In order to understand what the Paleocene Eocene Thermal Maximum is, it is necessary to know about the Paleocene Eocene Epochs.
Paleocene Epoch: The Paleocene Epoch is a geological time-scale that lasted from 66 to 56 million years ago. This was the time of dinosaurs and is famously marked for the extinction event of non-avian dinosaurs because of an asteroid impact, along with 75% of living species. The end of the epoch is marked by Paleocene Eocene Thermal Maximum.
Eocene Epoch: The Eocene Epoch is the geological time-scale that lasted from 56 million years ago i.e. the end of Paleocene Epoch, to 33.9 million years ago i.e. the beginning of Oligocene Epoch.
Paleocene Eocene Thermal Maximum (PETM): It is a time period in the geological history of Earth when there was globally more than 5℃ - 8℃ temperature rise across the Earth. It is also known as initial Eocene Thermal Maximum 1 (ETM).
Characterising Paleocene Eocene Thermal Maximum
Towards the end of the Paleocene Epoch and the beginning of Eocene Epoch there was a temperature rise of 5℃ - 8℃ globally. The temperature rise was observed across the event which lasted from 20,000 to 50,000 years. The entire period was warm and has been known for increased temperature for approximately 2,00,000 years. The exact age and the duration of the event is arguable but has been estimated to have begun and occurred around 55 million years ago, the boundary of Paleocene Eocene Epochs.
The starting of the Eocene Thermal Maximum has been said to have occurred because of the increased volcanic activity or volcanism and the uplift that is associated with the North Atlantic Igneous Province (a large area in north atlantic region centered on iceland) that lead to significant changes in the carbon cycle of Earth which in turn lead to the global temperature rise of 5℃ - 8℃. The period is marked by the high decrease observed in the amount of 13C stable isotope of carbon all over the world. This led to the decrease of the 13C/12C ratio of marine and terrestrial carbonates and organic carbon. From the combined data obtained of isotopes, 𝛿13C, 𝛿11B, 𝛿18O it is observed that approximately 12,000 Gigatonnes of carbon was released into the atmosphere for a period of 50,000 years, with 0.24 Gigatonnes per year average. This means that at least 44,000 Gigatonnes of carbon dioxide responsible for global warming was released into the atmosphere due to extreme geological events.
Changes During Paleocene Eocene Thermal Maximum
Apart from the rise in the carbon dioxide levels due to volcanic activity throughout the globe, there were several other changes that led to drastic changes in the animal and plant life both on land and in the ocean. These other changes are observed by the stratigraphic sections of the rock from this period that reveal numerous changes.
Many of the fossil records show major and significant changes and turnovers in the profile of organisms over the Earth. Examples include the changes in the marine realm, there was a mass extinction of the benthic foraminifera and a global expansion of the subtropical dinoflagellates. There was also an appearance of increase in the population of the planktic foraminifera and calcareous nannofossils. All these were observed during the beginning stages of PETM. The mammalian sea-animals also sprung and got the advantage of the wide resources available to them. There is trace evidence of the impacts of the rising temperatures and increase in the carbon dioxide content of the atmosphere leading to decrease in the amount of dissolved oxygen in the oceanic water, in turn affecting the life of deep-sea species. But one specific development is known about the increase in the population of heavily calcified algae and weakly calcified forams which occurred due to acidification of the water bodies.
Similar things were observed on land as well. The modern mammalian order including the primates suddenly appeared in the continents of Europe and North America. There is a widespread migration known for Asian mammals towards the north, because of the prevailing humid conditions. Also, the population of the mammalian species increased many fold during this time. Increase in the levels of CO2 may have facilitated the physical trait of dwarfing in turn encouraging speciation. Some 13,000 to 22,000 years after the initiation of the PETM, many animals belonging to the mammalian orders such as Artioctyla, horses and primates spread around the globe. The deposition of the sediments also changed the outcrops and many of the drill cores that spanned the time interval.
Recovery and Comparison With Current Global Warming
In order to understand the climatic conditions of olden times such as the Paleocene Epoch about 56 million years ago, climate proxies are used. Climate proxies are the preserved physical characteristics of the past, that show us visible proof and help us understand and reconstruct the possible behaviour of the climate during that time. From such climate proxies it is stated that the recovery might have occurred with an increase in the biological productivity. Increased biological productivity might have helped to form an appropriate carbon cycle and would have transported carbon to the deep oceans. This would have occurred with higher global temperatures, higher CO2 levels, and increased supply of nutrients from continental weathering (because of higher temperatures and higher rainfall) and depositions from volcanic eruptions.
The evidence of such an increased biological productivity leading to recovery is given by the bio-concentrated barium. But there is a possibility that the bio-concentrated barium may have been because of barium dissolved with methane which is a later event and a possibly recent event. The diversification of the plant species, especially the ones near the shores indicate that productivity increase in such areas as the weather there would have been warm and fertilized run-off which had outweighed the reduction of productivity in the deep ocean areas.
The initial Eocene Thermal Maximum 1 has been the centre point of investigation as an analog to understand the effects of global warming. Although, the adverse effects of global warming and the reason are much worse today as compared to the period of PETM. Humans emit 10 Gigatonnes of carbon dioxide into the atmosphere as compared to the 0.24 Gigatonnes produced per year during PETM. Another difference is that during the PETM the planet was ice-free and the effects of global warming on such areas isn't clear but the effects will be known by today as the water level rises all over the world due to melting glaciers and melting ice at the poles. Also, it is said that the cause of the PETM were the widespread volcanic eruptions and the K-T extinction event but they are still debatable and the cause, details and the overall significance of the event remain uncertain. It has already been established that the peak carbon addition to the ocean-atmosphere system during the PETM is much slower and lower than the carbon addition by human activities. Also, it is said that the current methane emission regime is similar to the one that occurred during the PETM. Thus, it is understandable what such emissions can lead to and the changes and impact that it can have on the species life and nature.