Rock magnetism and geochemistry of two plio-pleistocene Chinese loess-palaeosol sequences - implications for quantitative palaeoprecipitation reconstruction

Stratigraphic and spatial variations in the magnetic properties of Chinese loess are widely accepted as reflecting changes in the intensity of soil forming processes over time and space. Recently, several workers have used statistical correlations between the magnetic susceptibility and related parameters of modem loess plateau soils and modem climate to generate transfer functions to reconstruct past temporal and spatial trends in precipitation. Here we try to assess the viability of this approach by conducting magnetic and geochemical measurements of two long (similar to 2.5 My) loess-palaeosol sequences from different locations on the modem precipitation gradient across the loess plateau. One site, Duanjiapo, is located on the southern humid margin of the plateau; and the other site, Luochuan, is located in a drier modem environment some 160 kin further north. Our results indicate discrepancies between magnetic and geochemical indicators of weathering and soil forming intensity and that any attempt to predict palaeoprecipitation at Duanjiapo using published approaches would be significantly in error. Overall our results indicate a complex response of magnetic mineral transformations to climate, including the production of both ferrimagnetic and antiferromagnetic minerals within the same climatic interval, together with the destruction of magnetic material within some of the palaeosols. Therefore we suggest considerable caution should be exercised in generating quantitative climatic reconstructions from a single magnetic parameter. (c) 2005 Elsevier B.V. All rights reserved.