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

Surface water, pore water, vegetation and peat cores were sampled from a waterlogged ombrotrophic peat bog on the Shetland Isles, UK and analysed for different S forms and their isotopic composition, in an attempt to elucidate the biogeochemical processes affecting S during peat diagenesis. Surface waters show that inputs of S to the peat have a maritime-dominated isotopic composition close to +20‰CDT. Uptake of S by vegetation introduces a −10‰ shift in δ34S from these input values. Below the vegetation layer and down to 18 cm depth, bacterial image 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 image 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 not, vert, similar50 cm), bacterial reduction of pore-water image, 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.