Biotic and Abiotic Controls of the Barium Distribution in the Southern Ocean Water Column

Abstract

Besides mixing, the dissolved barium distribution in the oceanic water column is controlled to a significant extent also by particle-solute interaction. In vi tro uptake experiments involving diatom cultures suggest that passive uptake prevails over active uptake. The overall effect of these processes is that barium behaves as a non-limiting nutrient element with ratios of dissolved over particulate concentrations varying between 100 and 1000. Uptake of barium by plankton particles eventually results in the formation of barium sulphate (baryte). There is increasing evidence that this baryte formation is closely linked with the formation of micro-environments composed of detrital organic matter, in which BaSO4 saturation is eventually reached during the process of bacterial decay (Bishop, 1988; Dehairs et al., 1980). This mechanism sets the link between oceanic baryte and oceanic productivity and as a result oceanic baryte is now increasingly recognised to represent a good proxy for palaeoproductivity (Francois e ta / . , 1995). Using an extensive data set obtained mainly from the Southern Ocean we investigated the different main processes affecting the oceanic barium cycle. For particulate barium in surface waters regional variability can be important and this is clearly associated with variation of planktonic production and biomass such as occurring, for instance, across the Polar Front Zone (PFZ). We observed that on a time scale of approximately three weeks water column stocks of micro-crystalline baryte co-vary with seasonal change in surface water productivity (Dehairs et al. 1997). This phase lag of about three weeks is consistent with the time lapse required for baryte to be formed during in vitro experiments using decaying detritus from a diatom culture. This process Department of Analytical Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, B-1050, Brussels, Belgium