A two-tracer (210Po–234Th) approach to distinguish organic carbon and biogenic silica export flux in the Antarctic Circumpolar Current

We attempt to quantify and qualify the particle export from the surface water of the Antarctic Circumpolar Current during a spring phytoplankton bloom by the simultaneous use of the tracers 210Po and 234Th. We present data from the Southern Ocean JGOFS expedition in 1992 at about 6°W, from the marginal ice zone to the Polar Frontal region. Radionuclide export was calculated with a one-dimensional non-steady-state scavenging model. Rapidly changing activities of 210Pb and 210Po during the phytoplankton bloom and the application of the tracer pair 210Po/210Pb with particle-reactive parent and daughter required a new solution to the non-steady-state scavenging model. The observed fractionation of 210Po and 234Th on particles, dependent on particle composition (POC/biogenic silica ratio), corroborates the known preference of 210Po for cytoplasm. A combination of these two tracers can help to characterize the nature (i.e. organic carbon and biogenic silica content) of the material settling out of the mixed layer, and thus to arrive at a more detailed interpretation of export fluxes than is possible with 234Th alone. In the Polar Front region, where diatoms are dominant in the phytoplankton and where the highest export rates were observed, we find a preferential settling of biogenic silica when heavily silicified diatom species occur. In contrast, POC and biogenic silica are exported with comparable efficiency when diatom species with thinner frustules prevail. The export of biogenic opal and carbon is then closely coupled. In the southern Antarctic Circumpolar Current (sACC), where siliceous organisms are not dominant in the plankton, we find a preferential settling of siliceous material over POC.