Ventilation of the Black Sea anoxic zone: Evidence from the sulfur isotope composition of sulfate

Abstract

Assuming stability of the Black Sea system and conservative behavior of sulfate in relation to salinity outside the bottom convective layer (BCL), the influence of shelf-modified Mediterranean water (SMMW) on the water column of the Black Sea below the core of the cold intermediate layer (CIL) was estimated on the basis of variations in the sulfur isotope composition of sulfate. As a result of construction of the model of mixing of three water masses, it was shown that the SMMW fraction in the area of hydrogen sulfide onset at a salinity of 20.8–20.9 was 5–7 times higher than the amount of water produced by mixing of the CIL and the BCL. The SMMW fraction decreased with depth rapidly and was only 10% at a depth of 1000 m. Significant supply of SMMW to the pycnocline area provided a high renewal rate of water, which prevented accumulation of ³²S-rich sulfate resulted from hydrogen sulfide oxidation.