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

• Published in: Paleoceanography
• Main Authors: Gottschalk, Julia, Riveiros, Natalia Vazquez, Waelbroeck, Claire, Skinner, Luke C., Michel, Elisabeth, Duplessy, Jean-claude, Hodell, David, Mackensen, Andreas
• Format: Text
• Language: English
• Published: Amer Geophysical Union
• Subjects: envir; geo; Antarctic; Antarc*
• Online Access: https://doi.org/10.1002/2016PA003029; https://archimer.ifremer.fr/doc/00421/53207/54927.pdf; https://archimer.ifremer.fr/doc/00421/53207/54928.pdf; https://archimer.ifremer.fr/doc/00421/53207/

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