|The biochemistry of barium in the Southern Ocean|
Dehairs, F.A.; Goeyens, L. (1989). The biochemistry of barium in the Southern Ocean, in: Caschetto, S. (Ed.) Belgian scientific research programme on Antarctica: scientific results of phase I (10/1985-01/1989): 2. A: Marine geochemistry; B: Marine geophysics. pp. 08/1-100
In: Caschetto, S. (Ed.) (1989). Belgian scientific research programme on Antarctica: scientific results of phase I (10/1985-01/1989): 2. A: Marine geochemistry; B: Marine geophysics. Belgian Science Policy Office: Brussel. 184 pp., more
In this work some aspects of the barium biogeochemistry in the Southern Ocean were studied and the usefulness of dissolved barium as a tracer of new bottom and deep water formation and of particulate barium-barite as a tracer of passed biological activity was investigated. The data presented were obtained as a result of our participation to the INDIGO 3 campaign in the Indian Ocean sector (January and February, 1987). Dissolved barium was measured directly on seawater using ICP-OES techniques tested for reproducibility and accuracy. Occasional decreased barium concentrations in deep and bottorn water of the Prydz Bay area, indicate new bottom and deep water formation to occur. New bottom water was detectable on the shelf and slope for stations within Wild Canyon. In addition, an intermediate layer with slightly decreased dissolved barium content was observed to extend below the salinity maximum associated with the North Atlantic Deep Water core, indicating that during formation of new bottom water on the shelf, part of this water spreads out from the slope at intermediate depths. This intermediate layer occurs over most of the area south of the Polar Front. The presence of this intermediate layer, although well marked on the barium-salinity plots, is not always identified on the Tpot - salinity and the silicate -salinity diagrams. Furthermore, between the Antarctic continental slope and the Kerguelen Plateau, evidence was found of important local stocks of "new" (i.e. with decreased barium and silicate contents) bottom water. This situation stands in contrast to the limited amount (in terms of vertical and spatial extension) of new bottorn water observed on the shelf. It suggests (1) the existence of other more important sites of bottom water formation along the continent and of a specific bottom topography allowing this new bottom water to be channeled to the site where we observed it and /or (2) the process of new bottom water formation to be discontinuous. More research is needed to clarify the complex hydrology due to bottom and deep water formation in the Prydz Bay area, but our data show the usefulness of dissolved barium as an additional tool for resolving this complex hydrology. SEM- EMP investigations show pariculate barium to be carried mainly by discrete microcrystals of barite. This is consistent with earlier observations for other oceans. Total particulate barium was also measured by ICP-OES following a metaborate fusion and pellet redissolution in nitric acid medium. The main feature of the vertical particulate barium profiles is the coincidence of a maximum with the oxygen minimum. In the oxygen minimum region the oxygen decrease appears to be significantly anticorrelated with barite. This observation could be interpreted by considering the oxygen minimum layer to develop originally by advection to, and upwelling at the Divergence of deep oxygen poor water and subsequent spreading of this water to the North and North -East, following the general flow of the Antarctic Circumpolar Current. Sedimentation and decay of biogenic particles and oxidation of organic matter produced in local surface waters sustains an input of discrete barite crystals in the oxygen minimum layer and induces a local consumption of oxygen. Downstream, within the oxygen minimum layer these processes result in the increase of barite content and the decrease of oxygen content. Evidence is thus given for (1) the oxygen minimum layer to be the result, both of advection and local processes of production - consumption and (2) water in the oxygen minimum to flow in North and North -Eastdirection. The involvement of biological activity in barite production is clear. Evidence was found that this involvement is expressed through (1) a passive production mechanism, whereby heterotrophic oxidation of organic detritus creates local (i.e. within microenvironments) conditions of BaSO4 precipitation. and /or (2) an active production mechanism, whereby phytoplankton secretes intravacuolar barite. Passive production of barite is sustained by the observation in the euphotic layer of a positive relationship between the barite content and the remineralization rate of ammonium by heterotrophs. Active production of barite is sustained by the concurrence in the euphotic layer of occasional extreme barite contents with increased Chlorophyll a and decreased nitrate contents. The barite produced in the euphotic layer is carried downwards within larger particles (i.e. biogenic aggregates, fecal pellets). The largest fraction of these carriers breaks open a few hundred meter deeper, thereby releasing smaller particles whose transport is now mainly controlled by advection. The released discrete barite crystals tend to accumulate as a result of their conservative character relative to organic matter that is oxidized. Instantaneous rates of measured ammonium remineralization were observed to depend on oxygen content of the water. In the oxygen minimum layer and below remineralization rates therefore are inversely correlated with barite content. In the euphotic layer, however, barite appears to be positively correlated with instantaneous heterotrophic activity and also with phytoplankton biomass. It can thus be stated that in the euphotic layer barite occurrence is closely in phase with biological activity, while in the oxygen minimum water it is not but rather reflects and integrates former biological activity.