Last glacial maximum

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The earth at the last glacial maximum. Artist's impression

As last glacial maximum ( Engl. Last Glacial Maximum or abbreviated LGM ), the climate history section called, in which the glaciations of the last ice age (in Europe, the Vistula and Würm glaciation ) took their maximum extent. In the period from 24,500 to 18,000 BC Chr. Covered huge ice sheets large areas of North America , northern Europe and Asia . Their widespread distribution had profound effects on the earth's climate - for example, drop in temperature , aridity , desertification and a drop in global sea ​​levels .

Correlations

The last glacial maximum correlates temporally with the beginning of marine isotope stage 2 (MIS 2 - 25,600 to 12,100 BC). The Heinrich event H2 (around 22,000 BC), the Dansgaard-Oeschger event DO2 (around 21,300 BC), the Brandenburg phase and the Frankfurt phase of the Fennoscan Ice Sheet fall within its course . In the Alps, the Schlieren stage prevailed on the Rheingletscher (already a retreat stage) and on the Inn Glacier the Kirchseeon stage (ice edge). The main advance of the Laurentide Ice Sheet was the Shelby Phase on Lake Michigan , which is dated from 21,561 to 21,151.

Ice age maximum of the last two glacial periods in Europe: Weichsel / Würm complex and Saale / Riß complex. The glacier advances were interrupted by warmer periods during which the archaic people of Europe (the Neanderthals as the successor to Homo heidelbergensis ) spread beyond the permafrost limit to the north and northeast. From around 40,000 BC. The modern Cro-Magnon humans colonized these areas.

Development over time

Climate and postglacial expansion, here in the Middle East , over a longer period of time, 20 to 0 BP
The temporal position of the Last Ice Age Maximum (LGM) during the last 47,500 years

Using environmental parameters, the period of the last ice age maximum can be limited quite well. Generally speaking, the period can be thought to be 24,500 to 18,000 years BC. BC, with a maximum at 19,000 BC. The growth of the ice masses to their maximum extent in the previous period 31,000 to 24,500 BC. Chr. (English Ice build-up ) took place in response to a reduced radiation intensity during the summer in the northern hemisphere, to reduced surface temperatures in the Pacific and to a decrease in greenhouse gases, especially carbon dioxide. The minimum in the radiation intensity at 65 ° north latitude with 440 W / m² was around 22,000 BC. Run through. The Eiszerfall (Engl. Ice decay ) then began v 18000 to 17000. To become effective - the trigger was the now noticeable increase in radiation intensity during the northern summer, which led to a sudden rise in sea level . The collapse of the West Antarctic Ice Sheet was delayed between 13,000 and 12,000 BC. And caused an abrupt rise in sea level around 12,500 BC with the meltwater pulse 1A . Chr.

Environmental parameters

Greenhouse gases

The greenhouse gases had consistently minimum values ​​during the last ice age maximum. For example, carbon dioxide was 180 to 190 ppm , then from 16,000 BC. To rise steeply in its atmospheric concentration. The same behavior also shows methane , which showed 350 to 375 ppb (with a minimum at 17,500 BC) and then from 15,000 BC. Chr. Showed gradual increases in concentration (for comparison: the carbon dioxide values ​​at the beginning of the Holocene were already 270 ppm, the methane values ​​700 ppb). The nitrous oxide began to rise from its minimum value of 200 ppb only after 14,000 BC. Chr.

Oxygen isotopes

δ 2 H values ​​from Antarctica and δ 18 O values ​​from Greenland

Various series of measurements are available for the δ 18 O values , but they all show very similar behavior throughout. The Byrd ice core from Antarctica had a clear minimum of −44 ‰ SMOW centered around 19,000 BC. BC (with a clear trough between 20,000 and 17,000 BC), from 16,000 BC Then began the steady increase to modern values ​​(-34 ‰ SMOW at the beginning of the Holocene). The conditions in the ice cores of Greenland are not that clear - they had their minimum around 20,000 BC. BC ( GISP 2 at −42 and GRIP at −44 ‰ SMOW), the increase to the Holocene was not steady, but showed a relapse in the interval 15,000 to 12,000 BC. BC and the very clear setback of the Younger Dryas around 11,000 BC. A drill core from the Pacific has maxima (reverse ratios due to PDB values) at 24,000 and 16,000 years BC. With a plateau in between, which very nicely underlines the duration of the last Ice Age maximum (the steady decline to modern values ​​began from 16,000 BC).

The values ​​determined for atmospheric oxygen have a mirror-inverted behavior, they run through from 22,000 BC. A maximum with a peak value of 1.1 ‰ around 17,000 BC. And then decay from 13,000 BC. Quickly to a minimum of −0.4 ‰ in the Holocene.

On benthic organisms , data obtained show to 22,000 v. A steady increase in values ​​from 4.5 to 4.8 ‰ (benthos also behaves in the opposite way) with an interim minimum at 23,000 BC. Chr.

deuterium

The δ 2 H values , obtained from ice cores in the Antarctic (Vostok), run through from around 24,000 BC. Their trough averaged −486 ‰ with an absolute minimum of −490 ‰ at 22,500 BC. From 15,000 BC. They then rise steeply - with an interim decline during the Bölling and Alleröd interstadials - and reach just under −420 ‰ at the beginning of the Holocene. The results from EPICA Dom C (also Antarctica) run roughly parallel to this, but are around 40 ‰ higher.

carbon

The carbon isotopes demonstrate a very different behavior compared to the other environmental parameters. The δ 13 C values ​​maintain their high plateau of −6.45 from 22,000 to 15,500 BC. BC, but then they decay quite suddenly by around 0.3 ‰ to −6.7 ‰. Only at the beginning of the Holocene do they rise again and in the last 6000 years of the Holocene they reach a maximum of −6.35 ‰. At 600 ‰, the Δ 14 C values ​​are highest at the beginning of the last glacial maximum, and then steadily decrease to today's minimum of 0 ‰. This process was only interrupted by a brief, slight rise again during the Younger Dryas.

Temperatures

Temperature proxies for the past 40,000 years

For the high latitudes of the northern hemisphere, proxy data for the last glacial maximum show a very strong cooling with simultaneous southward shift of the boreal coniferous forest belt, which was also subject to a significant reduction in area. For example, an enormous cooling of 21 ± 2 ° C was determined for Greenland.

The high latitudes in the southern hemisphere were also significantly colder, with East Antarctica having temperatures 9 ± 2 ° C lower.

Drill cores from the Ocean Drilling Program from the Atlantic point to much colder and salty deep waters during the last Ice Age maximum.

For the annual average temperatures, Otto-Bliesner and Brady (2005) were able to use a new model calculation to determine a global cooling of 4.5 ° C compared to pre-industrial values ​​(before 1850) (or 5.4 ° C compared to today's values). For the tropical sea ​​surface temperatures, they calculated a decrease of 1.7 ° C (or 2.6 ° C) on average, for the tropical land masses a decrease of 2.6 ° C (or 3.5 ° C compared to today) . Overall, the highest temperature changes concerned the high latitudes due to the positive feedback from ice masses and snow cover (= polar amplification ). The subtropics recorded the smallest changes due to the negative feedback triggered by low-lying cloud cover .

Globally calculated Lunt u. a. (2006) a decrease of 4 ° C (from 13.2 to 9.2 ° C) for the average sea surface temperatures, for the deep waters (at 5000 meters water depth) a decrease of 1.6 ° C (from 1.3 to - 0.3 ° C). The temperature trough is situated between 22,000 and 16,000 BC. Chr.

Ice cover

The volume of ice cover rose from a relative minimum of 12 million cubic kilometers by 27,000 years BC. BC relatively quickly to a first maximum value of around 45 million cubic kilometers around 22,000 BC. And then to its absolute maximum of 53 million cubic kilometers around 18,000 BC. The following ice collapse led to another, intermittent peak value of 43 million cubic kilometers around 14,000 BC. At a level of around 10 million cubic kilometers at the beginning of the Holocene.

The pack ice cover was also much more extensive during the last ice age maximum, but more seasonal. It reached around 12% of the total ocean area, at the beginning of the Holocene this proportion was only 7% (for comparison, today's value of around 5%). The Antarctic pack ice carried out large, seasonal drift movements around the southern continent.

Sea level

The rise in sea levels since 22,000 BC Chr.

The low of the sea level was around 24,000 BC. Reached. In their model, based on coral data from Barbados , Peltier and Fairbanks (2006) calculate a subsidence of 118.7 meters for this point in time. Waelbroeck u. a. (2002) determined a comparable value of around 120 meters, which, according to them, was only realized later (around 18,000 BC). Lambeck and Chappell (2001) found even deeper subsidence using coral measurements, for example 145 meters for Barbados and 140 meters for the Joseph Bonaparte Gulf (both around 20,000 BC). From 14000 BC The rapid rise from 110 meters below sea ​​level to today's sea level began.

Climatological characterization

Map of the CLIMAP project with sea surface temperature changes and the extent of the ice masses during the last glacial maximum

For the formation of ice sheets or ice caps , a long-lasting drop in temperature is required with simultaneously increased precipitation (in the form of snow ). However, East Asia remained ice-free up to high altitudes, although the temperatures were quite comparable with those of the glaciated areas of North America and Europe. The reason for this was the ice sheets of Europe, which led to the formation of very extensive anticyclones with extremely dry air masses over their area , so that the downwind Siberia and Manchuria could only receive very little rainfall (one exception was Kamchatka , here westerly winds brought enough Moisture from the Sea of Japan ). Other factors that prevented continental glaciation in Asia were the relative warming of the Pacific Ocean due to the absence of the Oyashio current and the general west-east direction of the mountain ranges.

Possible distribution of the vegetation zones during the last glacial maximum

Even in warmer climatic regions of the world, the temperature and precipitation values ​​were lower during the last glacial maximum. Extreme conditions prevailed in South Australia and the Sahel with up to 90% less rainfall than today and with catastrophic effects on the local flora. The rainforest belts were not so badly affected, but here too there was a significant reduction in the tree population. In West Africa in particular , the tropical jungle only existed in individual refuges surrounded by grasslands. The Amazon rainforest was split into two large areas by a wide savannah . The rainforests of Southeast Asia are likely to have been affected in a similar way, here more and more deciduous forests spread at the expense of the rainforest, which could only hold on to the east and west end of the Sunda shelf . Only in Central America and in Chocó Colombia did the rainforest remain largely intact due to the high rainfall there.

Most of the desert belts expanded. Exceptions were the western United States , which recorded heavy rainfalls due to the shift of the jet stream in the current desert areas, so that large inland lakes such as Lake Bonneville in Utah could form, but also Afghanistan and Iran (here an inland lake was created in the Dasht-e -Kavir ). In Australia , wandering sand dunes covered half the continent, and in South America the Pampas and Gran Chaco fell to the drought. Today's subtropical areas such as eastern Australia , the Atlantic forests of Brazil and southern China lost a large part of their closed forests due to the drought, which gave way to open forest landscapes. In northern China , which remained unglaciated despite its cold climate, a mixture of tundra and open grassland established itself , and the tree cover shifted southward by at least 20 degrees of latitude.

Many of the areas that fell into desertification during the last Ice Age maximum had higher rainfall than today. An example is South Australia, the settlement of which by Aborigines in the period 60,000 to 40,000 BP was evidently associated with a period of damp.

Global impact

During the last ice age maximum, cold, dry and unfriendly climatic conditions prevailed on a large part of the earth; the atmosphere was often stormy and dust-laden , clearly recognizable in ice cores , which have a 20 to 25-fold increase in the dust load compared to today's level. This increased dust load can probably be attributed to several factors:

  • reduced vegetation
  • increased wind speeds
  • less dust-binding precipitation

In addition, the lower sea level has now exposed continental shelves and created new coastal plains.

Europe

The Fennoscan ice sheet largely covered Northern Europe, including Spitsbergen , Franz Josef Land , the shelves of the Barents Sea , the Kara Sea , Novaya Zemlya and parts of the Taimyr Peninsula . The whole of Iceland was covered by ice sheets and in Great Britain only the south of England remained ice-free. The north of mainland Great Britain lay under an ice cap that was possibly connected to the Fennoscan ice sheet through the dry North Sea ; the ice-free south was a cold desert. The southern edge of the ice ran through northeast Germany and Poland . To the south of it, permafrost had z. B. expanded to Szeged in southern Hungary .

Asia

Large parts of present-day Tibet , Baltistan and Ladakhs were icy during the Last Ice Age maximum (the extent of glaciation of the Tibetan Plateau is still controversial among scientists). Many smaller mountain glaciers formed even in Southeast Asia . The southern limit of the permafrost reached Beijing . Because of the sharp drop in sea level, there was a connection between today's islands and the mainland. For example, the Indonesian chain of islands as far as Borneo and Bali was connected to the Asian continent as Sundaland . This also included Palawan , whereas the Philippines were united into a single island, which was only separated from the mainland by the Sibutu Passage and the Mindoro Strait .

Africa and Middle East

In Africa and the Middle East , several smaller mountain glaciers formed. Sandy deserts such as the Sahara expanded very strongly.

The Persian Gulf is on average only 35 meters deep, between Abu Dhabi and Qatar usually only 15 meters. The Ur-Shatt (a confluence of the Euphrates and Tigris rivers) flowed through the Strait of Hormuz into the Gulf of Oman for thousands of years , bringing fresh water to the Persian Gulf. Bathymetric data suggest two paleo basins in the Persian Gulf, with the central basin having an area of ​​20,000 km² and roughly the length of Lake Malawi . Between 10,000 and 7000 BC The main part of the Persian Gulf had finally fallen dry. Not until 6000 BC Then the sea transgressed again into the Gulf region.

Australasia

Because of the low sea level, Australia , New Guinea, and Tasmania were united into a single large land mass known as the Sahul . Wallacea , which consists of several islands, pushed itself between the Southeast Asian continent and the Sahulland, considerably reducing the number and width of the separating waterways.

North America

In North America, essentially all of Canada was covered by ice, with the Laurentide Ice Sheet stretching in the west to the Missouri River , in the center to the Ohio River and in the east to Manhattan . The west coast of North America carried the Cordillera Ice Sheet , in Canada and Montana alpine glaciers advanced and the Rocky Mountains were in places covered by ice caps. The temperature gradients in north-south direction were so pronounced that the permafrost, apart from high altitudes, penetrated only insignificantly further south compared to the ice sheet. The last ice age maximum freezing forced the groups of people who originally immigrated from Northeast Siberia into refugia areas and thereby changed their gene pool through mutations and gene drift . This phenomenon established the older haplogroups within the Native Americans, whereas later migrations are responsible for the northern haplogroups.

In Hawaii , glacier deposits on Mauna Kea have been known for a long time. New investigations suggest that between 200,000 and 150,000 years ago, three different stages have been preserved on the volcano. Moraines formed around 70,000 years ago and between 40,000 and 13,000 years ago. Possible glacial deposits on Mauna Loa have now been covered by young lava flows .

South America

In South America , the Patagonian Ice Sheet covered the entire southern third of Chile with the neighboring areas of Argentina . On the west side of the Andes , the ice sheet reached the Pacific at 41 degrees south latitude in the Chacao Canal . Most of the west coast of Patagonia was also glaciated, but there may have been some plant refuge. On the east side of the Andes there were glacier lobes in the lowlands around Seno Skyring and Seno Otway , in the Bahía Inútil and in the Beagle Channel . On the Strait of Magellan , the ice reached as far as the Segunda Angostura .

Volcanic eruptions

Direct influence of the sun's intensity on the environmental parameters

In addition to the unchangeable astronomical conditions (solar activity controlled by Milanković cycles ) volcanic eruptions may well have had an additional, not insignificant influence on the climate development during the last ice age maximum. For example, the super eruption of the Oruanui falls in the Taupo Volcanic Zone of New Zealand , dated 24,000 and 20,600 years BC. In the period of the LGM. During this eruption, at least 1,170 km³ of ejecta was produced, almost a quarter of the eruption volume of the La Garita Caldera , one of the largest events ever known. During the LGM, the Eltviller Tuff in the Vulkaneifel , which Zöller u. a. (1987) to 20,000 to 19,000 years BC Is dated. It represents an important stratigraphic marker horizon in the loess sediments of Central Europe.

Cultural development

During the last Ice Age maximum, after Gravettia (from 31,000 BC), the Upper Paleolithic culture stage of the Solutréens (22,000 to 16,500 BC) with its characteristic leaf , notch and laurel leaf tips developed in southwestern Europe . Cultural innovations are boomerang , perforated rods , textiles and needles made from bone for sewing fur clothing. Fired clay figurines appear for the first time as art objects. The Solutréen is then replaced by the Magdalenian .

See also

Individual evidence

  1. ↑ Representation based on Thomas J. Crowley: Ice age terrestrial carbon changes revisited . In: Global Biogeochemical Cycles . 9, No. 3, 1995, pp. 377-389, doi: 10.1029 / 95GB01107 .
  2. ↑ Peter U. Clark, Arthur S. Dyke, Jeremy D. Shakun, Anders E. Carlson, Jorie Clark, Barbara Wohlfarth, Jerry X. Mitrovica, Steven W. Hostetler, A. Marshall McCabe: The Last Glacial Maximum . In: Science . tape 325 , no. 5941 , 2009, p. 710-714 .
  3. ↑ Mithen, Steven: After the Ice: a global human history, 20,000-5,000 BC . Harvard University Press, Cambridge MA 2004, ISBN 0-674-01570-3 , pp. 3 .
  4. ^ SR Hemming: Heinrich events: massive late Pleistocene detritus layers of the North Atlantic and their global climate imprint . In: Rev. Geophys. tape 42 , no. 1 , 2004, doi : 10.1029 / 2003RG000128 .
  5. ↑ Curry, BB a. a .: The DeKalb Mounds of northeastern Illinois: archives of deglacial history and postglacial environments . In: Quaternary Research . tape 74 , 2010, pp. 82-90 .
  6. ↑ Before Present is an age given to English before present "before today" and is used for uncalibrated 14 C data needed
  7. ↑ Intergovernmental Panel on Climate Change: Climate Change 2007: Working Group I: The Physical Science Basis. 6.4.1.2 What Do the Last Glacial Maximum and the Last Deglaciation Show? Ed .: IPCC Fourth Assessment Report: Climate Change 2007. 2007.
  8. ^ Peter U. Clark et al.: The Last Glacial Maximum . In: Science . tape 325 , no. 5941 , July 8, 2009, p. 710-714 , doi : 10.1126 / science.1172873 , PMID 19661421 .
  9. ↑ Blunier, T. u. a .: Synchronization of ice core records via atmospheric gases . In: Climate of the Past . tape 3 , 2007, p. 325-330 .
  10. ↑ Liesicki, LE and Raymo, ME: A Pliocene-Pleistocene stack of 57 globally distributed benthic d18O records . In: Paleoceanography . tape 20 , 2005.
  11. ↑ Schmitt, J. u. a .: Carbon Isotope Constraints on the Deglacial CO 2 Rise from Ice Cores . In: Science . tape 336 , 2012, p. 711-714 .
  12. ↑ Bigelow, NH et al. a .: Climate change and Arctic ecosystems: 1. Vegetation changes north of 55 ° N between the last glacial maximum, mid-Holocene, and present . In: Journal of Geophysical Research . tape 108 , 2003, p. 8170 , doi : 10.1029 / 2002JD002558 .
  13. ↑ D. Dahl-Jensen et al.: Past Temperatures Directly from the Greenland Ice Sheet . In: Science . tape 282 , no. 5387 , September 10, 1998, pp. 268-271 , doi : 10.1126 / science.282.5387.268 , PMID 9765146 .
  14. ↑ Barbara Stenni and others: An Oceanic Cold Reversal During the Last Deglaciation . In: Science . tape 293 , no. 5537 , September 14, 2001, p. 2074-2077 , doi : 10.1126 / science.1059702 , PMID 11557889 .
  15. ↑ Jess F. Adkins, Katherine McIntyre, Daniel P. Schrag: The Salinity, Temperature, and δ18O of the Glacial Deep Ocean . In: Science . tape 298 , no. 5599 , November 29, 2002, pp. 1769-1773 , doi : 10.1126 / science.1076252 , PMID 12459585 .
  16. ↑ Bette L. Otto-Bliesner, Esther C. Brady, Gabriel Clauzet, Robert Tomas, Samuel Levis, Zav Kothavala: Last Glacial Maximum and Holocene Climate in CCSM3 . In: Journal of Climate . tape 19 , no. 11 , June 1, 2006, p. 2526-2544 , doi : 10.1175 / JCLI3748.1 .
  17. ^ A b D. J. Lunt, MS Williamson, PJ Valdes, TM Lenton, R. Marsh: Comparing transient, accelerated, and equilibrium simulations of the last 30,000 years with the GENIE-1 model . In: Clim. Past . tape 2 , no. 2 , November 28, 2006, p. 221-235 , doi : 10.5194 / cp-2-221-2006 .
  18. ↑ A. de Vernal et al .: Reconstruction of sea-surface conditions at middle to high latitudes of the Northern Hemisphere during the Last Glacial Maximum (LGM) based on dinoflagellate cyst assemblages . In: Quaternary Science Reviews . tape 24 , no. 7–9 , April 2005, pp. 897-924 , doi : 10.1016 / j.quascirev.2004.06.014 .
  19. ^ Rainer Gersonde, Xavier Crosta, Andrea Abelmann, Leanne Armand: Sea-surface temperature and sea ice distribution of the Southern Ocean at the EPILOG Last Glacial Maximum — a circum-Antarctic view based on siliceous microfossil records . In: Quaternary Science Reviews . tape 24 , no. 7–9 , April 2005, pp. 869-896 , doi : 10.1016 / j.quascirev.2004.07.015 .
  20. ↑ WR Peltier, RG Fairbanks: Global glacial ice volume and Last Glacial Maximum duration from an extended Barbados sea level record . In: Quaternary Science Reviews . tape 25 , no. 23–24 , December 2006, pp. 3322-3337 , doi : 10.1016 / j.quascirev.2006.04.010 .
  21. ↑ C. Waelbroeck et al: Sea-level and deep water temperature changes derived from benthic foraminifera isotopic records . In: Quaternary Science Reviews . tape 21 , no. 1–3 , January 2002, pp. 295-305 , doi : 10.1016 / S0277-3791 (01) 00101-9 .
  22. ^ Kurt Lambeck, John Chappell: Sea Level Change Through the Last Glacial Cycle . In: Science . tape 292 , no. 5517 , April 27, 2001, p. 679-686 , doi : 10.1126 / science.1059549 , PMID 11326090 .
  23. ↑ Jan Mangerud et al .: Ice-dammed lakes and rerouting of the drainage of northern Eurasia during the Last Glaciation . In: Quaternary Science Reviews . tape 23 , no. 11–13 , June 2004, pp. 1313-1332 , doi : 10.1016 / j.quascirev.2003.12.009 .
  24. ↑ E. Sathiamurthy, HK Voris: Pleistocene Sea Level Maps for the Sunda Shelf . Ed .: The Field Museum. Chicago, IL 2006.
  25. ^ Ugo A. Perego et al .: The initial peopling of the Americas: A growing number of founding mitochondrial genomes from Beringia . In: Genome Research . tape 20 , no. 9 , January 9, 2010, p. 1174–1179 , doi : 10.1101 / gr.109231.110 , PMID 20587512 .
  26. ↑ Jorge Rabassa et al.: Quaternary of Tierra del Fuego, Southernmost South America: an updated review . In: Quaternary International . tape 68-71 , June 2000, pp. 217-240 , doi : 10.1016 / S1040-6182 (00) 00046-X .