Evaluating the utility of B/Ca ratios in planktic foraminifera as a proxy for the carbonate system: a case study of Globigerinoides ruber

B/Ca ratios in foraminifera have attracted considerable scientific attention as a proxy for past ocean carbonate system. However, the carbonate system controls on B/Ca ratios are not straightforward, with Δ[ CO₃^2-] ([ CO₃^2-]_{in situ} - [ CO₃^2-]_{at saturation}) correlating best with B/Ca ratios in benthic foraminifera, rather than pH, B(OH)₄^-/HCO₃^-, or B(OH)₄^-/DIC (as a simple model of boron speciation in seawater and incorporation into CaCO₃ would predict). Furthermore, culture experiments have shown that in planktic foraminifera properties such as salinity and [B]_SW can have profound effects on B/Ca ratios beyond those predicted by simple partition coefficients. Here, we investigate the controls on B/Ca ratios in G. ruber via a combination of culture experiments and core-top measurements, and add to a growing body of evidence that suggests B/Ca ratios in symbiont-bearing foraminiferal carbonate are not a straightforward proxy for past seawater carbonate system conditions. We find that while B/Ca ratios in culture experiments covary with pH, in open ocean sediments this relationship is not seen. In fact, our B/Ca data correlate best with [ PO₄^3-] (a previously undocumented association) and in most regions, salinity. These findings might suggest a precipitation rate or crystallographic control on boron incorporation into foraminiferal calcite. Regardless, our results underscore the need for caution when attempting to interpret B/Ca records in terms of the ocean carbonate system, at the very least in the case of mixed-layer planktic foraminifera.