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

• Published in: Earth and Planetary Science Letters
• Main Authors: Yu, J., Elderfield, H.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2007
• Subjects: 01 - Climate Change and Earth-Ocean Atmosphere Systems; North Atlantic
• Online Access: http://eprints.esc.cam.ac.uk/821/; http://eprints.esc.cam.ac.uk/821/1/yuelderfield_EPSL2007.pdf; https://doi.org/10.1016/j.epsl.2007.03.025

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 not, vert, similar 25–30 μmol/kg at intermediate depths and lower by not, vert, similar 20 μmol/kg in deeper waters, consistent with glacial water mass reorganization in the North Atlantic Ocean inferred from other paleochemical proxies.