THE Increasing FREQuENOY and intensity of glacial-interglacial cycles toward the end of the Pliocene (1.806-5.332 million years ago) set the stage for the Pleistocene epoch (11.8 thousand years ago-1.806 million years ago), which is the final phase of the Quaternary period. Some argue that the lower Pleistocene boundary may be set too late because the general trend toward significant cooling and glaciation had begun in the mid-late Pliocene (2.75 million years ago). Hence, the term Plio-Pleistocene may be used to delineate this transitional phase between the two epochs.

Strong glacial-interglacial phases are the key climatic features that characterize the Pleistocene epoch and have shaped much of the modern landscape. Glacial stages may be referred to as ice ages, and are used to describe a period of extensive ice sheet presence in the polar, high latitude continental, and alpine regions. Glacial phases are synonymous with reduced global temperatures. Quaternary glacial-interglacial cycles occurred with a 41,000-year periodicity, starting in the late Pliocene (2.75 million years ago) to mid-Pleistocene (1.11 million years ago), followed by a 100,000 year cycle in the mid- to late-Pleistocene. The most intensely studied glacial stage during the Pleistocene is the last glacial maximum (21,000 years ago).

Marine fossil material and isotopic proxies were used to simulate sea surface temperatures, sea ice, continental ice sheets, and albedo during the last glacial maximum, with results indicating that high latitudes in the northern hemisphere cooled by 7-11 degrees F (4-6 degrees C), while simulated sea temperatures increased by 2-5 degrees F (1-3 degrees C) in the Pacific and Indian oceans. Most recent evidence suggests that with the exception of Central America and the Indo-Pacific, the climate was much drier than today, due to the combination of reduced evaporation, greater coverage of land surfaces by ice sheets, and wind anomalies.

Glaciation was most extensive in the northern hemisphere, with 2-2.5 mi.- (3-4 km.-) thick ice sheets covering Canada and parts of the northern United States, Greenland, northern Europe, Russia, and perhaps to a lesser extent, the Tibetan Plateau. In the southern hemisphere, the glaciation of Antarctica that began in the Pliocene continued through to the last glacial maximum, the Andes were highly glaciated, the Pata-gonian Ice Sheet covered much of southern Chile, and small glaciers formed in Africa, the Middle East, and southeast Asia, where simultaneously, deserts were expanding. Sea levels may have been up to 426.5 ft. (130 m.) lower than today. The hydrologic and geological consequences of the last glacial maximum and other glacial stages are still evident, particularly at the higher latitudes of the northern hemisphere, where the abundance of fresh water is effectively the result of glacial retreat and runoff. Remnants of Pleistocene glaciers also remain in high-altitude tropical localities such as on Mount Kilimanjaro and the Peruvian Andes, but these glaciers are quickly retreating.

The causes of the Pliocene-Pleistocene glacial-interglacial cyclicity are largely attributed to climate forcing caused by variations in the Earth's orbital parameters (Milankovitch cycles), but the sequence of events is difficult to establish. However, there is strong evidence that greenhouse gas levels fell at the start of glacials and rose during the interglacial retreat of the ice sheets. So far, eight glacial cycles have been identified from cores in Antarctica dating back to 740,000 years ago, but currently, it is the Vostok ice core dating back to 420,000 years ago that provides the clearest perspective on the link between greenhouse gases and sea surface temperatures over the last four glacial-interglacial cycles. CO2 concentrations fall between 180-200 ppm during the coldest glacial periods, and 280-300 ppm during full interglacials, while methane concentrations were approximately 350 ppb during glacials, and roughly twice that amount during interglacials. Current thinking is that Pleistocene changes in greenhouse gas levels were probably caused by disturbance to the sources of these gases, of which the oceanic and terrestrial sources were most significant.

During the last glacial maximum, the presence of large ice sheets over the high latitudes of the Northern Hemisphere significantly reduced the amount of exposed vegetation, and combined with low atmospheric CO2 and other regional climatic changes, created biomes and vegetation assemblages that have no modern analogue. The Laurentide Ice Sheet completely covered Canada and the northern United States, with taiga, desert, and grassland ecotones dominating the mid-latitudes. At this time, woodland and shrub communities were also present, but highly fragmented. An exception to this is the Canadian and Alaskan Pacific coasts, where the continuity of woody flora remains largely unchanged from the last glacial maximum.

Substantial winter cooling reduced the global extent of tropics and subtropics and caused local extinctions, but equable areas may have acted as regional refuges for species that otherwise would have disappeared. The expansion of more arid ecosystems is well documented from pollen data showing that grasslands and shrub ecotones spread into previously tropical areas such as the Amazonian Basin, equatorial Africa and southern Asia. The persistence of rainforests in central North America and Indonesia during the last glacial maximum can be attributed to the consistently high rainfall in these regions. By contrast, over half of central Australia was desert, with tropical grasslands lying in the north, and scrub-woodland vegetation dominating the eastern and western regions.

Substantial evidence exists to support the hypotheses that Pleistocene fauna was dually affected by the climatic oscillations of the early-mid Quaternary, and the hunting activity of ancestral humans. The disappearance of species that had evolved in colder climates, such as the woolly mammoth, woolly rhinoceros, and musk ox, is most consistent with the appearance of humans in North America. By contrast, the Pleistocene extinction of Eurasian megafauna was likely due to climate. The disappearance of African and South American mammals is unresolved, but current evidence points to the arrival of humans as a key factor.

sEE ALsO: Glaciology; Ice Ages; Pliocene Era; Quatenary Era.

bibliography. K.D. Alverson, R.S. Bradley, and T.F. Peder-son, Paleoclimate, Global Change and the Future (SpringerVerlag, 2003); A.D. Barnosky, et al., "Assessing the Causes of Late Pleistocene Extinctions on the Continents," Science (v.306, 2004); J.A. Van Couvering, The Pleistocene Boundary and the Beginning of the Quaternary (Cambridge University Press, 1997).

Jarmila Pittermann University of California, Berkeley

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