RELATIONSHIPS BETWEEN SULFUR, ORGANIC-CARBON, AND IRON IN THE MODERN SEDIMENTS OF THE BLACK-SEA

The ratio of sedimentary S to organic C has been used as a diagnostic criterion for recognizing anoxic environments in the sedimentary record. Here we examine the relationship between pyrite S and organic C in the modern (Unit 1) sediments of the Black Sea to re-evaluate this suggested relationship. In box cores from shallow oxic or near-oxic water, pyrite S contents, ranging from 0.1 and 2.5% by weight, do not correlate with organic C contents. In cores from the deep anoxic basin, pyrite S concentrations range between 1.0 and 2.5% and are linearly related to organic C contents with an intercept that is not significantly different from zero. The S/C ratio is around 0.31 in these sediments, which is somewhat lower than the ratio of 0.36 considered to be characteristic of "normal" marine oxic sediments. Thus, the Black Sea anoxic sediments appear to be depleted in S relative to organic C. The degree of pyritization (DOP) of the sediments may be a better measure of anoxic conditions of sedimentation. The DOP values measured here show that pyrite formation is Fe limited, the correlation between organic C and pyrite S probably being induced by a positive correlation between fine-grained sediment components (the original Fe source for pyrite formation in the sediments) and organic matter. Such Fe-limitation is probably characteristic of anoxic basins where pyrite formation can take place in the water column and Fe oxyhydroxides do not accumulate in the surface sediments. The results on the sample set examined here are at variance with earlier data and interpretations for the Black Sea. In particular, the suggested S enrichment of anoxic Black Sea sediments was not found because of Fe limitation.