¹²⁹I, of which the half-life is 15.7 Ma, has presented its potential of geological dating in the range of 2-80 Ma. However, researches on dating in terrestrial system using ¹²⁹I were scarce due to its extremely low content and possibly complicated variation in the terrestrial environment.

By using the newly established "carrier free iodine coprecipitation" separation method, scientists from the Institute of Earth Environment (IEE) of the Chinese Academy of Sciences investigated the iodine isotopes, ¹²⁹I and ¹²⁷I, in the loess-paleosol sections from the Chinese Loess Plateau (CLP). Their findings were published in GEOCHIMICA ET COSMOCHIMICA ACTA.

¹²⁹I/¹²⁷I atomic ratio was found to approximately vary within (2.0±1.0)×10^-11 in most samples except those in the upper 1-2 m, which obviously undergone the anthropogenic product contamination. Scientists were surprised to find that the observed ¹²⁹I/¹²⁷I level in the deep layers was one order of magnitude higher than the initial value reported for the marine system.

Possible anthropogenic migration and excessive fissiogenic ¹²⁹I from uranium isotopes were proved to have negligible impact on the iodine isotopic ratio based on the estimation of migration distance as well as the measured uranium content. Low ¹²⁷I content in the terrestrial environment caused by the limited exchange between marine and continent was proposed to attribute to the higher ¹²⁹I/¹²⁷I ratio in the terrestrial environment.

These findings may provide new insight into the complex composition of natural iodine isotopes in the terrestrial system and help constraining the pre-nuclear ¹²⁹I level therein in the future for dating the terrestrial materials using ¹²⁹I.

Depth profile of iodine isotopes analyzed in Xifeng (XF) and Luochuan (LC) loess sections. Black dashed line represents the pre-nuclear level of ¹²⁹I/¹²⁷I in the CLP and the red dot-dashed line represents the initial value of ¹²⁹I/¹²⁷I in the marine system. (Image by FAN Yukun, et al)