Copper and iron isotope fractionation during weathering and pedogenesis: Insights from saprolite profiles.

Iron and copper isotopes are useful tools to track redox transformation and biogeochemical cycling in natural environment. To study the relationships of stable Fe and Cu isotopic variations with redox regime and biological processes during weathering and pedogenesis, we carried out Fe and Cu isotope analyses for two sets of basalt weathering profiles (South Carolina, USA and Hainan Island, China), which formed under different climatic conditions (subtropical vs. tropical). Unaltered parent rocks from both profiles have uniform δ 56 Fe and δ 65 Cu values close to the average of global basalts. In the South Carolina profile, δ 56 Fe values of saprolites vary from −0.01‰ to 0.92‰ in the lower (reduced) part and positively correlate with Fe 3+ /ΣFe ( R 2 = 0.90), whereas δ 65 Cu values are almost constant. By contrast, δ 56 Fe values are less variable and negatively correlate with Fe 3+ /ΣFe ( R 2 = 0.88) in the upper (oxidized) part, where large (4.85‰) δ 65 Cu variation is observed with most samples enriched in heavy isotopes. In the Hainan profile formed by extreme weathering under oxidized condition, δ 56 Fe values vary little (0.05–0.14‰), whereas δ 65 Cu values successively decrease from 0.32‰ to −0.12‰ with depth below 3 m and increase from −0.17‰ to 0.02‰ with depth above 3 m. Throughout the whole profile, δ 65 Cu positively correlate with Cu concentration and negatively correlate with the content of total organic carbon (TOC). Overall, the contrasting Fe isotopic patterns under different redox conditions suggest redox states play the key controls on Fe mobility and isotope fractionation. The negative correlation between δ 56 Fe and Fe 3+ /ΣFe in the oxidized part of the South Carolina profile may reflect addition of isotopically light Fe. This is demonstrated by leaching experiments, which show that Fe mineral pools extracted by 0.5 N HCl, representing poorly-crystalline Fe (hydr)-oxides, are enriched in light Fe isotopes. The systematic Cu isotopic variation in the Hainan profile reflects desorption and downward transport of isotopically heavy Cu, leaving the organically-bound Cu enriched in light isotope as supported by the negative correlation of δ 65 Cu with TOC ( R 2 = 0.88). The contrasting (mostly positive vs. negative) Cu isotopic signatures in the upper parts of these two profiles can be attributed to the different climatic conditions, e.g., high rainfall at a tropical climate in Hainan favors desorption and the development of organism, whereas relatively dry climate in South Carolina favors Cu re-precipitation from soil solutions and adsorption onto Fe (hydr)-oxides. Our results highlight the potential applications of Fe and Cu isotopes as great tracers of redox condition, ancient climate and biological cycling during chemical weathering and pedogenic translocation. [ABSTRACT FROM AUTHOR]