| Summary: | 碩士 === 國立成功大學 === 地球科學系碩博士班 === 95 === Uranium-238 is a radionuclide with a half-life of 4.468×109 yrs. It decays to a stable nuclide lead-206 through a series of short-lived radionuclides. Because of alpha-recoil effect, uranium-234 will exist preferentially on mineral surface or defect in mineral lattice when uranium-238 decays to uranium-234, causing preferential transport of uranium-234 to lakes or oceans when water flows through the rocks. On the other hand, uranium element exits in 4+ oxidation state in the primary igneous rocks and minerals, but uranium can be oxidized to 6+ states in oxidized environments. The 6+ oxidation state forms soluble uranyl complex ions （UO22+） transported to lakes during the weathering. This unique feature makes uranium concentration and uranium isotope ratio very useful to determine the changes of paleo-intensity of physical and chemical weatherings on the Earth surface. This work aims at using uranium isotopes in lacustrine diatomaceous microfossils as a proxy to investigate the past changes in the rate of chemical and physical weathering in Taiwan island during the past 10,000 years. Sediment cores were collected from Lake Shuang-Lien Pi in Taiwan and diatoms were separated from the sediments and cleaned to measure their U/Si and 234U/238U ratio by ICP-MS.
Measurements of 234U/238U in diatom tests, together with those of δ13C, δ18O, TC and TOC in lake sediments, provide rich information on the paleoenviromental and paleoclimatic changes in the Taiwan. Such multi-proxy approaches provide a useful calibration and validation on the proposed use of uranium isotope ratios as a proxy of paleo-weathering intensity on lands.
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