PLEISTOCENE EPOCH

the earlier and major of the two epochs that constitute the Quaternary Period of Earth 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. Originally, the Pleistocene Epoch was envisaged as covering the period of the Great (or Quaternary) Ice Age. The onset of this ice age, however, was marked by a sharp climatic cooling dated about 2,500,000 years ago, and it is now placed within the Pliocene Epoch. Additionally, the Holocene Epoch is now regarded as the latest interglacial stage of the ice age. Some authorities would like to extend the Pleistocene up to the present time, but others feel that the present subdivision of the Quaternary Period into Pleistocene and Holocene epochs is justified by the much-intensified role of humans during the latter epoch. At the height of the Pleistocene glacial ages, more than 30 percent of the land area of the world was covered by glacial ice. At present only about 10 percent is so covered, and much of the ice is in the higher latitudes. The same was probably true for the interglacial stages. Some differences between the present-day conditions and the conditions of at least one of the interglacials are apparent, however, from comparison of fossil faunal forms with the animals of today. Glacial cirques (theatre-like valley heads fashioned by the action of snowfields at the heads of individual glaciers in mountainous terrain) bear a rough general relation to the snow line (lower limit of perennial snow). Through measurements of the altitudes of cirques in many parts of the world, the approximate position of the snow line at the height of the latest glaciation has been determined. Wherever measured, this former snow line is lower than the snow line of todayat the equator as well as in polar latitudes. The subpolar climate belts were shifted toward the equator during the glacial episodes. This shift may have amounted to as much as 15 of latitude for the boreal, or northern, belt but somewhat less for the warmer belts. The pluvial conditions of the dry regions of middle and low latitudes support this conclusion in that they appear to show equatorward shifting of the middle-latitude belts of rain-bearing cyclonic storms. Fossil flora and fauna likewise indicate southward shifting of the cold northern climatic zone through many degrees of latitude. The areas within the Northern Hemisphere that were formerly glaciated include the mountainous western part of North America. This region was occupied by a vast complex of glaciers that, throughout most of the Canadian sector, formed a nearly continuous covering of ice. The vast area from the Atlantic to the Rocky Mountains, Canada and the northern part of the United States (as far south as New York City; Cincinnati, Ohio; Carbondale, Ill.; St. Louis, Mo.; Kansas City, Mo.; and Pierre, S.D.) were buried beneath the Laurentide Ice Sheet that had its source in Canada. This enormous continental glacier is thought to have originated on the Labrador-Ungava plateau and on the Arctic islands of Canada. Greenland and Iceland were almost entirely ice-covered. Nearly half of Europefrom the North Cape off the north coast of Norway south to Kiev on the banks of the Dnieperwas covered by the Scandinavian Ice Sheet. Much of Siberia was overspread by mountain glaciers and by the Siberian Ice Sheet on its northwestern plain. The Alps, the Caucasus, and the Pyrenees in Europe and most of the high mountains on other continents carried glaciers of varying dimensions. The Antarctic continent was even more nearly completely ice-covered than now, and the glaciers of the southern Andes spread westward to tidewater in Chile and eastward onto the pampas of Argentina. The highest mountains of Hawaii, Japan, and New Guinea supported valley glaciers. The effects of the glacial climate and continental ice sheets included the fall and rise of the sea level throughout the world. The moisture that formed glaciers came from the sea, resulting in the lowering of the sea level. Under warming climates the existing glaciers melted and large volumes of meltwater were returned to the sea, thereby raising its level. The alternation of glacial and interglacial episodes, therefore, resulted in fluctuation of the sea level through a range of more than a hundred metres, including levels both somewhat higher and much lower than today. Glaciation also caused extensive warping of the Earth's crust. The crust yields to excessive loading such as that induced by great ice sheets. In the substratum below the rocky crust, plastic flow transfers rock material outward away from the weighted area, causing basinlike subsidence. When the ice sheet melts, the load is removed, causing the crust to rebound. Such uplift is initially rapid, but it decreases with time. The eastern area of Hudson Bay is known to have been uplifted by over 300 m (985 feet) since the last major glaciation. In the dry regions in low and middle latitudes on all the continents, there is impressive evidence both of former lakes where none now exist and of the formerly much larger size of existing lakes. A good example of the latter is Lake Bonneville, the predecessor of the present-day Great Salt Lake in Utah in the western United States. At its highest stage some 15,000 years ago during the last glaciation, Lake Bonneville covered an area of approximately 51,000 square km (19,700 square miles) and was about 370 m (1,215 feet) deep. The flora and fauna of the Pleistocene began to resemble those of today. Angiosperms (flowering plants), particularly the deciduous forms, proliferated in the temperate and colder regions. New groups of land mammals appeared in Europe during the early Pleistocene; these included true elephants, zebrine horses, and cattle. Later in the epoch, there emerged various Arctic mammalian forms, such as reindeer, musk-oxen, and lemmings, followed by woolly mammoths, woolly rhinoceroses, and moose. North American fauna of the early and middle Pleistocene was quite similar. It included a large number of immigrants from Asia, notably mammoths, caballine horses, antelopes, and many rodents. By the late Pleistocene, bison, skunks, and bats appeared in North America. Among the significant mammals that evolved during the Pleistocene were humans. Homo erectus, usually considered to be the direct ancestor of modern humans, had appeared in Africa at the outset of the Pleistocene and became extinct late in the epoch. By the mid-Pleistocene, Homo sapiens, the species of modern humans, evolved in Africa and Europe. H. sapiens spread to Asia and the Americas before the end of the epoch. The striking difference between the present-day fauna of large terrestrial mammals and the far richer fauna that existed at the height of the last glaciation is the direct result of mass extinctions. In North America more than 30 genera of large mammals became extinct within a span of roughly 2,000 years during the late Pleistocene. The number in Eurasia was considerably smaller, as was the case in Africa and South America. The extinctions involved not only large mammalian forms but also large flightless birds (e.g., in New Zealand). Of the many causes that have been proposed by scientists for these faunal extinctions, the two most likely are changing environment with changing climate, and the disruption of the ecological pattern by early humans. 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 Pleistoceneduring the latter part of the Tertiary Period as well as during earlier periods of geologic timeand that glaciation is not unique to the Pleistocene. Additional reading General summaries of the physical and biological record of the Pleistocene Epoch are found in Tage Nilsson, The Pleistocene: Geology and Life in the Quaternary Ice Age (1983). Regional surveys include W.F. Ruddiman and H.E. Wright, Jr. (eds.), North America and Adjacent Oceans During the Last Deglaciation (1987); Stephen C. Porter and H.E. Wright, Jr. (eds.), Late-Quaternary Environments of the United States, 2 vol. (1983); N.J. Shackleton, R.G. West, and D.Q. Bowen (eds.), The Past Three Million Years: Evolution of Climatic Variability in the North Atlantic Region (1988); and A.A. Velichko, H.E. Wright, Jr., and C.W. Barnosky (eds.), Late Quaternary Environments of the Soviet Union, trans. from Russian (1984). Megafaunal extinctions at the end of the Pleistocene are explored in Paul S. Martin and Richard G. Klein (eds.), Quaternary Extinctions: A Prehistoric Revolution (1984). W. Hilton Johnson 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 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 cyclesthat 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 heatthat is to say, infrared radiationin 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. Summary of marine oxygen isotope records. 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 oceanatmosphere 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 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. Pleistocene fauna and flora The plants and animals 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. 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. 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 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, 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.