DECOUPLING OF DELTA-C-13(SIGMA-C02) AND PHOSPHATE IN RECENT WEDDELL SEA DEEP AND BOTTOM WATER - IMPLICATIONS FOR GLACIAL SOUTHERN-OCEAN PALEOCEANOGRAPHY

On a section between 72 degrees S and 42 degrees S and a transect betw een 60 degrees E and 10 degrees E through the Weddell Sea and the sout hernmost eastern Atlantic Ocean, the water column was sampled on 72 st ations, and the stable carbon isotopic composition of total dissolved inorganic carbon (delta(13)C Sigma(CO2)) as well as the stable oxygen isotopic composition of seawater (delta(18)O) was determined. These da ta were compared with potential temperature, salinity, dissolved oxyge n and phosphate data from the same stations. The observed delta(13)C S igma CO2/PO43- relationship in the deep Weddell Sea strongly differs f rom the global Redfield-driven deep water relationship. We attribute t his to enhanced thermodynamic fractionation at sites of bottom water f ormation that decouples the nutrient signal from the delta(13)C Sigma( CO2) signal not only in surface and intermediate water masses but also in deep and bottom water. Different, water-mass specific thermodynami c imprints due to different modes of bottom water formation are assume d to cause the observed deviation from the global delta(13)C Sigma CO2 /PO43- relationship in the deep Weddell Sea. The influence of increase d photosynthetic fractionation, i.e., a more negative than low-latitud e isotopic organic carbon composition, is shown to be minor. As a resu lt, Recent Weddell Sea deep and bottom water delta(13)C Sigma(CO2) is by 0.4-0.5 parts per thousand higher than expected if solely biologic fractionation would occur. A discussion of simple hypotheses of Weddel l Sea deep and bottom water formation during glacial times reveals tha t regardless of what scenario is considered, the thermodynamic imprint on Southern Ocean deep water would increase. This makes it difficult to explain low glacial delta(13)C values observed in benthic foraminif era from the subpolar Southern Ocean as being calcified in Antarctic s ource bottom water and thus is in support of hypotheses looking for ad ditional sites of deep water formation.