Characterizing and quantifying iron oxides in Chinese loess/paleosols: Implications for pedogenesis

Abstract

To accurately decode the paleoclimatic and paleoenvironmental significance of magnetic properties in Chinese loess/paleosols, neoformation of magnetic iron oxides via pedogenesis as well as the relationship between ferrimagnetic (maghemite and magnetite) and antiferromagnetic minerals (hematite and goethite) of both detrital and pedogenic origins should be well determined. To resolve this problem, magnetic iron oxides from a contiguous loess–paleosol sequence (lower part of the loess unit L1, paleosol unit S1, and the upper part of L2) at Luochuan, Center of the Chinese Loess Plateau, were well characterized and quantified by using an integrated approach including chemical dissolution, diffuse reflectance spectroscopy and rock magnetism methods. Results showed that these magnetic minerals have distinct coercivity and dissolution behavior, which is highly grain size-dependent. More specifically, the paleosol is enriched with nano-sized pedogenic hematites with a medium coercivity of ~130mT, which are also preferentially dissolved in a weak reductive environment. In contrast, the coarse-grained micro-sized detrital hematites with a medium coercivity of ~1T are dominant in loess. In addition, these coarse-grained detrital hematites are more stoichiometric than their pedogenic counterparts as evidenced by the higher band position on the diffuse reflectance spectrum. Goethite can be well distinguished from hematite by its higher coercivity (4–7T) at low temperature (10K). The consistent dissolution behavior of goethite for both loess and paleosol samples in citrate–bicarbonate–dithionite solution indicates that goethite could be less relevant to the pedogenic processes. The reductive dissolution of pedogenic maghemite lags the dissolution of pedogenic hematite and fine-grained goethite, which suggests that average grain size of pedogenic maghemite is relatively larger than those of pedogenic hematite and fine-grained goethite. Detrital magnetite is hard to dissolve and ~50% of its initial remanence survived after dissolution. On the basis of these contrasting behaviors, pedogenic hematite, fine grained goethite and maghemite content was further quantified. Our results suggest that single domain pedogenic hematite was produced after initial formation of pedogenic maghemite. This provides direct evidence for the recently proposed pathway in which ferrihydrite is transformed into a transient maghemite-like phase before its final transformation into hematite.