Potassium isotope composition in global loess: Origins and implications

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).