^(129)I年代学初始值研究进展

Progress on the study of initial value for ^(129)I dating

^(129)I由于具有较长半衰期,为2 Ma—80 Ma的地质定年提供了可能,因此,近几十年来,^(129)I定年的关键参数"初始值"受到了学术界关注。海洋系统中^(129)I初始值研究已较为成熟,并得到广泛认可和应用。陆地环境的认知有限,但一定程度上暗示了海陆间可能存在差异。由于环境中天然^(129)I和^(127)I来源的不同,影响地表碘同位素混合过程的环境因素是讨论陆地^(129)I初始值水平的关键,将为以后确定陆地环境的^(129)I初始值提供思路。

Background, aim, and scope Since ＇Age＇ has been playing an important role in the research on geoscience, geological dating is one of the fields that have attracted plenty of attentions and works. However, rare dating methods are available for age range from millions to tens of millions of years and each method has its applicability and limitation. Exploring and establishing a new geochronometer, especially an absolute dating method, is of great significance for geoscience research. Radionuclide iodine-129 has a long half-life （15.7 Ma）. This special feature provided a great potential of dating with range of 2 Ma--80 Ma. Since 129I decays expoentially with time, confirming the 129I/127I ratio at the preliminary stage of decay is critical to the establishment of 129I dating method. Early researches mainly focused on the initial value in marine system, while few studies on the terrestrial system are available. Interestingly, some recent analytical results coming from experiments on the pre-nuclear 129I level in the terrestrial environment showed difference. Meanwhile, some speculations are evoked by these observations and findings. Materials and methods To make a forward step of exploring the initial value of 129I on the entire earth surface and of understanding the possible mechanism of iodine isotopes circulation in the environment, we reviewed most of the relevant literatures in the past decades. Results 129I initial value started from estimations since 1960s. Experimental results in deep sea sediment gave an initial value to be 1.5× 10 ^-12. This value was widely accepted and applied for geological dating with samples formed in marine phase. Investigations on terrestrial environment including analysis of fluvial sediment, lake sediment, lignite, petrified wood and loess samples showed variations of the pre-nuclear level, most of the results being higher than that in the marine system. Illustrations were made for the indetermination and those unreliable results. Our work on the speciation analysis of 129I in loess profile presented the pre-nuclear level of 129I in Chinese loess deposits which is still an order of magnitude higher than the initial value in marine. The initial value in the terrestrial environment is in suspense, but there seems to be possibility of the clue in formation and possible variation of the iodine isotopes in the environment. Discussion Iodine can easily reach to isotopic equilibrium in the marine system, the main reservoir on the earth surface. However, it might be difficult for the iodine to transport to the terrestrial environment uniformly because of the different sources of 129I and 127I as well as the distance to the sea. These are all the possible factors that might influence the composition of the isotopes. Based on the knowledge from early studies and some recent observations, authors speculated that the different sources of 129I and 127I, together with the lack of stable iodine induced by the depletion of marine-source iodine during the transportation of air mass into the inner land, might be the main cause of the 129I/127I level difference between marine and terrestrial system. Conclusions Difference in the initial value of 129I in marine and terrestrial system might exists while it is still obscure in the terrestrial environment. Clue in the isotopic difference seems possible to be found in the work on natural 129I in Chinese loess. Recommendations and perspectives Our speculation about the cause of the iodine isotopic difference between marine and terrestrial system remained subjective and arbitrary. The initial value of 129I in the terrestrial environment and the mechanism of the transportation of iodine isotopes on the earth surface need further study.