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 Sigma CO2. Recently, attention has been focused on substantial productivity-linked delta(13)C depletions. To examine further the productivity effect on benthic delta(13)C deviations, we present data from the South Atlantic between 15 and 35 degrees 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, delta(13)C Sigma(CO2) 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 delta(13)C(Sigma CO2) values in upwelling areas, a depletion in the delta(13)C Of epibenthic foraminifer calcite is observed, which is most likely explained by the decay of organic matter, reducing the C-13/C-12 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. (C) 1999 Elsevier Science Ltd. Ail rights reserved.