Ancient global warming event sheds light on Earth's future

About 56 million years ago, our planet experienced one of the largest and fastest global warming events in the history. The so-called Paleocene-Eocene Thermal Maximum (PETM) was exceptional both in terms of its amplitude (with global temperatures rising by five to eight degrees Celsius) and its suddenness (about 5,000 years – a very short period on a geological scale). 

By analyzing sediments collected from the deep waters of the Gulf of Mexico, a team of scientists led by the University of Geneva (UNIGE) has now found that this event – which led to the extinction of a vast number of terrestrial and marine species – was characterized by an increase in rainfall seasonality, which led to the movement of massive quantities of clay into the ocean, making it uninhabitable for many species. 

Due to the similarities between PETM and current global warming – including possible causes such as high concentrations of carbon dioxide and methane in the atmosphere – the geological remains of this period are closely studied by scientists in order to estimate what could the future of our planet look like.

“The objective of our study was to investigate the influence of these climatic changes on sedimentary systems, i.e. on the processes of sediment formation and deposition, and to understand how these changes could have been transmitted from the atmosphere to the depths of the ocean,” said lead author Lucas Vimpere, a postdoctoral fellow in Earth and Environmental Sciences at UNIGE.

The scientists analyzed sediments collected from over eight kilometers deep in the Gulf of Mexico, a giant “sink” in which material eroded and transported from North America during million of years is discharged. “For reasons of cost and infrastructure, the sediments used to study the PETM are generally taken from shallow marine or continental environments. Thanks to the collaboration of an oil company, we were able to obtain a sample of unprecedented quality, without any alteration,” Vimpere explained.

Surprisingly, the analysis revealed that the ocean floor was composed first of a massive layer of clay and then a layer of sand. “At the time of the PETM, we thought that there had been more precipitation, and therefore more erosion, and that large quantities of sand had then been transported first by the fluvial systems into the oceans. However, thanks to our sample, we were able to determine that it was the clays and not the sands that were transported in the first instance,” said study senior author Sébastien Castelltort, a professor of Earth and Environmental Sciences at UNIGE.

Thus, this period does not seem to be marked by an increase in the annual rate of precipitation – as scientists previously assumed – but rather a rise in its seasonality and intensity, which led to an increased mobility of the river channels that transported large quantities of fluvial clays to the ocean depths, significantly harming a variety of marine species.

“The PETM is a potential analogue of current warming. As recent IPCC reports show, we are also now seeing an increase in the seasonality and intensity of rainfall. As our study shows, this is likely to destabilize sedimentary systems in the same way as during the PETM and with the same consequences for the oceans and living species,” Vimpere concluded.

The study is published in the journal Geology.

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By Andrei Ionescu, Earth.com Staff Writer

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