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

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