Microbial sulfate reduction in deep sediments of the Southwest Pacific (ODP Leg 181, Sites 1119-1125): evidence from stable sulfur isotope fractionation and pore water modeling

Interstitial water samples from seven ODP sites (Leg 181, Sites 1119–1125) of the southwestern Pacific Ocean have been analyzed for the stable sulfur isotopic composition of dissolved sulfate along with major and minor ions. Sulfate from the interstitial fluids (δ34S values between +20.7 and +60‰ vs. the SO2-based Vienna–Canyon Diablo troilite standard) was enriched in 34S with respect to modern sea water (δ34S≈+20.6‰) indicating that microbial sulfate reduction takes place to different extents at all investigated sites. Microbial sulfate reduction (MSR) was found at all sites, the intensity depending on the availability of organic matter which is controlled by paleo-sedimentation conditions (sedimentation rate, presence of turbidites) and productivity. Microbial net sulfate reduction was additionally confirmed by modeling interstitial water sulfate profiles. Areal net sulfate reduction rates up to 14 mmol m−2 yr−1 have been calculated which were positively related to sedimentation rates. Total reduced inorganic sulfur (TRIS; essentially pyrite) as a product of microbial sulfate reduction was isotopically characterized in squeeze cake samples and gave δ34S values between −51 and +9‰ indicating pyrite formation both close to the sediment–water interface and later diagenetic contributions