Isotopic composition of sulfur in precipitation within the Great Lakes Basin

Abstract

By monitoring both the isotope ratio variations and the concentrations of sulfur in bulk precipitation samples, the seasonal changes in the relative contributions of airborne sulfur from bacteriogenic and anthropogenic sources have been assessed. Although the δ³⁴ values during the winter months are generally higher than those of the summer months by about 4%, there is no corresponding seasonal variability in sulfur concentrations. In general, the average SO2 concentrations at urban stations are also higher during the winter months. Precipitation samples at urban sites usually have higher sulfur contents and are enriched in 34S compared to samples at rural and remote locations. On the basis of these observations, it is suggested that the bacteriogenic sulfur emissions within the basin may account for 10–30% of all the sulfur emitted in the basin. The data suggest that the biogenic release of sulfur from land areas may be smaller than the figures employed in many models of the global sulfur cycle.