QUANTIFYING PALEOCEAN ACIDIFICATION AT THE PETM FROM BORON PROXY RECORDS

Understanding the fate of marine organisms under paleocean acidification conditions analogous to today requires independent geochemical evidence for acidification and saturation changes in the past. The Paleocene-Eocene Thermal Maximum (~56 Ma) has been identified as a carbon cycle perturbation event that is characterized by massive deep-ocean carbonate dissolution, a reef crisis, benthic foraminifer extinction and some turnover changes in planktic communities. A recent study (Penman et al., Paleoceanography, 2014) quantified surface ocean pH changes at that time similar to acidification projections for the remainder of this century, albeit at a much slower rate. The planktic foraminiferal B/Ca record is qualitatively consistent with the boron isotope reconstruction, however, modern calibrations could not be applied because the seawater elemental composition of the Paleocene ocean differed from the modern ocean. In addition, potential symbiont bleaching may have further modified the geochemical signal.

We have conducted culture experiments with planktic foraminifers, where we simulated Paleocene seawater chemistry and tested the effects of light, pH and dissolved inorganic carbon (DIC) on the foraminiferal B/Ca records. These new calibrations reveal no or little influence of light on B/Ca, but the proxy is more sensitive to pH at the low [B], high DIC conditions predicted for the Paleocene/Eocene. We will use these new calibrations in conjunction with boron isotope pH estimates to further constrain the carbon cycle perturbation at the PETM.