Stable isotope evidence for the Bottom Convective Layer homogeneity in the Black Sea

Abstract

The Black Sea is the largest euxinic basin on the Earth. The anoxic zone consists of the upper part water mass stratified by density, and the lower water mass homogenized relative to density (depth >1750 m), named the Bottom Convective Layer. To assess homogeneity and possible exchange of matter across the upper and lower boundaries of the Bottom Convective Layer, new data on stable isotope composition of S, O and H were obtained. Samples were collected in August 2008 and March 2009 from two stations located in the eastern central part of the Black Sea.

Distribution of δ¹⁸O and δD values of water for the entire water column did not vary seasonally. Appreciable differences were marked for δD value variation in the picnocline area (water depth 200-400 m) and in the BCL 5 m above the bottom that might be caused by penetration of intrusions with elevated portion of shelf modified Mediterranean Water. Observed linear relationship between δ¹⁸O (or δD) and salinity indicates that mixing water and salt occurs at the same time, and the deep water of the Black Sea has two end members: the high-salinity Mediterranean seawater and freshwater input.

In the Bottom Convective Layer, the average δ³⁴S (H₂S) was -40.6 ± 0.5‰ and did not vary seasonally. At the bottom (depth > 2000 m), ³⁴S depletion down to –41.0‰ was observed. Our δ³⁴S (SO₄) data are by 2-3‰ higher than those measured previously for the Bottom Convective Layer. Sulfate from the aerobic zone with δ³⁴S (SO₄) = +21‰ corresponds to ocean water sulfate and that has not been subjected to sulfate reduction. Average δ³⁴S (SO₄) values for depths > 1250 m were found to be +23.0 ± 0.2‰ (1σ). Sulfur isotope composition of sulfate does not change in the Bottom Convective Layer and on its upper and lower boundaries, and does not depend on the season of observation.