Lead (Pb), a toxic heavy metal, poses significant health risks through ingestion and inhalation. Its natural geochemical cycle has been significantly altered by human activities since approximately 7000 BC. While Pb use dates back to the Neolithic period, emissions skyrocketed with the Industrial Revolution and the introduction of leaded gasoline. Although Pb emissions have decreased in North America and Europe after phasing out leaded gasoline, they remain high in Asia, particularly China. The Guliya ice cap in the Tibetan Plateau, previously studied for trace elements (TEs) including Pb, offers a unique opportunity to investigate Pb sources due to its remoteness and low industrial activity. However, previous studies lacked the isotopic resolution to effectively differentiate sources. This study leverages high-resolution Pb isotope measurements (204Pb, 206Pb, 207Pb, and 208Pb) from the Guliya ice core, combined with a Bayesian mixing model, to identify and quantify Pb sources before and after the Industrial Revolution, providing a more sensitive approach than previous TE-only studies.

Existing research on lead pollution has established its widespread presence and significant health impacts, particularly on children's development. Studies have traced anthropogenic lead emissions back to early civilizations but show a dramatic increase following the Industrial Revolution and the widespread adoption of leaded gasoline. While the decline in lead emissions in North America and Europe is well-documented, Asia, particularly China, continues to experience high levels of anthropogenic lead emissions. Previous research on the Guliya ice core has revealed periods of elevated trace element concentrations, including lead, but lacked the detailed isotopic analysis to pinpoint specific sources with confidence. This gap in knowledge highlights the need for this study.

This study analyzed 89 discrete ice core samples from the Guliya ice cap, covering periods from the Stone Age to 2015. Six new samples from the Paleolithic period were analyzed to establish a natural Pb isotopic background. Additional samples from the 1500s and 17 potential source area (PSA) dust samples from around the Tibetan Plateau were also included. Lead isotope ratios (204Pb, 206Pb, 207Pb, and 208Pb) were measured using a Thermo Scientific Neptune Plus™ High-Resolution MC-ICPMS. Changepoint detection analysis identified shifts in Pb isotope ratios. K-means clustering classified ice core samples into groups based on isotopic ratios. Three-isotope plots compared Guliya ice core samples with potential natural and anthropogenic sources. The Bayesian mixing model MixSIAR was employed to quantify the contributions of different Pb sources through time. Statistical tests, including the Wilcoxon rank-sum test, were used to analyze data. Potential source areas were grouped based on geographical location and k-means clustering analysis. The methodology employed advanced isotopic analysis techniques along with statistical modeling to robustly determine sources of lead pollution.

Changepoint analysis revealed a shift in Pb isotope ratios starting in 1949, with a more pronounced decrease after 1960 and 1974. K-means clustering separated samples into two main clusters: Cluster 1 (pre-1974, primarily natural sources) and Cluster 2 (post-1974, with anthropogenic contributions). Comparison with potential sources indicated that the Tibetan Plateau was the primary natural source of lead. For anthropogenic sources, Chinese coal and Pb/Zn ores, and Chinese gasoline initially showed a strong visual match with Cluster 2 samples on three-isotope plots; however, temporal trend analysis and MixSIAR modeling pointed towards gasoline combustion as the main anthropogenic source until 2007, after which coal combustion became dominant. MixSIAR results showed varying contributions from different sources, however, limitations in the data, particularly regarding the overlap in isotopic signatures between Chinese coal and the Tibetan Plateau PSA, suggest that the contributions from coal may be underestimated. The study also found that changes in atmospheric circulation between 1750 and 1935 may have affected Pb isotope ratios, suggesting shifts in natural Pb sources. The study successfully established a baseline of natural Pb isotope ratios by analyzing samples predating the Bronze Age. The study identified a clear temporal link between changes in Pb isotope ratios in the Guliya ice core and anthropogenic Pb emission changes in China.

This study successfully addresses the research question of identifying and quantifying Pb sources in the remote Guliya ice cap. The findings demonstrate that while natural sources from the Tibetan Plateau have been consistently significant, anthropogenic contributions have become increasingly important since the mid-20th century. The shift from gasoline combustion to coal combustion as the primary anthropogenic source is notable and reflects changing energy policies and industrial practices in China. The results underscore the long-range transport of pollutants and the ability of remote ice cores to provide valuable insights into global pollution trends. The study's methodology, using a combination of isotopic analysis and statistical modeling, offers a robust framework for future studies of trace metal pollution in other remote environments. These results have major implications for understanding the impact of Asian air pollution on the environment.

This study provides a comprehensive analysis of lead sources in the Guliya ice core, highlighting the transition from primarily Chinese gasoline combustion to coal combustion as the dominant anthropogenic source after 2007. The inclusion of 204Pb isotope data and the use of MixSIAR improved source apportionment. This study underscores the importance of considering both natural variability and anthropogenic pollution in remote areas. Further research should explore the regional impact of coal combustion in more detail and investigate other trace metals’ origins in the same ice core to expand understanding of past and present pollution.

The overlap in isotopic signatures between Chinese coal and the Tibetan Plateau PSA presented challenges in quantifying coal's contribution using the MixSIAR model. The relatively limited number of samples from certain periods (e.g., the Industrial Revolution period) might affect the accuracy of temporal trend analysis. While the study considered various potential sources, the possibility of other minor sources contributing to lead deposition in the Guliya ice cap cannot be entirely ruled out.