VARIABLE SHAPE OF MAGNETIC HYSTERESIS LOOPS IN THE CHINESE LOESS-PALEOSOL SEQUENCE

Shape of magnetic hysteresis loops of the Chinese loess-paleosol seque nce is variable with low-field susceptibility and is weakly constricte d in samples with intermediate susceptibility (similar to 1.0 x 10(-6) m(3)/kg). The analyses of the hysteresis loops show that both low- (f errimagnetic) and high-coercivity (antiferromagnetic) components are p resent and the ferrimagnetic component dominate the magnetic character istics. The ratio of ferrimagnetic over antiferromagnetic minerals (S ratio) and the superparamagnetic fraction increase with increasing sus ceptibility. Neither simple two-component mixtures of ferrimagnetic an d antiferromagnetic minerals nor of single-domain and superparamagneti c grains fully explain the constricted hysteresis loops. We interpret the variation of the loop shape with susceptibility in the following w ay. When the ratio of the ferrimagnetic to antiferromagnetic contribut ion is relatively low (low susceptibility), the broad loop is controll ed by lithogenic ferrimagnetic and antiferromagnetic minerals. For sam ples with intermediate susceptibility values, constricted shape origin ates from an addition of a broad loop from the lithogenic fraction and a narrow loop from a pedogenic fraction with high superparamagnetic c ontent. Then with further susceptibility increase, the constricted sha pe almost disappears and the loop is dominated by the pedogenic fracti on. The variation of hysteresis loop shape with susceptibility can be a useful indicator of the degree of pedogenesis for loess-paleosol sam ples.