Copper isotope signatures during red earth weathering and their significance for soil reticulation (Xuancheng, South China)

Copper isotopes (δ⁶⁵Cu) can provide critical insights in the study of weathering, pedogenic processes, and paleoenvironmental evolution. However, the behavior of Cu and its isotopic fractionation mechanisms in complex soil-forming systems are not well known. The Xuancheng profile in South China is characterized by a distinctive reticulate structure, consisting of white veins and red matrix. It is an excellent field laboratory for the study of Cu isotope variation in pedogenic systems with complex redox and developmental histories. Our results show that Cu is present mainly in clay minerals, and its isotopic composition exhibits limited variability, ranging from −0.29 ± 0.02 ‰ to −0.15 ± 0.02 ‰ (relative to NIST 976). In the modern soil layer (MSL) and homogenous red earth layer (HREL), bulk Cu isotopes (−0.22 to −0.15 ‰) are similar to those of the residual fraction (δ⁶⁵Cu_res = −0.29 to −0.21 ‰), suggesting that Cu in clay minerals is relatively stable during oxic weathering. Cu isotopic compositions in the middle and lower reticulated red earth layer (RREL) were affected by redox variations during the reticulation process. In the middle RREL (1.7–2.4 m), δ⁶⁵Cu variation was related to decomposition or transformation of iron oxides during weak reticulation. The clay assemblages, Cu isotopes of bulk soil and residual fractions indicate that Cu isotopic variation in the RREL was positively correlated with the presence of reticulation but not to the intensity of the reticulation process. Furthermore, the white veins contain isotopically heavier Cu than the red matrix of the LRREL, which was related to dissolution of iron oxides containing isotopically light Cu in the red matrix under seasonally wet-dry climate conditions. These results advance our understanding of the mechanisms of Cu-isotopic fractionation during chemical weathering in pedogenic environments.