鈾-238及釷-232為首的兩大衰變序列，經過多次α和β衰變，最終衰變成穩定核種鉛-206及鉛-208。在封閉系統中，母子核種間的活度會相等達到平衡，但由於開放環境中物理、化學風化和生物作用下造成的，造成自然界中存在著不平衡的現象。當α衰變時，由於核反衝效應（alpha-recoil）使子核種（如鈾-234）反彈至晶格缺陷或顆粒表面，使子核種相較於母核種（鈾-238）更容易淋洗釋放到水體中，使水體中（或自生相中）鈾-234／鈾-238的活度比值大於1，在岩屑相中則小於1。鈾在氧化環境時，會從不易溶解的四價態氧化成可溶解的六價態，並與水溶液中的碳酸根離子形成穩定的UO2(CO3)3-4，經由化學風化作用進入水相中。
　　本研究地點位於墨西哥灣邊緣海所採集的岩心，以6N鹽酸溶濾，分析鈾系及釷系的放射性同位素（鈾-238、鈾-234、釷-232、釷-230、釷-228、鐳-228、鐳-226、鉛-210、釙-210）於自生相和岩屑相中的活度變化，及各核素間的同位素活度比值作為示蹤，了解風化作用與邊緣海的地球化學過程。結果顯示自生相中的放射性活度較岩屑相高，且放射性同位素的不平衡現象，說明著沉積及地球化學過程所造成的元素吸附與遷移。而岩屑相的變化可視為陸源物質於傳輸搬運時所受風化作用的影響，自生相的變化則受風化作用與沉積後的地球化學過程影響。利用鉛-210定出的沉積速率約為每年0.09公分，質量累積沉積速率則為每年每平方公分0.04克。鐳同位素的富集與總有機碳隨深度的變化，於2公分深度發現氧化還原界面的存在。
　　利用鈾系放射性核素間的不平衡關係，與其在自生相和岩屑相上的分佈趨勢，有助於了解沉積物質的來源、沉積作用、風化歷史、地球化學過程，更可藉以說明大陸風化的速率與通量、成岩作用，以及沉積環境中生物地球化學行為。

　　In igneous rocks, U-238 reaches secular equilibrium with its daughter radionuclides, giving activity ratio of uranium series of unity one. When the rock is in contact with water, the α-recoil effect during the rock weathering usually causes the daughter nuclide (U-234) to be preferentially released into solution than the parent nuclide (U-238), such that the solution or authigenic minerals have U-234/U-238 of > 1, and the detrital phases have this ratio of < 1. In the marginal sea, the dissolved uranium with a constant U-234/U-238 ratio of 1.145, can be removed from sea water to sediments by adsorption, co-precipitation, or biological processes, via the reduction from the soluble U+6 to the less soluble U+4, contributing additional authigenic uranium to the sediments.
　　A systematic investigation on distribution of major U- and Th-decay series radioisotopes e.g. those of uranium (U-238, U-234), thorium (Th-232, Th-230, Th-228), radium (Ra-228, Ra-226), lead (Pb-210), and polonium (Po-210) etc., in the authigenic and detrital phases, respectively, for a sediment core GC234 S1#3 collected from the ocean margin of Gulf of Mexico. The sedimentation rate was determined via Pb-210 dating. The study aims at exploring the tracer utilities of U- and Th-series radioisotopes to understand the present and past changes in continental weathering and biogeochemical processes in marginal sea and sediments. The distributional patterns and disequilibrium relationships of the uranium series nuclides, provide new insight on the sources of sediment materials, the rate of continental weathering, sedimentation, as well as diagenetic, biogeochemical reactions in sediments.

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