| Summary: | 碩士 === 國立成功大學 === 地球科學系 === 103 === Continuous sediment accumulation in Antarctic lakes provides useful information on past environmental/climatic changes in polar region. To reconstruct such changes, a sediment core (L2-2) was collected from a lake basin in the Antarctica for the analysis of U- and Th-series isotopes and carbon contents. The measured excess 210Pb activities show a decreasing trend with depth, superimposed by about 7-8 cyclical changes through the core. The results give a mean sedimentation rate of ~4.68±1.81 cm/yr and a seasonal variability in excess 210Pb flux to the lake. Activity ratios of 234U/238U and 230Th/238U show an intense physical weathering in the study site proving the high sedimentation rate. Against conventional view, 238U activity do not show apparent variation through the core. On the contrary, 228Th/232Th ratios with lower errors and distinct variation indicates three times changes of redox conditions within 8 years which is related to precipitation and sea ice albedo. Different from previous researches, it is found in this study that 228Th/232Th ratio showed a more sensitive response in environmental redox conditions than conventional use of 238U. Against a typical view, we find no simple correlation between 238U activity and organic carbon content in core L2-2. Our study find out the existence of graphitic carbon in Antarctica and shows that carbonaceous materials in the core occur mainly in the form of graphitic carbon and labile organic carbon of lacustrine origin. Both 228Th/232Th and organic carbon content of lacustrine origin can be used as a paleo-redox indicator of the ancient lakes.
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