Analysis of iodine isotopes in travertine from Baishuitai, Yunnan Province, China

Travertine and tufa can be used as a high-resolution record for reconstructing the paleoclimate and paleoenvironment. It is not clear whether travertine is able to preserve the radionuclides for the reconstruction of modern human nuclear events. In this study, we developed and compared two sample preparation methods for the determination of ¹²⁹I and ¹²⁷I in travertine samples using direct acid dissolution and pyrolysis combustion. The analytical results of the levels of ¹²⁹I and ¹²⁷I as well as the ¹²⁹I/¹²⁷I ratios in travertine are presented here for the first time, and the hydrodynamic influence on the levels and distribution of iodine isotopes are discussed. It was found that the concentration of¹²⁷I in travertine was low in the winter but high in the summer. This temporal variation of ¹²⁷I is likely related to the level and species of iodine in the water supply by mixing geothermal water with rainfall, as well as the amount of iodine-containing impurity. The ¹²⁹I level clearly suggests that the Baishuitai travertine is endogenic and slightly affected by the amount of rainfall and biological impurities. The results of this study show that endogenic travertine can be used in reconstructing the influence of human nuclear activity with high resolution, and also imply that hypergenic tufa has great potential for such studies. Both archives have implications on the mechanism used for iodine incorporation into carbonate in the natural environment.