Lead isotopic composition of insoluble particles from widespread mountain glaciers in western China: Natural vs. anthropogenic sources

摘要

Stable lead (Pb) isotopic fingerprints provide opportunities to trace natural and anthropogenic Pb sources in the environment. In order to evaluate Pb deposition from different sources over mountainous areas of western China, Pb isotopic compositions were characterized from modern aeolian dust in 15 snowpit samples collected from 13 typical mountain glaciers between 2008 and 2010. Most of the snowpits sampled cover more than a whole year of accumulation and overlap with each other on deposition date. Pb isotopic variability among all the samples is small, varying in the range of 18.1399-18.9199 for ~(206)Pb/~(204)Pb, 15.5979-15.8743 for ~(207)Pb/~(204)Pb, 38.2272-39.9453 for ~(208)Pb/~(204)Pb, 1.1605-1.2009 for ~(206)Pb/~(207)Pb and 2.4433-2.5182 for ~(208)Pb/~(207)Pb. Three isotopic plots of the different Pb isotope ratios(~(207)Pb/~(204)Pb vs. ~(206)Pb/~(204)Pb ~(208)Pb/~(204)Pb vs. ~(206)Pb/~(204)Pb, and ~(208)Pb/~(207)Pb vs. ~(206)Pb/~(207)Pb) in all the sam- ples show identical geographic trends, with more radiogenic in the south and less in the north. This trend is consistent with the distribution of natural dust sources and supports the interpretation of a regional/ local source for insoluble particles (IP) to snow/glaciers in this region. Comparison with the Pb isotope results from potential dust sources, however, it shows that the Pb isotopic compositions of IP samples in snow samples are relatively less radiogenic. Parts of these less-radiogenic Pb isotopes are comparable with the ice core results during recent decades, which are shown to be influenced by anthropogenic sources. At sites located along the periphery of western China, the Pb isotopic compositions are much closer to anthropogenic results. Natural and anthropogenic Pb sources are roughly assessed using a simple binary model. The sites with a high anthropogenic fraction are at lower elevations and are relatively close to population centers.