TheOligocene[a] is a geologicepoch of thePaleogenePeriod that extends from about 33.9 million to 23 million years before the present (33.9±0.1 to23.04±0.05 Ma). 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 paleontologistHeinrich Ernst Beyrich[6][7] from his studies of marine beds in Belgium and Germany.[8] The name comes fromAncient Greekὀλίγος (olígos), meaning "few", andκαινός (kainós), meaning "new",[9] and refers to the sparsity ofextant forms ofmolluscs. The Oligocene is preceded by theEocene Epoch and is followed by theMiocene Epoch. The Oligocene is the third and final epoch of thePaleogene Period.
The Oligocene is often considered an important time of transition, a link between the archaic world of the tropical Eocene and the more modernecosystems of the Miocene.[10] Major changes during the Oligocene included a global expansion ofgrasslands, and a regression oftropical broad leafforests to theequatorial belt.
The start of the Oligocene is marked by a notableextinction event called theGrande Coupure; it featured the replacement ofEuropean fauna withAsianfauna, except for the endemicrodent andmarsupial families. By contrast, theOligocene–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.
The lower boundary of the Oligocene (itsGlobal Boundary Stratotype Section and Point or GSSP) is placed at the last appearance of theforaminiferan genusHantkenina in a quarry atMassignano,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).[11]
Neotethys during the Oligocene (Rupelian, 33.9–28.4 mya)
During the Oligocene Epoch, the continents continued todrift toward their present positions.[14][15]Antarctica became more isolated as deep ocean channels were established between Antarctica and Australia andSouth 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.[16] The timing of the formation of theDrake Passage between South America and Antarctica is also uncertain, with estimates ranging from 49 to 17 mya (early Eocene to Miocene),[17] but oceanic circulation through the Drake Passage may also have been in place by the end of the early Oligocene.[18][16] 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).[19]
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 theSan Andreas Fault andextensional tectonics in theBasin and Range province,[20] 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 theMogollon-Datil volcanic field to theSan Juan volcanic field, then through Utah and Nevada to the ancestral Northern Cascades. Huge ash deposits from these volcanoes created theWhite River andArikaree Groups of the High Plains, with their excellent fossil beds.[21]
TheAlps were rapidly rising inEurope as theAfrican plate continued to push north into theEurasian plate, isolating the remnants of theTethys Sea.[14][23] 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. TheObik Sea, which had separated Europe from Asia, retreated early in the Oligocene, creating a persistent land connection between the continents.[14] TheParatethys Sea stretched from what is now theBalkan Peninsula acrossCentral Asia to theTian Shan region of what is nowXinjiang.[24] There appears to have been a land bridge in the early Oligocene between North America and Europe, since thefaunas of the two regions are very similar.[25] However, towards the end of the Oligocene, there was a brief marine incursion in Europe.[26][27]
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.[28][29] TheTibetan Plateau may have reached nearly its present elevation by the late Oligocene.[30]
The Andes first became a major mountain chain in the Oligocene, as subduction became more direct into the coastline.[21][31]
Climate change during the last 65 million years[32]
Climate during the Oligocene reflected a general cooling trend following theEarly Eocene Climatic Optimum. This transformed the Earth's climate from a greenhouse to an icehouse climate.[33]
The Eocene-Oligocene transition was a major cooling event and reorganization of the biosphere,[34][35] being part of a broader trend of global cooling lasting from theBartonian to the Rupelian.[36][37] The transition is marked by the Oi1 event, an oxygen isotope excursion occurring approximately 33.55 million years ago,[38] during whichoxygen 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) ofglobal cooling.[33] 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 105 meters (344 ft), and ice sheets were 25% greater in extent than in the modern world.[39]
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.[40]Playa lakes of the Tibetan Plateau disappeared at the transition, pointing to cooling and aridification of central Asia.[41] Pollen and spore counts in marine sediments of 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.[42] 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.[43]
The best terrestrial record of Oligocene climate comes from North America, where temperatures dropped by 7 to 11 °C (13 to 20 °F) in the earliest Oligocene. This change is seen from Alaska to the Gulf Coast. Upper Eocenepaleosols reflect annual precipitation of over a meter of rain, but early Oligocene precipitation was less than half this.[44][45] 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.[46] Ice-rafted debris in the Norwegian-Greenland Sea indicated that glaciers had appeared in Greenland by the start of the Oligocene.[47]
Continental ice sheets in Antarctica reached sea level during the transition.[48][49][50] Glacially rafted debris of early Oligocene age in theWeddell Sea andKerguelen Plateau, in combination with Oi1 isotope shift, provides unambiguous evidence of a continental ice sheet on Antarctica by the early Oligocene.[51]
The causes of the Eocene-Oligocene transition are not yet fully understood.[52]The timing is wrong for this to be caused either by knownimpact events or by the volcanic activity on the Ethiopean Plateau.[53] Two other possible drivers of climate change, not mutually exclusive, have been proposed.[52] The first is thermal isolation of the continent of Antarctica by development of theAntarctic Circumpolar Current.[18][49][15] Deep sea cores from south of New Zealand suggest that cold deep-sea currents were present by the early Oligocene.[53] However, the timing of this event remains controversial.[54] The other possibility, for which there is considerable evidence, is a drop in atmosphericcarbon dioxide levels (pCO2) during the transition.[52][55][36] 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.[56] Climate modeling suggests that glaciation of Antarctica took place only when pCO2 dropped below a critical threshold value.[57]
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.[58]
Oligocene climate following the Eocene-Oligocene event is poorly known.[59] 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 75 meters (246 ft). This is reflected in a mid-Oligocene incision of continental shelves and unconformities in marine rocks around the world.[44]
Some evidence suggests that the climate remained warm at high latitudes[59][60] even as ice sheets experienced cyclical growth and retreat in response toorbital forcing and other climate drivers.[61] Other evidence indicates significant cooling at high latitudes.[48][62] 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.[63]
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.[64] However, Antarctica remained heavily glaciated during this warming period.[65][66] The late Oligocene warming is discernible in pollen counts from the Tibetan Plateau, which also show that theSouth Asian Monsoon had already developed by the late Oligocene.[67] Around 25.8 Ma, the South Asian Monsoon underwent an episode of major intensification brought on by the uplift of the Tibetan Plateau.[68]
The early Eocene climate was very warm, withcrocodilians and temperate plants thriving north of theArctic 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 thePhanerozoic. 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.[70]
The Oi1 event of the Eocene-Oligocene transition covered the continent of Antarctica with ice sheets, leavingNothofagus and mosses and ferns clinging to life around the periphery of Antarctica in tundra conditions.[57]
Angiosperms continued their expansion throughout the world as tropical and sub-tropical forests were replaced bytemperate deciduous forests. Openplains anddeserts became more common andgrasses expanded from their water-bank habitat in the Eocene moving out into open tracts.[71] The decline in pCO2 favoredC4 photosynthesis,[72] which is found only in angiosperms and is particularly characteristic of grasses.[73] However, even at the end of the period, grass was not quite common enough for modernsavannas.[71]
In North America, much of the dense forest was replaced by patchy scrubland with riparian forests.[44][45] Subtropical species dominated withcashews[74] andlychee trees present,[75] and temperate woody plants such asroses,beeches,[76] andpines[77] were common. Thelegumes spread,[78] whilesedges[79] andferns continued their ascent.[80]
In Europe, floral assemblages became increasingly affected by strengthening seasonality as it related to wildfire activity.[81]
InPakistan, the flora consisted mainly of dry but dense forests.[82] In northern China, there was a progressive ascendance of open, grassy environments.[83] The Ha Long megafossil flora from the Dong Ho Formation of Oligocene age shows that the Oligocene flora of what is nowVietnam was very similar to its present flora.[84]
Kelps make their first appearance in the fossil record during the earliest Oligocene.[85]
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 asdogs,nimravids, bears, weasels, and raccoons began to replace thecreodonts 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.[86] Many groups, such asequids,[87]entelodonts,rhinos,merycoidodonts, andcamelids, became more able to run during this time, adapting to the plains that were spreading as the Eocene rainforests receded.[88]Brontotheres died out in the Earliest Oligocene, andcreodonts died out outsideAfrica and theMiddle East at the end of the period.Multituberculates, an ancient lineage of primitive mammals that originated back in theJurassic, also became extinct in the Oligocene, aside from thegondwanatheres.[89]
The Eocene-Oligocene transition in Europe and Asia has been characterized as the Grande Coupure.[90] 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 asrhinoceroses andruminants, to enter Europe and drive endemic species to extinction.[86] Lesser faunal turnovers occurred simultaneously with the Oi2 event and towards the end of the Oligocene.[91] There was significant diversification of mammals in Eurasia, including the giantindricotheres, 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.[92] However, the indricotheres were an exception to a general tendency for Oligocene mammals to be much smaller than their Eocene counterparts.[71] The earliest deer, giraffes, pigs, and cattle appeared in the mid-Oligocene in Eurasia.[86] The firstfelid,Proailurus, originated in Asia during the late Oligocene and spread to Europe.[93]
There was only limited migration between Asia and North America.[86] The cooling of central North America at the Eocene-Oligocene transition resulted in a large turnover ofgastropods,amphibians, andreptiles. Mammals were much less affected.[46] Crocodilians and pond turtles replaced by dry land tortoises. Molluscs shifted to more drought-tolerant forms.[44] TheWhite River Fauna of central North America inhabited a semiarid prairie home and included entelodonts likeArchaeotherium, camelids (such asPoebrotherium), runningrhinoceratoids, three-toed equids (such asMesohippus),nimravids,protoceratids, and earlycanids likeHesperocyon.[94] Merycoidodonts, an endemic American group, were very diverse during this time.[95]
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 aspyrotheres andastrapotheres, as well aslitopterns andnotoungulates.Sebecosuchians,terror birds, and carnivorousmetatheres, like theborhyaenids remained the dominant predators.[96]
Africa was also relatively isolated and retained its endemic fauna. These includedmastodonts, hyraxes, arsinoitheres, and other archaic forms.[86]Egypt in the Oligocene was an environment of lush forested deltas.[97] 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.[98]
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.[99] 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.[86] The marine animals of Oligocene oceans resembled today's fauna, such as thebivalves. Calcareouscirratulids appeared in the Oligocene.[100]
The Oligocene saw the emergence of parrotfishes, as the centre of marine biodiversity shifted from the Central Tethys eastward into theIndo-Pacific.[101] 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. Thebaleen whales andtoothed whales had just appeared, and their ancestors, thearchaeocetecetaceans 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 therequiem sharks, which also appeared in this epoch. Earlydesmostylians, likeBehemotops, are known from the Oligocene.Pinnipeds appeared near the end of the epoch from anotter-like ancestor.[102]
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,[103] 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.
The opening and closing of ocean gateways: the opening of theDrake Passage; the opening of theTasmanian Gateway and the closing of theTethys seaway; along with the final formation of theGreenland–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.[104]
The Drake Passage is located betweenSouth America andAntarctica. Once the Tasmanian Gateway between Australia and Antarctica opened, all that kept Antarctica from being completely isolated by theSouthern Ocean was its connection to South America. As the South American continent moved north, the Drake Passage opened and enabled the formation of theAntarctic Circumpolar Current (ACC), which would have kept the cold waters of Antarctica circulating around that continent and strengthened the formation ofAntarctic Bottom Water (ABW).[104][105] 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,[106] 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.[107]
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[104] and subsequent global cooling, other studies have suggested that the δ18O signature is too strong for glaciation to be the main trigger for cooling.[107] 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,[103] which strongly implies that feedbacks and factors other than the ACC were integral to the rapid cooling.[103]
The latest hypothesized time for the opening of the Drake Passage is during the early Miocene.[103] 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.[103] 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 theShackleton fracture zone, has been shown in a recent study to be fairly young, only about 8 million years old.[105] 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.[108]
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.
TheTethys 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.[109] The closure of Tethys built some new mountains (the Zagros range) and drew down more carbon dioxide from the atmosphere, contributing to global cooling.[110]
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.[106] More information about the evolution of North Atlantic Deep Water will be given a few sections down.
Evidence for ocean-wide cooling during the Oligocene exists mostly in isotopic proxies. Patterns of extinction[111] and patterns of species migration[112] 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.[105][113]
Isotopic evidence suggests that during the early Oligocene, the main source of deep water was theNorth Pacific and theSouthern Ocean. As the Greenland-Iceland-Faroe Ridge sank and thereby connected the Norwegian–Greenland sea with the Atlantic Ocean, the deep water of theNorth Atlantic began to come into play as well. Computer models suggest that once this occurred, a more modern in appearancethermo-haline circulation started.[109]
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.[106]
The chilling of the South Ocean deep water began in earnest once the Tasmanian Gateway and the Drake Passage opened fully.[105] 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.
Haughton impact crater,Nunavut, Canada (23 Ma, crater 24 km (15 mi) diameter; now considered questionable as an Oligocene event; later analyses have concluded the crater dates to 39 Ma, placing the event in the Eocene.)[114][115]
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