Paleocene–Eocene Thermal Maximum Totally Explained

Everything about Paleocene-eocene Thermal Maximum totally explained

The Paleocene/Eocene boundary,, was marked by the most rapid and significant climatic perturbation 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 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.

Setting

The Paleocene-Eocene Thermal Maximum lasted around 20,000 years, and was superimposed on a 6 million year period of more gradual global warming,

The evidence

Our strongest evidence for climate change comes from a global, synchronous and uniform excursion in the record, of −2-3 ‰. Its magnitude is larger in terrestrial environments. This excursion implies the release of large amounts of into the ocean and atmosphere, and implies the release of at least 6,800 Pg C.
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's 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 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 10-20; the surface waters of the northernmost Arctic ocean warmed, seasonally at least, enough to support tropical lifeforms requiring surface temperatures of over 22°C.
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. Such a change would transport warm water to the deep oceans, enhancing further warming. resulting in the dissolution of deep water carbonates. This deep-water acidification can be observed in ocean cores, which show (where bioturbation hasn't 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 shallower waters, it's undeniable that increased levels result in a decreased oceanic pH, which has a profound negative effect on corals. Interestingly, no change in the distribution of calcareous nannoplankton such as the coccolithophores can be attributed to acidification during the PETM. and weakly calcified forams.

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. Our biggest aid in disentangling these factors comes from a consideration of the carbon isotope mass balance. We know the entire exogenic carbon cycle (for example 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's today - something which is very hard to confirm.

Volcanic activity

In order to balance the mass of carbon and produce the observed value, at least 1,500 Gt of carbon would have to be 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 Palæogene. 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 Gt 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 methane. Further phases of volcanic activity could have triggered the release of more methane, and caused other early Eocene warm events such as the ETM2. Even allowing for feedback processes, this would require at least 100 Gt of extra-terrestrial carbon it turns out they were created by bacteria. Further, an iridium anomaly - often an indicator of extra-terrestrial impact - observed in Spain is far too small to denote a comet impact.

Burning of peat

This combustion of prodigal quantities of peat was once postulated, but in order to produce the excursion observed, over 90% of the Earth's biomass would have to be combusted. Since plants in fact grew more voraciously during the period of the PETM, this theory has been discounted.

Orbital forcing

The presence of later (smaller) warming events of a global scale, such as the Elmo horizon (aka ETM2), has led the 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 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, so the pressure at which this clathrate configuration is stable falls - so shallow clathrates dissociate, releasing methane gas to make its way into the atmosphere. Since biogenic clathrates have a signature of −60 ‰ (inorganic clathrates are the still rather large −40 ‰), relatively small masses can produce large excursions. Further, methane is a potent greenhouse gas - as it's released into the atmosphere, so it causes warming, and as the ocean transports this to the bottom sediments, it destabilises more clathrates. It would take around 2,300 years for an increased temperature to diffuse warm the sea bed to a depth sufficient to cause clathrates' release - although the exact time frame is highly dependant on a number of poorly-constrained assumptions.
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 doesn't seem to hold true for all cores.
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.

Recovery

The record records a recovery time of around 150,000
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, 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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