The Environmental Magnetism Study Of Loess In North Xinjiang

The loess are the most widespread and most continuous late Cenozoic sediment in the land. They are good archives for paleoclimate and paleomagnetic changes. Rock magnetism is one of important methods to read the paleoenvironmental information. Over the past 30 years, loess environmental magnetism developed very rapidly. Magnetic susceptibility of loess-paleosol sequences has been used widely as a proxy in paleoclimate research, and makes a great contribution to Chinese loess paleoclimate research. But, until now, the linkage of magnetic properties and environmental conditions is not completely understood, which limited the understanding of paleoclimate and comparison of loess magnetism in different regions.The loess which distributed in Chinese inland arid region are good good archives for paleoclimate. So far. the study of loess magnetism in this region is quite rough. The thick loess in north flank of Tianshan Mountain which deposited on the river terraces preserved high-resolution climate. These thick loess sections provide us an opportunity to study the mechanism of magnetic susceptibility variation and the significance of magnetic parameters of loess in arid region. The loess magnetic results are also valuable for us to improve the understanding of the driving mechanism and the history of sandstorm evolution.This study collect loess samples form the north flank of Tianshan Mountain and Bole in Xinjiang. Six loess sections, four in the north flank of Tianshan Mountain and two in Bole, were chosen out for detailed rock magnetism study. Also, bulk sample grain size of all samples was analyzed. And CBD extraction, magnetic extraction, SEM, XRD of representational samples was analyzed. These data were used to discuss the mechanism of magnetic susceptibility variation and the significance of magnetic parameters of loess in arid region. Based on the analysis of magnetic properties and environmental conditions, the important finds and main conclusions are drawn as following:(1) The magnetite and maghemite are the main magnetic minerals in north Xinjiang, also, hematite and geothite are detected, but less important to magnetic properties of sample. The kinds of magnetic minerals in Xinjiang are the same as Chinese Loess Plateau (CLP). The magnetic minerals domain is coarse PSD and MD, SP and SD are rare. The content of ferrimagnetic minerals in north Xinjiang is higher than loess layers in CLP, but lower than paleosols in CLP. Anti-ferromagnetic minerals of north Xinjiang loess are less than CLP in content. In north Xinjiang, the variation of magnetic parameters fluctuate sharply, indicating that north Xinjiang loess were affected by Source region more evidently. Pedogenesis is mainly controlled by precipitation. In humid region, the the mechanism of magnetic susceptibility variation is similar to CLP.(2) In north Xinjiang, physical weathering is more important than chemical weathering. The content and grain size of ferrimagnetic minerals control the magnetic susceptibility fluctuating. The mechanism of magnetic susceptibility variation is different to CLP. Due to the weak weathering, magnetic minerals in loess are most of lithogenic origin. There is a good positive relationship between magnetic susceptibility and bulk sample grain size, suggesting that magnetic susceptibility and S1RM are similar to d (0.5), can be used as proxies for evolution of source region and wind vigor.(3) Though the magnetic parameters which indicated ferrimagnetic minerals domain are sensitive to the change of precipitation, below the critical precipitation, these parameters can be used to estimate the paleoprecipitation.(4) The bulk sample grain size of north Xinjiang loess is much coarser than CLP. The content of slit can be used as a proxy for the evolution of sandstorm. The altitude increasing, the bulk sample grain size fining, suggesting that the loess are wind-blown origin.(5) There is an excellent negative relationship between SIRM/x and d(0.5), namely, the bulk sample grain size and the ferrimagnetic minerals grain size are positive relation. SIRM/x can be used as a proxy for evolution of w ind vigor and source region change, and is more reliable than magnetic susceptibility.(6) In different regions, the magnetic parameters may have different environmental significance. Therefore great care should be taken when using these magnetic parameters for paleoclimatic reconstruction.