SULFUR ISOTOPIC INVESTIGATION OF A POLLUTED RAISED BOG AND THE UPTAKEOF POLLUTANT SULFUR BY SPHAGNUM

The sulphur content and sulphur isotopic composition of Sphagnum as we ll as anionic compositions and sulphur isotope ratios of rainwater inp uts and bog waters have been measured at Thorne Moors, a raised bog in eastern England. Rainwater sulphate isotopic composition shows the su lphur input at this site to be dominated by anthropogenic pollution fr om fossil fuel burning. Strong depletion of sulphate (low SO42-/Cl-) a nd enrichment in S-34 in sulphate occurs at depth in the bog porewater s due to bacterial sulphate reduction. Some surface waters have low SO 42-/Cl- and are S-34 enriched due to removal of sulphate by downward d iffusion into a sulphate-reducing zone. Other sites have high SO42-/Cl - which appears to result from oxidation of organically bound sulphur in the peat. Sulphur is present in Sphagnum at around 0.2% by weight a nd is depleted by 0 to -9 parts per thousand in the heavier S-34 isoto pe compared to sulphate. Comparison with similar data from pristine co astal sites shows that sulphur incorporation into Sphagnum is enhanced in the polluted site (as Sphagnum sulphur concentrations are higher a t lower total sulphur inputs) and that sulphur incorporation is accomp anied by a smaller isotopic shift than in the pristine sites. The data support a model of preferential incorporation of partially reduced su lphur species (probably HSO3-) into Sphagnum. In pristine sites these are only available as oxidation products of sulphide formed by sulphat e reduction and are S-32 depleted. In polluted sites this source is au gmented by sulphur(rV) species in atmospheric inputs and the resultant mixture is less depleted in S-32. Thus, in the polluted sites more HS O3- is available for uptake and the isotopic shift between Sphagnum an d aqueous sulphur species is smaller. (C) 1998 Elsevier Science Ltd. A il rights reserved.