Behaviour of sulphur during diagenesis of a maritime ombrotrophic peat from Yell, Shetland Islands, UK

Below the vegetation layer and down to 18 cm depth, bacterial SO42- reduction is the major control on S species distribution and isotopic composition within the solid peat and pore waters. In this part of the peat, preferential reduction of 32SO4 in pore water during metabolism produces isotopically light sulphide, which is incorporated into the solid phase in both inorganic and organic forms, while pore water SO42- becomes enriched in 34S. From 18 to 28 cm, organic S content falls relative to C and residual organic S becomes 34S-enriched, indicative of mineralization of organic S, a process which releases isotopically light S to the pore waters. Still deeper in the core (28 to ∼50 cm), bacterial reduction of pore-water SO42-, now enriched in 34S, results in addition of isotopically heavy S to the solid phase. Limited pore water data suggest that below 50 cm mineralization reactions again release S from the organic fraction of the peat.