Evidence for geographic variation in the diets of late Pleistocene and early Holocene Bison in North America, and differences from the diets of recent Bison

During the late Pleistocene and early Holocene, Bison was widely dispersed across North America and occupied most regions not covered by ice sheets. A dietary study on Bison paleopopulations from Alaska, New Mexico, Florida, and Texas was performed using two methods that relate dental wear patterns to diet, mesowear analysis and microwear analysis. These data were compared to a mixed sample of extant Bison from the North American central plains, extant wood Bison from Alberta (Canada) and a variety of other modern ungulates. Mesowear relates macroscopic molar facet shape to levels of dietary abrasion. The mesowear signature observed on fossil Bison differs significantly from the hyper-abrasive grazing diet of extant Bison. Tooth microwear examines wear on the surface of enamel at a microscopic scale. The microwear signal of fossil samples resembles to modern Bison, but the fossil samples show a greater diversity of features, suggesting that fossil Bison populations regularly consumed food items that are texturally inconsistent with the short-grass diet typical of modern plains Bison. Mesowear and microwear signals of fossil Bison samples most closely resemble a variety of typical mixed feeding ungulates, all with diets that are substantially less abrasive than what is typical for modern plains Bison. Furthermore, statistical tests suggest significant differences between the microwear signatures of the fossil samples, thus revealing geographic variability in Pleistocene Bison diets. This study reveals that fossils are of value in developing an understanding of the dietary breadth and ecological versatility of species that, in recent times, are rare, endangered, and occupy only a small remnant of their former ranges.     

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

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 CO₂ concentrations, temperature, and precipitation. It is argued that Lateglacial climate change, particularly increases in atmospheric CO₂, 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.     

Lubbock Lake: Late Quaternary cultural and environmental change on the Southern High Plains,USA

Lubbock Lake (Southern High Plains of Texas) contains a cultural, faunal, and floral record within a virtually complete geological record spanning the past 11 100+ years. More than 88 archaeological occurrences have been excavated from five major stratigraphic units. The Paleoindian record (11 500–6500yr BP) begins with Clovis-age occupation (ca. 11 100yr BP) found within fluvial deposits (stratum 1). Subsequent Paleoindian occupations are found in lake and marsh sediments (stratum 2). Archaic occupations (8500-2000yr BP) are contained within aeolian and marsh deposits (strata 3 and 4). Ceramic occupations (2000-500yr BP) are found on a soil developed in stratum 4, in marsh sediments (strata 4 and 5), and in slopewash and aeolian sediments (stratum 5). The Protohistoric (500-300yr BP) and Historic (300-100yr BP) remains are in slopewash, aeolian, and marsh sediments (stratum 5) and associated soils. The Southern High Plains remained a grasslands throughout the last 11 500 years and neither man nor bison abandoned the region. The successive local faunas reflect changing ecosystems under pluvial to arid to more mesic to semiarid conditions. The occupation of Lubbock Lake through time appears to have been by small groups of people for both economic and short-term residential uses. These hunter-gatherer peoples underwent adaptive change brought about by climatic stress and alterations to food resources.     

A high-resolution record of climate variability and landscape response from Kettle Lake,northern Great Plains,North America

A decadal-scale multiproxy record of minerals, pollen, and charcoal from Kettle Lake, North Dakota provides a high-resolution record of climate and vegetation change spanning the entire Holocene from the northern Great Plains (NGP) in North America. The chronology is established by over 50 AMS radiocarbon dates. This record exhibits millennial-scale trends evident in other lower-resolution studies, but with much more detail on short-term climate variability and on the rapidity and timing of major climatic shifts. As a proxy for precipitation, we utilize the rate of endogenic carbonate sedimentation, which depends on groundwater inflow, which in turn depends on precipitation. Independent cluster analyses of mineral and pollen data reveal major Holocene mode shifts at 10.73 ka (ka = cal yr BP), 9.25 ka, and 4.44 ka.The early Holocene, 11.7–9.25 ka, was generally wet, with perhaps a trend to higher evaporation associated with warming temperatures. A switch from calcite to aragonite deposition associated with a severe, but brief drought occurred at 10.73 ka. From 10.73 ka to 9.25 ka, climate was generally humid but punctuated at 100–300 yr intervals by brief droughts, including the most severe drought of the entire Holocene at 9.25 ka. This event was coeval with the 9.3–9.2 ka event in the Greenland ice cores and observed at a number of sites worldwide. In contrast, the prominent 8.2 ka event in Greenland is not remarkable at Kettle Lake. The prominence of the 9.25 event locally in the NGP may be due to a major drawdown and northward retreat of Lake Agassiz at this time, reducing its mesoclimatic effect on the NGP and thrusting the region into an insolation controlled regime.The mid-Holocene, 9.25–4.44 ka, was characterized by great variability in moisture on a multi-decadal scale, with severe droughts alternating with more humid periods. The high abundance of the weedy but drought intolerant Ambrosia generally during the mid-Holocene and specifically during the multi-decadal drought periods is seemingly paradoxical, but can be explained by high interannual variability of moisture overlaid on the multi-decadal variability.The late Holocene, 4.44 ka–present, was also characterized by multi-decadal variability in moisture, but was generally wetter than the mid-Holocene and the magnitude of variability was less. The trends in wet-dry mineral, pollen, and charcoal proxies were similar to the mid-Holocene, but late Holocene mineral-pollen assemblages are distinct from mid-Holocene. The shift to wetter climate in the late Holocene was more gradual than the abrupt shift to arid conditions 9.25 ka, which may explain the asymmetric retreat and readvance of forest along the eastern margin of the NGP.Precipitation variations in the NGP have been linked with Pacific and Atlantic sea-surface temperatures, and mid-Holocene drought in the NGP has been linked with sustained La Niña-like conditions in the Pacific. These linkages may explain the decadal- and millennial-scale variations seen in the NGP, but cause of the prominent century-scale variations remains elusive.     

Interpreting the response of a dryland river system to Late Quaternary climate change

A U-series calcrete chronology has been constructed for three Late Quaternary terrace units, termed the D1, D2 and D3 terraces in age descending order, from the Rio Aguas river system of the Sorbas basin, southeast Spain. The D1 terrace formed between 30,300±4400 year BP and 12,140±360 year BP, correlating well with the Last Glacial Maximum when rates of sediment supply would have increased greatly, because of higher rates of weathering, reduced vegetation cover and weak soil development. The D2 terrace formed between 12,800±1100 year BP and 9,600±530 year BP, correlating well with the Younger Dryas event. The D3 terrace could only be poorly constrained to the early Holocene and no unequivocal cause could be assigned to this period of aggradation. The sedimentology and geomorphology of the D2 terrace suggests, however, that the aggradation of this unit was a response to diapirism/karstic processes occurring within the underlying Messinian gypsum strata and the subsequent damming of the Aguas system. Therefore, despite its coincident occurrence with the Younger Dryas, aggradation of the D2 terrace is unrelated to climate change. The style of this response, controlled predominantly by the characteristics of the underlying bedrock, makes correlating the terrace record of the Aguas with other systems in the Mediterranean unreliable. This study, therefore, highlights the problems of correlating fluvial sequences in regions of variable tectonics, climatic history and bedrock geology and emphasises the need to properly understand the main controls on individual fluvial systems before any attempt is made to correlate their depositional histories.     

Climatic Implications of the Late Quaternary Alluvial Record of a Small Drainage Basin in the Central Great Plains

Four late-Quaternary alluvial fills and terraces are recognized in Wolf Creek basin, a small (163 km²) drainage in the Kansas River system of the central Great Plains. Two terraces were created during the late Pleistocene: the T-4 is a fill-top terrace underlain by sand and gravel fill (Fill I), and the T-3 is a strath terrace cut on the Cretaceous Dakota Sandstone. Both Fill II (early Holocene) and Fill III (late Holocene) are exposed beneath the T-2, a Holocene fill-top terrace. The T-1 complex, consisting of one cut and three fill-top terraces, is underlain by Fills III and IV. A poorly developed floodplain (T-0) has formed within the past 1000 yr. As valleys in Wolf Creek basin filled during the early Holocene, an interval of soil formation occurred about 6800 yr B.P. Early Holocene fill has been found only in the basin's upper reaches, indicating that extensive erosion during the middle Holocene removed most early-Holocene fill from the middle and lower reaches of the basin. Valley filling between 5000 and 1000 yr B.P. was interrupted by soil formation about 1800, 1500, and 1200 yr B.P. As much as 6 m of entrenchment has occurred in the past 1000 yr. Holocene events in Wolf Creek basin correlate well with those in other localities in the central Great Plains, indicating that widespread changes in climate, along with adjustments driven by complex response, influenced fluvial activity.     

Tropical cyclones,global climate change and the role of Quaternary studies

The number and types of late Quaternary records of tropical cyclones (TCs) and temperate storms have been increasing globally over the past 10 years. There are now numerous such records for the Atlantic Ocean (USA) and Gulf of Mexico and Caribbean Sea, South Pacific Ocean, and a fewer number from the northwest Pacific and Indian Ocean regions. The most obvious characteristic of these records is that many reveal extended alternating periods of greater and lesser TC activity over the past 6000 years. The length of these phases of relative inactivity and greater activity depends on the chronological resolution of the record, with the coarser‐resolution ones displaying multi‐century to millennial‐scale episodes and the high‐resolution records displaying decadal to centennial‐scale oscillations. In several instances the likely causes of these alternating periods of TC behaviour have been attributed to different phases of climate when El Niños and La Niñas dominated or to longer‐term variations in sea surface temperatures and possibly solar forcing. The picture emerging from these records is that TC behaviour is not entirely stochastic over the long term and that any simulations of long‐term TC behaviour need to account for these climatic influences. Incorporation of these observations, and the many more needed, is important for understanding the future behaviour of TCs. Copyright © 2011 John Wiley & Sons, Ltd.     

Order amid sparse data: Patterns of lake level changes in North America during the Late Quaternary

Water-level fluctuations in closed-basin lakes can be used to reconstruct past hydrological changes, and the recognition of spatially coherent patterns in lake behavior provides evidence for changes in climate. The geological records of water level in many lakes, particularly those in arid regions, are by nature incomplete. The fragmentary nature of the data poses special problems for comparison of records and identification of regions where lakes behave similarly. An unconventional method of assessing similarity in the behavior of lakes is used with multidimensional scaling to place lakes in a low-dimensional space. Weights are used to reflect the amount of information available for each particular comparison. The similarity measure is based on evidence for changes in lake depth between successive time intervals and on independent evidence for the direction of change at any given time. Groups (clusters) of lakes in the low-dimensional space are identified by mutual proximity. The method was applied to a set of 65 Late Quaternary lake-level records from North America. About one-third of the lakes had too little weight to be placeable, about one-third were in clusters, and about one-third showed unique behavior. Those lakes which clustered showed four distinct types of record, characteristic of well-defined geographic regions. This ability to distinguish spatially coherent patterns on internal evidence alone strengthens the basis for using lake-level records for regional palaeoclimatic reconstructions.     

Evolution of the Nile deep‐sea turbidite system during the Late Quaternary: influence of climate change on fan sedimentation

This paper presents an overview of the evolution of the Nile deep‐sea turbidite system during the last 200 kyr, over a series of glacial to interglacial cycles. Six individual deep‐sea fans were identified from an extensive field data set. Each fan comprises a canyon, channel system and terminal lobes. Two of these fan systems were possibly active at the same time, at least during some periods. Large‐scale slope failures destroyed channel segments and caused the formation of new submarine fan systems. These slope failures thus played an important role in the overall evolution of the turbidite system. During the last glacial maximum (ca 25 to 14·8 ka) the central and eastern parts of the Nile deep‐sea turbidite system were relatively inactive. This inactivity corresponds to a lowstand in sea‐level, and a period of arid climate and relatively low sediment discharge from the Nile fluvial system. Rapid accumulation of fluvial flood‐derived deposits occurred across the shallower part of the submarine delta during sea‐level rise between ca 14·8 and 5 ka. The most recent deep‐sea channel–lobe system was very active during this period of rising sea‐level, which is also associated with a wetter continental climate and increased sediment and water discharge from the Nile. Increased sediment deposition in shallower water areas led to occasional large‐scale slope failure. The Nile deep‐sea turbidite system was largely inactive after ca 5 ka. This widespread inactivity is due to retreat of the coastline away from the continental shelf break, and to a more arid continental climate and reduced discharge of sediment from the Nile. The Nile deep‐sea turbidite system may be more active during periods of rising and high sea‐level associated with wetter climates, than during lowstands, and may rapidly become largely inactive during highstands in sea‐level coupled with arid periods. These acute responses to climate change have produced sedimentary/stratigraphic features that diverge from traditional sequence models in their nature and timing. This large‐scale sedimentary system responded to monsoon‐driven climate change and sea‐level change in a system‐wide and contemporaneous manner.     

Ecosystem response to Lateglacial and early Holocene climate oscillations in the Great Lakes region of North America

Fossil pollen, plant macrofossils, gastropods, and elemental and stable-isotope geochemistry in a sediment core from Twiss Marl Pond, southern Ontario, Canada, were used to document climate oscillations during the Last Glacial–Interglacial transition (13,000–8500 ¹⁴C BP) and understand their ecological effects. Chronology was provided by AMS ¹⁴C dating and regional pollen correlation. Oxygen isotope (δ¹⁸O) results from mollusc shells, Chara-encrustations and bulk carbonates show a classic climate sequence of a warm Bølling–Allerød (BOA) at 12,500–10,920 ¹⁴C BP, a cold Younger Dryas (YD) at 10,920–10,000 ¹⁴C BP, the Holocene warming at 10,000 ¹⁴C BP, a brief Preboreal Oscillation (PB) at 9650 ¹⁴C BP, and a possible Gerzensee/Killarney (G/K) cooling shortly before 11,000 ¹⁴C BP.Clay sediments at the base of the core contain high herb and shrub pollen and abundant arctic/alpine plant macrofossils, indicating a treeless tundra with severe soil erosion in watershed. During the BOA warm period, authigenic marl began to be deposited, and Picea woodland became established. The establishment of Picea woodland after peaks of δ¹⁸O and of carbonate accumulation suggests a lagged response of upland vegetation to BOA warming. In contrast, the occurrence of warmth-loving aquatics Najas flexilis and Typha latifolia at that time indicates sensitive responses of aquatic plants. The YD cooling is indicated by a 1.5‰ negative excursion in δ¹⁸O, an increase in minerogenic matter and higher concentrations of erosion-derived elements (Al, Na, K, Ti and V). Pollen data show no forest transformation in response to YD cooling, which is attributed to the insensitive nonecotonal vegetation at that time. However, more openings in the forests and increased erosion in the watershed are indicated by a slight increase of herb pollen, high concentrations of erosion elements and a Pediastrum peak. The onset of the Holocene was marked by an abrupt increase of 2‰ in δ¹⁸O and the replacement of Picea woodland by Pinus-dominated forest. The Picea recurrence at 9650 ¹⁴C BP demonstrates sensitive response of ecotonal vegetation to the PB climate oscillation, which is also indicated by 0.4‰ negative excursion of δ¹⁸O. These new results suggest the importance of multiproxy records for reliable paleoclimate reconstruction.Reevaluation and revised chronologies of previously published sites (Gage Street, and Nichols Brook) in the eastern Great Lakes region show their major δ¹⁸O shifts correlative to the YD and PB oscillations as documented from Twiss Marl Pond and nearby Crawford Lake. The sequence and magnitude of climatic oscillations from these sites match in detail with records from the Atlantic Seaboard, suggesting that these oscillations are an expression of broad-scale, probably global, climate change rather than local meltwater-induced climate cooling.     

Taphonomy of two last glacial maximum mammoth sites in the central Great Plains of North America: A preliminary report on La Sena and Lovewell

Two mammoth sites from the central Great Plains of North America, each containing one adult Columbian mammoth (Mammuthus columbi), were excavated from Last Glacial Maximum (LGM) loess and fine-grained alluvial deposits, respectively. Taphonomic data from both sites indicate that the mammoth skeletons exhibit numerous spirally fractured limb elements. Dynamic loading points are present on midshafts of large limb bones. Bone flakes produced from the partial thickness of thick cortical bone are also present. Hypotheses of carnivore activity, mammoth trampling, and human-induced fracturing are evaluated as possible causes of the fractured limb bone. Testing the hypotheses using modern data from actualistic taphonomic studies of elephant skeletons, paleontological data from two proboscidean natural death sites, experimental data from elephant bone fracturing, and archaeologically derived data concerning late Pleistocene human modification of mammoth limb bone indicates that the first two hypotheses can be rejected, while the third hypothesis is supported.     

Late Quaternary environmental change of Yellow River Basin: An organic geochemical record in Bohai Sea (North China)

Bulk geochemical characterization (total organic carbon, grain size distribution, carbon isotope composition) and molecular biomarkers (lignin phenols, straight chain aliphatic hydrocarbons, glycerol dialkyl glycerol tetraethers) were analyzed for a 21 m core from the Bohai Sea (North China), spanning ca 21 ka BP. These paleo-proxies presented remarkable differences between the late glacial period and the Holocene, reflecting continental and coastal environments, respectively. Two peat layers were deposited during the period of ca 9000-8460 yr BP. Thereafter the core site has been consistently covered by seawater until recent reclamation of land from the sea. The occurrence of a total organic carbon maximum from ca 6000-3800 yr BP was attributed to delivery of organic carbon enriched sediments via the Yellow River, consistent with increased vegetation density and higher development of soil under warm and humid mid-Holocene climate conditions. The distributions of lignin phenol compositions and C₃₁/C₂₉n-alkane ratio suggested the largest expansion of woody plants between ca 5300 and 4000 yr BP, corresponding to the extremely favorable climatic conditions. Since ca 3800 yr BP, an abrupt increase in the C₃₁/C₂₉n-alkane ratio suggested higher abundance of grasses, consistent with a drying climate trend after the mid-Holocene. Since our coastal sediments close to the Yellow River outflow contain catchment-integrated environmental signals of the river basin, molecular proxies demonstrate that the variability of vegetation distributions in the Holocene is a widespread phenomenon in those areas adjacent to Yellow River Basin.     

Simulation of the climate of 18,000 years BP: Results for the North American/North Atlantic/European sector and comparison with the geologic record of North America

The large ice sheets in North America and Europe and the extensive sea-ice cover in the North Atlantic at the time of the last glacial maximum must have greatly modified the surface temperature patterns and, in turn, the location and intensity of the surface winds and jet streams. A general circulation model was used to simulate the January and July patterns of temperature, precipitation, and wind for 18 ka BP. Boundary conditions for the model, consisting of ice-sheet location and height, sea-ice location, and sea-surface temperature were prescribed from CLIMAP (1981). The model results are illustrated and described for the North American/North Atlantic/European sector. The jet stream splits around the North American ice-sheet, and the southern branch strengthens considerably (compared to present) over the southern portion of the United States, the sea-ice margin of the North Atlantic, and the southern edge of the European ice-sheet. Geologic evidence, principally from North America, of wind, temperature and moisture conditions is assessed from sand dune and loess records, estimates of snowline depression, pollen records and lake-level studies. The geologic evidence is generally compatible with the model simulation.     

北美五大湖区晚更新世劳伦泰德冰盖演化年代学综述及石英光释光测年的初步尝试

北美五大湖区的安大略湖北岸Don Valley Brickyard、Scarborough Bluffs、Bowmanville Bluffs剖面共同构成了北美东北部最长也是最厚的陆地第四纪沉积记录, 较完整地记录了晚更新世劳伦泰德冰盖(the Laurentide Ice Sheet)的演化. 晚更新世劳伦泰德冰盖演化的重建有赖于这些经典剖面中重要沉积地层单元的准确年代学控制. 传统的地层年代学主要是依靠少量¹⁴C年代, 将主要的混杂堆积单元(diamicton)解释为气候变冷环境下的冰川扩张, 并与指示全球冰量变化的深海氧同位素曲线一一比对建立起来的. 这样建立起来的年代学存在很大的不确定性. 20世纪80-90年代的少量热释光年代也不相吻合, 最近的13个长石红外释光定年则只集中于Bowmanville Bluffs的一个分层, 并未建立整个剖面的地层年代学, 使这些经典沉积剖面的年代学一直没有得到系统的建立. 应用石英光释光SAR-SGC法测试了Bowmanville Bluffs剖面Glaciofluvial Sand单元的2个冰水沉积样品, 年代结果分别为(41.6±3.8) ka、(48.1±4.4) ka, 分析表明这一年代结果偏老, 石英颗粒可能晒褪不完全. 由于大测片无法识别晒褪不完全的颗粒, 因此, 测试更多的剖面序列的光释光年代并尝试采用粗颗粒小测片或单颗粒技术解决样品颗粒晒褪不完全的问题将是必要的.     

Chronology of the impact of Quaternary climate change on continental environments in the Arabian Peninsula

The remains of former lakes show that in the past the Arabian Peninsula experienced much wetter conditions than today. The last of these humid periods dates to about 10 to 5.5 ka ago. The chronological framework for an earlier humid phase, radiocarbon dated to some 35–20 ka, is inconsistent with evidence from other records from the region. Possibly, these ages are significantly underestimating the true depositional age due to methodological problems. The earliest phase of dune accumulation known so far is dated to the penultimate glaciation maximum of the mid latitudes (ca. 150 ka). Subsequently, dune accumulation occurred around 110 ka, 65 ka and 20 ka ago. All these phases concur with rapid drops in global sea level that caused a drying out of the Persian Gulf basin and of the shelf of the Oman coast. In contrast to some previous interpretations, it is concluded here that aeolian deposition has been limited by sediment supply and not by preservation potential.     

Flamingos,salt lakes and volcanoes: hunting for evidence of past climate change on the high Altiplano of Bolivia

Hidden within the vast Bolivian Altiplano are archives of past climate change in the form of remarkable carbonate rocks surrounding lakes long since disappeared. Beyond the Salar de Uyuni, the largest salt flat in the world, lies a relatively untouched realm of volcanoes and salt lakes. Ancient shorelines from intervals in the Altiplano history, when large lakes were more abundant, may hold important information about a time when the climate in this region was punctuated by much wetter phases before present day aridity took a hold. Previous studies in this region have reconstructed robust chronological timelines for such events and highlight two large lake phases over the last 18 thousand years (the Tauca and Coipasa lake phases); however higher resolution climate data are scarce. Current studies on climate proxies from smaller lakes in southern Bolivia may shed light on some of these higher resolution climate events including El Niño–Southern Oscillation (ENSO) events. Laminated tufa found around the palaeoshorelines of the West Lípez Lakes is one such proxy, and can be analysed to investigate the potential roles of annual versus shorter‐term climatic variation in the evolving Altiplano climate at the time.     

Quaternary alluvial fans in the Gobi of southern Mongolia: evidence for neotectonics and climate change

Alluvial fans in southern Monglia occur along a group of narrow discontinuous mountain ranges which formed as transpressional uplifts along a series of strike-slip faults. They provide information on the nature of neotectonic activity in the eastern Gobi Altai range and on palaeoclimate change. Alluvial fan formation was dominated by various geomorphological processes largely controlled by climatic changes related to an increase in aridity throughout late Quaternary times. Their sedimentology shows that initially they experienced humid conditions, when the sedimentary environments were dominated by perennial streams, followed by a period of increasing aridity, during which coarse fanglomerates were deposited in alluvial fans by ephemerial streams and active-layer structures were produced by permafrost within the alluvial fan sediments. With climatic amelioration during early Holocene times, the permafrost degraded and fan incision and entrenchment dominated. Sedimentation was then confined to the upper reaches of the fans, adjacent to steep mountain slopes, and within the entrenched channels. The alluvial fans have been neotectonically deformed, faulted and their surface warped by small thrust faults that propagate from the mountain fronts into their forelands. Localised uplift rates are in the order of 0.1 to 1 m Ka⁻¹. © 1997 John Wiley & Sons, Ltd.     

The cause of the Late Cenozoic Northern Hemisphere glaciations: a climate change enigma

The ultimate cause of the onset of glaciations remains elusive, but in the case of northem hemisphere glaciation it is probable that several factors acted in combination. General global cooling resulted from reduction of atmospheric C0₂ by weathering of silicate rocks exposed by erosion of late Cenozoic uplifts. Uplifts in south Asia, southwestern North America and Scandinavia occurred at distances appropriate for the generation of quasi-permanent Rossby waves in the atmosphere. The resulting winds, given suitable moisture sources, were favourable for causing large-scale precipitation at mid-latitudes on the northern continents. Moisture sources were provided by the closure of the Central American isthmus. Gulf Stream flow increased, carrying warm subtropical waters to high latitudes. The Denmark Strait deepened permitting greater outflow of deep water from the Norwegian-Greenland Sea. The relative importance of each of these factors should be investigated by additional atmospheric and ocean climate model sensitivity studies.