Sulphur Isotopic Study of Two Pristine Sphagnum Bogs in the Western British Isles

Abstract

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 41 2%o in the heavier 34S 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 34S than can be accounted for by Sphagnum sulphur uptake alone. 4 Strong depletion of sulphate (very low SO2-/Cl-) accompanied by strong enrichment in 34S 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 32S isotope) can account for the sulphate depletion and 34S enrichment observed in the bog surface waters.