黄土和古土壤中磁性矿物的粒度分布

磁化率是一种重要的环境替代性指标，对中国黄土中古土壤磁性增强已有不少讨论．本文对采自吉县、西峰和西宁的S₁和L₂的代表性样品进行了粒度分离，并测量了各个不同粒级的磁性参数，对所获得的结果就不同粒级的颗粒对磁化率的贡献、黄土和古土壤中磁性矿物颗粒的粒度和古土壤磁性增强的原因进行了初步的探讨，指出古土壤形成过程中并没有像一些学者所认为的那样，有大量超顺磁颗粒生成，它们对古土壤磁化率增大的作用有限。

GRAIN SIZE DISTRIBUTION OF MAGNETIC MINERALS IN LOESS AND PALEOSOL

Magnetic susceptibility is a useful proxy in paleoclimatic studies. The origin of magnetic enhancement of paleosol is still with much uncertainty. It is, however, a key problem in cormctly explaining its paleoclimatic significance and in transferring the magnetic signals to paleoclimatic parameters.Two kinds of models have been proposed to explain the mechanism of magnetic enhancement i. e. depositional model and pedogenic model. The dilution of arelatively constant flux of fine-grained stronger magnetic dust from remote sources by weaker magnetic eolian silt of local source during cold episode was suggested by depositional model. In situ preduction of ultrafine grained strong magnetic particles by chemical, biochemical or biogenic achons during soil forming process was suggested by pedogenic model.A new technique is developed to study the grain size of magnetic minerals in loess and paleosol. The representative samples of S₁ and L₂ taken from Jixian, Xifeng,and Xining were subdivided into eight grain size groups by depositional method and by centrifugal method. By TEM examination, there is no smaller grains in each group.The separation of grain size was successful. The grain size of eight groups is > 10μm,10 - 2μm, 2 - 1μm, 1 - 0.5μm, 0.5 - 0.2μm, 0.2 - 0.1μm, 0.1 - 0.05μm, and < 0.05μm respectively. The products of grain size separation were carefully weighed,and percentages of each group to the total sample were calculated. Systematic measurements of magnetic parameters were camed out on each group.The results of magnetic measurement show that:（1） For the loess samples from three sites, values of magnetic susceptibility in different grain sizes are in the same order. There is no apparent difference between different groups. It is, however, that the magnetic susceptibility of the fine grains increases a great deal in the paleosols.（2） The contribution of each group to total susceptibility was calculated. The susceptibility of loess samples was mainly contributed by the > 10μm particles （up to 75% - 90% of total magnetic susceptibility）.The grains between 10 - 1μm play the second role. The grains less than 1μm contribute little to the total magnetic susceptibility （< 5%）. For the paleosol, > 10μm particles, which is 65% - 70% of total mass, contribute about 30% - 40% of total susceptibility only. Other grains play more important role to magnetic enhancement in paleosol. Each of them is over 5%.（3） It is, however, that the grains less than 0.05μm do not contribute much to the total magnetic susceptibility in paleosols as suggested by many authors. Their contribution is less than 5%.（4） The grain size distribution of magnetic particles in loess is mainly PSD and MD particles. The SD particle is an important component in paleosol.（5） The increase of magnetic particles with grain size of 10 - 1μm in paleosol is not from the eolian dust but from the weathering of paramagnetic iron-bearing silicates.Unfortunately we are still not very clear the evolution of iron-bearing minerals during the pedogenic processes.