Pleistocene


Pleistocene

The Pleistocene (IPAEng|'plaɪstəsi:n) is the epoch from 1.8 million to 10,000 years BP covering the world's recent period of repeated glaciations. The name "pleistocene" is derived from the Greek Polytonic|πλεῖστος ("pleistos" "most") and Polytonic|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.PDFlink| [http://www-qpg.geog.cam.ac.uk/people/gibbard/GTS2004Quat.pdf Gibbard, P. and van Kolfschoten, T. (2004) "The Pleistocene and Holocene Epochs" Chapter 22] |2.96 MiB "In" Gradstein, F. M., Ogg, James G., and Smith, A. Gilbert (eds.), "A Geologic Time Scale 2004" Cambridge University Press, Cambridge, ISBN 0521781426] 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.

Dating

The Pleistocene has been dated from 1.806 million (±5,000 years) to 11,550 years before present [Lourens, L., Hilgen, F., Shackleton, N.J., Laskar, J., Wilson, D., (2004) “The Neogene Period”. In: Gradstein, F., Ogg, J., Smith, A.G. (Eds.), "A Geologic Time Scale 2004". Cambridge: Cambridge University Press.] , 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 9600 BC (11550 calendar years BP).

The International Commission on Stratigraphy (a body of the International Union of Geological Sciences) has confirmed the time period for the Pleistocene but has not yet confirmed a type section, Global Boundary Stratotype Section and Point (GSSP), for the Pleistocene/Holocene boundary. The proposed section is the "North Greenland Ice Core Project" ice core 75° 06' N 42° 18' W. [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)]

The type section 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 Gelasian (end of the Pliocene). Some climatologists and geologists would therefore prefer a start date of around 2.58 million years BP.PDFlink| [http://www.inqua.tcd.ie/documents/QP%2016-1.pdf Clague, John "et al." (2006) "Open Letter by INQUA Executive Committee" "Quaternary Perspective, the INQUA Newsletter" International Union for Quaternary Research 16(1):] |1.30 MiB ] The name Plio-Pleistocene has in the past been used to mean the last ice age. But since only a part of the Pliocene is involved, the Quaternary was subsequently redefined to start 2.58 Ma. as more consistent with the data.PDFlink| [http://www.inqua.tcd.ie/documents/sdarticle.pdf Pillans, Brad (2004) "Update on Defining the Quaternary" "Quaternary Perspective, the INQUA Newsletter" International Union for Quaternary Research 14(2):] |869 KiB ]

The continuous climatic history from the Pliocene into the Pleistocene and Holocene was one reason for the International Commission on Stratigraphy to propose discontinuance of the use of the term "Quaternary", this proposal was strongly objected to by the International Union for Quaternary Research (INQUA). The ICS proposed that the "Quaternary" be considered a sub-era (sub-erathem) with its base at the base of the Pilocene Gelasian Stage GSSP at circa 2.6 Ma at Marine Isotope State 103. The boundary is not in dispute, but the sub-era status was rejected by INQUA. The matter remains under discussion with resolution expected to be reached by the ICS and INQUA in 2008.PDFlink| [http://www.inqua.tcd.ie/documents/sdarticle.pdf Clague, John J. "INQUA, IUGS, and the 32nd International Geological Congress" "Quaternary Perspective, the INQUA Newsletter" International Union for Quaternary Research 14(2):] |869 KiB ] Therefore, the Pleistocene is currently an epoch of both the longer Neogene and the shorter Quaternary.

The proposal of INQUA is to extend the beginning of the Pleistocene to the beginning of the Gelasian Stage, shortening the Pliocene, and ending the Neogene with the revised end of the Pliocene.

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 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. [National Geographic Channel, "Six Degrees Could Change The World," Mark Lynas interview. Retrieved February 14, 2008.]

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 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.

Major events

[
2, stored in bubbles from glacial ice of Antarctica] 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.

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.

North American Land Mammal Ages (NALMA) are Blancan (4.5–1.2), Irvingtonian (1.2–0.5) and Rancholabrean (0.5–0.01) in millions of years. The Blancan extends significantly back into the Pliocene.

South American Land Mammal Ages (SALMA) are Uquian (2.5–1.5), Ensenadan (1.5–0.3) and Lujanian (0.3–0.01) in millions of years. The Uquian extends significantly back into the Pliocene.

In Europe, the faunal stages are Calabrian (1.806–0.781), Sicilian (0.781–0.26) and Tyrrhenian (0.26–0.005). [ [http://www.stratigraphy.org/geowhen/region_all.html GeoWhen Database — Comparision of Regional Geologic Nomenclature] ;]

Humans during pleistocene

Scientific evidence [Rogers, A.R. and Jorde, L.B. (1995) "Genetic evidence on modern human origins" "Human Biology" 67: pp. 1–36] indicates that humans evolved into their present form during the Pleistocene. [Wall, J.D. and Przeworski, M. (2000) "When did the human population start increasing?" "Genetics" 155: pp. 1865–1874] 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. [Cann, R.L.; Stoneking, M. and Wilson, A.C.(1987) "Mitochondrial DNA and human evolution" "Nature" 325: pp. 31–36] [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.] [Templeton, A. (2002) "Out of Africa again and again" "Nature" 416: p. 45]

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 and has generally lost ground over the past century. [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 [http://dx.doi.org/10.1016/j.jhevol.2005.02.006 Abstract] ]

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 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 meters.

ee also

* Abbassia Pluvial
* Geologic time scale
* Glacial history of Minnesota
* Ice age
* List of fossil sites "(with link directory)"
* Mousterian Pluvial
* Paleontological Museum in Tocuila
* Pleistocene Park
* 1 E13 s

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

* [http://www.stratigraphy.org/vrica.htm Pliocene-Pleistocene boundary at Vrica, Italy (Map)]


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