Natural pedogenic pathway of iron oxides

Abstract

The exact formation pathway of iron oxides via pedogenesis has long been a contentious topic in geological and environmental studies [1]. Pedogenesis is crucial to accurately interpret the environmental significances of iron oxides in paleoclimatic archives, for example, the Chinese Loess/Paleosol sequences [2]. Several models have been developed but all lack direct constraints from natural records [2]. Hu et al. adopted a novel approach by integrating rock magnetism, dynamic dissolution and diffuse reflectance spectroscopy techniques [3]. They successfully separated and quantified major iron oxides of different origins (detrital and pedogenic) from a loess– paleosol–loesstransition sequence of the classic Luochuan Loess section in Shaanxi, China. Their major scientific contribution is 2-fold. First, they characterized, in detail, the pedogenic and detrital (or aeolian) hematite in loess/paleosols. Pedogenic hematite is nano-sized, preferentially dissolved in a weak reductive environment and, with a greater content of Al, can be easily demagnetized by the alternating field (AF) (remanence coercivity=∼130 mT). In contrast, the detrital counterpart is micro-sized, more resistant to dissolution, is almost stoichiometric and is resistant more to the AF demagnetization (remanence coercivity = ∼1 T). Second, based on the separation of iron oxides of different origins, the researchers quantified and described the relationships between different iron oxides. The results indicated that superparamagnetic (SP) pedogenic maghemite was Figure 1. Identification of pedogenic (Ped) and detrital (Det) hematite (Hm) in the Chinese loess/paleosol sequence. a) Schematic representation of Luochuan section; b) The depth variation of low field susceptibility, χ ; c) Hematite dissolution behavior in a typical loess sample; d) Isothermal remanent magnetization (IRM) decomposition of a typical loess sample, indicating the presence of detrital hematite with the central remanence coercivity around 1. 5 T. The stars defines the end of the first step where Ped–Hm is significantly dissolved, but the Det–Hm remains almost untouched; e) Hematite dissolution behavior in a typical paleosol sample; f) IRM decomposition of a typical paleosol sample, indicating the presence of both detrital and pedogenic hematitewith central remanence coercivity around 1 T and 130 mT, respectively. The black and color curves indicate the original and the dissolved samples, respectively; g) Relationship between the concentration indicator and the remanence indicator of pedogenic hematite, indicating the initial production of pedogenic hematite is in the SD region. The dashed line represents the case when SP hematites are largely produced initially. The solid line indicates the linear trend corresponding to the case that the initial production is in an SD state. (Revised from Hu et al. [3]).