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

Abstract 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.