Paleoclimatic significance of mineral magnetic properties of loess sediments in northeastern Qinghai-Tibetan Plateau
【摘要】：正Loess deposit in the northeastern Qing-hai -Tibetan Plateau offers an excellent record of climate and environment changes in this region.We have conducted multiparameter mineral magnetic analyses of samples of loess deposits from the Dongchuan,Lalakou and Panzishan sections. The methods used include magnetic susceptibility,frequency -dependent susceptibility,temperature-dependent susceptibility,isothermal remanent magnetization acquisition, and magnetic hysteresis loops.The results reveal that magnetite is the dominant contributor to magnetic susceptibility. Maghemite and hematite is also present in the loess layers and paleosol horizons.The higher concentration of maghemite in paleosols suggests that the formation of maghemite occurred during in situ pedogenesis,which plays an important role in the enhancement of the magnetic susceptibility. Similar to that in the Chinese Loess Plateau lying to the east of the Liupan Mountains,magnetic granulometry in the studied loess and paleosols is predominantly pseudo-single-domain(PSD).However,the mean grain-size of the ferrimagnetic minerals in loess is evidently coarser (larger PSD and multidomain-like) than in paleosols(mainly PSD).The magnetic susceptibility of loess and paleosols is positively correlated with the content of ultrafine magnetite /maghemite grains,and hence with the intensity of in situ pedogenesis.Therefore,the enhanced magnetic susceptibility in the studied loess-paleosol sequences can be interpreted as being due to climatically induced in situ pedogenesis.