Global Iodine-129 Deposition Records the Impacts of Anthropogenic Nuclear Activities

Since the mid-20th century, anthropogenic nuclear activities have released large amounts of radioactive iodine-129 (129I, half-life 15.7 million years). Owing to its high volatility and reactivity, 129I disperses globally, making it a powerful marker in Anthropocene research. This study comprehensively compared 129I records in sediments and ice cores from low-latitude Taal Lake (Philippines), mid-latitude Sihailongwan Maar Lake, Gonghai Lake and Mayinghai Lake (China), and high-latitude regions (Greenland, Swiss Alps) (Bautista et al., 2018; Bautista VII et al., 2023; Han et al., 2023; Reithmeier et al., 2006; Zhang et al., 2024, 2018). Four distinct stages were identified: (1) Pre-nuclear baseline period (before 1950): The 129I/127I atomic ratio was on the order of 10-12~10-11, dominated by natural sources. (2) Atmospheric nuclear weapons testing period (1950-1960): The 129I/127I ratio rapidly rose above 10-10, and 129I released by atmospheric nuclear tests was recorded in global deposition sequences. (3) Nuclear fuel reprocessing plants dominated period (after the 1960s): Emissions from European nuclear reprocessing facilities became the main source, whose signals propagated across the Northern Hemisphere via the westerlies. The East Asian winter monsoon further promoted deposition of this signal in mid-low latitudes of East Asia. (4) Superimposed period of nuclear accidents and marine re-release (after 1986): The signal of the Chernobyl accident was clearly recorded in some regions (e.g., peak at 1987.3 in the SE-Dome ice core, Greenland). Deposition records of the Fukushima nuclear accident are still lacking. Spatially, the 129I / 127I ratio in mid-high latitude sediments is two orders of magnitude higher than that in low-latitude regions, reflecting differences in emission source distribution and atmospheric transport pathways. In summary, the 129I deposition sequence provides a high-resolution stratigraphic signal of anthropogenic nuclear activities. Studies of sedimentary records in the Southern Hemisphere will further promote global comparisons of nuclear activities in the Anthropocene.