Coccolithophore cell size and the Paleogene decline in atmospheric CO2

Alkenone-based Cenozoic records of the partial pressure of atmospheric carbon dioxide (pCO2) are founded on the carbon isotope fractionation that occurred during marine photosynthesis (εp37:2). However, the magnitude of εp37:2 is also influenced by phytoplankton cell size — a consideration lacking in previous alkenone-based CO2 estimates. In this study, we reconstruct cell size trends in ancient alkenone-producing coccolithophores (the reticulofenestrids) to test the influence that cell size variability played in determining εp37:2 trends and pCO2 estimates during the middle Eocene to early Miocene. At the investigated deep-sea sites, the reticulofenestrids experienced high diversity and largest mean cell sizes during the late Eocene, followed by a long-term decrease in maximum cell size since the earliest Oligocene. Decreasing haptophyte cell sizes do not account for the long-term increase in the stable carbon isotopic composition of alkenones and associated decrease in εp37:2 values during the Paleogene, supporting the conclusion that the secular pattern of εp37:2 values is primarily controlled by decreasing CO2 concentration since the earliest Oligocene. Further, given the physiology of modern alkenone producers, and considering the timings of coccolithophorid cell size change, extinctions, and changes in reconstructed pCO2 and temperature, we speculate that the selection of smaller reticulofenestrid cells during the Oligocene primarily reflects an adaptive response to increased [CO2(aq)] limitation.