Pre-nuclear level of ¹²⁹I in Chinese loess-paleosol sections: A search for the natural ¹²⁹I level for dating in terrestrial environments

Due to its long half-life (15.7 Myr), radioactive ¹²⁹I has great potential for dating geologic materials as old as 100 Myr. Thus, knowing the natural level of ¹²⁹I is crucial to dating applications. The initial ratio of ¹²⁹I/¹²⁷I in the ocean has been quantified by a number of researchers who have reached a consensus value. However, the applicability of ¹²⁹I dating in the terrestrial environment remains problematic because the lack of an initial ¹²⁹I/¹²⁷I value. In this work, samples of loess-paleosol sections from the Chinese Loess Plateau (CLP) were analyzed for ¹²⁹I/¹²⁷I, aiming to provide an initial ¹²⁹I/¹²⁷I ratio that can be adopted for dating purposes in terrestrial environments. A value of (2.0 ± 1.0) × 10⁻¹¹ for the ¹²⁹I/¹²⁷I ratio was found in two investigated loess-paleosol sections from Xifeng and Luochuan, China. This ratio is one order of magnitude higher than the initial value reported for the marine environment. Alteration of the natural initial ¹²⁹I in the investigated samples due to the downward migration of anthropogenic ¹²⁹I and by excess fissiogenic ¹²⁹I from uranium was not supported. Consequently, the ¹²⁹I/¹²⁷I ratio measured is considered to be a pristine value, and the difference from that in the marine systems is attributed to an isotopic dilution effect.