Zagros Mountains, Iran, part of the ‘gypsum belt’ – a rich source of sulfur for the world’s oceans and climate. Credit: NASA.

When we think of the relation between geology and the Earth´s climate system we think of episodes of intense orogenic and volcanic activity, during which large amounts of carbon enter the atmosphere, leading to relatively sudden climate warming – separated by long quiet periods during which slow carbon sequestration processes lead to net cooling.

But new research by two researchers of the University of Toronto and the University of California at Santa Cruz shows plate tectonics can drive that other large climate forcer too – aerosol cooling.

A clear example in paleoclimatology of the first geological climate driver is the PETM, or Paleocene-Eocene Thermal Maximum – when increased volcanology led to higher CO2, warming, ocean clathrate destabilisation and methane release – followed by the high carbon high temperature period of the Eocene, which lasted from about 56-34 million years ago, with a notable temperature peak at the Early Eocene Climatic Optimum.

The Eocene cooling trend

After the early Eocene Earth’s climate gradually cooled for many millions of years, first enabling the formation of the Antarctic ice sheet (about 45 million years ago) and right until the Pleistocene ice ages (<2.6Ma) even brought glaciations to currently inhabited latitudes.

Here of course different factors were at play. During the Pliocene (5.3-2.6Ma) for instance the Panama Isthmus formed after which the AMOC thermohaline circulation got into a higher gear, possibly speeding up ocean carbon sequestration. And during the subsequent Pleistocene the Milankovitch variables were primarily responsible for the ice ages – nothing to do with geology, but astronomical forces instead.

Squeezing a gypsum belt

But much earlier – about 50 million years ago – something else happened that paved the way to enable this planetary cooling, the new research, published in Science, illustrates: the collision between India and Eurasia.

Although collisions usually mean erosion of carbon and carbonate rock formations – hence more CO2 – this collision also exposed the world’s largest gypsum deposit, which ranges from Oman through Pakistan to western India (once a salty high evaporation sea).

As much of the terrain was raised under the tectonic forces fluvial erosion in the gypsum-rich mountain ranges brought large quantities of sulfur in the world’s oceans, evidence of which was found in marine sediment records.

This increased sulfur concentration in turn affects many different climatologically relevant factors, like carbonate biology (dissolved gypsum is not only a source of sulfur, but also of calcium – a nutrient) and more importantly increased sulfate aerosol emissions, acting as a potent climate cooling force, and according to the researchers thereby ending the Eocene warm period.

© Rolf Schuttenhelm | www.bitsofscience.org