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December 2013 Vol.56 No.12: 2126–2134 doi: 10.1007/s11430-013-4705-3

Characteristics and environmental significance of Ca, Mg, and Sr in the soil infiltrating water overlying the Furong Cave, Chongqing, China LI JunYun1, LI TingYong1,2*, WANG JianLi1†, XIANG XiaoJing1, CHEN YunXuan1 & LI Xuan1 1

Key Laboratory of the Three Gorges Reservoir Region’s Eco-Environment, School of Geographical Sciences, Southwest University, Chongqing 400715, China; 2 State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, China Received September 29, 2012; accepted March 28, 2013; published online October 28, 2013

This paper explores the influence of the local meteoric conditions, the overlying soil on the concentrations of Ca, Mg, and Sr, and the ratios of Mg/Ca, Sr/Ca, and Mg/Sr in soil infiltrating water (SIW). An in situ monitoring program was carried out above the Furong Cave throughout 2010 to collect data on SIW, monthly temperature and rainfall, and the geochemical composition and granularity of soils. The discharge of SIW responded quickly to the local rainfall, and its residence time was the primary factor affecting the Ca, Mg, and Sr content of the SIW. The high concentrations of Ca, Mg, and Sr in the SIW during April should be attributed primarily to the prolonged residence of SIW in the soil during the local dry seasons of winter and early spring. The maximum Mg/Sr ratio also occurred in April. The ratio of Mg/Ca in SIW is positively correlated with prolonged residence time and with high temperatures, which do not strongly affect the ratio of Sr/Ca. The Mg/Ca ratio was lowest when the Sr/Ca ratio was highest because plant metabolism increased the absorption of Ca and Mg, but not Sr, and also because higher temperatures enhanced the dissolution of Mg more than that of Sr. These different responses of Mg and Sr to temperature increases resulted in high Mg/Sr ratios during July and August. Furong Cave, soil infiltrating water, Ca, Mg, Sr, ratios of elements Citation:

Li J Y, Li T Y, Wang J L, et al. Characteristics and environmental significance of Ca, Mg, and Sr in the soil infiltrating water overlying the Furong Cave, Chongqing, China. Science China: Earth Sciences, 2013, 56: 2126–2134, doi: 10.1007/s11430-013-4705-3

Trace elements in speleothems reflect environmental conditions such as regional rainfall (Banner et al., 1996; Treble et al., 2003; Musgrove et al., 2004), temperature (Katz, 1973; Goede et al., 1991), soil conditions (Lorens et al., 1981; Morse et al., 1990; Paquette et al., 1995), karst hydrology (Roberts et al., 1998), cave pCO2 (Huang et al., 2001), the growth rate of stalagmites (Gabitov et al., 2006; Borsato et al., 2007), the sources of elements (Goede et al., 1998; Frumkin et al., 2004; Zhou et al., 2009), and other environ*Corresponding author (email: [email protected]) †Corresponding author (email: [email protected])

© Science China Press and Springer-Verlag Berlin Heidelberg 2013

mental variables (Pingitore et al., 1986; Fairchild et al., 2006). Speleothems are playing an increasingly important role in the reconstruction of paleoclimate and paleoenvironment. Because of the complexity of multiple factors and regional sources of the trace elements, cave monitoring has been identified as an effective method to correctly understand the environmental significance of trace elements that have been recorded in the speleothems (Fairchild et al., 2009). Previous research has examined several factors that affect the Mg/Ca and Sr/Ca ratios in cave drip water, including the geochemical composition of local soil and bedrock, the flow path of groundwater (whether channel flow earth.scichina.com

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or infiltrating), water-rock interactions (whether calcite is dissolved or precipitated), temperature, growth-rate relative to the KD values of Mg and Sr respectively, and the intensity of prior calcite precipitation (PCP) (Trudgill et al., 1983a, 1983b; Fairchild et al., 1996; Baker et al., 1997; Karmann et al., 2007; Wong et al., 2011). The two most important determinants of the Mg/Ca and Sr/Ca ratios in drip water are water-rock interactions and the PCP process, which occurs as water is infiltrating the soil and bedrock (Fairchild et al., 2009). Soil is the primary source of metal elements in the water (Fairchild et al., 2006), but we need to improve our understanding of the geochemical changes that happen as meteoric precipitation filters through the soil zone. The composition of the soil directly influences the geochemical composition of soil water. For example, high levels of dolomite (rich in Mg) or aragonite (rich in Sr) in the clay increase the concentrations of Mg and Sr in drip water (Baker et al., 2000). The residence time, flow type, and amount of evaporation of soil water are influenced by the ground temperature, meteoric precipitation, and the physical properties of the soil (Shurbaji et al., 1995). In addition, the pCO2 is higher in soil air than in the atmosphere. Soil CO2 is the main source of CO2 in soil water, which controls the δ18O and δ13C composition of the soil water (Atkinson, 1977; Genty et al., 2001; McDermott, 2004). To fully understand the factors influencing the geochemical character of Karst cave drip water, the whole Karst system, including the atmospheric environment, the soil, and the Karst aquifer, should be studied integrally and systematically (Smart et al., 1986). Based on our cave monitoring work in the Furong Cave in Wulong County, Chongqing, and on our understanding of the process of rainfall infiltration into the soil zone (Fairchild et al., 2009; Li et al., 2007, 2008, 2011, 2012; Xiang et al., 2011; Yi et al., 2011), we examined four soil profiles above the Furong Cave. We monitored changes in trace elements in the SIW and analyzed their source and other influential factors by collecting soil and SIW samples. Specifically, we studied the influence of temperature, rain-

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fall, soil and plants on the concentrations of Ca, Mg, and Sr, the ratios of Mg/Ca, Sr/Ca, and Mg/Sr in the SIW. This work can help reconstruct the paleoenvironment from trace elements proxies that are stored in the speleothem.

1 1.1

Study site and methods Study site

The Furong Cave is located in the Karst valley near the town of Jiangkou in Wulong County, Chongqing (29°13′N, 107°54′E), approximately 4 km away from the junction of the Furong and Wujiang Rivers (Figure 1(a)). The bedrock above the Furong Cave is composed of dolomite limestone and dolomite. The rock formation over the main cave is of medium thickness with a dip angle of less than 25° and a trend toward the southeast (Zhu, 1994) (Figure 1(b)). The climate is a humid subtropical monsoon environment with four distinct seasons. The mean annual temperature from 1960 to 1985 was 17.9°C, and the average temperatures in January and July were 7°C and 28°C, respectively. The mean annual precipitation was 1082 mm from 1960 to 1980, with approximately 830 mm precipitation from May to October accounting for 77% of the annual total. The precipitation was only 47 mm during the winter months of December, January, and February, which represents only 4.3% of the annual total (Chen et al., 2006). The climate is characterized by a rainy spring, floods in summer, and an arid winter. The surface above the cave is covered in evergreen, broadleaved shrubs, and the soil is developed yellow mountain soil with a depth of 20–90 cm (Li et al., 2012). 1.2

Sample collection

We selected four soil profiles (SA–SD) in the mountain valley above the Furong Cave (Figure 1(b)). Water from both sides of the slope flows to the valley and infiltrates the Epikarst, providing the primary source for the dripwater in

Figure 1 (a) The location of the Furong Cave (revised from Li et al., 2008); (b) the distribution of sampling sites for the SIW above the Furong Cave (revised from Xiang et al., 2011); (c) the equipment for SIW collection.

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the Furong Cave. Soil samples were collected every 5 cm (10 cm in SC profile) from the bottom to the top of the soil profiles, and six bedrock samples were extracted from the base of the soil profiles. The mechanical composition and the elements including Ca, Mg, and Sr were measured in the soil and rock samples. The SIW collection apparatus was buried in the SA, SB, and SD profiles at 30, 25, and 30 cm, respectively. For the SC profile, SIW was collected at 10, 20, and 30 cm of depth. The SIW equipment consisted mainly of a polyethylene (PE) plate that was immersed in 1:1 HCl acid for 24 hours, washed with deionized water, and then dried. Three layers of gauze were washed, dried, and then placed over the plate. Silica wool was affixed to the center of the gauze to prevent soil from entering the plate. A small hole was drilled in the bottom of the plate to connect it to a PE receptacle (1000 mL) outside the profile with rubber tubing. To avoid microorganism activity that could affect the concentration of the elements in the infiltrating water, 2 mL of methyl aldehyde was added to the bottle. At the same time, liquid paraffin was added to prevent evaporation. Soil was backfilled to recover the profile after the equipment was buried. SIW was collected once a month since January 2010. The soil water and liquid paraffin were separated with a separating funnel in the field. Water samples were acidized with two or three drops of 1:1 HNO3. 1.3

Sample analysis

Soil and bedrock samples were air-dried naturally and then ground and sifted with a 100-mesh sieve. Approximately 0.5 g of the sample (to an accuracy of 0.0001 g) was placed into a polytetrafluoroethylene (PTFE) crucible and dissolved by HF, HClO4 and HNO3 (following the procedure described in reference (Lu, 1999)). The samples were then prepared as solutions for the analysis of element concentrations. The mechanical composition of the soil was analyzed at the Quaternary Sediments Analysis Laboratory in the School of Geographical Sciences at Southwest University

Figure 2


using the Mastersizer 2000 laser particle analyzer, which is produced by MALVERN Company in England. The measurement range is 0.02–2000 μm, and the precision is ±1%. The contents of Ca, Mg, and Sr in bedrock, soil, and SIW were measured using the Perkin-Elmer Optima 2100DV inductively coupled plasma emission spectrometer (ICP-OES, with a detection limit of 1 μg L1 and relative error ≤2%) in the Geochemistry and Isotope Laboratory in the School of Geographical Sciences at Southwest University (Li et al., 2006; Xiang et al., 2011).

2

Results and discussion

2.1 Distribution of Mg, Sr, and other elements in the karst surface The Ca, Mg, and Sr content has been analyzed by Xiang et al. (2011) in the bedrock, soil, and SIW above the Furong Cave, and the results showed that Ca>Mg>Sr. In addition, the average ratios of Mg/Sr were 1099, 486, and 530, respectively. Samples were rich in Mg and poor in Sr. The ratios of Mg/Sr in soil and SIW were similar, suggesting that soil was the primary source for the elements in the infiltrating water. The concentrations of Ca, Mg, Sr, the median size of the soil and the percentage of clay ( SC (20 cm) > SC (10 cm) (Figure 4). This could be explained by the leaching of soil elements by SIW during its passage through the soil zone. The contact time was prolonged with increased depth, and more material was dissolved into soil water. The contents of Ca, Mg and Sr in soil were arranged in the following order: humus horizon (0–10 cm) < illuvial horizon (10–50 cm) < parent rock horizon (>50 cm). This suggests that the increase in element content with soil depth should be attributed both to the prolonged contact time between the infiltrating water and the soil and to the variation in the chemical composition of the soil at different depths of the profile. In April, the content of Ca, Mg and Sr in infiltrating water from SC (10 cm) was higher than in the sample from SC (20 cm). However, Ca, Mg, and Sr were present in similar concentrations in May (Figure 4). This could have been affected by the temperature and the soil physical structure. For example, the SC soil profile is composed of dry

branches and fallen leaves with loose structure at the depth of 0–10 cm. At the depth of 10–20 cm, the soil was black or brown, and many tree roots were found. The temperature rose after April, and the soil temperature at 0–10 cm rose quickly. Microorganisms may have bloomed and secreted organic acids, which could have enhanced the dissolution of the infiltrating water. Although the deep soil responded to the rising surface temperature more slowly, the continual rise in temperature should eventually increase the temperature of deeper soil. It should also enhance the microbial activity and plant respiration, the organic acid content, and the CO2 concentration in soil air. Together, these enhance the dissolution ability of SIW. Finally, the SIW collected at 20 cm filtered through more soil than the SIW collected at 10 cm. Therefore, in May the SIW samples at 20 cm had higher concentrations of elements in the SC soil profile than the samples from 10 cm. The average ratios of Mg/Ca and 1000 × Sr/Ca in SIW were 0.413±0.093 and 0.716±0.152, respectively (excluding SB) (Table 1). For most of the soil profiles, the ratios of Mg/Ca and 1000×Sr/Ca changed each month, except for the samples from profile SC (30 cm) (Figure 5). Excluding the

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for several months during the dry winter and spring. The second peak of Mg/Ca ratios occurred in July, perhaps because of the rising soil temperature. In an open system, the solubility of CaCO3 can be reduced following an increase in temperature, even as the solubility of MgCO3 increases, driving up the ratio of Mg/Ca (Yuan et al., 1988). The peak ratios of Sr/Ca were measured in May and June (Table 1, Figure 5(a)–(e)), and there was no significant correlation between the Sr/Ca ratios and the regional precipitation and temperature. This is likely due to the following two reasons: (1) The vegetation absorbed more Ca and Mg than Sr during the rapid growing season of May and June (refer to section 3.3). (2) Because of its chemical properties, Sr is not as active as Ca or Mg. Additionally, Sr can be adsorbed strongly by the clay and organic matter in soil (Du et al., 1996; Li et al., 2007), and this adsorption effect means that Sr does not respond quickly to changes in temperature and precipitation. In addition, for the SIW from soil profile SB, the highest ratios of Mg/Ca and 1000 × Sr/Ca were in May, with the values of 1.202 and 5.83, respectively. These were clearly higher than the values of the SIW samples from other profiles. This suggests that there was PCP in the soil because in May, the content of Mg and Sr in SIW was increased, whereas the content of Ca was reduced (Treble et al., 2003; Fairchild et al., 2000; Banner et al., 1995). 2.3 The environmental implications of Mg/Ca, Sr/Ca and Mg/Sr ratios

Figure 5 Variation of Mg/Ca and Sr/Ca ratios in SIW. (a) SA; (b) SC (10 cm); (c) SC (20 cm); (d) SC (30 cm); (e) SD; (f) SB.

SB profile, the maximum ratio of Mg to Ca occurred in April or July (Table 1, Figure 5), which reflected the monthly variation in the precipitation and temperature in 2010 (Figure 3). The high ratio of Mg/Ca in April SIW samples can be attributed to the mixture of soil water that had high Ca and Mg because it was stored in the soil zone

Figure 6

The concentrations of Ca, Mg and Sr are positively correlated in SIW (Figure 6). The correlation coefficients of Ca to Mg, Sr to Ca, and Sr to Mg were 0.82, 0.88 and, 0.67, respectively. Previous research has shown that the residence time and the seepage distance for the infiltrating water determine the content of Ca, Mg and Sr in SIW. These findings underscore the importance of the contact time between water and soil in addition to the contact time between water and rock in the element concentrations of cave drip water (Hellstrom et al., 2000; Huang et al., 2001). There was an inverse correlation between Mg/Ca and Sr/Ca (R=-0.61) (Figure 7). Sr/Ca peak values corre-

The correlations between Mg and Ca (a), Sr and Ca (b), and Mg and Sr (c) in SIW.

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Figure 8

The variation of the SIW Mg/Sr ratio in 2010.

3 Conclusions Figure 7

The inverse correlation between Mg/Ca and Sr/Ca in the SIW.

sponded to the minimum Mg/Ca values, which were measured in May and June (Table 1, Figure 5). Roberts et al. (1998) discussed the Mg/Ca and Sr/Ca ratios in a stalagmite from the north of England and concluded that the low Mg/Ca and high Sr/Ca reflected the short resident time of groundwater in the epikarst zone. In the soil zone, this negative correlation might be attributable to the different levels of plant absorption of the different elements (Wang et al., 1984). Zhao (1997) thought that Ca and Mg were important nutritional elements for vegetation growth and that they could be strongly absorbed and accumulated by plants, which is not true of Sr (Liu et al., 1984). In the vigorous growing season with higher temperatures and more precipitation, the plants absorbed more Ca and Mg, reducing Ca and Mg in the SIW. In contrast, there was less absorption of Sr, leading to the synchronous occurrence of minimum Mg/Ca and maximum Sr/Ca. In a word, the absorption of elements by plants has an obvious influence on the ratios of Mg/Ca and Sr/Ca of infiltrating water. In the reconstruction of paleoclimate and paleoenvironment, the Mg/Sr ratio in speleothems has been used as an index of the temperature variation in cave drip water over long time scales and therefore as a reflection of the regional annual mean temperature. In addition, the speleothem Mg/Sr ratio has been used as an index to reconstruct the regional precipitation (Goede et al., 1991; Roberts et al., 1998; Fairchild et al., 2000, 2009; Ma et al., 2002; Huang et al., 2001). In this study, the peak value of Mg/Sr in April is related to the long residence time of infiltrating water in the soil during the dry months of winter and spring (Figure 8). Mg/Sr was also high in July, reflecting the higher air temperature and the annual maximum in precipitation (Figures 3 and 8). More Mg was also dissolved into soil water due to the high soil temperatures in July and August (Yuan et al., 1988), whereas Sr was not sensitive to temperature (Liu et al., 1984). This resulted in peak Mg/Sr ratios during July and August.

Based on our monitoring of SIW above the Furong Cave during 2010, we found that the geochemical composition of the SIW was directly influenced by the seasonal variation of surface temperature and precipitation, the seasonal changes in biological activity and the soil environment. The main conclusions can be outlined as follows. (1) The volume of SIW responded to local precipitation quickly and changed seasonally with the rainfall. During the dry seasons, the SIW remained in the soil zone and had prolonged contact with soil minerals, which resulted in an increase of Ca, Mg and Sr in the SIW. During the wet summer months, the increase of SIW volume meant reduced interaction between the soil minerals and SIW, so the concentrations of Ca, Mg, and Sr decreased. In addition, the thickness of the soil zone affected the residence time of SIW and has an influence on its chemical composition. (2) The highest values of the Mg/Ca and Mg/Sr ratios appeared in April and July, for different reasons. The high ratios in April water samples were mainly because of the prolonged residence time of SIW in the soil zone during the dry winter and spring months. In contrast, in July and August, the increased temperature and precipitation enhanced the biological activity of plants and microbes, increased the CO2 content in soil air, and strengthened the dissolution ability of SIW. The higher temperature and precipitation strengthened the dissolution of minerals in soil as well. All of these changes lead to the high values of Mg/Ca and Mg/Sr in SIW during July and August. We thank the reviewers for constructive and helpful reviews. We also thank Prof. Hongchun Li and Houyun Zhou for useful discussion and comments regarding the manuscript. We are grateful to Feng Huiwen, Li Jiubin, and Liu Wei for their help with field sampling and experimental work. This work was supported by National Natural Science Foundation of China (Grant Nos. 41302138, 40971122, 41030103 and 41172165), the Fundamental Research Funds for the Central Universities of Southwest University (Grant Nos. XDJK2013A012 and XDJK2009C106), State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences (Grant No. SKLLQG1310).

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