Magnetic properties of Lake Qinghai Sediment

Lake Qinghai, the largest lake in China, locates at an elevation of 3196 m, covers an area of 4400 km2, with an average depth of 21 m and a drainage area of about 29,660 km2. It sits near the northeastern margin of the Tibetan Plateau and is positioned on the conjunction region where East Asian winter and summer monsoon, Indian Monsoon, and Westerlies interact. Lake Qinghai Drilling Project (LQDP) initiated by Institute of Earth Environment, Chinese Academy of Sciences had extracted a series of shallow cores from Lake Qinghai basin in the summer of 2005. Upper 6m of Core 1F (36°48'40.7"N, 110°08'13.5"E) contains a relatively uniformed greenish gray silty clay sediment with several darker layers and with a grayish brown section at 4.2-5m, preliminary chronology work (Zhou et al.,2008) indicate the upper 6m sediment spans to about 14ka. Some typical samples are selected for magnetic properties tests (K-T, J-T and Hysteresis loops), environmental magnetism parameters (magnetic susceptibility, ARM, SIRM, S-rations) are measured at 1cm intervals of core 1F. Rock magnetism tests identified that hematite and magnetite are dominant magnetic minerals below 5m, which may imply a detrital origin of magnetic properties related to terrestrial, especially eolian sources. In contrast paramagnetic iron sulfides e.g pyrites, normally formed in reduced condition are common in upper 5m. Magnetic susceptibility value varies between 4-8 10-8m3/kg below 5m, while that of the upper 5m are even lower (about 1-4 10-8m3/kg). Such magnetic susceptibility value change is attributed to changes of magnetic minerals and sedimentary conditions, such as oxidation-reduction alternation and carbonate dilution in the lake. Comparison with grain size and other climate proxies show that Lake Qinghai was generally in a drier environment before Holocene and became a deeper lake since Holocene. Key words: Lake Qinghai, magnetic minerals and magnetic susceptibility