The Ferrimagnetic Mineral And Paleoenvironment Of Typical Chinese Loess Deposit

Chinese loess deposit is one of most important paleoclimate research object. The long history, high resolution, and continuous deposition loess sequence can provide quantitative information on the interactions between aerosols and climate. Due to measurement fast, easy, and economy, magnetic susceptibility has been one of most commonly used proxy of paleo-rainfall and paleo-air temperature. The magnetic susceptibility is controlled by strong magnetic minerals, such as magnetite and maghemite, which includes origin and secondary minerals. During the concentration of origin magnetic minerals being low and varying stable, the bulk magnetic susceptibility variation can be used to recover the paleoclimate which is recorded by loess sequences. However, as the concentration of origin magnetic minerals being high and variable, then the implication of magnetic susceptibility is complex which is not suitable to recover paleoclimate. So, to identify origin and secondary magnetic signals and understand their relationship with climate and environment are very important.In this study, we collected loess samples from the typical loess profile locating in Loess Plateau and Xinjiang province, and desert samples from potential source area. The environment magnetic investigation revealed that:magnetite and maghemite, the most important origin strong magnetic minerals, usually enrich in coarse silt particle which lead origin magnetic susceptibility value enhancing in coarse material. Additional, the most important secondary strong magnetic mineral is also magnetite in loess deposit, the quantity of which is controlled by rainfall rather than temperature. Actually, in loess deposit, the high temperature is not essential condition during secondary magnetite formation but maghemite and hematite.Basing on properties of origin strong magnetic minerals, this study traced the Chinese loess source. Firstly, we discovered the silt material presents obviously different magnetic characteristics with bulk sample in desert which is the major constitute of loess deposit. So, the silt material has more possible to be the source of loess deposit rather than bulk sample of desert sediment. By contrast magnetic properties of the origin magnetic material in loess and desert samples, it can be discovered that Northeast of Tibet Plateau is the most important potential source of Loess Plateau loess and the North Tienshan loess may be product from west of Zhungaer basin by presenting most similar magnetic characteristic between desert and loess in these area.In this study, the interglacial climate in Loess Plateau and Xinjiang area had been recovered by pedogenic strong magnetic material evidence. It show paleoclimate is out of phase evolution between Loess Plateau and Xinjiang area. Firstly, on orbital scale, the paleo-rainfall and paleo-wind intensity evolution is in phase and which must be forced by solar radiation. However, it must be noticed that last interglacial rainfall is distinct less than Holocene in Western Loess Plateau which indicates there exists another important forcing factors influence the climate evolution in this area. Similar with Loess Plateau area, it also shows three heavy rainfall events and three weak wind intensity events being recorded by Xinjiang loess during last interglacial. However, the paleorainfall and paleowind evolution is totally out of phase and the last interglacial rainfall is similar with Holocene in Xinjiang area which is obviously different with Western Loess Plateau. Secondly, on sub-orbital scale, the half precession cycle had been recorded by particle size parameters of Loess Plateau loess sequences but disappeared in pedogenic magnetic parameters, and aridity events occurred during last interglacial soil units developing, especially in S1S2 and SI S3. However, in Xinjiang area, the loess sequences never recorded half precession climate evolution cycle and aridity events both in paleorainfall and paleowind proxy parameters. Third, both Loess Plateau and Xinjiang area, S1S3 presents strongest pedogenic soil units during last interglacial. But it shows more distinct in Xinjiang. At last, comparison paleoclimate signal record by Holocene soil and SI S3 soil units in these two regions. In Loess Plateau, the last interglacial climate illustrated a similar evolution story with Holocene period. And both of their development delay to solar radiation peak and are interval by aridity events. In Xinjiang area, it illustrated a more complex evolution story than Holocene. The paleowind intensity change is a little early than paleorainfal during last interglacial, but this phenomenon disappears during Holocene.After all, concentration of strong magnetic material has close link with climate, environment, and parent material in loess deposit. After separating origin and pedogenic signals, environmental magnetic parameters will present more definite implication and supply more paleoclimate and paleoenvironment information to us.