Climate influence on Si abundance and Si isotopes in the Yimaguan loess–paleosol sequence

Abstract

The SiO₂/Al₂O₃ and SiO₂/TiO₂ ratios of loess–paleosol sequences in the Chinese Loess Plateau have been proposed as indicators recording changes of the East Asian winter monsoon. However, interference from silicon (Si) leaching induced by chemical weathering raises concerns about their applications to paleoclimate reconstruction. Silicon isotopes can serve as an effective tool for monitoring Si leaching, as the release of Si can lead to a decrease in δ³⁰Si values of soils. Here we use Si isotopes to evaluate previously published SiO₂/Al₂O₃ and SiO₂/TiO₂ ratios in loess–paleosol sequences from the Yimaguan profile, Gansu Province, Northwest China, and examine the potential alteration caused by chemical weathering. The SiO₂/Al₂O₃ and SiO₂/TiO₂ ratios show opposite trends to that of the frequency-dependent magnetic susceptibility (χ_fd) and the chemical index of alteration (CIA). However, the whole profile exhibits limited Si isotopic variation, with δ³⁰Si values ranging from −0.28‰ to −0.16‰. The homogeneous Si isotopic signature indicates that Si is not significantly leached during pedogenesis. Therefore, the SiO₂/Al₂O₃ and SiO₂/TiO₂ ratios are not modified by Si leaching due to chemical weathering; instead, they are mainly affected by the grain size sorting effect, which is controlled by the change of winter monsoon intensity. This suggests that SiO₂/Al₂O₃ and SiO₂/TiO₂ can serve as reliable indicators of paleoclimate changes, with an increase in the ratios reflecting the strengthening of the East Asian winter monsoon. Based on our observation of the Yimaguan loess–paleosol samples and the global loess data previously published, we also constrained the average Si isotopic composition of the upper continental crust (UCC) to be −0.22 ± 0.06‰ (2 s.d.).