Source controls on iron isotope variations of loess from the Chinese Loess Plateau and its implications

The loess composition is an ideal candidate for constraining the upper continental crust (UCC) composition. Iron, as the second most abundant metal element in the Earth’s crust, its isotope has been used to understand iron cycling. This study analyzed Fe isotope variations in loess and paleosol from the Chinese Loess Plateau (CLP), the largest loess deposit on Earth. Unlike previously reported constant δ⁵⁶Fe (0.09 ± 0.03‰; 2SD), we found significant variations (+0.05‰ to +0.24‰) across six CLP sections. Coarse-grained loess (GT32 > 26%) showed varied δ⁵⁶Fe values, which weakly correlate with GT32 and Th/U, reflecting impacts of inherited heterogeneous source compositions and provenance oxidization after short-distance transport. Differently, fine-grained loess (GT32 < 15%) well mixed after long-distance transport shows relatively constant δ⁵⁶Fe (+0.12 ± 0.03‰; n = 5, 1SD). Together with four coarse loess samples, δ⁵⁶Fe values of these loess negatively correlate with CaO/Al₂O₃ and CaO, indicating the contribution of carbonates inherited from source rocks to different extents. Thus, we recommend the average δ⁵⁶Fe of these samples (+0.10 ± 0.04‰; n = 9, 1SD) can represent the carbonate-bearing UCC Fe isotope composition, and Fe isotope compositions of UCC without carbonate should be even heavier considering carbonate lighter Fe isotope compositions.