Oligocene

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Oligocene Epoch
33.9–23.03 million years ago
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Ages in the Oligocene
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Ages of the Oligocene Epoch.
Axis scale: millions of years ago.
System/
Period
Series/
Epoch
Stage/
Age
Age (Ma)
Neogene Miocene Aquitanian younger
Paleogene Oligocene Chattian 23.027.8
Rupelian 27.833.9
Eocene Priabonian 33.937.8
Bartonian 37.841.2
Lutetian 41.247.8
Ypresian 47.856.0
Paleocene Thanetian 56.059.2
Selandian 59.261.6
Danian 61.666.0
Cretaceous Upper/
Late
Maastrichtian older
Subdivision of the Paleogene Period
according to the ICS, as of 2019. [2]

The Oligocene ( /ˈɒl.ɪ.ɡə.sn/ OL-ih-ghə-seen) [3] is a geologic epoch of the Paleogene Period and 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, 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; [4] [5] the name comes from the Ancient Greek ὀλίγος (olígos, "few") and καινός (kainós, "new"), [6] 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.

Contents

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. [7] Major changes during the Oligocene included a global expansion of grasslands, and 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 the warmer late Oligocene and the relatively cooler Miocene.

Subdivisions

Oligocene faunal stages from youngest to oldest are:

Chattian or late Oligocene ( 28.1    23.03 mya )
Rupelian or early Oligocene ( 33.9    28.1 mya)

Climate

All palaeotemps.png

The Paleogene Period general temperature decline is interrupted by an Oligocene 7-million-year stepwise climate change. A deeper 8.2 °C, 400,000-year temperature depression leads the 2 °C, seven-million-year stepwise climate change 33.5 Ma (million years ago). [8] [9] The stepwise climate change began 32.5 Ma and lasted through to 25.5 Ma, as depicted in the PaleoTemps chart. The Oligocene climate change was a global [10] increase in ice volume and a 55 m (181 feet) decrease in sea level (35.7–33.5 Ma) with a closely related (25.5–32.5 Ma) temperature depression. [11] The 7-million-year depression abruptly terminated within 1–2 million years of the La Garita Caldera eruption at 28–26 Ma. A deep 400,000-year glaciated Oligocene Miocene boundary event is recorded at McMurdo Sound and King George Island.

Paleogeography

Neotethys during the Oligocene (Rupelian, 33.9-28.4 mya) Mediterranean Rupelian.jpg
Neotethys during the Oligocene (Rupelian, 33.9–28.4 mya)

During this epoch, the continents continued to drift toward their present positions. Antarctica became more isolated and finally developed an ice cap.

Mountain building in western North America continued, and the Alps started to rise in Europe as the African plate continued to push north into the Eurasian plate, isolating the remnants of the Tethys Sea. A brief marine incursion marks the early Oligocene in Europe. Marine fossils from the Oligocene are rare in North America. 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. Sometime during the Oligocene, South America was finally detached from Antarctica and drifted north towards North America. It also allowed the Antarctic Circumpolar Current to flow, rapidly cooling the Antarctic continent.

Flora

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. However, even at the end of the period, grass was not quite common enough for modern savannas.

In North America, subtropical species dominated with cashews and lychee trees present, and temperate trees such as roses,[ clarification needed ] beeches, and pines were common. The legumes spread, while sedges, bulrushes, and ferns continued their ascent.

Fauna

Even more open landscapes allowed animals to grow to larger sizes than they had earlier in the Paleocene epoch 30 million years earlier. Marine faunas became fairly modern, as did terrestrial vertebrate fauna on the northern continents. This was probably more as a result of older forms dying out than as a result of more modern forms evolving. Many groups, such as equids, 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. The first felid, Proailurus , originated in Asia during the late Oligocene and spread to Europe. [12] South America was isolated from the other continents and had evolved a quite distinct fauna by the Oligocene. 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.

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 Oligocene was home to a wide variety of mammals. A good example of this would be the White River Fauna of central North America, which were formerly a semiarid prairie home to many different types of endemic mammals, including 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. In Asia during the Oligocene, a group of running rhinoceratoids gave rise to the indricotheres, like Paraceratherium , which were the largest land mammals ever to walk the Earth.

The marine animals of Oligocene oceans resembled today's fauna, such as the bivalves. Calcareous cirratulids appeared in the Oligocene. [13] 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. [14]

Oceans

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, [15] 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 effects of oceanic gateways on circulation

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 GreenlandIcelandFaroes 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. [16]

The Drake Passage

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). [16] [17] 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, [18] 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. [19]

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 [16] and subsequent global cooling, other studies have suggested that the δ18O signature is too strong for glaciation to be the main trigger for cooling. [19] 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, [15] which strongly implies that feedbacks and factors other than the ACC were integral to the rapid cooling. [15]

The late Oligocene opening of the Drake Passage

The latest hypothesized time for the opening of the Drake Passage is during the early Miocene. [15] 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 early Oligocene opening of the Drake Passage

The earliest hypothesized time for the opening of the Drake Passage is around 30 Ma. [15] 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. [17] 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.

Currently, an opening of the Drake Passage during the early Oligocene is favored.

The opening of the Tasman Gateway

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.

The Tethys Seaway closing

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. [20] The closure of Tethys built some new mountains (the Zagros range) and drew down more carbon dioxide from the atmosphere, contributing to global cooling. [21]

Greenland–Iceland–Faroes

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. [18] More information about the evolution of North Atlantic Deep Water will be given a few sections down.

Ocean cooling

Evidence for ocean-wide cooling during the Oligocene exists mostly in isotopic proxies. Patterns of extinction [22] and patterns of species migration [23] 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. [17] [24]

Deep water

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. [20]

North Atlantic deep water

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. [18]

South Ocean deep water

The chilling of the South Ocean deep water began in earnest once the Tasmanian Gateway and the Drake Passage opened fully. [17] 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 events

Recorded extraterrestrial impacts:

Supervolcanic explosions

La Garita Caldera (28 through 26 million years ago) [25]

See also

Related Research Articles

Antarctic Circumpolar Current Ocean current that flows clockwise from west to east around Antarctica

The Antarctic Circumpolar Current (ACC) is an ocean current that flows clockwise from west to east around Antarctica. An alternative name for the ACC is the West Wind Drift. The ACC is the dominant circulation feature of the Southern Ocean and has a mean transport estimated at 100-150 Sverdrups, or possibly even higher, making it the largest ocean current. The current is circumpolar due to the lack of any landmass connecting with Antarctica and this keeps warm ocean waters away from Antarctica, enabling that continent to maintain its huge ice sheet.

Cenozoic Third era of the Phanerozoic Eon 66 million years ago to present

The Cenozoic Era meaning "new life" is the current and most recent of the three Phanerozoic geological eras, following the Mesozoic Era and extending from 66 million years ago to the present day. It is generally believed to have started on the day of the Cretaceous–Paleogene extinction event when an asteroid hit the earth.

The Eocene Epoch is a geological epoch that lasted from about 56 to 34 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name Eocene comes from the Ancient Greek ἠώς and καινός and refers to the "dawn" of modern ('new') fauna that appeared during the epoch.

The Miocene is the first geological epoch of the Neogene Period and extends from about 23.03 to 5.333 million years ago (Ma). The Miocene was named by Charles Lyell; its name comes from the Greek words μείων and καινός and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene is preceded by the Oligocene and is followed by the Pliocene.

The Pliocene Epoch is the epoch in the geologic timescale that extends from 5.333 million to 2.58 million years BP. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch. Prior to the 2009 revision of the geologic time scale, which placed the four most recent major glaciations entirely within the Pleistocene, the Pliocene also included the Gelasian stage, which lasted from 2.588 to 1.806 million years ago, and is now included in the Pleistocene.

The Paleogene is a geologic period and system that spans 43 million years from the end of the Cretaceous Period 66 million years ago (Mya) to the beginning of the Neogene Period 23.03 Mya. It is the beginning of the Cenozoic Era of the present Phanerozoic Eon. The earlier term Tertiary Period was used to define the span of time now covered by the Paleogene and subsequent Neogene periods; despite no longer being recognised as a formal stratigraphic term, 'Tertiary' is still widely found in earth science literature and remains in informal use. The Paleogene is most notable for being the time during which mammals diversified from relatively small, simple forms into a large group of diverse animals in the wake of the Cretaceous–Paleogene extinction event that ended the preceding Cretaceous Period. The United States Geological Survey uses the abbreviation PE for the Paleogene, but the more commonly used abbreviation is PG with the PE being used for Paleocene.

The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 541 million years to the present, and began with the Cambrian Period when animals first developed hard shells preserved in the fossil record. The time before the Phanerozoic, called the Precambrian, is now divided into the Hadean, Archaean and Proterozoic eons.

Antarctic Peninsula peninsula

The Antarctic Peninsula, known as O'Higgins Land in Chile, Tierra de San Martin in Argentina, and originally known as the Palmer Peninsula in the US and as Graham Land in the United Kingdom, is the northernmost part of the mainland of Antarctica, located at the base of the Southern Hemisphere.

Antarctic ice sheet Polar ice cap

The Antarctic ice sheet is one of the two polar ice caps of the Earth. It covers about 98% of the Antarctic continent and is the largest single mass of ice on Earth. It covers an area of almost 14 million square kilometres and contains 26.5 million cubic kilometres of ice. A cubic kilometer of ice weighs approximately one metric gigaton, meaning that the ice sheet weighs 26,500,000 gigatons. Approximately 61 percent of all fresh water on the Earth is held in the Antarctic ice sheet, an amount equivalent to about 58 m of sea-level rise. In East Antarctica, the ice sheet rests on a major land mass, while in West Antarctica the bed can extend to more than 2,500 m below sea level.

Scotia Plate Minor oceanic tectonic plate between the South American and Antarctic Plates

The Scotia Plate is a tectonic plate on the edge of the South Atlantic and Southern Ocean. Thought to have formed during the early Eocene with the opening of the Drake Passage that separates South America from Antarctica, it is a minor plate whose movement is largely controlled by the two major plates that surround it: the South American Plate and Antarctic Plate.

Quaternary glaciation Series of alternating glacial and interglacial periods

The Quaternary glaciation, also known as the Pleistocene glaciation, is an alternating series of glacial and interglacial periods during the Quaternary period that began 2.58 Ma, and is ongoing. Although geologists describe the entire time period as an "ice age", in popular culture the term "ice age" is usually associated with just the most recent glacial period during the Pleistocene. Since the planet Earth still has ice sheets, geologists consider the Quaternary glaciation to be ongoing, with the Earth now experiencing an interglacial period.

Geological history of Earth The sequence of major geological events in Earths past

The geological history of Earth follows the major events in Earth's past based on the geological time scale, a system of chronological measurement based on the study of the planet's rock layers (stratigraphy). Earth formed about 4.54 billion years ago by accretion from the solar nebula, a disk-shaped mass of dust and gas left over from the formation of the Sun, which also created the rest of the Solar System.

Paratethys A large shallow sea that stretched from the region north of the Alps over Central Europe to the Aral Sea in Central Asia

The Paratethys ocean, Paratethys sea or just Paratethys was a large shallow inland sea that stretched from the region north of the Alps over Central Europe to the Aral Sea in Central Asia. The sea was formed during the Oxfordian stage of the Late Jurassic as an extension of the rift that formed the Central Atlantic Ocean and was isolated during the Oligocene epoch. It was separated from the Tethys Ocean to the south by the formation of the Alps, Carpathians, Dinarides, Taurus and Elburz mountains. During its long existence the Paratethys was at times reconnected with the Tethys or its successors, the Mediterranean Sea or Indian Ocean. From the Pliocene epoch onward, the Paratethys became progressively shallower. Today's Black Sea, Caspian Sea, Aral Sea, Lake Urmia, Namak Lake and others are remnants of the Paratethys Sea.

Throughout the history of the Earth, the planet's climate has been fluctuating between two dominant climate states: the greenhouse Earth and the icehouse Earth. These two climate states last for millions of years and should not be confused with glacial and interglacial periods, which occur only during an icehouse period and tend to last less than 1 million years. There are five known great glaciations in Earth's climate history; the main factors involved in changes of the paleoclimate are believed to be the concentration of atmospheric carbon dioxide, changes in the Earth's orbit, long-term changes in the solar constant, and oceanic and orogenic changes due to tectonic plate dynamics. Greenhouse and icehouse periods have profoundly shaped the evolution of life on Earth.

Natural history of New Zealand

The natural history of New Zealand began when the landmass Zealandia broke away from the supercontinent Gondwana in the Cretaceous period. Before this time, Zelandia shared its past with Australia and Antarctica. Since this separation, the New Zealand biota and landscape has evolved in near-isolation. The exclusively natural history of the country ended in about 1300 AD, when humans first settled, and the country's environmental history began. The period from 1300 AD to today coincides with the extinction of many of New Zealand's unique species that had evolved there.

Gondwana Neoproterozoic to Carboniferous supercontinent

Gondwana or Gondwanaland was a supercontinent that existed from the Neoproterozoic until the Jurassic.

Pangaea Supercontinent from the late Paleozoic to early Mesozoic eras

Pangaea or Pangea was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from earlier continental units approximately 335 million years ago, and it began to break apart about 175 million years ago. In contrast to the present Earth and its distribution of continental mass, Pangaea was centred on the Equator and surrounded by the superocean Panthalassa. Pangaea is the most recent supercontinent to have existed and the first to be reconstructed by geologists.

The East Tasman Plateau is a submerged microcontinent south east of Tasmania. Its area is 50,000 square kilometres (19,000 sq mi), and it is mostly from 2,500 to 3,000 metres deep. It is a circular piece of continental rocks surrounded by oceanic crust. Volcanism occurred there 36 million years ago. The East Tasman Plateau is separated from the island of Tasmania by 100 kilometres (62 mi) of deeper water, and the East Tasman Saddle is a higher ridge connecting the plateau to the Freycinet Peninsula region of the Tasmanian East Coast. This ridge runs north west from the plateau. South-west of the plateau is the L'Atalante Depression.

Late Cenozoic Ice Age An ice age of the last 34 million years, in particular in Antarctica

The Late Cenozoic Ice Age, or Antarctic Glaciation began 33.9 million years ago at the Eocene-Oligocene Boundary and is ongoing. It is Earth's current ice age or icehouse period. Its beginning is marked by the formation of the Antarctic ice sheets. The Late Cenozoic Ice Age gets its name due to the fact that it covers roughly the last half of Cenozoic era so far.

Amelia E. Shevenell American marine geologist

Amelia E. Shevenell is an American marine geologist who specializes in high-latitude paleoclimatology and paleoceanography. She is currently an Associate Professor in the College of Marine Science at the University of South Florida. She has made notable contributions to understanding the history of the Antarctic ice sheets and published in high-impact journals and, as a result, was awarded full membership of Sigma Xi. She has a long record of participation in international ocean drilling programs and has served in leadership positions of these organizations. Shevenell is the elected Geological Oceanography Council Member for The Oceanography Society (2019-2021).

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