Abstract
Although 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.
Highlights
B/Ca and U/Ca of planktonic foraminiferal calcite have previously been suggested to reflect the carbonate chemistry of seawater (Russell et al, 2004; Allen et al, 2011, 2012) and could potentially be used to reconstruct past ocean carbonate chemistry changes (Yu et al, 2007; Foster, 2008)
The range of B/Ca in G. truncatulinoides alone is equal to the combined range of G. ruber (p) and O. universa (∼110 μmol/mol), despite this species living in a more limited range of winter temperatures and [CO23−] (Fig. 1b–c)
Calcification rate and test size We find that B/Ca displays a strong positive correlation with area density, pointing to a calcification control on boron incorporation in G. ruber (p), O. universa and non-encrusted G. truncatulinoides (Fig. 4a)
Summary
B/Ca and U/Ca of planktonic foraminiferal calcite have previously been suggested to reflect the carbonate chemistry of seawater (Russell et al, 2004; Allen et al, 2011, 2012) and could potentially be used to reconstruct past ocean carbonate chemistry changes (Yu et al, 2007; Foster, 2008). Boron exists in seawater as two species, boric acid [B(OH3)] and borate ion [B(OH)−4 ], the proportions of which are pH dependent, (see equilibrium equation (1) below). Increasing pH leads to greater incorporation of B in the CaCO3 lattice due to increasing abundance of aqueous borate, (see equilibrium equation (2) below).
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