Extreme Copper Isotope Fractionation Driven by Redox Oscillation During Gleysols Weathering in Mun River Basin, Northeast Thailand

Abstract

Copper (Cu), a key nutrient for plants and humans, is ubiquitously involved in redox reactions in terrestrial ecosystems, and the biogeochemical behavior and isotope fractionation of Cu are controlled by redoximorphic conditions (Kumar et al., 2021). Gleysols that are formed in response to water-level fluctuations are recognized as redoximorphic soils, and play a crucial role in controlling the terrestrial biogeochemical behaviors of Cu (Fekiacova et al., 2015;Tabor et al., 2017). Under redox fluctuation, Fe/Mn oxyhydroxides and soil organic Abstract Copper (Cu) isotopes are utilized to track Cu geochemical cycling in weathered gleysols of tropical zones. A significant isotope fractionation of Cu in these soils is primarily redox-controlled; however, it is rarely reported how the frequency of redox fluctuations affects the soil Cu isotope signature. This study investigated the variations of Cu content and isotope fractionation in two low-humic gleysol profiles (S1 and S2) from a dry tropical savanna zone. Owing to redox oscillation during weathering, δ 65 Cu values in profile S2 showed a stronger positive correlation with the mafic index of alteration of reducing environment than S1, and isotopically light Cu is more retained in the Zone II of profile S2 than S1. As the frequency of redox fluctuation increased, the retained stable Cu(I) species and light Cu isotopes increased in the residual soils through re-adsorption or re-precipitation by iron oxyhydroxide (i.e., ferrihydrite). Importantly, an Mn-enriched zone was formed after reduction events in profile S2, and found to be enriched in light Cu isotopes. The heavier Cu fraction might be lost by adsorption on Fe oxyhydroxides in the Mn-rich zone, while the relatively light Cu might be retained through adsorption on Mn oxyhydroxides. Additionally, a significant Soil Organic Carbon (SOC) contribution to Cu was found due to the high δ 65 Cu-SOC correlation (R 2 = 0.80) in S1 (depth <1 m). Therefore, our study shows that the Cu isotope signature can respond to redox changes in the terrestrial ecosystem, and these Cu isotope signatures may have significant implications for assessing soil ecological vulnerability under future climate change scenarios.