ISOTOPIC COMPOSITION AND SPECIATION OF SULFUR IN THE MIOCENE MONTEREY FORMATION - REEVALUATION OF SULFUR REACTIONS DURING EARLY DIAGENESIS IN MARINE ENVIRONMENTS

The timing and pathways of early diagenetic sulfur transfer from dissolved species in pore waters to solid inorganic and organic compounds in sediments have been studied in the Miocene Monterey Formation, Santa Maria Basin (onshore), California. Correlation between concentrations of total organic carbon (TOC) and total sulfur (TS), in addition to concentrations of titanium, aluminum, total iron, and reactive iron, have been used to infer organic matter reactivity, redox conditions, and, relative rates of clastic and biogenic input for each lithofacies. Isotopic compositions of six sulfur species (acid-volatile, disulfide, kerogen, bitumen, sulfate, and elemental) have provided information regarding relative timing of sulfur incorporation, sulfate diffusivity in the upper centimeters of the sediments, and the sources of sulfur for individual species. Isotopically, the disulfide species expresses the greatest fractionation relative to estimated values of Miocene seawater sulfate (approximately +22 parts per thousand(CDT)). On average, disulfide is depleted in S-34 by 10.4 parts per thousand relative to kerogen, and by 9.9 parts per thousand relative to acid-volatile sulfide. The delta-S-34 of bitumen shows no systematic change relative to delta-S-34kerogen, suggesting the presence of migrated bitumen. Isotopic similarity of sulfate and elemental sulfur to sulfides and bitumen indicates that sulfate and elemental sulfur are chemical and/or biological oxidation products derived from sulfides and bitumen. Consistent ordering of isotopic values for sulfur species (disulfide < acid-volatile sulfide less-than-or-equal-to kerogen) indicates that pyrite precipitated nearest to the sediment-water interface under mildly reducing conditions and with little or no decrease in sulfate concentration relative to seawater. Enrichment of S-34 in acid-volatile sulfide and kerogen sulfur resulted from formation of these species at greater depths or in restricted micro-environments under more reducing conditions and with low concentrations of porewater sulfate. The formation of acid-volatile sulfide after pyrite but during early diagenesis is significant because it implies more strongly reducing conditions than are generally recognized in deep-water marine sediments.