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 18 1, 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 (delta(34)S values between +20.7 and +60parts per thousand vs. the SO-based Vienna-Canyon Diablo troilite standard) was enriched in S-34 with respect to modern sea water (delta(34)S approximate to +20.6parts per thousand) 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 delta(34)S values between -51 and +9parts per thousand indicating pyrite formation both close to the sediment-water interface and later diagenetic contributions. (C) 2004 Elsevier B.V. All rights reserved.