The Pleistocene is the best-known glacial period (Ice Age) of the earth's history. Its ice sheets at one time covered all of Antarctica, large parts of Europe, North America, and South America, and small areas in Asia. In North America they stretched over Greenland and Canada and over the United States as far south as a line drawn westward from Cape Cod through Long Island, New Jersey, and Pennsylvania, along the line of the Ohio and Missouri rivers to North Dakota, and through N Montana, Idaho, and Washington to the Pacific. The ice sheets of Europe radiated from Scandinavia and covered Finland, NW Russia, N Germany, and the British Isles. Glaciers distinct from the main sheets were formed in the Rockies and the Alps. In South America, Patagonia and the S Andes lay under an extension of the antarctic sheet, while in Asia the Caucasus, the Himalayas, and other mountain regions were glaciated.
The glaciation of the Pleistocene was not continuous but consisted of several glacial advances interrupted by interglacial stages, during which the ice retreated and a comparatively mild climate prevailed. In all probability there were actually only four glacial stages, the Iowan and Bradyan being included in the Wisconsin as one complex stage. Carbon-14 analysis of fossils shows that the last glacial period ended about 11,000 years ago.
The characteristic formation laid down in the glacial stages of the Pleistocene, as in all glacial periods, is the drift. The interglacial stages were marked by the weathering of the till of the drift to form a sticky, heavy soil called the gumbotil and by the deposition of peat and loess. Peat is plentiful in the Aftonian, Yarmouth, and Sangamon interglacial stages in North America.
The Pleistocene glaciers made important alterations in the topography of the glaciated regions, leveling hilly sections to low, rolling plains, both by erosion and by deposition of drift, eroding hollows that later became lakes, and forcing rivers to cut new channels by filling their former beds. Among the characteristic surface features formed in the Pleistocene are the drumlin, kame, esker, and moraine. The retreat of the ice after the Wisconsin glacial stage was followed by the formation, at the edge of the melting glaciers, of lakes, such as the extinct Lake Agassiz and the Great Lakes. The further retreat of the ice led to the flooding by the Atlantic of the NE United States and SE Canada, which had been depressed below sea level by the weight of the ice. In the areas of North America not covered by ice, the Pleistocene was marked chiefly by erosion, with only very slight marine transgressions over the coast.
During the various glacial stages many areas not covered with ice, including the arid and semiarid parts of the W United States, had periods of increased rainfall and lessened evaporation. Called pluvial periods, they were characterized by the spread of vegetation and the formation of many lakes. Heavy precipitation in the West was responsible for two great lakes—Lake Lahontan of Nevada and Lake Bonneville of Utah (which today forms the Great Salt and Utah lakes). During the Pleistocene, volcanic activity and warping of the earth's surface occurred on the Pacific coast. The cutting of the Grand Canyon took place chiefly in Pleistocene time.
Among the characteristic Pleistocene mammals of North America were at least four species of elephants, including the mastodon and the mammoth, true horses, of the same genus as the domestic horse though not of the same species, saber-tooth carnivores, large wolves, giant armadillos and ground sloths, bisons, camels, and wild pigs. Among the arctic mammals that ranged far south in the glacial stages were the musk ox in North America and the woolly mammoth in Europe. The Pleistocene saw the beginning of the trend toward the extinction of many mammal species, which continued into historic times. The Pleistocene is noted also for the first appearance of modern humans approximately 500,000 years ago and the migration of humans to the American continents.
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. The end of the Pleistocene corresponds with the end of the Paleolithic age used in archaeology.
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.
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. 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.
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. 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.
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.
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.
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.
|Region||Glacial 1||Glacial 2||Glacial 3||Glacial 4|
|Region||Interglacial 1||Interglacial 2||Interglacial 3|
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.
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.
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.
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.
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.
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 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.
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.
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) # 255/255/179
Period = from:0 till:1900000 TimeAxis = orientation:horizontal ScaleMajor = unit:year increment:1000000 start:0 ScaleMinor = unit:year increment:250000 start:0
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
bar:Events color:events align:right shift:(-5,-10)
width:7 mark:none color:events align:right shift:(-5,-4)
at:1400000 text:H. habilis
at:1800000 text:H. georgicus
at:94000 text:H. floresiensis
at:1800000 text:H. erectus
at:600000 text:H. heidelbergensis
at:230000 text:H. neanderthalensis
at:200000 text:H. sapiens
at:1400000 text:P. boisei
at:1400000 text:P. robustus
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: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:white