A core top assessment of proxies for the ocean carbonate system in surface-dwelling foraminifers

We have assessed the reliability of several foraminifer-hosted proxies of the ocean carbonate system (delta(11) B, B/Ca, and U/Ca) using Holocene samples from the Atlantic and Pacific oceans. We examined chemical variability over a range of test sizes for two surface-dwelling foraminifers (Globigerinoides sacculifer and Globigerinoides ruber). Measurements of delta B-11 in G. ruber show no significant relationship with test size in either Atlantic or Pacific sites and appear to provide a robust proxy of surface seawater pH. Likewise there is no significant variability in the delta B-11 of our Atlantic core top G. sacculifer, but we find that delta B-11 increases with increasing test size for G. sacculifer in the Pacific. These systematic differences in delta B-11 are inferred to be a consequence of isotopically light gametogenic calcite in G. sacculifer and its preferential preservation during postdepositional dissolution. The trace element ratio proxies of ocean carbonate equilibria, U/Ca and B/Ca, show systematic increases in both G. ruber and G. sacculifer with increasing test size, possibly as a result of changing growth rates. This behavior complicates their use in paleoceanographic reconstructions. In keeping with several previous studies we find that Mg/Ca ratios increase with increasing size fraction in our wellpreserved Atlantic G. sacculifer but not in G. ruber. In contrast to previous interpretations we suggest that these observations reflect a proportionally larger influence of compositionally distinct gametogenic calcite in small individuals compared to larger ones. As with delta B-11 this influences G. sacculifer but not G. ruber, which has negligible gametogenic calcite.