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

张信宝; 袁帅; 孙明明; 曾奕; 岳跃民; 张润川

doi:10.11932/karst20260101

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

doi: 10.11932/karst20260101

1.
中国科学院水利部成都山地灾害与环境研究所, 四川成都 610213
2.
中国科学院环江喀斯特生态系统观测研究站, 广西喀斯特生态过程与服务重点实验室, 广西环江 547100
3.
四川省辐射环境管理监测中心站, 四川成都 611139

基金项目: 国家重点研发计划项目(2022YFF1300700，2023YFF0806002)

详细信息

作者简介:
张信宝（1946－），男，研究员，博士生导师，主要从事土壤侵蚀与水土保持研究。E-mail：zxbao@imde.ac.cn

中图分类号: P588.245；P59
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出版历程
- 收稿日期: 2025-05-30
- 录用日期: 2025-07-07
- 修回日期: 2025-06-24
- 网络出版日期: 2026-05-27
- 刊出日期: 2026-02-01

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

1.
Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu, Sichuan 610213, China
2.
Huanjiang Observation and Research Station for Karst Ecosystems, Chinese Academy of Sciences, Huanjiang, Guangxi 547100, China
3.
Sichuan Management and Monitoring Center Station of Radioactive Environment, Chengdu, Sichuan 611139, China

摘要

摘要: 为了探讨喀斯特峰丛洼地坡地石灰土的成土物质来源和土楔形成机制，于广西环江古周洼地锥丘坡地分层采集了石灰岩坡地土楔土样和石灰岩围岩样品，测定了土样的粒度、容重、pH和土岩样品的²³⁸U、²³²Th、²²⁶Ra和⁴⁰K活度。结果表明：土楔剖面石灰土质地均一，粒度组成以粉（31.81%）、黏粒（58.82%）为主，容重0.94~1.01g·cm⁻³，pH 7.47~8.01。剖面的²³⁸U、²³²Th和²²⁶Ra活度平均值分别为111.91 Bq·kg⁻¹、100.17 Bq·kg⁻¹和74.03 Bq·kg⁻¹；⁴⁰K活度随深度增加略有增加，平均值341.55 Bq·kg⁻¹。围岩的²³⁸U活度低于检出限0.71Bq·kg⁻¹，²³²Th、²²⁶Ra和⁴⁰K分别为2.77 Bq·kg⁻¹、3.69 Bq·kg⁻¹和18.43 Bq·kg⁻¹。土体和围岩的²³²Th∶²²⁶Ra∶⁴⁰K活度比值分别为1∶0.74∶3.41和1∶1.34∶6.65，差别较大。围岩与土体的²³⁸U活度差差别更大。土楔石灰土剖面的粒度，容重几乎无变化，不同深度土层的²³⁸U与 ²³²Th、²²⁶Ra比值较一致，土体与围岩直接接触无过渡层，这些证据证实土楔是土壤蠕滑充填岩隙形成。而围岩与土体²³²Th∶²²⁶Ra∶⁴⁰K活度比值的不一致及²³⁸U活度的明显差异，表明土楔土体的主要成土物质可能不是石灰岩围岩原位就地风化产物。
- 石灰岩坡地土楔 /
- 成土物质来源 /
- 放射性核素活度 /
- 喀斯特峰丛洼地 /
- 土壤蠕滑充填
Abstract: 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.
- limestone slope soil wedge /
- pedogenic material sources /
- radioactive nuclide activity /
- karst peak-cluster depression /
- soil creep infilling

HTML

图 1 广西环江古周洼地石灰岩坡地土楔石灰土取样剖面

Figure 1. Sampling profile of the limestone slope soil wedge in Huanjiang Guzhou depression, Guangxi

下载: 全尺寸图片幻灯片

图 2 湖南长沙花岗岩风化壳

Figure 2. Granite weathering crust in Changsha, Hunan

下载: 全尺寸图片幻灯片

表 1 古周石灰岩坡地土楔石灰土剖面的粒度、容重和pH深度变化

Table 1. Variations in grain size, bulk density, and pH with depth in the limestone slope soil wedge profile, Guzhou

深度/cm	容重/ g·cm⁻³	²³⁸U/Bq·kg⁻¹	²³²Th/Bq·kg⁻¹	²²⁶Ra/Bq·kg⁻¹	⁴⁰K/Bq·kg⁻¹	粒度/% <2 µm	粒度/% 2~20 µm	pH
0~10	1.01	112.88±6.33	100.38±2.81	71.57±1.73	304.88±11.32	27.59	41.15	7.47
10~20	0.98	118.92±6.53	107.90±2.70	84.99±1.98	358.85±11.39	30.11	66.14	7.47
20~40		120.10±6.34	105.19±2.68	77.26±1.80	342.48±11.09	34.74	63.28	7.85
200	0.94	95.72±6.00	87.19±2.55	62.28±1.79	359.98±11.81	34.80	64.71	8.01
石灰岩		低于检出限0.71	2.77±0.55	3.69±0.50	18.43±3.05

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