河口是大陸物質通過河流運輸到海洋的主要途徑，存在著許多複雜的生物地球化學過程，而沉積物記錄著沉積環境的變化，我們能利用沉積物中的鈾釷系同位素活度及其比值來研究大陸風化及沉積環境的地球化學過程。本實驗樣品有長江口沉積物以及採集自濁水溪河口的沉積物岩心（JRD-N，JRD-S），也採集2004年敏督利颱風期間的濁水溪懸浮沉積物，採用前人的方法以6N 鹽酸分相實驗將沉積物進一步分離為自生相和碎屑相。在經過化學純化程序後，利用α能譜儀測量沉積物自生相和碎屑相中的鈾釷系列放射性同位素如238U、234U、232Th、230Th、228Th和210Pb。測量其放射性活度及活度比值，藉由其放射性不平衡的關係來探討濁水溪的過去環境變化和地球化學過程。
放射性活度在河流上下游沉積物有著明顯差異，下游的沉積物放射性活度低於上游的沉積物，對照著6N鹽酸分相後的碎屑相殘餘率，推斷下游部分含有較多放射性活度低的石英，在颱風期間的沖刷使得帶有大量石英的砂進入濁水溪下游，造成濁水溪沉積物放射性活度的稀釋效應，懸浮沉積物的放射性活度及其比值的變化主要受水動力影響。
利用碎屑相和自生相中的鈾和釷系列不平衡的分佈關係，提供了解風化過程和氧化還原環境的有用指標，並且不同於前人使用全溶法作分析，其結果代表整體趨勢分佈無法有效得知各來源之貢獻及影響。
232Th來自於陸源物質提供，從232Th在自生相與碎屑相中的比例可以看出化學風化的程度，當自生相所佔的比例越大的時候說明化學風化越加強烈，從放射性活度結果得知長江口大於濁水溪，而南側又大於北側；藉由物理風化會造成234U/238U比值在兩相間有不同趨勢，結合14C定年結果得知，物理風化強度在樣品在冰期時大於間冰期，冰期時氣候以乾冷為主以物理風化主導與放射性活度比值結果一致。
228Th/232Th和210Pb/230Th的比值主要受228Ra與226Ra在氧化還原環境的富集與損失所影響，由於228Ra與226Ra的半衰期時間長短差異能夠指示不同時間尺度的氧化還原環境。228Th/232Th指示短時間尺度的氧化還原環境，可能在近幾十年受地下水遷移影響，放射性活度比值結果說明濁水溪北側比南側氧化，而長江河口沉積環境則較為還原。210Pb/230Th的不平衡是226Ra, 222Rn以及雨水造成的過剩210Pb，濁水溪河口沉積物自生相中210Pb/230Th的比值較大，其中Rn屬於氣體不易再度富集，而雨水造成的過剩210Pb則因為由河口沉積物的定年結果看出，沉積年代久遠使得過剩的210Pb會衰變而不造成貢獻，所以其比值結果可看出沉積環境較為氧化有Ra的富集，造成自生相比值的增加，長江河口則相反，較為還原造成Ra的損失而比值下降。總結來說濁水河口沉積物的分析結果以物理風化主導且處於氧化條件，長江口則是化學風化主導的還原環境。

Estuaries are a major pathway of continental material transport through rivers to the oceans and lots of biogeochemical reactions occurred in these regions. The sediment records of a fluvial system are expected to bear signals of the changing sedimentary environments. The sample contains Yangtze River estuarine sediments and the sediment cores (JRD-N, JRD-S) collected from the Choshui River estuary, and collected the suspended sediment from typhoon induced continental weathering and erosions. Sediments sampled were further separated into authigenic and detrital phase with 6N HCl leaching experiment. Investigate the naturally-occurring uranium-series radioisotope e.g. those of uranium (238U and 234U), thorium (232Th, 230Th and 228Th), and lead (210Pb via 210Po) in the authigenic and detrital phase respectively with alpha-counting techniques following radiochemical purification procedures. Measurement of radioactivities and activity ratios in the detrital and authigenic phases of sediments to understand the past environmental change and geochemical process.
The activities in downstream are generally lower than these in upstream, and there are high negative correlation between activities and sediment flux. The results indicated that more sand be washed into the river in Typhoon Mindulle event, the numerous of silt were inserted to Choshui River with severe dilution effect. The change of radioactivities mainly caused by hydrodynamic.
The disibution patterns and disequilibrium relationships of uranium- and thorium-series disequilibrium in the detrital and authigenic phases provide the useful tool to understand weathering process and redox condition, and more valuable than based on total sediments.
232Th is a good proxy for lithogenic particles. The stronger continental chemical weathering could raise the thorium activities in authigenic phase. The chemical weathering intensity in Yangtze River was greater than in the southern Choshui River estuary and greater than in the northern Choshui River estuary. Highly physical weathering may trigger the fractionation between 234U and 238U, the physical weathering intensity in the glacial period was greater than in the interglacial period.
228Th/232Th and 210Pb/230Th ratios as a tracer to reconstruct the past changes of redox conditions. From the cored results, Yangtze River sediments deposited under anoxic conditions and Choshui River estuary was under oxygenated conditions.
In conclusion, water-sediment interaction in recent years, physical weathering dominated and under oxidizing condition in Choshui River estuarine sediments. The chemical weathering is the dominant mechanism in Yangtze River and sediments deposited under anoxic conditions.

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