Sulfur and oxygen isotopes of sulfate in precipitation and lakewater, Quebec, Canada

Stable isotope ratios of sulfur and oxygen of sulfate ion in precipitation and lakewater were monitored in six headwater lakes in Québec, Canada. The lakes studied are oligotrophic and located over a wide area (350 km) on the Canadian Shield, which is known to be sensitive to acidification.The δ34S of the sulfate ion in precipitation ranges from 3.1 to 6.7‰ for all locations during the sampling period, which is very close to the values found for the lakes, from 3.5 to 6.7‰. Moreover, the δ34S of sulfate showed seasonal variations which are parallel in time for all stations. This strongly suggests that precipitation is a major source of sulfate in the lakes that were investigated. However, the δ18Ovalues of precipitation sulfate are close to each other between stations and are consistently higher (+ 8.3 to 13.5‰) than that of the lakes (+ 1.7 to 8.5‰). In addition, there is no spatial or temporal trend in lake δ18O sulfate, suggesting that a transformation occurs to sulfate in the watersheds. We suggest the hypothesis that sulfate ion is reduced and reoxidized in the runoff and lake waters so that it acquires an oxygen isotopic label different from precipitation.