The EOCENE ( /ˈiːəˌsiːn, ˈiːoʊ-/ ) Epoch, lasting from 56
to 33.9 million years ago, is a major division of the geologic
timescale and the second epoch of the
Paleogene Period in the Cenozoic
Eocene spans the time from the end of the
Paleocene Epoch to
the beginning of the
Oligocene Epoch. The start of the
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 end is set at a major extinction
event called the Grande Coupure (the "Great Break" in continuity) or
Oligocene extinction event , which may be related to the
impact of one or more large bolides in Siberia and in what is now
Chesapeake Bay . As with other geologic periods , the strata that
define the start and end of the epoch are well identified, though
their exact dates are slightly uncertain.
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.
* 1 Subdivisions
* 2 Climate
* 2.1 Atmospheric greenhouse gas evolution
* 2.2 Early
Eocene and the equable climate problem
* 2.2.1 Large lakes
Ocean heat transport
* 2.2.3 Orbital parameters
Polar stratospheric clouds
* 2.3 Hyperthermals through the Early
* 2.4 Greenhouse to icehouse climate
* 3 Palaeogeography
* 4 Flora
* 5.1 Oceans
* 6 Eocene–
* 7 See also
* 8 References
* 9 Further reading
* 10 External links
Eocene epoch is conventionally divided into early, middle, and
late subdivisions. The corresponding rocks are referred to as lower,
middle, and upper Eocene. The
Ypresian stage constitutes the lower,
Priabonian stage the upper; and the
are united as the middle Eocene.
Eocene Epoch contained a wide variety of different climate
conditions that includes the warmest climate in the
Cenozoic Era and
ends in an icehouse climate. The evolution of the
Eocene climate began
with warming after the end of the Palaeocene-
Eocene Thermal Maximum
(PETM) at 56 million years ago to a maximum during the
at around 49 million years ago. 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. Following the maximum was a descent
into an icehouse climate from the
Eocene Optimum to the
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
Antarctic ice sheet began
to rapidly expand.
ATMOSPHERIC GREENHOUSE GAS EVOLUTION
Greenhouse gases, in particular carbon dioxide and methane , played a
significant role during the
Eocene in controlling the surface
temperature. The end of the PETM was met with a very large
sequestration of carbon dioxide in the form of methane clathrate ,
coal , and crude oil at the bottom of the
Arctic Ocean , that reduced
the atmospheric carbon dioxide. 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
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. 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. 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
During the early Eocene, methane was another greenhouse gas that had
a drastic effect on the climate. In comparison to carbon dioxide,
methane has much greater effect on temperature as methane is ~34 times
more effective per molecule than carbon dioxide on a 100-year scale
(it has a higher global warming potential ). 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. Due to the warmer
climate and sea level rise associated with the early Eocene, more
wetlands, more forests, and more coal deposits would be available for
methane release. Comparing the early
Eocene production of methane to
current levels of atmospheric methane, the early
Eocene would be able
to produce triple the amount of current methane production. 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 oxygen atmosphere 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
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. At the beginning of the middle
Eocene an event that
may have triggered or helped with the draw down of carbon dioxide was
Azolla event at around 49 million years ago. With the equable
climate during the early Eocene, warm temperatures in the arctic
allowed for the growth of azolla , which is a floating aquatic fern,
Arctic Ocean . Compared to current carbon dioxide levels, these
azolla grew rapidly in the enhanced carbon dioxide levels found in the
early Eocene. As these azolla sank into the
Arctic Ocean, they became
buried and sequestered their carbon into the seabed. This event could
have led to a draw down of atmospheric carbon dioxide of up to 470
ppm. Assuming the carbon dioxide concentrations were at 900 ppmv
prior to the
Azolla Event they would have dropped to 430 ppmv, or 30
ppmv more than they are today, after the
Azolla Event. Another event
during the middle
Eocene that was a sudden and temporary reversal of
the cooling conditions was the
Middle Eocene Climatic Optimum . At
around 41.5 million years ago, stable isotopic analysis of samples
Southern Ocean drilling sites indicated a warming event for 600
thousand years. A sharp increase in atmospheric carbon dioxide was
observed with a maximum of 4000 ppm: the highest amount of atmospheric
carbon dioxide detected during the Eocene. 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.
At the end of the Middle
Eocene Climatic Optimum, cooling and the
carbon dioxide drawdown continued through the late
Eocene and into the
Oligocene transition around 34 million years ago. Multiple
proxies, such as oxygen isotopes and alkenones , indicate that at the
Oligocene transition, the atmospheric carbon dioxide
concentration had decreased to around 750-800 ppm, approximately twice
that of present levels .
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 proxies support the presence
of a warmer equable climate being present during this period of time.
A few of these proxies include the presence of fossils native to warm
climates, such as crocodiles , located in the higher latitudes, the
presence in the high-latitudes of frost-intolerant flora such as palm
trees which cannot survive during sustained freezes, and fossils of
snakes found in the tropics that would require much higher average
temperatures to sustain them. Using isotope proxies to determine
ocean temperatures indicates sea surface temperatures in the tropics
as high as 35 °C (95 °F) and, relative to present day values, bottom
water temperatures that are 10 °C (18 °F) higher. 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
An issue arises, however, when trying to model the
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 20 °C (36 °F) 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.
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. 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.
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. 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.
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. Simulating the
Eocene by using an ice
free planet, eccentricity , 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.
Polar Stratospheric Clouds
Another method considered for producing the warm polar temperatures
were polar stratospheric clouds .
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).
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
To test the polar stratospheric clouds effects on the
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.
Methane would need
to be continually released and sustained to maintain the lower
stratospheric water vapor. Increasing amounts of ice and condensation
nuclei would be need to be high for the polar stratospheric cloud to
sustain itself and eventually expand.
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 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 4-8 °C (7.2-14.4 °F) at the surface of the
ocean. These hyperthermals led to increased perturbations in
planktonic and benthic foraminifera , with a higher rate of
sedimentation as a consequence of the warmer temperatures. 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.
GREENHOUSE TO ICEHOUSE CLIMATE
Eocene is not only known for containing the warmest period during
the Cenozoic, but it also marked the decline into an icehouse climate
and the rapid expansion of the
Antarctic ice sheet
Antarctic ice sheet . The transition
from a warming climate into a cooling climate began at ~49 million
years ago. Isotopes of carbon and oxygen indicate a shift to a global
cooling climate. The cause of the cooling has been attributed to a
significant decrease of >2000 ppm in atmospheric carbon dioxide
concentrations. One proposed cause of the reduction in carbon dioxide
during the warming to cooling transition was the
Azolla event . The
increased warmth at the poles, the isolated
Arctic basin during the
early Eocene, and the significantly high amounts of carbon dioxide
possibly led to azolla blooms across the
Arctic Ocean. The isolation
Arctic Ocean led to stagnant waters and as the azolla sank to
the sea floor, they became part of the sediments and effectively
sequestered the carbon. The ability for the azolla to sequester carbon
is exceptional, and the enhanced burial of azolla could have had a
significant effect on the world atmospheric carbon content and may
have been the event to begin the transition into an ice house climate.
Cooling after this event continued due to continual decrease in
atmospheric carbon dioxide from organic productivity and weathering
from mountain building .
Global cooling continued until there was a major reversal from
cooling to warming indicated in the
Southern Ocean at around 42-41
million years ago.
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. This
warming event is known as the Middle
Eocene Climatic Optimum. The
cause of the warming is considered to primarily be due to carbon
dioxide increases, since carbon isotope signatures rule out major
methane release during this short term warming. The increase in
atmospheric carbon dioxide is considered to be due to increased
seafloor spreading rates between
increased amounts of volcanism in the region. Another possible
atmospheric carbon dioxide increase could be during a sudden increase
with metamorphic release during the
Himalayan orogeny , 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
India could release significant amounts of carbon dioxide. This
warming is short lived, as benthic oxygen isotope records indicate a
return to cooling at ~40 million years ago.
Cooling continued throughout the rest of the late
Eocene into the
Oligocene transition. During the cooling period, benthic oxygen
isotopes show the possibility of ice creation and ice increase during
this later cooling. The end of the
Eocene and beginning of the
Oligocene is marked with the massive expansion of area of the
Antarctic ice sheet
Antarctic ice sheet that was a major step into the icehouse climate.
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. Another major contribution to the expansion of the ice sheet
was the creation of the
Antarctic circumpolar current
Antarctic circumpolar current . The creation
Antarctic circumpolar current
Antarctic circumpolar current would isolate the cold water
around the Antarctic, which would reduce heat transport to the
Antarctic along with create ocean gyres that result in the upwelling
of colder bottom waters. 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
Drake Passage , sediments indicate the opening
occurred ~41 million years ago while tectonics indicate that this
occurred ~32 million years ago.
During the Eocene, the continents continued to drift toward their
At the beginning of the period,
connected, and warm equatorial currents mixed with colder Antarctic
waters, distributing the heat around the planet and keeping global
temperatures high, but when
Australia split from the southern
continent around 45 Ma, the warm equatorial currents were routed away
from Antarctica. An isolated cold water channel developed between the
two continents. The Antarctic region cooled down, and the ocean
Antarctica began to freeze, sending cold water and
icefloes north, reinforcing the cooling.
The northern supercontinent of
Laurasia began to fragment, as Europe
North America drifted apart.
In western North America, mountain building started in the Eocene,
and huge lakes formed in the high flat basins among uplifts, resulting
in the deposition of the
Green River Formation lagerstätte .
At about 35 Ma, an asteroid impact on the eastern coast of North
America formed the
Chesapeake Bay impact crater .
In Europe, the Tethys Sea finally disappeared, while the uplift of
Alps isolated its final remnant, the Mediterranean , and created
another shallow sea with island archipelagos to the north. Though the
North Atlantic was opening, a land connection appears to have remained
North America and
Europe since the faunas of the two regions
are very similar.
India began its collision with
Asia , folding to initiate formation
It is hypothesized that the
Eocene hothouse world was caused by
runaway global warming from released methane clathrates deep in the
oceans . The clathrates were buried beneath mud that was disturbed as
the oceans warmed.
Methane (C H 4) has ten to twenty times the
greenhouse gas effect of carbon dioxide (CO2).
At the beginning of the Eocene, the high temperatures and warm oceans
created a moist, balmy environment, with forests spreading throughout
the Earth from pole to pole. Apart from the driest deserts , Earth
must have been entirely covered in forests.
Polar forests were quite extensive. Fossils and even preserved
remains of trees such as swamp cypress and dawn redwood from the
Eocene have been found on
Ellesmere Island in the
Arctic . Even at
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 have also been found in
Alaska . Tropical rainforests grew as far north as northern North
Palm trees were growing as far north as
Alaska and northern Europe
during the early Eocene, although they became less abundant as the
Dawn redwoods were far more extensive as well.
Cooling began mid-period, and by the end of the
interiors had begun to dry out, with forests thinning out 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
The cooling also brought seasonal 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
Eurasia and the Arctic, and rainforests held on only in
South America ,
Antarctica , which began the
Eocene fringed with a warm temperate to
sub-tropical rainforest , 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 .
Crassostrea gigantissima (Finch, 1824), a giant oyster from the
Fossil nummulitid foraminiferans showing
microspheric and megalospheric individuals;
Eocene of the United Arab
Emirates ; scale in mm.
The oldest known fossils of most of the modern mammal orders appear
within a brief period during the early Eocene. At the beginning of the
Eocene, several new mammal groups arrived in North America. These
modern mammals, like artiodactyls , perissodactyls and primates , had
features like long, thin legs , feet and hands capable of grasping, as
well as differentiated 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.
Both groups of modern ungulates (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 bats , proboscidians
(elephants), primates, rodents and marsupials . Older primitive forms
of mammals declined in variety and importance. Important
fauna fossil remains have been found in western North America, Europe,
Egypt and southeast
Asia . Marine fauna are best known
Asia and the southeast United States .
Reptile fossils from this time, such as fossils of pythons and
turtles , are abundant. The remains of
Titanoboa , a snake the length
of a school bus, was discovered in
South America along with other
large reptilian megafauna. During the Eocene, plants and marine
faunas became quite modern. Many modern bird orders first appeared in
Several rich fossil insect faunas are known from the Eocene, notably
Baltic amber found mainly along the south coast of the Baltic Sea
, amber from the
Paris Basin , France, the
Fur Formation ,
Bembridge Marls from the
Isle of Wight
Isle of Wight , England. Insects found in
Eocene deposits are mostly assignable to modern genera, though
frequently these genera do not occur in the area at present. For
instance the bibionid genus
Plecia is common in fossil faunas from
presently temperate areas, but only lives in the tropics and
Prorastomus , an early sirenian
Eocene oceans were warm and teeming with fish and other sea life.
The first carcharinid sharks evolved, as did early marine mammals,
Basilosaurus , an early species of whale that is thought to
be descended from land animals that existed earlier in the Eocene, the
hoofed predators called mesonychids , of which
Mesonyx was a member.
The first sirenians , relatives of the elephants , also evolved at
The end of the
Eocene was marked by the Eocene–
event , also known as the Grande Coupure.
Paleocene–Eocene Thermal Maximum
Paleocene–Eocene Thermal Maximum
Green River Formation in western North America
List of fossil sites
List of fossil sites (with link directory)
Fur Formation in Denmark
Messel Pit in Germany
Bolca in Italy
Wadi Al-Hitan in Egypt
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