THE SUBOXIC TRANSITION ZONE IN THE BLACK SEA

Abstract

Abstract The Black Sea is a classic marine anoxic basin. It has an oxygenated surface layer overlying a sulfide containing (anoxic) deep layer. At the interface between these layers there is a suboxic layer in which both oxygen and sulfide are extremely low and have no perceptible vertical gradients. This condition has evolved because of the superposition of the flux of organic matter which consumes oxygen during respiration on the strong density stratification on the water column. The density stratification is strong because water with high salinity enters the Black Sea from the Bosporus Strait and mixes with overlying cold intermediate layer (CIL) water that forms in the winter on the northwest shelf and in the center of the western and eastern gyres. The rate of CIL formation is also variable over 5 to 10 year periods in response to climate variability on that same time interval. This variability appears to be driven by the North Atlantic Oscillation. This mixture of Bosporus outflow and entrained cold intermediate layer water results in formation the Bosporus Plume which ventilates the layers of the Black Sea deeper than the CIL. New data about the biogeochemical distributions (oxygen, sulfide, nitrate and ammonium) were obtained during R/V Knorr research cruises in 2001 and 2003. The distributions in the upper layers reflect a classic example of the connection between climate forcing, physical regime, chemical fluxes and biological response.