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The Pleistocene ( ) is the epoch from 2.588 million to 12 000 years BP covering the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek (pleistos "most") and (kainos "new").

The Pleistocene epoch follows the Pliocene epoch and is followed by the Holocene epoch. The Pleistocene is the first epoch of the Quaternary period or 6th epoch of the Cenozoic Era. The end of the Pleistocene corresponds with the retreat of the last continental glacier. It also corresponds with the end of the Paleolithic age used in archaeology.

In the ICS timescale, the Pleistocene is divided into four stages or ages, the Gelasian, Calabrian, Ionian and Tarantian. All of these stages were defined in southern Europe. In addition to this international subdivision, various regional subdivisions are often used.

Dating

The Pleistocene has been dated from 2.588 million (±5 000 years) to 11 550 years before present (BP), with the end date expressed in radiocarbon years as 10 000 carbon-14 years BP. 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 9640 BCE (11590 calendar years BP).

In 2009 the International Union of Geological Sciences (IUGS) confirmed a change in time period for the Pleistocene, changing the start date from 1.8 to 2.588 million years BP, and accepted the base of the Gelasian as the base of the Pleistocene, namely the base of the Monte San Nicola GSSP. The IUGS has yet to approve a type section, Global Boundary Stratotype Section and Point (GSSP), for the upper Pleistocene/Holocene boundary (i.e. the upper boundary). The proposed section is the North Greenland Ice Core Project ice core 75° 06' N 42° 18' W. The lower boundary of the Pleistocene Series is formally defined magnetostratigraphically as the base of the Matuyama (C2r) chronozone, isotopic stage 103. Above this point there are notable extinctions of the calcareous nanofossils: Discoaster pentaradiatus and Discoaster surculus.

The Pleistocene covers the recent period of repeated glaciations. The name Plio-Pleistocene has in the past been used to mean the last ice age. The revised definition of the Quaternary, by pushing back the start date of the Pleistocene to 2.58 Ma, results in the inclusion within the Pleistocene of all the recent repeated glaciations.

Paleogeography and climate

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 Pacificmarker weakening or heading east, warm air rising near Perumarker, warm water spreading from the west Pacific and the Indian Oceanmarker to the east Pacific, and other El Niño markers.

Glacial features

Pleistocene climate was marked 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. Antarcticamarker 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 Zealandmarker and Tasmaniamarker. The current decaying glaciers of Mount Kenyamarker, Mount Kilimanjaromarker, and the Ruwenzori Rangemarker in east and central Africa were larger. Glaciers existed in the mountains of Ethiopiamarker 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 Britainmarker; the Alpine ice sheet on the Alps. Scattered domes stretched across Siberiamarker 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 Agassizmarker. Over 100 basins, now dry or nearly so, were overflowing in the American west. Lake Bonnevillemarker, for example, stood where Great Salt Lakemarker 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.

Major events

Over 11 major glacial events have been identified, as well as many minor glacial events. A major glacial 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. 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.

The glacials in the following tables show historical usages, are a simplification of a much more complex cycle of variation in climate and terrain, and are generally no longer used. These names have been abandoned in favor of numeric data because many of the correlations were found to be either inexact or incorrect and more than four major glacials have been recognized since the historical terminology was established.

Historical names of the "four major" glacials in four regions.
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 U.S. Nebraskan Kansan Illinoian Wisconsinan


Historical names of interglacials.
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 U.S. 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. Only a few of the names for pluvials in restricted regions have been strategraphically 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 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.

Oxygen isotope ratio cycles

In oxygen isotope ratio analysis, variations in the ratio of O18 to O16 (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 O18, which is included in the tests of the microorganisms (foraminifera) contributing the calcite.

A more recent version of the sampling process makes use of modern glacial ice cores. Although less rich in O18 than sea water, the snow that fell on the glacier year by year nevertheless contained O18 and O16 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 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 102 MIS stages beginning at about 2.588 MYA in the Early Pleistocene Gelasian. Early Pleistocene 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 85,000-11,000 YA. The largest glacials were 2, 6, 12, and 16; the warmest interglacials, 1, 5, 9 and 11. For matching of MIS numbers to named stages, see under the articles for those names.

Fauna

Both marine and continental faunas were essentially modern.
Early Pleistocene Animals
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, Irish elk, cave bears, and short-faced bear, began late in the Pleistocene and continued into the Holocene. Neanderthals also became extinct during this period. At the end of the last ice age, Cold-blooded animals, smaller mammals like wood mice, migratory birds, and swifter animals like whitetail deer had replaced the megafauna and migrated north.

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



Humans during the Pleistocene

Scientific evidence indicates that humans evolved into their present form during the Pleistocene. 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 Stage, spreading all over the ice-free world during the late Pleistocene.

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. While this would suggest that modifications in modern man may have been extensive and regionally based, the theory remains controversial and has generally lost ground over the past century. due to the advent of genetic evidence that directly contradicts it in favor of a single-origin theory.

Deposits

Pleistocene non-marine sediments are found primarily in fluvial deposits, lakebeds, slope and loess deposits as well as in the large amounts of material moved about by glaciers. Less common are cavedeposits, travertines and volcanic deposits (lavas, ashes). Pleistocene marine deposits are found primarily in shallow marine basins mostly (but with important exceptions) in areas within a few tens of kilometres 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 metres.

See also



References

  1. In Gradstein, F. M., Ogg, James G., and Smith, A. Gilbert (eds.), A Geologic Time Scale 2004 Cambridge University Press, Cambridge, ISBN 0521781426
  2. For the top of the series, see: , (2004) “The Neogene Period”. In: Gradstein, F., Ogg, J., Smith, A.G. (Eds.), A Geologic Time Scale 2004. Cambridge: Cambridge University Press.
  3. Riccardi, Alberto C. (30 June 2009) "IUGS ratified ICS Recommendation on redefinition of Pleistocene and formal definition of base of Quaternary" International Union of Geological Sciences
  4. Svensson, A., S. W. Nielsen, S. Kipfstuhl, S. J. Johnsen, J. P. Steffensen, M. Bigler, U. Ruth, and R. Röthlisberger (2005) "Visual stratigraphy of the North Greenland Ice Core Project (NorthGRIP) ice core during the last glacial period" Journal of Geophysical Research 110: (D02108)
  5. Gradstein, Felix M.; Ogg, James G. and Smith, A. Gilbert (eds.) (2005) A Geologic Time Scale 2004 Cambridge University Press, Cambridge, UK, p. 28, ISBN 0-521-78142-6
  6. Rio, D.; Sprovieri, R.; Castradori, D. and Di Stefano, E. (1998) "The Gelasian Stage (Upper Pliocene): a new unit of the global standard chronostratigraphic scale" Episodes 21: pp. 82-87
  7. National Geographic Channel, Six Degrees Could Change The World, Mark Lynas interview. Retrieved February 14, 2008.
  8. Richmond, G.M. and D.S. Fullerton, 1986, Summation of Quaternary glaciations in the United States of America. Quaternary Science Reviews. vol. 5, pp. 183-196.
  9. Roy, M., P.U. Clark, R.W. Barendregt, J.R., Glasmann, and R.J. Enkin, 2004, Glacial stratigraphy and paleomagnetism of late Cenozoic deposits of the north-central United States, PDF version, 1.2 MB. Geological Society of America Bulletin. vol. 116, no. 1-2; pp. 30-41; DOI: 10.1130/B25325.1
  10. Aber, J.S. (1991) "Glaciations of Kansas" Boreas 20(4): pp. 297-314 - (contains a summary of how and why the Nebraskan, Aftonian, Kansan, and Yarmouthian stages were abandoned by modern stratigraphers.)
  11. Rogers, A.R. and Jorde, L.B. (1995) "Genetic evidence on modern human origins" Human Biology 67: pp. 1–36
  12. Wall, J.D. and Przeworski, M. (2000) "When did the human population start increasing?" Genetics 155: pp. 1865–1874
  13. Cann, R.L.; Stoneking, M. and Wilson, A.C.(1987) "Mitochondrial DNA and human evolution" Nature 325: pp. 31–36
  14. Stringer, C.B. (1992) "Evolution of early modern humans" In: Jones, Steve; Martin, R. and Pilbeam, David R. (eds.) (1992) The Cambridge encyclopedia of human evolution Cambridge University Press, Cambridge, ISBN 0-521-32370-3, pp. 241–251.
  15. Templeton, A. (2002) "Out of Africa again and again" Nature 416: p. 45
  16. Eswarana, Vinayak; Harpendingb, Henry and Rogers, Alan R. (2005) "Genomics refutes an exclusively African origin of humans" Journal of Human Evolution 49(1): pp. 1–18 Abstract
  • 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



Hominin species during pleistocene
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