Characteristics Of Suspended Particles And Sediments From The Yangtze Estuary And Yangtze River Basin

The Yangtze River is the largest river in China, which shows a significant decrease of sediment load in the recent years. Such a change in sediment discharge have direct influence on the geomorphology and deposition process of the Yangtze Estuary. A knowledge of estuarine sediment composition is critical to the understanding of the source to sink process of the Yangtze River. In this paper, magnetic analyses were carried out on surface sediments and suspended particles from the Yangtze Estuary, Jialing River and some sites in the upper reach of the Yangtze River, with the purpose to explore the temporal variations of estuarine sediment composition.The results indicate that magnetic properties of surface tidal flat sediments (collected in September 2004-July 2005, August 2006-July 2009) from the Yangtze Estuary are all dominated by ferrimagnetic minerals. Due to the seasonal variation of particle size, magnetic properties show significant seasonal variations:from December to July of the following year, sediments show finer particle size and high values ofx, SIRM and HIRM; from August to November, sediments have coarser particle size and low magnetic value of the above mentioned magnetic parameters. The result indicates that magnetic properties of surface sediments can reflect the changes in sedimentation dynamics linked with tidal flat accretion-erosion cycle.Suspended particles from 6 sites (Xuliujing, Gaoqiao, Hengshan, Jiuduansha, Sheshan, Luchaogang) in the Yangtze Estuary are also dominated by ferrimagnetic minerals. Values ofχ, SIRM,χARM/SIRM from Sheshan show fluctuation with the spring-neap tidal variation, with higher concentration of ferromagnetic minerals in samples collected at spring tide due to stronger resuspension of sediments. Comparison to samples from Xuliujing, samples from Luchaogang show lower magnetic mineral contents and finer grain size, which is possibly caused by the mixing of fluvial material from the Yangtze River with magnetically weak resuspended sediments in the estuary.Magnetic measurements and particle size of three major tributaries of the Jialing River indicate that magnetic properties of the sediments are dominated by both magnetite and hematite, which reflects the fluvial input from the Mesozoic red beds widely distributed in Sichuan Basin. A combination of magnetic parameters ofχ, SIRM and S-300 can discriminate the sediments from the three tributaries very well. The samples from the downstream of Jialing River have significant similarity to samples form the main stream of Jialing River and Qujiang River.In comparison to suspended particles and sediments from the upper reach of the Yangtze River, sediments in the estuary show lower concentration of magnetic mineral. It suggests that the magnetically strong sediment of Jinsha River is diluted by the tributary and marine inputs during the transportation from the upper reach down to the estuary. The decline of sediment discharge from the upper reach of the Yangtze River in recent years could be the reason for the declines of magnetic parametersχand SIRM in sediments from the tidal flat collected at Luchaogang.