09:35 〜 09:50
[U03-03] The Application of Long-Lived Anthropogenic Uranium Isotopes to Geoscience Research
★Invited Papers
キーワード：人工放射性核種、ウラン同位体、サンゴコア、堆積物コア
A variety of natural and anthropogenic uranium isotopes are present in the environment, and conventionally the parent nuclides of the uranium and actinium series 238U and 235U, as well as the progeny 234U, have been used as tools for geochemical/science research. In recent years, developments in mass spectrometry have made it possible to measure ultra-trace amounts of radionuclides in the environment. In particular, it is possible to measure attograms (10-18 g) orders of nuclides by utilizing accelerator mass spectrometry (AMS), which in turn has promoted research on exotic nuclides in the environment. Of such nuclides, we have chosen to focus on 236U (half-life 2.342×107 years) and 233U (half-life 1.59×105 years), which were released into the environment as a result of human nuclear activities. We attempted to reconstruct when and how much of these nuclides were introduced into the study area by using corals and sediments. Further, we also attempted to use said nuclides as tracers for studying environmental dynamics and as age markers for defining the Anthropocene.
The introduction history of anthropogenic uranium reconstructed from coral cores and a sediment core differs depending on the uranium isotope, with the 236U/238U atom ratio increasing rapidly from the early 1950s, with a maximum observed in the mid-1950s and early 1960s. The maximum value of the 233U/238U atom ratio was found in the 1950s. In particular, 233U was produced mainly by nuclear reactions involving high-energy neutrons, which were subsequently released into the atmosphere, so it is expected to be available as an age-specific marker anywhere in the world. In addition, the 233U/236U ratio, as anthropogenic U, can be used as a tool for identifying origins and for studying environmental dynamics without having to consider changes in the ‘concentrations’ of uranium in the system; the ability for U to be deposited as sediments and/or incorporated into corals. Thus, it is expected that these isotope ratios will find further use in the research of environmental dynamics.
The introduction history of anthropogenic uranium reconstructed from coral cores and a sediment core differs depending on the uranium isotope, with the 236U/238U atom ratio increasing rapidly from the early 1950s, with a maximum observed in the mid-1950s and early 1960s. The maximum value of the 233U/238U atom ratio was found in the 1950s. In particular, 233U was produced mainly by nuclear reactions involving high-energy neutrons, which were subsequently released into the atmosphere, so it is expected to be available as an age-specific marker anywhere in the world. In addition, the 233U/236U ratio, as anthropogenic U, can be used as a tool for identifying origins and for studying environmental dynamics without having to consider changes in the ‘concentrations’ of uranium in the system; the ability for U to be deposited as sediments and/or incorporated into corals. Thus, it is expected that these isotope ratios will find further use in the research of environmental dynamics.