Last Glacial Period

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"Last glacial" redirects here. For the period of maximum glacier extent during this time, see Last Glacial Maximum.

A chronology of climatic events of importance for the Last Glacial Period, about the last 120,000 years
The Last Glacial Period caused a much lower global sea level

The Last Glacial Period (LGP), also known colloquially as the Last Ice Age or simply Ice Age,[1] occurred from the end of the Last Interglacial to the end of the Younger Dryas, encompassing the period c. 115,000 – c. 11,700 years ago.

The LGP is part of a larger sequence of glacial and interglacial periods known as the Quaternary glaciation which started around 2,588,000 years ago and is ongoing.[2] The definition of the Quaternary as beginning 2.58 million years ago (Mya) is based on the formation of the Arctic ice cap. The Antarctic ice sheet began to form earlier, at about 34 Mya, in the mid-Cenozoic (Eocene–Oligocene extinction event). The term Late Cenozoic Ice Age is used to include this early phase.[3] The previous ice age, the Penultimate Glacial Period, which ended about 128,000 years ago, was more severe than the Last Glacial Period in some areas such as Britain, but less severe in others.

During this last glacial period, alternating episodes of glacier advance, and retreat occurred. Within the last glacial period, the

epoch
.

From the point of view of human

Denisovans and Homo erectus
in Asia.

Origin and definition

An artist's impression of the last glacial period at glacial maximum[4]

The LGP is often colloquially referred to as the "last ice age", though the term ice age is not strictly defined, and on a longer geological perspective, the last few million years could be termed a single ice age given the continual presence of ice sheets near both poles. Glacials are somewhat better defined, as colder phases during which glaciers advance, separated by relatively warm interglacials. The end of the last glacial period, which was about 10,000 years ago, is often called the end of the ice age, although extensive year-round ice persists in Antarctica and Greenland. Over the past few million years, the glacial-interglacial cycles have been "paced" by periodic variations in the Earth's orbit via Milankovitch cycles.

The LGP has been intensively studied in North America, northern Eurasia, the

Eemian
) period.

Overview

Last glacial maximum
Last glacial period, as seen in ice core data from Antarctica and Greenland

Northern Hemisphere

Scandinavian ice sheet once again reached the northern parts of the British Isles, Germany, Poland, and Russia, extending as far east as the Taymyr Peninsula in western Siberia.[7]

The maximum extent of western Siberian glaciation was reached by about 18,000 to 17,000 BP, later than in Europe (22,000–18,000 BP).[8] Northeastern Siberia was not covered by a continental-scale ice sheet.[9] Instead, large, but restricted, icefield complexes covered mountain ranges within northeast Siberia, including the Kamchatka-Koryak Mountains.[10][11]

The Arctic Ocean between the huge ice sheets of America and Eurasia was not frozen throughout, but like today, probably was covered only by relatively shallow ice, subject to seasonal changes and riddled with icebergs calving from the surrounding ice sheets. According to the sediment composition retrieved from deep-sea cores, even times of seasonally open waters must have occurred.[12]

Outside the main ice sheets, widespread glaciation occurred on the highest mountains of the

Balkan mountains, the Caucasus, and the mountains of Turkey and Iran.[13]

In the Himalayas and the Tibetan Plateau, there is evidence that glaciers advanced considerably, particularly between 47,000 and 27,000 BP,[14] but the exact ages,[15][16] as well as the formation of a single contiguous ice sheet on the Tibetan Plateau, is controversial.[17][18][19]

Other areas of the Northern Hemisphere did not bear extensive ice sheets, but local glaciers were widespread at high altitudes. Parts of

Mount Atakor massif in southern Algeria, and several mountains in Ethiopia. Just south of the equator, an ice cap of several hundred square kilometers was present on the east African mountains in the Kilimanjaro massif, Mount Kenya, and the Rwenzori Mountains, which still bear relic glaciers today.[22]

Southern Hemisphere

Glaciation of the Southern Hemisphere was less extensive. Ice sheets existed in the Andes (Patagonian Ice Sheet), where six glacier advances between 33,500 and 13,900 BP in the Chilean Andes have been reported.[23] Antarctica was entirely glaciated, much like today, but unlike today the ice sheet left no uncovered area. In mainland Australia only a very small area in the vicinity of Mount Kosciuszko was glaciated, whereas in Tasmania glaciation was more widespread.[24] An ice sheet formed in New Zealand, covering all of the Southern Alps, where at least three glacial advances can be distinguished.[25]

Local ice caps existed in the highest mountains of the island of New Guinea, where temperatures were 5 to 6 °C colder than at present.[26][27] The main areas of Papua New Guinea where glaciers developed during the LGP were the Central Cordillera, the Owen Stanley Range, and the Saruwaged Range. Mount Giluwe in the Central Cordillera had a "more or less continuous ice cap covering about 188 km2 and extending down to 3200-3500 m".[26] In Western New Guinea, remnants of these glaciers are still preserved atop Puncak Jaya and Ngga Pilimsit.[27]

Small glaciers developed in a few favorable places in Southern Africa during the last glacial period.[28][A][B] These small glaciers would have been located in the Lesotho Highlands and parts of the Drakensberg.[30][31] The development of glaciers was likely aided in part due to shade provided by adjacent cliffs.[31] Various moraines and former glacial niches have been identified in the eastern Lesotho Highlands a few kilometres west of the Great Escarpment, at altitudes greater than 3,000 m on south-facing slopes.[30] Studies suggest that the annual average temperature in the mountains of Southern Africa was about 6 °C colder than at present, in line with temperature drops estimated for Tasmania and southern Patagonia during the same time. This resulted in an environment of relatively arid periglaciation without permafrost, but with deep seasonal freezing on south-facing slopes. Periglaciation in the eastern Drakensberg and Lesotho Highlands produced solifluction deposits and blockfields; including blockstreams and stone garlands.[28][29]

Deglaciation

A temperature rise marking the end of the most recent ice age, as derived from ice core data.
Main article: Holocene glacial retreat
See also: Bølling–Allerød warming, Meltwater pulse 1A, and Deglaciation

Scientists from the Center for Arctic Gas Hydrate, Environment and Climate at the University of Tromsø, published a study in June 2017[32] describing over a hundred

methane reservoirs
could eventually have the same fate.

Named local glaciations

Antarctica

During the last glacial period, Antarctica was blanketed by a massive ice sheet, much as it is today. The ice covered all land areas and extended into the ocean onto the middle and outer continental shelf.[33][34] Counterintuitively though, according to ice modeling done in 2002, ice over central East Antarctica was generally thinner than it is today.[35]

Europe

Devensian and Midlandian glaciation (Britain and Ireland)

British

Dee (Dēva in Latin), a river on the Welsh border near which deposits from the period are particularly well represented.[36]

The effects of this glaciation can be seen in many geological features of England, Wales, Scotland, and

Ipswichian stage and lying beneath those from the following Holocene, which is the current stage. This is sometimes called the Flandrian interglacial
in Britain.

The latter part of the Devensian includes pollen zones I–IV, the Allerød oscillation and Bølling oscillation, and the Oldest Dryas, Older Dryas, and Younger Dryas cold periods.

Weichselian glaciation (Scandinavia and northern Europe)

Main article: Weichselian glaciation
Europe during the last glacial period

Alternative names include Weichsel glaciation or Vistulian glaciation (referring to the Polish River

isotope stages is still in process.[37][38] During the glacial maximum in Scandinavia, only the western parts of Jutland were ice-free, and a large part of what is today the North Sea was dry land connecting Jutland with Britain (see Doggerland
).

The Baltic Sea, with its unique brackish water, is a result of meltwater from the Weichsel glaciation combining with saltwater from the North Sea when the straits between Sweden and Denmark opened. Initially, when the ice began melting about 10,300 BP, seawater filled the isostatically depressed area, a temporary marine incursion that geologists dub the Yoldia Sea. Then, as postglacial isostatic rebound lifted the region about 9500 BP, the deepest basin of the Baltic became a freshwater lake, in palaeological contexts referred to as Ancylus Lake, which is identifiable in the freshwater fauna found in sediment cores.

The lake was filled by glacial runoff, but as worldwide sea level continued rising, saltwater again breached the sill about 8000 BP, forming a marine Littorina Sea, which was followed by another freshwater phase before the present brackish marine system was established. "At its present state of development, the marine life of the Baltic Sea is less than about 4000 years old", Drs. Thulin and Andrushaitis remarked when reviewing these sequences in 2003.

Overlying ice had exerted pressure on the Earth's surface. As a result of melting ice, the land has continued to rise yearly in Scandinavia, mostly in northern Sweden and Finland, where the land is rising at a rate of as much as 8–9 mm per year, or 1 m in 100 years. This is important for archaeologists, since a site that was coastal in the Nordic Stone Age now is inland and can be dated by its relative distance from the present shore.

Würm glaciation (Alps)

Main article: Würm glaciation
Violet: the extent of the Alpine ice sheet in the Würm glaciation. Blue: extent in earlier ice ages.

The term Würm is derived from a river in the Alpine foreland, roughly marking the maximum glacier advance of this particular glacial period. The Alps were where the first systematic scientific research on ice ages was conducted by Louis Agassiz at the beginning of the 19th century. Here, the Würm glaciation of the LGP was intensively studied. Pollen analysis, the statistical analyses of microfossilized plant pollens found in geological deposits, chronicled the dramatic changes in the European environment during the Würm glaciation. During the height of Würm glaciation, c. 24,000 – c. 10,000 BP, most of western and central Europe and Eurasia was open steppe-tundra, while the Alps presented solid ice fields and montane glaciers. Scandinavia and much of Britain were under ice.

During the Würm, the Rhône Glacier covered the whole western Swiss plateau, reaching today's regions of Solothurn and Aargau. In the region of Bern, it merged with the Aar glacier. The Rhine Glacier is currently the subject of the most detailed studies. Glaciers of the Reuss and the Limmat advanced sometimes as far as the Jura. Montane and piedmont glaciers formed the land by grinding away virtually all traces of the older Günz and Mindel glaciation, by depositing base moraines and terminal moraines of different retraction phases and loess deposits, and by the proglacial rivers' shifting and redepositing gravels. Beneath the surface, they had profound and lasting influence on geothermal heat and the patterns of deep groundwater flow.

North America

Pinedale or Fraser glaciation (Rocky Mountains)

Bonneville Flood along the Snake River

The Pinedale (central Rocky Mountains) or Fraser (Cordilleran ice sheet) glaciation was the last of the major glaciations to appear in the Rocky Mountains in the United States. The Pinedale lasted from around 30,000 to 10,000 years ago, and was at its greatest extent between 23,500 and 21,000 years ago.[39] This glaciation was somewhat distinct from the main Wisconsin glaciation, as it was only loosely related to the giant ice sheets and was instead composed of mountain glaciers, merging into the Cordilleran ice sheet.[40]

The Cordilleran ice sheet produced features such as

USGS geologists estimate that the cycle of flooding and reformation of the lake lasted an average of 55 years and that the floods occurred about 40 times over the 2,000-year period starting 15,000 years ago.[41] Glacial lake outburst floods such as these are not uncommon today in Iceland
and other places.

Wisconsin glaciation

The

Bering land bridge potentially permitted migration of mammals, including people, to North America from Siberia
.

It radically altered the geography of North America north of the

grooves left by these glaciers can be easily observed. In southwestern Saskatchewan and southeastern Alberta, a suture zone between the Laurentide and Cordilleran ice sheets formed the Cypress Hills
, which is the northernmost point in North America that remained south of the continental ice sheets.

The Great Lakes are the result of glacial scour and pooling of meltwater at the rim of the receding ice. When the enormous mass of the continental ice sheet retreated, the Great Lakes began gradually moving south due to isostatic rebound of the north shore. Niagara Falls is also a product of the glaciation, as is the course of the Ohio River, which largely supplanted the prior Teays River.

With the assistance of several very broad glacial lakes, it released floods through the

gorge of the Upper Mississippi River
, which in turn was formed during an earlier glacial period.

In its retreat, the Wisconsin episode glaciation left

Connecticut River Valley
.

Tahoe, Tenaya, and Tioga, Sierra Nevada

In the

ice ages, were separated by warmer periods. These glacial maxima are called, from oldest to youngest, Tahoe, Tenaya, and Tioga.[42] The Tahoe reached its maximum extent perhaps about 70,000 years ago. Little is known about the Tenaya. The Tioga was the least severe and last of the Wisconsin episode. It began about 30,000 years ago, reached its greatest advance 21,000 years ago, and ended about 10,000 years ago.[citation needed
]

Greenland glaciation

In northwest Greenland, ice coverage attained a very early maximum in the LGP around 114,000. After this early maximum, ice coverage was similar to today until the end of the last glacial period. Towards the end, glaciers advanced once more before retreating to their present extent.[43] According to ice core data, the Greenland climate was dry during the LGP, with precipitation reaching perhaps only 20% of today's value.[44]

South America

Mérida glaciation (Venezuelan Andes)

A map showing the extent of the glaciated area in Venezuelan Andes during the Mérida glaciation

The name Mérida glaciation is proposed to designate the alpine glaciation that affected the central Venezuelan Andes during the Late Pleistocene. Two main moraine levels have been recognized - one with an elevation of 2,600–2,700 m (8,500–8,900 ft), and another with an elevation of 3,000–3,500 m (9,800–11,500 ft). The snow line during the last glacial advance was lowered approximately 1,200 m (3,900 ft) below the present snow line, which is 3,700 m (12,100 ft). The glaciated area in the Cordillera de Mérida was about 600 km2 (230 sq mi); this included these high areas, from southwest to northeast: Páramo de Tamá, Páramo Batallón, Páramo Los Conejos, Páramo Piedras Blancas, and Teta de Niquitao. Around 200 km2 (77 sq mi) of the total glaciated area was in the Sierra Nevada de Mérida, and of that amount, the largest concentration, 50 km2 (19 sq mi), was in the areas of Pico Bolívar, Pico Humboldt [4,942 m (16,214 ft)], and Pico Bonpland [4,983 m (16,348 ft)]. Radiocarbon dating indicates that the moraines are older than 10,000 BP, and probably older than 13,000 BP. The lower moraine level probably corresponds to the main Wisconsin glacial advance. The upper level probably represents the last glacial advance (Late Wisconsin).[45][46][47][48][49]

Llanquihue glaciation (Southern Andes)

Main article: Llanquihue glaciation
A map showing the extent of the Patagonian ice sheet in the Strait of Magellan area during the LGP: Selected modern settlements are shown with yellow dots.
Modelled maximum extent of the Antarctic ice sheet, 21,000 years before the present

The Llanquihue glaciation takes its name from Llanquihue Lake in southern Chile, which is a fan-shaped piedmont glacial lake. On the lake's western shores, large moraine systems occur, of which the innermost belong to the LGP. Llanquihue Lake's varves are a node point in southern Chile's varve geochronology. During the last glacial maximum, the Patagonian ice sheet extended over the Andes from about 35°S to Tierra del Fuego at 55°S. The western part appears to have been very active, with wet basal conditions, while the eastern part was cold-based.[50]

Cryogenic features such as

Valdivian temperate rain forest was reduced to scattered remnants on the western side of the Andes.[51]

See also

Historical names of the "four major" glacials in four regions
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 U.S. Nebraskan Kansan Illinoian
Wisconsinan

Notes

  1. ^ Prior to the 2010s, considerable debate arose on whether Southern Africa was glaciated during the last glacial cycle or not.[28][29]
  2. periglacial regolith that has not been reworked by glaciers.[29] Estimates of the mean annual temperature in Southern Africa during the last glacial maximum indicate the temperatures were not low enough to initiate or sustain a widespread glaciation. The former existence of rock glaciers or large glaciers is, according to the same study, ruled out, because of a lack of conclusive field evidence and the implausibility of the 10–17 °C temperature drop, relative to the present, that such features would imply.[28]

References

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  2. ^ Clayton, Lee; Attig, John W.; Mickelson, David M.; Johnson, Mark D.; Syverson, Kent M. "Glaciation of Wisconsin" (PDF). Dept. Geology, University of Wisconsin.
  3. ^ University of Houston–Clear Lake – Disasters Class Notes – Chapter 12: Climate Change sce.uhcl.edu/Pitts/disastersclassnotes/chapter_12_Climate_Change.doc
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  8. ^ Matti Saarnisto: Climate variability during the last interglacial-glacial cycle in NW Eurasia. Abstracts of PAGES – PEPIII: Past Climate Variability Through Europe and Africa, 2001 Archived April 6, 2008, at the Wayback Machine
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  13. ^ Williams, Richard S. Jr.; Ferrigno, Jane G. (1991). "Glaciers of the Middle East and Africa – Glaciers of Turkey" (PDF 2.5 Mb). U.S.Geological Survey Professional Paper 1386-G-1.
    Ferrigno, Jane G. (1991). "Glaciers of the Middle East and Africa – Glaciers of Iran" (PDF 1.25 Mb). U.S.Geological Survey Professional Paper 1386-G-2.
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  43. ^ Funder, Svend"Late Quaternary stratigraphy and glaciology in the Thule area, Northwest Greenland". MoG Geoscience. 22: 63. 1990. Archived from the original on June 6, 2007.
  44. ^ Johnsen, Sigfus J.; et al. (1992). "A "deep" ice core from East Greenland". MoG Geoscience. 29: 22. Archived from the original on June 6, 2007.
  45. doi:10.13140/RG.2.2.21325.38886/1
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  46. ^ Schubert, Carlos (1998). "Glaciers of Venezuela". US Geological Survey (USGS P 1386-I).
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  49. ^ Maximiliano, B.; Orlando, G.; Juan, C.; Ciro, S. "Glacial Quaternary geology of las Gonzales basin, páramo los conejos, Venezuelan andes".
  50. ^
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  51. ^ Adams, Jonathan. "South America during the last 150,000 years". Archived from the original on January 30, 2010.

Further reading

External links
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