The North Aral Sea ( Kazakh : Soltüstık Aral teñızı ) is the portion of the former Aral Sea that is fed by the Syr Darya River. It split from the South Aral Sea in 1987–1988 as water levels dropped due to river diversion for agriculture.
139-749: In 1925 a large site containing numerous fossils of the Oligocene was discovered near the village of Akespe by the northern shore of the Aral Sea. The Aral Sea began shrinking in the 1960s, when the Soviet Union decided that the two rivers feeding it, the Amu Darya and the Syr Darya , would be diverted to irrigate cotton and food crops in Kazakhstan and Uzbekistan . In 1987–1988, due to an accelerated loss of water,
278-648: A Phanerozoic low in diversity by the late Oligocene, and the temperate forests and jungles of the Eocene were replaced by forest and scrubland. The closing of the Tethys Seaway destroyed its tropical biota. The Oi1 event of the Eocene-Oligocene transition covered the continent of Antarctica with ice sheets, leaving Nothofagus and mosses and ferns clinging to life around the periphery of Antarctica in tundra conditions. Angiosperms continued their expansion throughout
417-668: A broader trend of global cooling lasting from the Bartonian to the Rupelian. The transition is marked by the Oi1 event, an oxygen isotope excursion occurring approximately 33.55 million years ago, during which oxygen isotope ratios decreased by 1.3 ‰ . About 0.3–0.4 ‰ of this is estimated to be due to major expansion of Antarctic ice sheets. The remaining 0.9 to 1.0 ‰ was due to about 5 to 6 °C (9 to 10 °F) of global cooling . The transition likely took place in three closely spaced steps over
556-487: A critical threshold value. Brachiopod oxygen isotope ratios from New Zealand suggest that a proto-Subtropical Convergence developed during the Early Oligocene, with northern New Zealand being subtropical and southern and eastern New Zealand being cooled by cold, subantarctic water. Oligocene climate following the Eocene-Oligocene event is poorly known. There were several pulses of glaciation in middle Oligocene, about
695-431: A large body of water is also present. In an attempt to try to mitigate the cooling polar temperatures, large lakes were proposed to mitigate seasonal climate changes. To replicate this case, a lake was inserted into North America and a climate model was run using varying carbon dioxide levels. The model runs concluded that while the lake did reduce the seasonality of the region greater than just an increase in carbon dioxide,
834-571: A major mountain chain in the Oligocene, as subduction became more direct into the coastline. Climate during the Oligocene reflected a general cooling trend following the Early Eocene Climatic Optimum . This transformed the Earth's climate from a greenhouse to an icehouse climate. The Eocene-Oligocene transition was a major cooling event and reorganization of the biosphere, being part of
973-531: A marine ecosystem)—one of the largest in the Cenozoic. This event happened around 55.8 Ma, and was one of the most significant periods of global change during the Cenozoic. The middle Eocene was characterized by the shift towards a cooler climate at the end of the EECO, around 47.8 Ma, which was briefly interrupted by another warming event called the middle Eocene climatic optimum (MECO). Lasting for about 400,000 years,
1112-556: A regression of tropical broad leaf forests to the equatorial belt . The start of the Oligocene is marked by a notable extinction event called the Grande Coupure ; it featured the replacement of European fauna with Asian fauna , except for the endemic rodent and marsupial families. By contrast, the Oligocene–Miocene boundary is not set at an easily identified worldwide event but rather at regional boundaries between
1251-503: A role in triggering the ETM2 and ETM3. An enhancement of the biological pump proved effective at sequestering excess carbon during the recovery phases of these hyperthermals. These hyperthermals led to increased perturbations in planktonic and benthic foraminifera , with a higher rate of fluvial sedimentation as a consequence of the warmer temperatures. Unlike the PETM, the lesser hyperthermals of
1390-520: A second dam further north, halfway between Kokaral and Aralsk , this city would thus regain its maritime space. The lake would then be in two parts, only the upper level, corresponding to the Saryshyganak bay, would return to the level before 1970. A supply channel would be necessary, for this several options are possible : a canal of around sixty km long coming from the Syr Daria, a channel coming from
1529-460: A semiarid prairie home and included entelodonts like Archaeotherium , camelids (such as Poebrotherium ), running rhinoceratoids , three-toed equids (such as Mesohippus ), nimravids , protoceratids , and early canids like Hesperocyon . Merycoidodonts, an endemic American group, were very diverse during this time. Australia and South America became geographically isolated and developed their own distinctive endemic fauna. These included
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#17328486674391668-526: A significant amount of water vapor is released. Another requirement for polar stratospheric clouds is cold temperatures to ensure condensation and cloud production. Polar stratospheric cloud production, since it requires the cold temperatures, is usually limited to nighttime and winter conditions. With this combination of wetter and colder conditions in the lower stratosphere, polar stratospheric clouds could have formed over wide areas in Polar Regions. To test
1807-637: A temporary constriction of the Drake Passage from sometime in the middle to late Oligocene (29 to 22 mya) to the middle Miocene (15 mya). The reorganization of the oceanic tectonic plates of the northeastern Pacific, which had begun in the Paleocene, culminated with the arrival of the Murray and Mendocino Fracture Zones at the North American subduction zone in the Oligocene. This initiated strike-slip movement along
1946-701: A wide variety of climate conditions that includes the warmest climate in the Cenozoic Era , and arguably the warmest time interval since the Permian-Triassic mass extinction and Early Triassic, and ends in an icehouse climate. The evolution of the Eocene climate began with warming after the end of the Paleocene–Eocene Thermal Maximum (PETM) at 56 Ma to a maximum during the Eocene Optimum at around 49 Ma. During this period of time, little to no ice
2085-514: Is a geological epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period in the modern Cenozoic Era . The name Eocene comes from the Ancient Greek Ἠώς ( Ēṓs , " Dawn ") and καινός ( kainós , "new") and refers to the "dawn" of modern ('new') fauna that appeared during the epoch. The Eocene spans the time from the end of
2224-454: Is an important factor in the creation of the primary Type II polar stratospheric clouds that were created in the early Eocene. Since water vapor is the only supporting substance used in Type II polar stratospheric clouds, the presence of water vapor in the lower stratosphere is necessary where in most situations the presence of water vapor in the lower stratosphere is rare. When methane is oxidized,
2363-638: Is considered to be primarily due to carbon dioxide increases, because carbon isotope signatures rule out major methane release during this short-term warming. A sharp increase in atmospheric carbon dioxide was observed with a maximum of 4,000 ppm: the highest amount of atmospheric carbon dioxide detected during the Eocene. Other studies suggest a more modest rise in carbon dioxide levels. The increase in atmospheric carbon dioxide has also been hypothesised to have been driven by increased seafloor spreading rates and metamorphic decarbonation reactions between Australia and Antarctica and increased amounts of volcanism in
2502-581: Is conventionally divided into early (56–47.8 Ma), middle (47.8–38 Ma), and late (38–33.9 Ma) subdivisions. The corresponding rocks are referred to as lower, middle, and upper Eocene. The Ypresian Stage constitutes the lower, the Priabonian Stage the upper; and the Lutetian and Bartonian stages are united as the middle Eocene. The Western North American floras of the Eocene were divided into four floral "stages" by Jack Wolfe ( 1968 ) based on work with
2641-617: Is discernible in pollen counts from the Tibetan Plateau, which also show that the South Asian Monsoon had already developed by the late Oligocene. Around 25.8 Ma, the South Asian Monsoon underwent an episode of major intensification brought on by the uplift of the Tibetan Plateau. A deep 400,000-year glaciated Oligocene-Miocene boundary event is recorded at McMurdo Sound and King George Island . The early Eocene climate
2780-619: Is preceded by the Eocene Epoch and is followed by the Miocene Epoch. The Oligocene is the third and final epoch of the Paleogene Period. The Oligocene is often considered an important time of transition, a link between the archaic world of the tropical Eocene and the more modern ecosystems of the Miocene. Major changes during the Oligocene included a global expansion of grasslands , and
2919-703: Is short lived, as benthic oxygen isotope records indicate a return to cooling at ~40 Ma. At the end of the MECO, the MLEC resumed. Cooling and the carbon dioxide drawdown continued through the late Eocene and into the Eocene–Oligocene transition around 34 Ma. The post-MECO cooling brought with it a major aridification trend in Asia, enhanced by retreating seas. A monsoonal climate remained predominant in East Asia. The cooling during
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#17328486674393058-478: Is that a deep channel was in place between the two continents by the end of the early Oligocene. The timing of the formation of the Drake Passage between South America and Antarctica is also uncertain, with estimates ranging from 49 to 17 mya (early Eocene to Miocene), but oceanic circulation through the Drake Passage may also have been in place by the end of the early Oligocene. This may have been interrupted by
3197-524: Is the period of time when the Antarctic ice sheet began to rapidly expand. Greenhouse gases, in particular carbon dioxide and methane , played a significant role during the Eocene in controlling the surface temperature. The end of the PETM was met with very large sequestration of carbon dioxide into the forms of methane clathrate , coal , and crude oil at the bottom of the Arctic Ocean , that reduced
3336-618: The Grande Coupure (the "Great Break" in continuity) or the Eocene–Oligocene extinction event , which may be related to the impact of one or more large bolides in Siberia and in what is now Chesapeake Bay . As with other geologic periods , the strata that define the start and end of the epoch are well identified, though their exact dates are slightly uncertain. The term "Eocene" is derived from Ancient Greek Ἠώς ( Ēṓs ) meaning "Dawn", and καινός kainos meaning "new" or "recent", as
3475-684: The Middle Eocene Climatic Optimum (MECO). At around 41.5 Ma, stable isotopic analysis of samples from Southern Ocean drilling sites indicated a warming event for 600,000 years. A similar shift in carbon isotopes is known from the Northern Hemisphere in the Scaglia Limestones of Italy. Oxygen isotope analysis showed a large negative change in the proportion of heavier oxygen isotopes to lighter oxygen isotopes, which indicates an increase in global temperatures. The warming
3614-522: The Paleocene Epoch to the beginning of the Oligocene Epoch. The start of the Eocene is marked by a brief period in which the concentration of the carbon isotope C in the atmosphere was exceptionally low in comparison with the more common isotope C . The average temperature of Earth at the beginning of the Eocene was about 27 degrees Celsius. The end is set at a major extinction event called
3753-728: The Puget Group fossils of King County, Washington . The four stages, Franklinian , Fultonian , Ravenian , and Kummerian covered the Early Eocene through early Oligocene, and three of the four were given informal early/late substages. Wolfe tentatively deemed the Franklinian as Early Eocene, the Fultonian as Middle Eocene, the Ravenian as Late, and the Kummerian as Early Oligocene. The beginning of
3892-795: The Red Sea and Gulf of Aden . The Alps were rapidly rising in Europe as the African Plate continued to push north into the Eurasian Plate , isolating the remnants of the Tethys Sea . Sea levels were lower in the Oligocene than in the early Eocene, exposing large coastal plains in Europe and the Gulf Coast and Atlantic Coast of North America. The Obik Sea , which had separated Europe from Asia, retreated early in
4031-616: The San Andreas Fault and extensional tectonics in the Basin and Range province , ended volcanism south of the Cascades, and produced clockwise rotation of many western North American terranes. The Rocky Mountains were at their peak. A new volcanic arc was established in western North America, far inland from the coast, reaching from central Mexico through the Mogollon-Datil volcanic field to
4170-698: The San Juan volcanic field , then through Utah and Nevada to the ancestral Northern Cascades. Huge ash deposits from these volcanoes created the White River and Arikaree Groups of the High Plains, with their excellent fossil beds. Between 31 and 26 mya, the Ethiopia-Yemen Continental Flood Basalts were emplaced by the East African large igneous province , which also initiated rifting along
4309-406: The amount of oxygen in the Earth's atmosphere more or less doubled. During the warming in the early Eocene between 55 and 52 Ma, there were a series of short-term changes of carbon isotope composition in the ocean. These isotope changes occurred due to the release of carbon from the ocean into the atmosphere that led to a temperature increase of 4–8 °C (7.2–14.4 °F) at the surface of
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4448-426: The archaeocete cetaceans began to decrease in diversity due to their lack of echolocation, which was very useful as the water became colder and cloudier. Other factors to their decline could include climate changes and competition with today's modern cetaceans and the requiem sharks , which also appeared in this epoch. Early desmostylians , like Behemotops , are known from the Oligocene. Pinnipeds appeared near
4587-599: The creodonts that had dominated the Paleocene in the Old World. Rodents and rabbits underwent tremendous diversification due to the increase in suitable habitats for ground-dwelling seed eaters, as habitats for squirrel-like nut- and fruit-eaters diminished. The primates, once present in Eurasia, were reduced in range to Africa and South America. Many groups, such as equids , entelodonts , rhinos , merycoidodonts , and camelids , became more able to run during this time, adapting to
4726-417: The proxy data . Using all different ranges of greenhouse gasses that occurred during the early Eocene, models were unable to produce the warming that was found at the poles and the reduced seasonality that occurs with winters at the poles being substantially warmer. The models, while accurately predicting the tropics, tend to produce significantly cooler temperatures of up to 20 °C (36 °F) colder than
4865-654: The southeast United States . After the Paleocene–Eocene Thermal Maximum, members of the Equoidea arose in North America and Europe, giving rise to some of the earliest equids such as Sifrhippus and basal European equoids such as the palaeothere Hyracotherium . Some of the later equoids were especially species-rich; Palaeotherium , ranging from small to very large in size, is known from as many as 16 species. Established large-sized mammals of
5004-522: The Antarctic Circumpolar Current. There is some evidence that it occurred much earlier, during the early Eocene. The other major oceanic gateway opening during this time was the Tasman, or Tasmanian, depending on the paper, gateway between Australia and Antarctica. The time frame for this opening is less disputed than the Drake Passage and is largely considered to have occurred around 34 Ma. As
5143-673: The Aral Sea split into northern and southern parts; the southern part was the former South Aral Sea . After the fall of the Soviet Union , the government of independent Kazakhstan decided to restore the northern lake fed by Syr Darya . In 2003, the lake was 30 m (98 ft) in depth and 2,550 km (985 sq mi) in area ( JAXA source: 3,200 km (1,240 sq mi)); by 2008 it had reached 42 m (138 ft) in depth and 3,300 km (1,270 sq mi) in area ( JAXA source: 3,600 km (1,390 sq mi)). The poorly built Dike Kokaral intended to contain
5282-634: The Arctic may have influenced this oceanic cooling, though the extent of this influence is still a matter of some significant dispute. The opening and closing of ocean gateways: the opening of the Drake Passage ; the opening of the Tasmanian Gateway and the closing of the Tethys seaway; along with the final formation of the Greenland – Iceland – Faroes Ridge; played vital parts in reshaping oceanic currents during
5421-603: The Azolla Event. This cooling trend at the end of the EECO has also been proposed to have been caused by increased siliceous plankton productivity and marine carbon burial, which also helped draw carbon dioxide out of the atmosphere. Cooling after this event, part of a trend known as the Middle-Late Eocene Cooling (MLEC), continued due to continual decrease in atmospheric carbon dioxide from organic productivity and weathering from mountain building . Many regions of
5560-528: The Dong Ho Formation of Oligocene age shows that the Oligocene flora of what is now Vietnam was very similar to its present flora. Kelps make their first appearance in the fossil record during the earliest Oligocene. Most extant mammal families had appeared by the end of the Oligocene. These included primitive three-toed horses, rhinoceroses, camels, deer, and peccaries. Carnivores such as dogs , nimravids , bears, weasels, and raccoons began to replace
5699-512: The Drake Passage opened and enabled the formation of the Antarctic Circumpolar Current (ACC), which would have kept the cold waters of Antarctica circulating around that continent and strengthened the formation of Antarctic Bottom Water (ABW). With the cold water concentrated around Antarctica, sea surface temperatures and, consequently, continental temperatures would have dropped. The onset of Antarctic glaciation occurred during
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5838-404: The EECO. Relative to present-day values, bottom water temperatures are 10 °C (18 °F) higher according to isotope proxies. With these bottom water temperatures, temperatures in areas where deep water forms near the poles are unable to be much cooler than the bottom water temperatures. An issue arises, however, when trying to model the Eocene and reproduce the results that are found with
5977-530: The Early Eocene had negligible consequences for terrestrial mammals. These Early Eocene hyperthermals produced a sustained period of extremely hot climate known as the Early Eocene Climatic Optimum (EECO). During the early and middle EECO, the superabundance of the euryhaline dinocyst Homotryblium in New Zealand indicates elevated ocean salinity in the region. One of the unique features of
6116-440: The Earth including the poles. Tropical forests extended across much of modern Africa, South America, Central America, India, South-east Asia and China. Paratropical forests grew over North America, Europe and Russia, with broad-leafed evergreen and broad-leafed deciduous forests at higher latitudes. Polar forests were quite extensive. Fossils and even preserved remains of trees such as swamp cypress and dawn redwood from
6255-409: The Eocene and Neogene for the Miocene and Pliocene in 1853. After decades of inconsistent usage, the newly formed International Commission on Stratigraphy (ICS), in 1969, standardized stratigraphy based on the prevailing opinions in Europe: the Cenozoic Era subdivided into the Tertiary and Quaternary sub-eras, and the Tertiary subdivided into the Paleogene and Neogene periods. In 1978, the Paleogene
6394-457: The Eocene have been found on Ellesmere Island in the Arctic . Even at that time, Ellesmere Island was only a few degrees in latitude further south than it is today. Fossils of subtropical and even tropical trees and plants from the Eocene also have been found in Greenland and Alaska . Tropical rainforests grew as far north as northern North America and Europe . Palm trees were growing as far north as Alaska and northern Europe during
6533-503: The Eocene include the Uintatherium , Arsinoitherium , and brontotheres , in which the former two, unlike the latter, did not belong to ungulates but groups that became extinct shortly after their establishments. Large terrestrial mammalian predators had already existed since the Paleocene, but new forms now arose like Hyaenodon and Daphoenus (the earliest lineage of a once-successful predatory family known as bear dogs ). Entelodonts meanwhile established themselves as some of
6672-544: The Eocene's climate as mentioned before was the equable and homogeneous climate that existed in the early parts of the Eocene. A multitude of proxies support the presence of a warmer equable climate being present during this period of time. A few of these proxies include the presence of fossils native to warm climates, such as crocodiles , located in the higher latitudes, the presence in the high latitudes of frost-intolerant flora such as palm trees which cannot survive during sustained freezes, and fossils of snakes found in
6811-426: The Eocene, and compression was replaced with crustal extension that ultimately gave rise to the Basin and Range Province . The Kishenehn Basin, around 1.5 km in elevation during the Lutetian, was uplifted to an altitude of 2.5 km by the Priabonian. Huge lakes formed in the high flat basins among uplifts, resulting in the deposition of the Green River Formation lagerstätte . At about 35 Ma, an asteroid impact on
6950-495: The Eocene-Oligocene transition are not yet fully understood. The timing is wrong for this to be caused either by known impact events or by the volcanic activity on the Ethiopean Plateau. Two other possible drivers of climate change, not mutually exclusive, have been proposed. The first is thermal isolation of the continent of Antarctica by development of the Antarctic Circumpolar Current . Deep sea cores from south of New Zealand suggest that cold deep-sea currents were present by
7089-399: The Eocene-Oligocene transition is the timing of the creation of the circulation is uncertain. For Drake Passage , sediments indicate the opening occurred ~41 Ma while tectonics indicate that this occurred ~32 Ma. Solar activity did not change significantly during the greenhouse-icehouse transition across the Eocene-Oligocene boundary. During the early-middle Eocene, forests covered most of
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#17328486674397228-445: The Grande Coupure. The lowering of sea levels closed the Turgai Strait across the Obik Sea, which had previously separated Asia from Europe. This allowed Asian mammals, such as rhinoceroses and ruminants , to enter Europe and drive endemic species to extinction. Lesser faunal turnovers occurred simultaneously with the Oi2 event and towards the end of the Oligocene. There was significant diversification of mammals in Eurasia, including
7367-419: The Kamyslybas or even a chanel coming from upper north avoiding arid regions. Oligocene The Oligocene ( IPA : / ˈ ɒ l ɪ ɡ ə s iː n , - ɡ oʊ -/ OL -ə-gə-seen, -goh- ) is a geologic epoch of the Paleogene Period that extends from about 33.9 million to 23 million years before the present ( 33.9 ± 0.1 to 23.03 ± 0.05 Ma ). As with other older geologic periods,
7506-439: The Kummerian was refined by Gregory Retallack et al (2004) as 40 Mya, with a refined end at the Eocene-Oligocene boundary where the younger Angoonian floral stage starts. During the Eocene, the continents continued to drift toward their present positions. At the beginning of the period, Australia and Antarctica remained connected, and warm equatorial currents may have mixed with colder Antarctic waters, distributing
7645-426: The MECO was responsible for a globally uniform 4° to 6°C warming of both the surface and deep oceans, as inferred from foraminiferal stable oxygen isotope records. The resumption of a long-term gradual cooling trend resulted in a glacial maximum at the late Eocene/early Oligocene boundary. The end of the Eocene was also marked by the Eocene–Oligocene extinction event , also known as the Grande Coupure . The Eocene
7784-625: The MECO. Both groups of modern ungulates (hoofed animals) became prevalent because of a major radiation between Europe and North America, along with carnivorous ungulates like Mesonyx . Early forms of many other modern mammalian orders appeared, including horses (most notably the Eohippus ), bats , proboscidians (elephants), primates, and rodents . Older primitive forms of mammals declined in variety and importance. Important Eocene land fauna fossil remains have been found in western North America, Europe, Patagonia , Egypt , and southeast Asia . Marine fauna are best known from South Asia and
7923-449: The New World and Old World monkeys. The South American continent was home to animals such as pyrotheres and astrapotheres , as well as litopterns and notoungulates . Sebecosuchians , terror birds , and carnivorous metatheres , like the borhyaenids remained the dominant predators. Africa was also relatively isolated and retained its endemic fauna. These included mastodonts, hyraxes, arsinoitheres, and other archaic forms. Egypt in
8062-405: The North American continent, and it reduced the seasonal variation of temperature by up to 75%. While orbital parameters did not produce the warming at the poles, the parameters did show a great effect on seasonality and needed to be considered. Another method considered for producing the warm polar temperatures were polar stratospheric clouds . Polar stratospheric clouds are clouds that occur in
8201-417: The North Aral Sea and save its fisheries failed twice, but in 2005, the government of Kazakhstan was able to fund a more robust design. Since then, water levels have risen faster than expected and fish stocks have increased. Plans to build a second dike to increase water levels further were due to begin in 2010, but have so far not materialized. There is now an ongoing effort in Kazakhstan to save and replenish
8340-518: The North Aral Sea. As part of this effort, a dam project was completed in 2005; in 2008, the water level in this lake had risen by 12 m (39 ft) from its level in 2003. Salinity has dropped, and fish are again found in sufficient numbers for some fishing to be viable, especially mackerel introduced at the Syr Darya river delta, though attempts to introduce tuna in 2009 failed. The Kokaral dam could be raised by several meters, which would restore this lake to its pre-1970 level. A discharge on
8479-459: The Norwegian-Greenland Sea indicate a drop in winter temperatures at high latitudes of about 5 °C (9.0 °F) just prior to the Oi1 event. Borehole dating from the Southeast Faroes drift indicates that deep-ocean circulation from the Arctic Ocean to the North Atlantic Ocean began in the early Oligocene. The best terrestrial record of Oligocene climate comes from North America, where temperatures dropped by 7 to 11 °C (13 to 20 °F) in
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#17328486674398618-436: The Norwegian-Greenland Sea indicated that glaciers had appeared in Greenland by the start of the Oligocene. Continental ice sheets in Antarctica reached sea level during the transition. Glacially rafted debris of early Oligocene age in the Weddell Sea and Kerguelen Plateau , in combination with Oi1 isotope shift, provides unambiguous evidence of a continental ice sheet on Antarctica by the early Oligocene. The causes of
8757-433: The Oligocene Epoch, the continents continued to drift toward their present positions. Antarctica became more isolated as deep ocean channels were established between Antarctica and Australia and South America . Australia had been very slowly rifting away from West Antarctica since the Jurassic, but the exact timing of the establishment of ocean channels between the two continents remains uncertain. However, one estimate
8896-401: The Oligocene exists mostly in isotopic proxies. Patterns of extinction and patterns of species migration can also be studied to gain insight into ocean conditions. For a while, it was thought that the glaciation of Antarctica may have significantly contributed to the cooling of the ocean, however, recent evidence tends to deny this. Isotopic evidence suggests that during the early Oligocene,
9035-402: The Oligocene was an environment of lush forested deltas. Nevertheless, the Early Oligocene saw a major reduction in the diversity of many Afro-Arabian mammal clades, including hyaenodonts, primates, and hystricognath and anomaluroid rodents. During the Oligocene, the Tethyan marine biodiversity hotspot collapsed as the Tethys Ocean contracted. The seas around Southeast Asia and Australia became
9174-407: The Oligocene, creating a persistent land connection between the continents. The Paratethys Sea stretched from what is now the Balkan Peninsula across Central Asia to the Tian Shan region of what is now Xinjiang . There appears to have been a land bridge in the early Oligocene between North America and Europe, since the faunas of the two regions are very similar. However, towards the end of
9313-440: The Oligocene, there was a brief marine incursion in Europe. The rise of the Himalayas during the Oligocene remains poorly understood. One recent hypothesis is that a separate microcontinent collided with south Asia in the early Eocene, and India itself did not collide with south Asia until the end of the Oligocene. The Tibetan Plateau may have reached nearly its present elevation by the late Oligocene. The Andes first became
9452-404: The Oligocene. The Oligocene saw the emergence of parrotfishes, as the centre of marine biodiversity shifted from the Central Tethys eastward into the Indo-Pacific. The fossil record of marine mammals is a little spotty during this time, and not as well known as the Eocene or Miocene, but some fossils have been found. The baleen whales and toothed whales had just appeared, and their ancestors,
9591-406: The Oligocene. As the continents shifted to a more modern configuration, so too did ocean circulation. The Drake Passage is located between South America and Antarctica . Once the Tasmanian Gateway between Australia and Antarctica opened, all that kept Antarctica from being completely isolated by the Southern Ocean was its connection to South America. As the South American continent moved north,
9730-415: The PETM event in the sea floor or wetland environments. For contrast, today the carbon dioxide levels are at 400 ppm or 0.04%. During the early Eocene, methane was another greenhouse gas that had a drastic effect on the climate. Methane has 30 times more of a warming effect than carbon dioxide on a 100-year scale (i.e., methane has a global warming potential of 29.8±11). Most of the methane released to
9869-399: The Shevchenko Bay, in the former Auzykokaral Strait, west of the former Kokaral Island, would allow available water to be used towards the other remnants of the Aral Sea, Barsakelmes Lake and the western basin, preventing this water from being lost in the sandy desert that the eastern part of the ancient sea has become, as is currently the case. Another project would be the construction of
10008-405: The actual determined temperature at the poles. This error has been classified as the "equable climate problem". To solve this problem, the solution would involve finding a process to warm the poles without warming the tropics. Some hypotheses and tests which attempt to find the process are listed below. Due to the nature of water as opposed to land, less temperature variability would be present if
10147-410: The addition of a large lake was unable to reduce the seasonality to the levels shown by the floral and faunal data. The transport of heat from the tropics to the poles, much like how ocean heat transport functions in modern times, was considered a possibility for the increased temperature and reduced seasonality for the poles. With the increased sea surface temperatures and the increased temperature of
10286-422: The amount of polar stratospheric clouds. While the polar stratospheric clouds could explain the reduction of the equator to pole temperature gradient and the increased temperatures at the poles during the early Eocene, there are a few drawbacks to maintaining polar stratospheric clouds for an extended period of time. Separate model runs were used to determine the sustainability of the polar stratospheric clouds. It
10425-456: The atmosphere during this period of time would have been from wetlands, swamps, and forests. The atmospheric methane concentration today is 0.000179% or 1.79 ppmv . As a result of the warmer climate and the sea level rise associated with the early Eocene, more wetlands, more forests, and more coal deposits would have been available for methane release. If we compare the early Eocene production of methane to current levels of atmospheric methane,
10564-515: The atmosphere may have been more important. Once the Antarctic region began to cool down, the ocean surrounding Antarctica began to freeze, sending cold water and icefloes north and reinforcing the cooling. The northern supercontinent of Laurasia began to fragment, as Europe , Greenland and North America drifted apart. In western North America, the Laramide Orogeny came to an end in
10703-538: The atmosphere, contributing to global cooling. The gradual separation of the clump of continental crust and the deepening of the tectonic ridge in the North Atlantic that would become Greenland, Iceland, and the Faroe Islands helped to increase the deep water flow in that area. More information about the evolution of North Atlantic Deep Water will be given a few sections down. Evidence for ocean-wide cooling during
10842-524: The atmospheric carbon dioxide. This event was similar in magnitude to the massive release of greenhouse gasses at the beginning of the PETM, and it is hypothesized that the sequestration was mainly due to organic carbon burial and weathering of silicates. For the early Eocene there is much discussion on how much carbon dioxide was in the atmosphere. This is due to numerous proxies representing different atmospheric carbon dioxide content. For example, diverse geochemical and paleontological proxies indicate that at
10981-719: The beginnings of sediment drift deposition in the North Atlantic, such as the Feni and Southeast Faroe drifts. The chilling of the South Ocean deep water began in earnest once the Tasmanian Gateway and the Drake Passage opened fully. Regardless of the time at which the opening of the Drake Passage occurred, the effect on the cooling of the Southern Ocean would have been the same. Eocene The Eocene ( IPA : / ˈ iː ə s iː n , ˈ iː oʊ -/ EE -ə-seen, EE -oh- )
11120-426: The boundary, such as the global oxygen isotope shift marking the expansion of Antarctic glaciation (the Oi1 event). The upper boundary of the Oligocene is defined by its GSSP at Carrosio , Italy , which coincides with the first appearance of the foraminiferan Paragloborotalia kugleri and with the base of magnetic polarity chronozone C6Cn.2n. Oligocene faunal stages from youngest to oldest are: During
11259-413: The decline into an icehouse climate and the rapid expansion of the Antarctic ice sheet . The transition from a warming climate into a cooling climate began at around 49 Ma. Isotopes of carbon and oxygen indicate a shift to a global cooling climate. The cause of the cooling has been attributed to a significant decrease of >2,000 ppm in atmospheric carbon dioxide concentrations. One proposed cause of
11398-437: The deep ocean water during the early Eocene, one common hypothesis was that due to these increases there would be a greater transport of heat from the tropics to the poles. Simulating these differences, the models produced lower heat transport due to the lower temperature gradients and were unsuccessful in producing an equable climate from only ocean heat transport. While typically seen as a control on ice growth and seasonality,
11537-410: The deep ocean. On top of that, MECO warming caused an increase in the respiration rates of pelagic heterotrophs , leading to a decreased proportion of primary productivity making its way down to the seafloor and causing a corresponding decline in populations of benthic foraminifera. An abrupt decrease in lakewater salinity in western North America occurred during this warming interval. This warming
11676-446: The difficulty may be that there were strong regional variations in the response to climate shifts. Evidence of a relatively warm Oligocene suggests an enigmatic climate state, neither hothouse nor icehouse. The late Oligocene (26.5 to 24 mya) likely saw a warming trend in spite of low pCO2 levels, though this appears to vary by region. However, Antarctica remained heavily glaciated during this warming period. The late Oligocene warming
11815-417: The earliest Oligocene. This change is seen from Alaska to the Gulf Coast. Upper Eocene paleosols reflect annual precipitation of over a meter of rain, but early Oligocene precipitation was less than half this. In central North America, the cooling was by 8.2 ± 3.1 °C over a period of 400,000 years, though there is little indication of significant increase in aridity during this interval. Ice-rafted debris in
11954-411: The early Eocene would have produced triple the amount of methane. The warm temperatures during the early Eocene could have increased methane production rates, and methane that is released into the atmosphere would in turn warm the troposphere, cool the stratosphere, and produce water vapor and carbon dioxide through oxidation. Biogenic production of methane produces carbon dioxide and water vapor along with
12093-888: The early Eocene, although they became less abundant as the climate cooled. Dawn redwoods were far more extensive as well. The earliest definitive Eucalyptus fossils were dated from 51.9 Ma, and were found in the Laguna del Hunco deposit in Chubut province in Argentina . Cooling began mid-period, and by the end of the Eocene continental interiors had begun to dry, with forests thinning considerably in some areas. The newly evolved grasses were still confined to river banks and lake shores, and had not yet expanded into plains and savannas . The cooling also brought seasonal changes. Deciduous trees, better able to cope with large temperature changes, began to overtake evergreen tropical species. By
12232-487: The early Oligocene, and the effect of the Drake Passage opening on this glaciation has been the subject of much research. However, some controversy still exists as to the exact timing of the passage opening, whether it occurred at the start of the Oligocene or nearer the end. Even so, many theories agree that at the Eocene/Oligocene (E/O) boundary, a yet shallow flow existed between South America and Antarctica, permitting
12371-496: The early Oligocene. However, the timing of this event remains controversial. The other possibility, for which there is considerable evidence, is a drop in atmospheric carbon dioxide levels ( pCO2 ) during the transition. The pCO2 is estimated to have dropped just before the transition, to 760 ppm at the peak of ice sheet growth, then rebounded slightly before resuming a more gradual fall. Climate modeling suggests that glaciation of Antarctica took place only when pCO2 dropped below
12510-617: The eastern coast of North America formed the Chesapeake Bay impact crater . The Tethys Ocean finally closed with the collision of Africa and Eurasia, while the uplift of the Alps isolated its final remnant, the Mediterranean , and created another shallow sea with island archipelagos to the north. Planktonic foraminifera in the northwestern Peri-Tethys are very similar to those of the Tethys in
12649-432: The end of the epoch from an otter -like ancestor. The Oligocene sees the beginnings of modern ocean circulation, with tectonic shifts causing the opening and closing of ocean gateways. Cooling of the oceans had already commenced by the Eocene/Oligocene boundary, and they continued to cool as the Oligocene progressed. The formation of permanent Antarctic ice sheets during the early Oligocene and possible glacial activity in
12788-486: The end of the period, deciduous forests covered large parts of the northern continents, including North America, Eurasia and the Arctic, and rainforests held on only in equatorial South America , Africa , India and Australia . Antarctica began the Eocene fringed with a warm temperate to sub-tropical rainforest . Pollen found in Prydz Bay from the Eocene suggest taiga forest existed there. It became much colder as
12927-502: The enhanced burial of azolla could have had a significant effect on the world atmospheric carbon content and may have been the event to begin the transition into an ice house climate. The azolla event could have led to a draw down of atmospheric carbon dioxide of up to 470 ppm. Assuming the carbon dioxide concentrations were at 900 ppmv prior to the Azolla Event they would have dropped to 430 ppmv, or 30 ppmv more than they are today, after
13066-466: The enhanced carbon dioxide levels found in the early Eocene. The isolation of the Arctic Ocean, evidenced by euxinia that occurred at this time, led to stagnant waters and as the azolla sank to the sea floor, they became part of the sediments on the seabed and effectively sequestered the carbon by locking it out of the atmosphere for good. The ability for the azolla to sequester carbon is exceptional, and
13205-661: The epoch saw the dawn of recent, or modern, life. Scottish geologist Charles Lyell (ignoring the Quaternary) divided the Tertiary Epoch into the Eocene, Miocene , Pliocene , and New Pliocene ( Holocene ) Periods in 1833. British geologist John Phillips proposed the Cenozoic in 1840 in place of the Tertiary, and Austrian paleontologist Moritz Hörnes introduced the Paleogene for
13344-411: The expansion of the ice sheet was the creation of the Antarctic Circumpolar Current . The creation of the Antarctic circumpolar current would isolate the cold water around the Antarctic, which would reduce heat transport to the Antarctic along with creating ocean gyres that result in the upwelling of colder bottom waters. The issue with this hypothesis of the consideration of this being a factor for
13483-783: The gateway widened, the Antarctic Circumpolar Current strengthened. The Tethys Seaway was not a gateway, but rather a sea in its own right. Its closing during the Oligocene had significant impact on both ocean circulation and climate. The collisions of the African plate with the European plate and of the Indian subcontinent with the Asian plate, cut off the Tethys Seaway that had provided a low-latitude ocean circulation. The closure of Tethys built some new mountains (the Zagros range) and drew down more carbon dioxide from
13622-479: The giant indricotheres , that grew up to 6 meters (20 ft) at the shoulder and weighed up to 20 tons. Paraceratherium was one of the largest land mammals ever to walk the Earth. However, the indricotheres were an exception to a general tendency for Oligocene mammals to be much smaller than their Eocene counterparts. The earliest deer, giraffes, pigs, and cattle appeared in the mid-Oligocene in Eurasia. The first felid , Proailurus , originated in Asia during
13761-399: The heat around the planet and keeping global temperatures high. When Australia split from the southern continent around 45 Ma, the warm equatorial currents were routed away from Antarctica. An isolated cold water channel developed between the two continents. However, modeling results call into question the thermal isolation model for late Eocene cooling, and decreasing carbon dioxide levels in
13900-402: The hot house to the cold house. The beginning of the Eocene is marked by the Paleocene–Eocene Thermal Maximum , a short period of intense warming and ocean acidification brought about by the release of carbon en masse into the atmosphere and ocean systems, which led to a mass extinction of 30–50% of benthic foraminifera (single-celled species which are used as bioindicators of the health of
14039-404: The initial stages of the opening of the Drake Passage ~38.5 Ma was not global, as evidenced by an absence of cooling in the North Atlantic. During the cooling period, benthic oxygen isotopes show the possibility of ice creation and ice increase during this later cooling. The end of the Eocene and beginning of the Oligocene is marked with the massive expansion of area of the Antarctic ice sheet that
14178-516: The largest omnivores. The first nimravids , including Dinictis , established themselves as amongst the first feliforms to appear. Their groups became highly successful and continued to live past the Eocene. Basilosaurus is a very well-known Eocene whale , but whales as a group had become very diverse during the Eocene, which is when the major transitions from being terrestrial to fully aquatic in cetaceans occurred. The first sirenians were evolving at this time, and would eventually evolve into
14317-456: The late Oligocene and spread to Europe. There was only limited migration between Asia and North America. The cooling of central North America at the Eocene-Oligocene transition resulted in a large turnover of gastropods, amphibians, and reptiles. Mammals were much less affected. Crocodilians and pond turtles replaced by dry land tortoises. Molluscs shifted to more drought-tolerant forms. The White River Fauna of central North America inhabited
14456-556: The lower stratosphere at very low temperatures. Polar stratospheric clouds have a great impact on radiative forcing. Due to their minimal albedo properties and their optical thickness, polar stratospheric clouds act similar to a greenhouse gas and trap outgoing longwave radiation. Different types of polar stratospheric clouds occur in the atmosphere: polar stratospheric clouds that are created due to interactions with nitric or sulfuric acid and water (Type I) or polar stratospheric clouds that are created with only water ice (Type II). Methane
14595-825: The main source of deep water was the North Pacific and the Southern Ocean . As the Greenland-Iceland-Faroe Ridge sank and thereby connected the Norwegian–Greenland sea with the Atlantic Ocean, the deep water of the North Atlantic began to come into play as well. Computer models suggest that once this occurred, a more modern in appearance thermo-haline circulation started. Evidence for the early Oligocene onset of chilled North Atlantic deep water lies in
14734-610: The maximum of global warmth the atmospheric carbon dioxide values were at 700–900 ppm , while model simulations suggest a concentration of 1,680 ppm fits best with deep sea, sea surface, and near-surface air temperatures of the time. Other proxies such as pedogenic (soil building) carbonate and marine boron isotopes indicate large changes of carbon dioxide of over 2,000 ppm over periods of time of less than 1 million years. This large influx of carbon dioxide could be attributed to volcanic out-gassing due to North Atlantic rifting or oxidation of methane stored in large reservoirs deposited from
14873-530: The members of the new mammal orders were small, under 10 kg; based on comparisons of tooth size, Eocene mammals were only 60% of the size of the primitive Palaeocene mammals that preceded them. They were also smaller than the mammals that followed them. It is assumed that the hot Eocene temperatures favored smaller animals that were better able to manage the heat. Rodents were widespread. East Asian rodent faunas declined in diversity when they shifted from ctenodactyloid-dominant to cricetid–dipodid-dominant after
15012-474: The methane, as well as yielding infrared radiation. The breakdown of methane in an atmosphere containing oxygen produces carbon monoxide, water vapor and infrared radiation. The carbon monoxide is not stable, so it eventually becomes carbon dioxide and in doing so releases yet more infrared radiation. Water vapor traps more infrared than does carbon dioxide. At about the beginning of the Eocene Epoch (55.8–33.9 Ma)
15151-557: The middle Lutetian but become completely disparate in the Bartonian, indicating biogeographic separation. Though the North Atlantic was opening, a land connection appears to have remained between North America and Europe since the faunas of the two regions are very similar. Eurasia was separated in three different landmasses 50 Ma; Western Europe, Balkanatolia and Asia. About 40 Ma, Balkanatolia and Asia were connected, while Europe
15290-399: The modern mammal orders appear within a brief period during the early Eocene . At the beginning of the Eocene, several new mammal groups arrived in North America. These modern mammals, like artiodactyls , perissodactyls , and primates , had features like long, thin legs , feet, and hands capable of grasping, as well as differentiated teeth adapted for chewing. Dwarf forms reigned. All
15429-502: The new dominant hotspot of marine biodiversity. At sea, 97% of marine snail species, 89% of clams, and 50% of echinoderms of the Gulf Coast did not survive past the earliest Oligocene. New species evolved, but the overall diversity diminished. Cold-water mollusks migrated around the Pacific Rim from Alaska and Siberia. The marine animals of Oligocene oceans resembled today's fauna, such as the bivalves . Calcareous cirratulids appeared in
15568-644: The ocean. Recent analysis of and research into these hyperthermals in the early Eocene has led to hypotheses that the hyperthermals are based on orbital parameters, in particular eccentricity and obliquity. The hyperthermals in the early Eocene, notably the Palaeocene–Eocene Thermal Maximum (PETM), the Eocene Thermal Maximum 2 (ETM2), and the Eocene Thermal Maximum 3 (ETM3), were analyzed and found that orbital control may have had
15707-566: The opening of the Drake Passage is around 30 Ma. One of the possible issues with this timing was the continental debris cluttering up the seaway between the two plates in question. This debris, along with what is known as the Shackleton Fracture Zone , has been shown in a recent study to be fairly young, only about 8 million years old. The study concludes that the Drake Passage would be free to allow significant deep water flow by around 31 Ma. This would have facilitated an earlier onset of
15846-468: The opening of the Drake Passage is during the early Miocene. Despite the shallow flow between South America and Antarctica, there was not enough of a deep water opening to allow for significant flow to create a true Antarctic Circumpolar Current. If the opening occurred as late as hypothesized, then the Antarctic Circumpolar Current could not have had much of an effect on early Oligocene cooling, as it would not have existed. The earliest hypothesized time for
15985-416: The orbital parameters were theorized as a possible control on continental temperatures and seasonality. Simulating the Eocene by using an ice free planet, eccentricity , obliquity , and precession were modified in different model runs to determine all the possible different scenarios that could occur and their effects on temperature. One particular case led to warmer winters and cooler summer by up to 30% in
16124-517: The period from 33.8 to 33.5 mya. By the end of the transition, sea levels had dropped by 105 meters (344 ft), and ice sheets were 25% greater in extent than in the modern world. The effects of the transition can be seen in the geological record at many locations around the world. Ice volumes rose as temperature and sea levels dropped. Playa lakes of the Tibetan Plateau disappeared at the transition, pointing to cooling and aridification of central Asia. Pollen and spore counts in marine sediments of
16263-416: The period progressed; the heat-loving tropical flora was wiped out, and by the beginning of the Oligocene, the continent hosted deciduous forests and vast stretches of tundra . During the Eocene, plants and marine faunas became quite modern. Many modern bird orders first appeared in the Eocene. The Eocene oceans were warm and teeming with fish and other sea life. The oldest known fossils of most of
16402-568: The period, grass was not quite common enough for modern savannas . In North America, much of the dense forest was replaced by patchy scrubland with riparian forests. Subtropical species dominated with cashews and lychee trees present, and temperate woody plants such as roses , beeches , and pines were common. The legumes spread, while sedges and ferns continued their ascent. In Europe, floral assemblages became increasingly affected by strengthening seasonality as it related to wildfire activity. The Ha Long megafossil flora from
16541-623: The plains that were spreading as the Eocene rainforests receded. Brontotheres died out in the Earliest Oligocene, and creodonts died out outside Africa and the Middle East at the end of the period. Multituberculates , an ancient lineage of primitive mammals that originated back in the Jurassic , also became extinct in the Oligocene, aside from the gondwanatheres . The Eocene-Oligocene transition in Europe and Asia has been characterized as
16680-438: The polar stratospheric clouds effects on the Eocene climate, models were run comparing the effects of polar stratospheric clouds at the poles to an increase in atmospheric carbon dioxide. The polar stratospheric clouds had a warming effect on the poles, increasing temperatures by up to 20 °C in the winter months. A multitude of feedbacks also occurred in the models due to the polar stratospheric clouds' presence. Any ice growth
16819-458: The reduction in carbon dioxide during the warming to cooling transition was the azolla event . With the equable climate during the early Eocene, warm temperatures in the arctic allowed for the growth of azolla , which is a floating aquatic fern, on the Arctic Ocean . The significantly high amounts of carbon dioxide also acted to facilitate azolla blooms across the Arctic Ocean. Compared to current carbon dioxide levels, these azolla grew rapidly in
16958-454: The region. One possible cause of atmospheric carbon dioxide increase could have been a sudden increase due to metamorphic release due to continental drift and collision of India with Asia and the resulting formation of the Himalayas ; however, data on the exact timing of metamorphic release of atmospheric carbon dioxide is not well resolved in the data. Recent studies have mentioned, however, that
17097-445: The removal of the ocean between Asia and India could have released significant amounts of carbon dioxide. Another hypothesis still implicates a diminished negative feedback of silicate weathering as a result of continental rocks having become less weatherable during the warm Early and Middle Eocene, allowing volcanically released carbon dioxide to persist in the atmosphere for longer. Yet another explanation hypothesises that MECO warming
17236-551: The rock beds that define the epoch are well identified but the exact dates of the start and end of the epoch are slightly uncertain. The name Oligocene was coined in 1854 by the German paleontologist Heinrich Ernst Beyrich from his studies of marine beds in Belgium and Germany. The name comes from Ancient Greek ὀλίγος ( olígos ) 'few' and καινός ( kainós ) 'new', and refers to the sparsity of extant forms of molluscs . The Oligocene
17375-478: The start of an Antarctic Circumpolar Current. Stemming from the issue of when the opening of the Drake Passage took place, is the dispute over how great of an influence the opening of the Drake Passage had on the global climate. While early researchers concluded that the advent of the ACC was highly important, perhaps even the trigger, for Antarctic glaciation and subsequent global cooling, other studies have suggested that
17514-583: The time of the Oi2 oxygen isotope shift. This led to the largest drop of sea level in past 100 million years, by about 75 meters (246 ft). This is reflected in a mid-Oligocene incision of continental shelves and unconformities in marine rocks around the world. Some evidence suggests that the climate remained warm at high latitudes even as ice sheets experienced cyclical growth and retreat in response to orbital forcing and other climate drivers. Other evidence indicates significant cooling at high latitudes. Part of
17653-399: The tropics that would require much higher average temperatures to sustain them. TEX 86 BAYSPAR measurements indicate extremely high sea surface temperatures of 40 °C (104 °F) to 45 °C (113 °F) at low latitudes, although clumped isotope analyses point to a maximum low latitude sea surface temperature of 36.3 °C (97.3 °F) ± 1.9 °C (35.4 °F) during
17792-476: The warmer late Oligocene and the relatively cooler Miocene. The lower boundary of the Oligocene (its Global Boundary Stratotype Section and Point or GSSP) is placed at the last appearance of the foraminiferan genus Hantkenina in a quarry at Massignano , Italy . However, this GSSP has been criticized as excluding the uppermost part of the type Eocene Priabonian Stage and because it is slightly earlier than important climate shifts that form natural markers for
17931-401: The world as tropical and sub- tropical forests were replaced by temperate deciduous forests . Open plains and deserts became more common and grasses expanded from their water-bank habitat in the Eocene moving out into open tracts. The decline in pCO2 favored C4 photosynthesis , which is found only in angiosperms and is particularly characteristic of grasses. However, even at the end of
18070-457: The world became more arid and cold over the course of the stage, such as the Fushun Basin. In East Asia, lake level changes were in sync with global sea level changes over the course of the MLEC. Global cooling continued until there was a major reversal from cooling to warming in the Bartonian. This warming event, signifying a sudden and temporary reversal of the cooling conditions, is known as
18209-414: The δ O signature is too strong for glaciation to be the main trigger for cooling. Through study of Pacific Ocean sediments, other researchers have shown that the transition from warm Eocene ocean temperatures to cool Oligocene ocean temperatures took only 300,000 years, which strongly implies that feedbacks and factors other than the ACC were integral to the rapid cooling. The latest hypothesized time for
18348-528: Was a major step into the icehouse climate. Multiple proxies, such as oxygen isotopes and alkenones , indicate that at the Eocene–Oligocene transition, the atmospheric carbon dioxide concentration had decreased to around 750–800 ppm, approximately twice that of present levels . Along with the decrease of atmospheric carbon dioxide reducing the global temperature, orbital factors in ice creation can be seen with 100,000-year and 400,000-year fluctuations in benthic oxygen isotope records. Another major contribution to
18487-470: Was caused by the simultaneous occurrence of minima in both the 400 kyr and 2.4 Myr eccentricity cycles. During the MECO, sea surface temperatures in the Tethys Ocean jumped to 32–36 °C, and Tethyan seawater became more dysoxic. A decline in carbonate accumulation at ocean depths of greater than three kilometres took place synchronously with the peak of the MECO, signifying ocean acidification took place in
18626-465: Was connected 34 Ma. The Fushun Basin contained large, suboxic lakes known as the paleo-Jijuntun Lakes. India collided with Asia , folding to initiate formation of the Himalayas . The incipient subcontinent collided with the Kohistan–Ladakh Arc around 50.2 Ma and with Karakoram around 40.4 Ma, with the final collision between Asia and India occurring ~40 Ma. The Eocene Epoch contained
18765-402: Was determined that in order to maintain the lower stratospheric water vapor, methane would need to be continually released and sustained. In addition, the amounts of ice and condensation nuclei would need to be high in order for the polar stratospheric cloud to sustain itself and eventually expand. The Eocene is not only known for containing the warmest period during the Cenozoic; it also marked
18904-452: Was officially defined as the Paleocene, Eocene, and Oligocene epochs; and the Neogene as the Miocene and Pliocene epochs. In 1989, Tertiary and Quaternary were removed from the time scale due to the arbitrary nature of their boundary, but Quaternary was reinstated in 2009. The Eocene is a dynamic epoch that represents global climatic transitions between two climatic extremes, transitioning from
19043-401: Was present on Earth with a smaller difference in temperature from the equator to the poles . Because of this the maximum sea level was 150 meters higher than current levels. Following the maximum was a descent into an icehouse climate from the Eocene Optimum to the Eocene–Oligocene transition at 34 Ma. During this decrease, ice began to reappear at the poles, and the Eocene–Oligocene transition
19182-437: Was slowed immensely and would lead to any present ice melting. Only the poles were affected with the change in temperature and the tropics were unaffected, which with an increase in atmospheric carbon dioxide would also cause the tropics to increase in temperature. Due to the warming of the troposphere from the increased greenhouse effect of the polar stratospheric clouds, the stratosphere would cool and would potentially increase
19321-583: Was very warm, with crocodilians and temperate plants thriving north of the Arctic Circle . The cooling trend that began in the middle Eocene continued into the Oligocene, bringing both poles well below freezing for the first time in the Phanerozoic . The cooling climate, together with the opening of some land bridges and the closing of others, led to a profound reorganization of the biosphere and loss of taxonomic diversity. Land animals and marine organisms reached
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