Element Geochemical Characteristics of a Soil Profile Developed On
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Acta Geochim (2016) 35(4):445–462 DOI 10.1007/s11631-016-0116-4 ORIGINAL ARTICLE Element geochemical characteristics of a soil profile developed on dolostone in central Guizhou, southern China: implications for parent materials 1 1 2 Xingxing Cao • Pan Wu • Zhenxing Cao Received: 29 December 2015 / Revised: 6 June 2016 / Accepted: 6 July 2016 / Published online: 18 July 2016 Ó Science Press, Institute of Geochemistry, CAS and Springer-Verlag Berlin Heidelberg 2016 Abstract This study presents bulk chemical compositions GdN/YbN and ternary diagrams for Sc, Th, Zr, and Ta of the Tongmuling soil profile, which developed on dolo- suggest that the overlying strata could also provide parent stone, and the overlying strata covering the bedrock in the materials for the genesis of terra rossa. central Guizhou province (southern China). The chemical weathering characteristics of the studied profile were Keywords Terra rossa Á Dolomite Á Parent materials Á investigated and the inheritance relationships between the Guizhou Á China terra rossa and overlying strata were discussed. The results show that there is no remarkable variation in the major elements and weathering indices from the rock–soil inter- 1 Introduction face to the topsoil, indicating that the studied profile was not typical for in situ crustal chemical weathering. The Globally, karsts account for approximately 15 % of the land terra rossa were mainly composed of SiO2,Al2O3 and area (Jiang et al. 2009). Terra rossa, a type of reddish silty Fe2O3. Compared with the insoluble residues and overlying clay soil that forms on hard carbonate rocks, is widely dis- strata, the terra rossa are characterized by an enrichment of tributed as a discontinuous layer ranging in thickness from a Y and Cs and depletion of Ba and Sr. The subsoil shows a few centimeters to several meters in karst regions (Durn et al. notable Ce negative anomaly, characterized by heavy rare 1999; Durn 2003; Merino et al. 2006). Because of the earth element enrichment (L/H = 1.55–3.74), whereas the tremendous differences in chemical composition between topsoil shows a positive Ce anomaly with light rare earth terra rossa and the underlying carbonates, the identification element enrichment (L/H = 5.93–9.14). According to of their parent materials has been debated over the past Laterite-forming capacity estimates, the terra rossa could several decades. Currently, there are three main hypotheses: not have only been formed from acid-insoluble residues (1) terra rossa originates only from the underlying carbonates from the bedrock; Al2O3 versus Fe2O3 and Nb plotted (Ahmad et al. 1966; Bronger et al. 1983; Ji et al. 2004a, b; against Ta show significant positive correlations between Feng et al. 2009; Wei et al. 2013); (2) terra rossa genesis is the terra rossa and overlying strata. The Eu/Eu* versus affected by external materials, such as volcanic ash, non- carbonate rocks, and eolian dust (Olson et al. 1980; Durn et al. 1999; Delgado et al. 2003; Mee et al. 2004; Cooke et al. Electronic supplementary material The online version of this 2007; Durn et al. 2007; Muhs and Budahn 2009; Liu et al. article (doi:10.1007/s11631-016-0116-4) contains supplementary material, which is available to authorized users. 2013; Sandler et al. 2015); (3) terra rossa is formed by metasomatic replacement processes (Zhu and Li 2002; & Pan Wu Merino and Banerjee 2008; Zhu et al. 2008; Lucke et al. [email protected] 2014). These hypotheses are proposed for sites with different 1 College of Resource and Environmental Engineering, geologic and climatic settings all over the world. Guizhou University, Guiyang 550025, Guizhou, China The Guizhou Province is located in southwestern China, 2 Graduate School of Science and Technology, Shizuoka at the center of the East Asian karst zone, where carbonate University, Shizuoka 422-8529, Japan terrains occupy more than 70 % of the land surface (Wu 123 446 Acta Geochim (2016) 35(4):445–462 et al. 2009). Previous studies indicated that the terra rossa 2.2 Sampling and analytical methods of the Guizhou karst plateau may have resulted from in situ carbonate weathering (Wang et al. 1999; Ji et al. 2.2.1 Profiles and sampling 2000, 2004a, b; Feng et al. 2009; Wei et al. 2013) rather than a metasomatic mechanism. However, the content of The soil profiles were sampled in November 2008 at the insoluble material in carbonates is very low. Approxi- Tongmuling Village (TML; 26°2202900N, E 106°4002700E; mately 1 % of acid insoluble residues were found in a Fig. 1b), southwest of Guiyang, the capital of Guizhou previously studied profile (Ji et al. 2004a); this concen- Province. They formed on the Anshun Formation, which tration is not sufficient to form the target profile. Hence, consists of early Triassic dolomites with approximately additional information regarding the parent materials of 0.68 % acid insoluble residues. The total thickness of the terra rossa in the Guizhou Province is still needed. bedrock is 50–301 m (Fig. 1c). The sampling site lies at According to previous research, the underlying carbon- the top of a karst hill and contains an 8-m-thick terra ates of this terra rossa were widely distributed in the Triassic, rossa (Fig. 2a). The soil layers covering the bedrock are with additional deposition of other strata since the later discontinuous, and there is no obvious transition layer Triassic (Team 1995). Therefore, the weathering of the strata from bedrock to soil. Field observations (photos of field covering the bedrock through time would be a possible investigation are shown in supplementary material) found source of terra rossa, and the weathering residues would not that at the weathering frontier, dolomite had weathered disappear without the action of a geological process. The into powder and, through the actions of water and gravity, purposes of this study are to investigate the major, trace and the terra rossa from the bottom of the TML profile in- rare earth elemental variations of a terra rossa profile to filled part of this powder (Fig. 2b). Before sampling, a pit determine if it is a product of in situ weathering, and to was dug from which the soil samples (TML4–19) were discuss the inherent relationship between the terra rossa and collected from the bottom to the top of the profile at a overlying strata. These results are relevant for the further 50-cm resolution. Bedrock (TML1), cataclastic dolomite study of terra rossa parent material in general. (TML2) and dolomite powder (TML3) were also col- lected. The overlying strata from the Triassic Guanling (T2g) and Yangliujing (T2yl) Formations were sampled, 2 Materials and methods and seven samples were collected from the Triassic Erqiao ? Sanqiao (CR1) and Jurassic Ziliujing clastic 2.1 Geologic setting and geography rock (CR2–7) Formations. The Guizhou Province lies on the eastern Yunnan–Guizhou 2.2.2 Experimental and chemical analysis methods Plateau in southwestern China (Fig. 1a). It is part of the Yangtze Platform and contains mainly Proterozoic clastic After air-drying, the rock and soil samples were ground sedimentary rocks and Paleozoic to Upper–Middle Triassic (200 mesh) by a ceramic mortar and pestle. The method to marine carbonate rocks; sediments deposited after the extract insoluble residues from dolomites was described in Triassic are mainly fluvial (Team 1995). The carbonate detail by Wang et al. (1999). Soil organic matter was rocks in southwestern China form one of the largest con- measured using the potassium dichromate volumetric tinuous karst distributions in the world with an approxi- method (NY/T 1121.6-2006). Values of pH were deter- 2 mate area of 500,000 km (Yuan 1992). In Guizhou mined from a mixture of soil to CO2-free pure water (1:2.5) Province, 109,000 km2 of the land area is karst landscape. with a pH analyzer (HORIBA). Major element concentra- The province has a subtropical monsoonal climate, in tions were measured using X-ray fluorescence (XRF) which most of the regions are mild and wet. The mean annual spectrometry (Philips, PW2404), according to the GB/ temperature is 8–12 °C, and the mean annual rainfall is T14506.28-93 silicate rock chemical analytical procedure. 850–1600 mm. Approximately 50 % of the rainfall occurs The concentrations of trace and rare earth elements were from June to August. The study area is in central Guizhou analyzed using quadrupole inductively-coupled plasma Province (Fig. 1a). The major geological structure is the mass spectrometry (Q-ICP-MS; PerkinElmer, ELAN DRC- Guiyang syncline. The core of the syncline stratum is the e). Organic matter and pH data were determined at the Jurassic Ziliujing Formation, and the flank Formations (from College of Resource and Environmental Engineering at youngest to oldest) are the Erqiao ? Sanqiao, Yangliujing, Guizhou University, while the terra rossa and bedrock Guanling, and Anshun (Fig. 1b). According to the strati- major element data were provided by the Beijing Research graphic relationship (Fig. 1c), the exposed Anshun Forma- Institute of Uranium Geology. The major elements of tion from the syncline wings could be associated with the overlying strata and all trace elements were identified at the weathering and erosion of the overlying strata. Institute of Geochemistry, Chinese Academy of Sciences. 123 Acta Geochim (2016) 35(4):445–462 447 Fig. 1 a Map showing the location of western Guizhou, China. b Geologic sketch of the study area and different lithologies. c Lithostratigraphic log above the Anshun Formation (modified from a 1:200,000 regional geological map, 1977) Fig. 2 a Photos of the sampling profiles and b the rock–soil interface 3 Results composed of CaO and MgO, while Al2O3, SiO2,Fe2O3 and loss-on-ignition (LOI) values account for 90 w%–97 w% 3.1 Major element of the total terra rossa, as well as acid insoluble residues from the bedrock.