Potassium isotope composition in global loess: Origins and implications

Abstract

Potassium (K) isotope compositions (δ41K) in global loess deposits can provide valuable insights into the average upper continental crust (UCC). However, current knowledge of δ41K in the UCC is limited due to the lithological complexity of the continental crust, which results in highly variable K isotope compositions. Accurately estimating the K isotope composition of the UCC remains a challenge. Here, we investigated the concentration, phase, and isotopic composition of K in loess samples from Asia, Europe, Oceania, North America, and South America to identify the dominant controls on loess δ41K and better constrain the average composition of the UCC. Our results show that the δ41K value in globally compiled loess ranges from −0.60 ± 0.08 ‰ to −0.33 ± 0.04 ‰, with an average of −0.46 ± 0.12 ‰ (2 S.D.). This loess-based δ41K value of the average UCC is comparable to the average δ41K value established from various crustal materials by Huang et al. (2020). Nonetheless, our new range is slightly narrower compared with the previous estimation. We infer the measured K isotope variation in loess reflects a result of eolian sorting rather than chemical weathering. The abundance of 39K-rich illite is the primary driver of the δ41K variability in these loess samples. We suggest that the K isotope composition in bulk loess record provide more reliable information of the grain size sorting effect and thus inferring regional wind patterns (e.g., past monsoon variation).