Abstract: Uranium and thorium, as lithophile, are ubiquitous in earth crust and bedrock. The weathering of bedrock makes uranium and thorium participate extensively in various surficial geochemical processes. Both weathering-pedogenesis and erosion of carbonate rocks are important sources of uranium-series radionuclides in karst-ecological systems. Also be key process of global biogeochemical cycling of uranium-series radionuclides. Bedrock and soil samples are collected from 6 typical carbonate rock profiles in Houzhai catchment. The content, distribution and characterization of mobility of uranium and thorium are analyzed

during weathering process of carbonate rocks. Carbonate rocks profiles are characterized by extreme mass defect which dominated by chemical-weathering. Chemical-weathering depletion fraction (CDF) is more than 70% due to dissolubility of carbonate rocks, and the CDF of pure limestone and dolomite is more than 90%. A sudden enrichment of uranium and thorium appear in the rocksoil interface because of chemical weathering and residual dissolution. Thorium-rich is more obvious than uranium, and both have secondary distribution in profile. Uranium is easily dissolved and more mobile, yet thorium is mostly adsorbed by soil particles less mobility in the process of weathering. The CDF indicates obvious difference in chemical mobility of uranium and thorium. Multi-stage variation of Th/U implies weathering intensity and more potential geochemical information during weatheringpedogenic process of carbonate rocks.

Key words: chemical weathering depletion fraction, mobility, enrichment of uranium and thorium, carbonate rocks