Calcification and growth processes in planktonic foraminifera complicate the use of B/Ca and U/Ca as carbonate chemistry proxies

AbstractAlthough boron and uranium to calcium ratios (B/Ca, U/Ca) in planktonic foraminifera have recently received much attention as potential proxies for ocean carbonate chemistry, the extent of a carbonate chemistry control on these ratios remains contentious. Here, we use bi-weekly sediment trap samples collected from the subtropical North Atlantic in combination with measured oceanographic data from the same location to evaluate the dominant oceanographic controls on B/Ca and U/Ca in three depth-stratified species of planktonic foraminifera. We also test the control of biological, growth-related, processes on planktonic foraminiferal B and U incorporation by using foraminifer test area density (μg/μm2) (a monitor of test thickness) and test size from the same samples. B/Ca and U/Ca show little or no significant correlation with carbonate system parameters both within this study and in comparison with other published works. We provide the first evidence for a strong positive relationship between area density (test thickness) and B/Ca, and reveal that this is consistent in all species studied, suggesting a likely role for calcification in controlling boron partitioning into foraminiferal calcite. This finding is consistent with previous observations of less efficient discrimination against trace element ‘impurities’ (such as B), at higher calcification rates. We observe little or no dependency of B/Ca on test size. In marked contrast, we find that U/Ca displays a strong species-specific dependency on test size in all species, but no relationship with test thickness, implicating some other biological control (possibly related to growth), rather than a calcification control, on U incorporation into foraminiferal calcite. Our results caution against the use of B/Ca and U/Ca in planktonic foraminifera as reliable proxies for the ocean carbonate system and recommend that future work should concentrate on improving the mechanistic understanding of how planktonic foraminifer calcification and growth rates regulate boron and uranium incorporation into the test