Ice age

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Antarctica ice sheet

An ice age is a section of the earth's history in which the mainland areas of at least one polar region are glaciated or covered by ice sheets . According to another, narrower and less common definition, the term Ice Age is only used when extensive glaciations occur in both the northern hemisphere and the southern hemisphere .

According to the first definition, the earth has been in the Cenozoic Ice Age for around 34 million years , as the Antarctic has been glaciated since then . According to the second definition, the current ice age only began about 2.7 million years ago, since the Arctic was also covered with ice. In terms of its duration, it corresponds approximately to the geological period of the Quaternary .

In addition to a number of shorter periods of ice that cannot be precisely determined, six ice ages are known from the history of the earth, each of which spanned several million years. In between there were periods of varying length with a more or less pronounced warm climate .

Ice Age and Ice Age

Schematic structure of an ice age

The term ice age has gone through a historical development that leads to confusion to this day. Originally it was introduced in 1837 by the German naturalist Karl Friedrich Schimper and was also called world winter in the linguistic usage at the time . He initially referred to the entire Quaternary . After the discovery of several alternating warm and cold periods , the word continued to be used on the one hand for the entire ice age and on the other hand as a name for the individual cold periods (glacials). Today, the colloquial language "Ice Age" usually means a cold period (a glacial), while this is avoided in technical language. Some technical terms use the term "Ice Age" with the meaning of Ice Age, for example Sturtic Ice Age .

A Eiszeitalter comprises both the cold periods, as well as the intervening warm periods (interglacials). A further subdivision is based on the terms stadial and interstadial . A stadial is a cold phase during a glacial or interglacial (usually associated with an increase in ice cover), while an interstadial is defined as a relatively short warm phase between two stadiums within a glacial (see for example Alleröd-Interstadial ). The subdivision into stadial / interstadial is mainly used for more recent icing phases; cold periods further in the past are less suitable for this, as a fine resolution of the respective epochs is no longer possible with increasing time interval.

The most recent Cenozoic Ice Age , which extends to the present day, began around 34 million years ago with the glaciation of the Antarctic regions and, in addition to the chronostratigraphic series Oligocene , Miocene and Pliocene, also includes the Quaternary , during the last glacial period (in the Alpine region Würm glacial period , in northern Germany / Northern Europe Vistula Glacial Period ) ended about 11,700 years ago. Since the Holocene is only a warm period within the Cenozoic Ice Age, further cold period cycles are likely to occur in the future under the conditions of the current climatic state . The cooling trend of approx. 0.12 ° C per millennium that has prevailed since the climatic optimum of the Holocene is regarded as a harbinger of a new cold period, which, however, is only expected in 30,000 to 50,000 years in the context of natural climatic changes. However, this development could be significantly changed by human ( anthropogenic ) influences on the climate system.

The ice ages in geological history

Historical overview

The systematic investigation of natural climate cycles began in the first half of the 19th century with the gradual reconstruction of the Quaternary cold ages. Indeed, as early as 1750, individual naturalists suggested that Central and Northern Europe must have been the scene of extensive freezing processes in the past, but their ideas initially found little attention. The possibility of extensive glaciation as a result of a climate shaped by the Ice Age was too revolutionary an idea to be accepted by science. Against the then firmly rooted belief in the biblical creation myth with the flood as a global " primal catastrophe", to which all known deposits including fossils were ascribed, the assumption of a primeval epoch was only gradually able to prevail and only gained acceptance with the development of geology into modern science on the ground. This led to the realization, which was accompanied by fierce controversy, that the early days of the earth encompassed considerably longer periods than the historically documented human history.

In the course of more intensive research, unusual relics in the form of moraines , drumlins and erratic blocks ( boulders ) were found, especially in the foothills of the Alps, in the northern German lowlands or in Scandinavia , which indicated a prolonged glaciation . In the first decades of the 19th century, the initially rough division of the earth's history into different geological periods emerged . In the further course these periods were classified in the geological time scale , although the true temporal dimensions were still greatly underestimated due to insufficient geochronological analysis methods. In addition, the first scientific descriptions of prehistoric habitats including their climatic conditions emerged . By the middle of the 19th century, the now more numerous proponents of the Ice Age theory had gathered so much evidence and “ climate witnesses ” for the existence of an earlier Ice Age that it was gradually becoming more difficult to ignore the arguments put forward. As one of the most tireless exponents, the Swiss Louis Agassiz campaigned for the scientific acceptance of the Ice Age idea. On numerous trips, combined with lectures in front of an academic audience, as well as through the publication of several books, he made a decisive contribution to the popularization of these findings. Other pioneers of early Quaternary research included Johann von Charpentier and Ignaz Venetz , who drew an increasingly differentiated picture of the Ice Age climate and the associated processes from around 1830. Around the same time, the ice age model received additional confirmation through the discovery of very old glacier cuts in Africa, Australia and India, which, according to current knowledge , are ascribed to permocarbon glaciation about 300 million years ago.

Portrait photography by James Croll (1821–1890)

Also in the early 19th century there was speculation about various astronomical causes of the Ice Ages. In 1824 , the Danish geologist Jens Esmark published the hypothesis that the orbit of the earth around the sun was strongly eccentric in prehistoric times and resembled that of a periodically recurring comet . In the 1830s, the French mathematician Siméon Denis Poisson suspected, on the basis of the then prevailing ether theory, that the universe was divided into warmer and colder regions through which the solar system moved over long periods of time. The first well-founded and well-founded Ice Age theory was formulated by the Scottish naturalist James Croll . Based on the calculations of the mathematician Joseph-Alphonse Adhémar and the astronomer Urbain Le Verrier , in a sensational paper in Philosophical Magazine in 1864 he advocated the idea that changes in the earth's orbit in connection with strong ice-albedo feedback are responsible for the formation of the Ice ages could be responsible. From around 1870 the possibility of cosmic or solar influences on the earth's climate was scientifically discussed on a broader basis.

Croll's theory was supported with concrete calculations in the first half of the 20th century by Milutin Milanković and Wladimir Köppen . The explanatory model, which has been created over many years, takes into account the changes in the earth's orbit (from slightly elliptical to almost circular), the inclination of the earth's axis and the gyration of the earth around its axis of rotation ( precession ) and its long-period fluctuations over several 10,000 years. Until the 1970s, however, only a few geoscientists believed that the Milanković cycles could be a (co-) cause of the Quaternary Ice Age . The turnaround began in 1976 with the widespread "Pacemaker Study" (currently over 4,000 citations) in the journal Science (with the participation of well-known scientists such as John Imbrie and Nicholas Shackleton ). Thereafter, the theory developed in a modified and expanded form to become an integral part of paleoclimatology and is often used in the reconstruction of the Quaternary climate history and increasingly also for the analysis of earlier geological periods.

At the beginning of the millennium, some hypotheses took the view that on the scale of Earth's history the climate had been changed not only by terrestrial factors, but also by varying cosmic radiation influences . According to this, the apparently regular cold periods of the Phanerozoic, for example, should correlate with equally regular spiral arm passages of the sun and its heliosphere . These and similar assumptions (such as the inclusion of supernovae and star formation rates ) led to controversial discussions and were largely cautiously accepted by science.

In the current geoscientific literature, the postulated cosmic effects, with the exception of the Milanković cycles and the solar constant that change over long periods of time, are a little-received niche topic. The geophysical, geological and biochemical components, which are mostly well documented from climatic history, are considered to be a valid and sufficient research basis in paleoclimatology and related disciplines.

Tabular representation of the various ice ages

Surname Beginning a million years ago Duration in million years aeon era period
Paleoproterozoic glaciation 2400 300 Proterozoic Paleoproterozoic Siderium , Rhyacium
Sturtic Ice Age 717 57 Proterozoic Neoproterozoic Cryogenium
Marino Ice Age 640 5 Proterozoic Neoproterozoic Cryogenium
Ordovician Ice Age /
also Brain Ice Age 1) 		460 30th Phanerozoic Paleozoic Ordovician , Silurian
Permocarbones Ice Age /
also Karoo Ice Age 		360/350 80 to 100 Phanerozoic Paleozoic Carbon , Perm
Cenozoic Ice Age /
Quaternary Ice Age		 34
2.6		 previously 34
previously 2.6		 Phanerozoic Cenozoic Oligocene , Miocene , Pliocene , Quaternary
Total duration of all cold periods: approx. 525
1)Occasionally in the literature as Andean-Saharan ice age called

Paleoproterozoic Ice Age

Although only incomplete proxy series are available for the Archean (4.0 to 2.5 billion years ago) , it is mostly assumed that a predominantly warm climate prevailed during this eon . However, there are indications of a cooling phase with possibly regional glaciations in the form of the Pongola glacial 2.9 billion years ago, but there is little reliable knowledge about its special characteristics. The paleoproterozoic glaciation (also known as the Huronian Ice Age ), which started 2.4 billion years ago and lasted for 300 million years, was the longest ice age in the history of the earth. Geological climate witnesses, including paleomagnetic evaluations from North America, Scandinavia, India and southern Africa indicate a global cold snap with a longer-lasting snowball earth event . Due to the large time interval, it is difficult to detect and is fraught with great uncertainties that the alternation of different cold and warm periods typical of the later Ice Ages is. On the other hand, the assumption that the Ice Age climate in the early Paleoproterozoic could be closely linked to the Great Oxygen Disaster (in the specialist literature Great Oxigenation Event ) is widely accepted .

At the beginning of the Paleoproterozoic , the terrestrial atmosphere had a relatively high methane concentration , but only small traces of free oxygen. Cyanobacteria produced large amounts of O 2 as a "waste product" of their metabolism by means of oxygen photosynthesis more than 3 billion years ago , but this was converted into trivalent iron ions during the oxidation of organic compounds, hydrogen sulfide and bivalent iron ions Fe 2+ Fe 3+ completely consumed. After this intensive oxidation phase was over, the excess oxygen began to accumulate both in the atmosphere and in the ocean. In marine biotopes, this process led to the mass extinction of anaerobic organisms , almost all of which fell victim to the toxic effects of oxygen. With the help of UV radiation, the oxygen in the atmosphere oxidized most of the methane deposits to carbon dioxide and water. Since methane has a significantly higher global warming potential than CO 2 , there was rapid climate change afterwards , and temperatures remained at ice age levels for 300 million years.

Main causes of the Paleoproterozoic Ice Age

  • Collapse of the methane concentration: The extensive depletion of atmospheric methane deposits resulted in a significant weakening of the greenhouse effect and thus a change in the radiation balance .
  • Weaker solar radiation : During its development as a main sequence star , the sun hadonly about 85 percent of its present luminosity in the early Paleoproterozoic . This radiation deficit was no longer fully compensated by the changed composition of the atmosphere and transferred the planet from the original warm-temperate climate to a state of global icing.

The glaciation phases in the Neoproterozoic

Fictional representation of a snowball earth stage, like in the Neoproterozoic, but with modern continents

After the decay of the Paleoproterozoic Ice Age , a relatively uneventful from today's perspective epoch that in the professional literature to as "boring billion" (English began The boring trillion is called). This phase ended in the cryogenium more than 700 million years ago, when a series of rapidly running plate tectonic processes with numerous geochemical and climatic turbulence probably led to the earth being frozen several times and almost completely up to the equator. The increased occurrence of glacial relics in low latitudes and on all paleocontinents led to the development of the relatively young snowball-earth hypothesis , which has also been popular outside of science and has been discussed intensively and sometimes controversially since the 1990s. Information on the duration, number and chronological sequence of the glacial cycles was long considered speculative and was sometimes based on fragmentarily documented reconstructions. In the meantime, however, due to the use of precise dating methods, more recent works give a more precise picture of the chronological classification of the various glacial phases (see table above). This also applies to the status of the Kaigas Ice Age (740  mya ) and the Gaskiers Ice Age (580 mya), which have been identified as regional and temporary cuts.

The geophysicist and climatologist Raymond Pierrehumbert characterized the Neoproterozoic as follows: The Phanerozoic seems, by comparison, to be a rather quiescent place (German: The Phanerozoic seems to be a rather quiet place ). In fact, the cryogenium (720 to 635 mya) in particular was a permanent geotectonic trouble spot due to the breakup of the supercontinent Rodinia . 900 million years ago Rodinia had united all land masses in itself and thus reached the maximum extent. Currently 100 million years later the first signs of decay occurred: In conjunction with several long active Superplumes including extensive release of flood basalts came to the plate boundaries, a number of widening grave fractures ( rifting ), which ushered in an increasing fragmentation of the continent. This disintegration process was followed immediately by the development of the new, but only “short-lived” supercontinent of Pannotia (also Greater Gondwana ) in the course of Pan-African orogeny (approx. 600 mya ). Although the individual theories differ in degree, it is unanimously assumed that the global glaciation of the earth during the Sturtic and Marino Ice Age is based on the interaction of various geological and geochemical components.

Many detailed questions about the exact freezing mechanisms and those factors that led to rewarming are so far only known in outline or not yet clarified in science. Self-reinforcing ice-albedo feedback can be assumed with sufficient certainty during the snowball-earth episodes, which had forced a worldwide cooling to at least -50 ° C. The natural carbon cycle almost came to a standstill in this way, and biomass production in the oceans sank to a minimum. This only changed when the unused atmospheric reservoir of volcanic CO 2 emissions reached an extremely high threshold, which tipped the permafrost climate and triggered a global thaw. According to this scenario, the earth changed from a deep-frozen “snowball” under chaotic environmental conditions ( heavy rain , hurricanes , sea ​​level rise of several hundred meters) into a super greenhouse with temperatures around 40 ° C for a short time.

Main causes of the icing phases in the Neoproterozoic

  • Various influencing factors: In general, a combination of different geological and geochemical components is assumed (including plate tectonics, superplume activities or flood basalt volcanism, extensive carbonate storage, extremely rapid weathering processes).
Perm-Trias-Ereignis Paläozän/Eozän-Temperaturmaximum Kreide-Paläogen-Grenze Kambrium Ordovizium Silur Devon (Geologie) Karbon Perm (Geologie) Trias (Geologie) Jura (Geologie) Kreide (Geologie) Paläogen Neogen Christopher Scotese
Clickable temperature profile in the Phanerozoic with the three ice ages described in this article (illustration somewhat simplified, based on Christopher R. Scotese, 2018).

Ordovician Ice Age

In the Ordovician the weathering effect intensified the spread of land plants.

The Ordovician Ice Age (also Andean-Sahara Ice Age or Hirnantische Glaciation ) began around 460 million years ago in the Upper Ordovician and ended in the early Silurian 430 million years ago. The movement of the great continent of Gondwana across the South Pole was reconstructed in chronological order using ice age deposits . The core area of ​​the glaciation was initially on the Arabian Plate or in today's Sahara , then migrated westward via the then continuous land connection towards South America ( Brazil and the lower Amazon region ) and expanded in a weaker form to the region of the as yet non-existent Andean chain .

The specialist literature of the last few decades has made a number of different and sometimes contradicting assumptions regarding the causes and structure of the Ordovician Ice Age . More recent studies assume that the typical CO 2 values for this era were set too high for a long time. For the Middle Ordovician, a carbon dioxide concentration below 3000 ppm is assumed today - if a greenhouse scenario is excluded, but against the background of a gradual cooling. This development is causally related to the spread of vegetation on the mainland. The continents were probably colonized by moss-like plants ( bryophytes ) and early fungus forms as early as the Middle Cambrian and continued increasingly in the Ordovician . The denser and more extensive plant cover developed into an elementary climate factor, as it contributed significantly to the accelerated chemical weathering of the earth's surface. This resulted in a reduction in atmospheric carbon dioxide and, in conjunction with other factors, global cooling. In a study published in 2019 it is assumed that around 466 million years ago an asteroid about 150 km in size orbiting between Mars and Jupiter was completely destroyed and partially pulverized by a collision with another celestial body. According to this hypothesis, the resulting interplanetary dust cloud was distributed in the inner solar system and dampened the solar radiation on earth, with the consequent effect of falling temperatures worldwide over a period of approximately two million years. In the opinion of the authors, the initially moderate climate change could have given the impetus for the emergence of new species with an increase in biodiversity in oceanic habitats .

An abrupt cold snap, presumably triggered by the crossing of a tipping point in the climate system and associated with the rapid expansion of sea ​​ice surfaces and continental ice sheets , occurred during the last Ordovician stage of the brain antium (445.2 to 443.4 mya), the surface temperature of equatorial oceans decreased by 8 ° C and the global mean temperature dropped from around 16 ° C to 11 to 13 ° C. At the same time, one of the most momentous mass extinctions in the history of the earth happened , with an estimated species loss of up to 85 percent. There is broad consensus among scientists that the biological crisis was due to a combination of factors, including, presumably, severe volcanism. Also in the Hirnantium there was the development of a longer oceanic anoxic event , reaching into the Silurian , which additionally destabilized the marine habitats.

Recently, the thesis has been advocated that it is not the glacial climate and the associated lowering of the sea level by more than 100 meters (with a decrease in shallow water biotopes) that are the primary extinction factors, but that serious geochemical changes leading to the release of toxic heavy metals such as arsenic , Lead or manganese and which also resulted in a substantial reduction in essential trace elements . On the basis of microfossils , a high incidence of malformed organisms can be demonstrated at the time in question, which suggest contamination by toxic substances.

Main causes of the Ordovician Ice Age

  • Weathering effects : The spread of extensive vegetation zones during the Ordovician period withdrew a number of elements from the soils, resulting in accelerated chemical weathering of the earth's surface with increased carbon sequestration.
  • Combination of different influencing factors: The weaker solar radiation compared to the present, the glaciation of the south polar mainland areas as well as the extensive deposition of organic carbon in the brain due to anoxic conditions in the oceans added up to a significant cooling factor.

Permocarbones Ice Age

The beginning and the exact end of the Permocarbon Ice Age (also Karoo Ice Age ) can only be narrowly defined. Already at the Devonian - Carboniferous transition (358.9 mya) there was a mass extinction and the collapse of several ecosystems with the Hangenberg event , combined with glaciations in the southern and western regions of the greater continent Gondwana and a drop in sea level of around 100 meters . Due to the extensive storage of organic carbon in black shale horizons , the atmospheric CO 2 concentration decreased by around 50 percent during the Upper Devonian crisis and was a maximum of 1000 ppm in the early Carboniferous. The cooling trend in the Middle Tournaisian , the first chronostratigraphic stage of the Carboniferous, which began after a brief warming phase , deviated significantly from the warm climate of the Devonian and possibly led to continental freezing in the Lower Carboniferous 350 million years ago (with a continuous reduction in the carbon dioxide content) the southern hemisphere extended to the 60th parallel.

An intensification of the cold-age conditions with the spread of continental ice sheets began in the Upper Mississippium 325 million years ago and affected large parts of Gondwana up to the 40th parallel south, including today's regions of South America, South Africa, Antarctica and Australia. This predominantly Ice Age environmental situation persisted throughout Pennsylvania (323.2 to 298.9 mya) and beyond into the early Permian . The analysis of rock conglomerates ( diamictite ) supports the assumption that temporary glaciations during the main phase of the Ice Age also occurred in higher-lying tropical regions. In the last 10 million years of the Carboniferous, different climatic conditions changed in rapid succession , apparently influenced by the cyclical changes in the Earth's orbit parameters , with strongly varying CO 2 concentrations between 150 and 700 ppm and corresponding fluctuations in the sea level ( glacial eustasia ). Taking into account the solar radiation at that time, which was around 2 to 3 percent weaker, the global average temperatures during a warm phase were 12 to 14 ° C and were at least 5 ° C lower during a cold period. According to a 2017 study, the CO 2 concentration continued to decrease in the earliest Permian period and for a short time dropped to a value of around 100 ppm. Accordingly, the earth system moved close to the tipping point that would have brought the planet into the climatic state of global icing, comparable to the snowball earth events in the Neoproterozoic .

Artist's impression of the carnivorous pelycosaur Dimetrodon from Unterperm.

In contrast to the falling CO 2 values, the oxygen content in the late Carboniferous reached the record level of 33 to 35 percent. The high O 2 concentration promoted the growth in size of various arthropods such as Arthropleura , but harbored the risk of large-scale forest fires. After the extent of vegetation suffered considerable losses several times during the glacial phases in the course of the Pennsylvania, the extensive collapse of the rainforests near the equator took place 305 million years ago in the Kasimovium due to the increasingly arid climate (in the specialist literature: Carboniferous Rainforest Collapse ). In the course of the first mass extinction of vegetation , the tropical forests were decimated to a few vegetation islands, and many wetlands and marshland also disappeared. Arthropods, most of the amphibians ( Temnospondyli ) and early reptiles with a semi-aquatic way of life were particularly affected by the loss of these biotopes . Due to the fragmentation of the habitats, the biodiversity of the terrestrial vertebrates ( Tetrapoda ) on the Carbon-Permian border fell significantly and initially remained low in the early Permian, before the biodiversity gradually increased again in the further course.

Compared to the other glacial phases of the Phanerozoic , the Permocarbone Ice Age has a number of peculiarities, especially in its spatial and temporal structure: several times, small ice centers alternated with intervals of large-scale glacier growth, followed by largely ice-free periods. The Middle Permian experienced a paradoxical situation towards the end of the Ice Age, when large parts of Australia - probably due to the influence of oceanic and atmospheric circulation patterns - were repeatedly covered by stable ice caps for millions of years , while in all other areas, including the southern polar region, there was no longer any noteworthy ice cover .

Main causes of the Permocarbon Ice Age

  • Geographical location: The position of the southern regions of Gondwana around the Antarctic, which changed little during the Carboniferous, was a major driver of glacier formation, since the polar near mainland iced up faster and more effectively than open sea zones and this process gained momentum through the ice-albedo feedback .
  • Carbon dioxide reduction : The increasing vegetation cover in the “hard coal age” of the carbon led to the spread of deep-rooted plants that split up the soil. The combination of increased soil erosion with extensive coalification processes withdrew large amounts of carbon from the atmosphere and caused the atmospheric CO 2 to drop to a hitherto unique low.
  • Forest fires: Due to the extremely high oxygen content, probably the most devastating forest and wildfires in the history of the earth occurred in the Upper Carboniferous, with the possible side effect of a global smoke and haze that dampens sunlight.
  • Plate tectonics: After the major continents Laurussia and Gondwana united to form the supercontinent Pangea and thus to a huge continental barrier about 310 million years ago, the exchange of water and heat in the equatorial ocean currents stagnated, which further intensified the prevailing cooling tendency.

The current ice age

Arathem system series Age
( mya )
K
ä
n
o
z
o
i
k
u
m 		quaternary Holocene 0
⬍
0.0117
Pleistocene 0.0117
⬍
2,588
Neogene Pliocene 2,588
⬍
5.333
Miocene 5.333
⬍
23.03
Paleogene Oligocene 23.03
⬍
33.9
Eocene 33.9
⬍
56
Paleocene 56
⬍
66
earlier earlier earlier

The Cenozoic Ice Age, which continues to this day (with the Quaternary Ice Age as the youngest section) began with the gradual glaciation of the continent of Antarctica at the beginning of the Oligocene . About 2.7 to 2.4 million years ago, in the vicinity of the Pliocene - Pleistocene border, increased ice formation also began in the Arctic . From this point on, longer cold periods (glacials) alternated with shorter warm periods ( interglacials ).

Until the later Eocene , Antarctica and South America were connected by a land bridge before the Drake Strait began to open. Because of this tectonic process, the Antarctic Circumpolar Current arose in the Southern Ocean , which cut off Antarctica from the supply of warmer seawater and probably initiated a global cooling process. The temperature of the oceans fell by 4 to 5 ° C down to the deeper regions, and the sea level dropped by about 30 meters within a relatively short time. At the same time, there was a steep drop in the atmospheric CO 2 concentration of up to 40 percent. The glaciation of the southern polar mainland around 34 million years ago when the CO 2 threshold was around 600 ppm marks the beginning of the Cenozoic Ice Age . In the course of the Pliocene , the Antarctic ice sheet reached its current size of 14 million km². In the period that followed, and increasingly since the beginning of the Quaternary, the mass of the ice cover increased steadily, up to a thickness of 4500 meters in places.

The formation of the Isthmus of Panama 2.76 million years ago formed the Gulf Stream , which from then on not only directed warm ocean currents to the north, but also caused an increase in humidity in the arctic regions. According to the current state of research, however, the influence of the Gulf Stream on icing processes (with increased precipitation potential in the Arctic) plays only a subordinate role. It is predominantly assumed that the glaciation of the Arctic, which expanded in the early Quaternary, is due to a significant decrease in the global CO 2 concentration.

Climate parameters of the last 420,000 years, determined from ice core analyzes of the Vostok station in Antarctica

During the Quaternary Ice Age , relatively warm periods alternated with very cold periods. The cold phases were characterized by massive glacier advances and covered significantly longer periods than the warm phases, which lasted around 15,000 years on average. Currently, a cycle from one warm period to the next takes a little more than 100,000 years and is therefore linked to changes in the earth's orbit ( eccentricity ) of the same length . This period occurred in full expression for the first time in the early Middle Pleistocene around 700,000 years ago. Before - that is, since the beginning of the Quaternary - the cycle duration was only 41,000 years and at that time correlated with the fluctuations of the earth's axis of rotation . This “jumping over” to a longer warm-cold cycle has long been considered one of the great puzzles of Quaternary research . A more recent study, based on the analysis of sediment cores, postulates as the main cause a significant weakening of the deep water circulation, especially in the subpolar regions of the southern ocean, which compared to the present 50 percent less carbon dioxide from the deep sea to the sea surface and from there into the atmosphere got.

Eleven interglacials have been identified for the past 800,000 years. The duration of these interglacials was normally around 10,000 to 30,000 years, only for the period of the marine isotope level 11c (MIS 11c) a maximum of 40,000 years is estimated. During the last glacial periods, the inland ice sheets and mountain glaciers increased significantly in size and volume and eventually covered about 32 percent of the mainland. Currently, only about 10 percent of the continental area is covered by glaciers. Large parts of Europe , Asia and North America were glaciated, especially in the northern hemisphere . Many traces of ice such as trough valleys , moraines and glacier cuts have been preserved there to this day.

The current interglacial, recorded on the geological time scale as the Holocene , is the most recent warm period of the Cenozoic Ice Age , with a previous duration of around 11,700 years. Even in the warmer epochs of an Ice Age, the climate remains at a relatively cool level in geological comparison. The ice cover of the polar regions and high mountains mostly remains, however, glacier advances up to middle latitudes are receding, and there is a clearly temperate climate in these areas with milder winters.

Towards the end of the last glacial period and partially in the early Holocene, a large part of the megafauna of America, Eurasia and Australia was wiped out in the course of the Quaternary extinction . The reasons for the delayed extinction on different continents are the subject of a scientific controversy, with more recent publications ascribing a clear preponderance to human influence.

Main causes of the Cenozoic Ice Age

  • Reduction of atmospheric carbon dioxide: The CO 2 reduction that began in the Middle Eocene due to various carbon-binding processes fell below several threshold values ​​in the second half of the Cenozoic , which led to accelerated cooling and ultimately to large-scale glaciations in both polar regions.
  • Oceanic circulation: The formation of the cold Antarctic Circumpolar Current , together with the exposed geographic location of Antarctica, contributed significantly to the ice cover of the continent.
  • Milanković cycles: The relatively weak, but reinforced by several feedbacks, the effect of the Earth's orbit parameters changing over longer periods of time gave the impetus for the periodic warm and cold periods during the Quaternary Ice Age . According to this, the fluctuations in the concentration of carbon dioxide, methane and nitrous oxide were about a third involved in the climatic changes in the warm and cold cycles, and according to another publication even half.

Glacial phases during a warm climate

During the approx. 541 million years of the Phanerozoic , the proportion of the three ice ages associated with this eon was around 30 percent, measured against the total duration of the earth's history (4.57 billion years) including the cold periods in the Precambrian, around 11 to 12 percent. However, this does not exclude the fact that relatively extensive glaciations have taken place in the polar regions over periods of several 100,000 years or longer under warmer environmental conditions. Even during a warm phase, the climate was never really stable and was often subject to larger and smaller fluctuations, among other things due to tectonic processes such as mountain formation ( orogenesis ) or the opening or closing of ocean straits, each associated with a shift, intensification or weakening of atmospheric and oceanic circulation patterns . Pronounced climatic changes were also caused by the megavolcanism of various igneous large provinces , in addition, disturbances of the organic carbon cycle in interaction with oceanic anoxic events formed an important climatic factor. In addition, the various components of the Milanković cycles ( precession , ecliptic skewness and eccentricity ) left behind detectable climatic signals, some of them over several hundred million years.

In the Mesozoic ( Mesozoic Era) and Cenozoic Era (New Earth Era), a number of time windows for the potential formation of glaciers and ice caps come into question. For one part, icing processes could definitely be proven, for another part only indications point to a possible glacial phase.

law

Arrangement of the continents in the Middle Jura

At the Triassic - Jurassic border (201.3 mya), one of the largest mass extinctions of the Phanerozoic occurred in connection with the advancing disintegration of the supercontinent Pangea, with a loss of species of around 70 percent. Along the plate edges of what is now North America and Europe, extensive rift fractures with the first marine ingressions arose, reaching as far as North Africa . This development, towards the gradual opening of the Central Atlantic later, resulted in the emergence of 11 million km² Central Atlantic magmatic province (English Central Atlantic Magmatic Province , abbreviated CAMP ), including their flood basalts of the richest of the known geological history. Further centers of volcanic activity emerged in the area of ​​South Africa and Proto-Antarctica in the form of the Karoo-Ferrar magma outflows with a main phase in the Middle Jurassic. These events were associated with a greatly increased ocean floor spreading rate , had lasting climatic effects and subsequently led to rapid warming and cooling phases with a duration of 0.5 to 1.0 million years each.

For the transition area from the Middle Jurassic to the Upper Jurassic or between the chronostratigraphic levels callovium (166.1 to 163.5 mya) and oxfordium (163.5 to 157.3 mya), several studies, after evaluating a series of proxy data, establish a rapid cooling, the drop the carbon dioxide concentration from 700 ppm to well below 500/400 ppm and an associated glaciation of the polar regions of the northern hemisphere. Other publications assume a moderate cooling and consider the existence of larger ice caps to be unlikely in this context. An important indicator of the occurrence of a glacial phase is the pronounced rise and fall in sea level, which, due to their very rapid succession, rule out tectonically induced changes in the ocean basin volume in most cases. The most comprehensive study of oceanic trends in the Jurassic to date comes to the conclusion that the pronounced sea level fluctuations (mostly in the range of 25 to 75 meters) remain a mystery without the existence of large ice sheets.

chalk

The 79 million years long Cretaceous period is seen in popular scientific publications as an archetypal symbol of a permanent tropical climate up to higher latitudes. However, this view is increasingly being called into question, also under the aspect that the CO 2 concentration - over the entire duration of the chalk - was in part overestimated and underestimated with regard to its range of fluctuation. It is true that in the climatic optimum of the Upper Cretaceous there was probably the strongest greenhouse phase of the Phanerozoic , but this was followed by a gradual cooling over millions of years, in the Maastrichtian (72.0 to 66.0 mya) due to the Dekkan-Trapp volcanism with abrupt climatic changes and two large cooling intervals at 71.6 to 69.6 mya and 67.9 to 66.4 mya. For these periods of time, different studies postulate a carbon dioxide level of approx. 420 to 650 ppm with relative agreement. This would roughly correspond to the threshold at which the glaciation of the Antarctic began on the Eocene-Oligocene border . However, in addition to palaeogeographic differences and the meridional temperature gradient, a number of other factors must be taken into account in this comparison . In principle, however, seasonal sea ice formation and the ice cover of high-altitude mainland regions in the southern polar region are not excluded by some studies. Without direct geological evidence, an icing scenario in the Maastrichtium is initially limited to the framework of modeling and theoretical considerations.

For the Lower Cretaceous, some cooling phases on a global level are assured, including three shorter intervals during the Valanginian (139.3–133.9 mya) and the Hauterivian (133.9–130.7 mya) as well as a longer temperature decline in the second half of the Aptium (126.3-112.9 mya). Until recently, the existence of glaciers in the vicinity of these time periods was generally considered unlikely. A broad geological study of South Australian regions published in 2019, however, came to the conclusion that more or less pronounced glaciers took place on the continent in the course of the Lower Cretaceous. This judgment is based on the evidence of tillites , dropstones , diamictite and glendonite crystals (see also Ikaite ), which were found in different stratigraphic layers of the early Cretaceous and whose formation can be traced back to glaciogenic processes.

The assumption made in a study of a south polar inland freezing of a maximum of 60 percent of the current Antarctic ice sheet under the tropical environmental conditions of the Turonian (93.9 to 89.7 mya) was discussed controversially in science and largely rejected.

Eocene to Miocene

Topographical representation of Greenland without ice cover

In the specialist literature, the view was taken for a long time that larger glacier and sea ​​ice formations in the Arctic took place for the first time near the Pliocene - Pleistocene transition (2.7 to 2.4 mya). In the meantime, more recent studies provide clear indications of glaciation processes of different lengths, which first appeared shortly after the climatic optimum of the Eocene (48/47 mya) and which were repeated several times in the following period. The extent to which the northern polar mainland areas at that time, and Greenland in particular, were covered by ice layers is still an open question. A temperature decrease postulated for the Arctic 41 million years ago could also be proven for the South Pole region, whereby Antarctica apparently recorded no or only very limited glacier formation up to the climatic break at the Eocene-Oligocene border (33.9 mya). On the other hand, finds of dropstones of Greenlandic origin in deep-sea sediments of the North Atlantic point to the temporary existence of continental ice 38 to 30 million years ago on Greenland.

The evaluation of marine carbonates from the tropical Pacific using the stable oxygen isotopes 18 O / 16 O supports several cooling scenarios for both poles from the Eocene to the early Oligocene. Analyzes of deep-sea drill cores from the Fram Strait and off South Greenland suggest that Greenland has been almost continuously covered by ice for the past 18 million years. However, the volume and extent of the ice caps of that time are still largely unclear, although the existence of icebergs (and thus also those of outlet glaciers ) is considered certain.

Web links

Commons : Ice Age  - Collection of images, videos and audio files
Wiktionary: Ice Age  - explanations of meanings, word origins, synonyms, translations

literature

English language works

  • William Ruddiman : Earth's climate, past and future. WH Freeman, New York 2002, ISBN 0-7167-3741-8
  • Fiona M. Hyden, Angela L. Coe: The Great Ice Age. The Open University, Walton Hall, Milton Keynes, 2nd Edition 2007, ISBN 978-0-7492-1908-6
  • Raymond T. Pierrehumbert: Principles of Planetary Climate. Cambridge University Press, 2010, ISBN 978-0-521-86556-2 .
  • Raymond S. Bradley : Paleoclimatology. Reconstructing Climates of the Quaternary. Academic Press (Elsevier Inc.) Oxford, Amsterdam, Waltham, San Diego, Third Edition 2015, ISBN 978-0-12-386913-5 .
  • George R. McGhee Jr .: Carboniferous Giants and Mass Extinction. The Late Paleozoic Ice Age World. Columbia University Press, New York 2018, ISBN 978-0-231-18097-9 .

German language works

Individual evidence

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