Barite formation in the Southern Ocean water column

The intensity of past biological activity can be correlated with the occurrence of suspended and sedimented barite (BaSO4). To date, there is a lack of information on the processes controlling the production of pelagic barite. During the EPOS-2 cruise (November-December 1988), situated in the Scotia Sea-Weddell Sea Confluence, suspended water samples were taken for analysis of total Ba by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and for scanning electron microscopy-electron microprobe (SEM-EMP).The vertical profiles of particulate barium generally show a Ba maximum between 200 and 500 m. SEM-EMP investigation on Scotia-Weddell Sea Confluence profiles provide a possible explanation for the origin of this subsurface Ba maximum. In the surface waters (i.e. above 200 m) barite is mainly contained within large bioaggregates. The fact that in the first 10-20m of the water column the barite particles in the bioaggregates appear as amorphous entities without a clear crystalline habit (in contrast to their form in greater depths) probably reflects the evolution from BaSO4 precipitation to barite crystallization. Below the surface layer barite in bioaggregates is present as microparticles with a crystalline habit. Deeper in the water column, below the first few hundred meters, barite crystals occur as free discrete particles. This suggests that in the subsurface zone (below 200m) the "aggregates" are dispersed, possibly as a result of bacterial activity on the organic matrix, thereby releasing the individual barite crystals. As these microcrystals, set free in the water column settle much more slowly than the carrier-aggregate and as barite dissolves relatively slowly, local accumulation of barite can result. In some cases high Ba concentrations are present in the first 10-20m of the water column, occasionally coinciding with biomass parameter maxima. This correlation suggests active precipitation by the living phytoplankton cell (i.e. intravacuolar barite formation), as already observed by others for cultures of marine algae (Pavlovales). Our SEM-EMP investigations on INDIGO 3 and EPOS-2 samples did not reveal any association of barite with identifiable phytoplankton individuals. Thus, SOM-EMP evidence favours passive precipitation in saturated microenvironments as the process involved in formation of pelagic barite