Iron isotope fractionation during granite weathering under different climates

Abstract

Climate controls chemical weathering of silicate rocks on the transport of iron (Fe) and its isotopes from continent to the ocean, impacting the global Fe geochemical cycle. However, it's elusive if Fe isotope fractionation during silicate weathering reflects variations in climate factors. This study examines two granite-derived regolith profiles; one in Beijing (BJ), representing a temperate climate, and the other in Guangdong (GD), representing a tropical climate, to investigate their mineralogy, Fe-bearing phases, element concentrations, and Fe isotope compositions. Our results show that, despite climate differences, the two granite weathering profiles have average δ56Febulk regolith values within analytical uncertainty (0.09 ± 0.02 ‰ vs. 0.12 ± 0.04 ‰, 2SD). The δ56Febulk regolith values of temperate and tropical regolith are similar to or slightly higher than those of their respective bedrocks and remain steady along the entire weathering profile. The limited variation of Fe isotope composition in weakly weathered temperate regolith likely reflects the dissolution of primary minerals rather than the formation of secondary minerals. The Rayleigh fractionation calculations also show a Δ56Fepore solution-regolith value of ∼0 ‰ between pore solution and regolith. In contrast, in the tropical profile, despite the abundance of secondary minerals and the differences in δ56Fe values among the extracted Fe-pools exceeding 0.68 ‰, only limited Fe isotope fractionation is observed in the bulk regolith (0.01 ‰ to 0.24 ‰). These variations are likely driven by the formation of Fe oxides, relying on the atomic distribution of Fe in hematite and goethite. The linear regression analysis estimates the apparent Fe isotope fractionation factor between hematite and goethite as 0.46 ± 0.07 ‰ (Δ56Fehematite-goethite, 1SE). These findings indicate that the sensitivity of Fe isotope fractionation in bulk regolith to variations in climate factors is relatively limited. However, combined with results from other weathering profiles in different climate zones, two models suggest that changes in δ56Fe values of easily leachable and silicate-bound Fe pools are likely influenced by climate factors such as temperature and precipitation. This work advances our understanding of the Fe isotope fractionation during silicate weathering and its potential climate connection on Earth's surface.