广西环江古周峰丛洼地石灰岩坡地土楔238U、232Th、226Ra、40K深度分布及物源分析OA

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(＜0.71 Bq·kg-1),while 232Th、26Ra and 40K registered only 2.77,3.69,and 18.43 Bq·kg-1 respectively,yielding a distinct 232Th∶226Ra∶40K ratio of 1∶1.34∶6.65.The soil-bedrock contrast was unequivocal:238U activity in the soil(95.72-120.10 Bq·kg-1)exceeded the bedrock level by more than 135 times,and the fundamental difference in 232Th∶226Ra∶40K ratios(1∶0.74∶3.41 in soil vs.1∶1.34∶6.65 in bedrock)provided definitive evidence.This stark geochemical disparity,particularly the absence of 238U and the divergent isotopic signatures in the bedrock,categorically refutes traditional models of residual accumulation via in situ weathering of the underlying pure carbonate limestone,as U/Th series nuclides primarily reside in silicate minerals which are negligible in such bedrock.Instead,the uniform soil texture,stable isotopic ratios across depths,and most critically,the abrupt soil-bedrock contact devoid of any transitional layers,collectively provide robust evidence supporting a formation mechanism driven by gravitational soil creep infilling pre-existing rock fissures. The elevated 238U activity and consistent Th/Ra ratios within the soil wedge point towards external material sources,likely comprising weathered residues from interbedded clastic layers within the mid-upper slopes and/or contributions from aeolian dust,analogous to the Saharan dust inputs documented in Mediterranean Terra Rossa formations.This mechanism stands in clear global context:unlike granitic weathering crusts characterized by gradual transitions through distinct weathering horizons(e.g.,soil → strongly weathered layer → weakly weathered layer →fresh bedrock),limestone soil wedges exhibit sharp,unconformable interfaces-a definitive hallmark of allochthonous infilling.While this aligns with the formation paradigm of Mediterranean Terra Rossa,it fundamentally contrasts with models proposing in situ karst weathering.Integrated evidence thus conclusively confirms that soil wedges in karst peak-cluster depressions form primarily through the gravitational infilling of rock fissures by externally sourced materials,not through the in-situ weathering of the immediate limestone substrate.Radionuclide activity ratios,particularly the highly diagnostic 238U/232Th contrast,serve as powerful tracers for discerning pedogenic sources in such complex terrains.These findings necessitate a redefinition of karst soil evolution paradigms,significantly underscoring the critical role of aeolian and slope-wash transported alluvial inputs in shaping subtropical karst ecosystems.