Stable Carbon Isotopes in Benthic Foraminifera: Proxies for Deep and Bottom Water Circulation and New Production

Abstract

The δ13C values recorded in benthic foraminiferal calcareous tests reflect the δ13C values of the dissolved inorganic carbon (δ13C∑CO2) of ambient deep and bottom water masses and as such are widely used as a nutrient proxy to reconstruct deep ocean paleocirculation. This paper summarizes new results on the thermodynamicly caused decoupling of nutrients and the δ13C∑CO2 signal in the Weddell Sea and its potential influence on paleoceanographic reconstruction. In addition, new data are presented on high latitude benthic foraminiferal (Cibicidoides spp.) δ13C and bottom water δ13C∑CO2 values, which prove a significant deviation from the ideal one-to-one relationship used in paleoceanography. Foraminiferal δ13C values by 0.2 - 0.6 %o lower than the ambient bottom water δ13C∑CO2 values coincide with areas of highly seasonal productivity, the development of plankton blooms, and subsequent rapid sedimentation of phytodetritus. On the one hand the phytodetritus triggers foraminiferal calcification and reproduction, but at the sametime it produces a 13C depleted microenvironment at the sediment/water interface. Benthic δ13C values of Cibicidoides spp. by 0.2 - 0.4 %o higher than bottom water values are found close to the Antarctic and the Arctic Eurasian continental margins. Therefore, we here speculate that during processes of bottom water formation in polynyas and brine formation on the continental shelves, water masses with enriched δ13C∑CO2 values spill down the continental slopes and influence the benthic foraminiferal δ13C record. Compensation for an artificial lowering of the Cibicidoides δ13C signal in areas of seasonally high primary production in the South Atlantic Ocean, allows for a paleoceanographic interpretation of very high glacial to interglacial amplitudes of up to 1.6 %o in benthic δ13C records in agreement with reconstructions based on other proxies. In particular, low Antarctic Cd values and regionally low δ13C values in glacial benthic foraminifera now can be consistently interpreted as indicating relatively low nutrient concentrations and thus relatively young Southern Ocean deep and bottom water masses compared to glacial Pacific deep water.