Understanding the magnetic properties of natural sediments, which include magnetic concentration, composition and grain size, is fundamental in paleomagnetic and paleoenvironmental studies. Systematic magnetic measurements and non-magnetic methods (e.g., transmission electron microscopy, TEM) provide detailed information of magnetic minerals. In this paper, we established a high efficient magnetic extraction method to separate magnetic minerals from surface sediments of Miyun lake in Beijing. We analyzed the bulk samples, magnetic extracts and residues. Saturation magnetization (M-s), volume magnetic susceptibility (kappa) and saturation isothermal remanent magnetization (SIRM) of the magnetic extracts is about 85%, 75% and 70% of the bulk samples, respectively. Multiple-parameter rock magnetic measurements indicated that the major magnetic minerals in sediments are multi-domain (MD) and single domain (SD) magnetite. TEM analyses revealed detrital MD titano-magnetite, biogenic SD magnetite (magnetofossils) and authigenic superparamagnetic (SP) magnetite. We propose that the combination of rock magnetism and TEM observations is useful to accurately identify the magnetic minerals in sediments.

Sediment; Rock magnetism; Transmission Electron Microscopy (TEM); Magnetic extraction; Biogenic magnetite

中国科学院大学; 岩石圈演化国家重点实验室; 中国科学院地质与地球物理研究所兰州油气资源研究中心; 中国科学院地质与地球物理研究所