河川匯集而成的入海口為上游風化後陸源物質經由河水或地下水進入海洋的管道，因此，河口成為全面性研究大陸對海洋的物質輸送及地球化學遷移的重要區域，為評估河流-河口的物質通量及生物地球化學行為，本研究在中國南方珠江河口採集沉積物岩芯並據深度不同來測量衰變系同位素以瞭解其分佈。由α譜儀測得沉積速率為4.4±1.2公分/年和γ譜儀所測得是3.8±1.2公分/年，藉此訂定此岩芯的年代。另外，Th-228相對於Th-232的虧損顯示，對Th-228的母核種Ra-228有地下水大量的遷移行為。Th-230/U-234比值多大於一，表示在流域內風化期間U相對於Th的流失較多，且在此岩芯中發現U-234富集於部分深度的層位中，顯示可能在還原環境下有自生鈾的貢獻；這結果與同層位的U-238/Th-232和U-234/U-238比值、總有機碳及Th-232通量出現類同性的變化，同時也伴隨著周期性乾濕季的氣候變化，也可能導因珠江地區的經濟相關產業的快速發展所造成的人為汙染。我們的研究顯示，放射性同位素於河流-河口沉積物的測量上，提供一個對於發生在大陸與海洋交界的物理、化學及生物過程極有效的工具。

Estuary is an important land-sea interface where active biogeochemical processes occur as a result of river in-fluxes of dissolved and particulate weathering materials from the continent to the ocean. To assess these in-fluxes and the associated biogeochemical processes, a core was collected from the estuary of Pearl River in south China and was measured for the distributions of the decay-series radioisotopes with depth. The sedimentation rate was dated by using excess Pb-210 to be 4.4±1.2 cm/yr (α-spectrometer) and 3.8±1.2 cm/yr (γ-spectrometer), respectively. The thorium isotope measurements exhibit significant deficiency of Th-228 relative to Th-232, implying a significant out-flux of Ra-228 out of sediments by subterranean groundwater discharge. The Th-230/U-234 activity ratios are mostly greater than one, resulting from preferential loss of uranium relative to thorium during the weathering of source rock in the drainage region. Excess U-234 relative to Th-230 as found at some depths indicates an enrichment of authigenic uranium in sediments under reducing conditions. These results, together with the measurements of U-238/Th-232 and U-234/U-238 in the same core, clearly recorded a concordant change in the continental weathering flux, the POC flux, and the redox conditions in Pear-River sediments during the past decades, a change which may attributed to the dual causes of the decadal wet/dry climate changes in the region as well as the anthropogenic contaminations of the Pearl River associated with the rapid regional industrial/economic development. Our study shows that the decay series radioisotopes in estuarine sediments provide a useful tool for understanding the physical, chemical, and biological processes occurred in the continents and/or at the land-sea boundary.

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