Iron in Oxidative and Reduction Processes in Marine Sands

The iron content and the ratio of bivalent to total iron in the labile acid-soluble fraction of iron were studied in sublittoral sands of Vostok Bay, Sea of Japan. Iron oxidation and reduction rates in sand sediment were measured in the laboratory. Trivalent iron almost always prevailed in the acid-extractable fraction in the natural environment. The most oxidized state of iron occurred in spring and was associated with a low water temperature and a high seawater oxygen content; the most reduced iron was found in fall during periods of low hydrodynamics and low oxygen concentration. In sand, iron is mainly reduced by bacteria; this process is slow and can be inhibited by adding chloroform. Oxidation of iron is mainly a chemical process and cannot be stopped by chloroform. In sand, the content of redox equivalents such as trivalent iron is much greater than in dissolved oxygen of pore water. It is assumed that labile iron in sands acts as a redox buffer, is oxidized by dissolved pore water oxygen at the periods of advective mixing, and is slowly reduced by benthic bacteria during anaerobic conditions.