Source: Wikipedia, the free encyclopedia.
56.0 – 33.9 Ma
δ13C values at the PETM[3]
Lower boundary GSSPDababiya section, Luxor, Egypt[3]
25°30′00″N 32°31′52″E / 25.5000°N 32.5311°E / 25.5000; 32.5311
Lower GSSP ratified2003[3]
Upper boundary definitionLAD of Planktonic Foraminifers Hantkenina and Cribrohantkenina
Upper boundary GSSPMassignano quarry section, Massignano, Ancona, Italy
43°31′58″N 13°36′04″E / 43.5328°N 13.6011°E / 43.5328; 13.6011
Upper GSSP ratified1992[4]

The Eocene (

Era. The name Eocene comes from the Ancient Greek ἠώς (ēṓs, "dawn") and καινός (kainós, "new") and refers to the "dawn" of modern ('new') fauna that appeared during the epoch.[7][8]

The Eocene spans the time from the end of the

geologic periods, the strata that define the start and end of the epoch are well identified,[9]
though their exact dates are slightly uncertain.


The term "Eocene" is derived from Ancient Greek ἠώς eos meaning "dawn", and καινός kainos meaning "new" or "recent", as the epoch saw the dawn of recent, or modern, life.

Scottish geologist

Moritz Hörnes had introduced the Paleogene for the Eocene and Neogene for the Miocene and Pliocene in 1853.[12] 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.[13] In 1978, the Paleogene was officially defined as the Paleocene, Eocene, and Oligocene epochs; and the Neogene as the Miocene and Pliocene epochs.[14] 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, which may lead to the reinstatement of the Tertiary in the future.[15]



The beginning of the Eocene is marked by the Paleocene–Eocene Thermal Maximum, a short period of intense warming and ocean acidification brought about by the release of carbon en masse into the atmosphere and ocean systems,[16] which led to a mass extinction of 30–50% of benthic foraminifera–single-celled species which are used as bioindicators of the health of a marine ecosystem—one of the largest in the Cenozoic.[17][18] This event happened around 55.8 mya, and was one of the most significant periods of global change during the Cenozoic.[16][19][20]

The end of the Eocene was marked by the Eocene–Oligocene extinction event, also known as the Grande Coupure.[21]


The Eocene is conventionally divided into early (56–47.8 million years ago), middle (47.8–38m), and late (38–33.9m) subdivisions.[22] The corresponding rocks are referred to as lower, middle, and upper Eocene. The Ypresian Stage constitutes the lower, the Priabonian Stage the upper; and the Lutetian and Bartonian stages are united as the middle Eocene.[citation needed]

Palaeogeography and tectonics

During the Eocene, the continents continued to drift toward their present positions.

At the beginning of the period,

icefloes north and reinforcing the cooling.[25]

The northern supercontinent of Laurasia began to fragment, as Europe, Greenland and North America drifted apart.[26]

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.[27][28] Huge lakes formed in the high flat basins among uplifts,[29] resulting in the deposition of the Green River Formation lagerstätte.[30]

At about 35 Ma, an asteroid impact on the eastern coast of North America formed the Chesapeake Bay impact crater.[31][32]

In Europe, the

Mediterranean, and created another shallow sea with island archipelagos to the north.[33] Though the North Atlantic was opening,[34] a land connection appears to have remained between North America and Europe since the faunas of the two regions are very similar.[35]

Eurasia was separated in three different landmasses 50 million years ago; Western Europe, Balkanatolia and Asia. About 40 million years ago, Balkanatolia and Asia were connected, while Europe was connected 34 million years ago.[36]

Himalayas.[37] India collided with the Kohistan–Ladakh Arc around 50.2 million years ago and with Karakoram around 40.4 million years ago, with the final collision between Asia and India occurring ~40 million years ago.[38][39]


The Eocene Epoch contained a wide variety of different climate conditions that includes the warmest climate in the

Cenozoic Era, and arguably the warmest time interval since the Permian-Triassic mass extinction and Early Triassic, and ends in an icehouse climate.[40] The evolution of the Eocene climate began with warming after the end of the Paleocene–Eocene Thermal Maximum (PETM) at 56 million years ago to a maximum during the Eocene Optimum at around 49 million years ago. Recent study show elevation-dependent temperature changes during the Eocene hothouse.[41] During this period of time, little to no ice was present on Earth with a smaller difference in temperature from the equator to the poles.[42] Because of this the maximum sea level was 150 meters higher than current levels.[43] Following the maximum was a descent into an icehouse climate from the Eocene Optimum to the Eocene-Oligocene transition at 34 million years ago. During this decrease, ice began to reappear at the poles, and the Eocene-Oligocene transition is the period of time where the Antarctic ice sheet began to rapidly expand.[citation needed

Atmospheric greenhouse gas evolution

Greenhouse gases, in particular

crude oil at the bottom of the Arctic Ocean, that reduced the atmospheric carbon dioxide.[44] This event was similar in magnitude to the massive release of greenhouse gasses at the beginning of the PETM, and it is hypothesized that the sequestration was mainly due to organic carbon burial and weathering of silicates. For the early Eocene there is much discussion on how much carbon dioxide was in the atmosphere. This is due to numerous proxies representing different atmospheric carbon dioxide content. For example, diverse geochemical and paleontological proxies indicate that at the maximum of global warmth the atmospheric carbon dioxide values were at 700–900 ppm[45] while other proxies such as pedogenic (soil building) carbonate and marine boron isotopes indicate large changes of carbon dioxide of over 2,000 ppm over periods of time of less than 1 million years.[46] Sources for this large influx of carbon dioxide could be attributed to volcanic out-gassing due to North Atlantic rifting or oxidation of methane stored in large reservoirs deposited from the PETM event in the sea floor or wetland environments.[45] For contrast, today the carbon dioxide levels
are at 400 ppm or 0.04%.

At about the beginning of the Eocene Epoch (55.8–33.9 million years ago) the amount of oxygen in the earth's atmosphere more or less doubled.[47]

During the early Eocene, methane was another greenhouse gas that had a drastic effect on the climate. The warming effect of one ton of methane dimensions unspecified is approximately 30 times the warming effect of one ton of carbon on a 100-year scale (i.e., methane has a global warming potential of 29.8±11).[48] Most of the methane released to the atmosphere during this period of time would have been from wetlands, swamps, and forests.[49] 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.

The middle to late Eocene marks not only the switch from warming to cooling, but also the change in carbon dioxide from increasing to decreasing. At the end of the Eocene Optimum, carbon dioxide began decreasing due to increased siliceous plankton productivity and marine carbon burial.

Middle Eocene Climatic Optimum (MECO).[51] At around 41.5 million years ago, stable isotopic analysis of samples from Southern Ocean drilling sites indicated a warming event for 600,000 years. 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.[52] The main hypothesis for such a radical transition was due to the continental drift and collision of the India continent with the Asia continent and the resulting formation of the Himalayas. Another hypothesis involves extensive sea floor rifting and metamorphic decarbonation reactions releasing considerable amounts of carbon dioxide to the atmosphere.[51] 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.[53]

At the end of the Middle Eocene Climatic Optimum, cooling and the carbon dioxide drawdown continued through the late Eocene and into the Eocene–Oligocene transition around 34 million years ago.[54] Multiple proxies, such as oxygen isotopes and alkenones, indicate that at the Eocene–Oligocene transition, the atmospheric carbon dioxide concentration had decreased to around 750–800 ppm, approximately twice that of present levels.[55][56]

Early Eocene and the equable climate problem

One of the unique features of the Eocene's climate as mentioned before was the equable and homogeneous climate that existed in the early parts of the Eocene. A multitude of

snakes found in the tropics that would require much higher average temperatures to sustain them.[58] TEX86 BAYSPAR measurements indicate extremely high sea surface temperatures of 40 °C (104 °F) to 45 °C (113 °F) at low latitudes,[60] although clumped isotope analyses point to a maximum low latitude sea surface temperature of 36.3 °C (97.3 °F) ± 1.9 °C (35.4 °F) during the Early Eocene Climatic Optimum.[61] Relative to present-day values, bottom water temperatures are 10 °C (18 °F) higher according to isotope proxies.[59] With these bottom water temperatures, temperatures in areas where deep water forms near the poles are unable to be much cooler than the bottom water temperatures.[citation needed

An issue arises, however, when trying to model the Eocene and reproduce the results that are found with the proxy data.[62] 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 20 °C (36 °F) colder than the actual determined temperature at the poles.[59] 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.[citation needed]

Large lakes

Due to the nature of water as opposed to land, less temperature variability would be present if a large body of water is also present. In an attempt to try to mitigate the cooling polar temperatures, large lakes were proposed to mitigate seasonal climate changes.[63] To replicate this case, a lake was inserted into North America and a climate model was run using varying carbon dioxide levels. The model runs concluded that while the lake did reduce the seasonality of the region greater than just an increase in carbon dioxide, the addition of a large lake was unable to reduce the seasonality to the levels shown by the floral and faunal data.[citation needed]

Ocean heat transport

The transport of heat from the tropics to the poles, much like how ocean heat transport functions in modern times, was considered a possibility for the increased temperature and reduced seasonality for the poles.[64] With the increased sea surface temperatures and the increased temperature of the deep ocean water during the early Eocene, one common hypothesis was that due to these increases there would be a greater transport of heat from the tropics to the poles. Simulating these differences, the models produced lower heat transport due to the lower temperature gradients and were unsuccessful in producing an equable climate from only ocean heat transport.[citation needed]

Orbital parameters

While typically seen as a control on ice growth and seasonality, the orbital parameters were theorized as a possible control on continental temperatures and seasonality.

obliquity, and precession were modified in different model runs to determine all the possible different scenarios that could occur and their effects on temperature. One particular case led to warmer winters and cooler summer by up to 30% in the North American continent, and it reduced the seasonal variation of temperature by up to 75%. While orbital parameters did not produce the warming at the poles, the parameters did show a great effect on seasonality and needed to be considered.[citation needed

Polar stratospheric clouds

Another method considered for producing the warm polar temperatures were polar stratospheric clouds.[66] Polar stratospheric clouds are clouds that occur in the lower stratosphere at very low temperatures. Polar stratospheric clouds have a great impact on radiative forcing. Due to their minimal albedo properties and their optical thickness, polar stratospheric clouds act similar to a greenhouse gas and traps outgoing longwave radiation. Different types of polar stratospheric clouds occur in the atmosphere: polar stratospheric clouds that are created due to interactions with nitric or sulfuric acid and water (Type I) or polar stratospheric clouds that are created with only water ice (Type II).[citation needed]

Methane is an important factor in the creation of the primary Type II polar stratospheric clouds that were created in the early Eocene.[49] 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.[citation needed]

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.[66] 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.[67] 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.[citation needed]

Hyperthermals through the early Eocene

During the warming in the early Eocene between 52 and 55 million years ago, there were a series of short-term changes of

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.[citation needed

Greenhouse to icehouse climate

The Eocene is not only known for containing the warmest period during the Cenozoic; it also marked the decline into an icehouse climate and the rapid expansion of the

Global cooling continued until there was a major reversal from cooling to warming indicated in the

Himalayan orogeny; however, data on the exact timing of metamorphic release of atmospheric carbon dioxide is not well resolved in the data.[51] Recent studies have mentioned, however, that the removal of the ocean between Asia and India could have released significant amounts of carbon dioxide.[52] This warming is short lived, as benthic oxygen isotope records indicate a return to cooling at ~40 million years ago.[55]

Cooling continued throughout the rest of the late Eocene into the Eocene-Oligocene transition. During the cooling period, benthic oxygen isotopes show the possibility of ice creation and ice increase during this later cooling.[45] The end of the Eocene and beginning of the Oligocene is marked with the massive expansion of area of the Antarctic ice sheet that was a major step into the icehouse climate.[56] Along with the decrease of atmospheric carbon dioxide reducing the global temperature, orbital factors in ice creation can be seen with 100,000-year and 400,000-year fluctuations in benthic oxygen isotope records.[70] Another major contribution to the expansion of the ice sheet was the creation of the Antarctic Circumpolar Current.[71] The creation of the Antarctic circumpolar current would isolate the cold water around the Antarctic, which would reduce heat transport to the Antarctic[72] along with creating ocean gyres that result in the upwelling of colder bottom waters.[71] The issue with this hypothesis of the consideration of this being a factor for the Eocene-Oligocene transition is the timing of the creation of the circulation is uncertain.[73] For Drake Passage, sediments indicate the opening occurred ~41 million years ago while tectonics indicate that this occurred ~32 million years ago.[citation needed]


During the early-middle Eocene, forests covered most of the Earth including the poles. Tropical forests extended across much of modern Africa, South America, Central America, India, South-east Asia and China.  Paratropical forests grew over North America, Europe and Russia, with broad-leafed evergreen and broad-leafed deciduous forests at higher latitudes.[74]

Polar forests were quite extensive.

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.[citation needed

Palm trees were growing as far north as Alaska and northern Europe during the early Eocene, although they became less abundant as the climate cooled.[75] Dawn redwoods were far more extensive as well.[76]

The earliest definitive

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 were still confined to river banks and lake shores, and had not yet expanded into plains and savannas.[citation needed

The cooling also brought seasonal changes. Deciduous trees, better able to cope with large temperature changes, began to overtake evergreen tropical species.[78] 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.[citation needed]

flora was wiped out, and by the beginning of the Oligocene, the continent hosted deciduous forests and vast stretches of tundra.[citation needed


During the Eocene, plants and marine faunas became quite modern. Many modern bird orders first appeared in the Eocene. The Eocene oceans were warm and teeming with fish and other sea life.


French National Museum of Natural History, Paris

The oldest known

teeth adapted for chewing. Dwarf forms reigned. All the members of the new mammal orders were small, under 10 kg; based on comparisons of tooth size, Eocene mammals were only 60% of the size of the primitive Palaeocene mammals that preceded them. They were also smaller than the mammals that followed them. It is assumed that the hot Eocene temperatures favored smaller animals that were better able to manage the heat.[citation needed

Both groups of modern


Established megafauna of the Eocene include the

, 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 began to take form as the terrestrial carnivores like the

to appear. Their groups became highly successful and continued to live past the Eocene.

Basilosaurus is a very 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 cetaceans occurred. The first sirenians were evolving at this time, and would eventually evolve into the extant manatees and dugongs.

It is thought that millions of years after the

Cretaceous-Paleogene extinction event, brain sizes of mammals now started to increase, "likely driven by a need for greater cognition in increasingly complex environments".[80][81][clarification needed


Primobucco, an early relative of the roller

Eocene birds include some enigmatic groups with resemblances to modern forms, some of which continued from the Paleocene. Bird taxa of the Eocene include carnivorous

psittaciforms, such as Messelasturidae, Halcyornithidae, large flightless forms such as Gastornis and Eleutherornis, long legged falcon Masillaraptor, ancient galliforms such as Gallinuloides, putative rail relatives of the family Songziidae, various pseudotooth birds such as Gigantornis, the ibis relative Rhynchaeites, primitive swifts of the genus Aegialornis, and primitive penguins such as Archaeospheniscus and Inkayacu.[citation needed


Reptile fossils from this time, such as fossils of

pythons and turtles, are abundant.[82]

Insects and arachnids

Several rich fossil insect faunas are known from the Eocene, notably the

Bembridge Marls from the Isle of Wight, England. Insects found in Eocene deposits mostly belong to genera that exist today, though their range has often shifted since the Eocene. For instance the bibionid genus Plecia is common in fossil faunas from presently temperate areas, but only lives in the tropics and subtropics today.[citation needed


See also


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  6. ^ See:
  7. ^ "Eocene". Online Etymology Dictionary.
  8. ^ 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).
  9. ^ Lyell, C. (1833). Principles of Geology. Vol. 3. Geological Society of London. p. 378.
  10. ^ Phillips, J. (1840). "Palæozoic series". Penny Cyclopaedia of the Society for the Diffusion of Useful Knowledge. Vol. 17. London, England: Charles Knight and Co. pp. 153–154.
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  32. ^ Torsvik & Cocks 2017, pp. 242–245.
  33. ^ Torsvik & Cocks 2017, pp. 251.
  34. .
  35. ^ CNRS (2022-03-01). "Balkanatolia: Forgotten Continent Discovered by Team of Paleontologists and Geologists". SciTechDaily. Retrieved 2023-02-06.
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  46. ^ O'Neil, Dennis (2012). "The First Primates". anthro.palomar.edu. Archived from the original on 2015-12-25. Retrieved 2014-05-13.
  47. ^ Forster, P.; Storelvmo, T.; Armour, K.; Collins, W. (2021). "Chapter 7: The Earth's energy budget, climate feedbacks, and climate sensitivity" (PDF). IPCC AR6 WG1 2021.
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  78. ^ Bender, Michael (2013). Paleoclimate. Princeton University Press. p. 108.
  79. ^ "Mammals' bodies outpaced their brains right after the dinosaurs died". Science News. 31 March 2022. Retrieved 14 May 2022.
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Further reading

External links