THE FREQUENCY-DEPENDENCE OF LOW-FIELD SUSCEPTIBILITY IN LOESS SEDIMENTS

The low held susceptibility variations in the Chinese and Tajik loess sequences reflect the palaeoclimatic fluctuations of the Quaternary on the continents in a very direct and complete way. It can be demonstrated that the fine-grained ferromagnetic mineral fraction is largely responsible for the climatically controlled susceptibility enhancement in the loess sediments. In order to gain more information about the origin and quality of the susceptibility enhancement we are scrutinizing the F-factor (F-D) which is conventionally used as a measure of the frequency dependence of the susceptibility. For qualitative estimates of the grain sizes present, F-D seems to be a rather misleading parameter because it is flawed when variable amounts of minerals with both frequency-independent and frequency-dependent susceptibility are present. We introduce a new concentration-independent F-factor (F-c) which takes the influence of the mineral fractions with frequency-independent susceptibility on the total susceptibility into consideration. F-c is fairly constant in all the investigated sections. This result is explained theoretically following Neel's (1949) fine-particle theory and Stephenson's (1971a) modelling of various single-domain grain distributions. It is found that the grain-size distribution is essentially the same throughout all loess sections investigated and the upper and lower grain-size limits are essentially invariable. In addition, susceptibility and its frequency dependence were measured at low temperatures on selected loess samples in order to determine the maximum grain size present in the fine-grained magnetic fraction.