Redox Processes in Anoxic Waters

Abstract

Anoxic waters are defined as those waters that have no dissolved oxygen (Richards 1965). This condition can occur in natural waters when the rate of consumption of oxygen exceeds the supply. The rate of oxidation of organic matter by bacteria is greater than the supply of oxygen from the atmosphere. The supply of O2 below the photic zone is dependent upon diffusion and advection. Anoxia normally occurs in enclosed basins where physical barriers (sills) and density stratification limit the advection of O2 to the deep waters (Grasshoff 1975). There are two types of anoxic basins. The most common occurs because of a strong halocline (salinity gradient) which is the result of a net outflow of low salinity water from a positive estuary. This is shown in Fig. 4.1. The halocline prevents low salinity oxic waters from mixing with the high salinity deep waters. Examples of this type of basin are the Black Sea, the Baltic Sea and many fjords such as the Framvaren in Norway. The second type of basin arises because of a strong thermocline preventing the mixing of surface and deep waters. The Cariaco Trench off the coast of Venezuela is an example of this type of basin. It is a deep trench with a maximum depth of 1400 m. The water is isohaline and isothermal from 600 m to the bottom. It is permanently anoxic below a depth of 350 m. The appearance of H2S above the thermocline is due to mixing. Both basins have a physical obstacle that prevents horizontal mixing of various water masses. In a fjord type basin, a shallow sill prevents the salty seawater rich in O2 from entering the basin and sinking to the bottom.