STABLE CARBON ISOTOPE RATIOS OF PLANKTON CARBON AND SINKING ORGANIC-MATTER FROM THE ATLANTIC SECTOR OF THE SOUTHERN-OCEAN

The stable carbon isotope composition of particulate organic carbon (POC) from plankton, sediment trap material and surface sediments from the Atlantic sector of the Southern Ocean were determined. Despite low and constant water temperatures, large variations in the delta-C-13 values of plankton were measured. C-13 enrichments of up to 10 parts per thousand coincided with a change in the diatom assemblage and a two-fold increase in primary production. Increased CO2 consumption as a result of rapid carbon fixation may result in diffusion limitation reducing the magnitude of the isotope fractionation. The delta-C-13 values of plankton from sea-ice cores display a relationship with the chlorophyll a content. High 'ice-algae' biomass, in combination with a limited exchange with the surrounding seawater, results in values of about -18 to -20 parts per thousand. It is assumed that these values are related to a reduced CO2 availability in the sea-ice system. In comparison with plankton, sinking krill faeces sampled by traps can be enriched by 2-5 parts per thousand in C-13 (e.g. central Bransfield Strait ). In contrast, the transport of particles in other faeces, diatom aggregates or chains results in minor isotope changes (e.g. Drake Passage, Powell Basin, NW Weddell Sea). A comparison between the delta-C-13 values of sinking matter and those of surface sediments reveals that C-13 enrichments of up to 3-4 parts per thousand may occur at the sediment-water boundary layer. These isotopic changes are attributed to high benthic respiration rates,