Pleistocene Era

THE INCREASING FREQUENCY and intensity of gla-cial-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 maybe 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 [Page 795]and have shaped much of the modern landscape. Glacial stages maybe 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 isotopie 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 hydrologie 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 inter-glacials. 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.

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