Comparative Studies On The Magnetic Properties Of Surface Sediments From The Changjiang Estuary And The Huanghe Estuary

In this paper, magnetic properties, geochemical and particle size composition of the surface sediments from the Changjiang (Yangtze River) Estuary and the Huanghe (Yellow River) Estuary are analyzed systematically, with the purpose to provide new indicators in distinghishing between sediments of the Changjiang and the Huanghe.To reflect magnetic properties of the sediments from the Changjiang and Huanghe fully, surface sediments were collected from the river channels and higher tidal flat of the estuaries. The Changjiang Estuary includes two regions, the river section from Datong to Jiangyin and the estuarine region, and the Huanghe Estuary includes the present river channel, the abandoned river channel, the Bohai Gulf and the Laizhou Gulf. Based on the results of particle size and magnetic measurements on all samples, selected samples were subjected to additional particle-size based rock magnetic and geochemical analyses.The main conclusions are as follows:The major sedimentary types are silts and clayey silts in the Changjiang Estuary, and clayey silts and sandy silts in the Huanghe Estuary. In comparison to the Huanghe Estuary, sediments of the Changjiang Estuary are coarser in the river bank deposition, and finer in the tidal flats, which is due to the differences in provenance and sedimentation dynamics.In both areas, magnetic properties of the sediments are dominated by ferrimagnetic magnetite, together with the contribution of anti-ferromagnetic minerals. Higher concentrations of ferrimagnetic minerals and anti-ferromagnetic minerals are observed for sediments of the Changjiang Estuary, so they display higher values of x, SIRM, SOFT, X_arm and HIRM. However, for sediments of the Huanghe Estuary, the relative contribution of anti-ferromagnetic minerals is higher, and therefore S_-100 is lower there.In both areas, the coarser magnetic minerals are highest in the fractions of very fine silt and fine silt, and the finer magnetic minerals in the clay fraction. Magnetic parameters X, SIRM, SOFT, X_arm, S_-100 and HIRM of the factions 2-4 μm, 4-8 μm, 8-16 μm, 16-32 μm and 63 μm are obviously higher in sediments of the Changjiang Estuary than in those of the Huanghe Estuary.There exist significant differences in the abundance of major and trace elementsin sediments of the Changjiang Estuary and the Huanghe Estuary. The contents of Fe, Ti and most of the trace elements (e.g., Cu, Zn and Cd) are higher in the Changjiang Estuary, while the contents of Ca, Na, Sr, Ba are higher in the Huanghe Estuary, which are similar to the conclusions obtained by previous studies.The new indicators for discriminating the sediments between the Changjiang Estuary and the Huanghe Estuary are suggested as follows: the bi-plot of S_-100 versus SIRM for bulk samples, magnetic parameters of the factions >63 μm, 8～16 μm and 4～8 μm, the bi-plot of Na versus Sr, ternary plot of Ca-Sr-Al, and bi-plots of S_-100 versus Ca and Sr, respectively.