LECTURE 8: THE PLEISTOCENE EPOCH
The Pleistocene (IPA: /'plaɪstəsi:n/) is the epoch from 1,808,000 to 11,550 years BP covering the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek πλεῖστος (pleistos "most") and καινός (kainos "new").
The Pleistocene epoch follows the Pliocene epoch and is followed by the Holocene epoch. The Pleistocene is the third epoch of the Neogene period or 6th epoch of the Cenozoic Era.[1] The end of the Pleistocene corresponds with the end of the Paleolithic age used in archaeology.
The Pleistocene is divided into the Early Pleistocene, Middle Pleistocene and Late Pleistocene, and numerous faunal stages.
The modern continents were essentially at their present positions during the Pleistocene, the plates upon which they sit probably having moved no more than 100 km relative to each other since the beginning of the period.
According to Mark Lynas (through collected data), the Pleistocene's overall climate could be characterized as a continuous El Niño with trade winds in the south Pacific weakening or heading east, warm air rising near Peru, warm water spreading from the west Pacific and the Indian Ocean to the east Pacific, and other El Niño markers.[7]
Pleistocene climate was characterized by repeated glacial cycles where continental glaciers pushed to the 40th parallel in some places. It is estimated that, at maximum glacial extent, 30% of the Earth's surface was covered by ice. In addition, a zone of permafrost stretched southward from the edge of the glacial sheet, a few hundred kilometres in North America, and several hundred in Eurasia. The mean annual temperature at the edge of the ice was −6 °C; at the edge of the permafrost, 0 °C.
Each glacial advance tied up huge volumes of water in continental ice sheets 1500–3000 m thick, resulting in temporary sea level drops of 100 m or more over the entire surface of the Earth. During interglacial times, such as at present, drowned coastlines were common, mitigated by isostatic or other emergent motion of some regions.
The effects of glaciation were global. Antarctica was ice-bound throughout the Pleistocene as well as the preceding Pliocene. The Andes were covered in the south by the Patagonian ice cap. There were glaciers in New Zealand and Tasmania. The current decaying glaciers of Mount Kenya, Mount Kilimanjaro, and the Ruwenzori Range in east and central Africa were larger. Glaciers existed in the mountains of Ethiopia and to the west in the Atlas mountains.
In the northern hemisphere, many glaciers fused into one. The Cordilleran ice sheet covered the North American northwest; the east was covered by the Laurentide. The Fenno-Scandian ice sheet rested on north Europe, including Great Britain; the Alpine ice sheet on the Alps. Scattered domes stretched across Siberia and the Arctic shelf. The northern seas were frozen.
South of the ice sheets large lakes accumulated because outlets were blocked and the cooler air slowed evaporation. North central North America was totally covered by Lake Agassiz. Over 100 basins, now dry or nearly so, were overflowing in the American west. Lake Bonneville, for example, stood where Great Salt Lake now does. In Eurasia, large lakes developed as a result of the runoff from the glaciers. Rivers were larger, had a more copious flow, and were braided. African lakes were fuller, apparently from decreased evaporation.
Deserts on the other hand were drier and more extensive. Rainfall was lower because of the decrease in oceanic and other evaporation.
Four major glacial events have been identified, as well as many minor intervening events. A major event is a general glacial excursion, termed a "glacial." Glacials are separated by "interglacials." During a glacial, the glacier experiences minor advances and retreats. The minor excursion is a "stadial"; times between stadials are "interstadials."
These events are defined differently in different regions of the glacial range, which have their own glacial history depending on latitude, terrain and climate. There is a general correspondence between glacials in different regions. Investigators often interchange the names if the glacial geology of a region is in the process of being defined. However, it is generally incorrect to apply the name of a glacial in one region to another.
For most of the 20th century only a few regions had been studied and the names were relatively few. Today the geologists of different nations are taking more of an interest in Pleistocene glaciology. As a consequence, the number of names is expanding rapidly and will continue to expand.
The glacials in the following table are a simplification of a more complex cycle of variation in climate and terrain. Many of the advances and stadials remain unnamed. Also, the terrestrial evidence for some of them has been erased or obscured by larger ones, but evidence remains from the study of cyclical climate changes.
Corresponding to the terms glacial and interglacial, the terms pluvial and interpluvial are in use (Latin: pluvia, rain). A pluvial is a warmer period of increased rainfall; an interpluvial, of decreased rainfall. Formerly a pluvial was thought to correspond to a glacial in regions not iced, and in some cases it does. Rainfall is cyclical also. Pluvials and interpluvials are widespread.
There is no systematic correspondence of pluvials to glacials, however. Moreover, regional pluvials do not correspond to each other globally. For example, some have used the term "Riss pluvial" in Egyptian contexts. Any coincidence is an accident of regional factors. Names for some pluvials in some regions have been defined.
The sum of transient factors acting at the Earth's surface is cyclical: climate, ocean currents and other movements, wind currents, temperature, etc. The waveform response comes from the underlying cyclical motions of the planet, which eventually drag all the transients into harmony with them. The repeated glaciations of the Pleistocene were caused by the same factors.
Glaciation in the Pleistocene was a series of glacials and interglacials, stadials and interstadials, mirroring periodic changes in climate. The main factor at work in climate cycling is now believed to be Milankovitch cycles. These are periodic variations in regional solar radiation caused by the sum of many repeating changes in the Earth's motion.
Milankovitch cycles cannot be the sole factor since they do not explain the start and end of the Pleistocene ice age, or of repeated ice ages. They seem to work best within the Pleistocene, predicting a glaciation once every 100,000 years.
In oxygen isotope ratio analysis, variations in the ratio of O-18 to O-16 (two isotopes of oxygen) by mass (measured by a mass spectrometer) present in the calcite of oceanic core samples is used as a diagnostic of ancient ocean temperature change and therefore of climate change. Cold oceans are richer in O-18, which is included in the shells of the microorganisms contributing the calcite.
A more recent version of the sampling process makes use of modern glacial ice cores. Although less rich in O-18 than sea water, the snow that fell on the glacier year by year nevertheless contained O-18 and O-16 in a ratio that depended on the mean annual temperature.
Temperature and climate change are cyclical when plotted on a graph of temperature versus time. Temperature coordinates are given in the form of a deviation from today's annual mean temperature, taken as zero. This sort of graph is based on another of isotope ratio versus time. Ratios are converted to a percentage difference (d) from the ratio found in standard mean ocean water (SMOW).
The graph in either form appears as a waveform with overtones. One half of a period is a Marine isoLecture stage (MIS). It indicates a glacial (below zero) or an interglacial (above zero). Overtones are stadials or interstadials.
According to this evidence, Earth experienced 44 MIS stages beginning at about 2.4 MYA in the Pliocene. Pliocene stages were shallow and frequent. The latest were the most intense and most widely spaced.
By convention, stages are numbered from the Holocene, which is MIS1. Glacials receive an even number; interglacials, odd. The first major glacial was MIS2-4 at about 850,000 YA. The largest glacials were 2, 6 and 12; the warmest interglacials, 1, 5, 9 and 11. For matching of MIS numbers to named stages, see under the articles for those names.
Both marine and continental faunas were essentially modern.
The severe climatic changes during the ice age had major impacts on the fauna and flora. With each advance of the ice, large areas of the continents became totally depopulated, and plants and animals retreating southward in front of the advancing glacier faced tremendous stress. The most severe stress resulted from drastic climatic changes, reduced living space, and curtailed food supply. A major extinction event of large mammals (megafauna), which included mammoths, mastodons, saber-toothed cats, glyptodons, ground sloths, and short-faced bears, began late in the Pleistocene and continued into the Holocene. Neanderthals also became extinct during this period.
The extinctions were especially severe in North America where native horses and camels were eliminated.
Scientific evidence[9] indicates that humans evolved into their present form during the Pleistocene.[10] In the beginning of the Pleistocene Paranthropus -species are still present, as well as early human ancestors, but during the lower Palaeolithic they disappeared, and the only hominin species found in fossilic records is Homo erectus for much of the Pleistocene. This species migrated through much of the old world, giving rise to many variations of humans. The Middle and late Palaeolithic saw the appearance of new types of humans, as well as the development of more elaborate tools than found in previous eras. According to mitochondrial timing techniques, modern humans migrated from Africa after the Riss glaciation in the middle palaeolithic during the Eemian interglacial, spreading all over the ice-free world during the late Pleistocene.[11][12][13]
While the ultimate “African Origin” view of hominid evolution has not been challenged, some researchers have posited that the last great expansion did not eliminate pre-existing populations of hominids so much as assimilate them upon contact with Homo sapiens sapiens. While this would suggest that modifications in modern man may have been extensive and regionally based, the theory remains controversial. [14]
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