Sulfate-Chlorinity Ratio in the Black Sea Water and its Variability over the Last 70 Years

A. V. Dubinin✉, M. N. Rimskaya-Korsakova, L. S. Semilova

P. P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russian Federation

✉ e-mail: dubinin_av@mail.ru

Abstract

Purpose. Microbial reduction of sulfate in the anoxic basins leads to a non-conservative behavior of sulfate relative to chlorinity. Sulfate deficiency indicates its consumption and a weak process of water renewal. The sulfate-chlorinity ratio has been repeatedly studied in the Black Sea waters, its 3% deficit was revealed in deep waters in the 1950s. In the light of possible impact of climate change on the water balance of the Black Sea, modern variations in the sulfate-chlorinity ratio are considered and compared with the results of the studies in the 20th century.

Methods and Results. Chlorinity was obtained by potentiometric titration with AgNO3 solution. Sulfates were measured gravimetrically after BaSO4 precipitation from two parallel seawater samples at the stations on the slope and in the center of the sea. The IAPSO seawater standard (Practical Salinity 34.993; Batch P162) was used as a reference sample. Chlorinity increases with depth from 9.77 to 12.32 g/kg with a change in conductometric salinity from 17.72 to 22.33. Sulfate concentration increases from 14.45 mmol/kg at the surface to 17.47–17.52 mmol/kg at the 200–600 m depths, at the bottom it decreases to 17.44 mmol/kg. The relationship between the sulfate concentration and chlorinity is linear up to chlorinity 12 g/kg, below 200 m sulfate concentration does not depend on chlorinity. The sulfate-chlorinity ratio on the sea surface is 0.1420 g/g. It monotonically decreases up to the depth 1200 m where it reaches its minimum value 0.1353 g/g. Below 1200 m, the SO4/Cl value varies within a small range from 0.1356 to 0.1361.

Conclusions. Comparison of data on chlorinity and sulfate concentration over the last 70 years (1952–2021) shows that within the precision of the analyses performed, chlorinity and sulfate concentration remain constant that reflects a stationary character of the Black Sea system over this period.

Keywords

sulfate, chlorinity, sulfate-chlorinity ratio, sulfate reduction, Black Sea

Acknowledgements

The authors are grateful to the staff members of the Southern Branch of IO RAS, V. V. Ocherednik and O. A. Ocherednik for their assistance in collecting material for the present study, as well as to the employees of the Geochemistry Laboratory of IO RAS, D. Yu. Grigoriev and E. N. Zologina. The study was carried out within the framework of the state assignment of IO RAS (theme No. FMWE-2021-0004).

Original russian text

Original Russian Text © A. V. Dubinin, M. N. Rimskaya-Korsakova, L. S. Semilova, 2022, published in MORSKOY GIDROFIZICHESKIY ZHURNAL, Vol. 38, Iss. 5, pp. 530–547 (2022)

For citation

Dubinin, A.V., Rimskaya-Korsakova, M.N. and Semilova, L.S., 2022. Sulfate-Chlorinity Ratio in the Black Sea Water and its Variability over the Last 70 Years. Physical Oceanography, 29(5), pp. 508-523. doi:10.22449/1573-160X-2022-5-508-523

DOI

10.22449/1573-160X-2022-5-508-523

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