(Table 1) Sulphur content and stable isotope ratios of ODP Leg 129 holes

The mineralogy of S-bearing phases, S-contents, and S-isotopic compositions of sediments and interstitial waters were determined for three cores drilled during Leg 129 in the western Pacific. Pelagic clays, chalks, and radiolarites generally contain 350-1200 ppm S, with essentially all S present as sulfate. Volcanogenic turbidite units contain about 400 ppm S, with about 10%-20% of total sulfur as pyrite-S and the remainder as sulfate. A single claystone from the base of Hole 802A has high pyrite-S content (1200 ppm) and contains a component of metalliferous sediment. The single analyzed chert has the highest sulfur content (4020 ppm) with essentially all sulfur present as pyrite-S. Sulfate in pore waters ranges from seawater concentrations to nearly zero, and has d34S values ranging from seawater values (+20 per mil) to slightly enriched in 34S (+28 per mil). Pyrite sulfur has 34S values of -4.8 per mil to -37.1 per mil. These general trends are attributed to bacterial reduction of seawater sulfate, but the data do not fit simple models for downward diffusion of sulfate or closed-system reduction of buried sulfate. These complications are due to variations in the d34S of seawater sulfate in the past, uptake of sulfate by thaumasite (Ca3Si(OH)6CO3SO4-12H2O) without isotopic fractionation in the Unit II tuff in Hole 802A, the presence of cherts that act as barriers to diffusion, and possible variations in the magnitude of isotopic fractionation during bacterial sulfate reduction.