Boron/Calcium and Boron Isotopes in planktonic foraminifera: A core-top perspective

Elemental Boron/Calcium (B/Ca) ratios in planktonic foraminifera have been suggested as a paleoceanographic proxy for surface seawater pH and ultimately pCO2 (Yu et al., 2007). In addition to pH, carbonate ion (Foster 2008) and temperature have been proposed to influence the incorporation of boron into calcitic tests. The application of the B/Ca proxy to down-core studies requires that the various controls be resolved. In this study Mg/Ca based thermometry is used to estimate the calcification temperature and compared with the B/Ca ratios of multiple species of planktonic foraminifera from recent core-top sediments. Initial results indicate a positive correlation between B/Ca ratios and Mg/Ca based temperature, similar to previous core-top studies. The sensitivity of B/Ca to temperature is greater in near-surface species (G.ruber and G.sacculifer) compared with thermocline and deeper-dwellers. A significant relationship between B/Ca and the carbonate system (i.e. pH and carbonate) is observed providing encouraging results for the proxy. Preliminary boron isotope results from the same samples as B/Ca will be presented and used to estimate seawater pH. To further ground-truth the core-top results boron isotopes and B/Ca records spanning the last 150 years will used to compare with the instrumental record of recent seawater pH decline.