Hydrography, nutrients, and carbon pools in the Pacific sector of the Southern Ocean: Implications for carbon flux

We investigated the hydrography, nutrients, and dissolved and particulate c arbon pools in the western Pacific sector of the Antarctic Circumpolar Curr ent (ACC) during austral summer 1996 to assess the region's role in the car bon cycle. Low fCO(2) values along two transects indicated that much of the study area was a sink for atmospheric CO2. The fCO(2) values were lowest n ear the Polar Front (PF) and the Subtropical Front (STF), concomitant with maxima of chlorophyll a and particulate and dissolved organic carbon. The l argest biomass accumulations did not occur at fronts, which had high surfac e geostrophic velocities (20-51 cm s(-1)), but in relatively low velocity r egions near fronts or in an eddy. Thus vertical motion and horizontal advec tion associated with fronts may have replenished nutrients in surface water s but also dispersed phytoplankton. Although surface waters north of the PF have been characterized as a "high nutrient-low chlorophyll" region, low s ilicic acid (Si) concentrations (2-4 muM) may limit production of large dia toms and therefore the potential carbon flux. Low concentrations (4-10 muM Si) at depths of winter mixing constrain the level of Si replenishment to s urface waters. It has been suggested that an increase in aeolian iron north of the PF may increase primary productivity and carbon export. Our results , however, indicate that while diatom growth and carbon export may be enhan ced, the extent ultimately would be limited by the vertical supply of Si. S outh of the PF, the primary mechanism by which carbon is exported to deep w ater appears to be through diatom flux. We suggest that north of the PF, pa rticulate and dissolved carbon may be exported primarily to intermediate de pths through subduction and diapycnal mixing associated with Subantarctic M ode Water and Antarctic Intermediate Water formation. These physical-biolog ical interactions and Si dynamics should be included in future biogeochemic al models to provide a more accurate prediction of carbon flux.