A core-top calibration of B/Ca in the benthic foraminifers Nuttallides umbonifera and Oridorsalis umbonatus: A proxy for Cenozoic bottom water carbonate saturation

Abstract We present modern B/Ca core-top calibrations for the epifaunal benthic foraminifer Nuttallides umbonifera and the infaunal Oridorsalis umbonatu s to test whether B/Ca values in these species can be used for the reconstruction of paleo-Δ[CO 3 2− ]. O. umbonatus originated in the Late Cretaceous and remains extant , whereas N. umbonifera originated in the Eocene and is the closest extant relative to Nuttallides truempyi , which ranges from the Late Cretaceous through the Eocene. We measured B/Ca in both species in 35 Holocene sediment samples from the Atlantic, Pacific and Southern Oceans. B/Ca values in epifaunal N. umbonifera (~ 85–175 μmol/mol) are consistently lower than values reported for epifaunal Cibicidoides (Cibicides) wuellerstorfi (130–250 μmol/mol), though the sensitivity of Δ[CO 3 2− ] on B/Ca in N. umbonifera (1.23 ± 0.15) is similar to that in C. wuellerstorfi (1.14 ± 0.048). In addition, we show that B/Ca values of paired N. umbonifera and its extinct ancestor, N. truempyi, from Eocene cores are indistinguishable within error. In contrast, both the B/Ca (35–85 μmol/mol) and sensitivity to Δ[CO 3 2− ] (0.29 ± 0.20) of core-top O. umbonatus are considerably lower (as in other infaunal species), and this offset extends into the Paleocene. Thus the B/Ca of N. umbonifera and its ancestor can be used to reconstruct bottom water Δ[CO 3 2− ], whereas O. umbonatus B/Ca appears to be buffered by porewater [CO 3 2− ] and suited for constraining long-term drift in seawater B/Ca.