Magnetic Properties Of Typical Estuarine And Coastal Sediments In China And Its Environmental Implications

Environmental magnetism is one of the most used methods in the field of geo sciences research. However, its application to Chinese estuarine and coastal study is still limited. Based on magnetic analysis of sediments from Luchaogang on tidal flat of Hangzhou Bay, coastal wetlands of Qi-Ao Island in Pearl River Estuary and South China Sea shelf deposits offshore Guangdong province, this thesis discusses their implications for sediment source ascription and hydrodynamic indication, early diagenesis and heavy metal pollutions. The main results are as follow:1. During the period of2007to2009, magnetic properties of sediments from Luchaogang site in Hangzhou Bay show similar pattern of periodic monthly changes. In general, fine sediments contain more ferrimagnetic mineral. In2010, although the periodic changes is different from that of the former three years, the relationship between magnetic properties and particle size is broadly the same, which indicates that particles size is the main factor influencing the monthly changes of magnetic properties. In2011, the relationship between χARM and the fraction of<16μm shows difference from that of former years, which indicates that sediment source probably changes, which could be a response of decreasing fluvial input from the Yangtze River into the Yangtze Estuary.2. Magnetic properties of core S201located in the continental shelf of northern South Sea indicate an apparent influence of early diagenesis. With increasing depth, ferrimagnetic mineral dissolves due to early diagenesis alteration. Superparamagnetic grains (SP) dissolved preferentially first due to smallest grain size, while single-domain (SD) ferrimagnetic grains persist in the upper part of the core and then decline with depth due to increasing reducing conditions. The persistence of SD grains in the upper part of the core is probably resulted from either the broking of coarse magnetic mineral or the generation of SD greigite. Antiferromagnetic mineral is resistant to dissolution and therefore shows increased proportion against ferrimagentic mineral. Electronic microscope examination shows the presence of pyrite, and their distribution is consistent with geochemical analysis of redox sensitive elements. The results here are generally in accordance with reports of magnetic mineral diagenesis elsewhere. However, the vertical distribution of magnetic mineral assemblages vary with regions.3. Magnetic susceptibility of sediments in coastal wetlands in Qi-Ao Island (Peral River Estuary) is mainly influenced by content of clay fraction. Magnetic mineral is mainly derived from natural source.χand SIRM doesn't show positive relationship with heavy metal pollution. However, it still need further research if χARM/χ can be used as indicator of heavy metal pollution. Therefore, before applying magnetic properties as indicator of heavy metal pollution, detailed mechanism between magnetic mineral and heavy metal pollution must be examined first.