Magnetic properties of muddy sediments on the northeastern continental shelves of China: Implication for provenance and transportation

摘要

Mud deposits are widespread in estuaries and on continental shelves, and contain important information regarding interactions between terrestrial and coastal processes. Magnetic properties of muddy sediments from the northeastern continental shelves of China, including shelves close to Korean coasts, were analyzed using bulk and size-fractionated samples to elucidate muddy sediment sources and transport. Types, concentration, and granulometry of magnetic minerals were determined and all samples were found to be dominated by magnetite. Muddy sediments from the Zhemin coast (off the Zhejiang and Fujian coasts) have the highest magnetic susceptibility (x) and saturation isothermal remanent magnetization (SIRM) values, while those from the central and southeastern South Yellow Sea and the western Korea Strait have lower values reflecting lower magnetic mineral concentrations. Single domain ferrimagnetic grains dominate the magnetic particles in the central South Yellow Sea, while superparamagnetic (SP) grains contribute significantly to the magnetic grains in the Bohai Sea. SP and coarse multi-domain (MD) grains dominate magnetic grain assemblages in the North Yellow Sea and along the Zhemin Coast. Rock sediment particle size is not a major factor affecting the magnetic properties of the mud deposits in the study area. Rather, terrigenous inputs and hydrodynamic transport processes primarily control the magnetic properties, and sediments from the Changjiang and Huanghe rivers make the most significant contribution. This observation applies to the whole study area, although terrigeneous magnetic mineral concentrations decrease with increasing distance from the river mouths due to both sorting and mixing with other sediments with weak magnetic signatures. Bacterial magnetosomes also play a role in the central South Yellow Sea.