismic Reflections and Sediment Character in Early to Middle Pleistocene Mudstone, Santa Barbara Basin, California  

NASA Astrophysics Data System (ADS)

High-resolution seismic reflection records and well logs from the Santa Barbara Channel suggest that large parts of the Pleistocene succession records climate variability on orbital to sub-orbital scales with remarkable sensitivity, much like the well-studied sediments of the last glacial cycle (ODP Site 893). Spectral analysis of seismic reflection data and gamma ray logs from stratigraphically similar Pleistocene sections finds similar cyclic character and shifts through the section. This correlation suggests that acoustic impedance and physical properties of sediment are linked by basin-scale, likely climatically-driven, oscillations in lithologic composition and fabric during deposition, and that seismic profiling can provide a method for remote identification and correlation of orbital- and sub-orbital-scale sedimentary cyclicity. Where it crops out along the northern shelf of the central Santa Barbara Channel, the early to middle Pleistocene succession (~1.8-1.2 Ma) is a bathyal hemipelagic mudstone with remarkably rhythmic planar bedding, finely laminated fabric, and well-preserved foraminifera, none of which have been significantly altered, or obscured by post-depositional diagenesis or tectonic deformation. Unlike the coarser, turbiditic successions in the central Ventura and Los Angeles basins, this sequence has the potential to record Quaternary global climate change at high resolution. Seismic reflection data (towed chirp) collected on the R/V Melville 2008 Cruise (MV08) penetrate 10's of meters below seafloor into a ~1 km-long sequence of south-dipping seismic reflectors. Sampling parallel to the seafloor permits acquisition of consistent signal amplitude for similar reflectors without spreading loss. Based on established age ranges for this section, sedimentation rates may range from 0.4 to 1.4 meters/kyr, therefore suggesting that the most powerful cycles are orbital- to sub-orbital-scale. Discrete sets of cycles with high power show an abrupt shift to shorter wavelengths midway through the section. Deep in the section, the strongest cycles indicated by spectral analysis are 50 and 16 meters thick, whereas up section, the strongest cycles are 20 and 12 meters thick. Nearby industry boreholes that penetrate a stratigraphically similar, 1500-meter-thick mudstone section, provide logs of natural gamma ray intensity with a higher sample interval (15 cm), allowing resolution and analysis of even higher frequency lithologic cycles. The strongest cycle deep in the section is 100 meters thick, and up section, the strongest cycle is 12 meters thick. This abrupt decrease in dominant cycle thickness midway through both the seismic and gamma ray records perhaps indicates a basin-wide shift in sedimentation. With improved chronostratigraphy based on Sr-isotope ratios and biostratigraphy, and comparison with paleoclimate proxy data, we will test if seismically resolved lithologic oscillations can be reliably interpreted as representing climatically driven Milankovitch cycles. This method may be used to evaluate the age and paleoceanographic potential of sedimentary strata before a coring vessel is deployed.

Peterson, C. D.; Behl, R. J.; Nicholson, C.; Lisiecki, L. E.; Sorlien, C. C.

2009-12-01

259

Paleotopography of glacial-age ice sheets  

SciTech Connect

This is technical comment and response to the subject of paleotophography of glacial age ice sheets. The model presented by Peltier reconstructing the paleotopography of glacial age ice sheets has implications for atmospheric general circulation models of ice age climate. In addition, the model suggests that the glacial-age Antarctic Ice Sheet was significantly larger than today`s. The commentor, Edwards, suggests there is a discrepancy between data from Papua New Guinea and the model results.

Edwards, R.L. [Univ. of Minnesota, Minneapolis, MN (United States)

1995-01-27

260

Migration of the subtropical front as a modulator of glacial climate.  

PubMed

Ice cores extracted from the Antarctic ice sheet suggest that glacial conditions, and the relationship between isotopically derived temperatures and atmospheric PCO(2) have been constant over the last 800,000 years of the Late Pleistocene epoch. But independent lines of evidence, such as the extent of Northern Hemisphere ice sheets, sea level and other temperature records, point towards a fluctuating severity of glacial periods, particularly during the more extreme glacial stadials centred around 340,000 and 420,000 years ago (marine isotope stages 10 and 12). Previously unidentified mechanisms therefore appear to have mediated the relationship between insolation, CO(2) and climate. Here we test whether northward migration of the subtropical front (STF) off the southeastern coast of South Africa acts as a gatekeeper for the Agulhas current, which controls the transport of heat and salt from the Indo-Pacific Ocean to the Atlantic Ocean. Using a new 800,000-year record of sea surface temperature and ocean productivity from ocean sediment core MD962077, we demonstrate that during cold stadials (particularly marine isotope stages 10 and 12), productivity peaked and sea surface temperature was up to 6 degrees C cooler than modern temperatures. This suggests that during these cooler stadials, the STF moved northward by up to 7 degrees latitude, nearly shutting off the Agulhas current. Our results, combined with faunal assemblages from the south Atlantic show that variable northwards migration of the Southern Hemisphere STF can modulate the severity of each glacial period by altering the strength of the Agulhas current carrying heat and salt to the Atlantic meridional overturning circulation. We show hence that the degree of northwards migration of the STF can partially decouple global climate from atmospheric partial pressure of carbon dioxide, P CO(2), and help to resolve the long-standing puzzle of differing glacial amplitudes within a consistent range of atmospheric PCO(2). PMID:19606147

Bard, Edouard; Rickaby, Rosalind E M

2009-07-16

261

Glacially conditioned specific stream powers in low-relief river catchments of the southern Laurentian Great Lakes  

NASA Astrophysics Data System (ADS)

Fluvial systems of the southern Laurentian Great Lakes region are carved into a complex glacial landscape shaped by continental ice and meltwater of the late Pleistocene. These glacially conditioned river catchments are typically small with drainage areas < 104 km2. A 10-m digital elevation model (DEM) is used to map the spatial distribution of stream gradient for 22 major river catchments of peninsular southern Ontario, which drain to base levels in the lower Great Lakes (Huron, St. Clair, Erie, and Ontario). Raw data from the DEM show stream gradients that exhibit multiscale variance from real and from artifact sources. Based on a vertical slice and multiple-pass moving-window averaging approach, slope data are generalised to the river reach scale (1-2 km) as a representative spatial scale for fluvial processes operating over Holocene timescales. Models of specific stream power are then compared with glacial landform and surface geology mapping. Inherited glacial signatures in river slope appear as deviations in a stream length-gradient index (SL/K index), where river reaches are frequently oversteepened or understeepened. Based on a slope-area analysis, and complementary to theories of channel pattern discrimination, constant stream power curves (with power-law exponent of - 0.4) provide a first-order approach to stratify river reaches in terms of glacial conditioning and expected planform morphologies. However, multiple-channel planform types are rare and localised in southern Ontario, indicating that oversteepened reaches with high stream powers may often be moderated by (1) sediment calibre, with cobble-beds from inherited glacial sediments; and/or (2) relative bank strength, with limited channel widening particularly in gravel and sand-bed channels. Further discrimination of glacially conditioned fluvial process domains will ultimately require consideration of alluvial floodplain characteristics in addition to general observations of river morphology and channel pattern.

Phillips, R. T. J.; Desloges, J. R.

2014-02-01

262

Connecting Soils and Glacial Geology  

NSDL National Science Digital Library

The goal of this activity is to provide students an opportunity to connect soil science to surficial geology by using a Soil Surveys. By the end of the activity, students should be able to use a Soil Survey to identify and interpret landforms and surficial features. This activity can be adapted to variety of process (ex. eolian deposits, glacial deposits, bedrock weathering, etc.). County-level soil surveys are available in both paper and online formats for the majority of the United States. Designed for a geomorphology course Has minimal/no quantitative component

Dolliver, Holly

263

Comparison of Late Pleistocene and Modern Glacier Extents in Central Nepal Based on Digital Elevation Data and Satellite Imagery  

NASA Astrophysics Data System (ADS)

Late Pleistocene and modern ice extents in central Nepal are compared to estimate equilibrium line altitude (ELA) depressions. New techniques are used for determining the former extent of glaciers based on quantitative, objective geomorphic analyses of a ˜90-m resolution digital elevation model (DEM). For every link of the drainage network, valley form is classified as glacial or fluvial based on cross-valley shape and slope statistics. Down-valley transitions from glacial to fluvial form indicate the former limits of glaciation in each valley. Landsat Multispectral Scanner imagery for the same region is used to map current glacier extents. For both full-glacial and modern cases, ELAs are computed from the glacier limits using the DEM and a toe-to-headwall altitude ratio of 0.5. Computed ELA depressions range from 100-900 m with a modal value of ˜650 m and a mean of ˜500 m, values consistent with previously published estimates for the central Himalaya but markedly smaller than estimates for many other regions. We suggest that this reflects reduced precipitation, rather than a small temperature depression, consistent with other evidence for a weaker monsoon under full-glacial conditions.

Duncan, Christopher C.; Klein, Andrew J.; Masek, Jeffrey G.; Isacks, Bryan L.

1998-05-01

264

Pleistocene history of the subarctic pacific: periodic and step-wise changes in temperature and precipitation  

SciTech Connect

Piston core V20-110 records the last 1.8 Ma of North Pacific conditions. Samples at 11 Ka intervals were analyzed for calcite, foraminifera, diatoms, and IRD. Data implies that precipitation varied on a 41-Ka cycle from latest Pliocene to 700 Ka. During the late Pleistocene precipitation, like temperature, has been dominated by a 100 Ka period. There are six distinct intervals, bounded by rapid, unidirectional changes: a) 1.8-1.6 Ma-mostly ice-free, warm (approx.15/sup 0/C), high precipitation, moderately well-mixed waters; b) 1.6-1.3 Ma-slightly cooler (approx.12/sup 0/C), precipitation increasing to maximum, waters well mixed; c) 1.3-1.1 Ma-change to winter precipitation, slightly colder, increased seasonal contrast. d) 1.1 Ma-700 Ka-beginning of glacial mode; periods of high annual precipitation and strong stratification alternate with lower precipitation and more mixing, temperatures cool (approx.10/sup 0/C); e) 700-300 Ka-strong 100-ka cycles, high winter precipitation and low temperatures (5-10/sup 0/C) during glacials; interglacials drier and warmer, more mixing; summer precipitation low throughout; f) 300-0 Ka-glacial maxima cold (<5/sup 0/C), dry, well-mixed; interglacial maxima cool (approx.10/sup 0/C), summer precipitation, well-mixed; transitions high winter precipitation and strong vertical stratification. CCD fluctuating close to 2700 m, being above during glacials and transitions, below only during peak interglacials.

Sancetta, C.

1985-01-01

265

Glacitectonic rafting and associated deformation of mid-Pleistocene glacigenic sediments, near Central Graben, central North Sea; results of a 2D High-Resolution Geophysical Survey  

NASA Astrophysics Data System (ADS)

Glacitectonic rafts are defined as dislocated slabs of bedrock or unconsolidated sediments, transported from their original position by glacial action. These relatively thin, slab-like bodies feature transport distances ranging from tens of meters to hundreds of kilometers. They occur as either single rafts, or multiple stacked bodies associated with a variety of ice-pushed landforms. Internally, rafts frequently appear undeformed although at a larger scale, they may be folded or cut by shear zones and brittle faults. However, the processes leading to the detachment, transport and subsequent emplacement of the rafts remain uncertain. This work describes the results of a geophysical 2D seismic survey of thrust-bound glacitectonic rafts and associated deformation structures, occurring within mid-Pleistocene glacigenic sediments of the Central Graben, central North Sea. The total shortened length of the rafted section is 2.4km, comprising a series of nine discrete rafts which individually range from 235m to 1018m in length. The principle basal detachment occurs at the erosive contact between Aberdeen Ground Formation and overlying Ling Bank Formation. The ice-proximal (northern) limit of rafting is defined by the presence of a large-scale palaeo-channel oriented perpendicular to the direction of rafting, composed of sediments of the Ling Bank Formation and the Forth Formation. The observed deformation structures infer a mean tectonic direction of 178°, indicating that they are associated with an active glacial advance from the north. The resulting deformation creates a minimum lateral shortening throughout the observed sequence of 35%, typifying a strongly compressional regieme associated with rafting. Throughout the surveyed area, structurally younger rafts are found to be emplaced towards the south, compared to the structurally older rafts which are emplaced towards the south-east. This distinction is suggested to be caused by early rafts creating an obstacle to transport for later stages of deformation, resulting in strike-slip basal detachment being associated with the later rafts. Localised distributions of high amplitude surfaces located adjacent to the primary detachment surface are identified through amplitude extraction techniques. These are indicative of migration and collection of gas along the inclined lower surfaces of rafted blocks. They represent a gas risk for drilling operations and demonstrate the significance and possible hazards of glacitectonic deformation to the exploration industry. A model for raft detachment and emplacement is proposed whereby; i) saturated sediments within the palaeo-channel are subject to pressurisation associated with overburden caused by over-riding ice, ii) elevated pore-water pressure develops along the principle detachment surface of the rafts, iii) early stages of deformation consist of ice-distal (southern) blocks becoming emplaced at relatively low angles of inclination, iv) with more proximal blocks accumulating as an imbricate thrust-stack sequence at relatively high angles of elevation. This interpretation suggests a significant subglacial hydrological control upon raft detachment and transport, with fluctuations between an extensional and compressive deformation regime caused by a switch from actively advancing glacial conditions to an oscillating ice-margin at this location. Tectono-stratigraphic evidence indicates that rafting occurring throughout the site is likely to be associated with a glacial advance of the Anglian (MIS 12).

Vaughan-Hirsch, David

2013-04-01

266

Development of a glacially dominated shelf-slope-fan system in tectonically active southeast Alaska: Results of IODP Expedition 341 core-log-seismic integrated studies at glacial cycle resolution  

NASA Astrophysics Data System (ADS)

Collision of the Yakutat microplate with North American formed the St. Elias Mountains in coastal Gulf of Alaska. While the tectonic driver for orogenesis has been ongoing since the Miocene, results from the Integrated Ocean Drilling Program Expedition 341 suggests that direct climatic perturbation of active orogenesis through glacial erosion is non-linear. Geophysical studies of the glaciated continental margin, slope, and adjacent deep-sea Surveyor Fan allow examination of the glaciated orogen from source to sink. Using high-resolution and crustal-scale seismic data and through comparison with other glaciated margins, we can identify key diagnostic seismic morphologies and facies indicative of glacial proximity and sediment routing. Expedition drilling results calibrated these images suggesting a timeline for initial advances of the Cordilleran ice sheet related glacial systems onto the shelf and a further timeline for the development of ice streams that reach the shelf edge. Comparisons can be made within this single margin between evolution of the tectonic-glacial system where erosion and sediment transport are occurring within a fold and thrust belt versus on a more stable shelf region. Onshore the Bering-Bagley glacial system in the west flows across the Yakataga fold and thrust belt, allowing examination of whether glacial erosion can cause tectonic feedbacks, whereas offshore the Bering-Bagley system interacts with the Pamplona Zone thrusts in a region of significant sediment accommodation. Results from Expedition 341 imply that timing of glacial advance to the shelf edge in this region may be driven by the necessity of filling up the accommodation through aggradation followed by progradation and thus is autogenic. In contrast the Malaspina-Hubbard glacial system to the east encountered significantly less accommodation and more directly responded to climatic forcing including showing outer shelf glacial occupation since the mid-Pleistocene transition-MPT to 100 kyr glacial-interglacial cycles. Examination of the sink for both of these systems, which includes the Surveyor Fan and Aleutian Trench wedge, demonstrates a clear climatic driver for sediment flux to the deep sea. The first appearance of ice-rafted debris at our distal drill site closely approximates the start of the Pleistocene and a doubling of sediment accumulation accompanies the MPT. Converting sediment volumes just within the deep-sea sinks back to erosion rates in the orogen and correlating with changes in exhumation rates from thermochronology demonstrates a lack of accelerated tectonic response to the intensification of Northern Hemisphere glaciations at the start of the Pleistocene but increased shortening and exhumation of sediments at the MPT. The form of tectonic response differs between out-of-sequence thrusting or antiformal stacking within the fold and thrust belt to the west and a near vertical advection of material in a tectonic aneurysm in the core of the orogen to the east.

Gulick, Sean; Jaeger, John; Mix, Alan; Swartz, John; Worthington, Lindsay; Reece, Robert

2014-05-01

267

Possible solar origin of the 1,470-year glacial climate cycle demonstrated in a coupled model  

Microsoft Academic Search

Many palaeoclimate records from the North Atlantic region show a pattern of rapid climate oscillations, the so-called Dansgaard-Oeschger events, with a quasi-periodicity of ~1,470years for the late glacial period. Various hypotheses have been suggested to explain these rapid temperature shifts, including internal oscillations in the climate system and external forcing, possibly from the Sun. But whereas pronounced solar cycles of

Holger Braun; Marcus Christl; Stefan Rahmstorf; Andrey Ganopolski; Augusto Mangini; Claudia Kubatzki; Kurt Roth; Bernd Kromer

2005-01-01

268

Glacial effects limiting mountain height.  

PubMed

The height of mountain ranges reflects the balance between tectonic rock uplift, crustal strength and surface denudation. Tectonic deformation and surface denudation are interdependent, however, and feedback mechanisms-in particular, the potential link to climate-are subjects of intense debate. Spatial variations in fluvial denudation rate caused by precipitation gradients are known to provide first-order controls on mountain range width, crustal deformation rates and rock uplift. Moreover, limits to crustal strength are thought to constrain the maximum elevation of large continental plateaus, such as those in Tibet and the central Andes. There are indications that the general height of mountain ranges is also directly influenced by the extent of glaciation through an efficient denudation mechanism known as the glacial buzzsaw. Here we use a global analysis of topography and show that variations in maximum mountain height correlate closely with climate-controlled gradients in snowline altitude for many high mountain ranges across orogenic ages and tectonic styles. With the aid of a numerical model, we further demonstrate how a combination of erosional destruction of topography above the snowline by glacier-sliding and commensurate isostatic landscape uplift caused by erosional unloading can explain observations of maximum mountain height by driving elevations towards an altitude window just below the snowline. The model thereby self-consistently produces the hypsometric signature of the glacial buzzsaw, and suggests that differences in the height of mountain ranges mainly reflect variations in local climate rather than tectonic forces. PMID:19675651

Egholm, D L; Nielsen, S B; Pedersen, V K; Lesemann, J-E

2009-08-13

269

The Mid-Pliocene sea-level conundrum: Glacial isostasy, eustasy and dynamic topography  

NASA Astrophysics Data System (ADS)

Determining eustatic sea level during the Mid-Pliocene warm period (?3.3 to 2.9 Ma) has been a central but elusive goal in the study of past warm climates. Estimates of eustatic sea level based on geologic data span a broad range; variation that we now recognize is due in part to geographically varying post-depositional displacement caused by glacial isostatic adjustment and dynamic topography. In this study, we combine field observations and glacial isostatic adjustment modeling to estimate the dynamic topography signal in three areas that are important to paleo-sea level studies of the Mid-Pliocene warm period (South Africa, West Australia and southeastern United States). We show that dynamic topography played a significant role in the post-depositional displacement of Pliocene, and even younger Pleistocene, shorelines. In this regard, we provide a robust paleo-sea level elevation data set, corrected for glacial isostatic adjustment, that can be used to evaluate predictions from mantle flow models of dynamic topography.

Rovere, A.; Raymo, M. E.; Mitrovica, J. X.; Hearty, P. J.; O'Leary, M. J.; Inglis, J. D.

2014-02-01

270

Combining cosmogenic radionuclides and amino acid racemization to date late Pliocene glacial deposits exposed on Baffin Island, Nunavut, Canada  

NASA Astrophysics Data System (ADS)

Sequences of glacial deposits spanning the Quaternary are valuable archives recording the effects of glaciation on landscapes through time, but determining the age of such deposits has long challenged geologists. The recent advances in cosmogenic radionuclide (CRN) measurement has made it possible to date some of these deposits, but dating buried glacial sediments in most settings remains problematic. Here we explore a new approach to date the oldest glacial deposits in the Plio-Pleistocene Clyde Foreland Formation of Baffin Island. This formation, approximately 40 m thick, includes interlayered shell-bearing marine, glaciomarine, and glacial sediments deposited along the northern margin of the Laurentide Ice Sheet and earlier continental ice sheets. Previous work on foraminifera assemblages suggests that the deposits span the last ?2 Ma. By combining CRN measurements (10Be and 26Al) from the glacial units and measurements of the D-alloisoleucine:L-isoleucine ratios (A/I) in valves of the mollusk Hiatella arctica in the marine units overlying a particular glacial deposit, we can calculate the age of the glacial deposit. Because the post-burial temperature history for the mollusks preserved in the Clyde Foreland Formation is poorly constrained, A/I ratios alone cannot be used to determine absolute ages. Instead, we use A/I ratios to identify sediment packages of discrete ages and define a step-wise burial history function for glacial units. A/I ratios of all packages (<0.3 for the total hydrolysate fraction) fall within the A/I interval characterized by linear racemization kinetics, so the age of each package in the burial history function can simply be defined as a fractional age with respect to the total burial age for the glacial deposit of interest. The long duration of burial (26Al/10Be as low as 1.6±0.6 at 2?) and low initial CRN inventories require that post-burial muogenic production is accounted for using the burial history function. We apply a numerical model to calculate the duration of burial from the measured CRN concentrations for a given inherited CRN inventory. But because this initial inventory is unknown, a single CRN sample/burial history combination will not provide a unique age solution. Instead, measurements from multiple localities where a particular glacial deposit has differing burial histories (i.e., the thickness of overlying units or ages of overlying units differ) are required to statistically determine the total burial age that most closely matches the observed CRN inventories and burial histories.

Refsnider, K. A.; Miller, G. H.

2009-12-01

271

Maximum glacial advance and deglaciation of the Pinar Valley (Sierra de Gredos, Central Spain) and its significance in the Mediterranean context  

NASA Astrophysics Data System (ADS)

Pinar Valley is located on the north side of Galana Peak (40° 15' 21? N; 5° 18' 00? W; 2564 m a.s.l.), Sierra de Gredos, in the center of the Iberian Peninsula. Surface exposure ages based on the in situ accumulation of 36Cl were obtained from six moraine boulders and from three bedrock outcrops to investigate the timing of both the last local glacial maximum and the deglaciation. The oldest moraines, which probably overrode older glacial deposits, are coeval to the global Last Glacial Maximum. Subsequently the Pinar glacier underwent minor pulsations close to its maximum position, followed by general glacier retreat after ~ 17-16 ka. Small cirque glaciers probably remained during the terminal Pleistocene but they had completely disappeared by 10 ka.

Palacios, David; Andrés, Nuria; Marcos, Javier; Vázquez-Selem, Lorenzo

2012-12-01

272

Ice-rafting from the British-Irish ice sheet since the earliest Pleistocene (2.6 million years ago): implications for long-term mid-latitudinal ice-sheet growth in the North Atlantic region  

NASA Astrophysics Data System (ADS)

The Plio-Pleistocene intensification of Northern Hemisphere continental ice-sheet development is known to have profoundly affected the global climate system. Evidence for early continental glaciation is preserved in sediments throughout the North Atlantic Ocean, where ice-rafted detritus (IRD) layers attest to the calving of sediment-loaded icebergs from circum-Atlantic ice sheets. So far, Early-Pleistocene IRD deposition has been attributed to the presence of high-latitudinal ice sheets, whereas the existence and extent of ice accumulation in more temperate, mid-latitudinal regions remains enigmatic. Here we present results from the multiproxy provenance analysis of a unique, Pleistocene-Holocene IRD sequence from the Irish NE Atlantic continental margin. There, the Challenger coral carbonate mound (IODP Expedition 307 site U1317) preserved an Early-Pleistocene record of 16 distinctive IRD events, deposited between ca 2.6 and 1.7 Ma. Strong and complex IRD signals are also identified during the mid-Pleistocene climate transition (ca 1.2 to 0.65 Ma) and throughout the Middle-Late Pleistocene interval. Radiogenic isotope source-fingerprinting, in combination with coarse lithic component analysis, indicates a dominant sediment source in the nearby British-Irish Isles, even for the oldest, Early-Pleistocene IRD deposits. Hence, our findings demonstrate, for the first time, repeated and substantial (i.e. marine-terminating) ice accumulation on the British-Irish Isles since the beginning of the Pleistocene. Contemporaneous expansion of both high- and mid-latitudinal ice sheets in the North Atlantic region is therefore implied at the onset of the Pleistocene. Moreover, it suggests the recurrent establishment of (climatically) favourable conditions for ice sheet inception, growth and instability in mid-latitudinal regions, even in the earliest stages of Northern Hemisphere glacial expansion and in an obliquity-driven climate system.

Thierens, M.; Pirlet, H.; Colin, C.; Latruwe, K.; Vanhaecke, F.; Lee, J. R.; Stuut, J.-B.; Titschack, J.; Huvenne, V. A. I.; Dorschel, B.; Wheeler, A. J.; Henriet, J.-P.

2012-06-01

273

Remote identification of a gravel laden Pleistocene river bed  

NASA Technical Reports Server (NTRS)

The abundance of gravel deposits is well known in certain areas across the Gulf of Mexico coastal plain, including lands within several National Forests. These Pleistocene gravels were deposited following periods of glacial buildup when ocean levels were down and the main river channels had cut deep gorges, leaving the subsidiary streams with increased gradients to reach the main channels. During the warm interglacial periods that followed each glaciation, melting ice brought heavy rainfall and torrents of runoff carrying huge sediment loads that separated into gravel banks below these steeper reaches where abraiding streams, developed. As the oceans rose again, filling in the main channels, these abraiding areas were gradually flattened and covered over by progressively finer material. Older terraces were uplifted by tectonic movements associated with the Gulf Coastal Plain, and the subsequent erosional processes gradually brought the gravels closer to the surface. The study area is located on the Kisatchie National Forest, in central Louisiana, near Alexandria. Details of the full study have been discussed elsewhere. The nearest source of chert is in the Ouachita Mountains located to the northeast. The Ouachita River flows south, out of these mountains, and in Pleistocene times probably carried these chert gravels into the vicinity of the present day Little River Basin which lies along the eastern boundary of the National Forest. Current day drainages cross the National Forest from west to east, emptying into the Little River on the east side. However, a north-south oriented ridge of hills along the west side of the Forest appears to be a recent uplift associated with the hinge line of the Mississippi River depositional basin further to the east, and 800,000 years ago, when these gravels were first deposited during the Williana interglacial period, the streams probably flowed east to west, from the Little River basin to the Red River basin on the west side of the Forest. Within the National Forest and north of Alexandria, along Fish Creek, and east and west of an area known as Breezy Hill, exist several small, worked out gravel pits on privately owned blocks of land, formerly used by the state and county road departments. The pattern presented by these pits gives the impression of a series of north-south drainages lacing through the Forest, probable tributaries to Fish Creek which flows south of east from the west side of the Forest to empty into the Little River. Because of this predominant north-south pattern, no consideration was given to areas between these drainages during early gravel exploration efforts.

Scholen, Douglas E.

1993-01-01

274

Global sensitivity analysis of Indian Monsoon during the Pleistocene  

NASA Astrophysics Data System (ADS)

The sensitivity of Indian Monsoon to the full spectrum of climatic conditions experienced during the Pleistocene is estimated using the climate model HadCM3. The methodology follows a global sensitivity analysis based on the emulator approach of Oakley and O'Hagan (2004) implemented following a three-step strategy: (1) develop an experiment plan, designed to efficiently sample a 5-dimensional input space spanning Pleistocene astronomical configurations (3 parameters), CO2 concentration and a Northern Hemisphere glaciation index, (2) develop, calibrate and validate an emulator of HadCM3, in order to estimate the response of the Indian Monsoon over the full input space spanned by the experiment design, and (3) estimate and interpret sensitivity diagnostics, including sensitivity measures, in order to synthesize the relative importance of input factors on monsoon dynamics, estimate the phase of the monsoon intensity response with respect to that of insolation, and detect potential non-linear phenomena. Specifically, we focus on four variables: summer (JJAS) temperature and precipitation over North India, and JJAS sea-surface temperature and mixed-layer depth over the north-western side of the Indian ocean. It is shown that precession controls the response of four variables: continental temperature in phase with June to July insolation, high glaciation favouring a late-phase response, sea-surface temperature in phase with May insolation, and continental precipitation in phase with July insolation, and mixed-layer depth in antiphase with the latter. CO2 variations controls temperature variance with an amplitude similar to that of precession. The effect of glaciation is dominated by the albedo forcing, and its effect on precipitation competes with that of precession. Obliquity is a secondary effect, negligible on most variables except sea-surface temperature. It is also shown that orography forcing reduces the glacial cooling, and even has a positive effect on precipitation. As regards the general methodology, it is shown that the emulator provides a powerful approach, not only to express model sensitivity, but also to estimate internal variability (based on the nugget term introduced in the correlation function of the emulator) and detect anomalous simulations.

Araya-Melo, P. A.; Crucifix, M.; Bounceur, N.

2014-04-01

275

Electrokinetic remediation of metal contaminated glacial tills  

Microsoft Academic Search

This paper presents the results of an experimental investigation which studied the feasibility of using the electrokinetic process to remediate contaminated clays of glacial origin, otherwise known as glacial tills. An overview of the electrokinetic phenomena, as well as previously performed laboratory and field investigations, is first presented. The methodology of the electrokinetic experiments which were conducted to investigate the

K. R. Reddy; A. B. Shirani

1997-01-01

276

Glacial biogeography of North American coho salmon (Oncorhynchus kisutch).  

PubMed

To study the glacial biogeography of coho we examined 20 microsatellite loci and mitochondrial DNA D-loop sequence in samples from Alaska to California. Microsatellite data divided our samples among five biogeographic regions: (1) Alaska and northern coastal British Columbia; (2) the Queen Charlotte Islands; (3) the mainland coast of British Columbia and northern Washington State; (4) the Thompson River; and (5) Oregon and California. The D-loop sequence data suggested three geographical regions: (1) Oregon and California; (2) the Thompson River; and (3) all the other sites north of the southern ice margin. Microsatellite data revealed no difference in the number of alleles in different regions, but mitochondrial DNA data revealed a cline of decreasing diversity from south to north. We suggest that the two signals presented by these different marker types illuminate two time frames in the history of this species. Endemic microsatellite diversity in Alaska and on the Queen Charlotte Islands provides evidence in favour of Fraser Glaciation refugia in these regions. The loss of mitochondrial variation from south to north suggests that one of the earlier, more extensive, Pleistocene glaciations eliminated coho from its northern range. PMID:11903891

Smith, C T; Nelson, R J; Wood, C C; Koop, B F

2001-12-01

277

Towards understanding the role of the Bering Strait in the Plio-Pleistocene climate change  

NASA Astrophysics Data System (ADS)

The Bering Strait is the gateway between Pacific and Arctic, plays an important role in global freshwater cycle and eventually influences on climate change. The glacial Pacific Ocean had well-ventilated and nutrient-depleted glacial North Pacific Intermediate Water (GNPIW) above ~2000 m. GNPIW is a thicker and more deeply penetrating water mass than today's North Pacific Intermediate Water (NPIW). The source of GNPIW was likely in the Bering Sea based on neodymium isotope evidence. During Heinrich Event 1 in the early deglacial period, deep water extending to a depth of ~2500 m formed in the North Pacific. Modeling simulations suggest that a closed Bering Strait is essentially required for salinity built-up in the North Pacific to form the deep water. First opening of the Bering Strait was suggested to be in the latest Miocene and have repeated opening and closure by eustatic sea-level change associated with climate changes. However, our knowledge on the history about the opening/closure is limited due to lack of appropriate samples. During the IODP Expedition 323, Pliocene to Pleistocene marine sediment was continuously recovered from the Bering Sea. At the northern Bering continental slope sites (U1343 and U1344), the period with high abundance of diatom species Neodenticula seminae is terminated around 0.8-0.9 Ma, which appears to coincide with the Mid-Pleistocene Transition (MPT). While Neodenticula seminae is an endemic and predominant diatom in the modern subarctic Pacific and the Aleutian basin of the Bering Sea, the occurrence of this species is reported from the sediments between 0.84 and 1.25 Ma in the northern North Atlantic. This suggests a significantly intensified Arctic Throughflow and adaptive environment for N. seminae production from the Bering Strait to the Greenland Sea during the MPT. The observations of hydrography and the modern re-appearance of this taxon since 1999 in the Labrador Sea due to the melting of the Arctic Sea ice also suggests the specific requirements by this species: low salinity and high dissolved silicon concentrations. During the glacial cycles of the last 800 kyr, a limited input of Pacific water into the Arctic Ocean or disconnection was suggested by decreased abundances of N. seminae. Furthermore, repeated abundance peaks of radiolarian species Cycladophora davisiana, indicating cold and well ventilated intermediate water, suggest enhancement of intermediate to deep water formation in the Bering Sea during glacial periods, when the Bering Strait was closed. This research used samples and data provided by the IODP. We acknowledge the IODP Expedition 323 Shipboard Scientists.

Okazaki, Y.; Onodera, J.; Teraishi, A.; Suto, I.

2011-12-01

278

Thermohaline Circulation Crisis and Changes Through the Mid-Pleistocene Transition  

NASA Astrophysics Data System (ADS)

The Mid-Pleistocene Transition (MPT) marked a fundamental change in glacial-interglacial periodicity, transitioning from ~41,000 to 100,000 year cycles, accompanied by higher amplitude climate variability. It occurred without a significant change in orbital forcing, and thus its causes are poorly understood. We report major changes in the pre- and post-MPT mode of the ocean thermohaline circulation (THC), and a THC crisis during the MPT, from Nd isotopes in ODP Sites 1088 (~42S, 2082m) and 1090 (~43S, 3702m). The core locations are at the transition between the South Atlantic and the Southern oceans, a major gateway for the exchange of northern- and southern-sourced water masses. The new data show that in the ';40-kyr world' prior to the MPT, NADW export was strong during both interglacials and glacials. At ~900 ka the THC system underwent a major crisis, with an unprecedented weakening in NADW export during Marine Isotope Stages (MIS) 22-24. The recovery of the THC system in the post-MPT ';100-kyr world' is characterized by strong THC during interglacials, similar to pre-MPT interglacials, but much weaker THC during glacials. The ';THC crisis' interval includes MIS 23, which is unique as an interglacial where the THC operated in the same weak mode as post-MPT glacials. The MIS 22-24 interval has been recognized as a time of abrupt atmospheric pCO2 drawdown (Hoenisch et al. 2009) and significant cooling of ocean deep water, and Antarctic ice sheet expansion (Elderfield et al. Science 2012). Our data indicate that THC changes played an important role as a primary driving force, and helped to generate a series of positive feedbacks. This drastic change in deep-ocean circulation had important implications for the coeval drawdown of atmospheric pCO2, and the absence of a strong THC system through a glacial-to-interglacial-to-glacial cycle had a major impact on high latitude ice sheet growth. We suggest that the weak NADW export during MIS 24-22 resulted in reduced vertical exchange between Antarctic surface and deep waters, which helped to induce the drop in atmospheric pCO2, and in turn generated significant cooling which facilitated ice sheet expansion. These impacts were amplified by anomalously low Southern Hemisphere summer insolation during MIS 23, which resulted in suppressed ice sheet melting. Increased sea-ice coverage around the Antarctic continent during this time period may have generated increased AABW formation, which would have further drawn down CO2 from the atmosphere.

Goldstein, S. L.; Pena, L.

2013-12-01

279

Age constraints on Pleistocene megafauna at Tight Entrance Cave in southwestern Australia  

NASA Astrophysics Data System (ADS)

A well-stratified succession of fossiliferous sediments occurs in Tight Entrance Cave, southwestern Australia. These infill deposits contain the remains of megafauna and have accumulated intermittently since the Middle Pleistocene: >137, 137-119 and 50-29 ka, according to the results of 14C, U-Th, ESR and OSL dating techniques. Megafaunal species richness was highest in the latest part of the penultimate glacial maximum and during the subsequent last interglacial (137-119 ka), but remains are less abundant following an apparent ˜70 ka depositional hiatus in the sequence. Most megafaunal specimens from the upper (<44 ka) units are fragmentary, and reworking from older strata cannot yet be ruled out. However, one specimen of Simosthenurus occidentalis (a large extinct kangaroo), a pair of articulated dentaries showing no signs of secondary transportation, was found within a sedimentary layer deposited between 48 and 50 ka. This represents one of the youngest demonstrably in situ occurrences of an Australian megafaunal taxon.

Ayliffe, Linda K.; Prideaux, Gavin J.; Bird, Michael I.; Grün, Rainer; Roberts, Richard G.; Gully, Grant A.; Jones, Rhys; Fifield, L. Keith; Cresswell, Richard G.

2008-09-01

280

Spatial and temporal patterns of Pleistocene biogenic sediment accumulation in the Gulf of Alaska  

NASA Astrophysics Data System (ADS)

Reconstructing the timing and nature of past changes in aquatic productivity in the Gulf of Alaska (GoA) can shed light on the primary processes driving biogeochemical cycling over geologic timescales. Today, Fe is an important micronutrient that limits primary productivity in surface waters beyond the continental shelf in much of the GoA. However, we have a relatively poor understanding of how Fe-delivery processes, combined with changing climate, environmental, and oceanographic conditions, interact to influence primary production over glacial-interglacial timescales. An important first step is to identify the spatial and temporal patterns of increased productivity in the sediment record. Here, we present sedimentologic and physical property data from IODP Expedition 341 and identify intervals where diatom ooze and diatom-rich mud lithofacies are prevalent during the Pleistocene. Among the Expedition 341 recovered cores, were high-recovery intervals in the outer (Site U1417) and inner (U1418) Surveyor Fan, and from a small slope basin at the edge of the continental shelf (Site U1419). In general, greenish gray diatomaceous ooze (containing >50 % diatoms in smear slides) and diatom-rich mud (>25% diatoms) is found in beds ranging in thickness from 20 to 150 cm, interbedded with gray mud that commonly contains lonestones. Ooze is occasionally found immediately overlying volcanic ash. Compared to non-biogenic mud, diatomaceous sediments are generally characterized by lower magnetic susceptibility, natural gamma ray, bulk density, and higher b* color reflectance. At Site U1417, we observe a frequent occurrence of diatomaceous ooze during the middle Pleistocene relative to the early and late Pleistocene. At Site U1418, intervals containing diatom ooze are less common than at U1417 and biogenic sediments are mainly observed within the late Pleistocene portion of the record. However, higher sedimentation rates at U1418 relative to U1417, and the co-occurrence of sand and interbedded mud and silt indicate that clastic sediment dilution may obscure biogenic sediment contribution. At Site U1419, two prominent ~5 m thick intervals of diatomaceous ooze are found (within the uppermost 5 m and between 80 and 90 m composite depth, respectively). Between these intervals are numerous 20 cm thick intervals of biogenic sediment that were likely deposited during the middle or late Pleistocene based on preliminary shipboard age models. Biogenic intervals observed at Expedition 341 sites may be related to increased productivity driven by a combination of the aforementioned processes, but additional chronological and geochemical constraints are needed from all sites to rule out the role that changing sedimentation rates and/or silica dissolution plays in controlling the distribution of ooze in these records.

Moy, C. M.; Bahlburg, H.; Childress, L. B.; Cowan, E. A.; Forwick, M.; Müller, J.; Ribeiro, F.; Ridgway, K. D.; Mix, A. C.

2013-12-01

281

Middle Pleistocene glaciations of the Russian North  

Microsoft Academic Search

Geological data on the pre-Eemian glaciations of northern Russia, including the latest results by the Russian–Norwegian PECHORA project, are synthesized in order to present evidence for comparison with other early glaciations around the Arctic. The bulk of evidence indicates that Arctic and Subarctic regions of European Russia, of western and central Siberia during the Middle Pleistocene were at least 4

Valery Astakhov

2004-01-01

282

Temporal labyrinths of eastern Eurasian Pleistocene humans.  

PubMed

One of the morphological features that has been identified as uniquely derived for the western Eurasian Neandertals concerns the relative sizes and positions of their semicircular canals. In particular, they exhibit a relatively small anterior canal, a relatively larger lateral one, and a more inferior position of the posterior one relative to the lateral one. These discussions have not included full paleontological data on eastern Eurasian Pleistocene human temporal labyrinths, which have the potential to provide a broader context for assessing Pleistocene Homo trait polarities. We present the temporal labyrinths of four eastern Eurasian Pleistocene Homo, one each of Early (Lantian 1), Middle (Hexian 1), and Late (Xujiayao 15) Pleistocene archaic humans and one early modern human (Liujiang 1). The labyrinths of the two earlier specimens and the most recent one conform to the proportions seen among western early and recent modern humans, reinforcing the modern human pattern as generally ancestral for the genus Homo. The labyrinth of Xujiayao 15 is in the middle of the Neandertal variation and separate from the other samples. This eastern Eurasian labyrinthine dichotomy occurs in the context of none of the distinctive Neandertal external temporal or other cranial features. As such, it raises questions regarding possible cranial and postcranial morphological correlates of Homo labyrinthine variation, the use of individual "Neandertal" features for documenting population affinities, and the nature of late archaic human variation across Eurasia. PMID:25002467

Wu, Xiu-Jie; Crevecoeur, Isabelle; Liu, Wu; Xing, Song; Trinkaus, Erik

2014-07-22

283

A new time-domain method of implementing the sea-level equation in glacial-isostatic adjustment  

NASA Astrophysics Data System (ADS)

The sea-level equation (SLE) describes the redistribution of glacial melt water in the oceans and the movement of the coastlines. Its implementation is complicated in conjunction with the Laplace-transform method conventionally used to model glacial-isostatic adjustment (GIA). The recently developed spectral-finite element method (Martinec, 2000) solves the field equations governing GIA in the time domain and, thus, eliminates the need of applying the Laplace-transform method. Moreover, the spectral finite-element approach allows us to solve the SLE when modelling GIA for a 3-D viscosity model. The present test is restricted to a radially symmetric, self-gravitating, incompressible earth model consisting of a fluid core, a Maxwell-viscoelastic lower and upper mantle, and an elastic lithosphere. The Pleistocene deglaciation is simulated using the global ice model ICE-3G (Tushingham and Peltier, 1990). To study the sensitivity of the GIA predictions, we consider different ocean models, i.e. approximations to the complete solution of the SLE. Our test confirms the importance of allowing for geoid undulations and for moving coastlines when calculating the redistribution of glacial melt water in GIA. Finally, we compare our predictions with different types of observational constraint from Canada / Fennoscandia and reduce a subset of Baltic tide-gauge measurements for GIA. \\vspace{2mm} {References} \\vspace*{-0pt} Marticec, Z., 2000. Spectral-finite element approach to three-dimensional viscoelastic relaxation in a spherical earth. Geophys. J. Int., 142, 117-141. Tushingham, A.M., Peltier, W.R., 1990. Ice-3G: a new global model of late Pleistocene deglaciation based upon geophysical predictions of post-glacial relative sea level change. J. Geophys. Res., 96, 4497-4523.

Hagedoorn, J. M.; Martinec, Z.; Wolf, D.

2003-04-01

284

Phylogeography and Pleistocene refugia of the adder (Vipera berus) as inferred from mitochondrial DNA sequence data.  

PubMed

In order to contribute to the debate about southern glacial refugia used by temperate species and more northern refugia used by boreal or cold-temperate species, we examined the phylogeography of a widespread snake species (Vipera berus) inhabiting Europe up to the Arctic Circle. The analysis of the mitochondrial DNA (mtDNA) sequence variation in 1043 bp of the cytochrome b gene and in 918 bp of the noncoding control region was performed with phylogenetic approaches. Our results suggest that both the duplicated control region and cytochrome b evolve at a similar rate in this species. Phylogenetic analysis showed that V. berus is divided into three major mitochondrial lineages, probably resulting from an Italian, a Balkan and a Northern (from France to Russia) refugial area in Eastern Europe, near the Carpathian Mountains. In addition, the Northern clade presents an important substructure, suggesting two sequential colonization events in Europe. First, the continent was colonized from the three main refugial areas mentioned above during the Lower-Mid Pleistocene. Second, recolonization of most of Europe most likely originated from several refugia located outside of the Mediterranean peninsulas (Carpathian region, east of the Carpathians, France and possibly Hungary) during the Mid-Late Pleistocene, while populations within the Italian and Balkan Peninsulas fluctuated only slightly in distribution range, with larger lowland populations during glacial times and with refugial mountain populations during interglacials, as in the present time. The phylogeographical structure revealed in our study suggests complex recolonization dynamics of the European continent by V. berus, characterized by latitudinal as well as altitudinal range shifts, driven by both climatic changes and competition with related species. PMID:16968280

Ursenbacher, S; Carlsson, M; Helfer, V; Tegelström, H; Fumagalli, L

2006-10-01

285

Phylogeography of the large white-bellied rat Niviventer excelsior suggests the influence of Pleistocene glaciations in the Hengduan mountains.  

PubMed

The Hengduan Mountains, situated in the southeastern Tibetan Plateau, have undergone dramatic geological and climatic changes over the Pleistocene epoch. Several studies have revealed that the mountains served as a refugium during the ice age. The large white-bellied rat Niviventer excelsior is a rodent endemic to the Hengduan Mountains, which makes it an appropriate species for investigating the influence of glacial movements on the genetic structure of mammals. In this study, we sequenced the partial mitochondrial DNA control region from 72 N. excelsior specimens collected from 20 localities. The results revealed very high levels of haplotype diversity (h = 0.947) and nucleotide diversity (pi = 0.101) in this species. No common haplotype was found to be shared in samples from all geographic regions. Demographic analyses suggested that N. excelsior populations had not been subject to either expansion or bottleneck. The phylogenetic relationships among the haplotypes have no correlation with their geographical origins, while topology revealed two major clades. We speculate that the populations of N. excelsior may have been restricted to two separate refugia during the Last Glacial Maximum (0.60-0.17 Mya), with one west and one east of the Shaluli Mountains. Between the two major refugia, there existed a more widely distributed network subrefugia, which conserved genetic variations in N. excelsior. These results indicated that complex topographic configuration in the Hengduan Mountains provided a network of refugia to maintain the high level of genetic diversity in Pleistocene glaciations. PMID:20528155

Chen, Weicai; Liu, Shaoying; Liu, Yang; Hao, Haibang; Zeng, Bo; Chen, Shunde; Peng, Hongyuan; Yue, Bisong; Zhang, Xiuyue

2010-06-01

286

Diagenesis in coastal carbonates related to Pleistocene sea level, Bermuda Platform  

SciTech Connect

Pleistocene glacioeustatic sea-level oscillation on the stable Bermuda Platform is expressed in a succession of shallow-water carbonates interrupted by lowstand unconformities. In Bermuda, the maximum highstands of the last 400,000 yr ranged within 10 m around the present level. Coastal carbonates of various highstands are exposed along the present shoreline. These carbonates were penetrated by meteoric and marine pore waters during lowstands and highstands following on deposition. Two representative Pleistocene shoreline sections were studied to see whether early diagenesis has recorded these pore-water changes. The sediments of both sections show multiple generations of cement. Optical and scanning electron microscopy, cathodoluminescence microscopy, X-ray diffraction, microprobe studies and stable-isotope analyses were used to determine the diagenetic environments involved. Regardless of the degree of substrate cementation, freshwater alteration was mainly vadose whereas marine cementation was either phreatic or vadose or both. Early diagenetic oscillation is easier recorded in coastal successions than in lagoonal sediments, mainly because marine cementation is more active nearshore.Because the coastal environment is prone to wave destruction, the potential for preserving these diagenetic features is usually low. Data published on tectonically unstable areas suggest that early diagenetic oscillation may characterize stable coastlines.

Vollbrecht, R.; Meischner, D. [Inst. fuer Geologie und Palaeontologie, Goettingen (Germany)

1996-01-01

287

Deciphering a non-glacial/glacial landscape mosaic in the northern Swedish mountains  

NASA Astrophysics Data System (ADS)

Relict surfaces contain information on past surface processes and long-term landscape evolution. A detailed investigation of relict non-glacial surfaces in a formerly glaciated mountain landscape of northern Sweden was completed, based on interpretation of colour infrared aerial photographs, analysis in a GIS, and fieldwork. Working backwards from landscape to process, surfaces were classified according to large- and small-scale morphologies that result from the operation of non-glacial processes, the degree of weathering, regolith characteristics, and the style of glacial modification. Surfaces were also compared in the GIS according to elevation, slope angle, and bedrock lithology. The study revealed five types of relict non-glacial surfaces but also two types of extensively weathered glacial surfaces that were transitional to relict non-glacial surfaces, illustrating spatially variable processes and rates of non-glacial and glacial landscape evolution. Rather than being static preglacial remnants, relict non-glacial surfaces are dynamic features that have continued to evolve during the Quaternary. The classification provides hypotheses for landscape evolution that can be field tested through, for example, terrestrial cosmogenic nuclide studies and geochemical analyses of fine matrix materials. The classification may be applicable to relict non-glacial surfaces in other formerly glaciated landscapes.

Goodfellow, B. W.; Stroeven, A. P.; Hättestrand, C.; Kleman, J.; Jansson, K. N.

2008-01-01

288

Canyon Creek: A late Pleistocene vertebrate locality in interior Alaska  

USGS Publications Warehouse

The Canyon Creek vertebrate-fossil locality is an extensive road cut near Fairbanks that exposes sediments that range in age from early Wisconsin to late Holocene. Tanana River gravel at the base of the section evidently formed during the Delta Glaciation of the north-central Alaska Range. Younger layers and lenses of fluvial sand are interbedded with arkosic gravel from Canyon Creek that contains tephra as well as fossil bones of an interstadial fauna about 40,000 years old. Solifluction deposits containing ventifacts, wedge casts, and rodent burrows formed during a subsequent period of periglacial activity that took place during the maximum phase of Donnelly Glaciation about 25,000-17,000 years ago. Overlying sheets of eolian sand are separated by a 9500-year-old paleosol that may correlate with a phase of early Holocene spruce expansion through central Alaska. The Pleistocene fauna from Canyon Creek consists of rodents (indicated by burrows), Mammuthus primigenius (woolly mammoth), Equus lambei (Yukon wild ass), Camelops hesternus (western camel), Bison sp. cf. B. crassicornis (large-horned bison), Ovis sp. cf. O. dalli (mountain sheep), Canis sp. cf. C. lupus (wolf), Lepus sp. cf. L. othus or L. arcticus (tundra hare), and Rangifer sp. (caribou). This assemblage suggests an open landscape in which trees and tall shrubs were either absent or confined to sheltered and moist sites. Camelops evidently was present in eastern Beringia during the middle Wisconsin interstadial interval but may have disappeared during the following glacial episode. The stratigraphic section at Canyon Creek appears to demonstrate that the Delta Glaciation of the north-central Alaska Range is at least in part of early Wisconsin age and was separated from the succeeding Donnelly Glaciation by an interstadial rather than interglacial episode. ?? 1981.

Weber, F. R.; Hamilton, T. D.; Hopkins, D. M.; Repenning, C. A.; Haas, H.

1981-01-01

289

Late Pleistocene voles (Arvicolinae, Rodentia) from the Baranica Cave (Serbia)  

NASA Astrophysics Data System (ADS)

Baranica is a cave system situated in the south-eastern part of Serbia, four kilometers south to Knjaževac, on the right bank of the Trgovi\\vski Timok. The investigations in Baranica were conducted from 1994 to 1997 by the Faculty of Philosophy from Belgrade and the National Museum of Knjaževac. Four geological layers of Quaternary age were recovered. The abundance of remains of both large and small mammals was noticed in the early phase of the research. In this paper, the remains of eight vole species are described: Arvicola terrestris (Linnaeus, 1758), Chionomys nivalis (Martins, 1842), Microtus (Microtus) arvalis (Pallas, 1778) and Microtus (Microtus) agrestis (Linnaeus, 1761), Microtus (Stenocranius) gregalis (Pallas, 1779), Microtus (Terricola) subterraneus (de Sélys-Longchamps, 1836), Clethrionomys glareolus (Schreber, 1780) and Lagurus lagurus (Pallas, 1773). Among them, steppe and open area inhabitants prevail. Based on the evolutionary level and dimensions of the Arvicola terrestris molars, as well as the overall characteristics of the fauna, it was concluded that the deposits were formed in the last glacial period of the Late Pleistocene. These conclusions are rather consistent with the absolute dating of large mammal bones (23.520 ± 110 B.P. for Layer 2 and 35.780 ± 320 B.P. for Layer 4).

Bogi?evi?, Katarina; Nenadi?, Draženko; Mihailovi?, Dušan

2012-02-01

290

Late Pleistocene environments of the western Noatak basin, northwestern Alaska  

USGS Publications Warehouse

Glacial Lake Noatak formed repeatedly during middle and late Pleistocene time as expanding glaciers from the DeLong Mountains blocked the Noatak River valley. Downcutting by the Noatak River has exposed thick sediment successions in bluffs up to 86 m high. Two river bluffs, Nk-26 and Nk-29A, contain correlative organic-rich flood-plain deposits that were formed during and after deposition of the Old Crow tephra at about the transition between oxygen isotope stage 6 and oxygen isotope stage 5, at the beginning of the last interglaciation. Both bluffs also contain older interglacial or interstadial flood-plain deposits of uncertain age. Pollen and beetle remains were recovered from the older and younger flood-plain deposits at each bluff. Pollen from the younger flood-plain deposits suggests tundra vegetation with local dominance of sedge. Juniperus abundances were locally high, especially around the time of Old Crow tephra deposition. Mutual climatic range (MCR) estimates from the insect fossil assemblages suggest that mean summer temperatures (Tmax) near the time of Old Crow tephra deposition were about 2 ??C colder than modern; mean winter temperatures were very similar to those of today. A younger sample from the same interglacial deposit yielded a Tmax estimate of 2 ??C warmer than modern, signaling interglacial warming. Pollen from the older interglacial deposit at Nk-29A suggests mesic tundra, with boreal forest more distant than it is today. MCR analysis of a possibly correlative older interglacial deposit at Nk-26 suggests a Tmax about 2 ??C below present.

Elias, S. A.; Hamilton, T. D.; Edwards, M. E.; Beget, J. E.; Krumhardt, A. P.; Lavoie, C.

1999-01-01

291

Glacial greenhouse-gas fluctuations controlled by ocean circulation changes.  

PubMed

Earth's climate and the concentrations of the atmospheric greenhouse gases carbon dioxide (CO(2)) and nitrous oxide (N(2)O) varied strongly on millennial timescales during past glacial periods. Large and rapid warming events in Greenland and the North Atlantic were followed by more gradual cooling, and are highly correlated with fluctuations of N(2)O as recorded in ice cores. Antarctic temperature variations, on the other hand, were smaller and more gradual, showed warming during the Greenland cold phase and cooling while the North Atlantic was warm, and were highly correlated with fluctuations in CO(2). Abrupt changes in the Atlantic meridional overturning circulation (AMOC) have often been invoked to explain the physical characteristics of these Dansgaard-Oeschger climate oscillations, but the mechanisms for the greenhouse-gas variations and their linkage to the AMOC have remained unclear. Here we present simulations with a coupled model of glacial climate and biogeochemical cycles, forced only with changes in the AMOC. The model simultaneously reproduces characteristic features of the Dansgaard-Oeschger temperature, as well as CO(2) and N(2)O fluctuations. Despite significant changes in the land carbon inventory, CO(2) variations on millennial timescales are dominated by slow changes in the deep ocean inventory of biologically sequestered carbon and are correlated with Antarctic temperature and Southern Ocean stratification. In contrast, N(2)O co-varies more rapidly with Greenland temperatures owing to fast adjustments of the thermocline oxygen budget. These results suggest that ocean circulation changes were the primary mechanism that drove glacial CO(2) and N(2)O fluctuations on millennial timescales. PMID:19020618

Schmittner, Andreas; Galbraith, Eric D

2008-11-20

292

Early and Middle Pleistocene river systems in eastern England: evidence from Leet Hill, southern Norfolk, England  

NASA Astrophysics Data System (ADS)

Pleistocene sediments at Leet Hill, southern Norfolk are examined in terms of their sedimentary structures, palaeocurrent indicators, clast and heavy mineral lithology and litho- and morphostratigraphic position. Colour of the quartzite and vein-quartz clasts is used to differentiate the Bytham and the Kesgrave sands and gravels, with the Bytham sands and gravels having a significantly higher proportion of coloured material. The Kirby Cane sands and gravels are the lower sedimentary unit and were deposited by the Bytham river, which drained a catchment extending into central England. At Leet Hill, erosion of the Kesgrave Sands and Gravels by the Bytham river has given the Kirby Cane sands and gravels a distinctive lithological assemblage. Trace clast lithologies suggest that the Kesgrave Sands and Gravels in the region of Leet Hill were deposited in a coastal location with an input from northern sources as well as southern and Welsh sources diagnostic of the Thames catchment. The glaciofluvial Leet Hill Sands and Gravels were deposited by outwash from the Anglian Scandinavian ice sheet. Initially the flow direction of the outwash was determined by the Bytham river valley, but this changed to a southerly direction once the valley had been infilled. This paper provides the first indication of the location of the boundary (Early Pleistocene coastline) between the fluvial Kesgrave Sands and Gravels and the marine equivalent reworked by coastal processes, and demonstrates the way the pre-glacial relief initially controlled patterns of glaciofluvial sedimentation during the early part of the Anglian glaciation.

Rose, James; Lee, Jeremy A.; Candy, Ian; Lewis, Simon G.

1999-07-01

293

The influence of Pleistocene refugia on the evolutionary history of the Japanese hare, Lepus brachyurus.  

PubMed

We performed a phylogeographic analysis of the Japanese hare, Lepus brachyurus, using the mitochondrial cytochrome b gene (1140 bp). In total, 119 haplotypes were recovered from 197 samples isolated from 82 localities on three main islands of the Japanese archipelago: Honshu, Sikoku, Kyushu, Sado Island and the Oki Islands. Results showed two distinct clades at a genetic distance of 3.5%, equivalent to an estimated 1.2 million years. The two clades, encompassing seven subclades, showed an apparent geographic affinity to Kyushu, Shikoku and the nearby area of Honshu (southern group) by one clade, whereas the other clade covered the remaining area of Honshu (northern group). The landscape shape interpolation analysis exhibited a higher genetic diversity in the southern parts of central Honshu (northern group) and Shikoku and Kyushu regions (southern group), suggesting the existence of multiple geographical origins of population expansion in each clade. The Bayesian skyline plot analysis showed that lineage diversifications occurred about 0.35, 0.20 and 0.05 million years ago (Mya), which coincide closely with the glacial-interglacial cycles during the Pleistocene. Therefore, we suggest that the Japanese hare population once inhabited northern and southern refugia, and subsequently developed several populations through local demographic fluctuations. The present day demarcation in the northern and southern geographic groups is considered to be a temporal remnant of Pleistocene population dynamics and the geographic boundary between them could move or fade away in time. PMID:20822403

Nunome, Mitsuo; Torii, Harumi; Matsuki, Rikyu; Kinoshita, Gohta; Suzuki, Hitoshi

2010-09-01

294

A late Pleistocene steppe bison ( Bison priscus) partial carcass from Tsiigehtchic, Northwest Territories, Canada  

NASA Astrophysics Data System (ADS)

A partial steppe bison ( Bison priscus) carcass was recovered at Tsiigehtchic, near the confluence of the Arctic Red and Mackenzie Rivers, Northwest Territories, Canada in September of 2007. The carcass includes a complete cranium with horn cores and sheaths, several complete post-cranial elements (many of which have some mummified soft tissue), intestines and a large piece of hide. A piece of metacarpal bone was subsampled and yielded an AMS radiocarbon age of 11,830 ± 45 14C yr BP (OxA-18549). Mitochondrial DNA sequenced from a hair sample confirms that Tsiigehtchic steppe bison ( Bison priscus) did not belong to the lineage that eventually gave rise to modern bison ( Bison bison). This is the first radiocarbon dated Bison priscus in the Mackenzie River valley, and to our knowledge, the first reported Pleistocene mammal soft tissue remains from the glaciated regions of northern Canada. Investigation of the recovery site indicates that the steppe bison was released from the permafrost during a landslide within unconsolidated glacial outwash gravel. These data indicate that the lower Mackenzie River valley was ice free and inhabited by steppe bison by ˜11,800 14C years ago. This date is important for the deglacial chronology of the Laurentide Ice Sheet and the opening of the northern portal to the Ice Free Corridor. The presence of steppe bison raises further potential for the discovery of more late Pleistocene fauna, and possibly archaeological evidence, in the region.

Zazula, Grant D.; MacKay, Glen; Andrews, Thomas D.; Shapiro, Beth; Letts, Brandon; Brock, Fiona

2009-12-01

295

Timing and dynamics of Late Pleistocene mammal extinctions in southwestern Australia.  

PubMed

Explaining the Late Pleistocene demise of many of the world's larger terrestrial vertebrates is arguably the most enduring and debated topic in Quaternary science. Australia lost >90% of its larger species by around 40 thousand years (ka) ago, but the relative importance of human impacts and increased aridity remains unclear. Resolving the debate has been hampered by a lack of sites spanning the last glacial cycle. Here we report on an exceptional faunal succession from Tight Entrance Cave, southwestern Australia, which shows persistence of a diverse mammal community for at least 100 ka leading up to the earliest regional evidence of humans at 49 ka. Within 10 millennia, all larger mammals except the gray kangaroo and thylacine are lost from the regional record. Stable-isotope, charcoal, and small-mammal records reveal evidence of environmental change from 70 ka, but the extinctions occurred well in advance of the most extreme climatic phase. We conclude that the arrival of humans was probably decisive in the southwestern Australian extinctions, but that changes in climate and fire activity may have played facilitating roles. One-factor explanations for the Pleistocene extinctions in Australia are likely oversimplistic. PMID:21127262

Prideaux, Gavin J; Gully, Grant A; Couzens, Aidan M C; Ayliffe, Linda K; Jankowski, Nathan R; Jacobs, Zenobia; Roberts, Richard G; Hellstrom, John C; Gagan, Michael K; Hatcher, Lindsay M

2010-12-21

296

Timing and dynamics of Late Pleistocene mammal extinctions in southwestern Australia  

PubMed Central

Explaining the Late Pleistocene demise of many of the world's larger terrestrial vertebrates is arguably the most enduring and debated topic in Quaternary science. Australia lost >90% of its larger species by around 40 thousand years (ka) ago, but the relative importance of human impacts and increased aridity remains unclear. Resolving the debate has been hampered by a lack of sites spanning the last glacial cycle. Here we report on an exceptional faunal succession from Tight Entrance Cave, southwestern Australia, which shows persistence of a diverse mammal community for at least 100 ka leading up to the earliest regional evidence of humans at 49 ka. Within 10 millennia, all larger mammals except the gray kangaroo and thylacine are lost from the regional record. Stable-isotope, charcoal, and small-mammal records reveal evidence of environmental change from 70 ka, but the extinctions occurred well in advance of the most extreme climatic phase. We conclude that the arrival of humans was probably decisive in the southwestern Australian extinctions, but that changes in climate and fire activity may have played facilitating roles. One-factor explanations for the Pleistocene extinctions in Australia are likely oversimplistic.

Prideaux, Gavin J.; Gully, Grant A.; Couzens, Aidan M. C.; Ayliffe, Linda K.; Jankowski, Nathan R.; Jacobs, Zenobia; Roberts, Richard G.; Hellstrom, John C.; Gagan, Michael K.; Hatcher, Lindsay M.

2010-01-01

297

Late Pleistocene Glaciation of Mono Basin, California  

NASA Astrophysics Data System (ADS)

Moraines in the canyons of Mono Basin are separable into relative-age groups on the basis of clast-sound velocities in exposed boulders, moraine morphology, and weathering features on boulder surfaces. Tioga, Tenaya, Tahoe, and Mono Basin moraine deposits each have distinct weathering characteristics and therefore constitute different relative-age groups. The Tioga glacial episode at June Lake may postdate ˜25,200(?)-yr-old basaltic lavas, and the Tenaya episode may have occurred ˜30,700 yr ago. A comparison of the glacial and lacustrine records of Mono Basin over the past 40,000 yr, based on new interpretations of radiometric ages, is consistent with the hypothesis that maximum glacial and maximum pluvial periods were not necessarily synchronous.

Bursik, Marcus I.; Gillespie, Alan R.

1993-01-01

298

Pleistocene climate and biome evolution modulated at orbital, millennial, and centennial time scales  

NASA Astrophysics Data System (ADS)

For the northern Andes we present a multi-proxy record of environmental and climatic change at millennial- to century-scale resolution of the full Pleistocene. The composite record includes the 540-m Funza core (2250-27 ka; 1050-yr resolution) from the Bogotá basin (~4°N, 2550 m asl, 2100 samples), the 58-mcd core (284-27 ka; 60-yr resolution) from the Fúquene basin (~5°N, 2540 m asl 4700 samples), and the 12-m core (last 14 ka; 25-yr resolution) from the La Cocha basin (1°N, 2780 m asl, 550 samples). At high elevations climatic variability is mainly driven by the 41-kyr component of orbital forcing changing into a dominant 100-kyr frequency during the last 0.9 Ma. High elevation intraAndean environments are mainly driven by temperature and atmospheric pCO2 while changes in moisture is an important driver of the Andean environments on the Amazonian flank. The Pleistocene is reflected by MIS 87 to 1, the last interglacial-glacial cycle by D/O-cycles 28 to 1 (and during MIS 7-6 another 15 D/O-style cycles), and the Holocene shows many events with an acceleration of climate change. Repeatedly the subpáramo shrub biome is temporarily lost suggesting vertical migration of forest exceeded the maximum migration capacity of the subpáramo biome. Continuous changes in altitidinal vegetation distribution caused mountains above ~1500 m were alternatingly covered by different biomes. Forests reached only ~125 ka modern species compositions indicating most of the Pleistocene record shows nonanalog vegetation associations, however not preventing modern ecological ranges can be applied to reconstruct past environments. Comparison with Greenland, Antarctic and marine climate records is demonstrated.

Hooghiemstra, H.

2013-05-01

299

Glacial Evidence in the Andes Mountains, Peru  

NSDL National Science Digital Library

This exercise brings the students to a landscape in the Peruvian Andes in Google Earth, where some alpine and niche glaciers still exist. Their task is to determine glacial extent in the past, and infer the kind of glacier(s) that had existed in the area. In order to answer the question, students need to identify the current ice extent, approximate the equilibrium line altitude based on dirty vs white ice (or the firn line, if visible) in summer time imagery, and document glacial landforms. They also need to recognize the correspondence between elevation and ice extent. Key words: glaciers, glacial landforms, Google Earth

Hasbargen, Les

300

Inter-specific gene flow dynamics during the Pleistocene-dated speciation of forest-dependent mosquitoes in Southeast Asia.  

PubMed

Tropical forests have undergone repeated fragmentation and expansion during Pleistocene glacial and interglacial periods, respectively. The effects of this repeated forest fragmentation in driving vicariance in tropical taxa have been well studied. However, relatively little is known about how often this process results in allopatric speciation, since it may be inhibited by recurrent gene flow during repeated secondary contact, or to what extent Pleistocene-dated speciation results from ecological specialization in the face of gene flow. Here, divergence times and gene flow between three closely-related mosquito species of the Anopheles dirus species complex endemic to the forests of Southeast Asia, are inferred using coalescent based Bayesian analysis. An Isolation with Migration model is applied to sequences of two mitochondrial and three nuclear genes, and 11 microsatellites. The divergence of An. scanloni has occurred despite unidirectional nuclear gene flow from this species into An. dirus. The inferred asymmetric gene flow may result from the unique evolutionary adaptation of An. scanloni to limestone karst habitat, and therefore the fitness advantage of this species over An. dirus in regions of sympatry. Mitochondrial introgression has led to the complete replacement of An. dirus haplotypes with those of An. baimaii through a recent (approximately 62 kya) selective sweep. Speciation of An. baimaii and An. dirus is inferred to have involved allopatric divergence throughout much of the Pleistocene. Secondary contact and bidirectional gene flow has occurred only within the last 100 000 years, by which time the process of allopatric speciation seems to have been largely completed. PMID:20444081

Morgan, Katy; Linton, Yvonne-Marie; Somboon, Pradya; Saikia, Prasanta; Dev, Vas; Socheat, Duong; Walton, Catherine

2010-06-01

301

Pleistocene paleoenvironmental reconstructions and mammalian evolution in South-East Asia: focus on fossil faunas from Thailand  

NASA Astrophysics Data System (ADS)

Mammalian faunal studies have provided various clues for a better reconstruction of hominid Quaternary paleoenvironments. In this work, two methods were used: (1) the cenogram method, based on a graphical representation of the mammalian community structure, and (2) the species richness of murine rodents to estimate climatic parameters. These methods were applied to Middle and Late Pleistocene mammalian faunas of South-East Asia, from South China to Indonesia. Special emphasis was laid on a fauna from north-east Thailand dated back to approximately 170,000 years (i.e. a glacial period). This Thai fauna seems characteristic of a slightly open forested environment intermediate between those of present-day central Myanmar and the northern part of South China. In the Thai fauna, the occurrence of both cool-loving mammalian taxa, currently living further north, and species of larger body size than their living counterparts, indicates cooler and probably drier climatic conditions than present-day climates in Thailand. These results are quite consistent with Middle Pleistocene palynological records from South China and eastern Java. From other less well-documented Pleistocene faunas, taken into account in this work, humid climatic conditions of interglacial periods were revealed from large mammalian taxa.

Tougard, C.; Montuire, S.

2006-01-01

302

Neurodynamic oscillators  

NASA Technical Reports Server (NTRS)

Oscillation of electrical activity has been found in many nervous systems, from invertebrates to vertebrates including man. There exists experimental evidence of very simple circuits with the capability of oscillation. Neurons with intrinsic oscillation have been found and also neural circuits where oscillation is a property of the network. These two types of oscillations coexist in many instances. It is nowadays hypothesized that behind synchronization and oscillation there is a system of coupled oscillators responsible for activities that range from locomotion and feature binding in vision to control of sleep and circadian rhythms. The huge knowledge that has been acquired on oscillators from the times of Lord Rayleigh has made the simulation of neural oscillators a very active endeavor. This has been enhanced with more recent physiological findings about small neural circuits by means of intracellular and extracellular recordings as well as imaging methods. The future of this interdisciplinary field looks very promising; some researchers are going into quantum mechanics with the idea of trying to provide a quantum description of the brain. In this work we describe some simulations using neuron models by means of which we form simple neural networks that have the capability of oscillation. We analyze the oscillatory activity with root locus method, cross-correlation histograms, and phase planes. In the more complicated neural network models there is the possibility of chaotic oscillatory activity and we study that by means of Lyapunov exponents. The companion paper shows an example of that kind.

Espinosa, Ismael; Gonzalez, Hortensia; Quiza, Jorge; Gonazalez, J. Jesus; Arroyo, Ruben; Lara, Ritaluz

1995-01-01

303

Discriminant Function Analysis of Spectral Gamma Data as a Tool for Regional Stratigraphic Correlation of Pleistocene Deposits  

NASA Astrophysics Data System (ADS)

The protection of groundwater resources is a growing environmental concern in North America. A key requirement in glacial terrains is an understanding of the subsurface distribution and lithologic characteristics of thick, regionally extensive Pleistocene till deposits, as they are a dominant control on regional groundwater flow and contaminant migration. Regional correlation is often problematic when based on lithologic or textural characteristics alone. This paper presents the results of one of the first studies that has utilized borehole and outcrop spectral gamma measurements as tool for regional correlation of tills. A hand-held Exploranium GPX-21 gamma spectrometer and 2SNA-1000 spectral sonde were used to obtain more than 400 measurements on Pleistocene tills and deltaic deposits in the Greater Toronto Area. To quantify separation of each unit, a discriminant function was applied to classify the data. Successful classification of greater than 75% of the stratigraphic units was achieved using this method. Crossplots and three-component plots show that the most sedimentary units have a unique spectral gamma signature. Clay-rich glaciolacustrine strata (e.g. Sunnybrook diamict, Thorncliffe and Scarborough clays) are characterized by more abundant Th, K and U when compared to sandy deltaic units and subglacial tills (e.g. Newmarket and Halton Tills). The element ratios for each unit show a regional consistency, except for the Halton Till, which has a variable spectral signature that reflects the composition of underlying deposits. This association reflects the sub-glacial reworking and incorporation of pre-existing deposits into the till during ice readvance. The results show that many Pleistocene units have unique spectral gamma signatures and that gamma is a useful attribute for regional stratigraphic studies in southern Ontario.

Collins, S. V.; Boyce, J. I.

2004-05-01

304

Catastrophic middle Pleistocene jökulhlaups in the upper Susquehanna River: Distinctive landforms from breakout floods in the central Appalachians  

NASA Astrophysics Data System (ADS)

Widespread till and moraines record excursions of middle-Pleistocene ice that flowed up-slope into several watersheds of the Valley and Ridge Province along the West Branch of the Susquehanna River. A unique landform assemblage was created by ice-damming and jökulhlaups emanating from high gradient mountain watersheds. This combination of topography formed by multiple eastward-plunging anticlinal ridges, and the upvalley advance of glaciers resulted in an ideal geomorphic condition for the formation of temporary ice-dammed lakes. Extensive low gradient (1°-2° slope) gravel surfaces dominate the mountain front geomorphology in this region and defy simple explanation. The geomorphic circumstances that occurred in tributaries to the West Branch Susquehanna River during middle Pleistocene glaciation are extremely rare and may be unique in the world. Failure of ice dams released sediment-rich water from lakes, entraining cobbles and boulders, and depositing them in elongated debris fans extending up to 9 km downstream from their mountain-front breakout points. Poorly developed imbrication is rare, but occasionally present in matrix-supported sediments resembling debris flow deposits. Clast weathering and soils are consistent with a middle Pleistocene age for the most recent flows, circa the 880-ka paleomagnetic date for glacial lake sediments north of the region on the West Branch Susquehanna River. Post-glacial stream incision has focused along the margins of fan surfaces, resulting in topographic inversion, leaving bouldery jökulhlaup surfaces up to 15 m above Holocene channels. Because of their coarse nature and high water tables, jökulhlaup surfaces are generally forested in contrast to agricultural land use in the valleys and, thus, are readily apparent from orbital imagery.

Kochel, R. Craig; Nickelsen, Richard P.; Eaton, L. Scott

2009-09-01

305

Mid-Pleistocene Lacustrine Records of Carbon and Nitrogen Elemental and Isotopic data from Valles Caldera, New Mexico, USA  

NASA Astrophysics Data System (ADS)

Lacustrine deposits dating back to the middle Pleistocene were recovered from Valles Caldera, located in Northern New Mexico. The core (82 m long) contained ~75 meters of lacustrine sediments. The age of the sediments was constrained by Ar-Ar dating of a primary tephra layer from the base of the core, in combination with correlation to the deep-sea marine isotope stage record along the length of the core. The lacustrine sediments span from ~350 k YBP to 552 k YBP covering two glacial-interglacial stages, from MIS 11 to MIS 14 and part of MIS 10. Total organic carbon (TOC) in the siderite free fraction (i.e. diagenetic siderite was chemically removed) ranged from <1% to 7% with the lowest values predominantly observed during glacial times and marked increases at the onset of interglacials. Particularly high TOC is observed during the long interglacial MIS 11. C/N ratios ranged from values of <2 to ~11 with lowest values in general observed during glacial times indicating increased input from aquatic sources and suggesting deepening of the lake as a result of wetter glacial climate. ?13C values measured in the siderite free fraction, ranged from -28‰ to -20‰ and remained more negative overall during glacial times. We are currently working on D/H analyses of leaf lipid biomarkers which may provide insight into the delivery of meteoric water to the region during these climatic stages, i.e. relative contributions of summer monsoonal (from the Gulfs of Mexico and California) vs. winter frontal precipitation (from the North Pacific).

Cisneros-Dozal, L.; Heikoop, J.; Fessenden, J.; Fawcett, P.; Kawka, O.; Sachs, J.

2007-12-01

306

A mid-Pleistocene record of monsoonal precipitation in southwestern US, insights into the occurrence of drought  

NASA Astrophysics Data System (ADS)

Accurate model predictions of drought require an understanding of human-induced as well as natural climate variability. Paleoclimate records of precipitation changes over glacial/interglacial periods are essential to gain insights into natural variability. However there is a lack of such records for the southwestern US, particularly of quantitative changes. Here we present a mid-Pleistocene record of changes in monsoonal precipitation in northern New Mexico spanning interglacials MIS 13 and 11, glacial MIS 12 and onset of MIS 10. We based our estimates on the reconstructed ?D record of precipitation in lacustrine sediments from the Valles Caldera, New Mexico. The ?D record of precipitation holds similarities to the global benthic ?18O record and the ?D record from the Antarctica Dome C ice core during glacial periods and at the glacial-interglacial boundaries. However marked differences to the global records during each of interglacials MIS 13 and 11 suggests the influence of a regional climate forcing on precipitation changes in the region. We show that monsoonal precipitation (as the proportion of annual precipitation) declined repeatedly during these interglacials from a maximum of up to 90% to less than 30%, resembling glacial periods. The variability in monsoonal precipitation during interglacial periods resembles a response to insolation cycles and its intensity correlates to the land-sea surface temperature contrast (LSTC). However, near the highest temperatures over land, there seems to be a failure of the LSTC mechanism leading to severe aridity. Our results suggest that current human-induced warming may worsen a natural state of monsoon minima leading to severe drought and this superimposed variability should be considered in climate predictions.

Cisneros Dozal, L.; Huang, Y.; Heikoop, J. M.; Fawcett, P. J.; Fessenden, J. E.; Anderson, R. S.; Meyers, P. A.; Larson, T. E.; Perkins, G.; Toney, J. L.; Werne, J. P.; Goff, F.; WoldeGabriel, G. W.; Allen, C. D.; Berke, M. A.

2012-12-01

307

Vegetation and climate changes in western Amazonia during a previous Interglacial- Glacial transition  

NASA Astrophysics Data System (ADS)

Amazonia is one of the most biodiverse areas of the world and its vegetation plays a crucial role in controlling the global climate through the regulation of the levels of atmospheric CO2. However, Amazonian ecosystems and their role in the climate system are threatened by ongoing the human impact (already estimated loss of 60% of the species in western Amazonia) and predicted climate change (+1.1-6.4oC by 2100). Unfortunately, there is absence of data relating to the ecological baseline function and response to global climate change of western Amazonian ecosystems in the absence of humans. To help anticipate the impact of future climate change predictions an improved understanding of the natural responses of tropical vegetation to known past climate change is required. Here we present the first study that shows the response of pristine tropical ecosystems in western Amazonia biodiversity hotspot to a major global climate change event (a Quaternary Interglacial-Glacial transition). Pleistocene lake/swamp sediments preserved at the Erazo study site (Lat. 00o 33’S, Long. 077o 52’W, 1927m alt.) today within tropical cloud forest vegetation provide a unique opportunity to examine the impact of past climate shifts. The sediment are >40,000 years old (radiocarbon infinite) and younger than 1 million years (presence of Alder biomarker) and consist of organic layers interbedded with volcanic ash (tephra). This study presents data from multiple proxies (fossil pollen, wood macrofossils and charcoal) to establish a comprehensive picture of regional and local vegetation change prior to human arrival. Our data show a change of vegetation from palm-dominated forest indicative of warm and wet conditions similar to the present at the base of this record, to a forest dominated by Podocarpus sp. suggesting cold and wet conditions at the top of the record. The transition between these two vegetation communities appears to be progressive with small sharp changes along the ecological succession. Fire activity appears to be minor through the record associated only with volcanic events (tephra layers). We conclude that western Amazonian vegetation was effected by Pleistocene global climate change. The Erazo record shows the progression of the vegetation from warm/wet Interglacial-like period similar to the present, to a colder and wetter Glacial-like period. This magnitude of change agrees with similar magnitude changes inferred for the last glacial-interglacial transition at 14,700-9,000 calendar years BP. We also establish for the first time that no natural fire occurred in western Amazonia in the absence of humans, under interglacial or glacial conditions, without volcanic eruptions acting as a source of ignition.

Cardenas, M. L.; Gosling, W. D.; Sherlock, S. C.; Poole, I.; Pennington, R. T.

2009-12-01

308

Isotopic composition of old ground water from Lake Agassiz: Implications for late Pleistocene climate  

SciTech Connect

A uniform oxygen isotope value of -25 per mil was obtained from old ground water at depths of 20 to 30 meters in a thick deposit of clay in the southern part of the glacial Lake Agassiz basin. The lake occupied parts of North Dakota and southern Manitoba at the end of the last glacial maximum and received water from the ice margin and the interior plains region of Canada. Ground water from thick late Pleistocene-age clay deposits elsewhere, a till in southern Saskatchewan, and a glaciolacustrine deposit in northern Ontario show the same value at similar depths. These sites are at about 50[degrees]N latitude, span a distance of 2000 kilometers, and like the Lake Agassiz sites, have a ground-water velocity of less than a few millimeters per year. The value of -25 per mil is characteristic of meltwater impounded in the southern basin of Lake Agassiz. This value corresponds to an estimated air temperature of -16[degrees]C, compared with the modern temperature of 0[degrees]C for this area. 15 refs., 5 figs.

Remenda, V.H.; Cherry, J.A.; Edwards, T.W.D. (Univ. of Waterloo, Ontario (Canada))

1994-12-23

309

Missing the forests for the trees: classical Pleistocene multiproxy records versus new geochemical tracers  

NASA Astrophysics Data System (ADS)

We present traditional paleoceanographic records from five DSDP/ODP cores ranging from 58.1N (Site 646), off W. Greenland to 41.5N (Site 607)on the Mid Atlantic Ridge, MAR(Fig. 1). The suite of proxies includes: grain size, clay mineral abundance, IRD and its source rock palynomorphs; foraminifera & their stable isotopes, SSTs and SSTw, coccoliths, dinocyst assemblages, SST and PP, and pollen markers of terrigenous carbon influx. Inter-site comparison shows major latitudinal shifts in intensity of glacial cooling cycles beginning in the Mid-Pleistocene (MIS 13-11), accompanied by floral and faunal re-organizations, shifts in carbon production and burial, and IRD sources. This reorganization clearly correlates with long-term shifts in Milankovitch orbital parameters. We compare these results with three sites where new proxies were used to reinterpret paleoceanography for the last 4 glacial cycles: Grand Banks site CH69-K09 (near site HU89-007-3) and MAR sites CHN82-2040 and IODP U1313 (re-occupation of DSDP site 607). New Fe and Ti markers of sediment remobilization confirm and refine our traditional palynological data. However, new interpretations of warmest and coldest conditions based on Mg/Ca-SSTs need to be re-evaluated against the old foraminifera-based SSTs from northern sites closer to icesheet margins, and in the context of updated, more precise surface current data.

Mudie, P.; Aksu, A. E.; McCarthy, F. M.; Rashid, H.

2009-12-01

310

Rapid thinning of the late Pleistocene Patagonian Ice Sheet followed migration of the Southern Westerlies  

PubMed Central

Here we present the first reconstruction of vertical ice-sheet profile changes from any of the Southern Hemisphere's mid-latitude Pleistocene ice sheets. We use cosmogenic radio-nuclide (CRN) exposure analysis to record the decay of the former Patagonian Ice Sheet (PIS) from the Last Glacial Maximum (LGM) and into the late glacial. Our samples, from mountains along an east-west transect to the east of the present North Patagonian Icefield (NPI), serve as ‘dipsticks' that allow us to reconstruct past changes in ice-sheet thickness, and demonstrates that the former PIS remained extensive and close to its LGM extent in this region until ~19.0?ka. After this time rapid ice-sheet thinning, initiated at ~18.1?ka, saw ice at or near its present dimension by 15.5?ka. We argue this rapid thinning was triggered by a combination of the rapid southward migration of the precipitation bearing Southern Hemisphere (SH) westerlies and regional warming.

Boex, J.; Fogwill, C.; Harrison, S.; Glasser, N. F.; Hein, A.; Schnabel, C.; Xu, S.

2013-01-01

311

Spawning sockeye salmon fossils in Pleistocene lake beds of Skokomish Valley, Washington  

NASA Astrophysics Data System (ADS)

An assemblage of fossil sockeye salmon was discovered in Pleistocene lake sediments along the South Fork Skokomish River, Olympic Peninsula, Washington. The fossils were abundant near the head of a former glacial lake at 115 m elevation. Large adult salmon are concentrated in a sequence of death assemblages that include individuals with enlarged breeding teeth and worn caudal fins indicating migration, nest digging, and spawning prior to death. The specimens were 4 yr old and 45-70 cm in total length, similar in size to modern sockeye salmon, not landlocked kokanee. The fossils possess most of the characteristics of sockeye salmon, Oncorhynchus nerka, but with several minor traits suggestive of pink salmon, O. gorbuscha. This suggests the degree of divergence of these species at about 1 million yr ago, when geological evidence indicates the salmon were deposited at the head of a proglacial lake impounded by the Salmon Springs advance of the Puget lobe ice sheet. Surficial geology and topography record a complicated history of glacial damming and river diversion that implies incision of the modern gorge of the South Fork Skokomish River after deposition of the fossil-bearing sediments.

Smith, Gerald R.; Montgomery, David R.; Peterson, N. Phil; Crowley, Bruce

2007-09-01

312

Parallel responses of bees to Pleistocene climate change in three isolated archipelagos of the southwestern Pacific.  

PubMed

The impacts of glacial cycles on the geographical distribution and size of populations have been explored for numerous terrestrial and marine taxa. However, most studies have focused on high latitudes, with only a few focused on the response of biota to the last glacial maximum (LGM) in equatorial regions. Here, we examine how population sizes of key bee fauna in the southwest Pacific archipelagos of Fiji, Vanuatu and Samoa have fluctuated over the Quaternary. We show that all three island faunas suffered massive population declines, roughly corresponding in time to the LGM, followed by rapid expansion post-LGM. Our data therefore suggest that Pleistocene climate change has had major impacts across a very broad tropical region. While other studies indicate widespread Holarctic effects of the LGM, our data suggest a much wider range of latitudes, extending to the tropics, where these climate change repercussions were important. As key pollinators, the inferred changes in these bee faunas may have been critical in the development of the diverse Pacific island flora. The magnitude of these responses indicates future climate change scenarios may have alarming consequences for Pacific island systems involving pollinator-dependent plant communities and agricultural crops. PMID:24807250

Groom, Scott V C; Stevens, Mark I; Schwarz, Michael P

2014-01-01

313

Glacial and marine chronology of Mars  

NASA Technical Reports Server (NTRS)

A summary is given of the glacial and marine chronology of Mars. Hydrological models of oceans and ice sheets, the cratering record, hydrological cycling, and episodic glaciation are discussed. Evidence for a Noachian ocean is evaluated.

Strom, Robert G.; Kargel, Jeffrey S.; Johnson, Natasha; Knight, Christine

1992-01-01

314

Galactic oscillations  

NASA Technical Reports Server (NTRS)

Long-lived oscillations that act like normal modes are described. The total kinetic energy is found to vary with time by amounts far in excess of the fluctuations expected from the virial theorem, and the variation shows periodic patterns that suggest oscillations. Experimental results indicate that oscillation amplitudes depend on the nature of the model. It is noted that it is difficult to answer questions about likely amplitudes in real galaxies with any confidence at the present time.

Miller, R. H.

1991-01-01

315

The Idea of Marine Exogenic Processes in Glacial and Contemporary Periods  

NASA Astrophysics Data System (ADS)

Ideas of exogenic processes on continental margin and in the open ocean of polar and moderate latitudes are based on the leading role of quaternary glaciation. Vast primary bottom relief and quaternary sediments' data, accumulated by the Russian marine research institutes, provided a possibility to have a new view on geomorphology formation of the North Atlantic, Norwegian - Greenland basins, West Arctic shelves and inland seas [Matishov, 1980, 1984; Matishov and Pavlova, 1990]. Analysis of bottom morphosculpture, including cartographic, geomorphologic, morpholithologic, seismoacoustic, and other methods, grounds our researches. As a result, previously held views on the forms' sculpturing and types have been reconsidered, as well as new theoretic principles of exogenic morphogenesis and vast continental glacial covers, spread onto the shelves and conditioning oceanic periglacial in deepwater parts of the ocean, have been developed. Glaciers of continental type repeatedly covered the continental shelves of Europe and North America in the period of quaternary glaciations [Markov et al., 1965; Matishov, 1980, 1986]. Reconstructing the genetic picture of bottom pre-glacier landscapes, large thawing waters' runoff valleys, sandr plains have been indicated, thus letting propose the idea of «periglacial shelves». There are no structures of analogous dimensions on land. Glacial morphogenesis, in many aspects, was determined by pre-quaternary structure-geomorphologic plan. Various glacial troughs, moraine ranges, water-glacier formations, now located at the depths from 50-200 to 400 m, are mapped on the glacial shelves in details [Matishov, 1984, 1987]. Capacities, substance composition, texture of moraine, fluvioglacial and glacial-marine sediments, composing the forms of glacial morphosculpture, have been ascertained. Most discussable is the problem of the Barents-Kara Sea shelf glaciation. Complex, but rather orderly Barents Sea shelf glacier morphosculpture, probably, was formed in the process of active spread of periphery parts of Scandinavian, Novaya Zemlya, Spitzbergen glacier covers from mainland to shelf. The fact is proved by detailed bathymetric maps, bottom relief regularities, lithology of subsurface moraines. Especially convincing are the newest radiocarbon dating of ancient coastlines of the Franz Josef Land, Novaya Zemlya, Kola Peninsula [Forman et al., 2004]. Cognition of marine and terrestrial ecosystems' evolution in contemporary and former periglacial zones requires quaternary geology and biology basic researches, reconstruction of Pleistocene and Holocene paleogeographic and paleoecological situations. Reconstruction of paleoclimate and paleobiogeocenoses (for instance, ancient soils) will let forecast dynamics of contemporary marine and terrestrial ecosystems in periglacial regions.

Matishov, G. G.

2009-04-01

316

Glacial erosion and expected permafrost thickness of Fennoscandia and adjacent regions  

NASA Astrophysics Data System (ADS)

Linked geological, geomorphological and tectonic features of Fennoscandia with adjacent regions of East-European plain and Barents-Kara shelf indirectly influenced the history of glacial grows and decays. The first-order bedrock landscape elements (often created or exhumed during pre-glacial Cenozoic stages) were the major factors that could partly control centers of ice nucleation and basal velocities, serve natural barriers shaping ice sheet margin during some time intervals, etc. On the hand, many landforms were powerfully modified by glacial and periglacial processes, in particular by strong glacial erosion with lithological and structural control. Quantitative estimation of Plio-Pleistocene erosion and deposition was performed combining regional geological-geomorphological analysis (GA) and modeling with rate-based time-scale reconstructions (RR), and mass-balance control. Of special GA importance was to compare and extract changes of preserved elements of pre-glacial Neogene topography from areas that underwent different duration of glacial activity, in comparison with bordering non-glaciated ones. More distinct radial glacial erosion pattern and larger basal ice velocities seem likely at the beginning of the early ice-age stage, with partial widening of pre-glacial drainage elements. Few wide lowlands with meandering rivers in permafrost condition could provoke early stage onset of topographic ice-streams. Over time, further complication of the pattern from radial to "spider web" is expected due to developing of topographic ice-streams. Worth to mention is progressive exhumation of resistant formations, additional complications of the pattern by fluvioglacial activity and glacial sedimentation, "pendulum" principle, with increasing amount of glacial and interglacial sedimentation in eroded material. Approximated variable permafrost distribution seems to be additional weighty aspect, changing erosion rates at some time intervals. To estimate mean annual temperatures and solve the Stefan's problem several known climate reconstructions were involved, but with account of possible ice-sheet related temperature depressions. In time-slices they were reinterpolated in agreement with changing the outlines of the ice sheets. Models of the basal sub-ice temperature based on relevant models for Greenland (Huybrechts P., 1996) and Antarctic ice sheets (Pattyn F., 2010) were accounted to estimate possible zonation and variability of warming effects of ice sheets. Expected lower permafrost thickness (first hundreds meters) and extent in the Barents region could be caused by unfavorable conditions and relatively high heat flow. Lowlands bearing major topographic ice streams were likely represented by taliks not affected by continuous permafrost or - depending on scenarios and parameters - were shortly affected by reduced permafrost with thick active layer. The same is expected for the Novaya Zemlya trench of the Kara Sea, while bordering shallow shelf parts were possibly characterized by thick permafrost, especially growing in time of eustatic ocean lowering. Permafrost in Fennoscandia and adjacent regions could be strongly variable but shortly relatively thick (hundreds meters) over large areas, including higher landscape on sedimentary cover west of Baltic - White Sea lowland. Linear taliks of discontinous permafrost zone on terrigenous sediments could contribute tunnel valley formation.

Amantov, Aleksey

2013-04-01

317

Observational and Model Constraints on Glacial Erosion  

NASA Astrophysics Data System (ADS)

Quantifying the controls on glacial erosion over geologic timescales is necessary to understand the role of Cenozoic climate change on the development of modern mountain belts. Unfortunately, understanding the spatial distribution of glacial erosion during repeated glaciations has proven difficult. We present results that integrate bedrock and detrital thermochronometer cooling ages with a glacial landscape evolution model. We use this to quantify the spatial distribution and temporal variability of glacial erosion in the Coast Mountains, British Columbia, Canada. A total of 100 apatite (U-Th)/He and 106 fission track single grain ages are presented from modern outwash of the Tiedemann Glacier whose catchment elevations range from 530-3960 m a.s.l.. Detrital thermochronometer ages utilize the tendency of thermochronometer cooling ages to increase with elevation and provide a sediment tracer for the elevation that eroded sediment is derived from. Bedrock ages used include 79 apatite (U-Th)/He ages collected in multiple catchments. Erosion rates derived from bedrock ages are compared to predicted erosion rates from a shallow-ice approximation glacial landscape evolution model of the region. Results from the observed distribution of detrital ages indicate that maximum glacial erosion occurs between elevations of 1200-1800m. Furthermore, near-uniform erosion is documented beneath the glacier with nearly all sediment derived from between elevations of 650- 3000 m a.s.l. Second, comparison of erosion rates derived from bedrock thermochronometer ages with the landscape evolution model suggest that a linear glacial sliding velocity is the primary control on erosion (r2=0.6). This result is important as it provides observational validation of the linear slide velocity erosion rule for million-year timescales. Finally, comparison of model and thermochronometer derived erosion rates reveals that active subglacial erosion occurs for only ~10-20% of a glacial-interglacial cycle. Taken together, these results highlight significant temporal variability in glacial erosion over a glacial-interglacial cycles as well as a new technique to quantify the spatial distribution of glacial erosion from sediments.

Ehlers, T. A.; Enkelmann, E.; Yanites, B. J.

2012-12-01

318

Glacial-lake deposits in the Mount Harper area, Yukon-Tanana Upland  

NASA Astrophysics Data System (ADS)

During the past year, two new C14 ages and pollen data threw light on some events in the Pleistocene history of the Yukon-Tanana Upland. Woody debris within a volcanic ash layer previously described (Weber, 1982) was dated, as well as some organic material from farther down in the same section. The locality with the new ages is an actively thawing 15-m-high cutbank in unconsolidated sediment on the Middle Fork of the Fortymile River about 16 km southeast of Mount Harper. Mount Harper, with an elevation of 1,994 m, is the highest point in the Yukon-Tanana Upland and has repeatedly supported active glacial systems. A detailed description of the sediments is given.

Weber, F. R.; Ager, T. A.

319

Glacial and nonglacial sediments of Matuyama paleomagnetic age on Banks Island, Canadian Arctic Archipelago  

NASA Astrophysics Data System (ADS)

Preliminary paleomagnetic investigations of unconsolidated sediments from Duck Hawk Bluffs on Banks Island, in the Canadian Arctic Archipelago, have defined magnetite-residing magnetizations that exhibit systematic polarity correlations between temporally equivalent units from spatially separate stratigraphic sections. The preglacial Worth Point Formation sediments and overlying Duck Hawk Bluff Formation sediments (including marine and glacial deposits laid down during the Banks Glaciation) have magnetically reversed directions and therefore are probably of Matuyama age (<730 Ka). Deposits of the younger Morgan Bluffs and Cape Collinson interglacials and Thomsen and Amundsen glaciations are normally magnetized and therefore of Brunhes age (<730 Ka). These sesults provide the first minimum-age estimate for the Worth Point Formation organic deposits and for the Banks Glaciation, the oldest and strongest glaciation recorded in the western Arctic. The new time framework will facilitate correlations with terrestrial sequences of Beringia and early Pleistocene Arctic Ocean sediments.

Vincent, Jean-Serge; Morris, William A.; Occhietti, Serge

1984-03-01

320

Pleistocene Glaciations on the Northwestern Tibet  

Microsoft Academic Search

As a result of its immense size and high elevation, the Tibetan plateau plays a major role in affecting global climatic changes, and in particular the Asian monsoon system. Consequently knowledge of its glacial evolution during the Quaternary is an essential parameter. However, the chronology and extent of Quaternary glaciations on the Tibetan plateau is still in debate. Based on

P. Kong; C. Na; F. Huang; D. Fink

2003-01-01

321

Middle Pleistocene palaeoenvironmental changes of the eastern Canary Islands - revealed by the Mála dune-palaeosol-sequence at Lanzarote (Canary Islands)  

NASA Astrophysics Data System (ADS)

The Canary Islands are located at the transition between the Mediterranean and the Saharan climate off NW-Africa. Thus, they are a key area for the investigation of palaeoenvironmental changes. Several terrestrial studies investigated the palaeoenvironmental development of that region during the later part of the last glacial cycle. However, apart from recent investigations of "vega" sediments on Lanzarote Island (Suchodoletz et al. 2010) the palaeoenvironmental evolution during the Middle Pleistocene is hardly studied yet, basically due to the lack of reliable geochronological data. The Mála dune-palaeosol-sequence is located in the north of Lanzarote. It consists of marine shell detritus originally blown out from the insular shelf during periods of low global sea level, and to a small part of Saharan dust and fine quartz sand. The aeolian layers are intercalated with up to eight silty-clayey palaeosol horizons. Unlike the dune sands, the soils indicate stable landscape conditions with trapping of Saharan dust. Using a combination of ESR and luminescence dating techniques, we are able to place this sequence into the Middle Pleistocene, in contrast to former investigations based on 14C datings postulating a Late Pleistocene age (Ortiz et al. 2006). As a consequence, clayey-silty palaeosols represent periods of stable landscape conditions in the Canarian region during the Middle Pleistocene, which we compare with marine palaeoclimatic studies from the area.

von Suchodoletz, H.; Zöller, L.; Hilgers, A.; Radtke, U.; Faust, D.

2012-04-01

322

Post-glacial recolonization of the Great Lakes region by the common gartersnake (Thamnophis sirtalis) inferred from mtDNA sequences.  

PubMed

Pleistocene events played an important role in the differentiation of North American vertebrate populations. Michigan, in particular, and the Great Lakes region, in general, were greatly influenced by the last glaciation. While several hypotheses regarding the recolonization of this region have been advanced, none have been strongly supported. We generated 148 complete ND2 mitochondrial DNA (mtDNA) sequences from common gartersnake (Thamnophis sirtalis) populations throughout the Great Lakes region to evaluate phylogeographic patterns and population structure and to determine whether the distribution of haplotypic variants is related to the post-Pleistocene retreat of the Wisconsinan glacier. The common gartersnake was utilized, as it is believed to have been one of the primary vertebrate invaders of the Great Lakes region following the most recent period of glacial retreat and because it has been a model species for a variety of evolutionary, ecological, behavioral, and physiological studies. Several genetically distinct evolutionary lineages were supported by both genealogical and molecular population genetic analyses, although to different degrees. The geographic distribution of the majority of these lineages is interpreted as reflecting post-glacial recolonization dynamics during the late Pleistocene. These findings generally support previous hypotheses of range expansion in this region. PMID:17174111

Placyk, John S; Burghardt, Gordon M; Small, Randall L; King, Richard B; Casper, Gary S; Robinson, Jace W

2007-05-01

323

Late Pleistocene glaciation of the Mt Giluwe volcano, Papua New Guinea  

USGS Publications Warehouse

The Mt Giluwe shield volcano was the largest area glaciated in Papua New Guinea during the Pleistocene. Despite minimal cooling of the sea surface during the last glacial maximum, glaciers reached elevations as low as 3200 m. To investigate changes in the extent of ice through time we have re-mapped evidence for glaciation on the southwest flank of Mt Giluwe. We find that an ice cap has formed on the flanks of the mountain on at least three, and probably four, separate occasions. To constrain the ages of these glaciations we present 39 new cosmogenic 36Cl exposure ages complemented by new radiocarbon dates. Direct dating of the moraines identifies that the maximum extent of glaciation on the mountain was not during the last glacial maximum as previously thought. In conjunction with existing potassium/argon and radiocarbon dating, we recognise four distinct glacial periods between 293-306 ka (Gogon Glaciation), 136-158 ka (Mengane Glaciation), centred at 62 ka (Komia Glaciation) and from >20.3-11.5 ka (Tongo Glaciation). The temperature difference relative to the present during the Tongo Glaciation is likely to be of the order of at least 5 ??C which is a minimum difference for the previous glaciations. During the Tongo Glaciation, ice was briefly at its maximum for less than 1000 years, but stayed near maximum levels for nearly 4000 years, until about 15.4 ka. Over the next 4000 years there was more rapid retreat with ice free conditions by the early Holocene. ?? 2011 Elsevier Ltd.

Barrows, T. T.; Hope, G. S.; Prentice, M. L.; Fifield, L. K.; Tims, S. G.

2011-01-01

324

Genetic consequences of Pleistocene glaciations for the tundra vole (Microtus oeconomus) in Beringia.  

PubMed

Repeated glacial events during the Pleistocene fragmented and displaced populations throughout the northern continents. Different models of the effects of these climate-driven events predict distinct phylogeographic and population genetic outcomes for high-latitude faunas. The role of glaciations in (i) promoting intraspecific genetic differentiation and (ii) influencing genetic diversity was tested within a phylogeographic framework using the rodent Microtus oeconomus. The spatial focus for the study was Beringia, which spans eastern Siberia and northwestern North America, and was a continental crossroads and potential high arctic refugium during glaciations. Variation in mitochondrial DNA (cytochrome b and control region; 214 individuals) and nuclear DNA (ALDH1 intron; 63 individuals) was investigated across the Beringian region. Close genetic relationships among populations on either side of the Bering Strait are consistent with a history of periodic land connections between North America and Asia. A genetic discontinuity observed in western Beringia between members of a Central Asian clade and a Beringian clade is geographically congruent with glacial advances and with phylogeographic discontinuities identified in other organisms. Divergent island populations in southern Alaska were probably initially isolated by glacial vicariance, but subsequent differentiation has resulted from insularity. Tests of the genetic effects of postglacial colonization were largely consistent with expansion accompanied by founder effect bottlenecking, which yields reduced diversity in populations from recently deglaciated areas. Evidence that populations in the Beringian clade share a history of expansion from a low-diversity ancestral population suggests that Beringia was colonized by a small founder population from central Asia, which subsequently expanded in isolation. PMID:14653795

Galbreath, Kurt E; Cook, Joseph A

2004-01-01

325

Middle Pleistocene glaciations ofthe Russian North  

Microsoft Academic Search

Geological data on the pre-Eemian glaciations ofnorthern Russia, including the latest results by the Russian-Norwegian PECHORA project, are synthesized in order to present evidence for comparison with other early glaciations around the Arctic. The bulk ofevidence indicates that Arctic and Subarctic regions ofEuropean Russia, ofwestern and central Siberia during the Middle Pleistocene were at least 4 times covered by large

Valery Astakhov

326

Simulated variations of eolian dust from inner Asian deserts at the mid-Pliocene, last glacial maximum, and present day: contributions from the regional tectonic uplift and global climate change  

Microsoft Academic Search

Northern Tibetan Plateau uplift and global climate change are regarded as two important factors responsible for a remarkable\\u000a increase in dust concentration originating from inner Asian deserts during the Pliocene–Pleistocene period. Dust cycles during\\u000a the mid-Pliocene, last glacial maximum (LGM), and present day are simulated with a global climate model, based on reconstructed\\u000a dust source scenarios, to evaluate the relative

Zhengguo Shi; Xiaodong Liu; Zhisheng An; Bingqi Yi; Ping Yang; Natalie Mahowald

327

Pleistocene calcrete deposits from southern Spain as indicators of climatic conditions and tectonic activity  

NASA Astrophysics Data System (ADS)

Quaternary calcrete horizons are common weathering products in arid and semi-arid regions of southern Spain. We have analysed a calcrete profile developed within poorly sorted gravels of an alluvial fan. These deposits were sourced from the Carrascoy Range, a fault generated mountain front located in the Internal Zones of the Betic Cordillera (South Spain). During the Pleistocene the climate in southern Spain was dry, either in the form of semi-arid/arid conditions or as seasonal moisture deficits. Alluvial channel incision trends appeared to be disrupted by episodes of alluvial aggradation produced during cold and dry glacial periods. At the top of the aggradational phases, pedogenic processes operated profusely, and, as a result, several calcretes (stage V mature calcrete profiles) were formed. We have analysed one of these calcrete profiles that appears subvertical within the forelimb of a regional fold in relation to the Carrascoy Fault activity. The calcrete consist of a densely cemented hardpan horizon (20 to 40 cm thick) overlain by a thin, 2-cm thick laminar crust. Below the hardpan horizon, carbonate concentrations gradually decrease to clast-coating textures. Calcretes form progressively and a wide range of carbonate phases occur within a single horizon, being the laminar crust the final stage of evolution within a mature pedogenic calcrete deposit, and, therefore, the carbonate within it postdates all the cement phases within the profile. The location of the latest cement phase of the calcrete deposit has been estimated by microscopic observations (to establish their suitability for dating) together with a detailed sedimentological analysis of the calcrete profile in the field. The laminar crust consists of less than 1 mm thick laminae characterized by the alternation of layers of micrite and layers of micrite with ooids, detrital grains and clays indicating environmental conditions in which sedimentation rates were low and episodic. By using radiometric 232Th/238U dating methods we obtain an age of formation of the laminar crust of ~209 Ka (upper part of the Middle Pleistocene). This age corresponds to the warm stage 7c within the glacial Riss period. As pointed out before, the studied calcrete appears subvertical as a result of the activity of the Carrascoy Fault, and, therefore, the fault was active only after the calcrete was formed. Our study permits, as well, to reliably asses the timing of changes in alluvial processes, to characterized this part of the stratigraphic succession as corresponding to an arid to semi-arid environment, and to conclude that this particular calcrete was developed during a relative European warm period within a glacial stage.

Herrero, Maria J.; Insua-Arevalo, Juan M.; Garcia-Mayordomo, Julian; Martin-Banda, Raquel

2014-05-01

328

Late Pleistocene Environments of the Central Ukraine  

NASA Astrophysics Data System (ADS)

The Vyazivok loess sequence from the Dnieper Plain, Ukraine, documents regional environmental changes during the late Pleistocene and Holocene. Pedological and palynological analyses and low-field magnetic susceptibility document changes from dense temperate forest during the last interglacial maximum to open, harsh, loess-steppe during the latest Pleistocene. The Vyazivok section overlies hillwash derived from a lower Pleistocene terrace and consists of two stratified soil complexes (Kaydaky and Pryluky; marine isotope stage [MIS] 5 equivalent) separated by a layer of eolian dust (Tyasmyn silt). The lower soils in both complexes formed within forest. These soils are overlain by the Uday (MIS4) and Bug (MIS2) loess units, which are separated by boreal soils of the Vytachiv (MIS3) complex. The coldest conditions within the record occurred in the youngest loess. Holocene soils cap the Bug loess. The Vyazivok section shows remarkable similarities with other classical loess sequences in western Europe, the Czech Republic, and Austria. The Kaydaky, Pryluky, and Vytachiv deposits, correlate with the PKIII, PKII, and PKI soil complexes, respectively, of the Czech Republic. The Tyasmyn and Prylyky silt layers correspond to marker horizons from central Europe.

Rousseau, Denis-Didier; Gerasimenko, Natalia; Matviischina, Zhanna; Kukla, George

2001-11-01

329

Pleistocene vertebrates of the Yukon Territory  

NASA Astrophysics Data System (ADS)

Unglaciated parts of the Yukon constitute one of the most important areas in North America for yielding Pleistocene vertebrate fossils. Nearly 30 vertebrate faunal localities are reviewed spanning a period of about 1.6 Ma (million years ago) to the close of the Pleistocene some 10 000 BP (radiocarbon years before present, taken as 1950). The vertebrate fossils represent at least 8 species of fishes, 1 amphibian, 41 species of birds and 83 species of mammals. Dominant among the large mammals are: steppe bison ( Bison priscus), horse ( Equus sp.), woolly mammoth ( Mammuthus primigenius), and caribou ( Rangifer tarandus) - signature species of the Mammoth Steppe fauna ( Fig. 1), which was widespread from the British Isles, through northern Europe, and Siberia to Alaska, Yukon and adjacent Northwest Territories. The Yukon faunas extend from Herschel Island in the north to Revenue Creek in the south and from the Alaskan border in the west to Ketza River in the east. The Yukon holds evidence of the earliest-known people in North America. Artifacts made from bison, mammoth and caribou bones from Bluefish Caves, Old Crow Basin and Dawson City areas show that people had a substantial knowledge of making and using bone tools at least by 25 000 BP, and possibly as early as 40 000 BP. A suggested chronological sequence of Yukon Pleistocene vertebrates ( Table 1) facilitates comparison of selected faunas and indicates the known duration of various taxa.

Harington, C. R.

2011-08-01

330

Palaeoenvironmental and cultural dynamics of the coast of Málaga (Andalusia, Spain) during the Upper Pleistocene and early Holocene  

NASA Astrophysics Data System (ADS)

The impact of late Upper Pleistocene climate change in the western Mediterranean region has been mainly documented through marine records. Archaeological and geomorphological continental records now available for the coast of Málaga complement these records for the second part of the last major glacial episode and the early stages of the Holocene. This paper provides an overview of the archaeological, chronological and palaeoenvironmental data from the end of the Middle Palaeolithic period to the Epipalaeolithic. Sequences from the two major sites, Nerja and Bajondillo, indicate a mosaic-type response of ecosystems to the rapidly shifting conditions of the Last Glacial episode. Terrestrial mammals, for example, show no major variations from present-day communities. Plant remains in contrast, demonstrates the existence of localised refuge areas for species that would not otherwise have survived in the more widely prevailing climatic conditions of the time, whereas remains of fish and, secondarily, birds demonstrate the existence of communities without any present-day analogues combining Mediterranean and Boreal (i.e., northern Atlantic) taxa. From an archaeological standpoint, the major cultural shift is the onset of marine fishing, beginning in the Solutrean. This coincided with the end of the Last Glacial Maximum (LGM) episode. The faunal record also testifies to the rising importance of marine resources more generally. Although the data have not been used systematically to test the validity of the Broad Spectrum Revolution (BSR) hypothesis, the hints for it in the area seem compelling from the start of the Solutrean onwards.

Cortés-Sánchez, Miguel; Morales-Muñiz, Arturo; Simón-Vallejo, María D.; Bergadà-Zapata, M. Mercè; Delgado-Huertas, Antonio; López-García, Pilar; López-Sáez, José A.; Lozano-Francisco, M. Carmen; Riquelme-Cantal, José A.; Roselló-Izquierdo, Eufrasia; Sánchez-Marco, Antonio; Vera-Peláez, José L.

2008-11-01

331

Evidence for an early glacial maximum in the French Vosges during the last glacial cycle  

Microsoft Academic Search

THE Last Glacial Maximum is observed to have occurred between 18,000 and 20,000 years ago throughout northern Europe. Here we present evidence for the surprising result that in the region of the French Vosges, the regional ice sheet advanced during the early part of the last glacial cycle, and this expansion was considerably larger than that which occurred during the

G. Seret; E. Dricot; G. Wansard

1990-01-01

332

Glacial Productivity Regimes in the Southern Ocean  

NASA Astrophysics Data System (ADS)

The contemporary Antarctic Circumpolar Current (ACC) is a weak CO2 sink despite high nutrient levels because productivity is limited by iron availability. During glacial periods much greater iron availability should have significantly enhanced productivity and CO2 drawdown. However, there is ongoing debate on the physical and biological mechanisms in the Southern Ocean causing the lowering of the glacial CO2 concentrations. Most paleochemical proxies indicate a latitudinal shift in the zone of enhanced glacial primary productivity and intensified CO2 drawdown but no overall increase in biogenic export. Other lines of explanation involve physical mechanisms restricting the Southern Ocean/atmosphere gas exchange via surface water stratification or sea ice coverage. However, the validity of such hypotheses to represent the major or sole mechanism steering the CO2 draw-down, has been questioned, based on theoretical grounds and numerical modeling, respectively. Here we present evidence for last glacial Southern Ocean conditions from biological proxies. We could detect resting spores of the diatom genus Chaetoceros as a proxy to indicate iron induced extensive diatom blooms across the entire Atlantic sector of the ACC, particularly in the seasonal sea-ice covered zone (SIZ) during the last glacial. In addition, we used the ecological information of the radiolarian Cycladophora davisiana gathered from plankton and surface sediment investigations of the Sea of Okhotsk to notify high glacial phytodetritus export in this area. The areal and downcore distribution patterns of these primary producer and phytodetritus feeding, deep living protozoans point to the occurrence of a high productive biological regime, dominated by fast growing thin-walled diatoms and non siliceous primary producer (e.g. Phaeocystis), which lead to high export of organic matter to the deep ocean. The dominance of the deep living radiolarian Cyladophora davisiana in glacial SIZ sediments indicates that organic carbon export to mesopelagic depths was at least ten-fold higher than today.

Abelmann, A.; Rainer, G.; Victor, S.

2006-12-01

333

Pleistocene glaciations of Central Asia: results from 10Be surface exposure ages of erratic boulders from the Pamir (Tajikistan), and the Alay–Turkestan range (Kyrgyzstan)  

Microsoft Academic Search

We have determined the timing of glaciations in the Pamir (Tajikistan) and the Alay–Turkestan Range (Kyrgyzstan) using 10Be surface exposure dating. Glacial advances in the area have occurred >93–136, ?60–80, (40–55), ?27–25, ?22–20, ?19–17, ?16–15, ?15–13, and 11–9calka BP. All Late Pleistocene glaciers in the Pamir, and the Alay–Turkestan Ranges have been valley glaciers except for the most extended glaciers

U. Abramowski; A. Bergau; D. Seebach; R. Zech; B. Glaser; P. Sosin; P. W. Kubik; W. Zech

2006-01-01

334

Sea surface temperatures from the southern Benguela region from the Pliocene and Pleistocene: tracking Agulhas Current input into the SE Atlantic  

NASA Astrophysics Data System (ADS)

The Pliocene and-Pleistocene epochs provide a way to understand the effect of past climate changes on key ocean currents. Here, we show results from ODP Site1087 (31.28'S, 15.19'E, 1374m water depth) to investigate changes in ocean circulation over the period of the mid-Pliocene warm period 3.0-3.5 Ma and compare these to the time of the 100 kyr Pleistocene glacial cycles. ODP 1087 is located in the South-eastern Atlantic Ocean, outside of the Benguela upwelling region; reconstructing the temperature history of the site will therefore provide an important data set from a part of the ocean that has few orbital-scale and continuous Pliocene temperature reconstructions. ODP 1087 can be used to investigate the history of the heat and salt transfer to the Atlantic Ocean from the Indian Ocean via the Agulhas Retroflection, which plays an important part in the global thermohaline circulation (Lutjeharms, 2007). Climate models and reconstructions for the most recent glacial-interglacial cycles have shown that changes to the strength of the heat transfer may cause major climatic changes and may play a role in transitions from glacial to interglacial events (Knorr & Lohmann, 2003). It is unknown how this transfer reacted to generally warmer global temperatures during the mid-Pliocene. Because the mid-Pliocene is seen as a model for future climate change it might provide a model for ocean circulations in a warmer world. Our approach is to apply several organic geochemistry proxies and foraminiferal analyses to reconstruct the history of ODP 1087. The UK37' index records differences in the unsaturated bonds in the C37 alkenones to reconstruct sea surface temperatures (Brassell et al., 1986). We present SSTs generated for the mid-Pliocene Warm period with a resolution of 4000 years. We compare this data to the time of the 100 kyr glacial cycles during the late Pleistocene. Even though ODP 1087 is located outside the Benguela upwelling system, it has lower Pliocene temperatures than sites to the north which are thought to be affected by the upwelling. However, our combination of alkenone and chlorin data indicates a potentially different productivity regime at this site in the Pliocene than exists in the late Pleistocene. To investigate the controls over these results we also present evidence from planktonic foraminifera assemblages to indicate the presence of warm salty Indian Ocean waters to the Atlantic (and thus active Agulhas Leakage in the Pliocene) and/or the presence of nutrient-rich upwelling waters.

Petrick, B. F.; McClymont, E.; Felder, S.; Lloyd, J. M.; Leng, M. J.

2011-12-01

335

Glacial to Holocene dynamics of Indonesian precipitation - New insights from plant-wax dD off Northwest Sumatra  

NASA Astrophysics Data System (ADS)

We used the stable hydrogen and stable carbon isotopic composition (dD and d13C, respectively) of terrestrial plant leaf waxes as a proxy for past rainfall variations over northwestern Indonesia. Our study site lies within the western boundary of the Indo-Pacific Warm Pool (IPWP), a key evaporative site for the global hydrologic cycle. At present, rainfall intensity in tropical Indonesia is influenced by the Pacific Ocean El Nino Southern Oscillation (ENSO) (see Kirono et al., 1999), the Indian Ocean Dipole (IOD) mode (Saji et al., 1999), and to some extend by the position of the Intertropical Convergence Zone (ITCZ) (e.g. Koutavas and Lynch-Stieglitz, 2005). Paleoclimate studies show that these systems have varied in the past, however, the impact of these changes on regional paelo-hydrology of Indonesia is yet unknown. We worked on marine sediment core SO189-144KL (1°09,300 N; 98°03,960 E) retrieved at 480 m water depth off Northwest Sumatra from the eastern Indian Ocean. Sediments consist of material from marine and terrestrial sources, and radiocarbon dating indicates an age of ~300 years at the core top and of ~24,000 years at the base. We used d13C and dD values of the n-C30 alkanoic acid as proxies for changes in vegetation composition (C3 vs. C4 plants) and rainfall variability on land, respectively. Values of d13C show only little variation and suggest persistent dominance of tropical trees throughout the past 24,000 years. Values of dD display distinct variability throughout the record, however, mean rainfall intensities during the late Last Glacial compare to those during the Holocene. This is in agreement with rather consistent vegetation at the study site but in sharp contrast with reconstructions of contemporaneous rainfall patterns at the nearby islands Borneo (Partin et al., 2007) and Flores (Griffiths et al., 2009), indicating multiple controls on regional hydrology of Indonesia. In combination with previous studies of late Pleistocene to Holocene ENSO and IOD variability, we further address the complex controls on Indonesian climate with emphasis of Holocene rainfall variability. References Griffiths, M.L., Drysdale, R.N., Gagan, M.K., Zhao, J.x., Ayliffe, L.K., Hellstrom, J.C., Hantoro, W.S., Frisia, S., Feng, Y.x., Cartwright, I., Pierre, E.S., Fischer, M.J., Suwargadi, B.W., 2009. Increasing Australian-Indonesian monsoon rainfall linked to early Holocene sea-level rise. Nature Geoscience 2, 636-639. Kirono, D.G.C., Tapper, N.J., McBride, J.L., 1999. Documenting Indonesian rainfall in the 1997/1998 El Nino event. Physical Geography 20, 422-435. Koutavas, A., Lynch-Stieglitz, J., 2005. Variability of the marine ITCZ over the eastern Pacific during the past 30,000 years: Regional perspective and global context. In: Bradley, R.S., Diaz, H.F. (Eds.), The Hadley Circulation: Present Past and Future. Springer, pp. 347-369. Partin, J.W., Cobb, K.M., Adkins, J.F., Clark, B., Fernandez, D.P., 2007. Millennial-scale trends in west Pacific warm pool hydrology since the Last Glacial Maximum. Nature 449, 452-455. Saji, N.H., Goswami, B.N., Vinayachandran, P.N., Yamagata, T., 1999. A dipole mode in the tropical Indian Ocean. Nature 401, 360-363.

Niedermeyer, E. M.; Mohtadi, M.; Sessions, A. L.; Feakins, S. J.

2012-12-01

336

QUANTIFICATION OF GLACIAL EROSION IN THE ALPS USING VERY LOW-TEMPERATURE THERMOCHRONOLOGY (OSL & AHe)  

NASA Astrophysics Data System (ADS)

The impact of glaciations on the topography of the Alps is still unclear: Long-term denudation rate determined by low-T thermochronology are in the range of 0.2 to 1 mm/yr, and increased during the Plio-Quaternary by 3 fold (Vernon et al., 2008). Such an increase is also documented by peri-alpine sediment budget (Kuhleman, 2000), with a similar increase in sediment yields since 5-3 Ma. This increase was considered as evidence of a climatically-driven surface process change, attributed to increased precipitation (Cederbom et al., 2004) and erosion by glacial processes (Champagnac et al., 2007). The timing of the onset of intense glacial erosion as well as its rates are still ambiguous. The glacial erosion seems to have accelerated around 0.9 Ma as suggested by the ten fold increase of incision rates of a valley in the Central Alps (Häuselmann et al., 2007), and by information about vegetation and sedimentologic changes (Muttoni et al., 2003). There is however no direct quantification of topographic change during the Plio-Quaternary. We present here how we use OSL-thermochronology, a new thermochronometer of exceptionally low closure temperature (about 30°-40°C) (Herman et al subm.), new {U-Th}/He on apatites data, and a glacial erosion model (Herman and Braun 2008) to estimate topographic changes in the Alps in response to glaciations. Because of their low closure temperature, OSL and AHe thermochronology enables quantification of events of less than 1 Ma at very small wavelength of the topography. We collected two vertical profiles, one in the Zermatt Valley (Valais) and one in Maurienne Valley (Savoy). We infer from these results changes in topography, date and quantify relief creation under glacial-interglacial cycles. Cederbom, C.E, et al., Climate induced rebound and exhumation of the European Alps. Geology 32, 709-712 (2000). Champagnac, J.-D., et al., Quaternary erosion-induced isostatic rebound in the western Alps. Geology 35, 195-198 (2007). Haüselmann P., et al.,et al. Abrupt glacial valley incision at 0.8 Ma dated from cave deposits in Switzerland. Geology 35, 33-42 (2007). Herman F. and Braun J. Evolution of the glacial landscape of the Southern Alps of New Zealand: Insights from a glacial erosion model, J. Geophys. Res., 113, F02009, doi:10.1029/2007JF000807 (2008). Herman F., Rhodes E.J. and Braun J. A new thermochronometer reveals steady state relief and exhumation in a small active orogen during the last glacial cycle, submitted. Kuhlemann J., et al., Quantifying tectonic versus erosive denudation by the sediment budget: the Miocene core complexes of the Alps, Tectonophysics 330, 1-23 (2000). Muttoni G., et al., Onset of major Pleistocene glaciations in the Alps. Geology 31, 989-992 (2003). Vernon, A.J., et al., Increase in late Neogene denudation of the European Alps confirmed by analysis of a fission-track thermochronology database. Earth and Planetary Science Letters, 270 (3-4), pp. 316-329 (2008).

Champagnac, J.; Herman, F.; Rhodes, E. J.; Fellin, M.; Jaiswal, M.; Schwenninger, J.; Reverman, R. L.

2009-12-01

337

Glacially generated overpressure on the New England continental shelf: Integration of full-waveform inversion and overpressure modeling  

NASA Astrophysics Data System (ADS)

zones of high-amplitude, discontinuous seismic reflections 100 km off the coast of Massachusetts, USA, have P wave velocities up to 190 m/s lower than those of adjacent sediments of equal depth (250 m below the sea floor). To investigate the origin of these low-velocity zones, we compare the detailed velocity structure across high-amplitude regions to adjacent, undisturbed regions through full-waveform inversion. We relate the full-waveform inversion velocities to effective stress and overpressure with a power law model. This model predicts localized overpressures up to 2.2 MPa associated with the high-amplitude reflections. To help understand the overpressure source, we model overpressure due to erosion, glacial loading, and sedimentation in one dimension. The modeling results show that ice loading from a late Pleistocene glaciation, ice loading from the Last Glacial Maximum, and rapid sedimentation contributed to the overpressure. Localized overpressure, however, is likely the result of focused fluid flow through a high-permeability layer below the region characterized by the high-amplitude reflections. These high overpressures may have also caused localized sediment deformation. Our forward models predict maximum overpressure during the Last Glacial Maximum due to loading by glaciers and rapid sedimentation, but these overpressures are dissipating in the modern, low sedimentation rate environment. This has important implications for our understanding continental shelf morphology, fluid flow, and submarine groundwater discharge off Massachusetts, as we show a mechanism related to Pleistocene ice sheets that may have created regions of anomalously high overpressure.

Siegel, Jacob; Lizarralde, Daniel; Dugan, Brandon; Person, Mark

2014-04-01

338

The drainage and glacial history of the Still River Valley, southwestern Connecticut  

USGS Publications Warehouse

The Still River is located in southwestern Connecticut. From its origin on the New York border, it passes through Danbury and flows northward to its junction with the Housatonic River in New Milford. Interpretation of the Still River's history is based on its surficial geology and bedrock topography. High bedrock surfaces to the south, east, and west of the river show that its preglacial direction was probably to the north. The Still River has developed along the easily eroded Inwood Marble as a subsequent tributary to the Housatonic. Pleistocene glaciation left a variety of deposits in the Still Valley. The oldest of these is the 'lower' till, of either Illinoian or Altonian age. This till unit is overlain in turn by the Woodfordian 'upper' till. The upper till has basal and ablation facies. Ice-contact deposits formed in the fringing stagnation zone of the last retreating ice sheet. As the glacier withdrew along the Still Valley, preglacial Lake Danbury was impounded against the highlands to the south. Glacial retreat opened progressively lower outlets for this lake. Its final stage was contained by a till (?) barrier at the Housatonic Gorge in New Milford. Filling of the lake by glacial outwash was soon followed by downcutting of the dam and establishment of the modern Housatonic and Still River channels.

Thompson, Woodrow B.

1971-01-01

339

Mite dispersal among the Southern Ocean Islands and Antarctica before the last glacial maximum.  

PubMed

It has long been maintained that the majority of terrestrial Antarctic species are relatively recent, post last glacial maximum, arrivals with perhaps a few microbial or protozoan taxa being substantially older. Recent studies have questioned this 'recolonization hypothesis', though the range of taxa examined has been limited. Here, we present the first large-scale study for mites, one of two dominant terrestrial arthropod groups in the region. Specifically, we provide a broad-scale molecular phylogeny of a biologically significant group of ameronothroid mites from across the maritime and sub-Antarctic regions. Applying different dating approaches, we show that divergences among the ameronothroid mite genera Podacarus, Alaskozetes and Halozetes significantly predate the Pleistocene and provide evidence of independent dispersals across the Antarctic Polar Front. Our data add to a growing body of evidence demonstrating that many taxa have survived glaciation of the Antarctic continent and the sub-Antarctic islands. Moreover, they also provide evidence of a relatively uncommon trend of dispersals from islands to continental mainlands. Within the ameronothroid mites, two distinct clades with specific habitat preferences (marine intertidal versus terrestrial/supralittoral) exist, supporting a model of within-habitat speciation rather than colonization from marine refugia to terrestrial habitats. The present results provide additional impetus for a search for terrestrial refugia in an area previously thought to have lacked ice-free ground during glacial maxima. PMID:20943685

Mortimer, E; Jansen van Vuuren, B; Lee, J E; Marshall, D J; Convey, P; Chown, S L

2011-04-22

340

Glacial-interglacial variability of the Greenland Ice Sheet in the Pliocene  

NASA Astrophysics Data System (ADS)

Significant built-up of ice caps on Greenland is believed to have begun in the Pliocene (5.33 - 2.58 Ma). The ice sheet has likely been highly dynamic, and paced by Milankovitch orbital cycles, variations in atmospheric carbon dioxide concentrations (pCO2 [ppmv]) and internal forcings, and feedbacks related to both local energy balance and far-field influences. The likely locations, extents and volumes of continental ice on Greenland during the Pliocene remain largely unconstrained. Feedback mechanisms involving coupling between ice sheets and sea ice have been proposed to have a significant impact on the Pleistocene climate-cryospheric system and the pacing of glacial-interglacial cycles. Here, we show the results of a sensitivity study exploring (i) the sensitivity of Northern Hemisphere sea ice to forcing and (ii) the impact of sea ice on northern-hemispheric climatic and cryospheric evolution through the Plio-Pleistocene. We investigate the threshold for the growth and decay of the Greenland Ice Sheet and the ice sheet's impact on sea ice extent and thickness with a coupled GCM-ice sheet model using combining sets of orbital parameters, and pCO2 levels. Additionally, we test the dynamic response of vegetation and assess the vegetation feedback under the prescribed orbits and pCO2. The paleoclimate simulations use both modern and ice-free isostatically equilibrated Greenland topography with modern (control) and cold and warm boreal summer orbits, and a range of pCO2 levels chosen to broadly represent both warm Pliocene and cold Pleistocene conditions. Model simulations are compared with available sea surface temperature and other proxy data on and around Greenland. We find that Pliocene Greenland (at elevated CO2 levels) is highly sensitive to orbital forcing. Sea ice-albedo feedback has important effects on the hydrological cycle and largely governs insulation receipts at the surface. Sea ice cover modifies moisture availability by controlling the air-sea fluxes and alters the atmospheric circulation and has far reaching effects by controlling heat and moisture transport. Vegetation changes over ice-free Greenland significantly impact summer temperature with consequences for glacial onset during colder orbital periods. Given our model results, we conclude that internal processes and feedbacks are capable of significantly modifying thresholds for inception and decay of ice on Greenland, especially in the Pliocene, when CO2 concentrations were comparable to today.

Koenig, S. J.; Deconto, R.; Pollard, D.

2010-12-01

341

Preliminary Results from a Late Pleistocene to Holocene Paleoclimate Study of the Lake Sediment Cores, Northern New Mexico  

NASA Astrophysics Data System (ADS)

We present the preliminary results from an integrated, paleoclimatic study of sediment cores collected from the Las Vegas National Wildlife Refuge (LVNWR) and surrounding region that bear on the late Pleistocene to Holocene paleoclimatic variations in northeastern NM. We collected sedimentologic, midge fossil, and rock magnetic data from sediment cores to characterize the materials, identify stratigraphic changes, document shifting lake levels, assess temperature changes, and infer paleoclimate conditions. Data from McAllister and Wallace Lake are encouraging and reveal depth dependent changes in fossil assemblages, grain size, and rock magnetic properties that we interpret to reflect climatic driven variations impacting the depositional system. We recognize three different types of chironomid subfamilies (Chironomini, Tanypodinae, and Orthocladiinae). Based on the fossil results, the water has been warm in the most recent years. Grain size distribution from the lower to upper core levels reveal that the amount of fine sand-sized sediment (0.125 mm diameter) increases while the amount of medium (0.25) to coarse (0.50) sand-sized sediment decreases implying that there may have been a reduction in stream energy and hence precipitation over the time period represented by the core. Bulk low-field magnetic susceptibility decreases by an order of magnitude from the surface to the base of the measured core suggesting a change in detrital magnetic influx into the lacustrian system. Curie point estimates indicate that the dominant magnetic mineral in all samples is cubic, low-Ti titanomagnetite phase. We postulate that concurrent with alpine glacial activity during the Pleistocene, the LVNWR and the transitional Great Plains region to the northeast was an expansive single lake or interconnected lake system, analogous to the Pleistocene lakes of the Estancia Basin (Lake Estancia) and the Tularosa Basin (Lake Otero) of central and southern NM. Following the end of glacial activity, these lacustrian systems shrank to their current condition of minor low-volume isolated lakes and numerous playas and pluvial bodies. We hypothesize that sediments from the LVNWR and surrounding playas contain an invaluable and untapped record of late Pleistocene to Holocene climatic change.

Cedillo, D. N.; Brister, A. R.; LoPresti, C. A.; Maldonado, M.; Pitrucha, R. M.; West, C.; Martinez, E.; Lineline, J.; Petronis, M. S.

2011-12-01

342

Late Pliocene to early Pleistocene changes in the North Atlantic Current and suborbital-scale sea-surface temperature variability  

NASA Astrophysics Data System (ADS)

The strength and latitudinal position of the North Atlantic Current, NAC, determines the position of the Arctic front and heat transport to the high northern latitudes with potentially important consequences for Northern Hemisphere glaciation. A southward shift in the NAC and reduced poleward heat transport is hypothesized to have triggered the last major climate transition in Earth's history—late Pliocene intensification of Northern Hemisphere glaciation (iNHG). In turn, iNHG is hypothesized to have led to the amplification of climate variability on suborbital time scales. To date, however, only a handful of adequately resolved records are available to test these two hypotheses. Here we present a new late Pliocene to earliest Pleistocene record from Integrated Ocean Drilling Program Site U1313 (North Atlantic, 41°N; 2.9 to 2.4 Ma). We use Mg/Ca-derived paleotemperature records in planktic foraminiferal calcite to investigate changes in summer sea-surface temperatures (SST) on orbital and suborbital time scales. Our results call into question the suggestion that significant weakening and/or southward shift of the NAC served as a trigger for Northern Hemisphere cooling and intensified continental ice sheet growth across iNHG. In contrast to the late Pleistocene, during iNHG, we find that the position of the NAC and Arctic Front probably lay well to the north of Site U1313 and that the amplitude of suborbital SST variability did not change on glacial-interglacial time scales. Conservative estimates of Late Pliocene to earliest Pleistocene interglacial summer SSTs in our record are up to 3°C warmer than present, while glacial summer SSTs are only 2°C to 3°C cooler. In fact, our interglacial summer SSTs are remarkably similar to those of the mid-Pliocene. Our findings indicate that iNHG must have involved amplifying feedback mechanisms that are tightly coupled to ice sheet growth but that these processes were insufficiently developed by the late Pliocene/earliest Pleistocene to have triggered large amplitude changes in suborbital climate in the midlatitude North Atlantic.

Friedrich, Oliver; Wilson, Paul A.; Bolton, Clara T.; Beer, Christopher J.; Schiebel, Ralf

2013-06-01

343

Power oscillator  

DOEpatents

An oscillator includes an amplifier having an input and an output, and an impedance transformation network connected between the input of the amplifier and the output of the amplifier, wherein the impedance transformation network is configured to provide suitable positive feedback from the output of the amplifier to the input of the amplifier to initiate and sustain an oscillating condition, and wherein the impedance transformation network is configured to protect the input of the amplifier from a destructive feedback signal. One example of the oscillator is a single active element device capable of providing over 70 watts of power at over 70% efficiency. Various control circuits may be employed to match the driving frequency of the oscillator to a plurality of tuning states of the lamp.

Gitsevich, Aleksandr (Montgomery Village, MD)

2001-01-01

344

Raindrop oscillations  

NASA Technical Reports Server (NTRS)

A model of the change in shape of a raindrop is presented. Raindrops measured by two orthogonal cameras were classified by shape and orientation to determine the nature of the oscillation. A physical model based on potential energy was then developed to study the amplitude variation of oscillating drops. The model results show that oscillations occur about the equilibrium axis ratio, but the time average axis ratio if significantly more spherical for large amplitudes because of asymmetry in the surface potential energy. A generalization of the model to oscillations produced by turbulence yields average axis ratios that are consistent with the camera measurements. The model results for average axis ratios were applied to rainfall studies with a dual polarized radar.

Beard, K. V.

1982-01-01

345

The impact of Pleistocene climate change on an ancient arctic-alpine plant: multiple lineages of disparate history in Oxyria digyna  

PubMed Central

The ranges of arctic–alpine species have shifted extensively with Pleistocene climate changes and glaciations. Using sequence data from the trnH-psbA and trnT-trnL chloroplast DNA spacer regions, we investigated the phylogeography of the widespread, ancient (>3 million years) arctic–alpine plant Oxyria digyna (Polygonaceae). We identified 45 haplotypes and six highly divergent major lineages; estimated ages of these lineages (time to most recent common ancestor, TMRCA) ranged from ?0.5 to 2.5 million years. One lineage is widespread in the arctic, a second is restricted to the southern Rocky Mountains of the western United States, and a third was found only in the Himalayan and Altai regions of Asia. Three other lineages are widespread in western North America, where they overlap extensively. The high genetic diversity and the presence of divergent major cpDNA lineages within Oxyria digyna reflect its age and suggest that it was widespread during much of its history. The distributions of individual lineages indicate repeated spread of Oxyria digyna through North America over multiple glacial cycles. During the Last Glacial Maximum it persisted in multiple refugia in western North America, including Beringia, south of the continental ice, and within the northern limits of the Cordilleran ice sheet. Our data contribute to a growing body of evidence that arctic–alpine species have migrated from different source regions over multiple glacial cycles and that cryptic refugia contributed to persistence through the Last Glacial Maximum.

Allen, Geraldine A; Marr, Kendrick L; McCormick, Laurie J; Hebda, Richard J

2012-01-01

346

The impact of Pleistocene climate change on an ancient arctic-alpine plant: multiple lineages of disparate history in Oxyria digyna.  

PubMed

The ranges of arctic-alpine species have shifted extensively with Pleistocene climate changes and glaciations. Using sequence data from the trnH-psbA and trnT-trnL chloroplast DNA spacer regions, we investigated the phylogeography of the widespread, ancient (>3 million years) arctic-alpine plant Oxyria digyna (Polygonaceae). We identified 45 haplotypes and six highly divergent major lineages; estimated ages of these lineages (time to most recent common ancestor, T(MRCA)) ranged from ?0.5 to 2.5 million years. One lineage is widespread in the arctic, a second is restricted to the southern Rocky Mountains of the western United States, and a third was found only in the Himalayan and Altai regions of Asia. Three other lineages are widespread in western North America, where they overlap extensively. The high genetic diversity and the presence of divergent major cpDNA lineages within Oxyria digyna reflect its age and suggest that it was widespread during much of its history. The distributions of individual lineages indicate repeated spread of Oxyria digyna through North America over multiple glacial cycles. During the Last Glacial Maximum it persisted in multiple refugia in western North America, including Beringia, south of the continental ice, and within the northern limits of the Cordilleran ice sheet. Our data contribute to a growing body of evidence that arctic-alpine species have migrated from different source regions over multiple glacial cycles and that cryptic refugia contributed to persistence through the Last Glacial Maximum. PMID:22822441

Allen, Geraldine A; Marr, Kendrick L; McCormick, Laurie J; Hebda, Richard J

2012-03-01

347

Abrupt climate change in the Black Sea basin during the last glacial period (10-60 kyr)  

NASA Astrophysics Data System (ADS)

As the most distant arm of the Atlantic Ocean, the Black Sea demonstrates an unparalleled feature: it oscillates between lacustrine and marine stages following, respectively, glacial-interglacial sea level changes. Today, the Black Sea is the world's largest anoxic basin. Coring efforts during the last years rather suggested an extensive glacial sediment cover on most of the Black Sea slope areas not reachable with conventional gravity and piston coring devices. Here we present new sediment cores retrieved from the tectonically formed Archangelsky Ridge in the southeastern Black Sea during the 2007 RV Meteor cruise M72/5, which provide a first view into a complete and undisturbed section of the last glacial period. Different independent stratigraphic approaches (radiocarbon dating, tephrochronology, paleomagnetics, tuning to Greenland ice cores) lead to a consistent age-depth model for the last glacial period. Various proxies from cores 24/25-GC1 suggest strong and immediate responses of the glacial Black Sea freshwater lake to the abrupt D-O climate oscillations of the last glacial period. Each abrupt warming initiated, like during Termination I, inorganic carbonate precipitation in the lake system. Subsequent stadials are marked by increasing IRD input suggesting more abundant coastal ice formation likely reflecting colder winter temperatures. Ostracod stable oxygen isotopes record the precipitation/runoff signal of the drainage basin but show a strongly smoothed signal characteristic to an 1-2 kyr mixing-time in the Black Sea basin with striking similarities to the Antarctic temperature and global ice volume records (Arz et al. 2007).

Arz, H. W.; Lamy, F.; Kwiecien, O.; Nowaczyk, N.; Plessen, B.; Röhl, U.; Ganopolski, A.

2009-04-01

348

Iceberg discharges of the last glacial period driven by oceanic circulation changes  

PubMed Central

Proxy data reveal the existence of episodes of increased deposition of ice-rafted detritus in the North Atlantic Ocean during the last glacial period interpreted as massive iceberg discharges from the Laurentide Ice Sheet. Although these have long been attributed to self-sustained ice sheet oscillations, growing evidence of the crucial role that the ocean plays both for past and future behavior of the cryosphere suggests a climatic control of these ice surges. Here, we present simulations of the last glacial period carried out with a hybrid ice sheet–ice shelf model forced by an oceanic warming index derived from proxy data that accounts for the impact of past ocean circulation changes on ocean temperatures. The model generates a time series of iceberg discharge that closely agrees with ice-rafted debris records over the past 80 ka, indicating that oceanic circulation variations were responsible for the enigmatic ice purges of the last ice age.

Alvarez-Solas, Jorge; Robinson, Alexander; Montoya, Marisa; Ritz, Catherine

2013-01-01

349

Early Pleistocene chronology of the Vrica section (Calabria, Italy)  

Microsoft Academic Search

Two alternative age models are presented for the Pleistocene part of the Pliocene-Pleistocene boundary stratotype section at Vrica (Calabria, southern Italy). Both age models, or chronologies, are based on the correlation of the characteristic sapropel pattern to the summer insolation time series of astronomical solution La90. The first age model is based on the assumption that the succession is continuous,

L. J. Lourens; F. J. Hilgen; I. Raffi; C. Vergnaud-Grazzini

1996-01-01

350

Early Pleistocene chronology of the Vrica Section (Calabria, Italy)  

Microsoft Academic Search

Two alternative age models are presented for the Pleistocene part of the Pliocene-Pleistocene boundary stratotype section at Vrica (Calabria, southern Italy). Both age models, or chronologies, are based on the correlation of the characteristic sapropel pattern to the summer insolation time series of astronomical solution La90. The first age model is based on the assumption that the succession is continuous,

L. J. Lourens; F. J. Hilgen; I. Raffi; C. Vergnaud-Grazzini

1996-01-01

351

Pleistocene to recent dietary shifts in California condors  

Microsoft Academic Search

We used carbon and nitrogen isotopes to investigate changes in the diet of California condors from the Pleistocene to the recent. During the Pleistocene, condors from California fed on both terrestrial megafauna and marine mammals. Early accounts reported condors feeding on the carcasses of marine mammals, but by the late 1700s, condor diets had shifted predominantly to terrestrial animals, following

C. P. Chamberlain; J. R. Waldbauer; K. Fox-Dobbs; S. D. Newsome; P. L. Koch; D. R. Smith; M. E. Church; S. D. Chamberlain; K. J. Sorenson; R. Risebrough

2005-01-01

352

Geophysical investigation of buried Pleistocene subglacial valleys in Northern Germany  

Microsoft Academic Search

Buried Pleistocene subglacial valleys are extensively used as groundwater reservoirs by waterworks in northern Germany, although little is known about the locations and size of these valleys and the internal structure of the sediment fill. This lack of knowledge about important groundwater reservoirs is a challenge for geophysics.This paper summarizes the geophysical investigation of two buried Pleistocene subglacial valleys in

Gerald Gabriel; Reinhard Kirsch; Bernhard Siemon; Helga Wiederhold

2003-01-01

353

Late Pleistocene stratigraphy of a carbonate platform margin, Exumas, Bahamas  

Microsoft Academic Search

Detailed field studies of the southern Exuma Cays on the eastern margin of the Great Bahama Bank show a complex history of late Pleistocene island construction. Pleistocene rocks include island core eolianites, overlain at island margins by fossil patch reefs and reef sands, which in turn are overlain by, and\\/or grade laterally into, talus breccia cones derived from the erosion

K. R. Aalto; Robert F. Dill

1996-01-01

354

Middle and Late Pleistocene fluvial systems in central Poland  

Microsoft Academic Search

This reconstruction of the fluvial palaeogeography of central Poland is based on an exhaustive and critical review of the published and archival data for the Middle and Late Pleistocene sediments of the area. The Warsaw Basin in central Poland was a major confluence area during the Middle and Late Pleistocene. The fluvial watersheds have been only slightly modified since that

Leszek Marks

2004-01-01

355

Comparative context of Plio-Pleistocene hominin brain evolution  

Microsoft Academic Search

One of the distinguishing features of Homo sapiens is its absolutely and relatively large brain. This feature is also seen in less extreme form in some fossil Homo species. However, are increases in brain size during the Plio-Pleistocene only seen in Homo, and is brain enlargement among Plio-Pleistocene primates confined to hominins? This study examines evidence for changes in brain

Sarah Elton; Laura C. Bishop; Bernard Wood

2001-01-01

356

Late Pleistocene marine chronology of the Gippsland Lakes region, Australia  

Microsoft Academic Search

Thermoluminescence dating is used to define the chronology of the coastal barriers of the Gippsland Lakes region, Australia. The area evidences ? long history of marine deposition extending back ?? the Middle Pleistocene. However, the majority of Pleistocene barriers have formed since the Last Interglacial during two phases at 59 to 72 ka and 40 to 48 ka corresponding to

E. A. Bryant; D. M. Price

1997-01-01

357

Electrical Resistivity Study of a Pleistocene Riverbed in Saltville, VA  

Microsoft Academic Search

A shallow capacitively coupled resistivity survey was performed in Saltville, VA, in an area of suspected buried Pleistocene river deposits. Previous excavations in the sediments beneath the Saltville valley floor had been performed to recover late Pleistocene megafaunal remains and possible Clovis-age human artifacts. These digs encountered a zone, one to two meters deep, of gravel-sized rock fragments, including some

R. B. Herman; R. C. Whisonant

2008-01-01

358

Paleoclimate in north-east Anatolia during the Quaternary deduced from glacial archives  

NASA Astrophysics Data System (ADS)

Today's climate of the northern part of the eastern Black Sea Mountains is humid with a mean annual precipitation over 2000 mm. The moisture is directly transported from the Black Sea. Field evidence indicate that this area was glaciated during past. Ice build-up can be therefore explained by a moisture transport system similar to today. Analogous to the north, the southern part of these mountains is relatively dry, although the field evidence in Çoruh Valley in Yusufeli, for instance, point to even more extensive glaciations. To accumulate more ice volume in the southern part, a different moisture transport than today is needed, for instance the moisture transport path may have changed in the past. But the time when this ice-build up (recorded by glacial deposits and erosional features) took place is still unclear. To construct the chronology of these glaciations with cosmogenic 10Be and 36Cl, we collected 14 samples from erratic boulders and glacially abraded bedrock in the Çoruh Valley in Yusufeli in the southern part and 41 samples in the Ba?yayla valley in the northern part. According to the existing data, glaciers advanced around 21 ka in the Kavron and Verçenik valleys in the northern part during the Last Glacial Maximum (LGM) (Akçar et al. 2007; 2008). While a Lateglacial advance