Pleistocene

The Pleistocene epoch () is part of the geologic timescale. The name of the pleistocene is derived from the Greek pleistos (most) and ceno (new). The Pleistocene 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. It lasted from 1.8 million to 12,000 years before the present.

The end of the Pleistocene corresponds with the end of the Paleolithic age used in archaeology.

Pleistocene dating

The Pleistocene has been dated in 2005 by the International Commission on Stratigraphy (a body of the International Union of Geological Sciences) from 1.81 million to 11,550 years before present, with the end date expressed in radiocarbon years. It covers most of the latest period of repeated glaciation, up to and including the Younger Dryas cold spell. The end of the Younger Dryas has been dated to about 9600 BC (11550 calendar years BP).

The GSSP for the start of the Pleistocene is in a reference section at Vrica, 4 km south of Crotone in Calabria, Southern Italy, a location whose exact dating has recently been confirmed by analysis of strontium and oxygen isotopes as well as by planktonic foraminifera.

The name was intended to cover the recent period of repeated glaciations; however, the start was set too late and some early cooling and glaciation are now reckoned to be in the end Pliocene. Some climatologists would therefore prefer a start date of around 2.5 million years BP. The name Plio-Pleistocene is in use to mean the last ice age.

The continuous climatic history from the Pliocene into the Pleistocene and Holocene was one reason for the International Commission on Stratigraphy to discourage the use of the term "Quaternary". Therefore, the Pleistocene is an epoch of the Neogene in current usage. (NOTE: The yellow table at the top of this page contains grossly outdated terminology!)

Pleistocene paleogeography and climate

The modern continents were essentially at their present positions during the Pleistocene, probably having moved no more than 100 km since.

Glacial features

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 kilometers 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 we are experiencing now, 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 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 due to blockage of outlets and decreased evaporation in the cooler air. 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. Due to the decrease in oceanic and other evaporation, rainfall was lower.

Major events

Four major glacial events have been identified, as well as many minor intervening events. A major event is a general glacial excursion, termed just 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. You would not refer to the Mindel as the Elsterian or vice versa.

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.

Four of the better known regions with the names of the glacials are listed in the table below. Fuller information including the dates is stated in the linked articles, which combine the same glaciation of different regions. A synthesis of the larger picture is shown under Timeline of glaciation.

It should be emphasized that these glacials 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 we know they existed from the study of cyclical climate changes.

Four of the better known regions with the names of the glacials.
Region	Glacial 1	Glacial 2	Glacial 3	Glacial 4
Alps	Günz	Mindel	Riss	Würm
North Europe	Eburonian	Elsterian	Saalian	Weichselian
British Isles	Beestonian	Anglian	Wolstonian	Devensian
Midwest of US	Nebraskan	Kansan	Illinoian	Wisconsin

The interglacials corresponding to prior glacials.
Region	Interglacial 1	Interglacial 2	Interglacial 3
Alps	Günz-Mindel	Mindel-Riss	Riss-Würm
North Europe	Waalian	Holsteinian	Eemian
British Isles	Cromerian	Hoxnian	Ipswichian
Midwest of US	Aftonian	Yarmouthian	Sangamonian

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.

Palaeocycles

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.

Milankovitch Cycles

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 a number of repeating changes in the Earth's motion.

Milankovitch cycles cannot be the sole factor, as they do not explain the start and end of the Pleistocene ice age, or repeated ice ages. They seem to work best within the Pleistocene, predicting a glaciation once every 100,000 years.

Oxygen Isotope Ratio Cycles

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 (δ) 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 isotopic 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 MIS22 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.

Pleistocene fauna

There are no faunal stages defined for the Pleistocene or Holocene. Both marine and continental faunas were essentially modern. It is believed by most scientists that humans evolved into modern man during the Pleistocene.

A major extinction event of large mammals (megafauna), which included mammoths, mastodons, saber-toothed cats, glyptodons and ground sloths, began late in the Pleistocene and continued into the Holocene.

The extinctions were especially severe in North America where native horses and camels were eliminated.

Pleistocene deposits

Pleistocene continental deposits are found primarily in lakebeds, loess deposits and caves as well as in the large amounts of material moved about by glaciers. Pleistocene marine deposits are found primarily in areas within a few tens of kilometers of the modern shoreline. In a few geologically active areas such as the Southern California coast, Pleistocene marine deposits may be found at elevations of several hundred meters.

References

Ogg, Jim; June, 2004, Overview of Global Boundary Stratotype Sections and Points (GSSP's) http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.

External links

Pliocene-Pleistocene boundary at Vrica, Italy (Map)