Last glacial period
The last cold period , also called the last glacial (or, somewhat ambiguously, the last ice age ) followed in the Young Pleistocene following the last warm period before the present day. It began about 115,000 years ago and ended with the beginning of the Holocene about 11,700 years ago. In the last glacial period, as in the previous glacial periods, there was a cooling of the climate all over the world, extensive glaciations , widespread flooding and a drop in sea level with the formation of land bridges .
The term ice age is easy to confuse with that of the ice age and should therefore be avoided.
Ice ages are longer phases in the earth's history in which the polar regions of the earth are glaciated, as is currently the case. The Cenozoic Ice Age is currently ruling .
Shorter climatic fluctuations within ice ages are called cold periods and warm periods .
- Warm periods within an ice age, in which continental ice sheets completely withdraw from the temperate latitudes , are also called interglacials . One such interglacial, the Holocene, currently prevails .
- Cold periods with extensive glaciations of the continents in the temperate latitudes are also called glacials . In common language they are often called Ice Ages .
- The end of the last glacial period or the last glacial thus marked the beginning of the present warm period or the present interglacial within the still ongoing Ice Age.
The last cold period lasted around 100,000 years, within this period there were again brief warm phases ( interstadials ) between cold phases ( stadials ). The glaciers repeatedly advanced and retreated several times, and the flora and fauna followed the fluctuations accordingly. Many species that could not survive in polar and boreal climates temporarily found new habitats in refuges in warmer regions. The last glacial maximum ( Last Glacial Maximum , LGM) was around 21,000 to 18,000 years ago. Although the temporal courses of temperatures and glaciations are similar worldwide, there are differences in the details from continent to continent.
Large areas of the earth are still shaped today by the consequences of the glaciations of this cold period.
Geologists traditionally work on a regional basis and therefore do not designate cold periods as global climatic and time periods, but in relation to a specific region in which they can be detected. This is particularly the case for the last glacial period. The cold period therefore has different names in different regions of the world. In the Alpine region it is called Würm - in North and Central Europe as the Vistula - in Eastern Europe as Waldai- in Siberia as Zyryanka- , to the British Isles as Devensian- in Ireland as Midlandian- in North America as Fraser , Pinedale - , Wisconsin or Wisconsinan , in Venezuela as Mérida , in Chile as Llanquihue and in New Zealand as Otira . The respective regional manifestations of the Cold Age are individually defined and dated accordingly and are also subdivided into individual subsections as well as stadials and interstadials.
If the end of the Pleistocene or the beginning of the Holocene is equated with the end of the last glacial period, then it is around 11,700 years b2k (before the reference year 2000), with an uncertainty of 99 years, based on the stratigraphic reference profile for the lower limit of the Holocene .
Global temperatures fell in the last cold period by several Kelvin compared to the Eem warm period before that. It is assumed that the cooling was stronger in high latitudes than near the equator . At the same time, the climate became drier because the amount of precipitation decreases when less water evaporates in the cold.
In the foothills of the Alps , the mean annual temperatures during the Würm glacial period were around 10 K colder than today. The global average temperature in the LGM was about 5 to 6 K lower than today.
Due to the gas inclusions in polar ice, it is known that the atmospheric concentration of the greenhouse gases carbon dioxide (CO 2 ) 70% and methane (CH 4 ) 50% of the pre-industrial value (CO 2 in the LGM: 200 ppmv , pre-industrial: 288 ppmv, today ( 2017): 405 ppmv; CH 4 in the LGM: 350 ppbv , pre-industrial: 750 ppbv, today: 1850 ppbv).
The warm and cold periods are defined differently than the isotope stages according to the marine oxygen isotope stratigraphy (MIS). Therefore, the beginning of the last glacial period falls in the middle of the heat isotope stage "MIS 5". This was followed by the cold isotope stage "MIS 4", the beginning of which is dated to approx. 71,000 years ago (according to Aitken & Stokes) or 74,000 years ago (according to Martinson et al.). Then the climate warmed up again slightly ("MIS 3", beginning approx. 60,000 years ago); however, this phase was not warm enough to be considered a warm period. Finally, an even stronger cooling followed (“MIS 2”, beginning approx. 24,000 years ago), in which the last glacial maximum then lies. The temperature rise at the end of the last glacial period was much faster.
Abrupt climatic fluctuations
Various abrupt climatic fluctuations can be detected during the last glacial period. There are various theories about their causes and periodicities, and to what extent they affect not only the northern but also the southern hemisphere, but no consensus as yet.
The Heinrich events , discovered in 1988, are evident in sediment cores of the North Atlantic Ocean. They mark thermal events in which glaciers and icebergs melted and the sediment of continental origin contained in this ice was deposited on the sea floor. Six to seven such Heinrich events are known.
The Dansgaard-Oeschger events are particularly evident in ice cores from Greenland. They present themselves in the northern hemisphere as periods of rapid warming (several Kelvin within a few decades) followed by slow cooling (within a few centuries). 23 such events have been found for the period 110,000 to 23,000 BP . There seems to be a connection between these and the Heinrich events.
About 74,000 years ago, the last eruption of the Toba super volcano led to a cooling of several Kelvin and a dramatic change in climate ( volcanic winter ). The Toba catastrophe theory , according to then the population to the Homo sapiens have reduced to a few thousand individuals. This could explain the low genetic diversity of today's humans (known as the “ genetic bottleneck ”).
The vegetation on earth changed according to the climate change. Large areas of the land not covered by ice became steppe and tundra , (cold) deserts and grasslands . The forest areas and also the tropical rainforests declined.
The fauna of the last cold period was characterized by large animals ( megafauna ), especially large mammal species, but also birds that are now extinct .
Mammoths , mastodons , saigas , giant deer , saber-toothed cats , cave lions , cave hyenas and cave bears lived in Eurasia . In North America there were other species such as prairie mammoths , the American mastodon , helmet musk ox , bush ox ( Euceratherium ), giant sloth and giant armadillos . In Australia lived rhino large marsupials such as the Diprotodon and zygomaturus which Beuteltapir palorchestes , the bag lion marsupial lion , the giant rat kangaroo propleopus , giant wombats , up to three meters high kangaroos , the large flightless bird genyornis and the huge monitor lizard Megalania .
During and especially at the end of the last glacial period, many of these species became extinct . This can be explained either with the environmental changes, the overhunting by humans or a combination of both causes.
The glaciations of the last glacial period covered northern Eurasia and North America with huge ice sheets , some of which were several kilometers thick. While around 10% of the earth's land area is covered by glacier ice today, it was 32% of the land area in the last glacial period.
The Fennoscan Ice Sheet (also known as the Scandinavian Ice Sheet ) covered Northern Europe, the adjacent Barents-Kara Ice Sheet covered parts of Northern Asia. The Laurentide Ice Sheet and the Cordillera Ice Sheet covered large parts of North America. In the southern hemisphere, the Patagonian Ice Sheet covered southern South America . Antarctica stayed under the Antarctic Ice Sheet , which still covers it today.
The great mountains were also glaciated, especially the Alps, the Himalayas and the Andes . Their glacier tongues united to form large areas of glacier and pushed far into the foreland. There were also glaciers in the mountain ranges of Africa , Japan , Taiwan , Tasmania and New Zealand . It is controversial whether the Tibetan plateau was also glaciated.
The glaciers of the Alps flowed into the foothills of the Alps and united to form a network of ice streams . Only the highest peaks still protruded from this.
The enormous weight of the ice sheets pushed the lithosphere downward. As the glaciers melted, these areas rose again, a process known as postglacial land elevation and which continues to this day.
Relics of the glaciations that are still visible today are “flat planed” terrain with swamps , large lakes , lake plateaus , flat seas , moraines , gravel fields / sand , glacial valleys and lakes at the edge of the glacier . See also glacial series .
The glaciations of the cold period led to strong, dry and cold falling winds near the glacier edges due to the cold air masses flowing down from them. These winds carried large amounts of loose sediment away from areas with little vegetation cover, which then accumulated elsewhere to form loess .
There were also more inland dunes and sand dunes than there are today. A relic of this is, for example, the Sandhills region in what is now the US state of Nebraska.
The last glacial period was marked by major floods despite the lower rainfall . Several rivers in North Asia that drain into the Arctic Ocean could no longer drain due to the ice sheet on their way and formed huge ice reservoirs . The largest of these lakes, the West Siberian Glacier Lake , was created in the West Siberian lowlands near the rivers Ob and Yenisei and extended over about 1500 km from north to south and just as far from west to east. To the west of the Urals there was an ice reservoir in the region of today's Komi Republic and one in today's White Sea . With the decline of the Scandinavian glacier, the Baltic ice reservoir was created and enlarged . The current connection of this body of water to the ocean was created over three intermediate phases (Yoldia Sea, Ancylus Sea , Littorina Sea ), salt water flowed in and today's Baltic Sea was created.
Inland lakes such as the Caspian Sea and the Aral Sea also rose significantly in the water level and enlarged to around twice their current area. It is believed that the Caspian Sea increased so much that it over the Aral-Caspian Depression with the Aral Sea and the Manych valley with the Black Sea (which during the cold period a freshwater lake with no connection to the Mediterranean was) to form a single huge water body was. It is possible that the West Siberian glacial lake drained itself over the chain Aral Sea - Caspian Sea - Black Sea to the Mediterranean Sea. How the Caspian seal and the Baikal seal got into the inland lakes is unclear and could be explained by the hypothesis of a water connection between the Arctic Ocean and these lakes.
At the end of the glacial period, catastrophic flooding occurred in various regions of the world . These are also called glacier runs when the dam of an ice reservoir breaks. The Missoula floods in North America with the drainage of the Lake Missoula ice reservoir are among the largest of these events . In Asia, there was a series of devastating glacier runs of a similar magnitude, the Altai Floods in what is now the Altai Republic . Other major floods were those of Lake Bonneville (in present-day Utah) ( Bonneville Flood ), at the Great Lakes , which are also relics of the cold ages, and to the northeast of it, in the Champlain Sea , where seawater penetrated far inland, which was previously depressed by the ice sheet .
Due to the enormous mass of water that was bound in the ice sheets, the sea level sank to more than 100 meters below its current level during the last glacial period. Shelf seas like the North Sea fell dry in large parts. This increased the land area of the continents and islands and land bridges were created that enabled animals and humans to reach areas that were later separated from each other by rising sea levels.
The Beringia land bridge connected Asia with North America and thus enabled the colonization of America . In Europe there was a land bridge between Ireland, the British Isles and mainland Europe, called Doggerland in the area of the North Sea . At the lowest sea level, many of today's Mediterranean islands were connected to the mainland.
In the Asia-Pacific region there was a Southeast Asian land bridge to the western part of Indonesia ( Sunda ), and another land bridge that connected New Guinea , Australia and Tasmania to form a land structure ( Sahul ). However, there was no land connection between the Sunda and Sahul, but a separation that can still be seen today from the Wallace Line . Hence, man must have found a way to cross the sea to get from Asia to Australia.
The Persian Gulf and the Gulf of Suez dried up in the last glacial period. India and Sri Lanka were believed to be linked by the Adam's Bridge .
With the sinking of the sea level, new islands also formed in the middle of the ocean, for example the Mascarene Plateau east of Madagascar, which is now 8 to 150 meters deep.
Man in the last glacial period
Modern humans living as hunters and gatherers spread during this cold period , coming from Africa , over all continents of the earth (with the exception of Antarctica). In contrast, the Neanderthals , who settled in Europe during the Eem warm period, died out in the last cold period around 30,000 years ago. Around 17,000 to 12,000 years ago, the first sedentary societies emerged in Asia Minor that practiced agriculture and livestock ( Neolithic Revolution ).
From the perspective of human archeology , the last cold period falls into the Paleolithic (Paleolithic). The beginning of the glacial period lies roughly in the middle of the Middle Paleolithic . The period from about 40,000 years ago to the end of the last glacial period is known as the Upper Paleolithic . Prehistory and early history deal with the archaeological material sources and the cultural development of man in this epoch .
Since humans mainly stayed near the coast, many of their settlement areas from this period are now below sea level and are therefore difficult to access from an archaeological point of view.
From fossils and genetic analyzes ( molecular clock ) it can be deduced that anatomically modern humans lived in Africa before and at the beginning of the last glacial period. Fossil sites from this period are Florisbad (South Africa = " Homo helmei "), Eliye Springs ( West Turkana , Kenya), Laetoli (Tanzania) and Djebel Irhoud (Morocco).
North Africa was subject to strong vegetation fluctuations in the last glacial period. At the beginning of the glacial period 120,000 to 110,000 years ago, the Sahara was a green savannah; then it became a desert. Another savannah phase followed 50,000 to 45,000 years ago. During the peak of the last glacial period, the Sahara expanded again as a huge desert even further south than it is today. After the glacial period, another and so far last fertile phase followed. Since then, the Sahara has increased again as the largest dry desert on earth.
Asia seems to have experienced two waves of human settlement in the last glacial period. From the first wave it is assumed that humans, coming from Africa, followed about 60,000 years ago over the Middle East of the south coast of Asia to Australia. There are practically no traces of this.
In a second wave of colonization that began around 40,000 years ago, humans spread over Asia. There are 40,000 year old traces in the interior of Southeast Asia, 30,000 years ago in China and 26,000 years ago in Northeast Asia.
Humans reached Australia about 50,000 to 60,000 years ago. The oldest human remains in Australia are those of Mungo Man and Mungo Lady , both dated to around 40,000 years ago. Other finds are estimated to be up to 60,000 years old, but these dates are controversial.
The oldest archaeological cultures in Europe are those of the Neanderthals .
The oldest culture of Homo sapiens, also known as Cro-Magnon man in this epoch , in Europe was the Aurignacian culture. It existed from about 40,000 (possibly 45,000) years to about 31,000 years ago. It overlapped with that of the Châtelperronien culture, the last culture of the Neanderthals.
The most important cold-age culture in Europe was the Gravettian culture that followed. Their traces are proven in the areas of today's France, southern Germany, Austria, the Czech Republic, Poland and the Ukraine and are dated to about the period from 28,000 to 22,000 years ago.
This was followed by the Solutréen culture in Western Europe during the last cold maximum of around 24,000 to 20,000 years ago . The Magdalenian culture existed around 15,000 years ago . The last cultural groups before the Holocene were the Hamburg culture about 15,000 to 14,000 years ago, the penknife groups , also known as the Azilien culture, about 14,000 to 13,000 years ago, the Bromme culture and the Ahrensburg culture (about 12,000 years ago) .
See also French Cantabrian cave art .
According to current research, the colonization of America by Paleo-Indians from Siberia took place over the Beringia land bridge in at least three waves of immigration. The first and by far the most important wave was around 15,500 years ago. The second wave brought the ancestors of the Na-Dene - which Diné - and Apaches - Indians . With the third wave came the ancestors of the Eskimos and Unungun .
The Monte Verde site in Chile is one of the oldest traces of human settlement on the American continent. At the end of the glacial period about 11,000 to 10,800 years ago, the Clovis culture was the first extensive culture in America.
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- J. Ehlers, PL Gibbard (Ed.): Quaternary Glaciations: Extent and Chronology 2: Part II North America . Elsevier, Amsterdam 2004, ISBN 0-444-51462-7 .
- J. Ehlers, PL Gibbard (Ed.): Quaternary Glaciations: Extent and Chronology 3: Part III: South America, Asia, Africa, Australia, Antarctica . Elsevier, Amsterdam 2004, ISBN 0-444-51593-3 .
- ↑ Glacials and interglacials of the last 800,000 years, correlating with CO 2 estimates from ice core data
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- ↑ Temperature diagram and the description for it
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- ↑ CJ Burrows, NT Moar: A mid Otira Glaciation palaeosol and flora from the Castle Hill Basin, Canterbury, New Zealand Archived from the original on February 27, 2008. (PDF 340 kB) In: New Zealand Journal of Botany . 34, No. 4, 1996, pp. 539-545. doi : 10.1080 / 0028825X.1996.10410134 . Retrieved July 23, 2012.
- ↑ F. Lehmkuhl: The Ice Age Glaciation of High Asia - Local Glaciations or Superordinate Ice Sheet? Archived from the original on July 7, 2007. Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. In: Geographical Rundschau . 55, No. 2, 2003, pp. 28-33. Retrieved July 23, 2012.
- ↑ Giant Siberian lake from the last glacial ( Memento of the original from December 13, 2006 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ J. Mangerud, V. Astakhov, M. Jakobsson, JI Svendsen: Huge Ice-age lakes in Russia (PDF; 192 kB) In: J. Quaternary Sci. , Vol. 16, 2001, pp. 773-777, ISSN 0267-8179 .
- ↑ DD Kvasov: The Late Quaternary History of Large Lakes and Inland Seas of Eastern Europe . Leningrad 1975, p. 248 ff.
- ↑ Steve Dutch: Pleistocene Glaciers and Geography . ( Memento of the original from February 6, 2014 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved November 30, 2006
- ↑ Keenan Lee: The Altai Flood . ( Memento from August 11, 2011 in the Internet Archive ) (PDF; 6.3 MB 2004)
- ↑ Map of Europe to the LGM of the Vistula Ice Age
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