Monday, 29 September 2014

Calculating the temperature of tropical waters during the Palaeocene-Eocene Thermal Maximum.

The Palaeocene Era was a period of significant global warming, culminating in the Palaeocene-Eocene Thermal Maximum, when around 2000 gigatons of isotopicly light carbon are thought to have been released into the atmosphere over a relatively short time interval (less than 60 000 years), due to melting of deep-sea methane hydrates and permafrost in high latitude and altitude soils. This resulted in a brief period of extreme global warming driven by the sudden input of more-or-less all available greenhouse gasses into the atmosphere, then followed by a period of cooling in the early Eocene as these gasses were lost from the atmosphere.

It should be noted that predictions of the results of human-induced global warming in the near future often predict more severe warming than occurred at the end of the Palaeocene; this is because these scenarios include not just the melting of permafrost and methane hydrates due to warming, but also the pumping of large amounts of carbon dioxide from the burning of fossil fuels (which did not occur during the Palaeocene), and therefore resulting in a much more severe warming, albeit from a cooler starting point.

Though we know that the temperature rose sharply around the globe at the end of Palaeocene, exactly how far it rose in many areas is unclear. One such area is the tropical oceans, which are important for the understanding of global climate patterns. The best way of determining sea temperatures in the past is generally accepted to be oxygen isotope analysis of minerals from the preserved shells of pelagic or planktonic organisms. Such organisms, if they lived close to the surface in open water can give a good measure of the temperature of the water as warmer water contains a higher proportion of isotopicly heavy oxygen. Inshore waters are prone to more extreme temperature fluctuations, as water trapped in enclosed bays can be heated more extremely, but this does not reflect the temperature over wider areas and is therefore not considered useful for understanding global climate patterns.

However this is difficult to study for tropical waters around the Palaeocene-Eocene Thermal Maximum, as the rise in atmospheric carbon dioxide led to oceanic acidification (carbon dioxide dissolves in water to form carbonic acid), leading to a rise in the carbonate compensation depth (the depth beneath which calcium carbonate will dissolve in water – calcium carbonate dissolves more readily at greater pressure, resulting in a pressure boundary rises as the ocean becomes more acidic and falls as the acidity does). Thus many end-Palaeocene deep water sediments, from which microfossil shells are usually extracted to perform oxygen isotope analysis, lack any such shells.

In a paper published in the journal Geology on 25 July 2014, a team of scientists led by Tracy Aze of the School of Earth and Ocean Sciences at CardiffUniversity describe the results of a study of planktonic Foraminifera shells from a location in Tanzania interpreted as having been an outer shelf environment 19˚ from the equator, that was remote from any land but had a depth of only about 300 m (i.e. above the carbonate compensation depth).

Azeet al. analysed specimens of the Foraminifera Acarinina, Morozovella, and Subbotina from across the late Palaeocene and early Eocene. This resulted in a maximum temperature estimate close to the Thermal Maximum of ~36-43˚C, however the sediments around the Thermal Maximum are extremely depleted in Foraminifera, suggesting that temperatures may have been even higher, resulting either in seawater too acidic to preserve calcium carbonate shells or too warm to have been inhabited by Foraminifera.

A Foraminifera of the genus Morozovella. Clay Kelly et al. (2001).

Studies in other areas have suggested that Foraminifera from equatorial and tropical regions migrated towards the poles around the Thermal Maximum, while experiments on modern members of the group suggest that these are unable to tolerate temperatures much above ~33˚C, so that even if Palaeocene Foraminifera were more tolerant of high temperatures, it is difficult to see them surviving in water much above ~43˚C.

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