Eocene emergence of highly-calcifying coccolithophores despite declining atmospheric CO2

Coccolithophores, a group of unicellular calcifying phytoplankton, have been major contributors to marine carbonate production since the calcite plates that they produce (coccoliths) first appeared in the fossil record over 200 million years ago (Ma). The response of this process to changes in environment on evolutionary timescales remains poorly understood, particularly in warm climates. Here we integrate a dataset consisting of carbon isotope ratios of size-separated coccolith calcite from marine sediments with a cell-scale model to interrogate cellular carbon fluxes and pCO2 through the Eocene (~55-34 Ma), Earth’s hottest interval of the last 100 million years. We show that the large coccolithophores that rose to dominate the oceans through the Eocene have higher calcification to carbon fixation ratios than their predecessors whilst the opposite is true for smaller coccolithophores. Our approach also provides independent support of multiproxy based evidence for generally pCO2 decline through the Eocene in step with temperature. Together, this complicates the emerging view that a general decline in pCO2 reduces calcification on evolutionary timescales.