UPPER BOUNDARY OF HYDROGEN-SULFIDE - IMPL ICATIONS FOR THE NEPHELOID REDOX LAYER IN WATERS OF THE CAUCASIAN SLOPE OF THE BLACK-SEA

A detailed description of the distribution patterns of oxygen, hydroge n sulfide, ammonium, and dissolved manganese in the interface region b etween oxic and anoxic waters is presented. The relationships of these parameters with the depth and density of the nepheloid redox layer (N RL) we:re studied. The calculated average vertical gradients of hydrog en sulfide (0.71 mu M/m), ammonium (0.19 mu M/m), and dissolved mangan ese (0.16 mu M/m) and their corresponding electron fluxes indicate tha t the major reductants (of which H2S is the dominant) may make unequal contributions to chemosynthetic production. This study and published data as well indicate that the NRL is related to biochemical redox pro cesses that are carried out by autotrophic microorganisms involved in the geochemical cycles of sulfur, nitrogen, and manganese (as well as iron and methane). Accordingly, the major contribution to the nepheloi d component of the redox zone is made by bacterial biomass, with hydro us manganese dioxide being the of second most important factor. The ro le of elemental sulfur in the NRL is insignificant.