The rate of lake acidification in four lakes north of Lake Superior and its relationship to downcore sulphur isotope ratios

Lake bottom sediments (0-12 cm cores) from three lakes downwind of a siderite sintering plant were analysed for sulphate, pyrite, sulphur, and diatoms to determine the effects of atmospheric pollution. A fourth, upwind, lake was used as a control. Contrary to expectations the pH of two of the lakes had increased over the last 30 years, possibly owing to contamination by alkaline fly ash, acid rain enhancing the weathering of their carbonate-rich greenstone basins, or bacterial reduction of sulphate. The pH of the third local lake had decreased from 5.8-5.2 over 30 years, whereas the control lake pH of 4.9 had not changed. Recently deposited sediments in the three local lakes were much higher in sulphur content than the older sediments. Variations in sulphur isotope ratios between the local and control lakes were attributed to fractionation by sulphate reducing bacteria stimulated by anthropogenic sulphur inputs from the sintering plant.