Sulfur speciation and isotopic composition in waters and sediments from river and seawater mixing zones

Studies of sulfur behavior in the water column and in sediments in river and seawater mixing zone were conducted in three areas of the Black and Azov Seas. These investigations showed constancy of sulfate concentrations versus chlorinity. Sulfur isotope composition in sulfates of surface, bottom, and pore waters depended on sulfate contents and salinity. The dependence was complicated by partial sulfate depletion in pore water due to bacterial sulfate reduction and also by alteration of isotope composition. Surface sediments in mixing zones are characterized by intensive sulfate reduction, great variability of content and isotopic composition of reduced sulfur, and a low mean isotopic fractionation factor of sulfur.

Isotope composition of sulfur, oxygen and carbon in Black Sea sediments

Inversion of isotopic composition in the SO4(2-)-H2S system is shown to be universal in Neoeuxine sediments and an explanation of its occurrence is proposed. Change in isotopic composition of sulfate sulfur in Black Sea waters over last 10-15 thousand years is reconstructed. Periods of alteration between aerobic and anaerobic situations are identified, the beginning of entry of Mediterranean waters into the basin is dated, presence of authigenic carbonates in sediments of the sea is established and amounts are determined. Methane generation from carbon dioxide is shown to have been replaced by its generation from acetate in the paleo-Black Sea period.

(Table 1) Sulfur content and sulfur-isotope ratios at DSDP Leg 82 Holes basalts

The sulfur contents of 21 basalt samples from four DSDP Leg 82 holes were determined and the isotopic compositions of sulfur were measured on 15 of them. Most of the basalts are altered and have sulfur contents of about 100 ppm. Isotopic ratios for sulfate and total sulfur range from +0.7 to +10.5 per mil, indicating almost complete leaching of the igneous sulfide in low-sulfur samples by alteration. Total sulfur content of some samples ranges between 960 and 1170 ppm, somewhat higher than expected for tholeiitic basalts. The isotope ratios of total sulfur in these samples are slightly shifted to values heavier than the generally assumed mantle ratio of zero, and this shift is thought to result from a secondary source of sulfur.