Carbon isotope offsets between benthic foraminifer species of the genus Cibicides (Cibicidoides) in the glacial sub‐Antarctic Atlantic

Abstract

AbstractEpibenthic foraminifer δ13C measurements are valuable for reconstructing past bottom water dissolved inorganic carbon δ13C (δ13CDIC), which are used to infer global ocean circulation patterns. Epibenthic δ13C, however, may also reflect the influence of 13C‐depleted phytodetritus, microhabitat changes, and/or variations in carbonate ion concentrations. Here we compare the δ13C of two benthic foraminifer species, Cibicides kullenbergi and Cibicides wuellerstorfi, and their morphotypes, in three sub‐Antarctic Atlantic sediment cores over several glacial‐interglacial transitions. These species are commonly assumed to be epibenthic, living above or directly below the sediment‐water interface. While this might be consistent with the small δ13C offset that we observe between these species during late Pleistocene interglacial periods (Δδ13C = −0.19 ± 0.31‰, N = 63), it is more difficult to reconcile with the significant δ13C offset that is found between these species during glacial periods (Δδ13C = −0.76 ± 0.44‰, N = 44). We test possible scenarios by analyzing Uvigerina spp. δ13C and benthic foraminifer abundances: (1) C. kullenbergi δ13C is biased to light values either due to microhabitat shifts or phytodetritus effects and (2) C. wuellerstorfi δ13C is biased to heavy values, relative to long‐term average conditions, for instance by recording the sporadic occurrence of less depleted deepwater δ13CDIC. Neither of these scenarios can be ruled out unequivocally. However, our findings emphasize that supposedly epibenthic foraminifer δ13C in the sub‐Antarctic Atlantic may reflect several factors rather than being solely a function of bottom water δ13CDIC. This could have a direct bearing on the interpretation of extremely light South Atlantic δ13C values at the Last Glacial Maximum.