Thearidification and cooling trends of the precedingNeogene were continued in the Pleistocene. The climate was strongly variable depending on the glacial cycle, oscillating between coldglacial periods and warmerinterglacials, with the sea levels being up to 120 metres (390 ft) lower than present at peak glaciation, allowing the periodic connection of Asia and North America via theBeringia land bridge, and the covering of most of northern North America by the largeLaurentide Ice Sheet.
At the end of the preceding Pliocene, the previously isolated North andSouth American continents were joined by theIsthmus of Panama, causinga faunal interchange between the two regions and changingocean circulation patterns, 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 humans of the genusHomo originated 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 Pleistocene witnessed the spread ofmodern humans outside Africa as well as the extinction of all other human species. Humans alsospread to the Australian continent andthe Americas for the first time.
The Late Pleistocene from around 50,000 years ago onwards was marked bythe extinction of most large terrestrial animals outside of Africa, an extinction event otherwise unprecedented in the geological record due to its extreme size bias. Most authors suggest that climate change, the expansion of modern humans, or the combination of both were the likely causes of the extinctions.
Evolution of temperature in the Post-Glacial period at the very end of the Pleistocene, according to Greenland ice cores[6]Temperature rise marking the end of the Pleistocene, as derived from Antarctic ice core data.
Charles Lyell introduced the term "Pleistocene" in 1839 to describe strata inSicily that 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 name "Pleistocene" ('most new' or 'newest') from theGreekπλεῖστος (pleīstos) 'most' andκαινός (kainós (Latinized ascænus) 'new').[7][8][9] This contrasts with the immediately precedingPliocene ("newer", fromπλείων (pleíōn, "more") andkainós) and the immediately subsequentHolocene ("wholly new" or "entirely new", fromὅλος (hólos, "whole") andkainós)epoch, which extends to the present time.
The Pleistocene has been dated from 2.580 million (±0.005) to 11,700 years BP[10] with the end date expressed inradiocarbon years as 10,000carbon-14 years BP.[11] It covers most of the latest period of repeatedglaciation, up to and including theYounger Dryas cold spell. The end of the Younger Dryas has been dated to about 9700 BCE (11,700 years before present). The end of the Younger Dryas is the official start of the currentHolocene Epoch. Although it is considered an epoch, the Holocene was not significantly different in terms of temperature change than previous Pleistocene interglacial intervals, although the rapid onset of theAnthropocene is unprecedented.[12] In theICS timescale, the Pleistocene is divided into fourstages orages, theGelasian,Calabrian,Chibanian (previously the unofficial "Middle Pleistocene"), andUpper Pleistocene (unofficially the "Tarantian").[13][14][note 1] In addition to these international subdivisions, various regional subdivisions are often used.
The namePlio-Pleistocene has, 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 baltica first appeared in the marine section at La Castella, Calabria, Italy.[20] 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.
Whileradiocarbon dating is well-suited for dates in the Holocene, its half life is too short for practical use in Pleistocene dating. Instead geologists usemarine isotope stages derived from oxygen isotopes to date materials deposited in the Pleistocene.
Pleistocene non-marinesediments are found primarily in fluvialdeposits, lakebeds, slope andloess deposits as well as in the large amounts of material moved about by glaciers. Less common arecave deposits,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 theSouthern California coast, Pleistocene marine deposits may be found at elevations of several hundred metres.
The maximum extent ofglacial ice in the north polar area during the Pleistocene Period
The moderncontinents were essentially at their present positions during the Pleistocene, theplates upon 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[21] during peak glaciation, exposing large areas of the presentcontinental shelf as dry land.
According toMark Lynas (through collected data), the Pleistocene's overall climate could be characterised as a continuousEl Niño withtrade winds in the southPacific weakening or heading east, warm air rising nearPeru, warm water spreading from the west Pacific and theIndian Ocean to the east Pacific, and other El Niño markers.[22]
Pleistocene climate was marked by repeated glacial cycles in whichcontinental glaciers pushed to the 40thparallel 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 ofpermafrost stretched 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 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. TheAndes were covered in the south by thePatagonian ice cap. There were glaciers inNew Zealand andTasmania. The current decaying glaciers ofMount Kenya,Mount Kilimanjaro, and theRuwenzori Range in east and central Africa were larger. Glaciers existed in the mountains ofEthiopia and to the west in theAtlas Mountains.
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 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 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 Sheet thinned by at least 500 meters, and that thinning since theLast Glacial Maximum is less than 50 meters and probably started after c. 14 ka.[23]
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 shorterinterglacials which lasted about 10,000–15,000 years. The last cold episode of thelast glacial period ended about 10,000 years ago.[24] Over 11 major glacial events have been identified, as well as many minor glacial events.[25] 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 showhistorical 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.[25][26][27]
Historical names of the "four major" glacials in four regions.
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.
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.
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.[28]
However, a 2020 study concluded that ice age terminations might have been influenced byobliquity since the Mid-Pleistocene Transition, which caused stronger summers in theNorthern Hemisphere.[29]
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'salbedo.[30]
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 asDansgaard-Oeschger events andHeinrich 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.
Inoxygen isotope ratio analysis, variations in the ratio of18 O to16 O (twoisotopes ofoxygen) bymass (measured by amass spectrometer) present in thecalcite of oceaniccore samples is 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 O than seawater, the snow that fell on the glacier year by year nevertheless contained18 O and16 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 from the ratio found in standard mean ocean water (SMOW).
The graph in either form appears as awaveform withovertones. 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 2.588MaBP in 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.
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 event of 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.Neanderthals also became extinct during this period. At the end of the last ice age,cold-blooded animals, smaller mammals likewood mice, migratory birds, and swifter animals likewhitetail deer had replaced the megafauna and migrated north. Late Pleistocenebighorn sheep were 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.[31]
The extinctions hardly affected Africa but were especially severe inNorth America where nativehorses andcamels were wiped out.
South American land mammal ages (SALMA) includeUquian (2.5–1.5),Ensenadan (1.5–0.3) andLujanian (0.3–0.01) in millions of years. The Uquian previously extended significantly back into the Pliocene, although the new definition places it entirely within the Pleistocene.
In July 2018, a team ofRussian scientists in collaboration withPrinceton University announced that they had brought two femalenematodes frozen 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.[32][33]
Theevolution ofanatomically modern humans took place during the Pleistocene.[35][36] At the beginning of the PleistoceneParanthropus species were still present, as well as early human ancestors, but during the lower Palaeolithic they disappeared, and the onlyhominin species found in fossilic records isHomo erectus for much of the Pleistocene.Acheuleanlithics appear along withHomo erectus, some 1.8 million years ago, replacing the more primitiveOldowan industry used byAustralopithecus garhi and by the earliest species ofHomo. TheMiddle Paleolithic saw more varied speciation withinHomo, including the appearance ofHomo sapiens about 300,000 years ago.[37] Artifacts associated withmodern human behavior are unambiguously attested starting 40,000–50,000 years ago.[38]
According to mitochondrial timing techniques,modern humans migrated from Africa after theRiss glaciation in the Middle Palaeolithic during theEemian Stage, spreading all over the ice-free world during the late Pleistocene.[39][40][41] A 2005 study posits that humans in this migration interbred witharchaic human forms already outside of Africa by the late Pleistocene, incorporating archaic human genetic material into the modern human gene pool.[42]
^The Upper Pleistocene is a subseries/subepoch rather than a stage/age but, in 2009, theIUGS decided that it will be replaced with a stage/age (currently unofficially/informally named theTarantian).[15]
^Lyell, Charles (1839).Nouveaux éléments de géologie (in French). Paris, France: Pitois-Levranet. p. 621. From p. 621:"Toutefois, en même temps … et de substituer à la dénomination de Nouveau Pliocène celle plus abrégée dePleistocène, tirée du grec pleiston,plus, et kainos,récent." (However, at the same time that it became necessary to subdivide the two periods mentioned above, I found that the terms intended to designate these subdivisions were of an inconvenient length, and I have proposed to use in the future the word "Pliocene" for "old Pliocene", and to substitute for the name "new Pliocene" this shorter "Pleistocene", drawn from the Greekpleiston (most) andkainos (recent).)
^ab"Major Divisions".Subcommission on Quaternary Stratigraphy. International Commission on Stratigraphy. Retrieved17 December 2024.
^For the top of the series, see: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.ISBN0-521-78142-6.
^Gradstein, Felix M.; Ogg, James G. and Smith, A. Gilbert (eds.) (2005)A Geologic Time Scale 2004 Cambridge University Press, Cambridge, UK, p. 28,ISBN0-521-78142-6
^Aber, J. S. (December 1991). "The Glaciation of Northeastern Kansas".Boreas.20 (4):297–314.Bibcode:1991Borea..20..297A.doi:10.1111/j.1502-3885.1991.tb00282.x. (contains a summary of how and why the Nebraskan, Aftonian, Kansan, and Yarmouthian stages were abandoned by modern stratigraphers).
^Petra Bajo; et al. (2020). "Persistent influence of obliquity on ice age terminations since the Middle Pleistocene transition".Science. Vol. 367, no. 6483. pp. 1235–1239.doi:10.1126/science.aaw1114.
^Shatilovich, A. V.; Tchesunov, A. V.; Neretina, T. V.; Grabarnik, I. P.; Gubin, S. V.; Vishnivetskaya, T. A.; Onstott, T. C.; Rivkina, E. M. (16 July 2018). "Viable Nematodes from Late Pleistocene Permafrost of the Kolyma River Lowland".Doklady Biological Sciences.480 (1):100–102.doi:10.1134/S0012496618030079.PMID30009350.S2CID49743808.
^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,ISBN0-521-32370-3, pp. 241–251.
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