Quaternary glaciation Article

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Northern Hemisphere glaciation during the Last Glacial Maximum. The creation of 3 to 4 km (1.9 to 2.5 mi) thick ice sheets equate to a global sea level drop of about 120 m (390 ft).

The Quaternary glaciation, also known as the Quaternary Ice Age [1] [2] or Pleistocene glaciation, is a series of glacial events separated by interglacial events during the Quaternary period that began 2.58 Ma (million years ago), and is ongoing. [3] It is one of five major glacial periods or " ice ages" in Earth's history since about 2.5 billion years ago; the preceding one, known as the Karoo Ice Age, lasted from about 360 to 260 Ma ( Carboniferous to Mid-Permian). Outside of these ice ages, the Earth seems to have been completely ice-free, even at high latitudes. [4]

The definition of the Quaternary as beginning 2.58 Ma is based on the formation of the Arctic ice cap. The Antarctic ice sheet began to form earlier, at about 34 Ma, in the mid- Cenozoic ( Eocene–Oligocene Boundary). The term Late Cenozoic Ice Age is used to include this early phase. [5]

During the Quaternary Glaciation, ice sheets expanded, notably from out of Antarctica and Greenland, and fluctuating ice sheets occurred elsewhere (for example, the Laurentide ice sheet). The major effects of the ice age were the erosion of land and the deposition of material, both over large parts of the continents; the modification of river systems; the creation of millions of lakes, including the development of pluvial lakes far from the ice margins; changes in sea level;, the isostatic adjustment of the Earth's crust; flooding; and abnormal winds. The ice sheets themselves, by raising the albedo (the extent to which the radiant energy of the Sun is reflected from Earth) created significant feedback to further cool the climate. These effects reshaped entire environments on land and in the oceans, and their associated biological communities.

Discovery

The Quaternary glaciation was the first ice age to be demonstrated to have occurred in the field of geology, thus proving ice ages were possible. This was achieved in the 18th and 19th centuries as part of the scientific revolution.

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 in order to reveal the extent of the ice sheets, the direction of their flow, and the locations of systems of meltwater channels. They also 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.

Since then, other, earlier ice ages have been identified, and the Quaternary event(s) constrained.[ clarification needed]

Description

Graph of reconstructed temperature (blue), CO2 (green), and dust (red) from the Vostok Station ice core for the past 420,000 years

The Current or Quaternary Ice Age is the last of five known major glacial periods, or ice ages, during Earth's history, the preceding ones being the Karoo Ice Age (360–260 Ma), Andean-Saharan (450–420 Ma), Cryogenian (720–635 Ma) and Huronian (2,400–2,100 Ma). [6] [7]

Within the Quaternary Period, or ice age, there were also periodic fluctuations of the total volume of land ice, the sea level, and global temperatures. During the colder episodes (referred to as glacial periods, or simply glacials) large ice sheets at least 4 km thick at their maximum existed in Europe, North America, and Siberia. The shorter and warmer intervals between glacials, when continental glaciers retreated, are referred to as interglacials. These are evidenced by buried soil profiles, peat beds, and lake and stream deposits separating the unsorted, unstratified deposits of glacial debris.

Initially the fluctuation period was about 41,000 years, but more recently it has slowed to about 100,000 years, as evidenced most clearly by ice cores for the past 800,000 years and marine sediment cores for the earlier period. Over the past 740,000 years there have been eight glacial cycles. [8]

The entire Quaternary Period, starting 2.58 Ma, is referred to as an ice age because at least one permanent large ice sheet—the Antarctic ice sheet—has existed continuously. There is uncertainty over how much of Greenland was covered by ice during each interglacial.

Currently, Earth is in an interglacial period, which marked the beginning of the Holocene epoch. The current interglacial began between 15,000 and 10,000 years ago; this 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 exist in Greenland, Antarctica and some mountainous regions.

During the glacial periods, the present (i.e. 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 was about 120 meters lower than present.

Causes

No completely satisfactory theory has been proposed to account for Earth's history of glaciation. The cause of glaciation may be related to several simultaneous factors, such as astronomical cycles, atmospheric composition, plate tectonics, and ocean currents. [9]

Astronomical cycles

The role of Earth's orbital changes in controlling climate was first advanced by James Croll in the late 19th century. [10] Later, Milutin Milanković, a Serbian geophysicist, elaborated on the theory and calculated that these irregularities in Earth's orbit could cause the climatic cycles now known as Milankovitch cycles. [11] They are the result of the additive behavior of several types of cyclical changes in Earth's orbital properties.

Relationship of Earth's orbit to periods of glaciation

Changes in the orbital eccentricity of Earth occur on a cycle of about 100,000 years. [12] The inclination, or tilt, of Earth's axis varies periodically between 22° and 24.5° in a cycle 41,000 years long. [12] The tilt of Earth's axis is responsible for the seasons; the greater the tilt, the greater the contrast between summer and winter temperatures. Precession of the equinoxes, or wobbles of Earth's spin axis, have a periodicity of 26,000 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 overall amount of solar heat Earth receives. 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 a sufficiently long and detailed chronology of the Quaternary temperature changes was worked out to test the theory adequately. [13] 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 these astronomical cycles have been in existence for many millions of years, but glaciation is a rare occurrence. Astronomical cycles correlate with glacial and interglacial periods, and their transitions, within a long-term ice age but do not initiate these long-term ice ages.

Atmospheric composition

One theory holds that decreases in atmospheric CO
2
, an important greenhouse gas, started the long-term cooling trend that eventually led to glaciation. The geochemical cycle of carbon indicates a decrease of more than a 90% in atmospheric CO2 since the middle of the Mesozoic Era. [14] An analysis of CO2 reconstructions from alkenone records shows that CO2 in the atmosphere declined before and during Antarctic glaciation, and supports a substantial CO2 decrease as the primary cause of Antarctic glaciation. [15]

CO2 levels also play an important role in the transitions between interglacials and glacials. High CO2 contents correspond to warm interglacial periods, and low CO2 to glacial periods. However, studies indicate that CO
2
may not be the primary cause of the interglacial-glacial transitions, but instead acts as a feedback. [16] The explanation for this observed CO
2
variation "remains a difficult attribution problem". [16]

Plate tectonics and ocean currents

An important component in the development of long-term ice ages is the positions of the continents. [17] These can control the circulation of the oceans and the atmosphere, affecting how ocean currents carry heat to high latitudes. Throughout most of geologic time, the North Pole appears to have been in a broad, open ocean that allowed major ocean currents to move unabated. Equatorial waters flowed into the polar regions, warming them. This produced mild, uniform climates that persisted throughout most of geologic time.

Throughout the Cenozoic Era, the large North American and South American continental plates moved westward from the Eurasian plate. This drift interlocked with the development of the Atlantic Ocean, trending north-south, with the North Pole in the small, nearly landlocked basin of the Arctic Ocean. The Isthmus of Panama developed at a convergent plate margin about 3 million years ago, and further separated oceanic circulation, closing the last strait, outside the polar regions, that had connected the Pacific and Atlantic Oceans. [18]

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 Earth well beyond the glacier margins. Directly or indirectly, the effects of glaciation were felt in every part of the world.

Lakes

A diagram of the formation of the Great Lakes

The Quaternary glaciation created more lakes than all other geologic processes combined. The reason is that a continental glacier completely disrupts the preglacial drainage system. The surface over which the glacier moved was scoured and eroded by the ice, leaving a myriad of 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 (10,000 ft) thick near the centers of maximum accumulation, but it tapered toward the glacier margins. Ice weight caused crustal subsidence, which 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 slope formed basins that have lasted for thousands of years. These basins became lakes or were invaded by the ocean. The Baltic Sea [19] [20] and the Great Lakes of North America [21] were formed primarily in this way.[ dubious ]

The numerous lakes of the Canadian Shield, Sweden, and Finland are thought to have originated at least partly from glaciers' selective erosion of weathered bedrock. [22] [23]

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 increased 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 leading to the sea. Instead, stream runoff 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, with less rain, the pluvial lakes shrank to form small salt flats.

Isostatic adjustment

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 per year or less. In northern Europe, this is clearly shown by the GPS data obtained by the BIFROST GPS network. [24] 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.

Winds

The presence of ice over so much of the continents greatly modified patterns of atmospheric circulation. 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 km2 (23,166 sq mi). [25] This region was a large, active dune field during the Pleistocene epoch, but today is largely stabilized by grass cover. [26] [27]

Ocean currents

Thick glaciers were heavy enough to reach the sea bottom in several important areas, thus blocking the passage of ocean water and thereby affecting ocean currents. In addition to direct effects, this caused feedback effects as ocean currents contribute to global heat transfer.

Records of prior glaciation

500 million years of climate change.

Glaciation has been a rare event in Earth's history, [28] but there is evidence of widespread glaciation during the late Paleozoic Era (300 to 200 Ma) and the late Precambrian (i.e. the Neoproterozoic Era, 800 to 600 Ma). [29] 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. [30] 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.

One of the best documented records 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 the late Precambrian, producing the Snowball Earth during the Cryogenian Period. [31]

Next glacial period

Increase in atmospheric CO
2
since the Industrial Revolution.

The warming trend following the Last Glacial Maximum, since about 20,000 years ago, has resulted in a sea level rise by about 130 metres. This warming trend has subsided about 6,000 years ago, and sea level has been comparatively stable since the Neolithic. The present interglacial period (the Holocene) has been fairly stable and warm, but the previous one was interrupted by numerous cold spells lasting hundreds of years. If the previous period was more typical than the present one, the period of stable climate, which allowed the Neolithic Revolution and by extension human civilization, may have been possible only because of a highly unusual period of stable temperature. [32]

Based on orbital models, the cooling trend initiated about 6,000 years ago will continue for another 23,000 years. [33] Slight changes in the Earth's orbital parameters might however indicate that, even without any human contribution, there will not be another glacial period for the next 50,000 years. [34] It is possible that the current cooling trend may be interrupted by an interstadial in about 60,000 years, with the next glacial maximum reached only in about 100,000 years. [35]

In the 1970s there was some concern with the possibility of global cooling, and suggested that the next glacial period could be rapidly approaching. The previous interglacial periods seemed to have lasted about 10,000 years each; a report in 1972 assuming that the present interglacial period would be equally long concluded, "it is likely that the present-day warm epoch will terminate relatively soon if man does not intervene". [36] Since then, concern has shifted to the effect that human impact is now seen as possibly delaying what would already be an unusually long warm period. Projection of the timeline for the next glacial maximum depend crucially on the amount of CO
2
in the atmosphere
. Models assuming increased CO
2
levels at 750 parts per million ( ppm; current levels are at 407 ppm [37]) have estimated the persistence of the current interglacial period for another 50,000 years. [38]

References

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External links

The dictionary definition of glaciation at Wiktionary

Causes