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The Eocene ( ) is a geological epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name "Eocene" comes from Ancient Greek ( ēṓs), meaning "dawn", and ( kainós), meaning "new", and refers to the "dawn" of modern ('new') fauna that appeared during the epoch.See:
The Eocene spans the time from the end of the Paleocene Epoch to the beginning of the Oligocene Epoch. The start of the Eocene is marked by a brief period in which the concentration of the carbon isotope 13C in the atmosphere was exceptionally low in comparison with the more common isotope 12C. The average temperature of Earth at the beginning of the Eocene was about 27 degrees Celsius. The end is set at a major extinction event called the Grande Coupure (the "Great Break" in continuity) or the Eocene–Oligocene extinction event, which may be related to the impact of one or more large in Siberia and in what is now Chesapeake Bay. As with other , the Stratum that define the start and end of the epoch are well identified,The extinction of the Hantkeninidae, a planktonic family of foraminifera became generally accepted as marking the Eocene-Oligocene boundary; in 1998 Massignano in Umbria, central Italy, was designated the Global Boundary Stratotype Section and Point (GSSP). though their exact dates are slightly uncertain.
Scottish geologist Charles Lyell (ignoring the Quaternary) divided the Tertiary Epoch into the Eocene, Miocene, Pliocene, and New Pliocene (Holocene) Periods in 1833. British geologist John Phillips proposed the Cenozoic in 1840 in place of the Tertiary, and Austrian paleontologist Moritz Hörnes introduced the Paleogene for the Eocene and Neogene for the Miocene and Pliocene in 1853. After decades of inconsistent usage, the newly formed International Commission on Stratigraphy (ICS), in 1969, standardized stratigraphy based on the prevailing opinions in Europe: the Cenozoic Era subdivided into the Tertiary and Quaternary sub-eras, and the Tertiary subdivided into the Paleogene and Neogene periods. In 1978, the Paleogene was officially defined as the Paleocene, Eocene, and Oligocene epochs; and the Neogene as the Miocene and Pliocene epochs. In 1989, Tertiary and Quaternary were removed from the time scale due to the arbitrary nature of their boundary, but Quaternary was reinstated in 2009.
The middle Eocene was characterized by the shift towards a cooler climate at the end of the Early Eocene Climatic Optimum, around 47.8 Ma, which was briefly interrupted by another warming event, the Middle Eocene Climatic Optimum. Lasting for about 400,000 years, the MECO was responsible for a globally uniform 4° to 6 °C warming of both the surface and deep oceans, as inferred from foraminiferal stable oxygen isotope records. The resumption of a long-term gradual cooling trend resulted in a glacial maximum at the late Eocene/early Oligocene boundary.
The end of the Eocene was also marked by the Eocene–Oligocene extinction event, also known as the Grande Coupure.
The Western North American floras of the Eocene were divided into four floral "stages" by Jack Wolfe (1968) based on work with the Puget Group fossils of King County, Washington. The four stages, Franklinian, Fultonian, Ravenian, and Kummerian covered the Early Eocene through early Oligocene, and three of the four were given informal early/late substages. Wolfe tentatively deemed the Franklinian as Early Eocene, the Fultonian as Middle Eocene, the Ravenian as Late, and the Kummerian as Early Oligocene. The beginning of the Kummerian was refined by Gregory Retallack et al (2004) as 40 Mya, with a refined end at the Eocene-Oligocene boundary where the younger Angoonian floral stage starts.
The northern supercontinent of Laurasia began to fragment, as Europe, Greenland and North America drifted apart. (2025). 9781107105324, Cambridge University Press. ISBN 9781107105324
In western North America, the Laramide Orogeny came to an end in the Eocene, and compression was replaced with crustal extension that ultimately gave rise to the Basin and Range Province. The Kishenehn Basin, around 1.5 km in elevation during the Lutetian, was uplifted to an altitude of 2.5 km by the Priabonian. Huge lakes formed in the high flat basins among uplifts, resulting in the deposition of the Green River Formation lagerstätte. (2025). 9781461354710 ISBN 9781461354710
At about 35 Ma, an asteroid impact on the eastern coast of North America formed the Chesapeake Bay impact crater. (2025). 9783642189005, Springer Berlin Heidelberg. ISBN 9783642189005
The Tethys Sea finally closed with the collision of Africa and Eurasia, while the uplift of the Alps isolated its final remnant, the Mediterranean, and created another shallow sea with island to the north. Planktonic foraminifera in the northwestern Peri-Tethys are very similar to those of the Tethys in the middle Lutetian but become completely disparate in the Bartonian, indicating biogeographic separation. Though the North Atlantic Ocean was opening, a land connection appears to have remained between North America and Europe since the faunas of the two regions are very similar. (2025). 9789400703711 ISBN 9789400703711
Eurasia was separated in three different landmasses 50 Ma; Western Europe, Balkanatolia and Asia. About 40 Ma, Balkanatolia and Asia were connected, while Europe was connected 34 Ma. The Fushun Basin contained large, suboxic lakes known as the paleo-Jijuntun Lakes.
Indian Plate collided with Asia, folding to initiate formation of the . The incipient subcontinent collided with the Kohistan–Ladakh Arc around 50.2 Ma and with Karakoram around 40.4 Ma, with the final collision between Asia and India occurring ~40 Ma.
During the early Eocene, methane was another greenhouse gas that had a drastic effect on the climate. Methane has 30 times more of a warming effect than carbon dioxide on a 100-year scale (i.e., methane has a global warming potential of 29.8±11). Most of the methane released to the atmosphere during this period of time would have been from wetlands, swamps, and forests. The atmospheric methane concentration today is 0.000179% or 1.79 ppmv. As a result of the warmer climate and the sea level rise associated with the early Eocene, more wetlands, more forests, and more coal deposits would have been available for methane release. If we compare the early Eocene production of methane to current levels of atmospheric methane, the early Eocene would have produced triple the amount of methane. The warm temperatures during the early Eocene could have increased methane production rates, and methane that is released into the atmosphere would in turn warm the troposphere, cool the stratosphere, and produce water vapor and carbon dioxide through oxidation. Biogenic production of methane produces carbon dioxide and water vapor along with the methane, as well as yielding infrared radiation. The breakdown of methane in an atmosphere containing oxygen produces carbon monoxide, water vapor and infrared radiation. The carbon monoxide is not stable, so it eventually becomes carbon dioxide and in doing so releases yet more infrared radiation. Water vapor traps more infrared than does carbon dioxide. At about the beginning of the Eocene Epoch (55.8–33.9 Ma) the amount of oxygen in the Earth's atmosphere more or less doubled.
During the warming in the early Eocene between 55 and 52 Ma, there were a series of short-term changes of carbon isotope composition in the ocean. These isotope changes occurred due to the release of carbon from the ocean into the atmosphere that led to a temperature increase of at the surface of the ocean. Recent analysis of and research into these hyperthermals in the early Eocene has led to hypotheses that the hyperthermals are based on orbital parameters, in particular eccentricity and obliquity. The hyperthermals in the early Eocene, notably the Palaeocene–Eocene Thermal Maximum (PETM), the Eocene Thermal Maximum 2 (ETM2), and the Eocene Thermal Maximum 3 (ETM3), were analyzed and found that orbital control may have had a role in triggering the ETM2 and ETM3. An enhancement of the biological pump proved effective at sequestering excess carbon during the recovery phases of these hyperthermals. These hyperthermals led to increased perturbations in planktonic and benthic foraminifera, with a higher rate of fluvial sedimentation as a consequence of the warmer temperatures. Unlike the PETM, the lesser hyperthermals of the Early Eocene had negligible consequences for terrestrial mammals. These Early Eocene hyperthermals produced a sustained period of extremely hot climate known as the Early Eocene Climatic Optimum (EECO). During the early and middle EECO, the superabundance of the euryhaline dinocyst Homotryblium in New Zealand indicates elevated ocean salinity in the region.
An issue arises, however, when trying to model the Eocene and reproduce the results that are found with the proxy data. Using all different ranges of greenhouse gasses that occurred during the early Eocene, models were unable to produce the warming that was found at the poles and the reduced seasonality that occurs with winters at the poles being substantially warmer. The models, while accurately predicting the tropics, tend to produce significantly cooler temperatures of up to colder than the actual determined temperature at the poles. This error has been classified as the "equable climate problem". To solve this problem, the solution would involve finding a process to warm the poles without warming the tropics. Some hypotheses and tests which attempt to find the process are listed below.
Methane is an important factor in the creation of the primary Type II polar stratospheric clouds that were created in the early Eocene. Since water vapor is the only supporting substance used in Type II polar stratospheric clouds, the presence of water vapor in the lower stratosphere is necessary where in most situations the presence of water vapor in the lower stratosphere is rare. When methane is oxidized, a significant amount of water vapor is released. Another requirement for polar stratospheric clouds is cold temperatures to ensure condensation and cloud production. Polar stratospheric cloud production, since it requires the cold temperatures, is usually limited to nighttime and winter conditions. With this combination of wetter and colder conditions in the lower stratosphere, polar stratospheric clouds could have formed over wide areas in Polar Regions.
To test the polar stratospheric clouds effects on the Eocene climate, models were run comparing the effects of polar stratospheric clouds at the poles to an increase in atmospheric carbon dioxide. The polar stratospheric clouds had a warming effect on the poles, increasing temperatures by up to 20 °C in the winter months. A multitude of feedbacks also occurred in the models due to the polar stratospheric clouds' presence. Any ice growth was slowed immensely and would lead to any present ice melting. Only the poles were affected with the change in temperature and the tropics were unaffected, which with an increase in atmospheric carbon dioxide would also cause the tropics to increase in temperature. Due to the warming of the troposphere from the increased greenhouse effect of the polar stratospheric clouds, the stratosphere would cool and would potentially increase the amount of polar stratospheric clouds.
While the polar stratospheric clouds could explain the reduction of the equator to pole temperature gradient and the increased temperatures at the poles during the early Eocene, there are a few drawbacks to maintaining polar stratospheric clouds for an extended period of time. Separate model runs were used to determine the sustainability of the polar stratospheric clouds. It was determined that in order to maintain the lower stratospheric water vapor, methane would need to be continually released and sustained. In addition, the amounts of ice and condensation nuclei would need to be high in order for the polar stratospheric cloud to sustain itself and eventually expand.
Global cooling continued until there was a major reversal from cooling to warming in the Bartonian. This warming event, signifying a sudden and temporary reversal of the cooling conditions, is known as the Middle Eocene Climatic Optimum (MECO). At around 41.5 Ma, stable isotopic analysis of samples from Southern Ocean drilling sites indicated a warming event for 600,000 years. A similar shift in carbon isotopes is known from the Northern Hemisphere in the Scaglia Limestones of Italy. Oxygen isotope analysis showed a large negative change in the proportion of heavier oxygen isotopes to lighter oxygen isotopes, which indicates an increase in global temperatures. The warming is considered to be primarily due to carbon dioxide increases, because carbon isotope signatures rule out major methane release during this short-term warming. A sharp increase in atmospheric carbon dioxide was observed with a maximum of 4,000 ppm: the highest amount of atmospheric carbon dioxide detected during the Eocene. Other studies suggest a more modest rise in carbon dioxide levels. The increase in atmospheric carbon dioxide has also been hypothesised to have been driven by increased seafloor spreading rates and metamorphic decarbonation reactions between Australia and Antarctica and increased amounts of volcanism in the region. One possible cause of atmospheric carbon dioxide increase could have been a sudden increase due to metamorphic release due to continental drift and collision of India with Asia and the resulting formation of the Himalayas; however, data on the exact timing of metamorphic release of atmospheric carbon dioxide is not well resolved in the data. Recent studies have mentioned, however, that the removal of the ocean between Asia and India could have released significant amounts of carbon dioxide. Another hypothesis still implicates a diminished negative feedback of silicate weathering as a result of continental rocks having become less weatherable during the warm Early and Middle Eocene, allowing volcanically released carbon dioxide to persist in the atmosphere for longer. Yet another explanation hypothesises that MECO warming was caused by the simultaneous occurrence of minima in both the 400 kyr and 2.4 Myr eccentricity cycles. During the MECO, sea surface temperatures in the Tethys Ocean jumped to 32–36 °C, and Tethyan seawater became more dysoxic. A decline in carbonate accumulation at ocean depths of greater than three kilometres took place synchronously with the peak of the MECO, signifying ocean acidification took place in the deep ocean. On top of that, MECO warming caused an increase in the respiration rates of Pelagic zone , leading to a decreased proportion of primary productivity making its way down to the seafloor and causing a corresponding decline in populations of benthic foraminifera. An abrupt decrease in lakewater salinity in western North America occurred during this warming interval. This warming is short lived, as benthic oxygen isotope records indicate a return to cooling at ~40 Ma.
Polar forests were quite extensive. and even preserved remains of trees such as Cupressaceae and Metasequoia from the Eocene have been found on Ellesmere Island in the Arctic. Even at that time, Ellesmere Island was only a few degrees in latitude further south than it is today. Fossils of subtropical and even tropical trees and plants from the Eocene also have been found in Greenland and Alaska. Tropical rainforests grew as far north as northern North America and Europe.
were growing as far north as Alaska and northern Europe during the early Eocene, although they became less abundant as the climate cooled. Dawn redwoods were far more extensive as well.
The earliest definitive Eucalyptus fossils were dated from 51.9 Ma, and were found in the Laguna del Hunco deposit in Chubut province in Argentina.
Cooling began mid-period, and by the end of the Eocene continental interiors had begun to dry, with forests thinning considerably in some areas. The newly evolved grasses began to expand during the climatic cooling and drying following the extreme warmth of the EECO, with subhumid being known from South America since the Middle Eocene.
The cooling also brought changes. Deciduous trees, better able to cope with large temperature changes, began to overtake evergreen tropical species. By the end of the period, deciduous forests covered large parts of the northern continents, including North America, Eurasia and the Arctic, and rainforests held on only in equatorial South America, Africa, India and Australia.
Antarctica began the Eocene fringed with a warm temperate to sub-tropical rainforest. Pollen found in Prydz Bay from the Eocene suggest taiga forest existed there. It became much colder as the period progressed; the heat-loving tropical flora was wiped out, and by the beginning of the Oligocene, the continent hosted deciduous forests and vast stretches of tundra.
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Rodents were widespread. East Asian rodent faunas declined in diversity when they shifted from ctenodactyloid-dominant to cricetid–dipodid-dominant after the MECO.
Both groups of modern (hoofed animals) became prevalent because of a major radiation between Europe and North America, along with carnivorous ungulates like Mesonyx. Early forms of many other modern mammalian orders appeared, including (most notably the Eohippus), , Proboscidea (elephants), primates, and . Older primitive forms of mammals declined in variety and importance. Important Eocene land fauna fossil remains have been found in western North America, Europe, Patagonia, Egypt, and southeast Asia. Marine fauna are best known from South Asia and the southeast United States.
After the Paleocene–Eocene Thermal Maximum, members of the Equoidea arose in North America and Europe, giving rise to some of the earliest such as Sifrhippus and basal European equoids such as the palaeothere Hyracotherium. Some of the later equoids were especially species-rich; Palaeotherium, ranging from small to very large in size, is known from as many as 16 species.
Established large-sized mammals of the Eocene include the Uintatherium, Arsinoitherium, and brontotheres, in which the former two, unlike the latter, did not belong to ungulates but groups that became extinct shortly after their establishments.
Large terrestrial mammalian predators had already existed since the Paleocene, but new forms now arose like Hyaenodon and Daphoenus (the earliest lineage of a once-successful predatory family known as bear dogs). Entelodonts meanwhile established themselves as some of the largest omnivores. The first nimravids, including Dinictis, established themselves as amongst the first feliforms to appear. Their groups became highly successful and continued to live past the Eocene.
Basilosaurus is a well-known Eocene whale, but whales as a group had become very diverse during the Eocene, which is when the major transitions from being terrestrial to fully aquatic in occurred. The first were evolving at this time, and would eventually evolve into the extant and .
Many Eocene birds in Central Europe evolved tuberculate vertebrae as an adaptation against predation, with flightless birds facing low predation pressure during this time as a result.
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