Quantifying production rates of tropical granitic regolith in Hainan Island, south China: A multi-stage U-series disequilibrium study

Abstract

Regolith, widely distributed on the Earth's surface, constitutes a significant compartment of the Critical Zone, resulting from intricate interactions among the atmosphere, lithosphere, hydrosphere, and biosphere. Regolith formation critically influences nutrient release, soil production, and long-term climate regulation. Regolith development is governed by two primary processes: production and denudation. An urgent need exists to comprehensively understand these processes to refine our understanding of Critical Zone functions. This study investigates an in-situ regolith profile developed on granitic bedrock from a tropical region (Sanya, China). We conducted geochemical analyses, encompassing major, trace elements and mineralogical compositions as well as U-series isotopes, and applied the U-series disequilibrium method to investigate the formation history of this profile. Alternatively, dividing the regolith profile into sub-weathering zones provides a better explanation for the geochemical results, and a multi-stage model based on this subdivision effectively interprets the evolution of deep regolith. Utilizing this multi-stage model, regolith production rates is derived from the “gain and loss” model, ranging from 1.27 ± 0.03 to 42.42 ± 24.24 m/Ma. The production rates first increase from surface until a maximum rate is reached at the depth of ∼ 160 cm and then decrease at further deeper horizons along the depth profile, and the variation of production rates follows a so-called “humped function”. This pioneering investigation into regolith production rates in the Chinese tropical region indicates that (1) the studied profile deviates from a steady state compared to the denudation rate derived from cosmogenic nuclides (10Be_in-situ); (2) subdividing the deep profile based on geochemical data and U-series isotopic activity ratios is imperative for accurately determining regolith production rates; and (3) the combination of U-series disequilibrium and cosmogenic nuclides robustly evaluates the quantitative evolution state of regolith over long time scales.