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

• Main Authors: Jimin Yu, Henry Elderfield
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 2007
• Subjects: boron; carbonate ion concentration; benthic foraminifera; proxy; North Atlantic
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.518.7101; http://www.ldeo.columbia.edu/~peter/Resources/Seminar/readings/Yu %26 Elderfield 2007.pdf

Boron/calcium ratios were measured in four benthic foraminiferal species (three calcitic: Cibicidoides wuellerstorfi, Cibici-doides 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. wuellerstorfiNC. mundulusNH. elegansNUvigerina 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±0.072μmol/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 [CO3 2−] 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.