South Atlantic and benthic foraminifer δ13C deviations: implications for reconstructing the Late Quaternary deep-water circulation

Benthic foraminifer species Cibicidoides wuellerstorfi and related genera are assumed to secrete calcite very close to the carbon isotope values of the ambient bottom-water ΣCO2. Recently, attention has been focused on substantial productivity-linked δ13C depletions. To examine further the productivity effect on benthic δ13C deviations, we present data from the South Atlantic between 15 and 35°S, including water samples from 10 hydrographic stations and related surface-sediment measurements on C. wuellerstorfi. We compare open-ocean data with observations in the Namibia Upwelling area. As a result, δ13CΣCO2 values as well as phosphate concentrations in water samples of the upwelling realm differ significantly from those of the open-ocean realm at least in the upper and mid-depth water masses (SACW, AAIW, UCDW). However, deviations from the Redfield fractionation, caused by air–sea fractionation, remain constant within each water mass, which means that the carbon isotope changes toward upwelling areas are exclusively determined by biological cycling. In addition to lower δ13CΣCO2 values in upwelling areas, a depletion in the δ13C of epibenthic foraminifer calcite is observed, which is most likely explained by the decay of organic matter, reducing the 13C/12C ratio in the pore water and influencing the carbon isotopic composition of the C. wuellerstorfi shells of highly productive areas. The paleoceanographic implication of this effect for reconstructing the Late Quaternary deep-water circulation is discussed using carbon isotope records of several sediment cores within and outside the Namibia upwelling area.