Isotopic and Diatom Evidence of Bacterial Sulphate Reduction in Sediments

Abstract

Abstract Sulphur contents and sulphur isotope ratios (δ34 So/00 values) in deep sediments (10–12 cms; circa 50–100 years before the present) and surface sediments (0–2 cm; circa 0–10 years before the present) from eight lakes located north of Lake Superior, Canada indicated that surface sediments had a significantly higher sulphur content (largely organic sulphur) per gram dry weight and a lower sulphur isotope ratio than the deeper layers. The increased sulphur content and decreased sulphur isotope ratios (δ34 S0/00 values) in the surficial sediments were much greater for the lakes near and downwind from the Wawa sintering plant than for more remote lakes reflecting the impact of the sintering plant emissions on the lake environment. A significant relationship was observed between the sulphate reducing bacterial populations and the magnitude of the isotope shift in the study lakes. It was evident from the data that the greater the abundance of sulphate reducers the greater was the surface sediment sulphate content and the degree of isotope fractionation in the anaerobic and organic rich sediments. The surface sediments of the eight study lakes revealed isotopic evidence of bacterial sulphate reduction. The deeper sediments, however, did not reveal such a condition due to the absence of sulphate reducers. The rate of lake acidification as inferred from the downcore diatom inferred pH profile for each of the study lakes was not significantly correlated with the amount of sulphur in the lake sediments, nor was it correlated with the density of sulphate reducing bacteria or shifts in the sulphur isotope ratios (δ34S).