Decoupling of δ13CΣCO 2 and phosphate in recent Weddell Sea deep and bottom water: Implications for glacial Southern Ocean paleoceanography

Abstract

On a section between 72°S and 42°S and a transect between 60°E and 10°E through the Weddell Sea and the southernmost eastern Atlantic Ocean, the water column was sampled on 72 stations, and the stable carbon isotopic composition of total dissolved inorganic carbon (δ13CΣCO2) as well as the stable oxygen isotopic composition of seawater (δ18O) was determined. These data were compared with potential temperature, salinity, dissolved oxygen and phosphate data from the same stations. The observed δ13CΣCO2/PO43− relationship in the deep Weddell Sea strongly differs from the global Redfield‐driven deep water relationship. We attribute this to enhanced thermodynamic fractionation at sites of bottom water formation that decouples the nutrient signal from the δ13CΣCO2 signal not only in surface and intermediate water masses but also in deep and bottom water. Different, water‐mass specific thermodynamic imprints due to different modes of bottom water formation are assumed to cause the observed deviation from the global δ13CΣCO2/PO43− relationship in the deep Weddell Sea. The influence of increased photosynthetic fractionation, i.e., a more negative than low‐latitude isotopic organic carbon composition, is shown to be minor. As a result, Recent Weddell Sea deep and bottom water δ13CΣCO2 is by 0.4–0.5‰ higher than expected if solely biologic fractionation would occur. A discussion of simple hypotheses of Weddell Sea deep and bottom water formation during glacial times reveals that regardless of what scenario is considered, the thermodynamic imprint on Southern Ocean deep water would increase. This makes it difficult to explain low glacial δ13C values observed in benthic foraminifera from the subpolar Southern Ocean as being calcified in Antarctic source bottom water and thus is in support of hypotheses looking for additional sites of deep water formation.