Carbon isotope offsets between benthic foraminifer species of the genus Cibicides (Cibicidoides) in the glacial sub‐Antarctic Atlantic. (20th December 2016) - Gottschalk, Julia
Abstract: Epibenthic foraminifer δ 13 C measurements are valuable for reconstructing past bottom water dissolved inorganic carbon δ 13 C (δ 13 CDIC ), 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 CDIC . 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 CDIC . This could have a direct bearing on the interpretation of extremely light South Atlantic δ 13 C values at the Last Glacial Maximum. Key Points: Glacial C. wuellerstorfi and C. kullenbergi δ 13 C offsets in the South Atlantic are observed Comparison of Uvigerina spp. δ 13 C and C. kullenbergi δ 13 C does not indicate strong habitat effects Glacial C. kullenbergi δ 13 C may be biased toward lower δ 13 C, and glacial C. wuellerstorfi δ 13 C may not represent average local conditions