Sulfur and iron speciation in surface sediments along the northwestern margin of the Black Sea

The speciation of sedimentary sulfur (pyrite, acid volatile sulfides (AVS), S(0) H(2)S, and sulfate) was analyzed in surface sediments recovered at different water depths from the northwestern margin of the Black Sea. Additionally, dissolved and dithionite-extractable iron were quantified, and the sulfur isotope ratios in pyrite were measured. Sulfur and iron cycling in surface sediments of the northwestern part of the Black Sea is largely influenced by (1) organic matter supply to the sediment, (2) availability of reactive iron compounds and (3) oxygen concentrations in the bottom waters. Biologically active, accumulating sediments just in front of the river deltas were characterized by high AVS contents and a fast depletion of sulfate concentration with depth, most likely due to high sulfate reduction rates (SRR). The delta (34)S values of pyrite in these sediments were relatively heavy (- 8 parts per thousand to -21 parts per thousand vs. V-CDT). On the central shelf, where benthic mineralization rates are lower, re-oxidation processes may become more important and result in pyrite extremely depleted in delta (34)S (-39 parts per thousand to -46 parts per thousand vs. V-CDT). A high variability in delta (34)S values of pyrite in sediments from the shelf-edge (- 6 parts per thousand to -46 parts per thousand vs. V-CDT) reflects characteristic fluctuations in the oxygen concentrations of bottom waters or varying sediment accumulation rates. During periods of oxic conditions or low sediment accumulation rates, re-oxidation processes became important resulting in low AVS concentrations and light delta (34)S values. Anoxic conditions in the bottom waters overlying shelf-edge sediments or periods of high accumulation rates are reflected in enhanced AVS contents and heavier sulfur isotope values. The sulfur and iron contents and the light and uniform pyrite isotopic composition (-37 parts per thousand to -39 parts per thousand vs. V-CDT) of sediments in the permanently anoxic deep sea (1494 m water depth) reflect the formation of pyrite in the upper part of the sulfidic water column and the anoxic surface sediment. The present study demonstrates that pyrite, which is extremely in (34)S, can be found in the Black Sea surface sediments that are positioned both above and below the chemocline, despite differences in biogeochemical and microbial controlling factors. (C) 2001 Elsevier Science B.V. All rights reserved.