Quantification of the contribution of pedogenic magnetic minerals to magnetic susceptibility of loess and paleosols on Chinese Loess Plateau: Paleoclimatic implications

Abstract

This study investigates the grain size–dependent properties of magnetic minerals in Chinese loess samples with an aim to better understand the contribution of pedogenesis to magnetic susceptibility enhancement of the paleosols. Samples of loess and paleosol were divided into two fractions, magnetic extracts and residues, by extraction in a weak magnetic field. Both the raw samples of loess and paleosol (i.e., before magnetic extraction) and the samples of residues after magnetic extraction were then separated into five grain size fractions by gravitational settling and centrifugation. Low‐field magnetic susceptibility (MS) and the frequency‐dependent MS of these samples were measured, and samples treated with citrate‐bicarbonate‐dithionite (CBD) to remove the iron oxides were also analyzed. For loess samples, coarse grains (≥2μm) contribute more than 50% of the magnetic signal. The ≤0.3 μm fraction, considered to be of pedogenic origin, contributes 22% (before magnetic extraction) or 36% (after magnetic extraction) of the magnetic signal in loess. A different pattern is found in the paleosol samples. For these samples, the ≤0.3 μm fraction contributes >60% (before magnetic extraction) or 71% (after extraction) to the bulk MS. In addition, 91% of the magnetic signal in paleosol S5 is CBD extractable. The CBD technique is able to extract pedogenic superfine‐grained magnetic particles and coarse dust magnetic particles. These results confirm that pedogenesis leads to enhanced MS and helps to quantify the multiple origins of the bulk magnetic material that defines loess and paleosol MS. This approach provides a reliable estimate of the contribution of pedogenic magnetic materials to MS. Our study indicates that multiple methods should be employed in analyzing paleoclimatic changes recorded by MS in the Chinese loess‐paleosol sequences.