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 [CO (sub 3) (super 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 ( approximately 85-175mu mol/mol) are consistently lower than values reported for epifaunal Cibicidoides (Cibicides) wuellerstorfi (130-250mu mol/mol), though the sensitivity of Delta [CO (sub 3) (super 2-) ] on B/Ca in N. umbonifera (1.23+ or -0.15) is similar to that in C. wuellerstorfi (1.14+ or -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 (35-85mu mol/mol) and sensitivity to Delta [CO (sub 3) (super 2-) ] (0.29+ or -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 [CO (sub 3) (super 2-) ], whereas O. umbonatus B/Ca appears to be buffered by porewater [CO (sub 3) (super 2-) ] and suited for constraining long-term drift in seawater B/Ca. Abstract Copyright (2011) Elsevier, B.V.