Oligocene
Template:Short description Template:Distinguish Template:Infobox geologic timespan The Oligocene (Template:IPAc-en Template:Respell)<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> is a geologic epoch of the Paleogene Period that extends from about 33.9 million to 23 million years before the present (Template:Val to Template:Val). As with other older geologic periods, 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<ref>Template:Cite journal From p. 664: "Der neue Name Oligocän mag sich zwischenstellen zwischen das ältere Eocän und das jüngere Miocän." (The new name Oligocene may be interposed between the older Eocene and the younger Miocene.)</ref><ref>Template:Cite book</ref> from his studies of marine beds in Belgium and Germany.Template:Sfn The name comes from Ancient Greek {{#invoke:Lang|lang}} (olígos) 'few' and {{#invoke:Lang|lang}} (kainós) 'new',<ref name=OnlineEtDict>Template:Cite encyclopedia</ref> and refers to the sparsity of extant forms of molluscs. The Oligocene 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.<ref>Haines, Tim; Walking with Beasts: A Prehistoric Safari, (New York: Dorling Kindersley Publishing, Inc., 1999)</ref> Major changes during the Oligocene included a global expansion of grasslands, and a regression of tropical broad leaf forests to the equatorial belt.
Template:Anchor 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 the warmer late Oligocene and the relatively cooler Miocene.
Boundaries and subdivisionsEdit
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 the boundary, such as the global oxygen isotope shift marking the expansion of Antarctic glaciation (the Oi1 event).Template:Sfn
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.<ref>Template:Cite journal</ref>
Oligocene faunal stages from youngest to oldest are:<ref name="stratigraphy379"/><ref>Template:Cite journal</ref>
| Chattian or late Oligocene | (Template:Ma– Template:Ma) |
| Rupelian or early Oligocene | (Template:Ma– Template:Ma) |
Tectonics and paleogeographyEdit
During the Oligocene Epoch, the continents continued to drift toward their present positions.Template:Sfn<ref name=Torsvik>Template:Cite book</ref> 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 is that a deep channel was in place between the two continents by the end of the early Oligocene.Template:Sfn 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),<ref>Template:Cite journal</ref> but oceanic circulation through the Drake Passage may also have been in place by the end of the early Oligocene.Template:SfnTemplate:Sfn This may have been interrupted by 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).<ref>Template:Cite journal</ref>
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 the San Andreas Fault and extensional tectonics in the Basin and Range province,Template:Sfn 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 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.Template:Sfn
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 the Red Sea and Gulf of Aden.Template:Sfn
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.Template:SfnTemplate:Sfn 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 the Oligocene, creating a persistent land connection between the continents.Template:Sfn The Paratethys Sea stretched from what is now the Balkan Peninsula across Central Asia to the Tian Shan region of what is now Xinjiang.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite book</ref> However, towards the end of the Oligocene, there was a brief marine incursion in Europe.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
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.<ref>Template:Cite journal</ref>Template:Sfn The Tibetan Plateau may have reached nearly its present elevation by the late Oligocene.<ref>Template:Cite journal</ref>
The Andes first became a major mountain chain in the Oligocene, as subduction became more direct into the coastline.Template:Sfn<ref name=Orme>Template:Cite book</ref>
ClimateEdit
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.Template:Sfn
Eocene-Oligocene transition and Oi1 eventEdit
The Eocene-Oligocene transition was a major cooling event and reorganization of the biosphere,<ref>Template:Cite book</ref><ref>Template:Cite book</ref> being part of a broader trend of global cooling lasting from the Bartonian to the Rupelian.<ref name="LauretanoEtAl2021">Template:Cite journal</ref><ref>Template:Cite journal</ref> The transition is marked by the Oi1 event, an oxygen isotope excursion occurring approximately 33.55 million years ago,<ref name="JovaneEtAl2006GGG">Template:Cite journal</ref> during which oxygen isotope ratios decreased by 1.3Template:Char. About 0.3–0.4Template:Char of this is estimated to be due to major expansion of Antarctic ice sheets. The remaining 0.9 to 1.0Template:Char was due to about Template:Convert of global cooling.Template:Sfn The transition likely took place in three closely spaced steps over the period from 33.8 to 33.5 mya. By the end of the transition, sea levels had dropped by Template:Convert, and ice sheets were 25% greater in extent than in the modern world.<ref>Template:Cite journal</ref>
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.<ref>Template:Cite journal</ref> Playa lakes of the Tibetan Plateau disappeared at the transition, pointing to cooling and aridification of central Asia.<ref>Template:Cite journal</ref> Pollen and spore counts in marine sediments of the Norwegian-Greenland Sea indicate a drop in winter temperatures at high latitudes of about Template:Convert just prior to the Oi1 event.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
The best terrestrial record of Oligocene climate comes from North America, where temperatures dropped by Template:Convert in 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.Template:Sfn<ref name="Late Eocene and Oligocene paleosols">Template:Cite journal</ref> 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.<ref name="Large temperature drop across the E">Template:Cite journal</ref> Ice-rafted debris in the Norwegian-Greenland Sea indicated that glaciers had appeared in Greenland by the start of the Oligocene.<ref>Template:Cite journal</ref>
Continental ice sheets in Antarctica reached sea level during the transition.Template:SfnTemplate:Sfn<ref>Template:Cite journal</ref> 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.Template:Sfn
The causes of the Eocene-Oligocene transition are not yet fully understood.Template:Sfn The timing is wrong for this to be caused either by known impact events or by the volcanic activity on the Ethiopean Plateau.Template:Sfn Two other possible drivers of climate change, not mutually exclusive, have been proposed.Template:Sfn The first is thermal isolation of the continent of Antarctica by development of the Antarctic Circumpolar Current.Template:SfnTemplate:Sfn<ref name=Torsvik/> Deep sea cores from south of New Zealand suggest that cold deep-sea currents were present by the early Oligocene.Template:Sfn However, the timing of this event remains controversial.<ref>Template:Cite journal</ref> The other possibility, for which there is considerable evidence, is a drop in atmospheric carbon dioxide levels (pCO2) during the transition.Template:Sfn<ref>Template:Cite journal</ref><ref name="LauretanoEtAl2021" /> 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.<ref>Template:Cite journal</ref> Climate modeling suggests that glaciation of Antarctica took place only when pCO2 dropped below a critical threshold value.Template:Sfn
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.<ref>Template:Cite journal</ref>
Middle Oligocene climate and the Oi2 eventEdit
Oligocene climate following the Eocene-Oligocene event is poorly known.Template:Sfn There were several pulses of glaciation in middle Oligocene, about the time of the Oi2 oxygen isotope shift. This led to the largest drop of sea level in past 100 million years, by about Template:Convert. This is reflected in a mid-Oligocene incision of continental shelves and unconformities in marine rocks around the world.Template:Sfn
Some evidence suggests that the climate remained warm at high latitudesTemplate:Sfn<ref>Template:Cite journal</ref> even as ice sheets experienced cyclical growth and retreat in response to orbital forcing and other climate drivers.<ref>Template:Cite journal</ref> Other evidence indicates significant cooling at high latitudes.Template:Sfn<ref>Template:Cite journal</ref> Part of 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.Template:Sfn
Late Oligocene warmingEdit
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.Template:Sfn However, Antarctica remained heavily glaciated during this warming period.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> The late Oligocene warming is discernible in pollen counts from the Tibetan Plateau, which also show that the South Asian Monsoon had already developed by the late Oligocene.<ref>Template:Cite journal</ref> Around 25.8 Ma, the South Asian Monsoon underwent an episode of major intensification brought on by the uplift of the Tibetan Plateau.<ref>Template:Cite journal</ref>
A deep 400,000-year glaciated Oligocene-Miocene boundary event is recorded at McMurdo Sound and King George Island.<ref>Template:Cite journal</ref>
BiosphereEdit
The early Eocene climate 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 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.Template:Sfn
FloraEdit
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.Template:Sfn
Angiosperms continued their expansion throughout 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.Template:Sfn The decline in pCO2 favored C4 photosynthesis,<ref>Template:Cite journal</ref> which is found only in angiosperms and is particularly characteristic of grasses.<ref>Template:Cite journal</ref> However, even at the end of the period, grass was not quite common enough for modern savannas.Template:Sfn
In North America, much of the dense forest was replaced by patchy scrubland with riparian forests.Template:Sfn<ref name="Late Eocene and Oligocene paleosols"/> Subtropical species dominated with cashews<ref>Template:Cite journal</ref> and lychee trees present,<ref>Template:Cite journal</ref> and temperate woody plants such as roses, beeches,<ref>Template:Cite journal</ref> and pinesTemplate:Sfn were common. The legumes spread,<ref>Template:Cite journal</ref> while sedges<ref>Template:Cite journal</ref> and ferns continued their ascent.<ref>Template:Cite journal</ref>
In Europe, floral assemblages became increasingly affected by strengthening seasonality as it related to wildfire activity.<ref>Template:Cite journal</ref>
In Pakistan, the flora consisted mainly of dry but dense forests.<ref>Template:Cite journal</ref> In northern China, there was a progressive ascendance of open, grassy environments.<ref>Template:Cite journal</ref> The Ha Long megafossil flora from the Dong Ho Formation of Oligocene age shows that the Oligocene flora of what is now Vietnam was very similar to its present flora.<ref>Template:Cite journal</ref>
Kelps make their first appearance in the fossil record during the earliest Oligocene.<ref>Template:Cite journal</ref>
FaunaEdit
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 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.Template:Sfn Many groups, such as equids,<ref>Template:Cite book</ref> entelodonts, rhinos, merycoidodonts, and camelids, became more able to run during this time, adapting to the plains that were spreading as the Eocene rainforests receded.<ref>Template:Cite book</ref> 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.<ref>Template:Cite journal</ref>
The Eocene-Oligocene transition in Europe and Asia has been characterized as the Grande Coupure.<ref>Template:Cite journal</ref> 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.Template:Sfn Lesser faunal turnovers occurred simultaneously with the Oi2 event and towards the end of the Oligocene.<ref>Template:Cite journal</ref> There was significant diversification of mammals in Eurasia, including the giant indricotheres, that grew up to Template:Convert at the shoulder and weighed up to 20 tons. Paraceratherium was one of the largest land mammals ever to walk the Earth.<ref>Template:Cite book</ref> However, the indricotheres were an exception to a general tendency for Oligocene mammals to be much smaller than their Eocene counterparts.Template:Sfn The earliest deer, giraffes, pigs, and cattle appeared in the mid-Oligocene in Eurasia.Template:Sfn The first felid, Proailurus, originated in Asia during the late Oligocene and spread to Europe.<ref name =Mott>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
There was only limited migration between Asia and North America.Template:Sfn 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.<ref name="Large temperature drop across the E"/> Crocodilians and pond turtles replaced by dry land tortoises. Molluscs shifted to more drought-tolerant forms.Template:Sfn The White River Fauna of central North America inhabited 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.<ref>Template:Cite book</ref> Merycoidodonts, an endemic American group, were very diverse during this time.<ref>Template:Cite book</ref>
Australia and South America became geographically isolated and developed their own distinctive endemic fauna. These included 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.<ref>Template:Cite journal</ref>
Africa was also relatively isolated and retained its endemic fauna. These included mastodonts, hyraxes, arsinoitheres, and other archaic forms.Template:Sfn Egypt in the Oligocene was an environment of lush forested deltas.<ref>Template:Cite book</ref> 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.<ref>Template:Cite journal</ref>
During the Oligocene, the Tethyan marine biodiversity hotspot collapsed as the Tethys Ocean contracted. The seas around Southeast Asia and Australia became the new dominant hotspot of marine biodiversity.<ref>Template:Cite journal</ref> 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.Template:Sfn The marine animals of Oligocene oceans resembled today's fauna, such as the bivalves. Calcareous cirratulids appeared in the Oligocene.<ref name=Vinn2009>Template:Cite journal</ref>
The Oligocene saw the emergence of parrotfishes, as the centre of marine biodiversity shifted from the Central Tethys eastward into the Indo-Pacific.<ref>Template:Cite journal</ref> 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, 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 the end of the epoch from an otter-like ancestor.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
- NMNH-USNMV15917Poebrotherium.jpg
- Merycoidodon Skull Oligocene Left Side.jpg
- Hoplophoneus primaevus (fossil false sabertooth cat) (Middle Oligocene; Nebraska, USA) 3 (32791323412).jpg
- Mesohippus barbouri Harvard.jpg
- Paraceratherium transouralicum skull.jpg
- Paleoparadoxia Natural History Museum of Los Angeles County 20110330.jpg
- Protoceras skeleton.jpg
- Archaeotherium mortoni.JPG
- Janjucetus hunderi skull.jpg
OceansEdit
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,<ref name="Lyle" /> 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 the Arctic may have influenced this oceanic cooling, though the extent of this influence is still a matter of some significant dispute.
Effects of oceanic gateways on circulationEdit
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 the Oligocene. As the continents shifted to a more modern configuration, so too did ocean circulation.<ref name="Proth" />
Drake PassageEdit
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, 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).<ref name="Proth" /><ref name="Mack" /> With the cold water concentrated around Antarctica, sea surface temperatures and, consequently, continental temperatures would have dropped. The onset of Antarctic glaciation occurred during the early Oligocene,<ref name="Via"/> 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 the start of an Antarctic Circumpolar Current.<ref name="Katz" />
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<ref name="Proth" /> and subsequent global cooling, other studies have suggested that the δ18O signature is too strong for glaciation to be the main trigger for cooling.<ref name="Katz" /> 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,<ref name="Lyle" /> which strongly implies that feedbacks and factors other than the ACC were integral to the rapid cooling.<ref name="Lyle" />
The latest hypothesized time for the opening of the Drake Passage is during the early Miocene.<ref name="Lyle" /> 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 the opening of the Drake Passage is around 30 Ma.Template:R 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.Template:R 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 the Antarctic Circumpolar Current. There is some evidence that it occurred much earlier, during the early Eocene.<ref name=Lagemaat2021>Template:Cite journal</ref>
Opening of the Tasman GatewayEdit
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 the gateway widened, the Antarctic Circumpolar Current strengthened.
Tethys Seaway closingEdit
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.<ref name="von" /> The closure of Tethys built some new mountains (the Zagros range) and drew down more carbon dioxide from the atmosphere, contributing to global cooling.<ref name="Allen" />
Greenland–Iceland–FaroesEdit
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.<ref name="Via" /> More information about the evolution of North Atlantic Deep Water will be given a few sections down.
Ocean coolingEdit
Evidence for ocean-wide cooling during the Oligocene exists mostly in isotopic proxies. Patterns of extinction<ref name="Green" /> and patterns of species migration<ref name="Bose" /> 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.<ref name="Mack" /><ref name="Hay" />
Deep waterEdit
_(Oligocene;_Argentina)_(32313481396).jpg){kind=link}
Isotopic evidence suggests that during the early Oligocene, 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.<ref name="von" />
Evidence for the early Oligocene onset of chilled North Atlantic deep water lies in the beginnings of sediment drift deposition in the North Atlantic, such as the Feni and Southeast Faroe drifts.<ref name="Via" />
The chilling of the South Ocean deep water began in earnest once the Tasmanian Gateway and the Drake Passage opened fully.<ref name="Mack" /> 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.
Impact eventsEdit
Recorded extraterrestrial impacts:
- Haughton impact crater, Nunavut, Canada (23 Ma, crater Template:Convert diameter; now considered questionable as an Oligocene event; later analyses have concluded the crater dates to 39 Ma, placing the event in the Eocene.)<ref name="EIDb">Template:Cite Earth Impact DB</ref><ref name=Sherlock2005>Template:Cite journal</ref>
Supervolcanic explosionsEdit
- La Garita Caldera (28–26 million years ago)<ref>Template:Cite book</ref>
- Wah Wah Springs Caldera (30 million years ago)
See alsoEdit
- List of fossil sites (with link directory)
- Turgai Sea
ReferencesEdit
- Ogg, Jim (June 2004). "Overview of Global Boundary Stratotype Sections and Points (GSSP's)". International Commission on Stratigraphy. Accessed April 30, 2006.
External linksEdit
Template:Sister project Template:Wikisource portal Template:EB1911 poster
- Palaeos: Oligocene
- UCMP Berkeley Oligocene Page
- Prehistoric Pictures, in the Public Domain
- Oligocene Leaf Fossils
- Olicgocene Fish Fossils
- PaleoMap Project: Oligocene
- Oligocene Microfossils: 300+ images of Foraminifera