The onset of anthropogenic activity recorded in lake sediments in the vicinity of the Horne smelter in Quebec, Canada: Sulfur isotope evidence

Contents and δ34S values of several S compounds, enumerations of S-reducing bacteria (SRB) and Fe-reducing bacteria (IRB), and Fe, Pb and In concentrations were determined for 210Pb-dated sediment cores from two lakes in Quebec, Canada. Both lakes are located approximately 70 km downwind of the Horne smelter and refinery in Rouyn-Noranda. Increases in Fe, Pb and In concentrations and a decrease in the δ34S values of total S in both lake sediment cores coincide with the start-up of the smelter in 1927. The shift towards more negative δ34S values was primarily caused by an increase in the extent of S isotope fractionation during bacterial (dissimilatory) SO4 reduction due to SO4 loading of the lakes after smelting began. Consequently, an enhanced accumulation of 32S-enriched reduced inorganic S compounds is evident in the sediments. δ34S values of organic S in the sediments decreased only slightly due to the smelter emissions between 1930 and 1980. Hence, due to the sulfide depositing mechanisms, S isotope ratios constitute a useful tracer recording the onset of S pollution in sediments of the two previously SO4-limited lakes investigated. In contrast, total S concentrations alone are not reliable indicators for anthropogenic S loading in lake sediment records.