SEDIMENTARY RECORD OF A SHOALING OF THE OXIC ANOXIC INTERFACE IN THE BLACK-SEA

Recent studies of hydrographic, nutrient and geochemical properties in the upper 200 m of the Black Sea have led to the hypothesis that the oxic/anoxic interface (chemocline) has shoaled by approximately 30 m during the past few decades. This conclusion remains disputed, however, because historical hydrographic data are not sufficient to distinguish between short-term variability and a long-term net upward displacement in the position of the chemocline. Sediments deposited on the shelf, where the oxic/anoxic interface impinges on the sea floor, provide a reliable record of the redox conditions of overlying bottom waters. Sediments deposited under anoxic conditions are invariably laminated, whereas sediments deposited under sustained conditions of oxygenated bottom waters are heavily bioturbated. A set of five cores collected along a transect across the shelf in the south central Black Sea was examined for evidence of a shoaling of the oxic/anoxic interface. Results indicate that there has not been a basin-wide 30 m shoaling of the interface during the past approximately 20 years as postulated from hydrographic data, although brief intervals during which the interface was depressed to a position as much as 30 m deeper than at present may have occurred. One core contained a clear record of a shoaling of the interface, as indicated by a transition from bioturbated to laminated sediments, but this transition occurred approximately 200 years ago.