Pleistocene Epoch

Earlier and longer of the two epochs that constitute the Quaternary Period.

The Pleistocene began с 1.8 million years ago and ended с 10,000 years ago. It was preceded by the Pliocene Epoch of the Tertiary Period and followed by the Holocene Epoch. At the height of the Pleistocene glacial ages, more than 30% of the land area of the Earth was covered by glacial ice; during the interglacial stages, probably only about 10% was covered. The animals of the Pleistocene began to resemble those of today, and new groups of land mammals, including humans, appeared. At the end of the epoch, mass extinctions occurred: in North America more than 30 genera of large mammals became extinct within a span of roughly 2,000 years. Of the many causes that have been proposed for these extinctions, the two most likely are changing environment with changing climate and disruption of the ecological pattern by early humans.

* * *

Introduction

      earlier and major of the two epochs that constitute the Quaternary Period of the Earth's history, and the time period during which a succession of glacial and interglacial climatic cycles occurred. The Pleistocene began about 1,600,000 years ago and ended roughly 10,000 years ago. It is preceded by the Pliocene Epoch of the Tertiary Period and is followed by the Holocene Epoch.

      The Pleistocene Epoch is best known as a time during which extensive ice sheets and other glaciers formed repeatedly on the landmasses and has been informally referred to as the “Great Ice Age.” Modern research, however, has shown that large glaciers had formed prior to the Pleistocene—during the latter part of the Tertiary Period as well as during earlier periods of geologic time—and that glaciation is not unique to the Pleistocene.

stratigraphy

Pliocene–Pleistocene boundary

      Definition of the base of the Pleistocene has had a long and controversial history. Because the epoch is best recognized for glaciation and climatic change, many have suggested that its lower boundary should be based on climatic criteria—for example, the oldest glacial deposits or the first occurrence of a fossil of a cold-climate life-form in the sediment record. Other criteria that have been used to define the Pliocene–Pleistocene include the appearance of humans, the appearance of certain vertebrate fossils in Europe, and the appearance or extinction of certain microfossils in deep-sea sediments. These criteria continue to be considered locally, and some workers advocate a climatic boundary at about 2.4 million years.

      Pre-Pleistocene intervals of time are defined on the basis of chronostratigraphic and geochronologic principles related to a marine sequence of strata. Following studies by a series of international working groups, correlation programs, and stratigraphic commissions, agreement was reached in 1985 to place the lower boundary of the Pleistocene series at the base of marine claystones that conformably overlie a specific marker bed in the Vrica section in Calabria. The boundary occurs near the level of several important marine biostratigraphic events and, more significantly, is just above the position of the magnetic reversal that marks the top of the Olduvai Normal Polarity Subzone, thus allowing worldwide correlation.

      The Pleistocene is subdivided into informal time units, the early, middle, and late Pleistocene. The early Pleistocene extends to the Brunhes–Matuyama paleomagnetic boundary at 730,000 years ago, and the middle Pleistocene extends to the end of the next to the last glaciation at about 130,000 years. The late Pleistocene includes the last interglacial–glacial cycle ending at the Holocene boundary 10,000 years ago.

Chronology and correlation

      The chronology of the Pleistocene originally developed through observation and study of the glacial succession, which in both Europe and the United States was found to contain either soils that developed under warm climatic conditions or marine deposits enclosed between glacial deposits. From these studies, as well as studies of river terraces in the Alps, a chronology was developed that suggested the Pleistocene consisted of four or five major glacial stages which were separated by interglacial stages with climates generally similar to those of today. Beginning with studies in the 1950s, a much better chronology and record of Pleistocene climatic events have evolved through analyses of deep-sea sediments, particularly from the oxygen isotope record of the shells of microorganisms that lived in the oceans.

Marine oxygen isotope record

      The isotopic record is based on the ratio of two oxygen isotopes, oxygen-16 (¹⁶O) and oxygen-18 (¹⁸O), which is determined on calcium carbonate from shells of microfossils that accumulated year by year on the seafloor. The ratio depends on two factors, the temperature and the isotopic composition of the seawater from which the organism secreted its shell. Shells secreted from colder water contain more oxygen-18 relative to oxygen-16 than do shells secreted from warmer water. The isotopic composition of the oceans has proved to be related to the storage of water in large ice sheets on land. Because molecules of oxygen-18 evaporate less readily and condense more readily, an air mass with oceanic water vapour becomes depleted in the heavier isotope (oxygen-18) as the air mass is cooled and loses water by precipitation. When moisture condenses and falls as snow, its isotopic composition is also dependent on the temperature of the air. Snow falling on a large ice sheet becomes isotopically lighter (i.e., has less oxygen-18) as one goes higher on the glacier surface where it is both colder and farther from the moisture source. As a result, large ice sheets store water that is relatively light (has more oxygen-16), and so during a major glaciation the ocean waters become relatively heavier (contain more oxygen-18) than during interglacial times when there is less global ice. Accordingly, the shells of marine organisms that formed during a glaciation contain more oxygen-18 than those that formed during an interglaciation. Although the exact relationship is not known, about 70 percent of the isotopic change in shell carbonate is the result of changes in the isotopic composition of seawater. Because the latter is directly related to the volume of ice on land, the marine oxygen isotope record is primarily a record of past glaciations on the continents (see figure).

      Long core samples taken in portions of the ocean where sedimentation rates were high and generally continuous and where water temperature changes were relatively small have revealed a long record of oxygen isotope changes that indicate repeated glaciations and interglaciations going back to the Pliocene. The record is relatively consistent from one core sample to the next and can be correlated throughout the oceans. Warmer periods (interglacials) are assigned odd numbers with the current warm interval, the Holocene, being 1, while the colder glacial periods are assigned even numbers. Subdivisions within isotopic stages are delineated by letters (see figure). The ages of the stage boundaries cannot be measured directly, but they can be estimated from available radiometric ages of the cores and from position with respect to both paleomagnetic boundaries and biostratigraphic markers, and also by using sedimentation rates relative to these data.

      The record for the last 730,000 years indicates that eight major glacial and interglacial events or climatic cycles of about 100,000 years' duration occurred during this interval. Similar cycles but of greater frequency and lesser magnitude continue back to the late Pliocene (see figure). An isotopic record from the North Atlantic suggests the first major glaciation in that region occurred about 2,400,000 years ago, some 800,000 years before the start of the Pleistocene. It also suggests that the first glaciation likely to have covered extensive areas of North America and Eurasia occurred about 850,000 years ago during oxygen isotope stage 22. The largest glaciations appear to have taken place during stages 2, 6, 12, and 16; the interglacials with the least global ice, and thus possibly the warmest, appear to be stages 1, 5, 9, and 11. The last interglaciation occurred during all of stage 5 or just substage 5e, depending on location; the last glaciation took place during stages 4, 3, and 2; and the current interglaciation falls during stage 1.

      The marine isotopic record is a continuous record, unlike most terrestrial records, which contain gaps because of erosion or lack of sedimentation and soil formation or a combination of these factors. Because of its continuity and its excellent record of climatic events on land (glaciations), the marine oxygen isotope record is the standard to which the terrestrial and other stratigraphic records are correlated. Correlations to it are based on available chronometric ages, on paleomagnetic data where available, and on attempts to match the terrestrial record and its interpretation with specific characteristics of the isotopic curve. Unfortunately, most terrestrial records contain few radiometric ages and are incomplete, and specific correlations, except for the most recent part of the record, are difficult and uncertain. A few terrestrial records, however, are exceptional and can be correlated with confidence.

Loess-paleosol (loess) records

      Central China is covered by deposits of windblown dust and silt, called loess. Locally the loess is more than 100 metres thick, mantling hillsides and forming loess plateaus and tablelands. The loess accumulated primarily during times that were colder and drier than present, and most of it was derived from desert areas to the west. The loess succession contains many colourful buried soils or paleosols that formed during periods which were both warmer and wetter than today. Thus, on stable tablelands with minimal erosion, the succession provides an exceptional climatic and chronological record that extends back 2.4 million years to the late Pliocene. In total, up to 44 climatic cycles have been delineated, with more frequent cycles occurring during the early Pleistocene. Although not directly related to glaciation, correlation with the marine oxygen isotope record is excellent, and many of the specific loess and soil units have similar climatic inferences, as do their correlative oxygen-18 stages.

      Another loess–paleosol succession occurs in the Czech Republic, Slovakia, and Austria, where loess blankets terraces of the major rivers that drained eastward and southward from the principal glaciated areas in the Alps and northern Europe. As in China, buried soils are common in the loess succession and, along with gastropod shells, provide paleoclimatic data and evidence for climatic change. The climatic cycles varied from cold and dry conditions when loess accumulated to warm and wet conditions with hardwood forests and well-developed soils. In the last 730,000 years, eight climatic cycles have been delineated; these correlate with the eight oxygen-18 cycles that occurred in the marine record during the same time interval. During the entire Pleistocene, about 17 glacial episodes alternated with 17 interglacials.

Glacial records

      Glacial till, which was directly deposited by glaciers, covers extensive areas of northern Eurasia and northern North America and occurs as well in many mountain regions and other areas that currently are not covered by glacial ice. Soils of warm climate origin buried between tills were recognized long ago and provided the basis for the development of the idea of multiple glaciation during the Pleistocene. However, because direct dating of the deposits generally is not possible and the glacial sequence is not complete as a result of erosion or nondeposition or a combination of the two, the development of long chronological records and correlation to the oxygen-18 record are difficult. Correlations generally are possible for the last two climatic cycles. They also are feasible in areas where the glacial succession contains interbedded volcanic rocks from which radiometric ages can be obtained.

       Classic Glacial/Cold and Interglacial/Warm Episodes*, TableIn the mid-continental region of the United States, early work recognized tills that were interpreted to represent four major glaciations and three major buried soils that were viewed as representing interglaciations (see Table (Classic Glacial/Cold and Interglacial/Warm Episodes*, Table)). Subsequent work showed that the glaciated record was more complex and that parts of the older record were miscorrelated. Consequently, the older portion of the record is informally referred to as the pre-Illinoian, and the older glacial and interglacial terms are no longer used except locally. Volcanic ash occurs within the succession in Iowa, Kansas, and Nebraska and is useful for correlation and dating. In one core, till occurs below ash that has been dated at about 2.2 million years old, suggesting late Pliocene glaciation. Other tills of the pre-Illinoian (Illinoian Glacial Stage) sequence probably are correlative with oxygen-18 stages 22, 16, and 12, and possibly others. The Illinoian correlates with oxygen-18 stage 6 and possibly stage 8, and the Sangamonian correlates with stage 5. The last glacial interval, the Wisconsinan (Wisconsin Glacial Stage), is subdivided into three parts, an early stade (substage) of glaciation, a middle interstadial, or time of restricted glaciation, and a late stade of glaciation. These intervals generally correlate with oxygen-18 stages 4, 3, and 2, respectively. Deposits of the early and middle Wisconsinan are poorly known in the mid-continental region of the United States; the area probably was not glaciated. Tills of the early Wisconsinan and even some that are correlative with oxygen-18 substages 5d or 5b, however, are common in the Canadian Arctic and on Baffin Island, where the ice sheet developed much earlier. It was not until the late Wisconsinan, about 18,000 years ago, that the southern ice sheet margin reached its maximum extent in the United States and eastern and western Canada. The ice sheet margin began to retreat and downwaste (i.e., thin out) soon after reaching its maximum position, and the United States was deglaciated by about 10,000 years ago. Hudson Bay, near the centre of the ice sheet, was open to the ocean by 8,000 years ago, and, except for the Barnes and Penny ice caps on Baffin Island, the ice sheet had dissipated from the upland areas of central Canada by 6,000 years ago, well into the Holocene and oxygen-18 stage 1.

       Classic Glacial/Cold and Interglacial/Warm Episodes*, TableA somewhat similar chronology has been developed for the glaciated areas of Eurasia and the British Isles based on a variety of criteria. In addition to tills and buried soils, marine deposits, permafrost features, and fossil pollen and beetles have been used to subdivide the succession on a climatic basis. As elsewhere, the earlier portion of the record is not well established, and correlations among different geographic areas, as well as to the marine oxygen-18 record, are uncertain (see Table (Classic Glacial/Cold and Interglacial/Warm Episodes*, Table)). The first cold period, known as the Pretiglian and based on pollen data from The Netherlands, began about 2.3 million years ago, soon after extensive ice-rafted material first appears in North Atlantic deep-sea cores. The Pretiglian was followed by a succession of warm and cold intervals, which also are based on pollen and on other flora and fauna evidence and which have been given different names in different areas. Although several old gravels with glacial erratics are known, the oldest major glacial episodes with extensive till deposits are the Elsterian (Elsterian Glacial Stage) in northern Germany and the Anglian in England. These glaciations probably are correlative with oxygen-18 stage 12, and local evidence suggests the possibility of earlier glacial events. Along coastal areas, these tills are overlain by the marine Holstein deposits, which also may represent more than one high sea-level stand. The next major glacial sequence is the Saalian (Saale Glacial Stage) of Germany, which is subdivided into the Drenthe and the Warthe; these probably correlate with oxygen-18 stages 8 and 6, respectively. Deposits and soils of the last interglaciation, the Eemian (Eemian Interglacial Stage) and Ipswichian, are correlative with oxygen-18 stage 5e, and those of the last glaciation, the Weichselian (Weichsel Glacial Stage) and Devensian, correlate with oxygen-18 stages 5d–a, 4, 3, and 2. As in central North America, tills and other deposits are well known only from the last part of this interval. The deglacial history generally is similar, except for a widespread but short interval of renewed glacial activity and cold climatic conditions that is known as the Younger Dryas in Scandinavia and Loch Lomond in the British Isles. This event occurred about 11,000 years ago, some 2,000 years before the dissipation of the ice sheet.

Ice-core records

      A relatively short but important late Pleistocene and Holocene climatic record is derived from ice cores (ice core) that have been taken from the ice sheets of Antarctica, Greenland, and Arctic Canada. The ice record in several cores extends back to the last interglaciation (oxygen-18 stage 5) and, in one case, to the next-to-the-last glaciation (stage 6). Although dating of the lower portions of the ice cores is difficult, annual layers of snow and ice can be counted in the upper parts and an accurate time scale reconstructed. Because the air temperature at the time when moisture condenses to fall as snow controls the oxygen and hydrogen isotopic composition of the snow, investigators are able to reconstruct temperature variations through isotopic studies of the ice cores. Data from the Vostok core taken from the East Antarctic Ice Sheet indicate that the climatic record of the Southern Hemisphere is similar to that interpreted from Northern Hemisphere records with respect to times of glaciation and interglaciation. It also is possible to measure the amount of microparticles (very fine dust) in the ice, and studies of this kind show that there are many more particles in the portions of the core that accumulated during periods of extensive glaciation, apparently reflecting greater atmospheric circulation and dust in the atmosphere at those times. Trapped air preserved in small bubbles in the ice gives an indication of the composition of the atmosphere at the time the ice (snow) accumulated. An important result from this work indicates that the amount of carbon dioxide in the atmosphere during the last glacial (stages 2, 3, and 4) was substantially less than during the Holocene (stage 1) and the last interglaciation (stage 5e). This observation has significant implications with respect to climate and climatic change during glacial and interglacial transitions.

Pleistocene events and environments

      Environments during the Pleistocene were dynamic and underwent dramatic change in response to cycles of climatic change and the development of large ice sheets. Essentially all regions of the Earth were influenced by these climatic events, but the magnitude and direction of environmental change varied from place to place. The best-known are those that occurred from the time of the last interglaciation, about 125,000 years ago, to the present.

Glaciation

      The growth of large ice sheets, ice caps, and long valley glaciers was among the most significant events of the Pleistocene. During times of extensive glaciation, more than 45 million square kilometres (or about 30 percent) of the Earth's land area were covered by glaciers, and portions of the northern oceans were either frozen over or had extensive ice shelves. In addition to the Antarctic and Greenland ice sheets, most of the glacial ice was located in the Northern Hemisphere, where large ice sheets extended to mid-latitude regions. The largest was the Laurentide Ice Sheet in North America, which at times stretched from the Canadian Rocky Mountains on the west to Nova Scotia and Newfoundland on the east and from southern Illinois on the south to the Canadian Arctic on the north. The other major ice sheet in North America was the Cordilleran Ice Sheet, which formed in the mountainous region from western Alaska to northern Washington. Glaciers and ice caps were more widespread in other mountainous areas of the western United States, Mexico, Central America, and Alaska, as well as on the islands of Arctic Canada where an ice sheet has been postulated.

      Although smaller in size, the Scandinavian Ice Sheet was similar to the Laurentide in character. At times, it covered most of Great Britain, where it incorporated several small British ice caps, and extended south across central Germany and Poland and then northeast across the northern Russian Plain to the Arctic Ocean. To the east in northern Siberia and on the Arctic Shelf of Eurasia, a number of small ice caps and domes developed in highland areas, and some of them may have coalesced to form ice sheets on the shallow shelf areas of the Arctic Ocean. Glaciers and small ice caps formed in the Alps and in the other high mountains of Europe and Asia. In the Southern Hemisphere, the Patagonia Ice Cap developed in the southern Andes, and ice caps and larger valley glaciers formed in the central and northern Andes. Glaciers also developed in New Zealand and on the higher mountains of Africa and Tasmania, including some located on the equator.

      The results of glaciation varied greatly, depending on regional and local conditions. Glacial processes were concentrated near the base of the glacier and in the marginal zone. Material eroded at the base was transported toward the margin, where it was deposited both at the glacier bed and in the marginal area. These processes resulted in the stripping of large quantities of material from the central zones of the ice sheet and the deposition of this material in the marginal zone and beyond the ice sheet. The Laurentide and Scandinavian ice sheets scoured and eroded bedrock terrain in their central areas, leaving behind many lakes and relatively thin glacial drift. On the other hand, the Central Lowland and the northern Great Plains of the United States and the western plains of Canada, as well as northern Germany and Poland, southern Sweden, and portions of eastern and northern Russia, contain relatively thick deposits of till and other glacial sediment. The landscape of such areas is flat to gently rolling. Today, these areas are among the great agricultural regions of the world, which is in large part attributable to glaciation.

      The effects in mountainous terrain were even more dramatic. Glacial processes were concentrated in the upper regions where snow accumulated and in the valleys through which the glaciers moved to lower elevations. These valley glaciers carved towering peaks (such as the Matterhorn in the Alps), large rock basins, and sweeping U-shaped valleys and left some of the most spectacular scenery on the Earth, with many high-level lakes and waterfalls. The lower portions of the valleys commonly contain ridges of glacial drift. Ridges of this sort that form along valley slopes are called lateral moraines, while those that loop across a valley at the lower end of a glacier are termed end moraines. The earliest observations and interpretations of more extensive Pleistocene glaciation were made on such deposits and landforms in the Alps during the early part of the 19th century.

Periglacial environments

      The environment around the ice sheets was markedly different from that of today in these formerly glaciated areas. Temperatures were much lower, and a zone of permafrost (perennially frozen ground) developed around the southern margin of the ice sheets in both North America and Eurasia. This zone was relatively narrow in central North America, on the order of 200 kilometres, but in Europe and Russia it extended many hundreds of kilometres south of the ice margin. Mean annual temperatures near the ice margin were about -6° C or colder and increased away from the ice margin to about 0° C near the southern extent of the permafrost. Compared to present-day conditions, the mean air temperature was on the order of 12° to 20° C colder near the ice margin. These conditions are indicated by ice-wedge casts and large-scale patterned ground, which are relict forms of ice wedges and tundra polygons that form today only in areas with continuous permafrost. Frost activity through freezing and thawing was intensified, and in areas of more relief talus accumulations and large block fields formed along escarpments and valley sides. Mass-wasting processes also were intensified and much material was eroded from slopes in periglacial areas. Deposits and landforms from such activity are known from the British Isles, northern Europe, and what was formerly the Soviet Union.

Lacustrine environments

      Large lakes (lake), usually many times bigger than their modern counterparts, were common during the Pleistocene. They fluctuated in level in response to the major climatic cycles or the opening and closing of outlets due to glaciation and vertical movements of land areas. Some lakes were closely tied to glaciation. In North America a series of large proglacial lakes formed around the margin of the Laurentide Ice Sheet during backwasting (recession) of the ice margin into Hudson Bay. The lakes were confined in part by the ice margin and in part by higher land to the south, east, and west. One of the largest was Lake Agassiz (Agassiz, Lake), which covered sizable areas of Manitoba, Ontario, and Saskatchewan and extended into North Dakota and Minnesota. The Great Lakes also formed as a result of glaciation as lobes of ice moved down preexisting lowlands and scoured out the weak rocks in the basins. Other lakes formed in the Champlain and Hudson valleys in eastern North America during deglaciation. Similar glacial lakes developed around the Scandinavian Ice Sheet and in other glaciated regions.

      Of equal interest was the development of large lakes in areas that today have arid to semiarid climatic regimes and generally lack lakes or have modern lakes that are much reduced in size and are saline in character. Such lakes are referred to as pluvial lakes, and the climate under which they existed is termed a pluvial climate. Most of these lakes existed in closed basins that lacked outlets, and thus their levels were related to relative amounts of precipitation and evaporation. A record of fluctuating lake levels is provided by ancient shorelines and beach deposits that are present along the slopes of the enclosing mountains as well as by the sediment and soil record preserved in the subsurface deposits of the lake basins. The history of lake fluctuations varies somewhat locally within a region but may be much different from one region of the world to another, depending on the local and regional climate.

      In the Great Basin of Utah, Nevada, California, and Oregon and in other areas of the western and southwestern United States and Mexico, about 100 basins contained lakes during the Pleistocene. The largest of these was Lake Bonneville (Bonneville, Lake), the predecessor of the modern Great Salt Lake in Utah. At its highest stage Lake Bonneville covered an area of about 52,000 square kilometres, and its maximum depth was approximately 370 metres. These conditions existed about 15,000 years ago during the interval of the last major Pleistocene glaciation. Lake Bonneville shrank rapidly in size and, by 12,000 years ago, had permanently shrunk to a point where it had become smaller than the Great Salt Lake. A long record of fluctuating lake levels is evident from a 930-metre core taken in the Searles Lake basin in California. Parts of the sediment record from the core sample indicate a deep lake with lacustrine silts and clays and freshwater fossils. Other parts contain unusual evaporite minerals which indicate that the lake was shallow and highly saline or even evidence of sediment exposure indicative of the complete desiccation of the lake. The inferred climatic record from the core is similar to the marine oxygen isotope record but differs in that it shows more variation in the amplitude of the climatic cycles.

      Pluvial lakes in these areas were most extensive during times of widespread glaciation in the Northern Hemisphere and were low or dry during times of reduced glacial cover. Paleoclimatic modeling suggests that the Laurentide Ice Sheet forced the polar jet stream south of its present-day position during glaciation. This brought more moisture from the Pacific into the desert areas of the southwestern United States, causing greater precipitation as well as producing more cloud cover, which, together with lower temperatures, resulted in less evaporation.

      Pluvial lakes also were common in other dry regions of the world, particularly in the subtropical zones, including eastern and northern Africa and portions of Australia, Asia, and the Middle East. Examples of these pluvial bodies are the Dead Sea in Jordan and Israel and Lake Chad (Chad, Lake) in the southern Sahara. The latter, now a shallow saline lake, covered some 300,000 square kilometres and was about six times the size of Lake Bonneville. A number of lakes in the rift valleys of East Africa (East African lakes) were larger and deeper than they are today. Among the better-known and better-understood are Lakes Rudolf, Victoria, Nakuru, Naivasha, Magadi, and Rukwa. Most of these lakes in the tropical and subtropical regions were not in phase with those in the Great Basin of North America. They were relatively high for some 20,000 or more years immediately before the last glaciation and again just after the last glaciation in the early Holocene. A long climatic record inferred from sediments in Lake George in southeastern Australia has characteristics similar to those of the marine oxygen isotope record. Alternating humid and arid climatic cycles were more rhythmic and of greater magnitude in the middle and late Pleistocene than earlier, and a major change in basin hydrology occurred approximately 2.5 million years ago.

Fluvial environments

      Rivers (river) and the valleys (valley) that they occupy were affected strongly by the changing climates of the Pleistocene. River channels and their sediment record are controlled in large part by the amount and type of load that is supplied by their drainage basins and the discharge or quantity of water available for flow. Both are closely related to climate, which not only includes precipitation, evaporation, and seasonality but also controls the extent of the vegetative cover of the land and the type and intensity of weathering processes. In addition, because of sea-level changes related to glaciation, the base level of rivers in coastal regions also fluctuated by significant amounts. As a result, river environments were dynamic and variable.

      This was true for most rivers, but particularly so for those rivers that drained large quantities of meltwater and sediment from the glacier margins. During glaciation, rivers of the latter kind developed braided-channel patterns in response to the input of large quantities of sediment derived from the melting glaciers and subglacial waters and to the large fluctuations in the quantity of water flowing at any one time, which varied because of seasonal and diurnal controls on the generation of meltwater. During times of glaciation many of these rivers deposited thick sequences of sand and gravel in their valleys; examples include those of the Hudson, Mississippi, and Ohio rivers in the United States and of the Thames, Elbe, Rhine, and Seine rivers in Europe. Similar valleys have been buried by younger glacial deposits and are no longer evident at the surface. They exist today as bedrock valleys with thick fills of fluvial sand and gravel or lacustrine silt in localities where lakes existed in the valleys as a result of glacial damming. The sand and gravel fill in the surface valleys provide aggregate material for construction, and much groundwater is derived from the fills of both surface and buried valleys.

      Some glacial valleys, as well as large upland areas, were sites of major catastrophic floods that resulted from the sudden drainage of proglacial and subglacial lakes. Such floods are known as jökulhlaups, an Icelandic term for subglacial lake outbursts. The largest and best-known floods of this type occurred in the Channeled Scabland of the Columbia Plateau region in eastern Washington state. Ice tongues flowing south from the Cordilleran Ice Sheet periodically dammed the Clark Fork River, forming glacial Lake Missoula. At times, Lake Missoula stretched more than 200 kilometres upvalley and was about 600 metres deep near the ice dam. Sudden failure of the ice dam released over 2,000 cubic kilometres of water, which flooded westward and southward across the Columbia Plateau and down the Columbia River valley. The floods cut through a loess cover into basalt and left a system of large dry channels with waterfalls, potholes, and longitudinal grooves in the basalt. Associated with the dry channels are huge, coarse gravel bars and giant current ripples. Other large catastrophic floods resulted from the sudden drainage of glacial Lake Agassiz and from the ancestral Great Lakes, as well as from some nonglacial lakes such as Lake Bonneville in the Great Basin (see above). During the Anglian–Elsterian glaciation in Europe a large ice-dammed lake formed in the North Sea, and large overflows from it initiated cutting of the Dover Straits. (Dover, Strait of)

      During the transition from glacial to interglacial conditions, river channel patterns evolved from braided to meandering as a result of decreased load and possibly discharge. Near glaciated areas, rivers eroded into glacial outwash and left a system of stream terraces along the sides of most valleys. These modern interglacial rivers are much smaller than their glacial counterparts and are underfit (i.e., appear too small) with respect to the large valleys in which they flow. In contrast, near coastal areas rivers actively built up their channels during the transition to interglacial conditions in response to rising sea level.

Coastal environments and sea-level (sea level) changes

      Coastal environments during the Pleistocene were controlled in large part by the fluctuating level of the sea as well as by local tectonic and environmental conditions. As a result of the many glaciations on land and the subsequent release of meltwater during interglacial times, sea level has fluctuated almost continuously between interglacial levels, like those of today, and levels during times of maximum glaciation, such as 18,000 years ago when sea level was more than 100 metres lower. At that time all the continental land areas were larger, and extensive areas of the world's continental shelves were exposed to weathering, soil formation, and fluvial and eolian activity and were inhabited by plants and animals. The Bering (Bering Strait) Shelf was exposed at this time and Siberia was connected to Alaska by a land bridge, thus allowing intercontinental migration of animals, including early humans. Rapid melting of the last large ice sheets resulted in a rising sea level that reached near modern level by the mid-Holocene, about 5,000 years ago. As a consequence, Pleistocene coastal environments are submerged below sea level in most parts of the world and are poorly known.

      Fortunately some coastal areas of the world were undergoing tectonic uplift during the Pleistocene, and as a result older shorelines and their deposits are exposed above modern sea level. Study of these deposits is important in understanding the recent sea-level record and in relating it to the record of glaciation. The most important are shorelines that contain coral reefs (coral reef), because it is possible to obtain radiometric ages on fossils in the reef complex. Two of the most important and best-dated records are on the island of Barbados in the Caribbean and along the Huron Peninsula of New Guinea. The latter area exposes a spectacular suite of coastal terraces due to steady and rapid uplift during the Pleistocene. Age determinations of the terraces indicate times of relatively high sea level and suggest that they occurred at intervals of about 20,000 years. The highest sea level prior to the modern level occurred about 125,000 years ago and correlates with the peak warm interval of the last interglaciation (oxygen-18 stage 5e). Sea level at that time was about six metres higher than it is today.

Eolian activity

      Eolian deposits are important in the Pleistocene record and indicate widespread wind action at certain times and in certain areas of the world. Mention has already been made of the importance of loess–paleosol (loess) records in working out regional chronologies and paleoclimatic history. Loess blankets large portions of the central and northwestern United States, Alaska, the east European plain of Russia, and southern Europe, where it is closely related to episodes of glaciation or to the cold periglacial climate beyond the ice sheet margins or to both. The loess was derived primarily from the broad floodplains of the braided rivers draining meltwater and sediment away from the glaciers as well as from newly exposed glacial drift. Locally, sand dunes and sheets of sand occur near the valley sources and in some cases cover large upland areas, as in central and northern Europe. The loess in China, on the other hand, is considered to have been deflated mostly from such desert areas as the Gobi.

      The deserts of the subtropical regions also experienced eolian activity during the Pleistocene. In Australia, the time of peak aridity and maximum dune activity (about 20,000 to 12,000 years ago) correlates with the time of peak glaciation in the Northern Hemisphere. This also was the case in the Sahara and other deserts in Africa, India, and the Middle East. One estimate is that the tropical arid zones were five times larger during times of peak glaciation. Sea level was lower at these times, the water was colder, and tropical cyclones were less extensive, resulting in decreased rainfall. These episodes of intensified eolian activity are recorded in other Pleistocene records. Ocean cores taken downwind of these regions contain windblown sediment in the portions of the core that accumulated during times of maximum eolian activity. In addition, microparticles occur in ice cores taken from the Greenland and Antarctic ice sheets and are concentrated at times of maximum glaciation and aridity in the subtropical deserts. At other times, the climate was less arid and the desert areas contracted, and vegetation developed to stabilize the dunes under more humid (pluvial) conditions.

Tectonic (tectonics) and isostatic movements

      The lithospheric plates continued to shift during the Pleistocene, but the continents essentially were in their modern position at the start of the epoch. Of more importance to subsequent Quaternary events were the late Tertiary tectonic movements that affected the evolution of climate toward that of the Quaternary. Among these were the formation of the Isthmus of Panama, which affected oceanic circulation, and the uplift of the Tibetan Plateau and broad regional areas of the western United States, which affected atmospheric circulation, particularly the position and configuration of the polar jet stream.

      Vertical movements of the Earth's crust also were caused by the formation and melting of large ice sheets. The area beneath an ice sheet subsides during glaciation because the crust is not able to sustain the weight of the glacier. These isostatic movements take place through the flow of material in the Earth's mantle, and the amount of subsidence amounts to about one-third the thickness of the ice sheet—for example, about one kilometre in the central area of the Laurentide Ice Sheet in Canada. Melting of the ice sheet removes the load and causes the ground to rise, or rebound. Such uplift is rapid at first but decreases with time. More than 300 metres of uplift has occurred in the eastern Hudson Bay area since that area was deglaciated. Substantial uplifting also took place prior to the complete melting of the ice sheets, and upward crustal movement continues today at a maximum rate of about 1.3 centimetres per year. A similar record of glacio-isostatic adjustments is encountered in Fennoscandia, where the greatest depression and subsequent uplift related to the Scandinavian Ice Sheet is located in the Gulf of Bothnia.

Pleistocene fauna and flora

      The plants (plant) and animals (animal) of the Pleistocene are, in many respects, similar to those living today, but important differences exist. Moreover, the spatial distribution of various Pleistocene fauna and flora types differed markedly from what it is at present. Changes in climate and environment caused large-scale migrations of both plants and animals, evolutionary adaptations, and in some cases extinction. Study of the biota provides not only data on the past paleoenvironments but also insights into the response of plants and animals to well-documented environmental change. Of particular importance is the evolution of the genus Homo during the Pleistocene and the extinction of large mammals at the end of the epoch.

Evolutionary changes

      Evolutionary changes during the Pleistocene generally were minor because of the short interval of time involved. They were greatest among the mammals (mammal). In fact, the epoch has been subdivided into mammalian ages on the basis of the appearance of certain immigrant or endemic forms.

      Mammalian evolution included the development of large forms, many of which became adapted to Arctic conditions. Among these were the woolly mammoth, woolly rhinoceros, musk ox, moose, reindeer, and others that inhabited the cold periglacial areas. Large mammals that inhabited the more temperate zones included the elephant, mastodon, bison, hippopotamus, wild hog, deer, giant beaver, horse, and ground sloth. The evolution of these as well as of much smaller forms was affected in part by three factors: (1) a generally cooler, more arid climate subject to periodic fluctuations, (2) new migration routes resulting largely from the emergence of intercontinental connections during times of lower sea level, and (3) a changing geography due to the uplift of plateaus and mountain building.

      The most significant biological development was the appearance and evolution of the genus Homo (Homo habilis). The oldest species, H. habilis, probably evolved from an australopithecine ancestor in the late Pliocene. The species was present in Africa by 2 million years ago and is known from sites as young as 1.5 million years old. Another extinct species, H. erectus, evolved in Africa, possibly from H. habilis, and is known from sites about 1.6 million years old. H. erectus spread to other parts of the Old World during the early Pleistocene and is known from northern China and Java by roughly 1 million years ago. Representatives of this group are known from many sites, and these beings constituted the dominant human (human being) species for more than a million years. The species H. sapiens, to which all modern humans belong, evolved in the later part of the middle Pleistocene, and early forms of the species are known from about 400,000 years ago. More modern forms of H. sapiens, the Neanderthals (Neanderthal), appeared approximately 100,000 years ago during the last interglaciation and are known from many sites in Europe and western Asia. They disappeared about 35,000 to 30,000 years ago, and by then populations with fully modern skeletons had evolved and were widespread in the Old World. Exactly when modern H. sapiens entered the New World remains controversial. It appears that fully evolved humans had migrated as far as Alaska from Siberia via the Bering land bridge by 30,000 years ago, and large numbers presumably moved south down the Canadian plains corridor between the Cordilleran and Laurentide ice sheets when it opened near the end of the last glaciation some 12,000 years ago. Conflicting and not fully accepted evidence at a few sites in the United States and in southern South America, however, suggests occupation of the continental interior prior to 30,000 years ago. If such findings are valid, the group of earlier immigrants may have arrived by small ocean-going craft from the Pacific Islands.

migration of plants and animals

      Changing environments in response to climatic variation caused drastic disruptions of faunas and floras both on land and in the oceans. These disruptions were greatest near the former ice sheets that extended far to the south and caused the southward displacement of climatic and vegetation zones. In the temperate zones of central Europe and the United States where deciduous forests exist today, vegetation was open and most closely resembled the northern tundra, with grasses, herbs, and few trees during glacial intervals. Farther south, a broad region of boreal forests with varying proportions of spruce and pine or a combination of both extended almost to the Mediterranean in Europe and northern Louisiana in North America. The vegetation succession has been documented by studies of fossil pollen, which accumulated year by year with other sediments in lakes and bogs beyond the ice margin. Although such floral migrations appear simple in concept, interpretation of the vegetation record is quite complicated because a number of the glacial pollen assemblages have no modern analogues—i.e., they contain mixtures of forms from different present-day climatic environments. Similar relationships also occur with vertebrate faunas: more temperate forms commonly occur together with more Arctic forms. Such “disharmonious” faunas suggest that glacial climatic and environmental conditions in some cases were totally unlike those of any modern environment. One explanation is that climatic conditions may have been more equable during glacial times and may have lacked the seasonal extremes of modern climates in such areas. Although overall temperatures were significantly lower, summers probably were much cooler because of the influence of the ice sheet, and winters, except very near the ice margin, lacked severe cold spells, as the ice sheet formed a barrier to Arctic air masses that today bring freezing conditions far to the south. Thus, plants and animals whose geographic ranges would ordinarily be controlled by either extreme seasonal warm or cold conditions were able to coexist during glacial times, and considerable community reorganization took place in response to climatic change during and following a glaciation.

      Similar responses to changing environments are well known from life in the oceans. Marine organisms closely reflect the temperature, depth, and salinity of the water in which they live, and studies of the fossil succession from deep-sea cores have allowed detailed reconstructions of oceanic conditions for the late Pleistocene. Planktonic foraminifers are most useful for determining sea-surface conditions, and changes in the distribution of polar, subpolar, subtropical, and tropical faunas have been used to map changing oceanic conditions. Changes in the North Atlantic Ocean were most dramatic because of the direct influence of the ice sheets to the west, north, and east. During episodes of glaciation, polar faunas extended south to about 45° N latitude, whereas during interglaciations these faunas occurred mostly north of 70° and subtropical faunas extended far to the north under the influence of the Gulf Stream.

Megafaunal extinctions (extinction)

      The end of the Pleistocene was marked by the extinction of many genera of large mammals, including mammoths, mastodons, ground sloths, and giant beavers. The extinction event is most distinct in North America, where 32 genera of large mammals vanished during an interval of about 2,000 years, centred on 11,000 BP. On other continents, fewer genera disappeared, and the extinctions were spread over a somewhat longer time span. Nonetheless, they still appear to be more common near the end of the Pleistocene than at any other time during the epoch. Except on islands, small mammals, along with reptiles and amphibians, generally were not affected by the extinction process. The cause of the extinctions has been vigorously debated, with two main hypotheses being advanced: (1) the extinctions were the result of overpredation by human hunters; and (2) they were the result of abrupt climatic and vegetation changes during the last glacial–interglacial transition.

      The first theory, the so-called overkill hypothesis, receives support from the coincidence in the timing of the mass extinction and the appearance of large numbers of human hunters, as evidenced by the Clovis complex, an ancient culture centred in North America. Clovis archaeological sites (concentrated in Arizona, New Mexico, and West Texas), with their distinctive projectile points, date between 10,000 and 12,000 years ago. Proponents of the hypothesis point out that these new immigrants from Eurasia were skilled hunters, that the North American fauna would not have been wary of this new group of predators, and that, once the number of large herbivores declined, large carnivores also would have been affected as their prey became extinct. In addition to direct slaughter, human disruption of the environment most likely contributed to the extinctions, particularly on other continents.

      Abrupt climatic change (climate change) also occurred at the time of the megafaunal extinctions, and so timing alone does not clearly differentiate one hypothesis from the other. The climatic-change hypothesis takes a number of forms but essentially focuses on the reorganization of vegetation, on the availability of food (including nutrient value), and on the general environmental disruption and stress that resulted as climates became more seasonal. It appears likely that the causes of extinction varied in different geographic areas under different conditions and that both climatic change and human activities played roles but of varying importance in different situations.

Cause of the climatic changes and glaciations

      Pleistocene climates and the cause of the climatic cycles that resulted in the development of large-scale continental ice sheets have been a topic of study and debate for more than 100 years. Many theories have been proposed to account for Quaternary glaciations, but most are deficient in view of current scientific knowledge about Pleistocene climates. One early theory, the theory of astronomical cycles, seems to explain much of the climatic record and is considered by most to best account for the fundamental cause or driving force of the climatic cycles.

      The astronomical theory is based on the geometry of the Earth's (Earth) orbit around the Sun, which affects how solar radiation is distributed over the surface of the planet. The latter is determined by three orbital parameters that have cyclic frequencies: (1) the eccentricity of the Earth's orbit (i.e., its departure from a circular orbit), with a frequency of about 100,000 years, (2) the obliquity, or tilt, of the Earth's axis away from a vertical drawn to the plane of the planet's orbit, with a frequency of 41,000 years, and (3) the precession, or wobble, of the Earth's axis, with frequencies of 19,000 and 23,000 years. Collectively these parameters determine the amount of radiation received at any latitude during any season; radiation curves have been calculated from them for different latitudes for the past 600,000 years. These curves vary systematically from the poles to the equator, with those in the higher latitudes being dominated by the 41,000-year tilt cycle and those in lower latitudes by the 19,000- and 23,000-year precession cycles. The astronomical theory places emphasis on summer insolation in the high-latitude areas of the Northern Hemisphere (about 55° N latitude). Glaciations are hypothesized to begin during times of low summer insolation when conditions should be most optimal for winter snow to last through the summer season.

      Dating of the marine terraces in Barbados and New Guinea and, more importantly, determining the chronology of glaciations as inferred from the marine oxygen isotope record were milestones in testing the astronomical theory. Early spectral analysis of the oxygen isotope record of cores from the deep ocean showed frequencies of climatic variation at essentially the same frequencies as the orbital cycles—that is to say, at 100,000 years, 43,000 years, 24,000 years, and 19,000 years. These results (reported in 1976), along with those of more recent analyses, provide firm evidence of a tie between orbital cycles and the Earth's recent climatic record. The variations in the Earth's orbit are generally considered the “pacemaker” of the ice ages.

      Although the planetary orbital cycles are the likely cause of the Pleistocene climatic cycles, the mechanisms and connections to the global climate are not fully understood, and important questions remain unanswered. The relatively small seasonal and latitudinal radiation variations alone cannot account for the magnitude of climatic change as experienced by the Earth during the Pleistocene. Clearly, feedback mechanisms must operate to amplify the insolation changes caused by the orbital parameters. One of these is albedo, the reflectivity of the Earth's surface. Increased snow cover in high-latitude areas would cause increased cooling. Another feedback mechanism is the decreased carbon dioxide content of the atmosphere during times of glaciation, as recorded in the bubbles of long ice cores. Variations in atmospheric carbon dioxide are essentially synchronous with global climatic change and thus in all likelihood played a significant role through the so-called greenhouse effect. (The latter phenomenon refers to the trapping of heat—that is to say, infrared radiation—in the lower levels of the atmosphere by carbon dioxide, water vapour, and certain other gases.) Another atmospheric effect is the increased amount of dust during glacial times, as borne out by ice core and loess records. All of these changes operate in the same direction, causing increased cooling during glacial times and warming during interglacial times.

      Other problems remain with respect to the astronomical theory. One is the dominance of the 100,000-year cycle in the Pleistocene climatic record, whereas the eccentricity cycle is the weakest among the orbital parameters. Another is the cause of the asymmetrical pattern of the climatic record. Ice ages appear to start slowly and take a long time to build up to maximum glaciation, only to terminate abruptly and go from maximum glacial to full interglacial conditions in less than 10,000 years (see figure). A third problem is the synchronous nature of the climatic record between the Northern and Southern hemispheres, which one would not expect from the orbital parameters because they operate in different directions in the two hemispheres.

      Different approaches have been taken to explain these questions. Most of these suggest that the Northern Hemisphere with its enormous continental ice sheets was the controlling area and that the ice sheets themselves with their complex dynamics may explain the 100,000-year climatic cycle. Others propose that major reorganizations of the ocean–atmosphere system must be called upon to explain the climatic record. These reorganizations are concerned with the transport of salt through the oceans and water vapour through the atmosphere and revolve around the existence and strength of deep oceanic currents in the Atlantic Ocean.

      Ongoing interdisciplinary research on Pleistocene paleoclimatology is focused on understanding the complex dynamics and interactions among the atmosphere, oceans, and ice sheets. Such research is expected to provide further insight into the cause of the climatic cycles, which is essential as scientists attempt to predict future climates in view of recent human-induced modifications of the climatic system.

W. Hilton Johnson

Additional Reading

A recent general summary of the physical and biological records of the Pleistocene Epoch is found in Bjørn G. Andersen and Harold W. Borns, Jr., The Ice Age World: An Introduction to Quaternary History and Research with Emphasis on North America and Northern Europe During the Last 2.5 Million Years (1994, reissued 1997); and Björn Kurtén and Elaine Anderson, Pleistocene Mammals of North America (1980).

* * *