Some excess carbon released during a period of global warming 56 million years ago may have been removed from the atmosphere by enhanced biological activity in the oceans, according to a paper published online in Nature Geoscience. The enhanced transfer of carbon from the surface ocean to depth may have helped remove carbon dioxide from the atmosphere and thus aided in cooling the climate.

Adina Paytan and colleagues used marine sediment records from around the globe to estimate the amount of carbon that was removed from the surface oceans during the Palaeocene-Eocene Thermal Maximum, about 56 million years ago. They found that carbon uptake and export to the mid-depths of the ocean increased during the interval of peak warmth. The authors argue that increased bacterial activity in the surface waters and mid-depths of the ocean at this time would have broken the marine organic carbon down into compounds that were not readily consumable. If so, carbon would have remained trapped in the waters of the mid-to-deep oceans for tens of thousands of years. Paytan and colleagues conclude that the continued consumption and storage of carbon would have removed carbon dioxide from the atmosphere, and helped return the planet to cooler temperatures.