A study on origin of limestone soil on dolomite slopes by fingerprint method of ²³⁸U, ²³²Th, ²²⁶Ra, ⁴⁰K nuclides

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.