Magnetic properties of sediments and their implications for sedimentary dynamic environment in the middle and lower reaches of the Qiantang River

Based on the particle size and magnetic characteristics of the sediments collected in the middle and lower reaches of the Qiantang River in January 2010 and August 2010, as well as the flow velocity data measured in August 2010, the temporal and spatial distribution of particle size and magnetism of the sediments are analyzed, and their indication significance to the sedimentary dynamic environment dicussed. Results show that the river bottom sediments are mainly composed of silt and clay in winter, but dominated by silt and sand in summer. The grain size of the sediments gradually gets fining from middle reaches to the Fuchunjiang Reservoir and becomes coarse from near-mouth section to estuary; the particle size is smaller in winter in general, indicating a stronger hydrodynamic environment in the estuary in summer. The content of magnetic minerals gradually decreases from the middle reaches to the lower reaches. The content of magnetic minerals in summer is higher than that in winter. Ferrimagnetic minerals dominate magnetic minerals, and there are more ferrimagnetic minerals in summer. χ_fd%,χ_ARM, χ_ARM/SIRM and χ_ARM/χ can be used as proxies for the sediments finer than 16 μm in the study area in winter, while χ_fd% and χ_ARM/χ be used as indicators for sediments finer than 32 μm in summer. χ_fd%, χ_ARM, χ_ARM/χ and χ_ARM/SIRM also show an increasing trend from middle reaches to the Fuchunjiang Reservoir, while show a decreasing trend from near-mouth section to estuary. This is closely related to the response of the sediment magnetic crystals to the sedimentary dynamic environment. χ_fd% and χ_ARM can reflect the sedimentary dynamic environment more effectively.