Control of surface exposure age on highly reactive iron of atmospheric dust

Abstract

Highly reactive iron (Fe_HR) oxyhydroxides in atmosphere dust play a critical role in determining the bioavailability of dust-derived iron in the ocean, thereby exerting substantial control over open-ocean biogeochemistry. However, the Fe_HR content in atmosphere dust exhibits significant variability across different landscapes, and the underlying factors contributing to this variability remain poorly understood. One hypothesis suggests that the Fe_HR content is controlled by chemical kinetics, which transform Fe_HR into more crystallized oxyhydroxides, but quantifying this process at the landscape scale has been challenging. In this study, we investigate potential factors controlling Fe_HR by analyzing its relationship with surface exposure age (determined by uranium comminution dating), chemical weathering intensity, lithology, and climatic conditions using sediment samples from major Asian deserts. Our results show that the ratio of Fe_HR to total iron (Fe_T) exhibits a strong first-order kinetic relationship with surface exposure age, and a weak negative correlation with chemical weathering intensity but no correlation with climatic condition and lithology, suggesting that kinetic process dictated by surface exposure age plays a pivotal role during the transformation of iron species. This finding underscores the significance of cold, rapidly eroding terrains in producing Fe_HR. The kinetic control of exposure age on iron reactivity may induce a positive feedback loop between atmospheric CO₂ levels and glacial activity, potentially contributing to the onset of ice ages and the associated increase in climate variability.