The Thermal Maximum Paleocene-Eocene (PETM), also called Initial Eocene Thermal Maximum, or Upper Paleocene Thermal Maximum was an abrupt climate change that marked the end of the Paleocene and the beginning of the Eocene. This is one of the periods of climatic change that was most significant in the Cenozoic, which suddenly altered ocean circulation and atmospheric, causing the extinction of many genera of benthic foraminifera and causing major changes in the mammalian land that marked the emergence of the lineages present.
In just 20,000 years, the average temperature of land is increased by 6 ° C, with a corresponding rise in sea level and warming of the oceans. Despite the warming could be triggered by many causes, it is believed that were the strong main volcanic activity and the emission of methane gas that was stored in clath-rates in ocean sediments, and that could increase warming by releasing large amounts of atmospheric carbon depleted in the isotope carbon-13 . In addition, atmospheric concentrations of CO 2 increased significantly, disrupting the cycle and causing the elevation of the lisoclina, and a shortage of oxygen in the deep ocean that eventually provoked the majority of marine extinctions.
At first, the absence of precise dating, the PETM was located in the late Paleocene, denominating Upper Paleocene Thermal Maximum. However, later adopted the name most Maximum text was the Paleocene-Eocene Thermal, as the boundary between the two periods was officially defined to coincide with the moment of greatest increase in carbon-12 and this fact the cause of climate event in question. However, in other publications to create more convenient to use the name of the Initial Eocene Thermal Maximum, as the absolute maximum temperatures are reached at the beginning of this period, after the release of carbon-12 to the atmosphere.
Given the uncertainties in radiometric dating, the maximum Paleocene-Eocene thermal occurred between 55.8 and 55.0 million years before our era. It lasted about 20,000 years, and was preceded by a longer period than 6 million years of gradual global warming that began in the mid-Paleocene, and reached its highest expression in the “Eocene Climatic Optimum” (several million years after the PETM). However, during this period, there were also several cooling events such as the Elmo event. During the first 1,000 years of the PETM, it is estimated that were released into the oceans and the atmosphere between 1,500 and 2,000 gigatons of carbon (~ 2 Gt / year), emission rate four times lower than that emitted in 2005 by human activity (7.8 Gt / year).
During the Eocene, the provision of the planet was significantly different. The Isthmus of Panama did not exercise even a bridge between North America and South America, allowing the transit of waters between the Atlantic Ocean and the Pacific. On the other hand, the Drake Passage was obstructed, preventing thermal isolation of Antarctica. This fact, coupled with high levels of CO2, indicating that there was no major ice sheets, so that the planet had no ice, then, almost in its entirety.
Evidence and timing
The strongest evidence to confirm the existence of climate change is provided by the negative change in the record of carbon-13, the isotope most common carbon, with a negative excursion, sudden and pronounced between -2 and -3 ‰. This massive injection of carbon depleted in carbon-13 involves the release of large amounts of carbon-12, at least 6800 Gt on the atmosphere and oceans during the 20,000 years it lasted.
The chronology of the relative decrease in carbon-13 in the PETM was calculated two different ways, complementary. The most important of these is the ODP Core 690 (made in the Weddell Sea); for the period is almost exclusively based on this record, though initially was calculated using an approximation taking into account a constant rate of sedimentation.
Later another different model came, assuming that the flow of helium-3 is constant, since this isotope of helium is produced by the sun constantly, and there is no reason to believe that major changes occur in the fluctuations of the solar wind during that brief period.
Both models have their shortcomings, but agree on major issues. Among the points that match, it is noteworthy that both agree that carbon release occurred in two stages, each lasting approximately 1,000 years, separated by a period of 20,000 years. The models differ mainly in the estimated recovery time, ranging from 150,000 for the first 20 and 30,000 years for the second model. Other theories suggest that the warming took place 3,000 years before the release of carbon -12, but the root causes remain uncertain. Studies have been conducted in the Spanish Pyrenees to confirm the increase in CO 2 during the PETM.
Graph showing the temperature record of the seafloor. The maximum Paleocene-Eocene Thermal is represented by the symbol “PETM.”
The average global temperature by 6 ° C increased dramatically over a period of just 20,000 years. This calculation is based on the values of Mg / Ca and the concentration of the isotope oxygen-18, which is the most used to calculate temperatures in the Eocene, as ice because of the small gain in safety calculations, remaining constant concentration of oceanic oxygen-18.
Further analysis focused on the composition of the flora, as well as the size and shape of their leaves, show a similar result: an increase of 5 ° C, besides revealing that at the start of the PETM the rains were scarce but, over time, progressively increased. Due to rising temperatures, the ice began to melt, causing the reduction in albedo, which in turn led to a rise of temperatures in a process of positive feedback. This caused the temperature increase was greater at the poles, reaching annual average temperatures between 10 and 20 ° C. 26 The heating of the surface water of the Arctic Ocean was such that came to harbor life forms typical of the tropics such as dinoflagellates , reaching temperatures above 22 ° C.
Temperature not only increased but so did the humidity, due to increased evaporation rates, more pronounced in the tropics. An isotope of hydrogen, the deuterium (2 H), reveals that this moisture was transported to the poles, thus explaining the heavy rains that occurred in the Arctic Ocean.
Despite the lack of ice, sea level rose significantly due to increased temperature. Proof of this is the displacement of palynomorphs (particle size of a grain of pollen) of the Arctic Ocean, reflecting a decrease of terrestrial organic matter compared to marine organic matter.
At the beginning of the PETM, the pattern of ocean circulation changed dramatically in a period of less than 5,000 years. The flow direction is reversed, causing for example that in the Atlantic Ocean bottom current flow from the north to the south, as had always happened in reverse. These effects lasted at least for 40,000 years. This change in the flow of hot water makes the deep ocean warming worse. The chemical composition of the oceans was also greatly altered.
In most parts of the oceans, especially in the North Atlantic Ocean, the bioturbation (the reprise of material, usually toxic, which is stored under the sediment) was almost nonexistent. This could be due to changing ocean circulation, which caused the ocean floor to increase its temperature, and thus harbored barely oxygen (anoxia). However, in some parts of the oceans bioturbation did not cease.
Another effect of the PETM on the ocean environment was to raise the limit of the lisoclina. The lisoclina indicates the depth to which spontaneously dissolved carbonate in the oceans. Today, this limit is 4 km below the ocean surface, which is very similar to the average depth of the oceans.
This depth depends, among other factors, temperature and the amount of CO 2 dissolved, so that both factors raised the lisoclina increasingly toward the ocean surface, causing the dissolution of carbonates in the deep water.
This acidification deep water can be seen in the strata of the ocean floor (if bioturbation has not been particularly active since in that case the evidence would be destroyed), it shows a quite pronounced change, going from a gray carbonates to reddish carbonates and clay , then returned back to the Grizzlies.
These evidences are much clearer in the North Atlantic Ocean than any other, it follows that the acidification was much more pronounced there. In some areas of the southeast Atlantic, the rise lisoclina reached 2 km in just a few thousand years.
The PETM occurred extinction of 35-50% of benthic foraminifera in a span of 1,000 years, the highest percentage in the mass extinction of Cretaceous-Tertiary which occurred about 10 million years ago. In contrast, planktonic foraminifera diversified, and dinoflagellates and mammals thrived. Also noteworthy is the growth of bacteria.
It is difficult to explain the extinctions of marine bottom organisms, since many of them were only regional, affecting mainly those distributed to the North Atlantic Ocean. This means that, unlike the temperature, you can not make general assumptions of the reduction of oxygen, or carbon due to corrosivity of unsaturated carbonates in the deep ocean.
The only factor is the increase in global temperature, and it seems that all the blame falls on this element. The North Atlantic regional extinctions are attributed, in general, the high level of anoxia in the deep waters.
Increased levels of CO 2 produced an acidification of surface waters, which was extremely harmful to corals. It has been shown experimentally that it is also very harmful to the plankton limestone.
However, the acids used in the laboratory to simulate the natural increase in acidity that would result from increased concentrations of CO 2 could have yielded misleading results. Proofs of this are the coccolithophores, which became more abundant in acidified waters.
Interestingly, the calcareous nannoplankton not attributed any change in its distribution by acidification during the PETM, as it did with the coccolithophores. Acidification, however, resulted in a significant increase in algae calcified, and also, to a lesser extent, foraminiferal limestone.
The increase in mammals is another interesting aspect. No evidence was found of any increase in the rate of extinction among terrestrial organisms. Many of the major orders of mammals, including artiodactyls, the horses and primates, emerged and spread rapidly around the globe between 13,000 and 22,000 years after the onset of the PETM. This diversification and dispersal of primates was an aspect key to human evolution.
Causes and Theories
There are many causes that could cause or intensify the PETM, which makes it difficult to clearly determine which of them have much impact. Global temperatures increased steadily worldwide, causing a series of events exacerbated by mechanisms of positive feedback. To determine these factors, it has resorted to the isotope mass balance of carbon, as carbon cycle can vary over relatively short periods. The relative concentration of carbon-13 fell between -2 ‰ and -3 ‰, and analyzing the carbon, we can consider what mass of the reserve would be required to produce the effect. The only assumption that is part of the mass of carbon contained in the atmosphere and oceans during the Paleogene was the same as the current, which is really difficult to confirm.
In order to produce such a disturbance in the concentration of carbon-13, according to this theory, the volcanoes should have expelled around 1,500 gigatons of carbon over the two periods of 1,000 years. For a more comprehensive view of this figure: it is a rate 200 times higher than the rest of the Paleogene, although this amount is unlikely, as they have not found evidence of volcanic activity of this magnitude in the entire history of the Earth.
However, about one million years before the PETM, a major volcanic activity began to ravage the east of Greenland, but alone can not explain the speed at which warming took place. Even in the case of the 1,500 gigatons had suddenly been pushed once, it would take other factors that had led to positive feedback mechanisms to make the alteration has been observed in the isotope of carbon.
Moreover, it has been suggested that surges of volcanic activity were associated with the activity of the rift continental ocean, which expelled magma hot on carbon-rich sediments, which would have triggered the release of methane. Other phases much more late volcanic activity would have caused the expulsion of methane gas as much, causing other periods during the Eocene global warming, such as ETM2 (stands for Eocene Thermal Maximum 2 , commonly Elmo event ).
Release of Methane Gas
Clathrates of methane in full combustion. It produces water and carbon dioxide in large quantities, being in all probability one of the main causes of the PETM.
None of the theories can explain, by itself, the tour of the isotope carbon-13 and the warming that occurred during the PETM. The positive feedback mechanism could further amplify the initial disturbance were the clathrate, according to the so-called clathrate gun hypothesis. The methane, which accumulates continuously in the sediments of the deep ocean due to the decomposing organic, is stable in water at a certain pressure and temperature, forming clusters in the state solid.
As the temperature increases, the pressure exerted decays, the configuration is no longer stable, and clathrates dissociate, causing the release of methane into the atmosphere. Since clathrates themselves have a -60 ‰ in the concentration of carbon-13 with respect to the atmosphere, small quantities of these materials could produce large changes in relative carbon-13.
In addition, methane is a potent greenhouse gas , about eight times more effective than carbon dioxide , so that, being expelled into the atmosphere, could cause a major global warming, in turn, warm the oceans and give rise more methane, destabilizing the system. It has been estimated that the ocean would have taken about 2,300 years to reach the temperature that would allow clathrates dissociate from its background, although this calculation is based on a series of assumptions.
For this hypothesis is valid, the oceans should show signs of warming before the carbon isotope excursion, as the methane takes a while until you manage to enter the atmosphere. Until relatively recently, the evidence showed that both peaks were simultaneous, minus support for the theory. However, recent studies have failed to detect a short period of time between the initial heating and the relative decrease in carbon-13.
Analysis of these records reveals another interesting fact: the planktonic foraminifera recorded small changes in the values of the isotope before the benthic foraminifera, which live in ocean sediments. The shells of these organisms reflect these changes to rust, so a gradual release of methane from the ocean floor would have to be oxidized first shells of benthic foraminifera. The fact that planktonic foraminifera were the first to show these signs of oxidation is that the methane was released so quickly that its oxidation exhausted all the oxygen of the seafloor, allowing, after this, methane reached the atmosphere without oxidation, which react with atmospheric oxygen. This analysis suggests that the process of methane release lasted about 10,000 years.
Impact of Comet
The orbital variations show the relationship between the eccentricity orbital (blue) and temperature (black). One theory proposes that relationship as one of the causes of the PETM.
Another theory is that a comet rich in carbon-12 hit on the land surface and initiated global warming. Even assuming that the size of the comet was found on the border to stop the catastrophe no footprint on the planet (according to the theory about 10 km), after the event and feedback processes occur, still would be required 100 gigatons of carbon would have to come extra terrestrial activity.
However, this theory still has some unresolved issues and does not explain in detail what happened. In theory, the comet would have caused the formation of layer clay of 9 meters thick extremely magnetized, but other sources believe that this layer was formed at a rate too slow to be a consequence of the impact, attributing its creation to the bacteria, which flourished during heating.
On the other hand, the failure of iridium (reliable indicator of impacts on the planet) has been observed in Spain is too small to confirm the impact of the comet.
Due to the existence of other global-scale climate changes, such as ETM2 (event Elmo), it has been hypothesized that these changes are repeated regularly, and are a result of orbital variations in the eccentricity of the orbit Earth. The proximity to the Sun that solar radiation was increased, and thus the temperature, the threshold for trespassing and unleash the various processes of positive feedback.
Burning of peat
It was to postulate a theory that the PETM was caused by the combustion of large amounts of peat, an organic material rich in carbon. However, to produce a relative decrease of carbon-13 that took place, it would need to burn 90% of the biomass land at that time. Since during the PETM plants grew wildly, this theory has been refuted.
The registration of the isotope carbon-13 shows a recovery time between 30,000 and 150,000 years, it is relatively short period when compared with the retention of carbon in the atmosphere today (between 100,000 and 200,000 years). Any satisfactory explanation of this rapid recovery time should include an effective feedback mechanism.
The most probable of recovery would come from an increase in biological productivity, rapidly transporting carbon to the ocean floor. This would have the help of global temperatures and high levels of CO 2, as well as an increase in nutrient supply (high temperatures and high rainfall would cause a continental erosion and volcanic activity may have provided more nutrients). Proof of increased biological productivity could be the barium; however, the increase of this element could also be due to the release of dissolved barium with seafloor methane. Furthermore, diversification is evidence that productivity increased on particularly in coastal areas where marine flora remained warm and fertile, offsetting the reduction in productivity in the ocean floor.