A study on origin of limestone soil on dolomite slopes by fingerprint method of 238U, 232Th, 226Ra, 40K nuclides
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摘要: 为了探讨白云岩坡地石灰土的物源，于广西环江木莲白云岩锥丘坡地分层采集了黑色石灰土和下伏岩土样品，测定了样品的238U、232Th、226Ra和40K活度。分析结果表明，白云岩坡地黑色石灰土的238U、232Th、226Ra、40K活度分别为84.71 Bq·kg−1, 71.50 Bq·kg−1, 137.81 Bq·kg−1 和327.86 Bq·kg−1。40K与这些核素的活度比值分别为1:0.26, 1:0.22, 1:0.42。白云岩的238U、232Th、226Ra、40K活度分别为27.08 Bq·kg−1, 2.42 Bq·kg−1, 2.13 Bq·kg−1和36.15 Bq·kg−1；40K与这些核素的活度比值分别为1：0.75, 1:0.07, 1:0.06。黑色石灰土与白云岩的238U、232Th、226Ra活度与40K活度比值不一致，很好地说明了黑色石灰土不是白云岩原位风化的产物。木莲白云岩坡地石灰土的238U、232Th、226Ra、40K的活度与古周石灰岩坡地石灰土的差别不大，暗示两地石灰土的成土物质主要来源可能相同，均可能来源于大气降尘。Abstract: The origins of limestone soils developed from the carbonate area in southwest China remains debated. Among the most popular recognition of in situ weathering, the aeolian origin of the red earth in southeast China still arose based on the grain-size distributions, rare-earth element (REE) patterns and major elemental composition of the red earth in the region compared to them of the loess and other aeolian deposits in northern China. In order to explore the origin of limestone soil on dolomite slopes in southwest China region, the black limestone soil and underlying rock and earth samples were collected from a slope of a dolomite cone hill in Mulian, Huanjiang, Guangxi, the dolomite cone hill was developed from the Devonian Upper Series. The activities of 238U, 232 Th, 226Ra and 40K nuclides of the samples were analyzed following the standard procedure using low background HPGe γ spectrometer by the Institute of Mountain Hazards and Environment, Chinese Academy of Sciences. The results showed that t he activities of 238U, 232Th, 226Ra and 40K nuclides in the black limestone soil were 84.71 Bq kg−1, 71.50 Bq kg−1, 137.81 Bq kg−1 and 327.86 Bq kg−1, respectively, which were similar to those in the limestone soil on the limestone (mostly calcite) slope in Guzhou, both of the sampling sites are in Huanjing county with the distance of less than 42 kilometers, confirming the general trends of the nuclide compositions in the region. These nuclide activities were higher than the underneath gravelly soil, sandy layer and dolomite rocks at Mulian sampling site. The activities of 238U, 232Th, 226Ra and 40K in dolomite are 27.08 Bq kg−1, 2.42 Bq kg−1, 2.13 Bq kg−1 and 36.15 Bq kg−1, respectively. The activtity ratios of 40K to 238U, 232Th, 226Ra for the black limestone soil are 1: 0.26, 1: 0.22, 1: 0.42, respectively, similar to the underneath yellow gravelly soil and significantly differing from the sandy layer and dolomite rocks. The activity ratios of 40K to 238U, 232Th, 226Ra in dolomite are were 1 : 0.75, 1 : 0.07, 1 : 0.06, respectively. The activity ratios of 238U, 232Th, 226Ra to 40K in the black limestone soil and in the dolomite rock were inconsistent. It indicated that the black lime soil was not the product of in-situ weathering of the underlying dolomite and could have other sources. Similar to the the Terra Rossa soils, for example, airborne dust from Africa plus tephra from the Lesser Antilles island arc are possible source materials for the terra rossa soils with Quaternary age at north coast of the Caribbean island of Jamaica. Furthermore, there is growing body of worldwide evidence (see the examples in Southwest China by Xiong et al.) of the importance of multiple parent materials, including far-traveled dust, to soil genesis. It is suggested that the limestone soils in both Mulian and Guzhou sites may have similar aeolian origins from the northern China.
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Key words:
- dolomite slope /
- black limestone soil /
- nuclear tracing /
- weathering /
- soil formation
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1 取样地剖面(下方为公路，自然开挖)
1. The profile of sampling site beside the road(road at the bottom,natural exvavation)
表 1 白云岩坡地石灰土剖面岩土的238U、232Th、226Ra、40K的活度（Bq·kg−1）及其比例关系
Table 1. Activities (Bq·kg−1) and proportional relationships of 238U、232Th、226Ra、40K in the soil profile of a dolomite slope
岩土样品 238U (238U:40K) 232Th (232Th: 40K) 226Ra(226Ra: 40K) 40K(40K: 40K) 黑色石灰土（0-10 cm） 84.71±7.54 (0.26) 71.50±2.68 (0.22) 137.81±9.52 (0.42) 327.86±4.06 (1.00) 灰黄色碎石土（10-40 cm） 65.95±5.75 (0.37) 30.57±1.71 (0.17) 66.07±6.38 (0.37) 177.51±2.78 (1.00) 糖沙层 42.90±3.82 (0.80) 2.29±0.09 (0.04) 6.29±0.44 (0.12) 53.68±0.67 (1.00) 白云岩 27.08±2.41 (0.75) 2.42±0.09 (0.07) 2.13±0.15 (0.06) 36.15±0.45 (1.00) 
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