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Oligocene Article by: Haq, Bilal U. Division of Ocean Sciences, National Science Foundation, Arlington, Virginia. Publication year: 2014 DOI: http://dx.doi.org/10.1036/1097­8542.468000 (http://dx.doi.org/10.1036/1097­8542.468000)

Content

Subdivisions Tectonics, oceans, and climate Life Bibliography Additional Readings

The third oldest of the seven geological epochs of the Cenozoic Era. It corresponds to an interval of geological time (and rocks deposited during that time) from the close of the Eocene Epoch to the beginning of the Miocene Epoch. The most recent geological time scales assign an age of 34 to 24 million years before present (MYBP) to the Oligocene Epoch. See also: Cenozoic (/content/cenozoic/118600); Eocene (/content/eocene/236400); Miocene (/content/miocene/427400)

In his early subdivision of the Tertiary published in 1833, Charles Lyell established four stratigraphic units, from the oldest to the youngest: Eocene, Miocene, and Older and Younger Pliocene. Stratigraphers in the Netherlands and Germany, however, kept describing strata that they considered to be intermediate in position and characteristics between Eocene and Miocene. They concluded that these sediments represented a major marine transgression in northern Europe at the close of the Eocene Epoch. This eventually led the German stratigrapher E. Beyrich in 1854 to propose the Oligocene as an independent subdivision of the Tertiary based on a sequence of marine, brackish­ water, and nonmarine sediments of the Mainz Basin in Germany. He proposed a new Oligocene Epoch and constructed it out of the younger part of Eocene and the older part of Miocene epochs of the Lyellian subdivisions of Tertiary.

An important event that characterizes the Oligocene Epoch was the development of extensive glaciation on the continent of Antarctica. Prior to that time, the world was largely ice­free through much of the Mesozoic and early Tertiary. A significant amount of ice is now known to have existed on the Antarctic continent since at least the beginning of the Oligocene, when the Earth was ushered into its most recent phase of ice­house conditions. This in turn created revolutions in the global climatic and hydrographic systems, with important repercussions for the marine and terrestrial biota. The changes include steepened latitudinal and vertical thermal gradients affecting major fluctuations in global climates, and the shift in the route of global dispersal of marine biota from an ancestral equatorial Tethys seaway, which had become severely restricted by Oligocene time, to the newly initiated circum­ Antarctic circulation. See also: Glacial history (/content/glacial­history/800690)

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Subdivisions

In modern time scales, this epoch is subdivided into two series, a Lower and an Upper Oligocene. Northern European Rupelian and Chattian stratigraphic stages are designated to be time­equivalent to the Early and Late Oligocene, respectively (Fig. 1). Worldwide, the epoch represents an overall regressive sequence when there was a drawdown of global sea level, with relatively deeper, marine facies in the Early Oligocene and shallower­water to nonmarine facies in the Late Oligocene. See also: Facies (geology) (/content/facies­geology/250000)

Fig. 1 Oligocene stages and their temporal equivalents.

The Rupelian Stage was proposed by A. Dumont in 1849, based on sedimentary strata in Belgium. The Tongrian Stage, also described from Belgium by Dumont in 1839, was originally regarded to be, in part, older than the Rupelian by Beyrich, who considered that the Tongrian and Rupelian together constituted his new epochal unit, the Oligocene. In 1983, K. Mayer­Eymar erected the Lattorfian Stage with a type locality in Saxony, Germany. Mayer­ Eymar considered the Lattorfian to be temporally equivalent to the Early Oligocene. Both the Tongrian and Lattorfian strata were later shown to range downward into the uppermost Eocene, and their use has largely been abandoned in favor of an expanded notion of the Rupelian. Because the Rupelian at its type section spans

http://www.accessscience.com/content/oligocene/468000 2/7 4/21/2016 Oligocene ­ AccessScience from McGraw­Hill Education somewhat less than the time interval included in the Early Oligocene, some French stratigraphers prefer the Stampian Stage to Rupelian. The Stampian was described by A. D'Orbigny in 1852 from marine and lacustrine beds at Etampes in France.

The Chattian, elected by consensus as the standard stage for the Late Oligocene, was based on marine sand beds near Kassel, Germany, and proposed by T. Fuchs in 1894. Fuchs considered the Chattian to be younger than Rupelian and older than the Aquitanian Stage (which was later placed in the Lower Miocene). Later studies have shown that there is a temporal gap between the top of Chattian and the base of Aquitanian as defined at their stratotype sections. However, in stratigraphy it is normal practice to extend the formal concept of standard stages to include such gaps. Thus, to accommodate the gap between the uppermost Eocene stage of Priabonian and the Oligocene Rupelian, and the gap between Chattian and Aquitanian, the formal concepts of Rupelian and Aquitanian were extended downward by later stratigraphers to bridge the intervening lacunae.

Other regional stages of the Oligocene include the Egerian, sometimes used in central and eastern European countries, which is in part equivalent to the Chattian, but may extend into the lower part of Miocene. In Russia the Caucasian Stage is considered to be an equivalent to the Egerian. Marine Oligocene in California is subdivided into the Refugian and Zemorrian stages, the former stage encompassing only the earliest Oligocene, and the latter spanning the bulk of the epoch. In the Gulf coast of the United States the Oligocene extends from the upper part of the Jacksonian Stage to Vicksburgian, Chickasawhayan, and lower part of Anahuacan stages. Temporally, the latter stage extends into the Miocene. Australian stratigraphers include the Willungian, Janjukian, and lower part of Longfordian stages in their Oligocene, and the New Zealanders now consider the epoch to span three local stages: Whaingaroan, Duntroonian, and Waitakian.

Paleontologists who study mammal and other vertebrate fossils often use their own subdivisions to express the ages of terrestrial assemblages. Workers in Europe consider the Oligocene to span three subdivisions, Suevian, Arvernian, and lower part of Agenian. North American vertebrate paleontologists include Orellan, Whiteneyan, and the lower two­thirds of the Arikareean ages in their concept of the Oligocene. See also: Paleontology (/content/paleontology/484100); Stratigraphy (/content/stratigraphy/659000)

Tectonics, oceans, and climate

Radical changes occurred in the Oligocene Epoch that revolutionized global oceanographic and climatic conditions. The most prominent change was the shift of circumglobal circulation (and a major means of biotic dispersal) from the equatorial to the southern high­latitude regions. Both the restriction of the equatorial flow between the Indian and the Atlantic oceans through the ancestral Tethys seaway, and the opening of the oceanic gateway at Drake Passage occurred during the Oligocene. These changes had important repercussions that led to the entry of the Earth into predominantly ice­house conditions that continue to the present. The cooling of the polar regions in the Oligocene led to the accentuation of latitudinal and vertical thermal gradients and an increase in seasonality. This resulted in a fundamental shift in the bottom­water regime, from density­driven warm, saline bottom waters in the Cretaceous and Paleocene­Eocene, to cold bottom waters, largely driven by thermal contrast. The accentuated latitudinal thermal contrast also resulted in the expansion of the erosive activity of bottom waters. See also: Cretaceous (/content/cretaceous/167800)

http://www.accessscience.com/content/oligocene/468000 3/7 4/21/2016 Oligocene ­ AccessScience from McGraw­Hill Education The separation of Svalbard and Greenland in the latest Eocene and the breaching of the Rio Grande Rise in the South Atlantic in the earliest Oligocene set the stage for the crossing of the important climatic threshold near the Eocene­Oligocene boundary. It has been suggested that after the development of the connection between the Arctic and the Norwegian Sea that allowed the supply of cold deep water to the Atlantic and the southern high latitudes, more moisture became available around Antarctica. This, combined with the partial isolation of Antarctica, may have led to large­scale freezing at sea level and the initiation of the formation of Antarctic Bottom Water. There is evidence that the temperature of the bottom water dropped by 4–5°C (7–9°F) near this boundary, and the change from warm to cold bottom water may date back to this time. The development of the psychrosphere (deeper, cold layer of the ocean) manifests itself in the presence of widespread erosional hiatuses in the eastern Indian and southwestern Pacific oceans, where scouring by cold bottom waters has stripped away much of the older sediments. The vigorous activity of the deep water is also indicated by widespread drift sediments in the North Atlantic.

Another important tectonic event that had major significance for the oceanic­climatic conditions was the breaching of the straits between South America and Antarctica at the Drake Passage in the mid­Oligocene and complete geographic isolation of Antarctica. This eliminated the last barrier in the path of the circum­Antarctic circulation. The development of the Cirum­Antarctic Current led to further thermal isolation of Antarctica that was conducive to the development of an extensive ice cap on the continent. Overall, there was a dramatic increase in the areas of the desert through the Oligocene and younger epochs that can be ascribed to the cooling higher latitudes and the development of polar ice caps. See also: Paleoclimatology (/content/paleoclimatology/483500); Paleogeography (/content/paleogeography/483800); Plate tectonics (/content/plate­tectonics/527000)

In Oligocene time the global surface circulation patterns had essentially evolved the major features of the modern oceans (Fig. 2). One major difference was that in the equatorial region an open Panama isthmus permitted the exchange of waters between the Pacific and the Atlantic oceans. However, by the Early Oligocene the flow of the Tethys Current from east to west had already become sharply reduced, intermittent, and restricted to a narrow passage southwest of the Indian plate, following the collision of the subcontinent with the Asian mainland, initial uplift of the Himalayas in the Eocene, and subsequent raising of the Tibetan Plateau. The convergence between India and Asia and between Africa and Europe closed the Tethys seaway, leaving behind smaller remnants that include the Mediterranean, Black, and Caspian seas.

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Fig. 2 Paleogeography and oceans of the Oligocene. (After B. U. Haq and F. W. B. van Eysinga, Geological Time Table, Elsevier, 1998)

The worldwide cooling trend that began in the latest Eocene but became accentuated in the Oligocene may have been in part caused by the long­term effect of the uplift of the Tibetan Plateau, which was significant by Oligocene time. The raised plateau is thought to be able to deflect the atmospheric jet stream more vigorously, leading to the strengthening of the summer monsoon, and increased rainfall and weathering in the Himalayas. Since increased weathering and dissolution of carbonates results in greater carbon dioxide drawdown from the atmosphere, the reduced levels of carbon dioxide may have provided a condition that helped the Earth enter into a renewed ice­ house phase.

By mid­Oligocene the Circum­Antarctic Current was well established. The ice shelves around Antarctica were the dominant sources of cold bottom waters. Northern sources of cold, deep water had also become operative, and the surface exchange between North Atlantic and the Arctic through Norwegian­Greenland and Labrador passages had become active. There is indirect evidence that suggests the initiation of an ice cover in the formerly ice­free Arctic by Oligocene time. By this time the central Arctic Ocean had become largely landlocked, and the connections to the open sea were restricted enough that the input of freshwater from Eurasian rivers, combined with salinity stratification characteristic of isolated basins, may have allowed winter ice to form. This, however, does not seem to have affected the deeper Arctic waters, as indicated by the presence of benthic faunas well up to the late Miocene.

As a consequence of the cooling higher latitudes and expanding ice sheets, the sea­level history of the Oligocene is one of repeated regressions that were followed by smaller transgressions that successively covered less epicontinental areas than before. The first major sea­level fall occurred near the Eocene­Oligocene boundary, which most likely dates back to the first extensive ice accumulation on Antarctica. A second more pronounced global sea­level fall is indicated in the mid­Oligocene, around 30 MYBP, when the sea level dropped about 150 m (500 ft) and continental margins around the world were laid bare to extensive erosion. This eustatic fall event was also in response to a significant amount of accumulation on the Antarctic ice cap. Later Oligocene eustatic fluctuations were of lesser magnitude, indicating less extensive ice­sheet fluctuations. In the early Miocene a major transgression occurred, and the sea covered much of the coastal areas that were exposed during the Oligocene.

http://www.accessscience.com/content/oligocene/468000 5/7 4/21/2016 Oligocene ­ AccessScience from McGraw­Hill Education The mid­Oligocene sea­level lowering was accompanied by extensive shelf erosion and stream incision on many continental margins. Prominent canyons were initiated on many of the exposed shelves that remained operative for several million years. The mid­Oligocene eustatic retreat of the seas was characteristically long­lasting. Although this event was followed by minor eustatic recoveries, the sea level never rose back to pre­Oligocene levels, and recovered fully only after the close of Oligocene Epoch. This implies that during the long lowstand of the sea the submarine canyons were kept active continuously, to be filled back only after the full eustatic recovery in the early Miocene. See also: Continental margin (/content/continental­margin/159100); Paleoceanography (/content/paleoceanography/483300); Submarine canyons (/content/submarine­canyons/664300)

Life

Owing to accentuated thermal gradients and seasonality in the Oligocene (compared to previous Tertiary epochs), marine biotic provinces became more fragmented. Extreme climates, with greater diurnal and seasonal temperature contrasts, are held responsible for reduced diversities in marine plankton. The marine micro and macro fauna and flora of the Oligocene have strong affinities with those of the late Eocene. The typically Paleogene assemblages gradually become extinct during the Oligocene. The boundary between Oligocene and Miocene represents a complete changeover to Neogene faunal elements. Thus, the Oligocene was characterized by transitional faunal features between the Paleogene and the Neogene.

Planktonic foraminifera, which had diversified rapidly in the Eocene, were severely reduced in diversity in the Oligocene. Calcareous nannofossils that had also proliferated earlier, suffered reduction in diversities, though somewhat less severe than the planktonic foraminifera. Benthic fauna fared better and some lineages actually diversified, perhaps due to greater niche fragmentation. However, the last of the Nummulites disappeared in the Late Oligocene. The discocyclinids, dominant in the Eocene, were followed by lepidocyclinids in the Oligocene, and later, in the Miocene, gave way to the miogypsinids.

An evolutionary leap was made by the mammals across the Eocene­Oligocene boundary. The Eocene perissodactyls and prosimians saw their last days, giving way to rhinocerids, tapirs, and wild boarlike hog species with strong incisors and large canines. Hyaenodon was an Oligocene carnivore with strong canines and sharp molars, much like those of the modern cat species. Grasslands still supported large browsers, and a new group of perissodactyls, typified by the Titanotherium (Fig. 3), appeared and climaxed, to later die out near the close of Oligocene. The horses that had first appeared in the Eocene continued to increase in size and became three­toed, typified by Mesohippus. Other hoofed mammals also diversified during the Oligocene. Elephants made their first appearance near the Eocene­Oligocene boundary and developed a short trunk and two pairs of tusks. An early simian, Propliopithecus, made its first appearance in Oligocene, and is considered ancestral to the modern family of gibbons. The general uniformity of mammalian fauna in the Oligocene suggests that the widespread regressions of the sea most likely resulted in land bridges that reconnected some of the Northern Hemispheric land masses, which may have led to transmigrations of some families of mammals between North America, Asia, and Africa. Birds had achieved some of their modern characteristics, and at least 10 modern genera had already made their appearance by the close of Oligocene time. See also: Aves (/content/aves/065700); Geologic time scale (/content/geologic­time­scale/286500); Mammalia (/content/mammalia/402500); Paleoecology (/content/paleoecology/483700); Perissodactyla (/content/perissodactyla/499400)

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Fig. 3 Oligocene Titanotherium (=Brontotherium) of North America, a large browsing mammal. The titanotheres disappeared abruptly at the close of the Oligocene Epoch. (After R. A. Stirton, Time, Life and Man, Wiley, 1959)

Bilal U. Haq

Bibliography

B. U. Haq, J. Hardenbol, and P. R. Vail, Chronology of fluctuation sea levels since the Triassic, Science, 235:1156– 1167, 1987 DOI: 10.1126/science.235.4793.1156 (http://dx.doi.org/10.1126/science.235.4793.1156)

B. U. Haq and F. W. B. van Eysinga, Geological Time Table, Elsevier, 1998

C. Pomerol, The Cenozoic Era: Tertiary and Quaternary, Ellis Horwood, 1982

Additional Readings

J. Cowie, Climate Change: Biological and Human Aspects, 2d ed., Cambridge University Press, Cambridge, UK, 2013

C. Fioroni et al., Revised middle Eocene­­upper Oligocene calcareous nannofossil biozonation for the Southern Ocean, Revue de Micropaléontologie, 55(2):53–70, 2012 DOI: 10.1016/j.revmic.2012.03.001 (http://dx.doi.org/10.1016/j.revmic.2012.03.001)

J. G. Fleagle, Primate Adaptation and Evolution, 3d ed., Academic Press, San Diego, CA, 2013

F. Florindo and M. Siegert, Antarctic Climate Evolution, Elsevier, Amsterdam, Netherlands, 2009

University of California Museum of Paleontology: Geologic Time Scale (http://www.ucmp.berkeley.edu/exhibit/geology.html)

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