Temporal and spatial variations in magnetic properties of suspended particular matter in the Yangtze River drainage and their implications

Abstract

As the largest river of China, the Yangtze River transports large amounts of sediments to the adjacent oceans. Provenance of these ancient marine sediments can only be deciphered when the source-to-sink process of modern sediments in the Yangtze River is fully understood. Many methods have been used to study the provenance of river sediment and an environmental magnetic method is applied in this study because of its fast, nondestructive advantages. Magnetic properties of suspended particulate matter (SPM) along the Yangtze River were measured to provide a holistic understanding about magnetic properties of sediments in this river and its controlling factors. The results indicate that the dominant magnetic mineral in SPM is magnetite, with a small contribution of hematite and goethite. Significant spatial variation was observed in most of the magnetic parameters, which primarily reflects the distribution of major geologic units along the drainage area of the river. Anthropogenic influences are also recorded in the magnetic parameters. The Three Gorges Dam results in a dramatic decrease of magnetic minerals in the downstream reaches, since its construction in 2003. In addition, small variations in magnetic properties of SPM are found along water depth, together with a clear seasonal shift at Datong station. This seasonal variation of magnetic properties of SPM is driven by variability in both hydrology and source contributions. This complicates the use of magnetic parameters for provenance studies. Magnetic properties of sediments in rivers are capable of tracing provenance areas, but caution must be taken into account.