Depth distribution of 238U、232Th、226Ra、40K from a soil wedge on limestone slope in Guzhou karst peak-cluster depression of Huanjiang and its origin analysis

The longstanding debate regarding the origin of pedogenic materials in karst regions—whether derived from in situ weathering of carbonate bedrock or allochthonous inputs—motivated this investigation into soil wedge formation within the karst peak-cluster depression of Huanjiang Guzhou depression, Guangxi, China. To resolve this controversy, we analyzed distributions of natural radionuclides (238U、232Th、226Ra和40K), grain size, bulk density, and pH in a limestone slope soil wedge to trace material sources and elucidate formation mechanisms. Stratified soil samples (0−10, 10−20, 20−40 cm, and 2 m depth) and adjacent limestone bedrock were collected from a 3-m-deep soil wedge (24°55′0.20″N, 107°57′4.35″E), with bulk density measured in situ via the ring-knife method. Grain size distribution was determined using laser diffraction (Mastersizer 2 000) following H2O2 and HCl pretreatment, while radioisotope activities were quantified through low-background γ-spectrometry (HPGe detector, LOAX model) at characteristic energy peaks: 92.8 keV (238U), 583 keV (232Th), 609 keV (226Ra), and 1460 keV (40K). The pH is measured by a pH meter.Results revealed remarkably homogeneous soil physicochemical properties: silt (average 31.81%) and clay (58.82%) dominated the texture, while bulk density (0.94−1.01 g·cm−3) and pH (7.47−8.01) exhibited minimal variation across depths, though the surface layer (0−10 cm) showed slightly lower clay content (27.59%) and higher bulk density (1.01 g·cm−3). Radionuclide activities within the soil wedge demonstrated significant stability: 238U, 232Th, and 226Ra averaged 111.91, 100.17, and 74.03 Bq·kg−1 respectively with negligible depth variation, while 40K activity exhibited a slight increase with depth (average 341.55 Bq·kg−1). Crucially, isotopic ratios remained consistent vertically, with 238U∶ 232Th∶ 226Ra∶ 40K ≈ 1∶0.89∶0.66∶2.86 (0−40 cm) and 1∶0.91∶0.65∶3.76 (2 m), indicating profound vertical homogeneity. In stark contrast, the underlying bedrock exhibited extremely low radionuclide activities: 238U was undetectable (