Modeling redox cycling across the suboxic–anoxic interface zone in the Black Sea

The reactions controlling the suboxic–anoxic interface structure in the Black Sea are investigated with a prognostic, one-dimensional vertically resolved diffusion-reaction model involving O2, NO3−, NH4+, HS−, S0, Mn2+, MnO2. All reactions are expressed in a second-order form, and values for the rate constants are estimated from laboratory and field measurements made during the 1988 RV Knorr expedition. The model successfully simulates the vertical profiles of O2, N, S and Mn species in the region between upper and lower boundaries of the model, which were specified at depths corresponding to σt∼ 15.50 kg/m3 and σt∼16.50 kg/m3. The model identifies an approximately 30 m thick suboxic layer with oxygen concentrations less than 5 μM and zero sulfide concentrations between σt∼15.55 kg/m3 and σt∼16.05 kg/m3. Dissolved oxygen decreases to trace concentrations above the zone of nitrate reduction. Hydrogen sulfide begins to increase downward into deeper levels of the anoxic pool starting at σt∼16.0 kg/m3, where nitrate becomes undetectable. Dissolved manganese and ammonium also increase beneath the suboxic layer. The position at which sulfide concentrations appear coincides with the particulate manganese peak, reflecting the paramount role of manganese cycling in the redox processes. This structure is found to have a fairly persistent character for a wide range of rate constants. Oxidation reactions by oxygen alone are not sufficient to provide a realistic interface structure in the absence of particulate manganese formed by oxidation of Mn2+ by NO3−. A transient lateral oxygen supply into sulfide rich waters alters the anoxic–suboxic structure by rapidly depleting local sulfide concentrations at the depths of oxygen injection.