Isotopic composition of ¹²⁹I in marine sediments from Beppu Bay: assessing its feasibility as an Anthropocene marker

Anthropogenic ¹²⁹I has been used as a powerful tool for tracing human nuclear activities. This study measured ¹²⁷I concentrations and ¹²⁹I/¹²⁷I ratios in a sediment core from Beppu Bay, Japan, to evaluate the potential of ¹²⁹I as a key marker for the Anthropocene. The ¹²⁹I/¹²⁷I ratios ranged from 1.16 × 10^-11 to 1.11 × 10^-10 and showed a clear increase in the 1950s, consistent with the beginning of global nuclear activities. This variation agrees with previously reported records from East Asia. Owing to the high sedimentation rate, well-preserved lamination, and redox conditions in Beppu Bay, the core retained both global increase of anthropogenic ¹²⁹I and several distinct local short-term peaks of local origin. The timeline of these peaks are likely to coincides to local volcanic eruptions and extreme weather events that enhanced ¹²⁹I/¹²⁷I, and slightly elevated ¹²⁹I/¹²⁷I ratio in pre-nuclear layers suggest limited post-depositional migration under reducing conditions. Overall, the Beppu Bay core provides a high-resolution and well-dated record of ¹²⁹I deposition influenced by both global and local signals. These results support the potential of ¹²⁹I as a key marker in coastal marine environments for defining the Anthropocene.