Paleocene / Eocene temperature maximum

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The Paleocene / Eocene - Thermal Maximum (PETM) approximately 55.8 million years ago was a geologically speaking very short but extreme warming phase, the duration is estimated depending on the scientific analysis of 170,000 to 200,000 years. The global temperature increase at that time took place on the basis of an already existing warm climate and was associated with a greatly increased input of greenhouse gases into the earth's atmosphere and the world's oceans . During the PETM the global temperature rose within probably 4,000 years by an average of 6 ° C (according to other studies briefly by up to 8 ° C) from about 18 ° C in the late Paleocene to at least 24 ° C at the beginning of the Eocene .

The thermal anomaly at the Paleozon-Eocene boundary was associated with a marked decrease in the concentration of the stable carbon isotope 13 C. This indicates that at the beginning of the PETM a large amount of 13 C-depleted carbon was distributed in the atmosphere and hydrosphere . In the meantime, various sediment samples and isotope studies have provided meaningful information about the changed environmental conditions in both the tropical and the higher latitudes of the northern and southern hemisphere . In this way, for example, through the ratio of the carbon isotopes 13 C and 12 C, a clear decline in vegetation in connection with pronounced periods of drought during the heat anomaly could be demonstrated.

In the geosciences and especially in paleoclimatology , the PETM is often analyzed under the aspect of what effects a massive carbon input , limited to a few millennia, has on the climate system . Here often comparison with the current anthropogenic carbon dioxide - emissions and increase its concentration in the atmosphere (see: Carbon dioxide in Earth's atmosphere ) pulled.

Duration of the heating phase

There are a number of different and sometimes contradicting statements in science about the time required from the start of warming to the attainment of the maximum temperature. While until recently a "lead time" of around 18,000 years was considered a realistic value, a publication published in 2013 refers to a sediment sequence in the Marlboro clay of the Salisbury Embayment , which, according to isotope measurements , released 3,000 gigatons of carbon in just 13 years suggests. However, this thesis found little support in the scientific literature and led to several critical statements. The authors of a study published in March 2016 estimated the duration of the warming phase on the basis of a comparison between the carbon signature δ 13 C and the oxygen signature δ 18 O at approximately 4,000 years. According to this, the annual carbon input in the order of magnitude of 0.6 to 1.1 petagrams was parallel to the associated warming. Since the thermally relatively sluggish climate system, including the oceans, reacted to the rise in the atmospheric greenhouse gas concentration without any significant delay, carbon injection that took place within a few years is excluded.

Recent studies seem to confirm the assumption that during a global hot climate , the climate sensitivity increased accordingly. For the PETM, taking into account all short- and long-term feedback factors, a climate sensitivity in the range of 3.7 to 6.5 ° C is postulated.

After the PETM subsided and a longer " recovery phase ", 2 million years later with the Eocene Thermal Maximum 2 (ETM-2, 53.6 mya) there was another strong global warming with a duration of also a maximum of 200,000 Years. This was followed by three shorter and less pronounced heat anomalies in the period between 53.3 and 52.8 million years ago.

Climatic and biological consequences of the PETM

Shells of benthic (= living on the sea floor) foraminifera from North America

Various studies show that the oceans stored considerable amounts of heat during the PETM. For subpolar waters (western Siberian Sea) 27 ° C were determined, and sediment drill cores from the coastal region off Tanzania show temperatures up to a maximum of 40 ° C. In connection with a considerable input of carbon dioxide, this caused an acidification of the oceans down to the deeper layers and the creation of anoxic milieus. All in all, in the course of the PETM, a development occurred in the oceans that, at least in the beginning, showed strong similarity to an oceanic anoxic event . This process was favored by a significant weakening or relocation of the deep water currents and by the increased inundation of continental weathering products into the oceans due to rapid erosion processes. In addition, during the course of the maximum temperature, the sea level rose by 3 to 5 meters due to the thermal expansion (thermal expansion) of the ocean water. There were no ice sheets that could have melted.

Even if the climatic emergency of the PETM was short-lived according to geological standards, it had a lasting impact on the biodiversity and paleoecology of the entire planet. The expansion of the tropical climatic zone up to higher latitudes led to large-scale migration of flora and fauna . Although the water vapor content of the air and thus the tendency towards precipitation increase with increasing temperatures , there was apparently an arid climate in many areas during the PETM , combined with a decrease in plant diversity, including the development of drought stress symptoms. It is assumed that regions close to the polar recorded an increased intensity of precipitation, in contrast to this, periods of drought occurred mainly in the subtropics.

Rapid morphological changes and evolutionary adaptations did not only take place in terrestrial habitats , but also in many cases in the ocean. Here there was a mass extinction of the benthic foraminifera with a species loss of between 30 and 50 percent, with a high probability due to the warming of the deeper oceanic layers by around 4 to 5 ° C and an associated oxygen deficit. In addition, the acidification of seawater with a relatively strong decrease in the pH value played a decisive role in the destabilization of marine biotopes . Affected by this, but only partially threatened with extinction, were organisms settled in the deep sea (sea urchins, mussels, snails) as well as almost all plankton groups .

The meridional temperature gradient (the temperature gradient from the equator to the polar regions) was considerably flatter at the time of the PETM than in the rest of the Cenozoic . This also applies to the near-surface regions of the oceans. The temperature difference of the oceans between equatorial and polar regions was 17 ° C (currently: 22 ° C) over large parts of the Paleocene and decreased to 6 ° C during the PETM. P. 436 As a result, a warm, temperate climate prevailed in the polar regions.

Some families and genera reacted to the rapidly increasing warming of the mammals with a marked tendency to short stature (English dwarfing ). This affected both predatory life forms such as the extinct Creodonta and Oxyaenidae as well as the early representatives of the equine species . In addition, it was possible to demonstrate with the help of Ichnofossils that smaller life forms (for example insects or worms of the class Clitellata ) also adapted to the changed environmental conditions and lost up to 46 percent of their original size. The reasons for the reduction in body size are the periods of drought associated with the extremely warm climate and the resulting lack of sufficient food with a corresponding effect on herbivores and indirectly on carnivores . In addition, the direct influence of the tropical climate and strongly fluctuating precipitation rates, according to several studies, favored phenotypic reactions and microevolutionary changes with regard to growth in size.

A tendency to "dwarf" also recorded many marine species, including the ostracods (shellfish). This development in particular most likely resulted from the warming and acidification of the deep-sea regions and an associated disruption of the remineralization processes of organic carbon. Comparable but somewhat less pronounced biological developments were also found for the duration of the subsequent Eocene Thermal Maximum 2 .

Possible causes and related mechanisms

system series step ≈ age ( mya )
higher higher higher younger
Paleogene Oligocene Chattium 23.03
⬍
28.1
Rupelium 28.1
⬍
33.9
Eocene Priobonium 33.9
⬍
38
Bartonium 38
⬍
41.3
lutetium 41.3
⬍
47.8
Ypresium 47.8
⬍
56
Paleocene Thanetium 56
⬍
59.2
Seelandium 59.2
⬍
61.6
Danium 61.6
⬍
66
deeper deeper deeper older

The discovery of the heat anomaly of the PETM is relatively recent and happened rather accidentally in the late 1980s. The original objective of the researchers involved was to collect more precise data on the mass extinction at the Cretaceous-Paleogene border using sediment samples as part of the Ocean Drilling Program . P. 435 When analyzing the drill cores, which also included the period of the Paleocene-Eocene transition, indications of an abrupt warming of the deep sea at that time were found around 56 million years ago. The new findings were first published in scientific form in 1991 in the journal Nature .

In the following decades, the Paleocene / Eocene temperature maximum developed into one of the focal points of paleoclimatological research, mostly on an interdisciplinary basis and documented in several thousand studies. Many details on the complex of questions of the PETM have now been clarified relatively comprehensively with the help of isotope analyzes and the evaluation of a wide range of proxy data. The answer to the key question about the exact cause of the PETM is still pending. The following sections describe the hypotheses that are currently the focus of scientific discussion.

Plate tectonics and volcanism

As the primary cause of the abrupt warming at the start of the PETM several studies favor the North Atlantic Magmatic Province United (English North Atlantic Igneous Province , also Thulean Plateau ), which was created during the formation and expansion of the North Atlantic. The igneous or volcanic processes began as early as the lower Paleocene (about 64 to 63 mya), extended in a much weaker form into the early Miocene and recorded several increased activity cycles, including 57 to 53 million years ago, with alternating intrusive and effusive phases occurred along the diverging plate edges. The flood basalts rising from the earth's mantle had an area of ​​approximately 1.3 to 1.5 million km² and covered parts of Greenland, Iceland, Norway, Ireland and Scotland. However, the extent and the exact timing of volcanic emissions and their effects on the Earth's climate system at that time are still controversial and the subject of scientific controversy. While some studies postulate a CO 2 release of several thousand gigatons from volcanic sources within a narrow time window, other studies contradict this view by emphasizing that the PETM is mainly related to the release of methane.

According to this hypothesis, extensive methane hydrate deposits were destabilized by the expansion of the North Atlantic, possibly in connection with earthquakes and subsea landslides , whereupon large amounts of the released gas were distributed in the atmosphere.

Methane hypothesis

In the specialist literature of the last few decades there have been widely differing data from 300 to well over 2,000 ppm for the atmospheric CO 2 concentration immediately before the start of the PETM. More recent studies calculated a corridor between 840 and a maximum of 1,680 ppm as the most likely variable, whereby a CO 2 increase of only a few hundred ppm was postulated for the time of the temperature maximum . On the other hand, there is also the opinion of a carbon dioxide increase of around 70 percent compared to the pre-PETM period. But even this significant increase can only partially explain the global temperature increase of around 6 ° C. In general, science therefore assumes an additional climate factor in the form of a methane release from submarine methane hydrate deposits.

Methane is produced in the ocean by the biochemical process of methanogenesis . If the water is supersaturated with methane and under high pressure and at low temperatures, the gas can crystallize into stable methane hydrate, mainly on the continental shelf from a minimum depth of around 300 meters. The specific properties and the extensive deposits of methane hydrate led science to the widespread assumption that methane hydrate reservoirs that became unstable during the PETM released large amounts of greenhouse gas into the atmosphere and thus contributed significantly to the warming effect (although methane in the atmosphere was only one has a short residence time or half-life of 12 years and is oxidized by the oxygen to carbon dioxide and water). However, this relatively simple thesis did not withstand a critical examination in all points and was expanded to a thought model including alternative carbonaceous deposits and sediment layers ("Seafloor hypothesis"). In doing so, it had to be taken into account that the carbon input took place less in short bursts, but rather more continuously over millennia.

Impact hypothesis

During the 22 million years of the Eocene , an above-average number of meteorite or comet impacts occurred on earth, such as the Montagnais or Chesapeake Bay impact . However, this accumulation was not a periodic event in the sense of the nemesis hypothesis , but happened purely by chance according to a statistical analysis published in 2017. The possibility that major impact events could have had a significant impact on the PETM and the subsequent heat anomalies is largely inconsistent with the available data and is considered an outsider thesis in science.

Possible influence of the Milanković cycles

Long-term diagram (2 million years) for the eccentricity of the earth's orbit

A study published in 2012 is based on the cyclical changes in the Earth's orbit parameters as the cause of the Eocene temperature maxima. The hypothesis is based on the assumption that with a pronounced eccentricity of the earth's orbit and a simultaneous maximum inclination of the earth's axis, the associated warming trend led to enormous amounts of greenhouse gases from thawing permafrost reaching the atmosphere within a short time, especially in the Antarctic . After an intense warm climate phase in which a large part of the emitted carbon dioxide was bound again through accelerated weathering processes, the earth slowly cooled before the cycle controlled by the orbital parameters started again.

According to the current state of knowledge, it seems questionable whether under the climatic conditions near the Paleocene-Eocene border (extension of subtropical climatic zones up to higher latitudes, flat temperature gradient and polar amplification ) there was a significant amount of Antarctic permafrost. The same applies to the polar near mainland in the north. Evidence of cooling in the South Pole region only occurred in the period after the heat anomalies (52 mya) and increasingly with seasonal snowfall in the Middle Eocene (41 mya). Regardless of this, a possibly central role of the Milanković cycles in climate change events and especially on the course of the carbon cycle is increasingly becoming the focus of research.

Special position of the PETM

In the search for a consistent model of the Paleocene / Eocene temperature maximum , a research approach may be that the PETM and the Eocene Thermal Maximum 2 were followed by three shorter and less pronounced thermal anomalies within a short period of time. This could be an indication of a geophysical constellation that was repeated several times over a period of just under 3 million years. In the following series of the Cenozoic up to the geological present, comparable events no longer occurred. The early Eocene is therefore of particular importance from a paleoclimatological point of view.

The Ypresium , the lowest chronostratigraphic stage of the Eocene, runs almost parallel to the so-called Eocene climatic optimum , an epoch characterized by subtropical to tropical conditions that ended about 49 to 48 million years ago without the extreme climate of the embedded thermal anomalies being reached again . After that, a slow and at first almost creeping cooling trend began (partly due to the Azolla event ), which accelerated significantly at the Eocene-Oligocene transition (33.9 to 33.7 mya), as well as a large extinction of species and a rapid decline in the atmospheric CO 2 concentration recorded and at the same time marked the beginning of the Cenozoic Ice Age with the formation of the Antarctic Ice Sheet .

Känozoikum Kreide-Paläogen-Grenze Eocene Thermal Maximum 2 Miozän#Klima, Ozeane und Vegetation Eem-Warmzeit Letzteiszeitliches Maximum Atlantikum Jüngere Dryaszeit Globale Erwärmung Paläogen Neogen Quartär (Geologie) Paläozän Eozän Oligozän Miozän Pliozän Pleistozän Holozän Christopher Scotese James E. Hansen James E. Hansen James E. Hansen EPICA EPICA Greenland Ice Core Project Delta-O-18 Repräsentativer Konzentrationspfad
Clickable diagram of the temperature development in the Cenozoic including a warming scenario on the basis of the extended representative concentration path ECP 6.0 up to the year 2300. The climate anomaly of the PETM is shown at the top left.

Relevance today

According to unanimous scientific judgment, the Paleocene / Eocene temperature maximum was the most significant and fastest occurring natural warming phase of the entire Cenozoic, i.e. the last 66 million years. In contrast to comparable geological temperature anomalies, such as the one at the Permian-Triassic border , the greenhouse gas concentration rose massively during the PETM without an adequate release of nitrogen oxides , sulfur dioxide and hydrogen sulfide . As a result, parallels to current global warming are increasingly being drawn in the specialist literature , combined with the question of whether the PETM represents a “blueprint” for future climate developments. In this context, however, some authors suggest that the current, very rapid environmental changes, including a possible destabilization of the biosphere , could lead to a specific climatic condition for which no equivalent exists in the known history of the earth.

The amount of carbon released at the Paleocene-Eocene border is estimated in the more recent specialist literature at 3,000 to approximately 7,000 gigatons, with individual studies even estimating a volume in the region of 10,000 gigatons. On the other hand, the anthropogenic carbon emissions to date amount to around 645 gigatons (that's over 2,300 gigatons of CO 2 ). There is broad consensus in research that the annual average of greenhouse gas emissions during the 21st century has exceeded those of the PETM by around ten times. The associated climate change will intensify correspondingly quickly if the release of carbon dioxide or methane is not drastically reduced in the next few decades. If this does not succeed, there is a high probability that the development will follow a course similar to that of 55.8 million years ago. There are indications that at that time most greenhouse gases were released in a relatively early stage of the Paleocene / Eocene temperature maximum, remained in the atmosphere for millennia without a significant decrease in concentration and were then only decomposed very slowly. This observation largely corresponds with the knowledge about the current climate development. With this, from a CO 2 level of 500 ppm and above, a self- reinforcing effect of the temperature increase in the context of a longer warm period is expected, which, among other things, would lead to the failure of a complete Ice Age cycle in around 30,000 to 50,000 years.

Apart from the hardly comparable ecosystems of the two epochs, an essential difference between the beginning of the PETM and the present is that the base temperatures on which the subsequent warming was based differ relatively strongly. The average temperature for the ice-free world of the late Paleocene was 18 ° C, while the global temperature for the 20th century was 14-15 ° C. This allows the conclusion that anthropogenically induced climate change will not reach the extreme climate of the PETM in the foreseeable future or with moderate use of resources. However, the occurrence of a possible "worst-case scenario" is likely to be serious, above all due to the sometimes incalculable influence of the tipping elements in the earth system in connection with the shift in the climate and vegetation zones and the extensive melting of the West Antarctic and Greenland ice sheets and the corresponding rise the sea level by several tens of meters. The “stocks” of methane hydrate on the seabed of over 10 trillion tons (10,000 gigatons), which could be increasingly destabilized if the current warming trend is maintained, represent a further hazard potential.

See also

literature

Web links

Individual evidence

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  2. ^ Peter Stassen, Robert P. Speijer, Ellen Thomas: Unsettled puzzle of the Marlboro clays . In: PNAS . 111, No. 12, 2014, pp. E1066 – E1067. doi : 10.1073 / pnas.1321839111 .
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