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 H2S, and sulfate) was analyzed in surface sediments recovered at diffe rent water depths from the northwestern margin of the Black Sea. Additional ly, dissolved and dithionite-extractable iron were quantified, and the sulf ur isotope ratios in pyrite were measured. Sulfur and iron cycling in surfa ce sediments of the northwestern part of the Black Sea is largely influence d by (1) organic matter supply to the sediment, (2) availability of reactiv e iron compounds and (3) oxygen concentrations in the bottom waters. Biolog ically active, accumulating sediments just in front of the river deltas wer e characterized by high AVS contents and a fast depletion of sulfate concen tration with depth, most likely due to high sulfate reduction rates (SRR). The delta S-34 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 S-34 (-39 parts per thousand to -46 parts per thousand vs. V-CDT). A high variability in delta S-34 values of pyrite in sediments from the shelf-edge (- 6 parts per thousand to -46 parts per thousand vs. V-CDT) reflects char acteristic fluctuations in the oxygen concentrations of bottom waters or va rying sediment accumulation rates. During periods of oxic conditions or low sediment accumulation rates, re-oxidation processes became important resul ting in low AVS concentrations and light delta S-34 values. Anoxic conditio ns in the bottom waters overlying shelf-edge sediments or periods of high a ccumulation rates are reflected in enhanced AVS contents and heavier sulfur isotope values. The sulfur and iron contents and the light and uniform pyr ite 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 de pth) reflect the formation of pyrite in the upper part of the sulfidic wate r column and the anoxic surface sediment. The present study demonstrates th at pyrite, which is extremely in S-34, can be found in the Black Sea surfac e sediments that are positioned both above and below the chemocline, despit e differences in biogeochemical and microbial controlling factors. (C) 2001 Elsevier Science B.V. All rights reserved.