Sulfur isotopic trends and pathways of iron sulfide formation in upper Holocene sediments of the anoxic Black Sea

Down-core trends for extents of iron sulfidation in upper Holocene microlaminated deposits of the Black Sea abyssal plain imply that most of the pyrite in these sediments is formed in the sulfidic water column and/or very close to the sediment-water interface. Sulfur isotopic data for pyrite within microlaminated muds from two localities in the deep basin show striking uniformity between sites and down core, with a mean delta(34)S(CDT) value of -37.2 +/- 0.9 parts per thousand (1 sigma, n = 18). These isotopic trends support the hypothesis of early pyrite formation. Isotopic compositions of pyrite sulfur in the microlaminated bottom sediments are similar to those reported for particulate reduced sulfur collected in middepth sediment traps and to delta(34)S Values for water-column dissolved sulfide within a narrow zone immediately below the O-2/H2S interface. These three sulfur reservoirs are distinctly different from the isotopic composition of ambient dissolved sulfide of the deep water column (>175 m), with delta(34)S values for the deep dissolved sulfide reservoir being consistently lower by similar to 2 to 4 parts per thousand. These isotopic relationships suggest that iron sulfidation is occurring dominantly within a narrow, shallow region at the top of the sulfidic water mass. Isotopic data from a fine-grained turbidite collected in the deep basin and from rapidly accumulating, iron monosulfide-rich muds on the anoxic upper slope are enriched in S-34 relative to the microlaminated deposits. The enrichments on the basin margin reflect a large component of iron sulfide formed during rapid burial. A broad suite of geochemical data, including sulfur isotopic results, are consistent with an upper-slope sediment source for the ubiquitous muddy turbidites of the deep basin. The collective effects of reworking, transport, and redeposition result in a complete conversion of AVS to pyrite with little additional iron sulfidation. This conclusion is supported by a detailed isotopic mass balance. The present isotopic study also highlights fundamental differences in the reactivities of the iron reservoirs distributed throughout the anoxic basin. Sediments of the upper slope and the genetically-linked turbidites of the deep basin show similar extents of iron sulfidation that are significantly lower than those of the microlaminated deposits of the deep-basin floor, suggesting dramatically shorter timescales of iron reactivity for the microlaminated sediments of the deep basin. Importantly, isotopic patterns from the modem Black Sea should assist greatly in the recognition and interpretation of ancient anoxia in the stratigraphic record. Copyright (C) 1997 Elsevier Science Ltd.