Comparison of hysteresis, thermomagnetic and low-temperature magnetic properties of particle-size fractions from loess and palaeosol samples in Central Asia and the Chinese Loess Plateau

Abstract

Hysteresis, thermomagnetic and low-temperature magnetic experiments on particle-size fractioned samples from the Chinese Loess Plateau (CLP) can be used to better characterize the magnetic mineralogy and magnetic granulometry of Chinese loess/palaeosols. However, a systematic study of the grain-size-dependent magnetic mineralogy of the Central Asian loess deposits has not been undertaken. In this paper, four size fractions of 17 loess and palaeosol samples from Central Asia and the CLP were subjected to aforementioned rock magnetic measurements. Our findings are as follows: (1) In Central Asia, the fractionated samples from loess and palaeosol couplets exhibit no obvious differences in their magnetic mineralogy due to weak pedogenesis. (2) Thermomagnetic analyses suggest that the content of maghemite in the clay fraction of palaeosols from the CLP is one or two orders of magnitude larger than that of the loess samples from the CLP and Central Asia. This result does not support the view that maghemite in the loess/palaeosol sequences of the CLP originated mainly from eolian sources. (3) Both hysteresis and low-temperature magnetic experiments demonstrate that detrital ferrimagnetic grains aremostly enriched in the 20-75 μmfraction of loess/palaeosols from Central Asia and the CLP. The relative paucity of coarser magnetic grains in the >75 μm fractions indicates that a positive correlation does not always exist between the magnetic concentration parameters and the sedimentological particle size in Chinese loess deposits. (4) The regional variations in the magnetic properties of the 20-75 μm fraction suggest that the supply of clastic sediments is the main control on the magnetic properties of loess deposits in Central Asia.