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Pleistocene

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Not to be confused withPlasticine.

Pleistocene
2.58 – 0.0117Ma
↓
Map of the sea levels during the Last Glacial Maximum (glaciers not shown)
Chronology
Etymology
Name formalityFormal
Usage information
Celestial bodyEarth
Regional usageGlobal (ICS)
Definition
Chronological unitEpoch
Stratigraphic unitSeries
Time span formalityFormal
Lower boundary definition
Lower boundary GSSPMonte San Nicola Section,Gela,Sicily,Italy
37°08′49″N14°12′13″E/ 37.1469°N 14.2035°E/37.1469; 14.2035
Lower GSSP ratified2009 (as base of Quaternary and Pleistocene)[3]
Upper boundary definitionEnd of theYounger Dryasstadial
Upper boundary GSSPNGRIP2ice core,Greenland
75°06′00″N42°19′12″W/ 75.1000°N 42.3200°W/75.1000; -42.3200
Upper GSSP ratified2008 (as base of Holocene)[4]
Part ofa serieson
Human history
andprehistory
↑beforeHomo(Pliocene epoch)
Prehistory
Stone Age
Lower Paleolithic
Middle Paleolithic
EarlyHomo sapiens
Upper Paleolithic
Behavioral modernity
Neolithic
Cradle of civilization
Protohistory
Copper Age
Bronze Age
Bronze Age collapse
Iron Age
Recorded history
Ancient history
Post-classical history
Modern history
↓Future(Holocene epoch)

ThePleistocene(/ˈplaɪstəˌsiːn,-stoʊ-/PLY-stə-seen, -⁠stoh-;[5][6]often referred to colloquially as theIce Age) is the geologicalepochthat lasted fromc. 2.58 millionto 11,700 years ago, spanning the Earth's most recent period of repeatedglaciations.Before a change was finally confirmed in 2009 by theInternational Union of Geological Sciences,the cutoff of the Pleistocene and the precedingPliocenewas regarded as being 1.806 million yearsBefore Present(BP). Publications from earlier years may use either definition of the period. The end of the Pleistocene corresponds with the end of thelast glacial periodand also with the end of thePaleolithicage used inarchaeology.The name is a combination ofAncient Greekπλεῖστος(pleîstos) 'most' andκαινός(kainós;Latinizedascænus) 'new'.

At the end of the preceding Pliocene, the previously isolated North and South American continents were joined by theIsthmus of Panama,causinga faunal interchangebetween the two regions and changingocean circulationpatterns, with the onset of glaciation in the Northern Hemisphere occurring around 2.7 million years ago. During the Early Pleistocene (2.58–0.8 Ma),archaic humansof the genusHomooriginated in Africa and spread throughoutAfro-Eurasia.The end of the Early Pleistocene is marked by theMid-Pleistocene Transition,with the cyclicity of glacial cycles changing from 41,000-year cycles to asymmetric 100,000-year cycles, making the climate variation more extreme. TheLate Pleistocenewitnessed the spread ofmodern humansoutside of Africa as well as the extinction of all other human species. Humans alsospread to the Australian continentandthe Americasfor the first time, co-incident withthe extinction of most large-bodied animalsin these regions.

The aridification and cooling trends of the precedingNeogenewere continued in the Pleistocene. The climate was strongly variable depending on the glacial cycle, with the sea levels being up to 120 metres (390 ft) lower than present at peak glaciation, allowing the connection of Asia and North America viaBeringiaand the covering of most of northern North America by theLaurentide Ice Sheet.

Etymology

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Evolution of temperature in the Post-Glacial period at the very end of the Pleistocene, according to Greenland ice cores[7]
Temperature rise marking the end of the Pleistocene, as derived from Antarctic ice core data.

Charles Lyellintroduced the term "Pleistocene" in 1839 to describe strata inSicilythat had at least 70% of their molluscan fauna still living today. This distinguished it from the olderPliocene Epoch,which Lyell had originally thought to be the youngest fossil rock layer. He constructed the namePleistocene('most new' or 'newest') from theGreekπλεῖστος (pleīstos) 'most' and καινός (kainós(Latinizedascænus) 'new').[8][9][10]This contrasts with the immediately precedingPliocene( "newer", from πλείων (pleíōn,"more" ) andkainós) and the immediately subsequentHolocene( "wholly new" or "entirely new", from ὅλος (hólos,"whole" ) andkainós)epoch,which extends to the present time.

Dating

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The Pleistocene has been dated from 2.580 million (±0.005) to 11,650 years BP[11]with the end date expressed inradiocarbon yearsas 10,000carbon-14years BP.[12]It covers most of the latest period of repeatedglaciation,up to and including theYounger Dryascold spell. The end of the Younger Dryas has been dated to about 9640 BCE (11,654 calendar years BP). The end of the Younger Dryas is the official start of the currentHolocene Epoch.Although it is considered an epoch, the Holocene is not significantly different from previous interglacial intervals within the Pleistocene.[13]In theICStimescale, the Pleistocene is divided into fourstagesorages,theGelasian,Calabrian,Chibanian(previously the unofficial "Middle Pleistocene" ), andUpper Pleistocene(unofficially the "Tarantian" ).[14][15][note 1]In addition to these international subdivisions, various regional subdivisions are often used.

In 2009 theInternational Union of Geological Sciences(IUGS) confirmed a change in time period for the Pleistocene, changing the start date from 1.806 to 2.588 million years BP, and accepted the base of theGelasianas the base of the Pleistocene, namely the base of the Monte San NicolaGSSP.[17]The start date has now been rounded down to 2.580 million years BP.[11]The IUGS has yet to approve atype section,Global Boundary Stratotype Section and Point(GSSP), for the upper Pleistocene/Holocene boundary (i.e.the upper boundary). The proposed section is theNorth Greenland Ice Core Projectice core 75° 06' N 42° 18' W.[18]The lower boundary of the Pleistocene Series is formally definedmagnetostratigraphicallyas the base of the Matuyama (C2r)chronozone,isotopic stage 103. Above this point there are notable extinctions of thecalcareous nannofossils:Discoaster pentaradiatusandDiscoaster surculus.[19][20]The Pleistocene covers the recent period of repeated glaciations.

The namePlio-Pleistocenehas, in the past, been used to mean the last ice age. Formerly, the boundary between the two epochs was drawn at the time when the foraminiferal speciesHyalinea balticafirst appeared in the marine section at La Castella, Calabria, Italy.[21]However, the revised definition of theQuaternary,by pushing back the start date of the Pleistocene to 2.58 Ma, results in the inclusion of all the recent repeated glaciations within the Pleistocene.

Radiocarbon dating is considered to be inaccurate beyond around 50,000 years ago.Marine isotope stages(MIS) derived from Oxygen isotopes are often used for giving approximate dates.

Deposits

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Pleistocene non-marinesedimentsare found primarily in fluvialdeposits,lakebeds, slope andloessdeposits as well as in the large amounts of material moved about by glaciers. Less common arecavedeposits,travertinesand 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 theSouthern Californiacoast, Pleistocene marine deposits may be found at elevations of several hundred metres.

Further information on the New York-New Jersey border Pleistocene geologic unit:Gardener's Clay Formation

Paleogeography and climate

[edit]
The maximum extent ofglacial icein the north polar area during the Pleistocene Period

The moderncontinentswere essentially at their present positions during the Pleistocene, theplatesupon which they sit probably having moved no more than 100 km (62 mi) relative to each other since the beginning of the period. In glacial periods, the sea level would drop by up to 120 m (390 ft) lower than today[22]during peak glaciation, exposing large areas of the presentcontinental shelfas dry land.

According toMark Lynas(through collected data), the Pleistocene's overall climate could be characterised as a continuousEl Niñowithtrade windsin the southPacificweakening or heading east, warm air rising nearPeru,warm water spreading from the west Pacific and theIndian Oceanto the east Pacific, and other El Niño markers.[23]

Glacial features

[edit]

Pleistocene climate was marked by repeated glacial cycles in whichcontinental glacierspushed to the 40thparallelin some places. It is estimated that, at maximum glacial extent, 30% of the Earth's surface was covered by ice. In addition, a zone ofpermafroststretched southward from the edge of the glacial sheet, a few hundred kilometres inNorth America,and several hundred inEurasia.The mean annual temperature at the edge of the ice was −6 °C (21 °F); at the edge of the permafrost, 0 °C (32 °F).

Each glacial advance tied up huge volumes of water in continental ice sheets 1,500 to 3,000 metres (4,900–9,800 ft) thick, resulting in temporary sea-level drops of 100 metres (300 ft) or more over the entire surface of the Earth. During interglacial times, such as at present,drowned coastlineswere common, mitigated by isostatic or other emergent motion of some regions.

The effects of glaciation were global.Antarcticawas ice-bound throughout the Pleistocene as well as the preceding Pliocene. TheAndeswere covered in the south by thePatagonianice cap. There were glaciers inNew ZealandandTasmania.The current decaying glaciers ofMount Kenya,Mount Kilimanjaro,and theRuwenzori Rangein east and central Africa were larger. Glaciers existed in the mountains ofEthiopiaand to the west in theAtlas Mountains.

In the northern hemisphere, many glaciers fused into one. TheCordilleran Ice Sheetcovered the North American northwest; the east was covered by theLaurentide.TheFenno-Scandian ice sheetrested onnorthern Europe,including much of Great Britain; theAlpine ice sheeton theAlps.Scattered domes stretched acrossSiberiaand 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 byLake Agassiz.Over a hundred basins, now dry or nearly so, were overflowing in the North American west.Lake Bonneville,for example, stood whereGreat Salt Lakenow does. In Eurasia, large lakes developed as a result of the runoff from the glaciers. Rivers were larger, had a more copious flow, and werebraided.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, theEast Antarctic Ice Sheetthinned by at least 500 meters, and that thinning since theLast Glacial Maximumis less than 50 meters and probably started after ca 14 ka.[24]

Major events

[edit]
Further information:Timeline of glaciation
Ice ages as reflected inatmospheric CO2,stored in the bubbles from glacial ice ofAntarctica

During the 2.5 million years of the Pleistocene, numerous cold phases calledglacials(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 shorterinterglacialswhich lasted about 10,000–15,000 years. The last cold episode of thelast glacial periodended about 10,000 years ago.[25]Over 11 major glacial events have been identified, as well as many minor glacial events.[26]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 showhistoricalusages, 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 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.[26][27][28]

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 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

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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.

TheMid-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.[29]

However, a 2020 study concluded that ice age terminations might have been influenced byobliquitysince the Mid-Pleistocene Transition, which caused stronger summers in theNorthern Hemisphere.[30]

Milankovitch cycles

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Main article: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 beMilankovitch 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.[31]

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 events and Heinrich events. 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

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Main article:Oxygen isotope ratio cycle

Inoxygen isotope ratioanalysis, variations in the ratio of18
Oto16
O(twoisotopesofoxygen) bymass(measured by amass spectrometer) present in thecalciteof oceaniccore samplesis used as a diagnostic of ancient ocean temperature change and therefore of climate change. Cold oceans are richer in18
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 in18
Othan seawater, the snow that fell on the glacier year by year nevertheless contained18
Oand16
Oin 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 from the ratio found in standard mean ocean water (SMOW).

The graph in either form appears as awaveformwithovertones.One half of a period is aMarine 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 2.588MaBPin the Early PleistoceneGelasian.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.

Fauna

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See also:Quaternary extinction event

Both marine and continental faunas were essentially modern but with many more large land mammals such asMammoths,Mastodons,Diprotodons,Smilodons,tigers,lions,Aurochs,short-facedbears,giant sloths,species withinGigantopithecusand others. Isolated landmasses such asAustralia,Madagascar,New Zealandand islands in the Pacific saw the evolution of large birds and even reptiles such as theElephant bird,moa,Haast's eagle,Quinkana,MegalaniaandMeiolania.

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 depopulated, and plants and animals retreating southwards 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 majorextinction eventof largemammals(megafauna), which includedmammoths,mastodons,saber-toothed cats,glyptodons,thewoolly rhinoceros,variousgiraffids,such as theSivatherium;ground sloths,Irish elk,cave lions,cave bears,Gomphotheres,American lions,dire wolves,andshort-faced bears,began late in the Pleistocene and continued into the Holocene.Neanderthalsalso became extinct during this period. At the end of the last ice age,cold-bloodedanimals, smaller mammals likewood mice,migratory birds, and swifter animals likewhitetail deerhad replaced the megafauna and migrated north. Late Pleistocenebighorn sheepwere more slender and had longer legs than their descendants today. Scientists believe that the change in predator fauna after the late Pleistocene extinctions resulted in a change of body shape as the species adapted for increased power rather than speed.[32]

The extinctions hardly affected Africa but were especially severe inNorth Americawhere nativehorsesandcamelswere wiped out.

Various schemes for subdividing the Pleistocene

In July 2018, a team ofRussianscientists in collaboration withPrinceton Universityannounced that they had brought two femalenematodesfrozen inpermafrost,from around 42,000 years ago, back to life. The two nematodes, at the time, were the oldest confirmed living animals on the planet.[33][34]

Humans

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Main articles:Human evolution,Paleolithic,andSettlement of the Americas

Theevolutionofanatomically modern humanstook place during the Pleistocene.[35][36]At the beginning of the PleistoceneParanthropusspecies were still present, as well as early human ancestors, but during the lower Palaeolithic they disappeared, and the onlyhomininspecies found in fossilic records isHomo erectusfor much of the Pleistocene.Acheuleanlithicsappear along withHomo erectus,some 1.8 million years ago, replacing the more primitiveOldowanindustry used byA. garhiand by the earliest species ofHomo.TheMiddle Paleolithicsaw more varied speciation withinHomo,including the appearance ofHomo sapiensabout 300,000 years ago.[37]

According to mitochondrial timing techniques,modern humansmigrated from Africa after theRiss glaciationin the Middle Palaeolithic during theEemian Stage,spreading all over the ice-free world during the late Pleistocene.[38][39][40]A 2005 study posits that humans in this migration interbred witharchaic humanforms already outside of Africa by the late Pleistocene, incorporating archaic human genetic material into the modern human gene pool.[41]


Homininspecies during Pleistocene
Homo (genus)AustralopithecusAustralopithecus sedibaAustralopithecus africanusHomo floresiensisHomo neanderthalensisHomo sapiensHomo heidelbergensisHomo erectusHomo nalediHomo habilisHolocenePleistocenePliocene


See also

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Explanatory notes

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  1. ^The Upper Pleistocene is a subseries/subepoch rather than a stage/age but, in 2009, theIUGSdecided that it will be replaced with a stage/age (currently unofficially/informally named theTarantian).[16]

References

[edit]
  1. ^Cohen, K. M.; Finney, S. C.; Gibbard, P. L.; Fan, J.-X. (January 2020)."International Chronostratigraphic Chart"(PDF).International Commission on Stratigraphy.Retrieved23 February2020.
  2. ^Mike Walker; et al. (December 2018)."Formal ratification of the subdivision of the Holocene Series/Epoch (Quaternary System/Period)"(PDF).Episodes.41(4). Subcommission on Quaternary Stratigraphy (SQS): 213–223.doi:10.18814/epiiugs/2018/018016.Retrieved11 November2019.
  3. ^Gibbard, Philip; Head, Martin (September 2010)."The newly-ratified definition of the Quaternary System/Period and redefinition of the Pleistocene Series/Epoch, and comparison of proposals advanced prior to formal ratification"(PDF).Episodes.33(3): 152–158.doi:10.18814/epiiugs/2010/v33i3/002.Retrieved8 December2020.
  4. ^Walker, Mike; Johnse, Sigfus; Rasmussen, Sune; Steffensen, Jørgen-Peder; Popp, Trevor; Gibbard, Phillip; et al. (June 2008)."The Global Stratotype Section and Point (GSSP) for the base of the Holocene Series/Epoch (Quaternary System/Period) in the NGRIP ice core".Episodes.31(2): 264–267.doi:10.18814/epiiugs/2008/v31i2/016.hdl:10289/920.
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  6. ^"Pleistocene".Dictionary Unabridged(Online). n.d.
  7. ^Zalloua, Pierre A.; Matisoo-Smith, Elizabeth (6 January 2017)."Mapping Post-Glacial expansions: The Peopling of Southwest Asia".Scientific Reports.7:40338.Bibcode:2017NatSR...740338P.doi:10.1038/srep40338.ISSN2045-2322.PMC5216412.PMID28059138.
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