Benthic stable oxygen and stable carbon isotopes records from the sub-Antarctic Atlantic over several glacial-interglacial transitions


Epibenthic foraminifer d13C measurements are valuable for reconstructing past bottom water dissolved inorganic carbon d13C (d13CDIC), which are used to infer global ocean circulation patterns. Epibenthic d13C, however, may also reflect the influence of 13C-depleted phytodetritus, microhabitat changes, and/or variations in carbonate ion concentrations. Here, we compare the d13C 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 d13C offset that we observe between these species during late Pleistocene interglacial periods (Dd13C=-0.19±0.31 per mil, N=63), it is more difficult to reconcile with the significant d13C offset that is found between these species during glacial periods (Dd13C=-0.76±0.44 per mil, N=44). We test possible scenarios by analysing Uvigerina spp. d13C and benthic foraminifer abundances: 1) C. kullenbergi d13C is biased to light values, either due to microhabitat shifts or phytodetritus effects; and 2) C. wuellerstorfi d13C is biased to heavy values, relative to long-term average conditions, for instance by recording the sporadic occurrence of less depleted deep water d13CDIC. Neither of these scenarios can be ruled out unequivocally. However, our findings emphasize that supposedly epibenthic foraminifer d13C in the sub-Antarctic Atlantic may reflect several factors rather than being a sole function of bottom water d13CDIC, which directly bear on the interpretation of extremely light South Atlantic d13C values at the last glacial maximum

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