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 [HCO3−], higher [CO32−] and [B(OH)4−]), but decreases with increasing total dissolved inorganic carbon (DIC) (higher [HCO3−] and [CO32−], 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.

► B/Ca increases with salinity in culture experiments with three planktic foraminifer species.
► B/Ca depends on the relative abundance of dissolved B and C species in seawater.
► There is no discernable B/Ca change with temperature in cultured planktic species.