Testing geochemical proxy relationships under variable paleo-seawater chemical compositions
DESCRIPTION: This project funded culture experiments to study the effect of Paleocene seawater composition on the B/Ca ratio and photosynthetic activity in planktic foraminifers.
OUTCOMES: B/Ca increases with boron concentration in seawater in G. sacculifer, G. ruber, and O. universa. None of these species show a measurable B/Ca response to varying calcium concentrations, which suggests that in contrast to inorganic precipitation studies, modifications to calcite growth rate do not significantly affect B/Ca in foraminifera. Paleocene experiments in G. sacculifer and O. universa have additionally confirmed the DIC effect on B/Ca, and we have created another set of calibrations for application to the PETM records of Penman et al. (2014, Paleoceanography) from this species for the relative influence of pH and DIC changes. In addition, geochemical proxies are often used to infer growth habitats and vital effects in planktic foraminifers. As such, it has been interpreted from B/Ca and boron isotopes that G. ruber inhabits a shallower growth habitat than G. sacculifer and O. universa, and/or that its symbionts are more active than in G. sacculifer and O. universa. Our FRRF data show that all three species have similar fluorescence, and Chlorophyll a concentrations and symbiont counts are comparable between all three species, which suggests that we need to consider other parameters, e.g. gametogenic calcite precipitation in G. sacculifer and O. universa or differential boron discrimination in different foraminifer species, to cause the observed geochemical offsets.