The Pleistocene is the geological epoch which lasted from about 2,588,000 to 11,700 years
ago, spanning the world's recent period of repeated glaciations.
Charles Lyell introduced this term in 1839 to describe strata in Sicily that had at least
70% of their molluscan fauna still living today. This distinguished it from the older
Pliocene Epoch, which Lyell had originally thought to be the youngest fossil rock layer.
He constructed the name "Pleistocene" from the Greek πλεῖστος, pleīstos, "most",
and καινός, kainós, "new"; this contrasting with the immediately preceding Pleiocene,
and the immediately subsequent Holocene epoch, which extends to the present time.
The Pleistocene is the first epoch of the Quaternary Period or sixth epoch of the Cenozoic
Era. The end of the Pleistocene corresponds with the end of the last glacial period. It
also corresponds with the end of the Paleolithic age used in archaeology. In the ICS timescale,
the Pleistocene is divided into four stages or ages, the Gelasian, Calabrian, Ionian and
Tarantian. All of these stages were defined in southern Europe. In addition to this international
subdivision, various regional subdivisions are often used.
Before a change finally confirmed in 2009 by the International Union of Geological Sciences,
the time boundary between the Pleistocene and the preceding Pliocene was regarded as
being at 1.806 million years before the present, as opposed to the currently accepted 2.588
million years BP: publications from the preceding years may use either definition of the period.
Dating The Pleistocene has been dated from 2.588
million to 11,700 years before present, 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 9640 BC. It was not until after the development of radiocarbon dating, however,
that Pleistocene archaeological excavations shifted to stratified caves and rock-shelters
as opposed to open-air river-terrace sites. In 2009 the International Union of Geological
Sciences 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 the Gelasian as the base
of the Pleistocene, namely the base of the Monte San Nicola GSSP. The IUGS has yet to
approve a type section, Global Boundary Stratotype Section and Point, for the upper Pleistocene/Holocene
boundary. The proposed section is the North Greenland Ice Core Project ice core 75° 06'
N 42° 18' W. The lower boundary of the Pleistocene Series is formally defined magnetostratigraphically
as the base of the Matuyama chronozone, isotopic stage 103. Above this point there are notable
extinctions of the calcareous nanofossils: Discoaster pentaradiatus and Discoaster surculus.
The Pleistocene covers the recent period of repeated glaciations. The name Plio-Pleistocene
has in the past been used to mean the last ice age. The revised definition of the Quaternary,
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.
Paleogeography and climate
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, 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.
Glacial features Pleistocene climate was marked by repeated
glacial cycles in which 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 1,500 to
3,000 metres thick, resulting in temporary sea-level drops of 100 metres 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 northern Europe, including 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 totally 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. 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
Over 11 major glacial events have been identified, as well as many minor glacial events. 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. These names have been abandoned
in favor of numeric data because many of the correlations were found to be either inexact
or incorrect and more than four major glacials have been recognized since the historical
terminology was established. Corresponding to the terms glacial and interglacial,
the terms pluvial and interpluvial are in use. 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. Only a few of the names for pluvials in restricted
regions have been strategraphically 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
and planetary solar radiation reaching the Earth caused by several repeating changes
in the Earth's motion. 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. 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. The timing of our present interglacial interval to that of the previous interglacial,
beginning about 130,000 years ago, suggests that the next glacial would likely begin in
about 3,000 years. Oxygen isotope ratio cycles
In oxygen isotope ratio analysis, variations in the ratio of 18O to 16O by mass 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 18O, which is included
in the tests 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 18O than sea water, the snow that fell on the glacier year by year nevertheless
contained 18O and 16O 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 from the ratio found in standard
mean ocean water. The graph in either form appears as a waveform
with overtones. One half of a period is a Marine isotopic stage. It indicates a glacial
or an interglacial. Overtones are stadials or interstadials.
According to this evidence, Earth experienced 102 MIS stages beginning at about 2.588 Ma
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; interglacials, odd. 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, 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 and many animals, specifically, mammals
were much larger than their modern relatives .
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, which included mammoths,
mastodons, saber-toothed cats, glyptodons, ground sloths, Irish elk, cave bears, and
short-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 like wood mice, migratory birds, and swifter animals like whitetail
deer had replaced the megafauna and migrated north.
The extinctions were especially severe in North America where native horses and camels
were eliminated. Asian land mammal ages include Zhoukoudianian,
Nihewanian, and Yushean. European land mammal ages include Gelasian.
North American land mammal ages include Blancan, Irvingtonian and Rancholabrean in millions
of years. The Blancan extends significantly back into the Pliocene.
South American land mammal ages include Uquian, Ensenadan and Lujanian in millions of years.
The Uquian previously extended significantly back into the Pliocene, although the new definition
places it entirely within the Pleistocene. Humans
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 hominid species found in fossilic records is Homo erectus for much of the Pleistocene.
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 Stage, spreading all over the ice-free world during the late Pleistocene.
A 2005 study posits that humans in this migration interbred with archaic human forms already
outside of Africa by the late Pleistocene, incorporating archaic human genetic material
into the modern human gene pool. Deposits
Pleistocene non-marine sediments are found primarily in fluvial deposits, lakebeds, slope
and loess deposits as well as in the large amounts of material moved about by glaciers.
Less common are cave deposits, travertines and volcanic deposits. Pleistocene marine
deposits are found primarily in shallow marine basins mostly in areas within a few tens of
kilometers 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
Climate state Geologic time scale
Pleistocene megafauna Timeline of glaciation
References
Ogg, Jim; June, 2004, Overview of Global Boundary Stratotype Sections and Points (GSSP's, Stratigraphy.org,
Accessed April 30, 2006. External links
The SMU-in-Taos Research Publications digital collection of anthropological and archaeological
monographs contains Late Pleistocene environments of the southern high plains.
Pleistocene Microfossils: 50+ images of Foraminifera