A sulfur budget for the Black Sea anoxic zone

A budget for the sulfur cycle in the Black Sea is proposed which incorporat es specific biogeochemical process rates. The average sulfide production in the water column is estimated to be 30-50 Tg yr(-1) occurring essentially in the layer between 500 and 2000 m. About 3.2-5.2 Tg sulfide yr(-1) form d uring sulfate reduction in surface sediments of the anoxic zone. Total sulf ur burial in anoxic sediments of 1 Tg yr(-1) consists of 10-70% (ca. 40-50% is the average) water column formed (syngenetic) component, the rest being diagenetic pyrite. As a maximum, between 3 and 5 Tg yr(-1) contribute sulf ide to the bottom water or diffuse downward in the sediment. About 20-50 Tg yr(-1) sulfide is oxidized mostly at the chemocline and about 10-20% of th is amount (4.4-9.2 Tg yr(-1)) below the chemocline by the oxygen of the Low er Bosphorus Current. A model simulating the vertical distribution of sulfi de in the Black Sea water column shows net consumption in the upper layers down to ca. 500 m, essentially due to oxidation at the chemocline, and net production down to the bottom. On the basis of the calculated budget anoxic conditions in the Black Sea are sustained by the balance between sulfide p roduction in the anoxic water column and oxidation at the chemocline. On av erage the residence time of sulfide in the anoxic zone is about 90-150 yr, comparable to the water exchange time between oxic and anoxic zones. Hydrop hysical control on the sulfur cycle appears to be the main factor regulatin g the extent of anoxic conditions in the Black Sea water column, rather tha n rates of biogeochemical processes. (C) 2001 Elsevier Science Ltd. All rig hts reserved.