Editing Pleistocene (section)

==Paleogeography and climate==
[[File:Pleistocene north ice map.jpg|thumb|left|The maximum extent of [[ice age|glacial ice]] in the north polar area during the Pleistocene Period]]

The modern [[continent]]s were essentially at their present positions during the Pleistocene, the [[tectonic plate|plates]] upon which they sit probably having moved no more than {{convert|100|km|abbr=on}} relative to each other since the beginning of the period. In glacial periods, the sea level would drop by up to {{convert|120|m|abbr=on}} lower than today<ref>{{cite journal |last1=von der Heyden |first1=Sophie |title=Disentangling population structure in marine species |journal=Nature Reviews Genetics |date=17 Apr 2023 |volume=24 |issue=Sep 2023 |page=589 |doi=10.1038/s41576-023-00606-9 |pmid=37069255 |s2cid=258189561 |url=https://www.nature.com/articles/s41576-023-00606-9 |access-date=16 Aug 2023}}</ref> during peak glaciation, exposing large areas of the present [[continental shelf]] as dry land.

According to [[Mark Lynas]] (through collected data), the Pleistocene's overall climate could be characterised as a continuous [[El Niño]] with [[trade winds]] in the south [[Pacific Ocean|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.<ref>[[National Geographic Channel]], ''Six Degrees Could Change The World,'' Mark Lynas interview. Retrieved 14 February 2008.</ref>

===Glacial features===
{{More citations needed section|date=September 2018}}

Pleistocene climate was marked by repeated glacial cycles in which [[continental glacier]]s pushed to the 40th [[parallel (latitude)|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 {{convert|-6|°C|0}}; at the edge of the permafrost, {{convert|0|°C|0}}.

Each glacial advance tied up huge volumes of water in continental ice sheets {{convert|1500|to(-)|3000|m}} thick, resulting in temporary sea-level drops of {{convert|100|m|sigfig=1}} or more over the entire surface of the Earth. During interglacial times, such as at present, [[drowned coastline]]s 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 [[Patagonia]]n 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 [[northern Europe]], including much of Great Britain; the [[Alpine ice sheet]] on the [[Alps]]. Scattered domes stretched across [[Siberia]] and the Arctic shelf. The northern seas were ice-covered.

South of the ice sheets large lakes accumulated because outlets were blocked and the cooler air slowed evaporation. When the Laurentide Ice Sheet retreated, north-central North America was completely covered by [[Lake Agassiz]]. Over a hundred basins, now dry or nearly so, were overflowing in the North 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 river|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 decreases in oceanic and other evaporation.

It has been estimated that during the Pleistocene, the [[East Antarctic Ice Sheet]] thinned by at least 500 meters, and that thinning since the [[Last Glacial Maximum]] is less than 50 meters and probably started after c. 14 ka.<ref>{{cite journal|doi=10.1016/j.quascirev.2014.05.007|journal=Quaternary Science Reviews|first1=Yusuke |last1=Suganuma |first2=Hideki |last2=Miura |first3=Albert |last3=Zondervan |first4=Jun'ichi |last4=Okuno |date=August 2014|volume=97|title=East Antarctic deglaciation and the link to global cooling during the Quaternary: evidence from glacial geomorphology and 10Be surface exposure dating of the Sør Rondane Mountains, Dronning Maud Land|pages=102–120|bibcode=2014QSRv...97..102S|doi-access=free}}</ref>

===Major events===
{{Further|Timeline of glaciation}}
[[File:Co2 glacial cycles 800k.png|upright=1.15|thumb|Ice ages as reflected in [[Carbon dioxide in Earth's atmosphere|atmospheric CO<sub>2</sub>]], stored in the bubbles from glacial ice of [[Antarctica]]]]

During the 2.5&nbsp;million years of the Pleistocene, numerous cold phases called [[Glacial period|glacials]] ([[Quaternary glaciation|Quaternary ice age]]), or significant advances of continental ice sheets, in Europe and North America, occurred at intervals of approximately 40,000 to 100,000 years. The long glacial periods were separated by more temperate and shorter [[interglacial]]s which lasted about 10,000–15,000 years. The last cold episode of the [[last glacial period]] ended about 10,000 years ago.<ref>{{cite magazine|url=https://www.nationalgeographic.com/science/prehistoric-world/quaternary|archive-url=https://web.archive.org/web/20170320053318/http://www.nationalgeographic.com/science/prehistoric-world/quaternary/|url-status=dead|archive-date=20 March 2017|title=Quaternary Period|magazine=National Geographic|date=6 January 2017}}</ref> Over 11 major glacial events have been identified, as well as many minor glacial events.<ref name="RichmondOther1">{{cite journal | last1 = Richmond | first1 = G.M. | last2 = Fullerton | first2 = D.S. | year = 1986 | title = Summation of Quaternary glaciations in the United States of America | journal = Quaternary Science Reviews | volume = 5 | pages = 183–196 | doi=10.1016/0277-3791(86)90184-8| bibcode = 1986QSRv....5..183R }}</ref> 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. The headings "Glacial 1" to "Glacial 4" are designations indicating the four most recent glacials, with "Glacial 4" being the most recent. These names have been abandoned in favour of numeric data because many of the correlations were found to be either inexact or incorrect and more than four major glacials have been recognised since the historical terminology was established.<ref name="RichmondOther1"/><ref name="RoyOther20041">{{cite journal | last1 = Roy | first1 = M. | last2 = Clark | first2 = P.U. | last3 = Barendregt | first3 = R.W. | last4 = Glasmann | last5 = Enkin | first5 = R.J. | year = 2004 | title = Glacial stratigraphy and paleomagnetism of late Cenozoic deposits of the north-central United States | url = http://geo.oregonstate.edu/files/geo/Royetal-GSAB-2004.pdf | journal = Geological Society of America Bulletin | volume = 116 | issue = 1–2 | pages = 30–41 | doi = 10.1130/B25325.1 | access-date = 20 March 2010 | archive-url = https://web.archive.org/web/20180928051015/http://geo.oregonstate.edu/files/geo/Royetal-GSAB-2004.pdf | archive-date = 28 September 2018  | bibcode = 2004GSAB..116...30R }}</ref><ref>{{cite journal |last=Aber |first=J. S. |date=December 1991 |title=The Glaciation of Northeastern Kansas |journal=Boreas |volume=20 |issue=4 |pages=297–314 |doi=10.1111/j.1502-3885.1991.tb00282.x |bibcode=1991Borea..20..297A }} (contains a summary of how and why the Nebraskan, Aftonian, Kansan, and Yarmouthian stages were abandoned by modern stratigraphers).</ref>

{| class="wikitable"
|+ Historical names of the "four major" glacials in four regions.
! Region
! Glacial 1
! Glacial 2
! Glacial 3
! Glacial 4
|-
| '''Alps'''
| [[Gunz glaciation|Günz]]
| [[Mindel glaciation|Mindel]]
| [[Riss glaciation|Riss]]
| [[Würm glaciation|Würm]]
|-
| '''North Europe'''
| [[Eburonian]]
| [[Elsterian]]
| [[Saalian]]
| [[Weichselian]]
|-
| '''British Isles'''
| [[Beestonian stage|Beestonian]]
| [[Anglian Stage|Anglian]]
| [[Wolstonian Stage|Wolstonian]]
| [[Devensian]]
|-
| '''Midwest U.S.'''
| [[Pre-Illinoian|Nebraskan]]
| [[Kansan glaciation|Kansan]]
| [[Illinoian (stage)|Illinoian]]
| [[Wisconsinian Glaciation|Wisconsinan]]
|}

{| class="wikitable"
|+ Historical names of interglacials.
! Region
! Interglacial 1
! Interglacial 2
! Interglacial 3
|-
| '''Alps'''
| [[Cromerian Stage|Günz-Mindel]]
| [[Hoxnian Stage|Mindel-Riss]]
| [[Eemian Stage|Riss-Würm]]
|-
| '''North Europe'''
| Waalian
| Holsteinian
| [[Eemian Stage|Eemian]]
|-
| '''British Isles'''
| [[Cromerian Stage|Cromerian]]
| [[Hoxnian Stage|Hoxnian]]
| [[Eemian Stage|Ipswichian]]
|-
| '''Midwest U.S.'''
| [[Pre-Illinoian|Aftonian]]
| [[Yarmouthian Interglacial (Stage)|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 is 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 between 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 stratigraphically defined.

===Palaeocycles===
[[File:Earth 1.00Ma.png|thumb|right|Map of Earth as it appeared 1 million years ago during the Pleistocene epoch, Calabrian stage]]
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.

The [[Mid-Pleistocene Transition]], approximately one million years ago, saw a change from low-amplitude glacial cycles with a dominant periodicity of 41,000 years to asymmetric high-amplitude cycles dominated by a periodicity of 100,000 years.<ref name="Willeit">{{cite journal|title=Mid-Pleistocene transition in glacial cycles explained by declining CO2 and regolith removal &#124; Science Advances|journal=Science Advances|volume=5|issue=4|pages=eaav7337|doi=10.1126/sciadv.aav7337|last1=Willeit|first1=M.|last2=Ganopolski|first2=A.|last3=Calov|first3=R.|last4=Brovkin|first4=V.|year=2019|pmid=30949580|pmc=6447376}}</ref>

However, a 2020 study concluded that ice age terminations might have been influenced by [[Axial tilt|obliquity]] since the Mid-Pleistocene Transition, which caused stronger summers in the [[Northern Hemisphere]].<ref>{{cite news<!--|author=Petra Bajo |author2=Russell N. Drysdale |author3=Jon D. Woodhead |author4=John C. Hellstrom |author5=David Hodell |author6=Patrizia Ferretti |author7=Antje H. L. Voelker |author8=Giovanni Zanchetta |author9=Teresa Rodrigues |author10=Eric Wolff |author11=Jonathan Tyler |author12=Silvia Frisia |author13=Christoph Spötl |author14=Anthony E. Fallick--> |author=Petra Bajo|display-authors=etal|title=Persistent influence of obliquity on ice age terminations since the Middle Pleistocene transition|work=Science|year=2020|volume=367|issue=6483|pages=1235–1239|doi=10.1126/science.aaw1114}}</ref>

====Milankovitch cycles====
{{Main|Milankovitch cycles}}

Glaciation in the Pleistocene was a series of glacials and interglacials, stadials and interstadials, mirroring periodic climate changes. The main factor at work in climate cycling is now believed to be [[Milankovitch cycles]]. These are periodic variations in regional and planetary solar radiation reaching the Earth caused by several repeating changes in the Earth's motion. The effects of Milankovitch cycles were enhanced by various positive feedbacks related to increases in atmospheric carbon dioxide concentrations and Earth's [[albedo]].<ref>{{cite journal |last1=Lee |first1=Kyung Eun |last2=Clemens |first2=Steven C. |last3=Kubota |first3=Yoshimi |last4=Timmermann |first4=Axel |last5=Holbourn |first5=Ann |last6=Yeh |first6=Sang-Wook |last7=Bae |first7=Si Woong |last8=Ko |first8=Tae Wook |date=30 September 2021 |title=Roles of insolation forcing and CO2 forcing on Late Pleistocene seasonal sea surface temperatures |journal=[[Nature Communications]] |volume=12 |issue=1 |page=5742 |doi=10.1038/s41467-021-26051-y |pmc=8484283 |pmid=34593821 |bibcode=2021NatCo..12.5742L }}</ref>

Milankovitch cycles cannot be the sole factor responsible for the variations in climate since they explain neither the long-term cooling trend over the Plio-Pleistocene nor the millennial variations in the Greenland Ice Cores known as [[Dansgaard-Oeschger event]]s and [[Heinrich event]]s. Milankovitch pacing seems to best explain glaciation events with periodicity of 100,000, 40,000, and 20,000 years. Such a pattern seems to fit the information on climate change found in oxygen isotope cores.

====Oxygen isotope ratio cycles====
{{Main|Oxygen isotope ratio cycle}}

In [[Oxygen isotope ratio cycle|oxygen isotope ratio]] analysis, variations in the ratio of {{chem|18|O}} to {{chem|16|O}} (two [[isotopes]] of [[oxygen]]) by [[mass]] (measured by a [[mass spectrometer]]) present in the [[calcite]] of oceanic [[core sample]]s is used as a diagnostic of ancient ocean temperature change and therefore of climate change. Cold oceans are richer in {{chem|18|O}}, 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 {{chem|18|O}} than seawater, the snow that fell on the glacier year by year nevertheless contained {{chem|18|O}} and {{chem|16|O}} 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 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 [[megaannum|Ma]] [[Before Present|BP]] 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 and interglacials receive an odd number. The first major glacial was MIS2-4 at about 85–11 ka BP. The largest glacials were 2, 6, 12, and 16. The warmest interglacials were 1, 5, 9 and 11. For matching of MIS numbers to named stages, see under the articles for those names.