BIOGEOCHEMISTRY OF THE BLACK SEA ANOXIC ZONE WITH A REFERENCE TO SULPHUR CYCLE

Abstract

The Black Sea is the largest anoxic basin on earth. Hydrogen sulphide inventory in the sea is about 4,600 Tg. This review discusses the evolution and contemporary physical and chemical characteristics of the Black Sea anoxic zone. We present hydrogen sulphide concentrations at different depths in the sea and discuss mechanisms of physical mixing in the anoxic interior. Special emphasis is given to recently discovered bottom convective layer located at depths below 1700--1750 m and concentrations of dissolved sulphide and other chemical species in this zone. The mechanism of double diffusion driven by geothermal heat flux is the main mixing process there. Mesoscale physical dynamics associated with the Main Rim Current has important imlications for the dissolved sulphide spatial distribution and, in general, for ventilation of the anoxic interior. We suggest that, together with the Bosporus influx, the nearshore convergence zone supplies dissolved oxygen into the anoxic zone and leads to the formation of inorganic sulphur intermediates. A detailed analysis of total alkalinity and its components in the Black Sea and other euxinic basins shows unambigously that sulfate reduction is the main process of organic matter anaerobic mineralization in the water column. This result is confirmed by the sulphur budget, whose components are discussed at the end.