Age models, isotopes, and calcium carbonate of two sediment cores from the Southern Ocean

We used benthic foraminiferal assemblages and the stable carbon isotopic composition of benthic foraminiferal tests to interpret glacial-to-interglacial contrasts in two gravity cores from the lower bathyal Antarctic continental margin at 69°S, and the abyssal Agulhas Basin at 43°S. As a Recent analogue, sediment surface samples from an eastern Atlantic Ocean and Weddell Sea transect between 20° and 70°S were discussed. In the investigated area, the benthic foraminiferal assemblages reflect both the ocean circulation and surface productivity. Also at most stations from a belt with seasonally high surface productivity between 48°S and 55°S, the d13C values of epibenthic Cibicidoides, including F. wuellerstorfi are depleted relative to the d13CsumCO2 of the bottom water and hence do not follow the 1:1 relationship established from more northern areas. This bears implications for the interpretation of large glacial/interglacial d13C shifts from the Southern Ocean: Significant parts of this shift can be caused by a northward migration of high productivity belts associated with the Polar Front and the winter sea-ice limit rather than indicating nutrient-rich glacial Southern Ocean deep and bottom water. During interglacial climatic optima, seasonally open surface water accompanied by relatively high opal and very low carbonate accumulation characterizes the Antarctic continental margin environment. The Recent benthic foraminiferal fauna indicates moderate productivity, but during peak warm periods (18O stages: 11, 9, 7.5, 7.3, 5.5 and 1.1) very low numbers of benthic foraminifera are inferred to represent maximum organic matter fluxes with severe calcite dissolution on the sea floor. Equally high d13C values in surface and bottom water as inferred from planktic and benthic foraminifera, may indicate deep convection and bottom water formation during interglacials. In contrast, during glacials, very low opal accumulation, moderate carbonate accumulation, a benthic fauna that is presently associated with low productivity, as well as different benthic and planktic d13C values are consistent with both a reduced primary productivity and a stratified water column, suggesting suppressed bottom water generation. In the Agulhas Basin high carbonate and low organic carbon accumulation reflect the late Holocene position of the site investigated well north of the present-day Polar Front. Low Holocene d13C values of 0.3 per mil and a benthic foraminiferal fauna that indicates a southern calcite corrosive bottom water mass is in agreement with the injection of North Atlantic Deep Water into Circumpolar Deep Water at intermediate depths, which does not affect bottom waters of this basin. During glacial periods, a specific southern fauna, associated with high productivity today, low carbonate, high sediment and organic carbon accumulation, and by 1.1 per mil lower d13C values indicate a bottom water mass of southern origin, a northward shift of the high productivity belt by 7° latitude, and strongly diminished injection of NADW into the Southern Ocean