Characteristics Of Sediments From The Main Tributaries Of The Yangtze River And Their Implication For The Provenance Identification Of The Sediments Of The Yangtze Estuary

The Yangtze River is the third largest river in the world and delivers huge amount of sediment into the sea (4.33×10⁸ t, Datong Station, 1995-2000), which makes an important contribution to the development of China's coast, the East China Sea continental shelf and the global fluvial material flux to the oceans. Recognition on the source of the Yangtze River Estuary sediments forms one of the focuses of the land-ocean interaction study. In this paper, magnetic and geochemical analyses were carried out on surface sediments from the Jinshajiang River, Jialingjiang River and Hanjiang River (the three main tributaries of the Yangtze River in terms of sediment discharge), with the purpose to discriminate sediments from the three rivers and provide diagnostic method for identifying the provenance of the Yangtze Estuary sediments.In addition to bulk magnetic and geochemical analyses, all the samples were separated into the > 63μm and < 63μm fractions in consideration of sediment sorting during transport. Furthermore, two representative samples from each region were subjected to more detailed particle size separation, resulting in seven particle size fractions. The obtained size fractions were then subjected to the above mentioned measurements.The results indicate that the magnetic properties of surface sediments from the three rivers are all dominated by ferrimagnetic minerals, together with the contributions of anti-ferromagnetic minerals. Higher values ofχ, SIRM, HIRM andχ_ARM for sediments of the Jinshajiang River suggests higher concentrations of ferrimagnetic and anti-ferromagnetic minerals, which are related to widespread distribution of basalts in this river basin. The Jialingjiang River sediments display lowest S_-300 and lower S_-100 values, manifesting higher contribution of anti-ferromagnetic minerals. The widespread occurrence of Mesozoic red beds can lead to the higher contribution of anti-ferromagnetic minerals in this river catchment. Compared to the Jialingjiang River, higher values of SIRM andχ, and highest S_-100 in the Hanjiang River may be due to the rock types of metamorphic rock and granite in the basin. In the case of the mean values ofχ, SIRM andχ_ARM, higher values occur in the <63μm fraction in the Jinshajiang and Hanjiang River, but in the > 63μm fraction of the Jialingjiang River sediment. Concentration-related parameters (χ, SIRM, HIRM,χ_ARM) peak in the fractions of 4 - 16μm, 2-4μm and > 63μm fraction in the Jinshajiang, Jialingjiang and Hanjiang River sediments, respectively.In general, the composition of major and trace elements are similar between the sediments of the Jialingjiang River and Hanjiang River. The concentrations of the major elements (e.g., Fe, Ca, Mg and Ti) and trace elements (e.g., V, Ni, Pb, Zn and Cd) show the highest abundance in the sediments of the Jialingjiang River, while the highest value of Na in the Hanjiang River. Al, K, Ba, Sr and Mn show no apparent difference among the three rivers. The lower enrichment factors of Na and Sr are consistent with their mobile features during the weathering process. Most of the elements decrease with increasing particle size, while Na and Ca in the Hanjiang River display an opposite trend. Ti and Sr (except for the Hanjiang River) are relatively similar across the size factions. The majority of Ca is associated with carbonates in the Jinshajiang and Jialingjiang River.The following bi-variable plots can discriminate sediments from the three rivers efficiently: (1) bulk samples (bi-plots of S_-100 versus SIRM, S_-300, Ti, respectively); (2) < 63μm fraction (bi-plots of S_-100 versus SIRM, S_-300, Mg, Ti, respectively and Ca_HCl/Mg_HCl versus Mg); (3) > 63μm fraction (bi-plots of S_-100 versus SIRM, S_-300, respectively).Quantitative modeling reveals that the Jinshajiang and the Jialingjiang River contribute dominantly to the sediments at Datong station (the starting point of marine influence) in the < 63μm fraction, whereas the Hanjiang River contributes more in the > 63μm fraction. In the 2 - 4μm, 4-8μm and 8-16μm fractions, the Jinshajiang River contributes about 50%, while the Jialingjiang River serves more than half of the volumes in the < 2μm fraction.