Illustration realized in the framework of a collaboration between the Image/Recit option of the HEAD (Haute École d'Art et de Design) - Genève and the Faculty of Sciences of the University of Geneva.

Historically, the Earth has known periods that were both colder and warmer than today. A better understanding of past climate dynamics will improve the modelling of upcoming changes. The Paleocene-Eocene Thermal Maximum (PETM) is one of the greatest (+5-8°C) and fastest (5,000 years) climate warming event in the recent Earth's history that occurred 56 million years ago. It would have been triggered by a high concentration of two greenhouse gases, the famous CO₂ (carbon dioxide) and methane. The origin of these emissions is still debated, and several phenomena have been evoked to explain it: a bustling volcanism in the North Atlantic, the release of methane hydrates trapped within the permafrost through its sudden melting, and a meteorite impact.  

For this study, a long cylindrical piece of the Earth’s crust, called core, was drilled out from the seafloor, at a depth of 8,350metres below the sea level in the deep Gulf of Mexico. In studying this core, sediments dated from the PETM were identified. We were able to conclude that a large amount of material coming from the North American continent were spilled in a short amount of time at the bottom of the ocean. These sediments are the product of the severe erosion of the Rocky Mountains by wind or rainwater, and were transported by rivers over thousands of kilometres before finally reaching the ocean at the river mouths. As such, they entombed precious information on the climate and the landscapes around rivers on the continent at the time. 

 

During the initial warming phase of the PETM, the climate became more arid and the temperate regions of North America turned into desertic environments. The thriving lush vegetation was considerably reduced, plants became scarcer, and soils were dehydrated and hardened. Ultimately, these changes increased the resistance of soils against water infiltration, which facilitated the erosion and transport of surface sediments by rivers.  

 

The PETM was marked by periods of intense precipitation that were concentrated in short rainy seasons. As a consequence, the river channels became highly mobile and eroded large quantities of clays that were deposited on the adjacent flooding plains. These clays were then transported downstream and made their way to the bottom of the gulf where they were finally deposited on the ocean floor. This phenomenon led to ocean waters becoming increasingly turbid, or muddy. Turbidity reduces the amount of light in water and is associated with a heavier abrasion. As such, it is often considered  very harmful to most marine life, especially coral reefs, which support an enormous diversity of species. A period of recovery followed, which was marked by a resumed transport of sand over clays by the rivers, thereby reducing the turbidity of ocean waters and giving room for marine life to bloom again. 

 

Because of the many similarities shared by the PETM and the current global warming, some valuable conclusions can be drawn for the future. A rapid heating of the atmosphere is likely to remodel temperate regions into drylands that get exposed to periods of extreme seasonal rainfall. This shift in precipitation regimes has proven to destabilise sedimentary systems across entire continents, from mountain tops to ocean floors, and highlights the inter-influence of the different environments of the Earth system.

Read also the press release from the University of Geneva