Mechanisms of sulfur introduction chemically controlled:: δ34S imprint

Organic sulfur in marine sediment is S-34 enriched relative to the co-existing pyrite. This phenomenon is still enigmatic. Timing of the sulfur incorporation, immobilization and different sulfur species involved are part of the explanations. The reduced sulfur species incorporation into organic matter (OM) is generally assumed to have negligible delta(34)S fractionation. This assumption has never been confirmed by laboratory experimental data. The present study measures the delta(34)S changes resulting from reduced sulfur species (sulfides and polysulfide anions) incorporation into organic model compounds in an aquatic and low temperature (25 degreesC) system that simulates diagenetic marine environment. In addition, we also investigate the delta(34)S fractionation and the isotope chemical mixing in the formation of polysulfide anions produced from elemental sulfur and sulfide anions. The results showed total isotope mixing between the two species in the formation of polysulfides. Acidification of the polysulfides solution caused delta(34)S fractionation between the released elemental sulfur and H2S. The incorporation of polysulfides and sulfides into carbonyl groups, caused S-34 enrichment relative to the starting polysulfides and sulfide of 4-5parts per thousand. The S-34 enrichment of the sulfurized carbonyl groups showed a minimal effect by temperature (0-70 degreesC) and is not affected by salinity, polysulfides composition, reaction time or solubility in water. The incorporation of polysulfides and sulfides into brominated organic compounds was negligibly S-34 enriched. The chemical mechanisms controlling the polysulfides incorporation into OM depend mostly on the functional groups and determine the S-34 enrichment of the sulfurized OM. The results presented in this study can explain part of the difference between pyrite delta(34)S and sulfurized OM delta(34)S in natural marine sediments. (C) 2004 Elsevier Ltd. All rights reserved.