The 'Pleistocene'
epoch () on the
geologic timescale is the period from 1,808,000 to 11,550 years
BP. The Pleistocene epoch had been intended to cover 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 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.
Dating
The Pleistocene has been dated from 1.806 million (±5,000 years) to 11,550 years before present
[2], 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 75º 06' N 42º 18' W.
[3]
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.
[4] 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.
[5]
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.
[6] 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 maximum extent of
glacial ice in the north polar area during Pleistocene time.
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.
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
Ice ages as reflected in atmospheric CO
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.
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 U.S.' | 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 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. 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 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 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 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 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.
Fauna
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.
Pleistocene of South America showing ''Megatherium'' and two ''Glyptodon''
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).
[7]
Hominini during 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 lower palaeolithic they disappear, 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 human. Middle and late palaeolithic saw the appearance of new types of human, as well as the development of more elaborate tools than previously present.¨According to mitochondrial timing techniques, the
modern human species 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.
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.
See also
★
Abbassia Pluvial
★
Geologic time scale
★
Glacial history of Minnesota
★
Ice age
★
List of fossil sites ''(with link directory)''
★
Mousterian Pluvial
★
Pleistocene Park
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)
'Hominin species during pleistocene'[ edit ]| ImageSize = width:700 height:400PlotArea = left:20 right:70 bottom:20 top:0AlignBars = justifyColors = id:period1 value:rgb(1,1,0.7) # light yellow id:period2 value:rgb(0.7,0.7,1) # light blue id:period3 value:rgb(0.7,1,0.7) # light green id:events value:rgb(1,0.3,1) # light purple id:Miocene value:rgb(1,0.8706,0) # 255/222/0 id:Pliocene value:rgb(0.9961,0.9216,0.6745) # 254/235/172 id:Pleistocene value:rgb(1,0.9216,0.3843) # 255/235/98 id:Holocene value:rgb(1,1,0.7020) # 255/255/179Period = from:0 till:1900000TimeAxis = orientation:horizontalScaleMajor = unit:year increment:1000000 start:0ScaleMinor = unit:year increment:250000 start:0BarData = bar:Timelines bar:buffer bar:Events bar:bar0 bar:bar1 bar:bar2 bar:bar3 bar:bar4 bar:bar5 bar:bar6 bar:bar7 bar:bar8 bar:bar9 bar:bar10 bar:bar11 bar:bar12 bar:bar13 bar:bar14 bar:bar15 bar:bar16 bar:bar17 bar:bar18 bar:bar19 bar:bar20 bar:bar21 bar:bar22 bar:bar23 bar:bar24 bar:bar25 bar:bar26 bar:bar27 bar:bar28 bar:bar29 bar:bar30 bar:bar31 bar:bar32 bar:bar33PlotData= width:25 mark:(line,red) textcolor:black bar:Timelines align:right shift:(-75,0) bar:Timelines align:center shift:none from:1806000 till:end color:Pliocene text:Pliocene from:11500 till:1806000 color:Pleistocene text:Pleistocene# from:0 till:11500 color:Holocene align:left text:Holocene bar:Events color:events align:right shift:(-5,-10) width:7 mark:none color:events align:right shift:(-5,-4) bar:bar2 bar:bar4 align:left shift:(5,-4) bar:bar9 bar:bar11 bar:bar13 bar:bar15 bar:bar17 bar:bar19 bar:bar21 bar:bar23 from:1400000 till:end at:1400000 text:H. habilis bar:bar25 from:1800000 till:1800000 at:1800000 text:H. georgicus bar:bar25 from:13000 till:94000 at:94000 text:H. floresiensis bar:bar27 from:300000 till:1800000 at:1800000 text:H. erectus bar:bar29 from:250000 till:600000 at:600000 text:H. heidelbergensis bar:bar31 from:29000 till:230000 at:230000 text:H. neanderthalensis bar:bar33 from:0 till:200000 at:200000 text:H. sapiens bar:bar10 bar:bar8 from:1400000 till:end at:1400000 text:P. boisei bar:bar6 from:1200000 till:end at:1400000 text:P. robustus mark:none bar:bar4 at:1200000 text:Genus Paranthropus bar:bar8 from:1165000 till:end width:47 color:period1 bar:bar8 from:1235000 till:end width:37 color:white bar:bar7 bar:bar10 bar:bar10 bar:bar21 bar:bar17 bar:bar17 bar:bar33 at:1000000 text:Genus Homo bar:bar28 from:0 till:end width:93 color:period3 bar:bar28 from:50000 till:end width:83 color:whiteTextData = pos:(29,14) text:"Years" |
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