Particle size separation and evidence for pedogenesis in samples from the Chinese Loess Plateau spanning the past 22 m.y.

A comparison between particle size distributions determined by laser diffraction and the pipette method confirms previous studies that point to a variable underestimate of the clay fraction using laser diffraction, relative to the values obtained by pipette analysis. In order to yield four particle-sized fractions (<2 mu m, 2-4 mu m, 4-8 mu m, >8 mu m) for further analysis, the pipette method was used on loess and paleosol samples from sites on the Chinese Loess Plateau spanning the past 22 m.y. In all cases, a subsidiary mode in the <2 mu m fraction is identified. The clear separation of this from the coarse mode in the >8 mu m fraction provides a basis for evaluating the relative contributions of pedogenic and detrital components. Pretreatment using acetic rather than hydrochloric acid for carbonate removal results in no significant loss of ferrimagnetic minerals and allows magnetic characterization of the separate fractions. This confirms the isolation of almost all the finest, pedogenic ferrimagnetic grains in the clay fraction and their negligible contribution to the magnetic properties of the coarse fractions in all the samples from early Miocene time onward. Because similar conclusions may be drawn from previous studies of late Pleistocene samples from the east-central and extreme western parts of the Chinese Loess Plateau, we conclude that this approach provides a basis for separately characterizing eolian deposition and pedogenesis throughout the region and for the whole time span of loess accumulation.