PARTLY PEDOGENIC ORIGIN OF MAGNETIC VARIATIONS IN CHINESE LOESS

QUATERNARY sequences of interbedded aeolian silts (loess) and buried soils (palaeosols) potentially provide one of the best terrestrial records of past climates1-5. Magnetic susceptibility variations in loess and palaeosol sequences from China are strongly correlated with climate-induced fluctuations of oxygen isotope ratios in deep-sea sediments2-4,6-8. As an explanation of this correlation, it has been suggested that the recorded variations in magnetic susceptibility depend primarily on the degree of dilution by non-magnetic bulk loess constituents of a uniform magnetic assemblage derived from remote but unknown sources6-8. It has also been suggested that such a model, involving changes in the accumulation rate of loess superimposed on a constant flux of magnetic particles, provides a relative geological timescale through the control of loess deposition by astronomically modulated climate fluctuations7. Here we report a preliminary test of this model using rock magnetic properties especially sensitive to variations in magnetic grain size. Our results show that palaeosols are characterized by much finer magnetic grain size assemblages than are the intervening loess units. This suggests that a simple model based on constant magnetic influx and dilution by variable amounts of non-magnetic loess is inadequate. Our magnetic measurements establish the close comparability of the fine grained magnetic minerals in the palaeosol samples to those in contemporary soils and thus point to a partially pedogenic origin for the magnetic mineral assemblages in the palaeosols. © 1990 Nature Publishing Group.