Evaluation of the biogeochemical controls on B/Ca of Globigerinoides ruber white from the Oceanic Flux Program, Bermuda

Abstract

Boron to calcium (B/Ca) ratios in planktonic foraminifera is suggested to be a proxy for the surface oceanic carbonate system. The reliability of the proxy has been questioned due to conflicting reports from culture and sediment calibrations on the influence of temperature on B/Ca. To assess this issue, B/Ca and Mg/Ca ratios and δO18 were measured in tests of the symbiont-bearing surface dwelling planktonic foraminifer Globigerinoides ruber white collected by the Oceanic Flux Program (OFP) time series sediment traps, located approximately 75 km SE of Bermuda. B/Ca ratios in the 300–400 μm size fraction of G. ruber were approximately 10–20% higher than those in the 200–300 μm size fraction. In contrast, Mg/Ca ratios and δO18 values do not exhibit any relationship to test size, which indicates that the size effect observed for B/Ca is not due to differences in depth habitat but to vital effects. Seasonal variation in the B/Ca ratio was similar in both size fractions and ranged by ∼20–30 μmol/mol. This range is larger than that predicted by seasonal variations in seawater [B(OH)−4/HCO−3], the proposed boron species incorporated into planktonic foraminiferal calcite, and indicates that B/Ca in G. ruber is influenced by an additional variable (s). The seasonal cycle of B/Ca in G. ruber white was more strongly correlated with light intensity than with temperature. Both observations suggest that the presence of symbionts in G. ruber and seasonal variability in their photosynthetic activity act to modify the internal pH during calcification, by up to ∼0.2 units relative to ambient seawater.