Pathways of south-derived iodine-129 intrusion into Tibet as revealed by its spatial distribution

The Tibetan Plateau plays a vital role in regional ecological stability and exerts significant influence on the global climate system. Intrusion of radioactive pollutant from potential sources would pose radiation threat to its ecological environment. Preliminary findings suggest that iodine-129 (¹²⁹I) from Indian nuclear activities may have reached southern Tibet, providing a good tracer for studying the pathway of possible gaseous radioactive pollutants to the plateau, which is essential for assessing the radiation risk and hazard in the future. Here, we investigated the distribution of ¹²⁹I in the central-southern Tibet and found higher ¹²⁹I concentrations and ¹²⁹I/¹²⁷I ratios in southeastern Tibet (up to 1.70 × 10⁸ atoms/g and 1.34 × 10^-8 for ¹²⁹I concentration and ¹²⁹I/¹²⁷I ratios in top soils, respectively), with strong correlations between iodine isotopes and environmental factors including altitude and rainfall. These findings indicate that southern water vapor, driven by the Indian summer monsoon, transported gaseous ¹²⁹I emissions from South Asia to the plateau, primarily through the Yarlung Tsangpo Grand Canyon. A minor contribution also ascended along the southern edge of the plateau. Elevated ¹²⁹I/¹²⁷I ratios in plants compared to soils underscore the role of atmospheric iodine uptake. While current ¹²⁹I levels pose no immediate radiological risk, the potential for bioaccumulation and long-term ecological impacts highlights the need for sustained monitoring.