Benthic foraminiferal B/Ca ratios reflect deep water carbonate saturation state

Boron/calcium ratios were measured in four benthic foraminiferal species (three calcitic: Cibicidoides wuellerstorfi, Cibicidoides mundulus, and Uvigerina spp., and one aragonitic: Hoeglundina elegans) from 108 core-top samples located globally. Comparison of coexisting species shows: B/Ca of C. wuellerstorfi > C. mundulus > H. elegans > Uvigerina spp., suggestive of strong “vital effects” on benthic foraminiferal B/Ca. A dissolution effect on benthic B/Ca is not observed. Core-top data show large intra-species variations (50–130 μmol/mol) in B/Ca. Within a single species, benthic foraminiferal B/Ca show a simple linear correlation with deep water Δ[CO32−], providing a proxy for past deep water [CO32−] reconstructions. Empirical sensitivities of Δ[CO32−] on B/Ca have been established to be 1.14 ± 0.048 and 0.69 ± 0.072 μmol/mol per μmol/kg for C. wuellerstorfi and C. mundulus, respectively. The uncertainties associated with reconstructing bottom water Δ[CO32−] using B/Ca in C. wuellerstorfi and C. mundulus are about ± 10 μmol/kg. A preliminary application shows that the Last Glacial Maximum (LGM) B/Ca ratios were increased by 12% at 1–2 km and decreased by 12% at 3.5–4.0 km relative to Holocene values in the North Atlantic Ocean. This implies that the LGM [CO32−] was higher by ∼ 25–30 μmol/kg at intermediate depths and lower by ∼ 20 μmol/kg in deeper waters, consistent with glacial water mass reorganization in the North Atlantic Ocean inferred from other paleochemical proxies.