The planktic foraminiferal B/Ca proxy for seawater carbonate chemistry: A critical evaluation

Abstract

The ratio of boron to calcium (B/Ca) in the carbonate tests of planktic foraminifers has been proposed as a proxy for surface ocean pH or carbonate ion concentration ([CO32−]), with a possible secondary influence of temperature. In both cultured and wild specimens, B/Ca generally increases with [B(OH)4−/HCO3−], consistent with the proxy's theoretical basis. However, close examination of the available data reveals that calibrations using the empirical boron partition coefficient, KD=[B/Ca]calcite/[B(OH)4−/HCO3−]seawater, can be driven by independent relationships between [B(OH)4−/HCO3−] and other environmental parameters, not by B/Ca itself. If the influence of B/Ca on a calibration is negligible, it follows that combining that calibration with down-core B/Ca can not yield new information about past ocean conditions. In this study, we evaluate existing calibrations and down-core records with the aim of establishing a framework that allows consistent calibration methods to yield accurate proxy reconstructions from the fossil record. While many issues still need to be addressed, B/Ca does respond to seawater chemistry, and does change across some major climate transitions. Because it may provide insight into major carbon cycle perturbations, we make specific recommendations on how to tackle current uncertainties and develop B/Ca into a more robust proxy.