Study On Provenance And Component Changes Of The Yangtze River Sediment: An Environmental Magnetism Application

Under the background of intensification of human activities in the Yangtze Basin and declining sediment load into the sea, to seek the host factors which impact the sediment transport dynamic and morphology evolution in coastal zone, it still need to focus on sediment source and composition change first. Human activities, including land use and intensification, influence sediment load in the Yangtze Catchment more than before, even the main stream and tributaries involved.Wide-range, high-quality samples for analysis, is the basis of such research work. With particle size measurement, magnetic measurement, grain size separation experiment and statistical methods combined, considering surface soil erosion and fluvial sediment transport process as a system, analyzing magnetic characteristics of topsoil, fluvial sediment and estuarine sediment, this study will discuss the source and the composition changes of the Yangtze River sediment. The main conclusions are showed as follows:1 The magnetic properties of topsoil, sediment from tributary, mainstream and estuary at the Yangtze Basin:The result of magnetic measurement of topsoil and sediment which taken from Jinshajiang (Shigu) to Yangtze Estuary, show that x values of topsoil are 9～1955* 10-8m3/kg, SIRM values are 2136～235654*10-6Am2/kg, XARM values are 23～8185* 10-8m3/kg, HIRM values are 15～7716*10-6 Am2/kg, S-300 values are 40～100%, Xfd% values are 0.9～15%, XARM/SIRM values are 0.06～1.44*10-3m/A. The x values of fluvial sediment are 2.2-665.1*10-8m3/kg, SIRM values are 192～84820*10-6Am2/kg, XARM values are 4.9～1600.5*10-8m3/kg, HIRM values are 37～1578*10-6Am2/kg, S-300 values are 75-100%, Xfd% values are 0.1～11.4%, XARM/SIRM values are 0.07～1.35*10-3m/A. The result indicates that much difference of magnetic properties within all samples exists. The content of magnetic minerals in topsoil is much higher than that in sediment. Besides, the content of magnetic minerals in sediment from upper basin is much higher than that in sediment from lower basin. The sediment from northern basin have much more magnetic minerals than that from southern basin.2 A discussion of magnetic properties of fluvial sediment and topsoil:Basalts are widely distributed around the upper basin and provide a good "background". Therefore, the content of ferrimagnetic minerals of fluvial sediment and topsoil in this region are very high, such as fluvial samples from Jinshajiang river, Yalongjiang river, Daduhe river, Minjiang river, Niulanjiang river. Purple soil and Southern red earth provide abundant of anti-ferrimagnetic minerals in the Jialingjiang and Wujiang basin, respectively. So HIRM values of the sediment from Chishuihe river, Wujiang river, Jialingjiang river and Fujiang river are relatively high. Granite, slate and carbonate rock are the primary rock type in the middle-lower basin, with low magnetic mineral content. As a result, magnetic mineral contents of downstream tributaries'sediment which are taken from Xiangjiang river, Yuanjiang river, Ganjiang river and Fuhe river are low too.3 The longitude change of magnetic properties of fluvial sediment along the mainstream, and its implication for the provenance identification:From upper reach to lower reach, the type and grain size of magnetic mineral change little. However, magnetic mineral's content decreases gradually. TGD'impact is the main factors which control magnetic mineral'change. Based on magnetic properties of bulk sample,<63μm fraction and>63μm fraction, using Q-mode cluster analysis, combining with corresponding physical process, to discuss the main source of Yangtze River sediment. The result indicates upper reach area (Nanxi station && Jiangjin station), magnetic properties of its bulk sample and<63μm fraction, are similar with topsoil along the river bank. It indicates that topsoil play a important role to the source of mainstream sediment. Jinshajiang river is the main source of Yangtze River sediment.<63μm fraction of Jinshajiang sediment can influence magnetic properties of middle reach's fine sediment to a large extent. Then, Minjiang, Jialingjiang, Wujiang rivers, Dongting Lake and Poyang Lake'fine-grain sediment make a relatively increasing contribution to fine-grain sediment of middle-lower reach. Hanjiang river sediment's imput, have a significantly impact on the low reach sediment (mainly coarse-grain).