Wiki Entry:
Paleocene–Eocene Thermal Maximum
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Climate change during the last 65 million years as expressed by the oxygen isotope composition of benthic foraminifera. The Paleocene-Eocene Thermal Maximum (PETM) is characterized by a brief but prominent negative excursion, attributed to rapid warming. Note that the excursion is understated in this graph due to the smoothing of data.
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The most extreme change in Earth surface conditions during the Cenozoic Era began at the temporal boundary between the Paleocene and Eocene epochs 55.8
million years ago. This event, the Paleocene–Eocene Thermal Maximum (PETM, alternatively "Eocene thermal maximum 1" (ETM1), and formerly known as the "Initial Eocene" or "Late Paleocene Thermal Maximum",[1] (IETM/LPTM)), was associated with rapid (in geological terms) global warming, profound changes in ecosystems, and major perturbations in the carbon cycle.
Global temperatures rose by about 6°C (11°F) over a period of approximately 20,000 years. Many benthic foraminifera and terrestrial mammals went extinct, but numerous modern mammalian orders emerged. The event is linked to a prominent negative excursion in carbon stable isotope (δ13C) records from across the globe, and dissolution of carbonate deposited on all ocean basins. The latter observations strongly suggest that a massive input of 13C-depleted carbon entered the hydrosphere or atmosphere 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 millenial time scales.
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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 was closed, perhaps preventing the thermal isolation of Antarctica. Although various proxies for past atmospheric CO
2 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.
[5]Earth surface temperatures increased by about 6°C from the late Paleocene through the early Eocene, culminating in the "Early Eocene Climatic Optimum" (EECO).[5] 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 (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 of the Jurassic) are open issues.
[edit] 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 δ18O 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 δ18O very likely signifies a rise in ocean temperature.[6] The temperature rise is also supported by analyses of foraminifera Mg/Ca and ratios of certain organic compounds (TEX86).
Due to the positive feedback effect of melting ice reducing albedo, temperature increases would have been greatest at the poles, which reached an average annual temperature of 10 to 20 °C (50 to 68 °F);[7] the surface waters of the northernmost[8] Arctic ocean warmed, seasonally at least, enough to support tropical lifeforms[9] requiring surface temperatures of over 22°C.[10]
[edit] 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 (δ13C) 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 δ13C excursion, the amount of carbonate dissolution on the seafloor, or ideally both. However, the shift in the δ13C 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‰.[11] 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 [12]
The timing of the PETM δ13C excursion has been calculated in two complementary ways. The iconic core covering this time period is the ODP's Core 690, and the timing is based exclusively on this core's record. The original timing was calculated assuming a constant sedimentation rate.[13] 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.[14] Both models have their failings, but agree on a few points. Importantly, they both detect two steps in the drop of δ13C, 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[13] to 30,000[14] years. There is other evidence to suggest that warming predated the δ13C excursion by some 3,000 years.[15]
[edit] Effects
The climate would also have become much wetter, with the increase in evaporation rates peaking in the tropics. Deuterium isotopes reveal that much more of this moisture was transported polewards than normal.[16] 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.[16]
[edit] Sea level
Despite the global lack of ice, the sea level would have risen due to thermal expansion.[10] Evidence for this can be found in the shifting palynomorph assemblages of the Arctic ocean, which reflect a relative decrease in terrestrial organic material compared to marine organic matter.[10]
[edit] Circulation
At the start of the PETM, the ocean circulation patterns changed radically in the course of under 5,000 years.[17] Global-scale current directions reversed; for example, deep water in the Atlantic flowed from north to south instead of the usual south to north.[17] This "backwards" flow persisted for 40,000 years.[17] Such a change would transport warm water to the deep oceans, enhancing further warming.[17]
[edit] Lysocline
The lysocline 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,[18] resulting in the dissolution of deep water carbonates. This deep-water acidification can be observed in ocean cores, which show (where bioturbation has not destroyed the signal) an abrupt change from grey carbonate ooze to red clays (followed by a gradual grading back to grey).[19] 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.[3] In parts of the southeast Atlantic, the lysocline rose by 2 km in just a few thousand years.[3]
