Shallow remineralization in the Weddell Gyre

The region influenced by the Polar Front in the Southern Ocean is characterized by relatively high productivity, which is mirrored in strong depletions of 234 Th in the surface water, a good tracer of export production, and by high accumulation rates on the underlying seabed. Farther south, the Weddell Sea is generally considered a low productivity region with very low export fluxes. This finding is based on satellite observations, sediment accumulation rates, trap deployments, and phytoplankton distribution. If this would be true, 234 Th should be close to equilibrium with its parent. However, in a series of high‐resolution transects of 234 Th/ 238 U across the Antarctic Circumpolar Current (ACC), 234 Th was found to be depleted by 10–15% throughout the clear Weddell Gyre, only to reach equilibrium in sea‐ice covered regions of the coastal zone. Vertical profiles showed that the depletion was limited to the upper mixed layer and was balanced by an enrichment of similar magnitude at 100–250m depth. This implies that the export of particles below 250 m is negligible. Such shallow remineralization is in line with the discrepancies between biogenic silica production rates and sediment trap data observed in the Weddell and Ross Seas. These observations in the Weddell Sea are fully consistent with our inverse modeling results for both organic carbon and opal, and they are not inconsistent with TCO 2 and oxygen sections that show a TCO 2 enriched, oxygen reduced shallow subsurface layer. This blue ocean, characterized by upwelling of CO 2 ‐enriched deep waters, supports sufficient productivity to be a net sink for CO 2 to abyssal depths [ Hoppema et al. , 1999]. No record of this productivity and export is stored in the underlying sediment, which has important palaeoceanographic consequences.