Pleistocene events and environments

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 km (roughly 17,400,000 square miles), or about 30 percent of Earth’s land area, was covered by glaciers, and portions of the northern oceans either were 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 midlatitude 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 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 km, 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 (21.2 °F) or colder and increased away from the ice margin to about 0 °C (32 °F) near the southern extent of the permafrost. Compared with present-day conditions, the mean air temperature was on the order of 12 to 20 °C (53.6 to 68 °F) 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, 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, 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, the predecessor of the modern Great Salt Lake in Utah. At its highest stage Lake Bonneville covered an area of about 52,000 square km (nearly 20,100 square miles), and its maximum depth was approximately 370 metres (about 1,214 feet). 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 (3,051-foot) 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 in the southern Sahara. The latter, now a shallow saline lake, covered some 300,000 square km and was about six times the size of Lake Bonneville. A number of lakes in the rift valleys of East Africa 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.