宍道湖における17世紀初頭の汽水から淡水への環境変化 : コアのCT画像解析と堆積,C・N・S元素分析による検討

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Geohistorical studies show that the environment of Lake Shinji changed from brackish to fresh water around the seventeenth century again. This environmental change is detectable by CT (computed tomography) scanning images and C.N.S elemental analyses of core samples. CT scanning images of core samples suggest that the sediments can be divided into two units, Unit I and II. The uppermost unit I is composed of fine- to very fine-sand and silt layers, with bioturbation and shell fragments. Fine- to very fine-sand layers can be distinguished clearly on cr scanning image pictures. The underlying unit 11 is represented by silt layers. and lacks bioturbation and shell fragments. In this unit, sedimentary structures comprising 2-3 mm thick laminations can be distinguished on cr scanning image pictures. C.N.S elemental analyses of Unit I show that sulfur and total organic carbon contents are low, suggesting a fresh and oxic water environment. Unit II exhibits higher sulfur and total organic carbon contents, which indicate brackish and anoxic water conditions. This change may have been brought about by the eastward migration of the Hii River after large floods around A.D. 1630. CT scanning images reveal the existence of laminated structures in Unit II, as shown in the color photo. These structures may represent rhythmical environmental changes. The structures are not visible with the naked eye, and were also not revealed by previous soft-X ray analysis. CT image analysis has thus proved to be a useful method for analyzing muddy soft sediments.