The Paleocene/Eocene boundary, , was marked by the most rapid and significant climatic disturbance of the Cenozoic Era. A sudden global warming event, leading to the Paleocene-Eocene Thermal Maximum (PETM, alternatively (ETM1), and formerly known as the "Initial Eocene" or "", (IETM/LPTM)), is associated with changes in oceanic and atmospheric circulation, the extinction of numerous deep-sea benthic foraminifera, and a major turnover in mammalian life on land which is coincident with the emergence of many of today's major mammalian orders.
The event saw global temperatures rise by around 6 over 20,000 years, with a corresponding rise in sea level as the whole of the oceans warmed. Atmospheric carbon dioxide concentrations rose, causing a shallowing of the lysocline. Regional deep water may have played a part in marine extinctions. The event is linked to a negative excursion in the isotope record, which occurs in two short (~1,000 year) pulses. These probably represent degassing of clathrates ("methane ice" deposits), which accentuated a pre-existing warming trend. The release of these clathrates, and ultimately the event itself, may have been triggered by a range of causes. Evidence currently seems to favour an increase in volcanic activity as the main perpetrator.
The globe was subtly different during the Eocene. The Panama Isthmus did not yet connect North and South America, allowing circulation between the Pacific and Atlantic oceans. Further, the Drake Passage was shut, preventing the thermal isolation of Antarctica. This, combined with higher levels, meant that there were no significant ice sheets - the globe was essentially ice free.
The timing of the PETM 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. This model was improved using the assumption that 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 , each lasting about 1000 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 excursion by some 3,000 years.
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. This would have resulted in the largely isolated Arctic ocean taking a more freshwater character as northern hemisphere rainfall was channelled towards it.
The deep sea extinctions are difficult to explain, as many were regional in extent (mainly affecting the north Atlantic): this means that we cannot appeal to general hypotheses such as a temperature-related reduction in oxygen availability, or increased corrosiveness due to carbonate-undersaturated deep waters. 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.
In shallower waters, it's undeniable that increased levels result in a decreased oceanic pH, which has a profound negative effect on corals.Langdon, C.; Takahashi, T.; Sweeney, C.; Chipman, D.; Goddard, J.; Marubini, F.; Aceves, H.; Barnett, H.; Atkinson, M.J. (2000). "Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef". Global Biogeochemical Cycles 14 (2): 639–654. Retrieved on 2008-02-28. Experiments suggest it is also very harmful to calcifying plankton.Riebesell, U.; Zondervan, I.; Rost, B.; Tortell, P.D.; Zeebe, R.E.; Morel, F.M.M. (2000). "Reduced calcification of marine plankton in response to increased atmospheric However, the strong acids used to simulate the natural increase in acidity which would result from elevated concentrations may have given misleading results, and the most recent evidence is that coccolithophores (E. huxleyi at least) become more, not less, calcified and abundant in acidic waters. Interestingly, no change in the distribution of calcareous nannoplankton 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 levels may have promoted dwarfing - which may (perhaps?) have encouraged speciation. Many major mammalian orders, including the Artiodactyla, horses and primates, appeared as if from nowhere, and spread across the globe, 13,000 to 22,000 years after the initiation of the PETM.
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. Our 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 −2-3 ‰ perturbation in , 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 Palæogene as it is today - something which is very hard to confirm.
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 . Further phases of volcanic activity could have triggered the release of more methane, and caused other early Eocene warm events such as the ETM2.
In order for the clathrate hypothesis to work, the oceans must show signs of being warmer slightly before the carbon isotope excursion - because it would take some time for the methane to become mixed into the system and -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 work has managed to detect a short gap between the initial warming and the excursion. Chemical markers of surface temperature 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: plantktonic (floating) forams record the shift to lighter isotope values earlier than benthic (bottom dwelling) forams. The lighter (lower ) 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' tests lighter 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.
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 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. However, diversifications suggest that productivity increased in near-shore environments, which would have been warm and fertilised by run-off - outweighing the reduction in productivity in the deep oceans.
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