15. Sulfur in Pacific Deep-sea Sediments (leg 129) and Implications for Cycling of Sediment in Subduction Zones

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 δ 3 4 S values ranging from seawater values (+20‰) to slightly enriched in 3 S (+28‰). Pyrite sulfur has 3 S values of -4.8‰to -37. l‰ 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 δ 3 4 S of seawater sulfate in the past, uptake of sulfate by thaumasite (Ca3Si(OH)6CO3Sθ4-12H2O) 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. The average sediment from Leg 129 contains around 850 ppm S (85% as sulfate, 15% as sulfide), with a bulk δ 3 4 S of+12‰ The bulk sulfur isotopic composition of deep-sea sediment is highly dependent on the proportion of pyrite-sulfur present, which varies depending on the sedimentary environment and history. The sedimentary sulfur component during subduction thus likely varies from near seawater values (+20‰) to possibly strongly negative values (-20‰).