Distribution of barium in the Weddell Gyre: Impact of circulation and biogeochemical processes

The Southern Ocean data base of dissolved barium (Bad) has been augmented with two densely sampled sections across the Weddell Gyre sampled by the icebreaker FS Polarstern during February and March 2005. Bad was found to be relatively high in the surface layer as compared to the adjacent waters north (Antarctic Circumpolar Current) and east (Antarctic Zone of the Indian sector). Compared to the inflowing water into the Weddell Gyre and also to the surface water, the deep water is characterized by a significant Bad enrichment. Depletion of Bad in the surface layer is associated with well-known biogeochemical processes, but we speculate that also sea-ice formation leads to the extraction of Ba from solution via barite (BaSO4) precipitation. The particulate Ba settles down the water column and redissolves in deeper water where undersaturation of barite is prevalent. In the bottom layer, enrichment of Bad occurs, exhibited as a Bad maximum, which is caused by Ba efflux from the sediments. In recently formed Weddell Sea Bottom Water, though, a Bad minimum is observed, imposed by the shelf water component of bottom water, which has relatively low Bad concentration. Like in other Southern Ocean regions, throughout the water column a strong correlation exists between Bad and dissolved silicate, although the relationship is different from that in the Antarctic regions to the east. In a tentative budget, the deep water is found to be supplied with Bad mainly by the surface layer, but also to some extent by the bottom layer. Weddell Sea Deep Water, enriched in Bad, is transferred out of the Weddell Gyre into the ACC as Antarctic Bottom Water, making the gyre a source of Bad to the deep and abyssal world oceans.