DETERMINATION OF REDUCED SULFUR SPECIES IN SEDIMENTS - AN EVALUATION AND MODIFIED TECHNIQUE

Previous studies have indicated that the separation and quantification of different species of reduced sulphur in sediments have suffered fr om uncertainty in selectivity and recovery of the acid volatile sulphu r pool and pyrite sulphur pool. This has probably resulted in either o verestimation or underestimation of the sedimentary sulphur pools. Thi s could be caused by differences in reagents and conditions of the ana lysis and by improper in situ sample pre-treatment, but the major prob lem is caused by the diversity of standard materials which were used a nd the discrepancy with real samples in natural systems. The effects o f SnCl2, ZnAc and heat on digestion recovery have been investigated us ing anaerobic sediments and sulphide-rich concretions from a modem sal t marsh environment in north Norfolk. UK. Acid volatile sulphur (AVS) and more refractory sulphide phases, termed pyrite sulphur (PS) here, are the major forms of reduced sulphur, but elemental sulphur (ES) is also significant. We found that AVS would be mostly transferred to the ES pool as a result of oxidation without in situ ZnAc treatment, and ES would be recovered in a hot Cr distillation step if not extracted b efore AVS and Cr extractions. A three-step sequential digestion proced ure, after prior elemental S extraction, on ZnAc-treated samples was u sed to distinguish these different reduced species. Modifications have also been made to the process of distillation and collection of relea sed H2S in order to retrieve enough precipitated S for later isotopic analysis. Pyrite sulphur, as defined by this extraction scheme, is div ided into a less mature fraction, as distilled by cold Cr reduction, a nd a mature fraction, by hot Cr reduction, This method could be used a s an index of the degree of the maturity of iron sulphide phases in re cent sedimentary environments. (C) 1997 Elsevier Science B.V.