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 mu mol/mol) in B/Ca. Within a single species, benthic foraminiferal B/Ca show a simple linear correlation with deep water A[CO32-], providing a proxy for past deep water [CO32-] reconstructions. Empirical sensitivities of [CO32-] on B/Ca have been established to be 1.14 +/- 0.048 and 0.69 +/- 0.072 mu mol/mol per mu mol/kg for C. wuellerstorfi and C. mundulus, respectively. The uncertainties associated with reconstructing bottom water Delta[CO32-] using B/Ca in C. wuellerstorfi and C mundulus are about +/- 10 mu mol/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 similar to 25-30 mu mol/kg at intermediate depths and lower by similar to 20 mu mol/kg in deeper waters, consistent with glacial water mass reorganization in the North Atlantic Ocean inferred from other paleochemical proxies. (C) 2007 Elsevier B.V. All rights reserved.