鈾釷同位素比值的演變在地球化學研究被公認為非常重要的指標，湖泊或水庫沉積物中同位素比值就像指紋般記錄了陸地上過去與現在的水文與氣候之變化，可用來追蹤陸地上物理侵蝕、化學風化的來源和過程。
　　本研究主要以alpha能譜分析儀偵測翡翠水庫上、中、下游河水流域沉積物自生相中天然衰變系放射性核種鈾、釷及其同位素之活度。研究由二部份組成：（1）沉積物過剩Pb-210定年及其環境意義和（2）以自生相U-234/U-238、Th-230/U-234、Th-228/Th-232比值探討翡翠水庫鈾釷同位素地球化學行為及遷移機制。
　　本研究與中研院環變研究中心合作從翡翠水庫三個不同地點取得岩芯樣品，其中下游火燒樟岩芯樣品較長，並且與氣候的周期性變化相一致。岩芯實驗結果顯示過剩Pb-210在三個岩芯中均一致呈現周期性變化，因為下游火燒樟岩芯較長，其過剩Pb-210隨深度指數性衰變並得到其沈積速率為0.81±0.05cm/yr，由過剩Pb-210層位比對法可得上、中游的沈積速率分別為0.63±0.04cm/yr與1.05±0.06cm/yr。過剩Pb-210結果還顯示非常高的過剩Pb-210值在三個岩芯中均有出現並且和颱風所帶來高降雨量呈現正相關。
　　研究發現翡翠水庫年降雨量增大時，由於alpha recoil效應間接造成岩芯自生相中U-234/U-238的比值呈現異常偏高，反應岩石的物理風化與侵蝕速率增大。Th-230 /Th-232活度比在 0.42~0.63 之間，接近或略小於台灣陸地岩石之U-238 /Th-232 活度比值（大約是0.6），顯示自生相從水體中吸附Th-230的量可忽略不計，反而鈾從顆粒中淋洗是造成自生相Th-230耗損的主要原因。自生相Th-228/Th-232比值多大於1，反映Th-228在自生相中富集，其原因主要是Ra在水中溶解和可移動性較高，以及碎屑相Th-232衰變時的alpha recoil效應所致。

　　The variation of isotope ratios of uranium and thorium has been reganised in lake or reservoir sediments ratios can be as used as an fingerprints,an important index in the studies of geochemistry.The isotope also to mark the past and present hydrological and climatic history changes in the continents. It can also be used to track the sources and processes of physical corrosions and chemical weathering of rocks.
　　This research investigates the activities of the natural decay-series U,Th and Pb radioisotopes in the authigenic phase of sediments from the upper,middle and lower reaches of the Fei-tsui reservoir by using alpha spectrometry. The research consists of two parts: (1) （210Pbex）and its application to sediment dating and studies of environmental changes. (2) investigation of the ratio of U-234/U-238, Th-230/U-234, and Th-228/Th-232 in authigenic sediments to understand geochemical behavior and transport mechanisms of uranium and thorium isotopes in The Fei-Tsui Reservior.
　　There are consistent periodical changes of results show that 210Pbex in three sediment cores,in concordance with the changes of the climate and hydrology in the region.Because of the longer record of core HSZ（Huo-shao-zhang）,in the lower stream, 210Pbex exhibits a general trend of exponential decrease with depth,giving the sedimentation rate of 0.81±0.05cm/yr by using stratigraphic comparison of 210Pbex, the sedimentation rate in the upper and middle streams is estimated to be 0.63±0.04cm/yr and 1.05±0.06cm/yr, respectively.
　　210Pbex measurements also show abnormally high values in some depths of the three cores and these abnormally high 210Pbex values seem to positively correlate with the changes of typhoon-related high rainfall precipitation in the area.
　　This study found that U-234/U-238 ratio in authigenic phase of the Fei-Tsui reservior sediments increases with increasing annual rainfall precipitation in the region,suggesting increased rates of physical weathering and erosion of rock in association with the typhoon-caused greater rainfall storms. Th-230/Th-232 ratios range from 0.42 ~ 0.63,close to or slightly smaller than its ratio in original rocks（~0.6）, suggesting that negligible Th-230 adsorption from reservoir water.The depletion of Th-230 in authigenic phases is mainly attributable to the leaching by the water. Th-228/Th-232 ratios are mostly＞1, reflecting Ra-228 enrichment in the authigenic phases,caused by higher solubility and mobility of Ra isotopes in waters and or alpha recoil input from Th-232 decay in detrital phases.

中文部份
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