1. Department of Earth Sciences, Parks Road, University of Oxford, Oxford OX1 3PR, UK
2. Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02540-1541, USA
3. Earth Sciences Department, University of California, Santa Cruz, California 95064 , USA

Correspondence to: Santo Bains¹ Correspondence and requests for data should be addressed to S.B. (e-mail: Email: santo.bains@earth.ox.ac.uk).

The onset of the Palaeocene/Eocene thermal maximum (about 55 Myr ago) was marked by global surface temperatures warming by 5–7 °C over approximately 30,000 yr (ref. 1), probably because of enhanced mantle outgassing^{2, 3} and the pulsed release of 1,500 gigatonnes of methane carbon from decomposing gas-hydrate reservoirs^{4, 5, 6, 7}. The aftermath of this rapid, intense and global warming event may be the best example in the geological record of the response of the Earth to high atmospheric carbon dioxide concentrations and high temperatures. This response has been suggested to include an intensified flux of organic carbon from the ocean surface to the deep ocean and its subsequent burial through biogeochemical feedback mechanisms⁸. Here we present firm evidence for this view from two ocean drilling cores, which record the largest accumulation rates of biogenic barium—indicative of export palaeoproductivity—at times of maximum global temperatures and peak excursion values of ¹³C. The unusually rapid return of ¹³C to values similar to those before the methane release⁷ and the apparent coupling of the accumulation rates of biogenic barium to temperature, suggests that the enhanced deposition of organic matter to the deep sea may have efficiently cooled this greenhouse climate by the rapid removal of excess carbon dioxide from the atmosphere.