Characteristics and paleoclimate significance of authigenic ferrimagnetic minerals in the Xuancheng red earth, southern China

Abstract

Soil magnetic records in Quaternary red earth (QRE) deposits contain a valuable record of paleoclimate information, providing insights into controls on Earth's climate system in the past and potentially helping to predict its response to perturbations in the future. Here, analysis of the environmental magnetism and mineralogy of the Xuancheng QRE (Anhui Province, South China) shows that magnetic variation was strongly linked to production of authigenic ferrimagnetic minerals such as maghemite. Fine-grained maghemite formed during the weathering-related transformation of iron-bearing illite to vermiculite, generating aggregates of vermiculite or mixed-layer illite-vermiculite. Enclosure of authigenic ferrimagnetic minerals by illite and vermiculite-group minerals inhibits their chemical weathering, but this protective effect can be lost due to intensified weathering, allowing the authigenic ferrimagnetic minerals to transform into more stable antiferromagnetic minerals. Upsection within the Xuancheng QRE, the content of authigenic ferrimagnetic minerals gradually increases, as revealed by SIRM and SIRM/χ, indicating a decrease in weathering intensity as the regional climate evolved from warm and wet in the middle Pleistocene to relatively cooler and drier today. Thus, this study improves our understanding of the relationship between soil magnetic properties and paleoclimate evolution in QRE deposits.

Keywords: Clay minerals; Fe oxides; Maghemite; Magnetite; Quaternary; Vermiculite.

Fig. 1

Lithologic characteristics of the Xuancheng profile. Arrows indicate the sample locations in the profile. Colors per Munsell color chart. Optically stimulated luminescence (OSL) ages from Hong et al..

Fig. 2

κ-T curves of samples from the Xuancheng profile ((a,b) represent samples from the modern soil layer and homogenous red earth layer, respectively; (c,d) represent samples from the upper and lower reticulated red earth layer, respectively). Red and blue lines represent heating and cooling curves, respectively. Gray and black arrows indicate peaks near 300 and 500 °C, respectively.

Fig. 3

Micromorphology and energy dispersive spectroscopy (EDS) analysis of iron minerals in magnetic extracts from the Xuancheng profile ((a-c) from modern soil layer; (d) from homogenous red earth layer; (e,f) from reticulated red earth layer). M, magnetite; Mh, maghemite; H, hematite; Ilm, ilmenite; I, illite; I/V, mixed-layer illite-vermiculite. Lowercase letters on the photos correspond to EDS analysis.

Fig. 4

X-ray diffraction patterns of samples from the Xuancheng profile. (a) XRD patterns for bulk sample and clay-fraction mounts of a representative sample (XC-19) using various preparation techniques (Air: Air-dried; Mg-25 °C: Mg-saturated at 25 °C; Mg-Gly: Mg-saturated and glycol solvation; K-25 °C: K-saturated at 25 °C; K-400 °C: K-saturated and heated to 400 °C; K-600 °C: K-saturated and heated to 600 °C). (b) XRD patterns of bulk samples and magnetic extracts of representative samples (XC-19, XC-52, XC-73 and XC-133). Mineralogic legends for XRD peaks shown in upper right, vermiculite (PDF: 16–0613), illite (PDF: 43–0685), illite-vermiculite (PDF: 35–0652), kaolinite (PDF: 14–0164), quartz (PDF: 46–1045), magnetite (PDF: 19–0629), maghemite (PDF: 39–1346), hematite (PDF: 33–0664), ilmenite (PDF: 29–0733), plagioclase (PDF: 41–1486) and orthoclase (PDF: 31–0966).

Fig. 5

χ_fd% and χ_ARM/SIRM crossplots of samples from the Xuancheng profile. χ_fd% cannot be quantitatively estimated according to particle size frequency, but semi-quantitative estimates of the SP fractions (i.e., 10%, 50% and 75% SP) are based on Dearing.

Fig. 6

A model of autogenetic iron mineral transformation during the development of Quaternary red earth in southern China. In the initial period of formation of the reticulated red earth layer, a warm and wet climate promotes formation of clay mineral-ferrimagnetic mineral aggregates. Seasonality of precipitation not only favors the development of reticulation structures but also destroys the protection of ferrimagnetic minerals by clay minerals, leading to the transformation of maghemite to hematite and leaching of some authigenic hematite. In contrast, a relatively cooler and drier climate during development of the homogenous red earth layer and modern soil layer was conducive to preservation of clay mineral-ferrimagnetic mineral aggregates, enhancing their magnetic susceptibility relative to the reticulated red earth layer.