Elsevier

Chemical Geology

Volume 567, 20 April 2021, 120081
Chemical Geology

Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond
Distribution and secondary enrichment of heavy metal elements in karstic soils with high geochemical background in Guangxi, China

https://doi.org/10.1016/j.chemgeo.2021.120081Get rights and content

Highlights

  • Geological background controlled spatial distribution of soil metal concentrations.

  • Secondary enrichment and bedrock inheritance caused Cd enrichment in karstic soil.

  • Secondary enrichment caused the other metal (As, Cr, Cu, Hg, Ni, Pb, and Zn) enrichment in karstic soil.

  • 2% of Cd being retained in situ during the weathering of carbonate bedrock.

  • Soil Fe/Al/Mn oxides, organic carbon and clay content controlled metal accumulation in soils.

Abstract

Karst environments are globally significant, and the soils that develop on these landscapes often have unique geochemical features, including elevated–levels of metals. However, it has not been determined whether the enrichment of metals in soils is mainly controlled by parent rocks or secondary enrichment during weathering. In this paper, we studied the control of the accumulation of heavy metals in soils by parent materials and/or the process of soil formation. We examined the un–weathered carbonate and clastic bedrocks, as well as surface soils in a typical karst area in Guangxi of China. The spatial distribution of heavy metal (Cd, As, Cr, Cu, Hg, Ni, Pb, and Zn) concentrations in soils was mainly controlled by the geological background. Except for Cd, the concentrations of heavy metals in carbonate rocks were much lower than those in clastic rocks, whereas the soils derived from the carbonate rocks were highly enriched with heavy metals than those in clastic area. The enrichment of Cd in surface soils from the carbonate substrates was the result of the dual effect of secondary enrichment and parent rock inheritance, whereas the enrichment of other metals was mainly the result of secondary enrichment. During the weathering process of carbonate bedrock, great majority of the quantities of inherent heavy metals were leached, with only 2% of Cd being retained in situ. The high concentrations of soil Fe, Al, and Mn oxides, organic carbon and clay content (Al2O3/SiO2) were closely related to the enrichment of heavy metals in studied surface soils.

Introduction

The distribution and cycling dynamics of metal elements have attracted significant attention (Zhao et al., 2010) as many of them threaten environment and human health due to their toxicity, non–biodegradability and persistence in soils (Kim et al., 2015; Yajima et al., 2015). The concentration of metal elements in the overlying laterite (“red soil”) developed on karst landscapes is generally high, with most researches concluding that the enrichment in soils was related to the parent rocks (Johnson et al., 1996; Miko et al., 2000; Zhang and Lalor, 2002; Halamić et al., 2012; Yamasaki et al., 2013; Wen et al., 2020a). In karstic area, the land is usually covered by clay-rich laterite but with different thicknesses (Wang et al., 1999; Ji et al., 2004). Several hypotheses have been proposed to explain the formation of the overlying laterite in karst areas, including: (1) “Soluble–residual theory”, the accumulation of insoluble residues from the underlying carbonate rocks (Summerson, 1959; Glazovskaya and Parfenova, 1974; Isphording, 1978; Ji et al., 2004), (2) “Dissolution–metasomatism theory”, dissolution of carbonate rocks being accompanied by metasomatism with the materials carried by groundwater (Li et al., 2001), and (3) “Extraneous sedimentation theory”, the weathering of extraneous materials such as, fine–grained eolian sediments (Macleod, 1980; Inoue et al., 1993), volcanic ash (Comer, 1974; Isphording et al., 1995), and fluvial sediments (the weathering products of insoluble rocks outside karst areas; Fu et al., 2013), that are deposited during the formation of the original carbonate bedrock.

The global area of carbonate rocks exposed accounts for about 12% of the land area, and carbonate rocks exposed are mainly distributed in tropical and subtropical regions, such as the Mediterranean coast, North Africa, South Asia, southwestern China, northern Australia, western North America, and southwestern South America (Wang et al., 1999; Jin et al., 2013). China, known for stereotypical karst landforms, is covered by 2,000,000 km2 of karst, accounting for about one–fifth of the land area (Wen et al., 1994). It is mainly distributed in Guangxi, Guizhou and Yunnan provinces of southwestern China, located in the center of East Asia karst region, one of the three major karst regions in the world (Zhao et al., 2006). In addition, there are also some karst areas distributed in Sichuan, Hubei, Hunan, Jiangxi, Guangdong and other provinces in China, and most of the limestones in China are Devonian–Triassic (Zhu et al., 1984). The laterite with thickness of large variations is also widely distributed in the karst areas of China. It has been reported that the karstic soil with high geochemical background was significantly enriched with heavy metals, whose bioavailability however is generally low (Wen et al., 2020b). Meanwhile, an inheritance relationship in mineralogy and trace elements geochemistry between the overlying soils and underlying carbonate rocks has been reported (Wang et al., 1999; Ji et al., 2004).

Systematic soil sampling surveys and elemental analyses have been conducted to varying degrees of element combination size and area coverage in most countries and regions (Lalor et al., 1995; Johnson et al., 1996; Miko et al., 2000; Zhang and Lalor, 2002; Takeda et al., 2004; Halamić et al., 2012; Wang et al., 2012; Yamasaki et al., 2013; Zheng et al., 2018). Most of these surveys focused on exploring element geochemical characteristics at relatively large regional scale. Regional soil with single lithology has been rarely investigated due to the fact of complex lithology and/or significant contribution of anthropogenic contamination at large areas being explored. As a result, there leaves many unsolved problems, particularly for soils derived from carbonate rocks in karst areas.

Pollution of heavy metals in soils is increasing due to the impact of human activities, including agricultural activities (pesticide spraying, fertilization, irrigation, etc.; Ramadan and Mandil, 2009; Vodyanitskii et al., 2010; Mazur and Mazur, 2016), industrial and mining activities (Intamo et al., 2015; Fatoba et al., 2016; Ma et al., 2020), and urban residential activities (Narwal et al., 1993; Filippelli et al., 2018). In addition to human activities, both secondary enrichment of parent material and the process of soil formation influence the enrichment of metals in soil. Metals are apparently accumulated in karstic soils to varying degrees, in spite of the fact that the concentration distribution and migration characteristics of metals in soils derived from different parent rocks during the weathering process are different. Identification of the main controlling factors for this remarkable accumulation is thus very important. It's still unclear that whether the accumulation is due to parent rock inheritance, secondary enrichment in the weathering process, or the joint effect of these two factors. To address this question, we elaborately selected a typical area (covering ~1100 km2) with two adjacent sub-areas of mono-lithology (carbonate vs clastic) in Guangxi, southwestern China. Samples of un–weathered carbonate and clastic bedrocks, and surface soils in the study area were intensively collected for analysis. The objective of this study is to unravel the geochemical behavior of metal elements during the weathering process of carbonate rocks and clastic rocks, and to ascertain the main influencing factors on the accumulation of metals in karstic soils.

Section snippets

Sampling sites

The study area is located in Guigang City of Guangxi, southwestern of China (Fig. 1). Its climate is subtropical monsoon, with an annual rainfall of 1400–1600 mm (Zhou and Tang, 2013). The parent rocks are carbonate rocks and clastic rocks. Carbonate rocks are located in northwest of the study area, including Carboniferous and Devonian dolomite and limestone, and a small amount of Permian limestone. Clastic rocks are located in southeast, and are mainly Cretaceous sandstone, siltstone, Devonian

Surface soils

Statistical results of total concentration of 19 components in surface soils (0–20 cm) in the carbonate and clastic sub–areas are shown in Table 1. Surface soils in the carbonate and clastic sub–areas were weak acidic–neutral and acidic, with soil pH medians of 6.49 and 5.46. The average concentrations of SiO2, Al2O3, TFe2O3, and Mn in the soils from the two substrates were considerably higher than those of the Chinese surface soils (Hou et al., 2020), whereas the pH, and CaO, Na2O, K2O, and

Conclusions

In this study, the spatial distributions of heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn) were highly consistent with the geographical distribution of the carbonate and clastic substrates. In comparison with the upper continental crust, the enrichment or loss tendencies of elements in soils in the two sub–areas were consistent, with the loss of alkali metals and alkaline earth metal elements, such as Ca, Na, Mg, and K, whereas Cd, Ti, Fe, Zn, As, and Hg were 2–fold enriched. Except Cd,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study was financially supported by the National Key R&D Program of China (Granted No. 2017YFD0800300), the project of Geochemical study on selenium and heavy metal elements in central–eastern area of Guangxi, China (2015–2016), Study on the genesis and ecological effect of Se, Ge and Cd in soil of Guangxi, China (2017–2019), Ecological and geochemical survey and study on the heavy metals in typical soil of Guangxi, China (2018–2019), Pollution identification and ecological risk assessment

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