Sulphur isotopic study of two pristine Sphagnum bogs in the western British Isles

1 The sulphur content and sulphur isotopic composition of Sphagnum as well as anionic compositions and sulphur isotope ratios of sulphate in rainwater inputs and of bog waters were measured in two areas in the western British Isles. 2 Since rainwater sulphate isotopic composition can distinguish between natural and pollutant sulphur sources the results show that during the summer months a significant proportion of sulphur is derived from anthropogenic pollution. 3 Sulphur is present in Sphagnum at around 0.1% by weight and is depleted by P 12 parts per thousand in the heavier S-34 isotope relative to the sulphate in environmental water. Sulphur uptake into growing Sphagnum accounts for 5-10% of the total sulphur input to the bogs. Bog surface waters are more strongly depleted in sulphate and more strongly enriched in S-34 than can be accounted for by Sphagnum sulphur uptake alone. 4 Strong depletion of sulphate (very low SO42-/Cl-) accompanied by strong enrichment in S-34 occurs at depth in the bogs due to bacterial sulphate reduction. Transport of sulphate into the deeper part of the bog along a concentration gradient (which is steeper for the S-32 isotope) can account for the sulphate depletion and S-34 enrichment observed in the bog surface waters.