Paleocene-Eocene Thermal Maximum

Paleocene-Eocene Thermal Maximum

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The most extreme change in Earth surface conditions during the Cenozoic Era
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...

 began at the temporal boundary between the Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...

 and Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

 epochs
Epoch (geology)
An epoch is a subdivision of the geologic timescale based on rock layering. In order, the higher subdivisions are periods, eras and eons. We are currently living in the Holocene epoch...

 . This event, the Paleocene–Eocene Thermal Maximum (PETM, alternatively (ETM1), and formerly known as the "Initial Eocene" or "", (IETM/LPTM)), was associated with rapid (in geological terms) global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

, profound changes in ecosystems, and major perturbations in the carbon cycle
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...

.

Global temperatures rose by about 6°C (11°F) over a period of approximately 20,000 years. Many benthic
Benthos
Benthos is the community of organisms which live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.Many organisms...

 foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...

 and terrestrial mammals went extinct, but numerous modern mammalian orders
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 emerged. The event is linked to a prominent negative excursion in carbon stable isotope
Stable isotope
Stable isotopes are chemical isotopes that may or may not be radioactive, but if radioactive, have half-lives too long to be measured.Only 90 nuclides from the first 40 elements are energetically stable to any kind of decay save proton decay, in theory...

 (
The most extreme change in Earth surface conditions during the Cenozoic Era
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...

 began at the temporal boundary between the Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...

 and Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

 epochs
Epoch (geology)
An epoch is a subdivision of the geologic timescale based on rock layering. In order, the higher subdivisions are periods, eras and eons. We are currently living in the Holocene epoch...

 {{Ma|eocene}}. This event, the Paleocene–Eocene Thermal Maximum (PETM, alternatively {{nowrap|"Eocene thermal maximum 1"}} (ETM1), and formerly known as the "Initial Eocene" or "{{nowrap|Late Paleocene Thermal Maximum}}", (IETM/LPTM)), was associated with rapid (in geological terms) global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

, profound changes in ecosystems, and major perturbations in the carbon cycle
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...

.

Global temperatures rose by about 6°C (11°F) over a period of approximately 20,000 years. Many benthic
Benthos
Benthos is the community of organisms which live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.Many organisms...

 foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...

 and terrestrial mammals went extinct, but numerous modern mammalian orders
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 emerged. The event is linked to a prominent negative excursion in carbon stable isotope
Stable isotope
Stable isotopes are chemical isotopes that may or may not be radioactive, but if radioactive, have half-lives too long to be measured.Only 90 nuclides from the first 40 elements are energetically stable to any kind of decay save proton decay, in theory...

 (
The most extreme change in Earth surface conditions during the Cenozoic Era
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...

 began at the temporal boundary between the Paleocene
Paleocene
The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...

 and Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

 epochs
Epoch (geology)
An epoch is a subdivision of the geologic timescale based on rock layering. In order, the higher subdivisions are periods, eras and eons. We are currently living in the Holocene epoch...

 {{Ma|eocene}}. This event, the Paleocene–Eocene Thermal Maximum (PETM, alternatively {{nowrap|"Eocene thermal maximum 1"}} (ETM1), and formerly known as the "Initial Eocene" or "{{nowrap|Late Paleocene Thermal Maximum}}", (IETM/LPTM)), was associated with rapid (in geological terms) global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

, profound changes in ecosystems, and major perturbations in the carbon cycle
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...

.

Global temperatures rose by about 6°C (11°F) over a period of approximately 20,000 years. Many benthic
Benthos
Benthos is the community of organisms which live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.Many organisms...

 foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...

 and terrestrial mammals went extinct, but numerous modern mammalian orders
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 emerged. The event is linked to a prominent negative excursion in carbon stable isotope
Stable isotope
Stable isotopes are chemical isotopes that may or may not be radioactive, but if radioactive, have half-lives too long to be measured.Only 90 nuclides from the first 40 elements are energetically stable to any kind of decay save proton decay, in theory...

 ({{delta) records from across the globe, and dissolution of carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

 deposited on all ocean basins. The latter observations strongly suggest that a massive input of 13C
Carbon-13
Carbon-13 is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.1% of all natural carbon on Earth.- Detection by mass spectrometry :...

-depleted carbon entered the hydrosphere
Hydrosphere
A hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet....

 or atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 at the start of the PETM. Recently, geoscientists have begun to investigate the PETM in order to better understand the fate and transport of increasing greenhouse-gas emissions over millennial time scales.

{{-}}
{{include timeline|cenozoic}}

Setting


The configuration of oceans and continents was somewhat different during the Eocene. The Panama Isthmus did not yet connect North and South America, which allowed circulation between the Pacific and Atlantic oceans. Further, the Drake Passage
Drake Passage
The Drake Passage or Mar de Hoces—Sea of Hoces—is the body of water between the southern tip of South America at Cape Horn, Chile and the South Shetland Islands of Antarctica...

 was closed, perhaps preventing the thermal isolation of Antarctica. Although various proxies for past atmospheric {{co2}} levels in the Eocene do not agree in absolute terms, all suggest that levels then were much higher than at present. In any case, there were no significant ice sheets during this time.

Earth surface temperatures increased by about 6°C from the late Paleocene through the early Eocene, culminating in the "Early Eocene Climatic Optimum" (EECO). Superimposed on this long-term, gradual warming were at least two (and likely more) "hyperthermals". These can be defined as geologically brief (<200,000 year) events characterized by rapid global warming, major changes in the environment, and massive carbon addition. Of these, the PETM was the most extreme and perhaps the first (at least within the Cenozoic). Another hyperthermal clearly occurred at approximately 53.7 Ma, and is now called ETM-2
Eocene Thermal Maximum 2
Eocene Thermal Maximum 2 , also called H-1 or the Elmo event, was a transient period of global warming that occurred approximately 53.7 million years ago...

 (also referred to as H-1, or the Elmo event). However, additional hyperthermals likely occurred at about 53.6 Ma (H-2), 53.3 (I-1), 53.2 (I-2) and 52.8 Ma (informally called K, X or ETM-3). The number, nomenclature, absolute ages, and relative global impact of the Eocene hyperthermals are the source of considerable current research. Whether they only occurred during the long-term warming, and whether they are causally related to apparently similar events in older intervals of the geological record (e.g., the Toarcian turnover
Toarcian turnover
The term Toarcian turnover, alternatively the Toarcian extinction, the Pliensbachian-Toarcian extinction, or the Early Jurassic extinction, refers to the wave of extinctions that marked the end of the Pliensbachian stage and the start of the Toarcian stage of the Early Jurassic period, c...

 of the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

) are open issues.

Evidence for global warming


Average global temperatures increased by ~6°C (11°F) within about 20,000 years. This is based on several lines of evidence. There is a prominent (>1‰) negative excursion in the {{delta|18|O|link}} of foraminifera shells, both those made in surface and deep ocean water. Because there was a paucity of continental ice in the early Paleogene, the shift in {{delta|18|O|}} very likely signifies a rise in ocean temperature.
The temperature rise is also supported by analyses of foraminifera Mg/Ca and ratios of certain organic compounds ({{#if:link|TEX86|TEX86}}).

Due to the positive feedback effect of melting ice reducing albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

, temperature increases would have been greatest at the poles, which reached an average annual temperature of 10 to 20 °C (50 to 68 F); the surface waters of the northernmost Arctic ocean warmed, seasonally at least, enough to support tropical lifeforms requiring surface temperatures of over 22°C.

Evidence for carbon addition


Clear evidence for massive addition of 13C-depleted carbon at the onset of the PETM comes from two observations. First, a prominent negative excursion in the carbon isotope composition ({{delta|13|C|}}) of carbon-bearing phases characterizes the PETM in numerous widespread locations from a range of environments. Second, carbonate dissolution marks the PETM in sections from the deep-sea.

The total mass of carbon injected to the ocean and atmosphere during the PETM remains the source of debate. In theory, it can be estimated from the magnitude of the {{delta|13|C|}} excursion, the amount of carbonate dissolution on the seafloor, or ideally both. However, the shift in the {{delta|13|C|}} across the PETM depends on the location and the carbon-bearing phase analyzed. In some records of bulk carbonate, it is about 2‰; in some records of terrestrial carbonate or organic matter it exceeds 6‰.
Carbonate dissolution also varies throughout different ocean basins. It is extreme in parts of the north and central Atlantic Ocean but far less pronounced in the Pacific Ocean. With available information, estimates of the carbon addition range from about 2500 to over 6800 gigatons

The timing of the PETM {{delta|13|C|}} excursion has been calculated in two complementary ways. The iconic core covering this time period is the ODP
Ocean Drilling Program
The Ocean Drilling Program was an international cooperative effort to explore and study the composition and structure of the Earth's ocean basins. ODP, which began in 1985, was the direct successor to the highly successful Deep Sea Drilling Project initiated in 1968 by the United States...

's Core 690, and the timing is based exclusively on this core's record. The original timing was calculated assuming a constant sedimentation rate.
This model was improved using the assumption that 3He flux is constant; this cosmogenic nuclide is produced at a (roughly) constant rate by the sun, and there is little reason to assume large fluctuations in the solar wind across this short time period. Both models have their failings, but agree on a few points. Importantly, they both detect two steps in the drop of {{delta|13|C|}}, each lasting about 1,000 years, and separated by about 20,000 years. The models diverge most in their estimate of the recovery time, which ranges from 150,000 to 30,000 years. There is other evidence to suggest that warming predated the {{delta|13|C|}} excursion by some 3,000 years.

Effects


The climate would also have become much wetter, with the increase in evaporation rates peaking in the tropics. Deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

 isotopes reveal that much more of this moisture was transported polewards than normal. This would have resulted in the largely isolated Arctic ocean's taking on a more freshwater character as northern hemisphere rainfall was channelled towards it.

Sea level


Despite the global lack of ice, the sea level would have risen due to thermal expansion.
Evidence for this can be found in the shifting palynomorph
Palynomorph
Palynomorph is the geological term used to describe a particle of a size between five and 500 micrometres, found in rock deposits and composed of organic material such as chitin, pseudochitin and sporopollenin...

 assemblages of the Arctic ocean, which reflect a relative decrease in terrestrial organic material compared to marine organic matter.

Circulation


At the start of the PETM, the ocean circulation patterns changed radically in the course of under 5,000 years. Global-scale current directions reversed due to a shift in overturning from the southern hemisphere to northern hemisphere overturning. This "backwards" flow persisted for 40,000 years. Such a change would transport warm water to the deep oceans, enhancing further warming.

Lysocline


The lysocline
Lysocline
The lysocline is a term used in geology, geochemistry and marine biology to denote the depth in the ocean below which the rate of dissolution of calcite increases dramatically....

 marks the depth at which carbonate starts to dissolve (above the lysocline, carbonate is oversaturated): today, this is at about 4 km, comparable to the median depth of the oceans. This depth depends on (among other things) temperature and the amount of {{co2}} dissolved in the ocean. Adding {{co2}} initially shallows the lysocline, resulting in the dissolution of deep water carbonates. This deep-water acidification can be observed in ocean cores, which show (where bioturbation
Bioturbation
In oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...

 has not destroyed the signal) an abrupt change from grey carbonate ooze to red clays (followed by a gradual grading back to grey). It is far more pronounced in north Atlantic cores than elsewhere, suggesting that acidification was more concentrated here, related to a greater rise in the level of the lysocline. In parts of the southeast Atlantic, the lysocline rose by 2 km in just a few thousand years.

Anoxia?


In parts of the oceans, especially the north Atlantic Ocean, bioturbation
Bioturbation
In oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...

 is absent. This may be due to bottom-water anoxia
Anoxic event
Oceanic anoxic events or anoxic events occur when the Earth's oceans become completely depleted of oxygen below the surface levels. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past. Anoxic events may have caused...

, or by changing ocean circulation patterns changing the temperatures of the bottom water. However, many ocean basins remain bioturbated through the PETM.

Life


The PETM is accompanied by a mass extinction of 35-50% of {{wict|benthic}} foraminifera
Foraminifera
The Foraminifera , or forams for short, are a large group of amoeboid protists which are among the commonest plankton species. They have reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net...

 (especially in deeper waters) over the course of ~1,000 years - the group suffering more than during the dinosaur-slaying K-T extinction. Contrarily, planktonic foraminifera diversified, and dinoflagellates bloomed. Success was also enjoyed by the mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s, who radiated profusely around this time.

The deep-sea extinctions are difficult to explain, as many were regional in extent (mainly affecting the north Atlantic). General hypotheses such as a temperature-related reduction in oxygen availability, or increased corrosiveness due to carbonate-undersaturated deep waters, are insufficient as explanations. The only factor which was global in extent was an increase in temperature, and it appears that the majority of the blame must rest upon its shoulders. Regional extinctions in the North Atlantic can be attributed to increased deep-sea anoxia, which could be due to the slowdown of overturning ocean currents, or the release and rapid oxidation of large amounts of methane.{{Verify source|date=April 2008}}

In shallower waters, it's undeniable that increased {{co2}} levels result in a decreased oceanic pH, which has a profound negative effect on corals. Experiments suggest it is also very harmful to calcifying plankton. However, the strong acids used to simulate the natural increase in acidity which would result from elevated {{co2}} concentrations may have given misleading results, and the most recent evidence is that coccolithophore
Coccolithophore
Coccolithophores are single-celled algae, protists, and phytoplankton belonging to the division of haptophytes. They are distinguished by special calcium carbonate plates of uncertain function called coccoliths , which are important microfossils...

s (E. huxleyi at least) become more, not less, calcified and abundant in acidic waters. Interestingly, no change in the distribution of calcareous nanoplankton such as the coccolithophores can be attributed to acidification during the PETM. Acidification did lead to an abundance of heavily calcified algae and weakly calcified forams.

The increase in mammalian abundance is intriguing. There is no evidence of any increased extinction rate among the terrestrial biota. Increased {{co2}} levels may have promoted dwarfing – which may (perhaps?) have encouraged speciation. Many major mammalian orders – including the Artiodactyla, horses, and primates – appeared and spread across the globe 13,000 to 22,000 years after the initiation of the PETM.

Possible causes


Discriminating between different causes of the PETM is difficult. Temperatures were rising globally at a steady pace, and a mechanism must be invoked to produce a sudden spike which may have been accentuated by positive feedbacks. The biggest aid in disentangling these factors comes from a consideration of the carbon isotope mass balance. We know the entire exogenic carbon cycle (i.e. the carbon contained within the oceans and atmosphere, which can change on short timescales) underwent a −0.2 % to −0.3 % perturbation in {{delta|13|C|}}, and by considering the isotopic signatures of other carbon reserves, can consider what mass of the reserve would be necessary to produce this effect. The assumption underpinning this approach is that the mass of exogenic carbon was the same in the Paleogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...

 as it is today - something which is very hard to confirm.

Volcanic activity


In order to balance the mass of carbon and produce the observed {{delta|13|C|}} value, at least 1,500 gigatons of carbon would have to have been degassed from the mantle via volcanoes over the course of the two 1,000 year steps. To put this in perspective, this is about 200 times the background rate of degassing for the rest of the Paleogene. There is no indication that such a burst of volcanic activity has occurred at any point in Earth's history. However, substantial volcanism had been active in East Greenland for around the preceding million years or so, but this struggles to explain the rapidity of the PETM. Even if the bulk of the 1,500 gigatons of carbon was released in a single pulse, further feedbacks would be necessary to produce the observed isotopic excursion.

On the other hand, there are suggestions that surges of activity occurred in the later stages of the volcanism and associated continental rifting. Intrusions of hot magma into carbon-rich sediments may have triggered the degassing of isotopically light methane in sufficient volumes to cause global warming and the observed isotope anomaly. This hypothesis is documented by the presence of extensive intrusive sill complexes and thousands of kilometer-sized hydrothermal vent complexes in sedimentary basins on the mid-Norwegian margin and west of Shetland. Volcanic eruptions of a large magnitude can impact global climate, reducing the amount of solar radiation reaching the Earth's surface, lowering temperatures in the troposphere, and changing atmospheric circulation patterns. Large-scale volcanic activity may last only a few days, but the massive outpouring of gases and ash can influence climate patterns for years. Sulfuric gases convert to sulfate aerosols, sub-micron droplets containing about 75 percent sulfuric acid. Following eruptions, these aerosol particles can linger as long as three to four years in the stratosphere.
Further phases of volcanic activity could have triggered the release of more methane, and caused other early Eocene warm events such as the ETM2
Eocene Thermal Maximum 2
Eocene Thermal Maximum 2 , also called H-1 or the Elmo event, was a transient period of global warming that occurred approximately 53.7 million years ago...

.
It has also been suggested that volcanic activity around the Caribbean may have disrupted the circulation of oceanic currents, amplifying the magnitude of climate change.

Comet impact


A briefly popular theory held that a 12C-rich comet struck the earth and initiated the warming event. A cometary impact coincident with the P/E boundary can also help explain some enigmatic features associated with this event, such as the iridium anomaly at Zumaya, the abrupt appearance of kaolinitic clays with abundant magnetic nanoparticles on the coastal shelf of New Jersey, and especially the nearly simultaneous onset of the carbon isotope excursion and the thermal maximum. Indeed, a key feature and testable prediction of a comet impact is that it should produce virtually instantaneous environmental effects in the atmosphere and surface ocean with later repercussions in the deeper ocean. Even allowing for feedback processes, this would require at least 100 gigatons of extraterrestrial carbon. Such a catastrophic impact should have left its mark on the globe. Unfortunately, the evidence put forward does not stand up to scrutiny. An unusual 9-meter-thick clay layer supposedly formed soon after the impact, containing unusual amounts of magnetism, but it formed too slowly for these magnetic particles to have been a result of the comet's impact -
it turns out they were created by bacteria. Further, an iridium anomaly
Iridium anomaly
The term iridium anomaly commonly refers to an unusual abundance of the chemical element iridium in a layer of rock strata, often taken as evidence of an extraterrestrial impact event because of the case of such an anomaly at the Cretaceous–Tertiary boundary...

 - often an indicator of extraterrestrial impact - observed in Spain is far too small to denote a comet impact.

Burning of peat


This combustion of prodigious quantities of peat
Peat
Peat is an accumulation of partially decayed vegetation matter or histosol. Peat forms in wetland bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world...

 was once postulated, because there was likely a greater mass of carbon stored as living terrestrial biomass during the Paleocene than there is today since plants in fact grew more vigorously during the period of the PETM. This theory was refuted, because in order to produce the {{delta|13|C|}} excursion observed, over 90% of the Earth's biomass would have to have been combusted. However, the Paleocene is also recognized as a time of significant peat accumulation worldwide. A comprehensive search failed to find evidence for the combustion of fossil organic matter, in the form of soot or similar particulate carbon.

Orbital forcing


The presence of later (smaller) warming events of a global scale, such as the Elmo horizon (aka ETM2
Eocene Thermal Maximum 2
Eocene Thermal Maximum 2 , also called H-1 or the Elmo event, was a transient period of global warming that occurred approximately 53.7 million years ago...

), has led to the hypothesis that the events repeat on a regular basis, driven by maxima in the 400,000 and 100,000 year eccentricity cycles in the Earth's orbit. The current warming period is believed to last another 50,000 years due to a minimum in the eccentricity of the Earth's orbit. Orbital increase in insolation (and thus temperature) would force the system over a threshold and unleash positive feedbacks.

Methane release


None of the above causes are alone sufficient to cause the carbon isotope excursion or warming observed at the PETM. The most obvious feedback mechanism that could amplify the initial perturbation is that of clathrates. At certain temperature and pressure conditions, methane – which is being produced continually by decomposing microbes in sea bottom sediments – is stable in a complex with water, which forms ice-like cages trapping the methane in solid form. As temperature rises, the pressure required to keep this clathrate configuration stable increases, so shallow clathrates dissociate, releasing methane gas to make its way into the atmosphere. Since biogenic clathrates have a {{delta|13|C|}} signature of −60 ‰ (inorganic clathrates are the still rather large −40 ‰), relatively small masses can produce large {{delta|13|C|}} excursions. Further, methane is a potent greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

 as it is released into the atmosphere, so it causes warming, and as the ocean transports this warmth to the bottom sediments, it destabilises more clathrates. It would take around 2,300 years for an increased temperature to diffuse warmth into the sea bed to a depth sufficient to cause a release of clathrates, although the exact time-frame is highly dependent on a number of poorly constrained assumptions. Ocean warming due to flooding and pressure changes due to a sea-level drop may have caused clathrates to become unstable and release methane. This can take place over as short of a period as a few thousand years. The reverse process, that of fixing methane in clathrates, occurs over a larger scale of tens of thousands of years.

In order for the clathrate hypothesis to work, the oceans must show signs of having been warmer slightly before the carbon isotope excursion, because it would take some time for the methane to become mixed into the system and {{delta|13|C|}}-reduced carbon to be returned to the deep ocean sedimentary record. Until recently, the evidence suggested that the two peaks were in fact simultaneous, weakening the support for the methane theory. But recent (2002) work has managed to detect a short gap between the initial warming and the {{delta|13|C|}} excursion. Chemical markers of surface temperature ({{#if:link|TEX86|TEX86}}) also indicate that warming occurred around 3,000 years before the carbon isotope excursion, but this does not seem to hold true for all cores. Notably, deeper (non-surface) waters do not appear to display evidence of this time gap.

Analysis of these records reveals another interesting fact: planktonic (floating) forams record the shift to lighter isotope values earlier than benthic (bottom dwelling) forams. The lighter (lower {{delta|13|C|}}) methanogenic carbon can only be incorporated into the forams' shells after it has been oxidised. A gradual release of the gas would allow it to be oxidised in the deep ocean, which would make benthic forams show lighter values earlier. The fact that the planktonic forams are the first to show the signal suggests that the methane was released so rapidly that its oxidation used up all the oxygen at depth in the water column, allowing some methane to reach the atmosphere unoxidised, where atmospheric oxygen would react with it. This observation also allows us to constrain the duration of methane release to under around 10,000 years.

There is a debate about whether there was enough amount of methane hydrate as a major carbon source, and a recent paper proposed that the amount can be enough. The present-day global methane hydrate reserve is poorly constrained, but mostly considered between 2,000 ~ 10,000 Gt. However, because the global ocean bottom temperatures were ~6 degree C higher than today which induces much smaller volume of sediment hosting gas hydrate than today, global hydrate amount before PETM was thought much less than present-day estimates. So many scientists called the source of carbon for PETM as a mystery. However, a recent paper using numerical simulations suggests that enhanced organic carbon sedimentation and methanogenesis could have compensated for the smaller volume of hydrate stability. Currently there is no other good choice as appropriate as methane hydrate, so to resolve the hydrate amount problem is very helpful.

Global methane hydrate reserve may have played as a capacitor for global carbon cycle. Methane was slowly generated from organic matter in marine sediment and hydrate phase accumulated. At some certain conditions hydrate dissociation was triggered, and possibly due to some certain positive feedback, methane was released very quickly.

Ocean circulation


The large scale patterns of ocean circulation are important when considering how heat was transported through the oceans. Our understanding of these patterns is still in a preliminary stage. Models show that there are possible mechanisms to quickly transport heat to the shallow, clathrate-containing ocean shelves, given the right bathymetric profile, but the models cannot yet match the distribution of data we observe. "Warming accompanying a south-to-north switch in deepwater formation would produce sufficient warming to destabilize seafloor gas hydrates over most of the world ocean to a water depth of at least 1900 m." (K. Bice and J. Marotzke) This destabilization could have resulted in the release of more than 2000 gigatons of methane carbon from clathrate zone of the ocean floor.

Recovery


The {{delta|13|C|}} record records a duration of around 170,000 to 120,000 years, relatively rapid compared to the residence time of carbon in the modern atmosphere (100-200 thousand years). A satisfactory explanation of this rapid recovery must incorporate a feedback system.

The most likely method of recovery invokes an increase in biological productivity, transporting carbon to the deep ocean. This would be assisted by higher global temperatures and {{co2}} levels, as well as an increased nutrient supply (which would result from higher continental weathering due to higher temperatures and rainfall; volcanics may have provided further nutrients). Evidence for higher biological productivity comes in the form of biogenic Barium. However, this proxy may instead reflect the addition of Barium dissolved in methane. Diversifications suggest that productivity increased in near-shore environments, which would have been warm and fertilized by run-off - outweighing the reduction in productivity in the deep oceans.

See also

  • Abrupt climate change
    Abrupt climate change
    An abrupt climate change occurs when the climate system is forced to transition to a new state at a rate that is determined by the climate system itself, and which is more rapid than the rate of change of the external forcing...

  • Azolla event
    Azolla event
    The Azolla event occurred in the middle Eocene period, around , when blooms of the freshwater fern Azolla are thought to have happened in the Arctic Ocean...

  • Clathrate gun hypothesis
    Clathrate gun hypothesis
    The clathrate gun hypothesis is the popular name given to the hypothesis that rises in sea temperatures can trigger the sudden release of methane from methane clathrate compounds buried in seabeds and permafrost which, because the methane itself is a powerful greenhouse gas, leads to further...

  • Climate sensitivity
    Climate sensitivity
    Climate sensitivity is a measure of how responsive the temperature of the climate system is to a change in the radiative forcing. It is usually expressed as the temperature change associated with a doubling of the concentration of carbon dioxide in Earth's atmosphere.The equilibrium climate...

  • Eocene
    Eocene
    The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

  • Eocene Thermal Maximum 2
    Eocene Thermal Maximum 2
    Eocene Thermal Maximum 2 , also called H-1 or the Elmo event, was a transient period of global warming that occurred approximately 53.7 million years ago...

  • Paleocene
    Paleocene
    The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...

  • Paleogene
    Paleogene
    The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...

  • Runaway climate change
    Runaway climate change
    Runaway climate change describes a theoretical scenario in which the climate system passes a threshold or tipping point, after which internal positive feedback effects cause the climate to continue changing without further external forcings...


Further reading


{{cite journal | url = http://www.palaeontologyonline.com/articles/2011/the-paleocene-eocene-thermal-maximum/ | author = Jardine, P | journal = Palaeontology Online | title=Editing Paleocene–Eocene Thermal Maximum }}

External links



{{global warming}}