Extent and intensity of the anoxic zone in basins and fjords

Abstract

A model is presented that describes the extent and intensity of the anoxic zone of semi-enclosed basins. Vertical profiles of oxygen and hydrogen sulfide can be modeled by normalizing depth to the sill and bottom depths of the basin, and oxygen and sulfide to the oxygen concentration at sill depth. Profile shapes are controlled by three model parameters that express the ratio of in situ respiration to diffusion, the ratio of oxygen diffusion to advection and the ratio of sulfide production (in oxygen equivalents) to oxygen diffusion. Anoxic basins fall into three categories when compared in terms of their normalized interface depth and normalized bottom sulfide concentrations. Analysis of the three groups suggests that one group represents basins not in equilibrium. The other two groups represent basins in equilibrium; one, with relatively linear sulfide profiles, dominated by sulfide production in the sediments, and the other, with curved sulfide profiles, in which sulfide production in both the water column and sediments is important.