The
Pleistocene ( ) is the
epoch from 2.588 million to 12 000 years
BP covering the world's recent period
of repeated
glaciations. The name
pleistocene is derived from the Greek (
pleistos
"most") and (
kainos "new").
The Pleistocene epoch follows the
Pliocene
epoch and is followed by the
Holocene
epoch. The Pleistocene is the
first epoch of the
Quaternary period or 6th epoch of the
Cenozoic Era. The end
of the Pleistocene corresponds with the retreat of the
last continental glacier. 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.
Dating
The Pleistocene has been dated from 2.588 million (±5 000 years) to
11 550 years
before present (BP),
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 BCE (11590 calendar
years BP).
In 2009 the
International Union
of Geological Sciences (IUGS) confirmed a change in time period
for the Pleistocene, changing the start date from 1.8 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 (GSSP), for the upper
Pleistocene/Holocene boundary (
i.e. the upper 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 (C2r)
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 within the
Pleistocene of all the recent repeated glaciations.
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 (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.
Glacial features
Pleistocene climate was marked by repeated glacial cycles where
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 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.
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 (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. 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 solar
radiation caused by the sum of many repeating changes in the
Earth's motion.
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.
Oxygen isotope ratio cycles
In
oxygen isotope ratio
analysis, variations in the ratio of O
18 to
O
16 (two
isotopes of
oxygen) by
mass (measured by a
mass spectrometer) 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 O
18, 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 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 102 MIS stages
beginning at about 2.588 MYA 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 85,000-11,000 YA. 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.
Early Pleistocene Animals
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,
Irish elk,
cave bears, and
short-faced
bear, 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.
Humans during the Pleistocene
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 Stage, 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. 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. due to the advent of genetic evidence that directly
contradicts it in favor of a single-origin theory.
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
cavedeposits,
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 the
Southern California coast, Pleistocene
marine deposits may be found at elevations of several hundred
metres.
See also
References
- In Gradstein, F. M., Ogg, James G., and Smith, A.
Gilbert (eds.), A Geologic Time Scale 2004 Cambridge
University Press, Cambridge, ISBN 0521781426
- For the top of the series, see: , (2004) “The Neogene Period”.
In: Gradstein, F., Ogg, J., Smith, A.G. (Eds.), A Geologic Time
Scale 2004. Cambridge: Cambridge University Press.
- Riccardi, Alberto C. (30 June 2009) "IUGS ratified ICS Recommendation on redefinition
of Pleistocene and formal definition of base of Quaternary"
International Union of Geological Sciences
- Svensson, A., S. W. Nielsen, S. Kipfstuhl, S. J. Johnsen, J. P.
Steffensen, M. Bigler, U. Ruth, and R. Röthlisberger (2005) "Visual
stratigraphy of the North Greenland Ice Core Project (NorthGRIP)
ice core during the last glacial period" Journal of Geophysical
Research 110: (D02108)
- Gradstein, Felix M.; Ogg, James G. and Smith, A. Gilbert (eds.)
(2005) A Geologic Time Scale 2004 Cambridge University
Press, Cambridge, UK, p. 28, ISBN 0-521-78142-6
- Rio, D.; Sprovieri, R.; Castradori, D. and Di Stefano, E.
(1998) "The Gelasian Stage (Upper Pliocene): a new unit of the
global standard chronostratigraphic scale" Episodes 21:
pp. 82-87
- National Geographic Channel,
Six Degrees Could Change The World, Mark Lynas interview.
Retrieved February 14, 2008.
- Richmond, G.M. and D.S. Fullerton, 1986, Summation of
Quaternary glaciations in the United States of America.
Quaternary Science Reviews. vol. 5, pp. 183-196.
- Roy, M., P.U. Clark, R.W. Barendregt, J.R., Glasmann, and R.J.
Enkin, 2004, Glacial stratigraphy and paleomagnetism of late
Cenozoic deposits of the north-central United States, PDF
version, 1.2 MB. Geological Society of America Bulletin. vol. 116,
no. 1-2; pp. 30-41; DOI: 10.1130/B25325.1
- Aber, J.S. (1991) "Glaciations of Kansas" Boreas
20(4): pp. 297-314 - (contains a summary of how and why the
Nebraskan, Aftonian, Kansan, and Yarmouthian stages were abandoned
by modern stratigraphers.)
- Rogers, A.R. and Jorde, L.B. (1995) "Genetic evidence on modern
human origins" Human Biology 67: pp. 1–36
- Wall, J.D. and Przeworski, M. (2000) "When did the human
population start increasing?" Genetics 155: pp.
1865–1874
- Cann, R.L.; Stoneking, M. and Wilson, A.C.(1987) "Mitochondrial
DNA and human evolution" Nature 325: pp. 31–36
- 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, ISBN 0-521-32370-3, pp.
241–251.
- Templeton, A. (2002) "Out of Africa again and again"
Nature 416: p. 45
- Eswarana, Vinayak; Harpendingb, Henry and Rogers, Alan R.
(2005) "Genomics refutes an exclusively African origin of humans"
Journal of Human Evolution 49(1): pp. 1–18 Abstract
- Ogg, Jim; June, 2004, Overview of Global Boundary
Stratotype Sections and Points (GSSP's)
http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.
External links
Hominin species during
pleistocene
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