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

The reactions controlling the suboxic-anoxic interface structure in the Bla ck Sea are investigated with a prognostic, one-dimensional vertically resol ved diffusion-reaction model involving O-2, NO3-, NH4+, HS-, S-0, Mn2+, MnO 2. All reactions are expressed in a second-order form, and values for the r ate constants are estimated from laboratory and field measurements made dur ing the 1988 RV Knorr expedition. The model successfully simulates the vert ical profiles of O-2, N, S and Mn species in the region between upper and l ower boundaries of the model, which were specified at depths corresponding to sigma (t) similar to 15.50 kg/m(3) and sigma (t) similar to 16.50 kg/m(3 ). The model identifies an approximately 30 m thick suboxic layer with oxyg en concentrations less than 5 muM and zero sulfide concentrations between s igma (t) similar to 15.55 kg/m(3) and sigma (t) similar to 16.05 kg/m(3). D issolved 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 sigma (t) similar to 16.0 kg/m(3), where ni trate becomes undetectable. Dissolved manganese and ammonium also increase beneath the suboxic layer. The position at which sulfide concentrations app ear 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. O xidation reactions by oxygen alone are not sufficient to provide a realisti c interface structure in the absence of particulate manganese formed by oxi dation of Mn2+ by NO3-. A transient lateral oxygen supply into sulfide rich waters alters the anoxic-suboxic structure by rapidly depleting local sulf ide concentrations at the depths of oxygen injection. (C) 2000 Elsevier Sci ence Ltd. All rights reserved.