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 mic rolaminated 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 dat a for pyrite within microlaminated muds from two localities in the dee p basin show striking uniformity between sites and down core, with a m ean 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 pyrit e formation. Isotopic compositions of pyrite sulfur in the microlamina ted bottom sediments are similar to those reported for particulate red uced sulfur collected in middepth sediment traps and to delta(34)S Val ues for water-column dissolved sulfide within a narrow zone immediatel y below the O-2/H2S interface. These three sulfur reservoirs are disti nctly different from the isotopic composition of ambient dissolved sul fide 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 tha t iron sulfidation is occurring dominantly within a narrow, shallow re gion at the top of the sulfidic water mass. Isotopic data from a fine- grained turbidite collected in the deep basin and from rapidly accumul ating, iron monosulfide-rich muds on the anoxic upper slope are enrich ed in S-34 relative to the microlaminated deposits. The enrichments on the basin margin reflect a large component of iron sulfide formed dur ing rapid burial. A broad suite of geochemical data, including sulfur isotopic results, are consistent with an upper-slope sediment source f or the ubiquitous muddy turbidites of the deep basin. The collective e ffects of reworking, transport, and redeposition result in a complete conversion of AVS to pyrite with little additional iron sulfidation. T his conclusion is supported by a detailed isotopic mass balance. The p resent isotopic study also highlights fundamental differences in the r eactivities of the iron reservoirs distributed throughout the anoxic b asin. Sediments of the upper slope and the genetically-linked turbidit es 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 r eactivity for the microlaminated sediments of the deep basin. Importan tly, isotopic patterns from the modem Black Sea should assist greatly in the recognition and interpretation of ancient anoxia in the stratig raphic record. Copyright (C) 1997 Elsevier Science Ltd.