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

We present modern B/Ca core-top calibrations for the epifaunal benthic foraminifer Nuttallides umbonifera and the infaunal Oridorsalis umbonatus to test whether B/Ca values in these species can be used for the reconstruction of paleo-Delta[CO32-]. 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 (similar to 85-175 mu mol/mol) are consistently lower than values reported for epifaunal Cibicidoides (Cibicides) wuellerstorfi (130-250 mu mol/mol), though the sensitivity of Delta[CO32-] 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 (3585 mu mol/mol) and sensitivity to Delta[CO32-] (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 Delta[CO32-], whereas O. umbonatus B/Ca appears to be buffered by porewater [CO32-] and suited for constraining long-term drift in seawater B/Ca. (C) 2011 Elsevier B.V. All rights reserved.