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

Abstract

We have assessed the reliability of several foraminifer‐hosted proxies of the ocean carbonate system ( δ 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 δ 11 B 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 δ 11 B of our Atlantic core top G. sacculifer , but we find that δ 11 B increases with increasing test size for G. sacculifer in the Pacific. These systematic differences in δ 11 B 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 well‐preserved 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 δ 11 B this influences G. sacculifer but not G. ruber , which has negligible gametogenic calcite.