The behavior of barium in anoxic marine waters

Abstract

The present day distributions of Ba in the water columns at three anoxic marine sites, namely the Cariaco Trench, Framvaren Fjord, and Black Sea, are presented. Dissolved Ba levels generally increase with depth, ranging from 45–85, 64–280, and 180–460 nM in surface and bottom waters for the three basins, respectively. Small maxima are observed in the vicinity of the in redox interface in both the Framvaren Fjord and Black Sea. Comparison of the dissolved and particulate Ba, Fe, and Mn distributions show that the maxima do not result from adsorption onto freshly precipitated Fe and/or Mn oxyhydroxides. As for the open ocean, Ba cycling in all three basins is dominated by its uptake, primarily in the form of barite, into particulate matter associated with productivity in surface waters, followed by its regeneration at depth or in the sediments. Microbiological activity near the redox interface promotes the breakdown of settling particulate matter and the release of barite just above the O 2 H 2S interface in the Black Sea, and most likely in the Framvaren Fjord, thus providing in part for the observed maxima. Dissolution of such barite in the marginal sediments of these basins probably also contributes to the maxima. Thermodynamic calculations show deep Black Sea Ba concentrations exceed saturation with respect to pure barite by at least a factor of 2. However, the uniformity of the deep water concentrations suggests thermodynamic control by some phase; it is likely that impurities, incorporated into barite during its rapid formation near the surface in microenvironments provided by decaying organisms, are responsible for the levels observed. Additional factors controlling the distributions of Ba in each basin are also discussed.