GEOCHEMICAL CHARACTERISTICS OF THE SURFACE TIDAL FLAT SEDIMENTS AT THE YUANTUOJIAO POINT, NORTH BRANCH OF CHANGJIANG RIVER AND IMPLICATIONS FOR PROVENANCE

The North Branch of the Changjiang River, to the north of the Chongming Island, was once the main channel of the estuary. Human reclamation and the south ward migration of the main channel led to the narrowing and shallowing of the North Branch. The Yuantuojiao Point is the intersection of the North Branch of the Changjiang River and the South Yellow Sea coastal line. The specific geographic location and the complexity of regional morphological dynamics jointly give birth to the typical silt-mud tidal flat. 22 surface sediments samples were collected on the areas of breeding, salt marshes and the mudflat at the Yuantuojiao Point. 19 elements both in contents and distributions were analyzed for the surface sediments. The main sources of sediments of the tidal flat near the Yuantuojiao Point were discussed according to the correlation analysis and elements enrichment coefficients. The study results indicate that the change trend is of difference among Na, Ca, Ti, P and Sr, but that of the other 14 elements has better consistency. The macroelements and microelements contents of the surface sediments have the highest on the salt marshes areas, the lower on the mudflat areas, and the lowest on the breeding areas. Calculation results of correlation analysis and enrichment coefficients of elements show that most elements of the surface sediments at the Yuantuojiao Point may have more coherent sources, and their average contents were similar to that of the Yellow Sea. Based on the grain size trend of surface sediments near the Yuantuojiao Point, the extension directions of alongshore bars and coastal current, and the comprehensive analysis of the muddy tidal flat sedimentary area, the south of Hengda levee at the southern part of submarine radial sand ridges, it is concluded that the sediments of the tidal flat near the Yuantuojiao Point is derived mainly from the southern part of the submarine radial sand ridges of the South Yellow Sea.