Climate: Lessons from the latest global warmi

56 million years ago, Earth experienced one of the largest and fastest global warming events in its history: the Paleocene-Eocene Thermal Maximum (PETM), which bears similarities to current and future warming. In this episode, global temperatures increased by 5-8°C. It was marked by an increase in the seasonality of rainfall that resulted in large amounts of clay entering the ocean, rendering it uninhabitable for certain living species. This scenario could repeat itself today. A team from the University of Geneva (UNIGE) found this out thanks to the analysis of sediments from the deep waters of the Gulf of Mexico. These results can be found in the journal geology.

The Paleocene-Eocene Thermal Maximum (PETM), which occurred 56 million years ago, is the largest and most rapid climatic disturbance of the Cenozoic (65.5 million years ago to present). This episode was exceptional both in terms of its amplitude (5-8°C increase) and its suddenness (5,000 years, a very short time on a geological scale) and was characterized by a warming of temperatures on a global scale. It lasted about 200,000 years and led to numerous marine and terrestrial extinctions.

It would have been caused by high concentrations of carbon dioxide – the famous CO2 – and methane in the atmosphere, two powerful greenhouse gases. As is currently the case, these gases may have been released by several phenomena, certainly in combination: the release of methane hydrates trapped on the seabed, the sudden and significant melting of the permafrost and the injection of magma into the organic sediments of the Western edge of Norway. The origin of these processes is still debated. A meteorite impact and/or the effects of intense volcanic activity deep in the North Atlantic could be responsible.

A geological “archive” of unprecedented quality

Because of the many similarities between the PETM and current warming, the geological remains of this period are being closely studied by scientists. A UNIGE team is now reporting new elements. “The aim of our study was to investigate the influence of these climatic changes on sediment systems, i.e. on the processes of sediment formation and deposition, and to understand how these changes could be transmitted from the atmosphere to the depths of the ocean,” explains Lucas Vimpere, postdoctoral researcher in the Earth and Environmental Sciences Section of UNIGE’s Faculty of Science and first author of the study.

The researchers analyzed sediments from more than 8 km deep in the Gulf of Mexico. This basin acts as a huge “sink” into which material eroded and transported from the North American continent is drained over millions of years. “Due to cost and infrastructure reasons, the sediments used to study the PETM are generally from shallow marine or continental environments. Thanks to the collaboration with an oil company, we were able to obtain a sample of unprecedented quality, without any modification,” says the researcher. The 543 meter long core contains a 180 meter thick PETM sediment record, making it the world’s most complete geological ‘archive’ of this period.

More clay on the seabed

UNIGE scientists found that it consisted first of a large layer of clay and then of sand, a counterintuitive result. “At the time of the PETM, we thought that there was more precipitation and therefore more erosion, and then large amounts of sand were transported first from the river systems into the oceans. However, thanks to our sample, we were able to determine that it was primarily the clays that were transported and not the sands”, explains Sébastien Castelltort, full professor in the Earth and Environmental Sciences section of the UNIGE Faculty of Natural Sciences , and last author of the study.

It was found that the period was not characterized by an increase in the annual precipitation rate, but by an increase in its seasonality and intensity. “This led to increased mobility of the river channels – the deepest parts of a river – which in turn transported large amounts of river clays deposited on the adjacent alluvial plains to the depths of the ocean. We can now consider the presence of clay in deep basins as an indication of increased seasonality in precipitation,” says Lucas Vimpere. The phenomenon has led to an increase in ocean turbidity, which is harmful to marine life, particularly coral.

“The PETM is a potential analogue of current warming. As recent IPCC reports show, we are now also seeing an increase in seasonality and intensity of precipitation. As our study shows, this is likely to destabilize sediment systems in the same way as it did during PETM, and with the same consequences for the oceans and living species,” explains Lucas Vimpere. This new data can now be integrated into modeling aimed at predicting the evolution and consequences of global warming.