Quaternary glaciation

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The Quaternary glaciation, also known as the Pleistocene glaciation or simply the ice age, refers to the period of the last few million years (2.58 Ma to present) in which a permanent ice sheet was established in Antarctica and probably Greenland, and fluctuating ice sheets have occurred elsewhere (e.g. the Laurentide). The major effects of the ice age were glacial erosion and deposition over large parts of the continents that modified river systems, creation of millions of lakes, changes in sea level, development of pluvial lakes far from the ice margins, isostatic adjustment of the crust, abnormal winds, its impact on the oceans, major flooding, and modifications of biologic communities. The ice sheets themselves, by modifying the albedo, constitutes a major feedback on the climate.

During the Quaternary Period, the total volume of land ice, sea level and global temperature has fluctuated initially on 41,000- and more recently on 100,000-year time scales, as evidenced most clearly by ice cores for the past 800,000 years and marine sediment cores for the earlier period. There have been approximately 80 glacial cycles over this time. All of this time is referred to as an ice age because at least one permanent large ice sheet—Antarctica—has existed continuously. There is uncertainty over how much of Greenland was present during the previous and earlier interglacials. During the colder episodes—referred to as glacial periods—large ice sheets also existed in Europe, North America, and Siberia. The shorter and warmer intervals between glacials are referred to as interglacials.

Currently, we are in a interglacial period, which marked the beginning of the Holocene epoch. The current interglacial began between 10,000 and 15,000 years ago, which caused the ice sheets from the last glacial period to begin to disappear. Remnants of these last glaciers, now occupying about 10% of the world's land surface, still exists in Greenland and Antarctica. Global warming has exacerbated the retreat of these glaciers.

During the glacial periods, what we see as the normal, ie interglacial, hydrologic system was completely interrupted throughout large areas of the world and was considerably modified in others. Due to the volume of ice on land, sea level is approximately 120 meters lower than present. The evidence of such an event in the recent past is robust. Over the last century, extensive field observations have provided evidence that continental glaciers covered large parts of Europe, North America, and Siberia. Maps of glacial features were compiled after many years of fieldwork by hundreds of geologists who mapped the location and orientation of drumlins, eskers, moraines, striations, and glacial stream channels. These maps revealed the extent of the ice sheets, the direction of flow, and the locations of systems of meltwater channels, and they allowed scientists to decipher a history of multiple advances and retreats of the ice. Even before the theory of worldwide glaciation was generally accepted, many observers recognized that more than a single advance and retreat of the ice had occurred. Extensive evidence now shows that a number of periods of growth and retreat of continental glaciers occurred during the ice age, called glacials and interglacials. The interglacial periods of warm climate are represented by buried soil profiles, peat beds, and lake and stream deposits separating the unsorted, unstratified deposits of glacial debris.

[edit] Records of prior glaciation

Main article: Timeline of glaciation

See also: Ice age and paleoclimatology

Glaciation has been a rare event in Earth's history,^[1] but there is evidence of widespread glaciation during the late Paleozoic Era (200 to 300 Ma) and during late Precambrian (i.e. in the Neoproterozoic Era, 600 to 800 Ma).^[2] Before the current ice age, which began 2 to 3 Ma, Earth's climate was typically mild and uniform for long periods of time. This climatic history is implied by the types of fossil plants and animals and by the characteristics of sediments preserved in the stratigraphic record.^[3] There are, however, widespread glacial deposits, recording several major periods of ancient glaciation in various parts of the geologic record. Such evidence suggests major periods of glaciation prior to the current Quaternary glaciation.

The best documented record of pre-Quaternary glaciation, called the Karoo Ice Age, is found in the late Paleozoic rocks in South Africa, India, South America, Antarctica, and Australia. Exposures of ancient glacial deposits are numerous in these areas. Deposits of even older glacial sediment exist on every continent except South America. These indicate that two other periods of widespread glaciation occurred during late the Precambrian, producing the Snowball Earth during the Cryogenian Period.^[4]

[edit] Causes

Further information: Ice age

No completely satisfactory theory has been proposed to account for Earth's history of glaciation. The cause of glaciation may be related to several simultaneously occurring factors, such as astronomical cycles, plate tectonics, atmospheric composition, and ocean currents.^[5]

[edit] Astronomical cycles

Main articles: Milankovitch cycles and orbital forcing

See also: 100,000-year problem

The role of Earth's orbital changes in controlling climate was first advanced by James Croll in the late 1800s.^[6] Later, Milutin Milanković, a Serbian geophysicist, elaborated on the theory and calculated these irregularities in Earth's orbit could cause the climatic cycles known as Milankovitch cycles.^[7] They are the result of the additive behavior of several types of cyclical changes in Earth's orbital properties.

Changes in the orbital eccentricity of Earth occur on a cycle of about 100,000 years.^[8] The inclination, or tilt, of Earth's axis varies periodically between 22° and 24.5°.^[8] (The tilt of Earth's axis is responsible for the seasons; the greater the tilt, the greater the contrast between summer and winter temperatures.) Changes in the tilt occur in a cycle 41,000 years long.^[8] Precession of the equinoxes, or wobbles on Earth's spin axis, complete every 21,700 years. According to the Milankovitch theory, these factors cause a periodic cooling of Earth, with the coldest part in the cycle occurring about every 40,000 years. The main effect of the Milankovitch cycles is to change the contrast between the seasons, not the amount of solar heat Earth receives. These cycles within cycles predict that during maximum glacial advances, winter and summer temperatures are lower. The result is less ice melting than accumulating, and glaciers build up.

Milankovitch worked out the ideas of climatic cycles in the 1920s and 1930s, but it was not until the 1970s that sufficiently long and detailed chronology of the Quaternary temperature changes as worked out to test the theory adequately.^[9] Studies of deep-sea cores, and the fossils contained in them indicate that the fluctuation of climate during the last few hundred thousand years is remarkably close to that predicted by Milankovitch.

A problem with the theory is that the astronomical cycles have been in existence for billions of years, but glaciation is a rare occurrence. Other factors must also be involved that caused Earth's temperature to drop below a critical threshold. Once that occurs, Milankovitch cycles will act to force the planet in and out of glacial periods.

[edit] Atmospheric composition

One theory holds that decreases in atmospheric carbon dioxide (CO₂), an important greenhouse gas, started the long-term cooling trend that eventually led to glaciation. Recent studies of the CO₂ content of gas bubbles preserved in the Greenland ice cores lend support to this idea. High CO₂ contents correspond to warm interglacial periods, and low CO₂ to glacial periods. The geochemical cycle of carbon indicate more than a 10-fold decrease in atmospheric CO₂ since the middle of the Mesozoic Era.^[10] However, its unclear what caused the decline in CO₂ levels, and whether this decline is the cause of global cooling or if it is the result.

[edit] Effects

The presence of so much ice upon the continents had a profound effect upon almost every aspect of Earth's hydrologic system. The most obvious effects are the spectacular mountain scenery and other continental landscapes fashioned both by glacial erosion and deposition instead of running water. Entirely new landscapes covering millions of square kilometers were formed in a relatively short period of geologic time. In addition, the vast bodies of glacial ice affected the Earth well beyond the glacier margins. Directly or indirectly, the effects of glaciation were felt in every part of the world.

[edit] Lakes

Further information: Lake

The Quaternary glaciation created more lakes than all other geologic processes combined. The reason is because a continental glacier completely disrupts the preglacial drainage system. The surface over which the glacier moved was scoured and eroded by the ice, leaving myriad closed, undrained depressions in the bedrock. These depressions filled with water and became lakes.

Very large lakes were created along the glacial margins. The ice on both North America and Europe was about 3,000 m (9,843 ft) thick near the centers of maximum accumulation, but it tapered toward the glacier margins. Crustal subsidence was greatest beneath the thickest accumulation of ice. As the ice melted, rebound of the crust lagged behind, producing a regional slope toward the ice. This sloped formed basins that have lasted for thousands of years. These basins became lakes or were invaded by the ocean. The Great Lakes^[11] and the Baltic Sea of northern Europe^[12][13] were formed primarily in this way.

[edit] Pluvial lakes

The climatic conditions that cause glaciation had an indirect effect on arid and semiarid regions far removed from the large ice sheets. The increased precipitation that fed the glaciers also increase the runoff of major rivers and intermittent streams, resulting in the growth and development of large pluvial lakes. Most pluvial lakes developed in relatively arid regions where there typically was insufficient rain to establish a drainage system to the sea. Instead, stream runoff in those areas flowed into closed basins and formed playa lakes. With increased rainfall, the playa lakes enlarged and overflowed. Pluvial lakes were most extensive during glacial periods. During interglacial stages, when less precipitation fell, the pluvial lakes shrank to form small salt flats.

[edit] Isostatic adjustment

Main article: Post-glacial rebound

Major isostatic adjustments of the lithosphere during the Quaternary glaciation were caused by the weight of the ice, which depressed the continents. In Canada, a large area around Hudson Bay was depressed below sea level, as was the area in Europe around the Baltic Sea. The land has been rebounding from these depressions since the ice melted. Some of these isostatic movements triggered large earthquakes in Scandinavia about 9,000 years ago. These earthquakes are unique in that they are not associated with plate tectonics.

Studies have shown that the uplift has taken place in two distinct stages. The initial uplift following deglaciation was rapid (called "elastic"), and took place as the ice was being unloaded. After this "elastic" phase, uplift proceed by "slow viscous flow" so the rate decreased exponentially after that. Today, typical uplift rates are of the order of 1 cm/year or less. In northern Europe, this is clearly shown by the GPS data obtained by the BIFROST GPS network.^[14] Studies suggest that rebound will continue for about at least another 10,000 years. The total uplift from the end of deglaciation depends on the local ice load and could be several hundred meters near the center of rebound.

[edit] Winds

The presence of ice over so much of the continents greatly modified patterns of atmospheric circulation.^[15] Winds near the glacial margins were strong and persistent because of the abundance of dense, cold air coming off the glacier fields. These winds picked up and transported large quantities of loose, fine grained sediment brought down by the glaciers. This dust accumulated as loess (wind-blown silt), forming irregular blankets over much of the Missouri River valley, central Europe, and northern China.

Sand dunes were much more widespread and active in many areas during the early Quaternary period. A good example is the Sand Hills region in Nebraska, USA, which covers an area of about 60,000 square kilometers (23,166 sq mi).^[16] This region was a large, active dune field during the Pleistocene epoch, but today are largely stabilized by grass cover.^[17][18]

[edit] References

[edit] External links

• http://www.geo.ua.edu/intro03/ice.html
• http://maps.unomaha.edu/maher/2003MBfieldtrip/Moakreport/Pleistgeo.html
• http://jasonlowder.googlepages.com/Hewitt1996.pdf
• http://geology.geoscienceworld.org/cgi/content/abstract/23/6/483
• After the Ice Age: The Return of Life to Glaciated North America by E.C. Pielou
• http://jgs.geoscienceworld.org/cgi/content/abstract/142/3/417
• http://www.fs.fed.us/r10/tongass/forest_facts/resources/geology/icefields.htm
• http://www.zoology.ubc.ca/~etaylor/413www/glaciation.html
• http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter6.pdf

Causes

• http://www.ncdc.noaa.gov/paleo/milankovitch.html
• http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/milankovitch_2.html