New Constraints on Evolutionary Changes in Coccolithophorid Cell Geometry: Implications for Past pCO2 Reconstructions

Reconstructions of atmospheric pCO2, based on δ13C values of diunsaturated alkenones and estimates of the total carbon isotopic fractionation during photosynthetic carbon fixation (ɛp37:2), show a distinct decline in pCO2 during the Oligocene, as well as regionally confined, short-term fluctuations in ɛp37:2 during the Miocene. Additional to surface water [CO2 (aq)], ɛp37:2 values are controlled by algal growth rates and cell geometry; specifically the ratio of the cell volume to surface area of the cell. Here, we confirm for the first time that substantial evolutionary changes in coccolithophorid cell geometry, based on coccolith morphology, warrant a re-evaluation of pCO2 reconstructions and interpretations of ɛp37:2 trends. A portion of the ɛp37:2 signal during specific events in the Miocene can be strongly explained by changes in cell geometry, resolving apparent contradictions between ɛp37:2 values and independent nutrient-proxy data from the Miocene record. However, the long term signal cannot be explained by the observed size variability, which in fact implies a steeper slope of change in ɛp37:2 during the Oligocene and possibly higher pCO2 estimates during the late-middle Eocene.