ROCK-MAGNETIC CHARACTERISTICS OF THE TIANSHAN LOESS AND THE MECHANISM FOR ENHANCING MAGNETIC SUSCEPTIBILITY

The large area of eolian loess distributed over the northern slope of the middle Tianshan between the altitude of 2 400~2 700 m has provided a good opportunity to study the loss rock magnetic properties of Central Asia and the evolutionary process of the extreme arid areas of NW China. Our research profile is located on the sixth terrace of Qingshui river on the northern slope of Tianshan. We collected 580 of bulk samples for analysis of the rock magnetic susceptibility at a 10~20 cm interval and selected several samples for analyses of other magnetic parameters. Results show that the magnetic properties of the Tianshan Loess are mainly controlled by low-coercivity ferrimagnetic minerals (such as magnetite and maghemite), significantly different from the loess sequence of the Loess Plateau, and the contribution of ultrafine pedogenically-produced magnetic grains to loess magnetic susceptibility is very limited, while the increased coarse ferrimagnetic minerals from the source region play a decisive role in increasing magnetic susceptibility in the Tianshan region. The gradual increasing trend in magnetic susceptibility and the coarsening upwards trend of grain size since 0.8Ma may be the response to the aridification process in the inland Asia since the mid-Pleistocene.