Provenance of Fe in Chinese Deserts: Evidence from the geochemistry and mineralogy of soil particles

Abstract

Determining the categories and content of Fe containing silicate minerals in the desert soils and sediments of northern China is important for understanding the Fe cycle and for correctly interpreting the paleoenvironmental signals from the Loess Plateau downwind. Using a mineral liberation analyzer (MLA), we established the modal mineral abundances of a complete set of Fe containing mineral species in the fine sand to silt fractions of surface samples from Chinese deserts. Results show that the most common discrete particles containing Fe are the silicate phases (~15 wt%), while the weathering product (i.e., Fe oxides) phases are minor and their abundances vary greatly (0.38–4.15 wt%). The degree of weathering has a substantial particle size dependence. The fine fraction (5–20 µm) generally has a relatively low Fe/Si ratio, possibly due to weathering, while the Fe/Si ratio of the coarse fractions (20–45 µm, 45–63 µm, 63–75 µm, and >75 µm) are relatively invariant. Abundant Fe is contained within the coarse fractions, which are associated with the local geology. The strong relationship between the Fe and Mg bound within the silicate phases further supports the conclusion that the correlated deviations in the Fe/Si and Mg/Si ratios originate from the provenance bedrock (i.e., siliciclastic, low-grade metamorphic, and possibly high-grade metamorphic, rocks). Since Mg tends to be easily removed during weathering, the Fe/Mg ratios of silicate species largely reflect the weathering intensity. We used the regression equation for the relationship between the local precipitation amount and the Fe/Mg ratios of the soils to produce a novel precipitation proxy. Compared with classical provenance studies conducted using coarse sand and larger rock fragments, the MLA methodology is more applicable to the fine sand and silt fractions; thus, it contributes to the development of weathering and provenance indices based on geochemistry and mineralogy.