Carbon export during the Spring Bloom at the Antarctic Polar Front, determined with the natural tracer 234Th

Profiles of particulate and dissolved 234Th were obtained during the JGOFS Southern Ocean expedition on R.V. Polarstern during October/November 1992. Measurements were made on three transects across the Antarctic Circumpolar Current, from the Polar Front (PF) in the north to the Weddell Sea/ACC boundary in the south. The dissolved 234Th238U ratio in surface waters gradually decreased during the development of the plankton bloom at the Polar Front. In the period between the first two transects, the 234Th activity removed from the dissolved phase had shifted to particles that had been produced, and as a result, the total activity ratio remained unchanged. The decrease in dissolved 234Th corresponds with decreases in dissolved nutrients and PC02, and with increases in chlorophyll and plankton biomass. uring the third transect was the total activity of 234Th clearly reduced. The 234Th activity missing in the upper 100 m amounted to approximately 6 × 104 dpm m-2 at the Polar Front. In the 22-day period between the second and third transect, the 234Th export flux averaged 3200 dpm m−2 day−1. The ratio of organic carbon to 234Th on suspended particles was lower near the Polar Front than to the south, which we attribute to the higher abundance of empty diatom frustules at the Polar Front. With an average Corg/234Th ratio on suspended particles in surface water of about 20 μmol dpm−1, and assuming that the Corg/234 Th ratio on exported particles is 30–60% of this value, we estimate that 0.43–0.86 mol C m−2 had been removed over the 22-day period from the surface ocean by sinking particles. production was negligible in the Antarctic Zone including the ice edge, but during the later stage of the bloom in the Polar Front region, it amounted to 12–24% of primary production or 25–50% of the net CO2 uptake as estimated from a CO2 budget.