| Magnetic susceptibility is an integrated indicator of the mineral content information, it's also affected by the change of magnetic mineral content, particle size of magneticparticles, magnetic mineral species and the measuring environment(temperature, frequency). However, for a long time, the frequency characteristics of the magnetic susceptibility was only in the theory. Due to the limitations of the experimental conditions, researches on this direction was still defective. In this paper, according to the variations of rainfall gradient, we have chosen four loess-paleosols sequencesections on Chinese Loess Plateau(Weinan, Xifeng, Dingbian, Jingyuan) and studied the samples since the last interglacial. By measuring broadband magnetic susceptibility and frequency magnetic susceptibility under eight altering current magnetic fieldranging from 125-16000 Hz, and through theoretical calculations, we tried to understandhow magnetic particles change with particle sizes under different frequencies. Based on that and combining environmental magnetism proxies such as?-T ? ARM and S-ratio and so on, we have revealed characteristics of broadband magnetic susceptibility under different pedogenesis conditions. This could helps us better understand broadband magnetic susceptibility and further study the implication of section magnetic susceptibility, and provide useful parameters revealthe influence factors onrevolution of Asian monsoon in different parts of Chinese Loess Plateau. The main conclusions are as follows:1) As for the frequency characteristic of magnetic susceptibility, the experiment results are quite consistent with theoretical researching results. Beneath the change of relaxation time, magnetic susceptibility decreases exponentially with the increasing of testing frequency, which also confirms the validity of our research.2) There is a good corresponding between broadband magnetic susceptibility and classic parameter of environmental magnetism. The content and grain size of magnetic particles reflected by environmental magnetism parameters are varying distinctly, meanwhile, this variation trend is certainly reflected in the distribution diagrams of particle content and broadband frequency magnetic susceptibility too.3) Compared with traditional parameters of environmental magnetism, broadband magnetic susceptibility and frequency magnetic susceptibility are deeper and more detailed in revealing the distribution features of grain size. The information about pedogenic magnetic mineral of fine particles is better expressed by highfrequency section, and the information about magnetism source of coarse particles is embodied in low-frequency section. Moreover, as the testing frequency getting broader, the magnetic information captured is getting more abundant too. Thus, broadband magnetic susceptibility is considerably suitable for samples with low magnetic susceptibility in use.4) Combined with the content and changing trend of magnetic mineral, the frequency properties of magnetic particles could better show the degree of pedogenesis and related paleoenvironmental messages. Fine samples withmore magnetic particles are more sensitive under high frequency magnetic susceptibility, and possessed of higher degree of pedogenesis form more SP, SD particles, which all indicate wet and warm paleoclimate. On the other hand, samples with more coarsemagnetic particles are better reflected under low frequency magnetic susceptibility, and possessed lower degree of pedogenesis, which all indicate dry and cold paleoclimate.5) Distributed along the variations of precipitation gradient belt from southeast to northwest in the Loess Plateau, we have quantitatively analyzed particle volume proportion and fraction at Weinan, Xifeng, Dingbian, Jinyuan profile, and concluded that absolute value of content of fine particles and different value between coarse and fine particles can demonstrate paleoclimate information and the degree of pedogenesis well. |
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