| Summary: | 碩士 === 國立成功大學 === 地球科學系 === 105 === 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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