Production and Evolution of Granitic Regolith Under Cold Temperate Climate in Northeast China: A Perspective From 238U-234U-230Th Disequilibrium

To investigate the long-term evolution of granitic regolith under cold temperate climate, we examined a 300 cm-thick regolith profile in the Oroqen Autonomous Banner, northeast China. We analyzed the mineralogy and U-series isotopic compositions of bulk regolith samples. Measurements of (²³⁴U/²³⁸U), (²³⁰Th/²³⁸U), and (²³⁰Th/²³⁴U) isotopic activity ratios indicate U-series disequilibrium, with complex variations in depth, ranging from 0.949 to 0.989, 0.906 to 1.036, and 0.926 to 1.059, respectively. The conventional “gain and loss” model could not be applied across the entire profile in a single simulation. By subdividing the profile into three subzones based on elemental and mineralogical depth variations, the “gain and loss” model was applicable to two subzones, excluding the middle portion. U-series disequilibrium-derived regolith production rates were 1.42 ± 0.03 m/Ma and 5.97 ± 3.98 m/Ma for these subzones. When compared to denudation rates (∼34 m/Ma) determined from in situ cosmogenic nuclides (¹⁰Be and ²⁶Al), the regolith production rates were substantially lower, suggesting that the profile is in a non-steady state. Our findings highlight the necessity of subdividing regolith profiles when applying the “gain and loss” model, and demonstrate the value of integrating U-series disequilibrium with in situ cosmogenic nuclides for assessing regolith evolution over long timescales. The evolution of regolith thickness, as a controlling factor of production rate, also has a significant impact on whether there is a coupling between the regolith production rate and the denudation rate.