Quaternary

The Quaternary period is the geologic time period from the end of the Pliocene Epoch roughly 1.8-1.6 million years ago to the present. The Quaternary includes 2 geologic subdivisions — the Pleistocene (1.6 million years ago to 10,000 years ago) and the Holocene Epochs (10,000 years ago to present). This period is marked by cycles of glaciations, in which a complete glacial and interglacial cycle has lasted approximately 100,000 years.

In a recent revision of the international classification of geological time periods, the Quaternary was subsumed into the Neogene. The move has met with some resistance from geologists.

Tertiary sub-era			Quaternary sub-era
Neogene period
Miocene		Pliocene	Pleistocene	Holocene
Aquitanian	Burdigalian	Zanclean	Early
Langhian	Serravallian	Piacenzian	Middle
Tortonian	Messinian	Gelasian	Late

Overview

The term Quaternary ("fourth") was proposed by Jules Desnoyers in 1829 to address sediments of France's Seine Basin that seemed clearly to be younger than Tertiary Period rocks. The Quaternary Period follows the Tertiary Period and extends to the present. The Quaternary roughly covers the time span of recent glaciations, including the last glacial retreat. An occasional alternative usage places the start of the Quaternary at the onset of North Pole glaciation approximately 3 million years ago and includes portions of the upper Pliocene. Some people do not recognize the Quaternary and consider it an informal term included in the Neogene, as can be seen from the 2003 edition of the International Stratigraphic Chart, published by the International Commission on Stratigraphy.

The 1.8-1.6 million years of the Quaternary represents the time which recognizable humans existed. Over this short a time period, the total amount of continental drift was less than 100 km, which is largely irrelevant to paleontology. Nonetheless, the geological record is preserved in greater detail than that for earlier periods, and is most relatable to the maps of today, revealing in the second half of the twentieth century its own series of extraordinary landform changes. The major geographical changes during this time period included emergence of the Strait of Bosphorus and Skaggerak during glacial epochs, which respectively turned the Black Sea and Baltic Sea into fresh water, followed by their flooding by rising sea level; the periodic filling of the English Channel, forming a land bridge between Britain and Europe; the periodic closing of the Bering Strait, forming the land bridge between Asia and North America; and the periodic flash flooding of Scablands of the American Northwest by glacial water. The Great Lakes and other major lakes of Canada, and Hudson's Bay, are also just the results of the last cycle, and are temporary. Following every other ice age within the Quaternary, there was a different pattern of lakes and bays.

The climate was one of periodic glaciations with continental glaciers moving as far from the poles as 40 degrees latitude. Few major new animals evolved, again presumably because of the short—in geologic terms—duration of the period. There was a major extinction of large mammals in Northern areas at the end of the Pleistocene Epoch.

Many forms such as saber-toothed cats, mammoths, mastodons, glyptodonts, etc., became extinct worldwide. Others, including horses, camels and cheetahs became extinct in North America.

The Quaternary glacial period

In 1821, a Swiss engineer, Ignaz Venetz, presented an article in which he suggested the presence of traces of the passage of a glacier at a considerable distance from the Alps. This idea was initially disputed by another Swiss scientist, Louis Agassiz, but when he undertook to disprove it, he ended up affirming his colleague's theory. A year later Agassiz raised the hypothesis of a great glacial period that would have had long-reaching general effects. This idea gained him international fame.

In time, thanks to the refinement of geology and geochemical analysis of ice cores and ocean cores, it was verified that there were several periods of forward and backward movement of the glaciers and that past temperatures on Earth were very different from today. During this time, thick glaciers advanced and retreated over much of North America and Europe, parts of South America and Asia, and all of Antarctica. The Great Lakes form and giant flourish in parts of North America and Eurasia not covered in ice. These mammals become extinct when the Ice Age ended about 10,000 years ago. Modern humans evolved about 100,000 years ago.

Paleocycles

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. Milankovitch cycles influence climate by increasing or decreasing the amount sunlight received by certain parts of the globe through time. These changes include a change in the precession of the equinoxes, the tilt of the Earth's axis, and how round versus elliptical the Earth's orbit is (eccentricity). These vary on time scales of 21,000, 41,000 and 100,000 years, respectively. The dominance of the 100,000 year time scale of the Pleistocene glaciations over the last 700,000 years leads many scientists to believe that the eccentricity cycle played a significant role in the climate of this time. Before this time, the ~41,000-year obliquity cycle appeared to dominate. Some scientists remain skeptical of these connections, but a recent paper by Huybers and Wunsch (2005) found that obliquity and eccentricity played a statistically significant role in the glacial cycles.

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 shells of the microorganisms contributing the calcite.

A more recent version of the sampling process makes use of 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. In this thermodynamic process, a higher ratio of O-18 to O-16 indicates that conditions were warmer and a lower ratio of O-18 to O-16 indicates that conditions were cooler.

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 (δ) 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 MIS22 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.

Pleistocene

The name Pleistocene is derived from the Greek pleistos (most) and ceno (new). The Pleistocene 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. It lasted from 1.8 million to 12,000 years before the present.

The end of the Pleistocene corresponds with the end of the Paleolithic age used in archaeology.

Pleistocene dating

The Pleistocene has been dated in 2005 by the International Commission on Stratigraphy (a body of the International Union of Geological Sciences) from 1.81 million to 11,550 years before present, with the end date expressed in radiocarbon years. 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 9600 B.C.E. (11550 calendar years BP).

The 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 end Pliocene. Some climatologists would therefore prefer a start date of around 2.5 million years BP. The name Plio-Pleistocene is in use to mean the last ice age.

The continuous climatic history from the Pliocene into the Pleistocene and Holocene was one reason for the International Commission on Stratigraphy to discourage the use of the term "Quaternary". Therefore, the Pleistocene is an epoch of the Neogene in current usage.

Pleistocene paleogeography and climate

The maximum extent of glacial ice in the north polar area during Pleistocene time.

The modern continents were essentially at their present positions during the Pleistocene, probably having moved no more than 100 km since.

Glacial features

Pleistocene climate was characterized by repeated glacial cycles where continental glaciers pushed to the 40th parallel latitude 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 kilometers 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 we are experiencing now, 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 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 ice sheet. 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 due to blockage of outlets and decreased evaporation in the cooler air. 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.

Increased dust accumulation in ice cores from Greenland and Antarctica suggests that conditions were drier and windier, as much of the water was tied up in ice caps. A decrease in oceanic and other evaporation because of colder air temperatures, resulted in drier deserts that were far more extensive.

Major events

Four major glacial events have been identified, as well as many minor intervening events. A major event is a general glacial excursion, termed just 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. You would not refer to the Mindel as the Elsterian or vice versa.

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.

Four of the better known regions with the names of the glacials are listed in the table below. Fuller information including the dates is stated in the linked articles, which combine the same glaciation of different regions. A synthesis of the larger picture is shown under Timeline of glaciation.

It should be emphasized that these glacials 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 we know they existed from the study of cyclical climate changes.

Four of the better known regions with the names of the glacials.

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 of US	Nebraskan	Kansan	Illinoian	Wisconsin

The interglacials corresponding to prior glacials.

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 of US	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, 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.

Pleistocene fauna

There are no faunal stages defined for the Pleistocene or Holocene. Both marine and continental faunas were essentially modern. It is believed by most scientists that humans evolved into modern man during the Pleistocene.

Pleistocene deposits

Pleistocene continental deposits are found primarily in lakebeds, loess deposits and caves as well as in the large amounts of material moved about by glaciers. Pleistocene marine deposits are found primarily 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.

Holocene Climate

The end of the Pleistocene is marked as the beginning of significant climate warming at around 10,000 years before present (BP). The time period from that point forward is known as the Holocene. During this time, three distinct changes occurred. The first of these is a significant rise in carbon dioxide (from 210 ppm to 280ppm), which was reported from trapped gas bubbles in ice cores (Neftel et al. 1982). The second change that was seen worldwide at around this time was a change in the species assemblage of foraminifera, microscopic oceanic microorganisms, found in ocean sediments. This change around 11k B.P. indicates an increase in ocean temperatures (Broecker et al. 1960). The third major change during this time (12k B.P. to 10k B.P.) was the extinction of a number of large mammals in North America (Kurten and Andersen 1980). The extinctions were especially severe in North America where native horses and camels were eliminated. Palynologists noted abrupt worldwide changes in vegetation during this time, with forests replacing tundra. The end of the Pleistocene also marks the end of a abrupt climate reversal known as the Younger Dryas (12.7 to 11.5 ky BP), where following deglaciation and climate warming, temperatures rapidly dipped back down, turning forested landscape back in to tundra. Almost as rapidly as the climate cooled, the warm temperatures were restored.

References

ISBN links support NWE through referral fees

• Broecker, W.S. Ewing, M. and Heezen, B.K. 1960. Evidence for an abrupt change in climate close to 11,000 years ago. American Jounral of Science. 258:429-448.
• Kurten, B, Anderson, E. 1980. Pleistocene mammals of North America. Columbia University Press: New York.
• Huybers, P. and Carl Wunsch. 2005. Obliquity pacing of the late glacial terminations. Nature. 434: 491-494.
• Neftel, A. Schwander, J. Stauffer,B. and Zumbrunn, R. 1982. Ice core sample measurements five atmosphereic CO2 content during the past 40,0000 yr. Nature. 295: 220-3.
• Pielou, E.C. 1991. After the Ice Age: the return of life to glaciated North America. University of Chicago Press:Chicago.
• The "Quaternary glacial period" section was derived from the article :es:Glacier in the Spanish-language Wikipedia, which was accessed in the version of July 24, 2005.
• 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

• Jan Zalasiewicz, The Guardian, June 30, 2005, "Repackaging time"

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Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

• Quaternary  ^history

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