Hydrogen Sulfide in the Black Sea

Abstract

The Black Sea hydrogen sulfide inventory is about 4 600×1012 g, which makes this sea the largest anoxic basin on earth. Anoxic conditions in the basin have been established 7 500 years ago. This review presents the contemporary inventory of dissolved sulfide and sulfur intermediates and discusses mechanisms of physical mixing in the anoxic interior. Special emphasis is given to concentrations of dissolved sulfide and other chemical species in the bottom convective layer located at depths below 1 700–1 750 m. The width and concentrations of dissolved sulfide in the bottom layer are directly proportional to the heat flow from the bottom. The mechanism of double diffusion driven by geothermal heat flux is the main mixing process in bottom waters. Hydrogen sulfide production in the water column by sulfate-reducing bacteria is the main source of dissolved sulfide, and sulfate reduction is the dominant process of organic matter mineralization in the Black Sea anoxic zone. Sulfur budget calculations suggest that Bosporus flux cannot be considered to be the major sulfur sink and factor for deep basin ventilation. Mesoscale physical dynamics along the periphery of the basin as well as pycnocline erosion during exceptionally severe winters are probably major ventilation mechanisms for the anoxic zone. The sulfur isotopic studies of the water column also support the importance of ventilation processes below the oxic/anoxic interface. Physical mixing processes and global climate change impact on the thermohaline structure of the Black Sea water column control the magnitude and direction of the processes within the sulfur cycle.