Shallow remineralization in the Weddell Gyre

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The region influenced by the Polar Front in the Southern Ocean is characterized by relatively high productivity, which is mirrored instrong depletions of 234Th in the surface water, a good tracer of export production, and by high accumulation rates on the underlyingseabed. Farther south, the Weddell Sea is generally considered a low productivity region with very low export fluxes. This finding is basedon satellite observations, sediment accumulation rates, trap deployments, and phytoplankton distribution. If this would be true, 234Thshould be close to equilibrium with its parent. However, in a series of high-resolution transects of 234Th/238U across the AntarcticCircumpolar Current (ACC), 234Th was found to be depleted by 10-15% throughout the clear Weddell Gyre, only to reach equilibrium insea-ice covered regions of the coastal zone. Vertical profiles showed that the depletion was limited to the upper mixed layer and wasbalanced 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 observedin the Weddell and Ross Seas. These observations in the Weddell Sea are fully consistent with our inverse modeling results for bothorganic carbon and opal, and they are not inconsistent with TCO2 and oxygen sections that show a TCO2 enriched, oxygen reducedshallow subsurface layer. This blue ocean, characterized by upwelling of CO2-enriched deep waters, supports sufficient productivity tobe a net sink for CO2 to abyssal depths [Hoppema et al., 1999]. No record of this productivity and export is stored in the underlyingsediment, which has important palaeoceanographic consequences.

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Usbeck, R. , Rutgers v. d. Loeff, M. , Hoppema, M. and Schlitzer, R. (2002): Shallow remineralization in the Weddell Gyre , Geochemistry geophysics geosystems, Vol. 3, No. 1.1029/2001GC000182, 10 .

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