A 2000-year record of polycyclic aromatic hydrocarbon deposition in Lake Basongcuo, southeast Tibet: Source apportionment and implications for human impacts.

The Tibetan Plateau (TP), sometimes referred to as the "Third Pole," is a highly sensitive ecosystem. Understanding historical organic pollutant inputs and their responses to climate conditions and human activities is essential for assessing future environmental sustainability in the region. Here, we present a 2000-year record of polycyclic aromatic hydrocarbons (PAHs) in sediments from Lake Basongcuo, southeast TP, and discuss their variations and sources through time, and influencing factors. Analyses revealed four zones of increasing PAH concentrations, from Zone I (1900-1250 cal yr BP) to IV (50 cal yr BP-present). Source apportionment via diagnostic ratios and Positive Matrix Factorization (PMF) modelling showed that PAH sources shifted from similar proportions of petrogenic, biomass and high-temperature combustion in Zone Ⅰ to biomass-dominated sources (31.4%), with emerging coal combustion, in Zone Ⅱ (1250-650 cal yr BP). High-temperature combustion subsequently rose to prominence (35.5%) in Zone III (650-50 cal yr BP) and persisted as the major source (31.5%) in Zone IV. Our findings also highlight the evolving roles of climate conditions and human activities as PAH drivers. Zone I PAH inputs were predominantly climate-driven, evidenced by wildfires during arid periods of the weakened Indian Summer Monsoon. A discernible human signal emerged in Zone II, with coal combustion from introduced smelting technologies, which intensified in Zone III through sustained metal-smelting and religious practices. Ultimately, human impacts became the predominant driver in Zone IV, associated with transportation and tourism. Three PAH-identified human-activity intervals identified in the sediments align temporally with three episodes of historically documented regional development, Tubo fragmentation, Yuan-Ming centralisation, and modernization. This illustrates that sediment PAHs are reliable proxies for millennial-scale human activities in the southeastern TP.