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.