ABUNDANCE AND ISOTOPIC COMPOSITION OF ORGANIC AND INORGANIC SULFUR SPECIES IN LAMINATED AND BIOTURBATED SEDIMENTS FROM HOLE 893A, SANTA BARBARA BASIN

Volker Brόchert, Lisa M. Pratt, Thomas F. Anderson, and Stephen R. Hoffmann

ABSTRACT

Organic carbon and total sulfur have been analyzed in 29 core samples from Ocean Drilling Program (ODP) Hole 893A spanning 191 m of marine sediment that record variable paleoceanographic conditions in the Santa Barbara Basin during the past 160,000 yr. Sulfur isotope ratios have been determined for six species of sulfur (pyrite S, acid-volatile S, elemental S, kerogen S, bitumen S, and sulfate S) in order to establish an isotope budget for sulfur in laminated and nonlaminated/bioturbated sediments. Variations in the relative abundance of specific sulfur species correlate with sedimentological properties in the cores. Abundances of total, acid-volatile, and pyrite sulfur are highest in laminated intervals. Abundances of elemental sulfur, bitumen, and kerogen sulfur are higher in nonlaminated samples and show a strong negative correlation with abundances of pyrite sulfur. Sulfur isotopic compositions of pyrite generally are depleted in 34S and vary between –39‰ and –15‰ versus the Caρon Diablo Troilite standard (CDT). The highest isotopic ratios for pyrites occur in nonlaminated samples. The isotopic composition of acid-volatile sulfur is consistently more enriched in 34S than pyrite sulfur, and is not well correlated with the isotopic variation of pyrite sulfur. Isotopic compositions of elemental sulfur vary between –22‰ and +14‰ vs. CDT with the lowest isotopic values in nonlaminated samples and the highest d-values in laminated samples. Isotopic compositions of bitumen and kerogen sulfur range between –2‰ and 6‰ vs. CDT suggesting formation of organic sulfur either later than mineral sulfur or with involvement of different fractionation effects related to sulfur oxidation and incorporation into organic matter.

Levels of benthic oxygenation affected both partitioning and isotopic fractionation of sulfur during early diagenesis. Higher abundances of elemental and organically bound sulfur in bioturbated and structureless sediments may be related to higher concentrations of geologically transient species such as polysulfides, polythionates, and thiosulfate. Calculation of the isotopic composition of total sulfur and assessment of the fraction of sulfur retained in the sediments indicates that, with one possible exception, never more than 60% of dissolved sulfate was retained in the sediment. Sulfate reduction rates did not exceed rates of sulfate supply. Thus, sulfur cycling in the marine sediment recovered from ODP Hole 893A is not analogous to the sulfate-limited system in the Miocene Monterey Formation.

Date of initial receipt: 2 August 1994
Date of acceptance: 8 February 1995


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