South Atlantic and benthic foraminifer delta C-13 deviations: implicationsfor reconstructing the Late Quaternary deep-water circulation

Benthic foraminifer species Cibicidoides wuellerstorfi and related genera a re assumed to secrete calcite very close to the carbon isotope values of th e ambient bottom-water Sigma CO2. Recently, attention has been focused on s ubstantial productivity-linked delta(13)C depletions. To examine further th e productivity effect on benthic delta(13)C deviations, we present data fro m the South Atlantic between 15 and 35 degrees S, including water samples f rom 10 hydrographic stations and related surface-sediment measurements on C . wuellerstorfi. We compare open-ocean data with observations in the Namibi a Upwelling area. As a result, delta(13)C Sigma(CO2) values as well as phos phate concentrations in water samples of the upwelling realm differ signifi cantly from those of the open-ocean realm at least in the upper and mid-dep th water masses (SACW, AAIW, UCDW). However, deviations from the Redfield f ractionation, caused by air-sea fractionation, remain constant within each water mass, which means that the carbon isotope changes toward upwelling ar eas are exclusively determined by biological cycling. In addition to lower delta(13)C(Sigma CO2) values in upwelling areas, a depletion in the delta(1 3)C Of epibenthic foraminifer calcite is observed, which is most likely exp lained 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, wuelle rstorfi shells of highly productive areas. The paleoceanographic implicatio n of this effect for reconstructing the Late Quaternary deep-water circulat ion is discussed using carbon isotope records of several sediment cores wit hin and outside the Namibia upwelling area. (C) 1999 Elsevier Science Ltd. Ail rights reserved.