Environmental controls on B/Ca in calcite tests of the tropical planktic foraminifer species Globigerinoides ruber and Globigerinoides sacculifer

The ratio of boron to calcium (B/Ca) in the calcite tests of planktic foraminifers may serve as a proxy for past seawater chemistry, but controls on B incorporation are not yet certain. Here we present the results of laboratory culture experiments with live specimens of Globigerinoides ruber (pink) and Globigerinoides sacculifer, which provide new insight into B incorporation controls. We find that in G. sacculifer, B/Ca increases with increasing pH (lower [HCO ₃ ^-], higher [CO ₃ ^2-] and [B(OH) ₄ ^-]), but decreases with increasing total dissolved inorganic carbon (DIC) (higher [HCO ₃ ^-] and [CO ₃ ^2-], constant [B(OH) ₄ ^-]). This suggests competition between aqueous boron and carbon species for inclusion into the calcite lattice. Similar to previous experiments with the subtropical-temperate Orbulina universa, B/Ca increases with salinity, but not with temperature. We evaluate possible carbonate system control parameters, and compare our tropical culture calibrations with new and published core-top data.