Characterizing the sediments from tributaries and estimating their mixing ratio in the sediments from Yangtze River mouth, using ESR and CI of quartz

Yangtze River is the longest river in East Asia and has 8 major tributaries. Over 400 million people lives in its drainage, and the flood of Yangtze River strongly affects the human activity. Because of its huge drainage area, middle to lower reaches of Yangtze River has long history of floods. Thus, it is meaningful to reconstruct temporal and spatial distribution of floods in the past to understand their causes, magnitudes, and controlling factors. To reconstruct the behaviours of paleofloods, it is useful to utilize the sediments from the rivermouth of Yangtze, identifying flood deposits, and reconstructing their provenance and its changes through time. In this study, Electron Spin Resonance(ESR) and Crystallinity Index(CI) of quartz are used as proxies to distinguish the sediments from each tributary of Yangtze River. ESR signal intensity is roughly proportional to the age of the mother rock, and the values of CI reflect the type of the mother rock. We analyzed the river bed sediments from each tributary and mainstream. ESR signal intensity of the tributaries in the upper reach shows low values(0-2). In contrast, ESR signal intensity of the tributaries in the middle reach tends to show higher values(7-10). The values of Yangtze mainstream show an increasing trend from the upper to lower reaches. As to CI, the upper reach shows low values (8.2-8.3) and some tributaries in middle reaches show higher values(around 9). The sediments from each tributaries can be distinguished using these two proxies. To test whether it is possible to specify the tributaries where the flood source water comes by examining the provenance of the sediments near the river mouth, we conducted a simple numeral calculation in three cases. The first case is the normal condition, the second case assumes the flood in the upper reaches, and the third case is the flood in the middle reaches. The result shows the detectable difference among these three cases. This suggests we can distinguish the changes in major precipitation area within the tributaries of the Yangtze River from the flood sediments near the mouth of Yangtze.