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Response of dissolved inorganic carbon (DIC) and δ13CDIC to changes in climate and land cover in SW China karst catchments
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ScienceDirect Geochimica et Cosmochimica Acta 165 (2015) 123–136 www.elsevier.com/locate/gca
Response of dissolved inorganic carbon (DIC) and d13CDIC to changes in climate and land cover in SW China karst catchments Min Zhao a,b,c, Zaihua Liu a,b,⇑, Hong-Chun Li c,⇑, Cheng Zeng a,b, Rui Yang a,b, Bo Chen a,b, Hao Yan a,b b
a State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, CAS, Guiyang 550002, China Puding Comprehensive Karst Research and Experimental Station, Institute of Geochemistry, CAS and Science and Technology Department of Guizhou Province, Puding 562100, China c Department of Geosciences, National Taiwan University, Taipei 10617, Taiwan
Received 9 September 2014; accepted in revised form 26 May 2015; available online 1 June 2015
Abstract Monthly hydrochemical data and d13C of dissolved inorganic carbon (DIC) in karst water samples from September 2007 to October 2012 were obtained to reveal the controlling mechanisms on DIC geochemistry and d13CDIC under different conditions of climate and land cover in three karst catchments: Banzhai, Dengzhanhe and Chenqi, in Guizhou Province, SW China. DIC of karst water at the Banzhai site comes mainly from carbonate dissolution under open system conditions with soil CO2 produced by root respiration and organic carbon decomposition with lowest d13C values under its dense virgin forest coverage. Weaker carbonate bedrock dissolution due to sparse and thin soil cover results in lower d13CDIC, pCO2, DIC and EC, and lower cation and anion concentrations. At the Chenqi site, larger soil CO2 input from a thick layer of soil results in high pCO2 and DIC, and low pH, SIc and d13CDIC in the karst water. At the Dengzhanhe site, a lesser soil CO2 input due to stronger karst rock desertification and strong gypsum dissolution contribute to higher d13CDIC, high EC and high cation and anion concentrations. Soil CO2 inputs, controlled by biological activity and available soil moisture, carbonate bedrock dissolution, dilution and degassing effects, vary seasonally following rainfall and temperature changes. Consequently, there are seasonal cycles in hydrochemistry and d13CDIC of the karst water, with high pCO2 and low pH, EC, SIc, and d13CDIC values in the warm and rainy seasons, and vice versa during the cold and dry seasons. A strongly positive shift (>3&) in d13CDIC occurred in the drought year, 2011, indicating that d13CDIC in groundwater systems can be an effective indicator of environmental and/or climate changes. Ó 2015 Elsevier Ltd. All rights reserved.
1. INTRODUCTION ⇑ Corresponding authors at: State Key Laboratory of Environ-
mental Geochemistry, Institute of Geochemistry, CAS, Guiyang 550002, China. Tel.: +86 851 85895263 (Z. Liu), Department of Geosciences, National Taiwan University, Taipei 10617, Taiwan. Tel.: +886 2 33662929 (H.-C. Li). E-mail addresses: [email protected] (Z. Liu), [email protected] (H.-C. Li). http://dx.doi.org/10.1016/j.gca.2015.05.041 0016-7037/Ó 2015 Elsevier Ltd. All rights reserved.
Recent debate on the contribution of karst processes to the global carbon budget is stimulating discussion upon dissolved inorganic carbon (DIC) behavior in karst aquatic systems (Liu et al., 2010). Utilization of DIC in photosynthesis in karst waters depends on hydrochemical and biological conditions in a system. In order to study the carbon budget and its controlling factors in a karst area, monitoring
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spatiotemporal hydrochemical and isotopic changes of the system is essential to investigate sources and timing of groundwater recharge, water-rock interaction along flow paths, and mixing of distinct groundwater sources (Kanducˇ et al., 2012). In addition, d18O and d13C of travertine deposits are considered as high-resolution paleoclimatic records because of their high depositional rates (Sun et al., 2014). Interpretation of travertine d13C records requires more fundamental study of the factors controlling DIC behavior and d13CDIC change in the systems. Using DIC concentrations and their d13C values together with other chemical measurements, previous studies have evaluated the sources, sinks and fluxes of carbon in natural waters (Taylor and Fox, 1996; Yang et al., 1996; Atekwana and Krishnamurthy, 1998). Many studies have shown that the DIC in surface waters has three main sources: soil CO2, dissolution of carbonate minerals, and atmospheric CO2 exchanged through the air–water interface (Yang et al., 1996; Atekwana and Krishnamurthy, 1998; Amiotte-Suchet et al., 1999; Aucour et al., 1999; Das et al., 2005). The processes not only vary seasonally but also control carbon input and output in the aquatic system, creating variations in DIC and d13CDIC. Although investigations of DIC concentration and d13CDIC in freshwater environments have been carried out recently by many researchers (Han et al., 2010; Zhao et al., 2010; Kanducˇ et al., 2012; Tsypin and Macpherson, 2012), studies on variations of inorganic carbon systems in the karst catchments on multiple time scales (e.g., the annual, seasonal and storm-event scale) are still very limited (He´lie et al., 2002; Liu et al., 2004, 2007; Guo et al., 2008; Li et al., 2010; Zhao et al., 2010). Thus, our understanding of the DIC and d13CDIC variations and their controlling factors at seasonal to long term scales in karst aquatic systems needs further investigation, especially under different land covers in different karst areas due to changes in biological activities, nutrient loads and the processes of mutual interaction that may occur (Harris, 1999). This study presents our measurements of temperature, rainfall, pH, conductivity, DIC concentration and d13CDIC in the underground karst rivers and springs of three small catchments with different land covers between September 2007 and October 2012. The inter-annual and seasonal variations over five hydrological cycles enable us to better understand the behavior of DIC geochemistry in small karst catchments. Our ultimate goal is to reveal the source and behavior of DIC and the factors controlling d13CDIC shifts in the catchments. The results will be used to estimate the effects of different land covers and climate changes on carbon budgets. This study also provides basic information for analysis of d13C records in travertine that can be used as good indicators of abnormal climatic conditions (e.g., annual precipitation and rainfall intensity) and land cover changes in the past (Matsuoka et al., 2001; Liu et al., 2011; Sun et al., 2014). 2. STUDY AREAS AND MONITORING SITES The study areas are located in central and south Guizhou Province, the center of the well-known karst
regions of SW China (Fig. 1). Under the typical monsoonal climates in the area the numerous karst aquatic systems may have a significant influence on the global carbon budget. One of the three study sites, Banzhai (25°120 –25°150 N, 107°550 –108°050 E), is situated in Maolan National Nature Reserved Park in Libo County, southeastern Guizhou, containing dense evergreen virgin forests growing on rugged peak-cluster karst topography (Fig. 1). The drainage area of Banzhai is 19 km2 with >90% forest coverage. The mean annual air temperature is 18.3 °C and annual precipitation is 1750 mm with 80% summer monsoonal rain. The lithology is chiefly dolomitic limestone of Middle and Lower Carboniferous age (Han et al., 2010). The monitoring station is built on the exit of an underground river (Fig. 1). Although this area has abundant vegetation coverage, the soil cover overlying carbonate rocks is sparse and thin with small farmlands (5% in area) in the bottoms of karst dolines. Thus, the Banzhai site represents an environment with abundant vegetation but barren soil. Chenqi and Dengzhanhe karst spring catchments are close to each other and located in Puding County, central Guizhou (Fig. 1). The main lithology is limestone and dolomite of the Guanlin Formation, middle Triassic (Zhao et al., 2009). Both catchments are characterized by a humid subtropical monsoon climate, with annual precipitation of 1400 mm/yr and mean annual air temperature of 16.5 °C (Zhao et al., 2010). Morphologically the two catchments are karst valleys with a variety of farmlands, including dry farming fields, rice paddies, shrubs and fruit orchards. However, there are large differences in the distribution and proportion of land use types in the two catchments (Zhao et al., 2009, 2010). The monitoring station at the Chenqi site was built on the small outlet of a karst spring (diameter of 0.5 m) surrounded by dense farmland before October 2009 (Fig. 1). The measured drainage area of this karst spring was about 1.31 km2 at that time. However, the station was enlarged and connected with other surface water sources via a canal after October 2009 (Fig. 1). Therefore, the hydrochemical record and d13CDIC values for Chenqi are used only from September 2007 to October 2009. The monitoring station at Dengzhanhe site is built on a karst spring pool about 3 m2 in size (Fig. 1). The catchment of this spring is 2.82 km2. The environment around this station is sparse and thin in soil and vegetation. In summary, the ecological environments represented by the three study sites can be classified as (1) abundant virgin forest vegetation but with barren soil at Banzhai; (2) thick soil with intensive land usage (i.e., strong farming activity) at Chenqi; and (3) less vegetation and thin soil coverage at Dengzhanhe. 3. MONITORING, SAMPLING AND LABORATORY ANALYSIS 3.1. Continuous monitoring and measurement of hydrochemistry Monthly water temperature (T), pH, and electrical conductivity (EC) with resolution of 0.01 °C, 0.01 pH and
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Fig. 1. The location of the study areas in Guizhou Province, China (modified after Zhao et al., 2010).
0.01 ls/cm, respectively were measured by hand-held water quality meter (WTW MultiLine P3 pH/LF-SET: Germany) from September 2007 to October 2012. A CTDP300 multi-parameter data logger (Greenspan Co., Australia) was used at each site to record water temperature and rainfall every 15 min with resolutions of 0.01 °C and 0.05 mm respectively. In situ titration with the Aquamerck Alkalinity Test and Hardness Test was carried out monthly to measure [HCO 3]
2+ and [Ca2+] of water. The resolution for [HCO 3 ] and [Ca ] is 6 and 1 mg/L respectively. Two sets of 60 ml samples were transferred in acid-washed hydroplastic bottles for chemical analysis of cations and anions after filtering through 0.45 lm Millipore filters. The samples for cation analysis were acidified to pH <2.0 using concentrated nitric acid. Concentrations of Na+, K+, Ca2+, and Mg2+ were determined by inductively coupled plasma optical emission
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spectrometer (ICP-OES). Concentrations of SO2 4 , Cl , and NO were analyzed by a Dionex ICS-90 ion chromatogra3 phy (IC).
3.2. d13CDIC measurement Water samples for d13CDIC analysis were collected in 600 ml hydroplastic bottles which contained a saturated BaCl2 solution. By injecting 2 N NaOH into the sealed bottles to increase pH to >12, all dissolved inorganic carbon was precipitated as BaCO3 (Liu et al., 2003; Zhao et al., 2010). During the process the bottles were filled completely to avoid CO2 exchange with ambient air, and then stored in a refrigerator before analysis. The BaCO3 precipitate was filtered, washed and dried, then analyzed in the laboratory with a GV IsoPrim IRMS. The carbon isotope data are reported on VPDB (&) scale, with a standard deviation (1r) of 0.15&.
4. CALCULATING CO2 PARTIAL PRESSURE AND THE CALCITE SATURATION INDEX CO2 partial pressure (pCO2) and the calcite saturation index (SIc) are calculated from a geochemical model with pH, temperature and concentrations of seven major ions (Liu et al., 2004, 2007; Zhao et al., 2010). Since the host rocks in the regions are limestone and dolomite, interca2 lated with gypsum strata, Ca2+, Mg2+, HCO 3 and SO4 are the major ions. With the recorded temperature and pH, as well as the measured ion concentrations, calculations of pCO2 and SIc were processed through the program WATSPEC (Wigley, 1977). 5. RESULTS AND DISCUSSIONS Time series data of rainfall, T, pH, EC, HCO 3 , pCO2, SIc and d13CDIC for Banzhai are shown in Fig. 2 and the
Fig. 2. Variations in hydrochemistry and d13CDIC of the Banzhai underground river in relation to changes in rainfall and water temperature. RS: rainy season; DS: dry season.
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Fig. 3. Variations in hydrochemistry and d13CDIC of the Chenqi and Dengzhanhe karst springs in relation to changes in rainfall and water temperature. RS: rainy season; DS: dry season; DZH: Dengzhanhe; CQ: Chenqi.
data sets for both Chenqi and Dengzhanhe sites are plotted in Fig. 3. Seasonal variations appear in all parameters in the figures. On top of the seasonal variations there are also longer-term trends in some parameters. For instance, increasing SIc and d13CDIC trends and decreasing pCO2 trends from 2007 to 2012 can be seen at Banzhai and Dengzhanhe sites (Figs. 2 and 3). The seasonal variations and multi-year trends shed the light on our understanding of DIC behavior in the karst water and provide fundamental information on interpretation of travertine d13C paleo-proxy records. Table 1 lists the minimum, maximum and mean values of the measured major cations and anions in the karst waters at the three study sites. Table 2 shows the minimum, maximum and mean values of the measured 13 water temperature, pH, EC, HCO 3 , and d CDIC, as well as the calculated pCO2 and SIc values at each site. Besides the seasonal variations and multi-year trends, the hydrochemical and d13CDIC data sets show some different features among these sites. We shall discuss them below.
5.1. Comparison of climate records and the 2011 drought Fig. 4 presents the comparison of rainfall records at Banzhai, Chenqi and Dengzhanhe with the rainfall records from the nearest weather stations – Dushan for Banzhai and Anshun for Chenqi and Dengzhanhe, indicating that the study sites reflect local climatic conditions very well. The rainfall records in Fig. 4 shows a severe drought in 2011 due to a summer monsoon failure. The drought caused serious damage in southwestern China, especially Guizhou and Yunnan provinces. Fig. 5 compares monthly air temperatures at the Dushan and Anshun weather stations with the water temperature recorded at the three study sites. Note first that the water temperature (measured only once each month) follows the air temperature, showing strong seasonal variations but with much smaller range. Second, the average annual air temperature at Banzhai is 1.5 °C higher than at Dengzhanhe. This difference is reflected in the water temperature except the April 2010
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Table 1 Concentrations of major cations and anions in the karst water samples at the three study sites. Site
Item
Ca2+ mg/L
Mg2+ mg/L
K+ mg/L
Na+ mg/L
HCO 3 mg/L
Cl mg/L
SO2 4 mg/L
NO 3 mg/L
Banzhai
Min Max Mean RS DS
54 102 74 73 75
7.9 21.7 16.7 17.3 16.7
0.18 0.75 0.44 0.43 0.44
0.21 0.78 0.57 0.60 0.51
176.9 268.4 227.8 225.7 233.8
0.31 2.25 1.15 1.14 1.17
5.2 21.6 14.6 14.3 15.3
0.5 13.0 3.8 3.8 3.4
Chenqi
Min Max Mean RS DS
84 140 106 103 111
9.5 26.0 15.6 14.3 17.0
0.71 8.20 1.70 1.35 2.09
0.87 6.50 1.94 1.28 2.66
170.8 311.1 232.6 234.6 230.0
3.41 9.39 6.52 6.45 6.60
38.0 211.4 98.1 70.2 128.3
2.6 26.9 11.3 12.4 10.0
Dengzhanhe
Min Max Mean RS DS
62 250 159 138 189
0.6 56.3 26.4 23.1 31.0
0.56 3.44 1.73 1.66 1.85
0.28 3.41 1.71 1.47 2.08
158.6 268.4 213.4 205.5 225.7
2.47 9.09 5.60 6.37 4.59
33.9 840.8 367.5 249.4 527.3
0.8 39.3 14.6 15.7 12.5
RS = rainy season from May to October; DS = dry season from November to April.
Table 2 Minimum, maximum and annual mean values of water temperature (water T), pH, electrical conductivity (EC), saturation index (SIC), CO2 partial pressure (pCO2) and d13CDIC at the three study sites. HCO 3 (mg/L)
SIC
340.0 409.0 373.9 41 375.6 373.6
176.9 268.4 227.8 41 225.7 233.8
6.94 8.15 7.36 24 7.23 7.48
418.0 743.0 571.5 24 553.3 589.8
7.25 8.14 7.65 44 7.53 7.78
374.0 1393.0 882.0 44 735.4 1042.6
Site
Item
Water T °C
pH
Banzhai
Min Max Mean N RS mean DS mean
16.2 19.5 18.2 41 18.6 17.7
7.37 8.36 7.82 41 7.73 7.99
Chenqi
Min Max Mean N RS mean DS mean
11.4 18.7 16.2 24 17.6 14.8
Dengzhanhe
Min Max Mean N RS mean DS mean
14.9 21.2 17.1 44 17.5 16.6
EC (ls/cm)
pCO2a (pa)
d13CDIC &
0.13 1.00 0.50 38 0.38 0.66
86 881 330 38 413 227
14.73 9.11 12.78 44 13.18 12.26
170.8 311.1 232.6 23 234.6 230.0
0.27 0.92 0.14 22 0.01 0.30
144 2799 1045 22 1291 749
13.74 9.85 11.54 21 11.78 11.28
158.6 268.4 213.4 44 206.1 221.3
0.28 1.00 0.43 44 0.27 0.60
140 1194 455 44 550 350
12.19 2.85 8.22 42 9.33 7.11
N = number of samples; RS = rainy season; DS = dry season. a Calculated CO2 partial pressure of water by WATSPEC (Wigley, 1977).
and August 2010 measurements at Dengzhanhe, which might be influenced by some anomalously warm water temperatures occurring on a daily scale. Third, the water temperatures had 1–2 months time lags behind the air temperatures (Fig. 5). Finally, the water temperature at Chenqi was a few degrees colder than that at Dengzhanhe site while the air temperature at the two sites should be the same because the two sites are only 1.2 km apart and at the same elevation. The colder water temperature at Chenqi may be due to higher mean height of catchment area. Although the elevations (1312 m a.s.l.) of the two
sites at the monitoring spots are similar, the area of lower elevation (1325–1375 m a.s.l.) is much smaller than that of higher elevation (1375–1525 m a.s.l.) in Chenqi catchment area, whereas the lower elevation area in Dengzhanhe catchment area is similar to the higher elevation area. These comparisons indicate that the karst waters at the three sites come chiefly from surface runoff and shallow groundwater. The discrepancies between air temperature and water temperature in terms of timing and variation affect pCO2 and SIc though the seasonal trends are similar.
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Fig. 4. Comparison of monitored rainfall records in the study sites with the rainfall records from nearby weather stations. BZ: Banzhai; DZH: Dengzhanhe; CQ: Chenqi.
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dolomite (Table 1). The major anion at Banzhai is 2 HCO 3 , but SO4 becomes dominant at Dengzhanhe and Chenqi due to interbedded gypsum strata. Karst waters at Banzhai thus are the Ca–HCO3 type, whereas the waters at Dengzhanhe are Ca–HCO3–SO4 (Table 1). The dissolution of gypsum provides abundant Ca2+ and SO2 ions, 4 which in turn will influence SIc, carbonate precipitation. Thus, the hydrochemical conditions of Banzhai and Dengzhanhe site are distinctly different. The concentrations of Ca2+, Mg2+ and SO2 at 4 Dengzhanhe are significantly higher than at Chenqi despite having the same bedrock lithology and climatic conditions. The cause of the hydrochemical differences may be manifold, such as soil pCO2, carbonate dissolution and precipitation, CO2 degassing and water residence time, etc. With higher soil pCO2, lower water temperature and pH at Chenqi, the host rock dissolution would be stronger (Fig. 3). But, this is not the case, meaning that the effect of soil pCO2 on host rock dissolution is not the reason for lower ion concentrations at Chenqi. As bedrock lithology and climatic conditions are the same for both Chenqi and Dengzhanhe, the situation of dissolution, CaCO3 precipitation and CO2 degassing are unlikely the reasons for lower ion concentrations at Chenqi. Instead, water residence time that is related to geomorphology and size of catchment may be the reason. The main course of groundwater flow of Dengzhanhe site is 2.2 km in length while that at Chenqi is only 400 m. The morphology of Chenqi is shovel-like in shape with steep slopes on three sides (Zhao et al., 2010). With shorter groundwater passage and fast water flow on the steep slopes, groundwater residence time at Chenqi site is short. This is why the concentrations of Ca2+, Mg2+ and SO2 4 and pH, EC and SIc in the Chenqi karst spring are lower than those in the Dengzhanhe karst spring (Fig. 3, Tables 1 and 2). Concentrations of K+, Na+, Cl and NO 3 at Chenqi and Dengzhanhe are higher than those at Banzhai (Table 1), indicating that the farming impacts at the former are stronger. Because these ions are not chiefly from limestone dissolution, they may be introduced by stronger farming activities (e.g., fertilizing). 5.3. pCO2 and d13CDIC in the karst water - effects of soil CO2 and carbonate dissolution
Fig. 5. Comparison of measured water temperature records at the study sites with the air temperature records from nearby weather stations. BZ: Banzhai; DZH: Dengzhanhe; CQ: Chenqi.
In addition, note that the winter air temperature of 2010– 2011 was anomalously cold, which might be one reason for the 2011 drought. 5.2. Hydrochemistry of the karst water The major cations in the study areas are Ca2+ and significant amounts of Mg2+ from dissolution of limestone and
Since the karst waters at the three sites have pH ranging from 6.94 to 8.36 with an average value of 7.64 (Table 2), HCO 3 is the main inorganic carbon species in water (Clark and Fritz, 1997). As it is dominant, HCO 3 can be used to represent DIC in the karst catchments (He´lie et al., 2002; Das et al., 2005). In Table 2, concentrations of HCO 3 at the three sites are similar, but the calculated pCO2 and measured d13CDIC are quite different. pCO2 is mainly determined by HCO 3 concentration and pH, with lower pH indicating higher pCO2. The HCO 3 concentrations at both Chenqi and Dengzhanhe sites are similar, thus calculated pCO2 depends mainly on pH. The measured pH values at Chenqi site were lower than those at Dengzhanhe except for the months of January 2009– March 2009. Therefore, the calculated pCO2 values at
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Chenqi were much higher than at Dengzhanhe during the most period, September 2007–October 2009. For the dry months of January 2009–March 2009, pCO2 was similar (Fig. 3). The higher calculated pCO2 in Chenqi karst spring was caused by lower pH which can be attributed either to higher soil CO2 input, or to less carbonate dissolution in the groundwater. d13CDIC may tell us the dominant factor. DIC in the karst water derives mainly from soil CO2 and dissolution of carbonate bedrock. Because the d13C of soil CO2 (24& in general) is much lower than that of carbonate bedrock (0–2&) (Cerling, 1984; Fritz et al., 1989; Cane and Clark, 1999), the d13CDIC in a karst water depends on the mixing ratio of the two sources. In Fig. 3, d13CDIC values in the Chenqi record were lower than those of Dengzhanhe regardless of pH and HCO 3 concentration, even though when pH was higher during January 2009–March 2009 and HCO 3 concentration was higher during the rainy seasons of 2008 and 2009. This indicates that more CO2 derived from soil CO2 at Chenqi. The thick soil there maintains higher CO2 with lower d13C, so that greater soil CO2 always produces lower d13CDIC in the karst water. For instance, during the rainy seasons of 2008 and 2009, EC values at both Chenqi and Dengzhanhe were similar, indicating the same amount of carbonate dissolution but the HCO 3 concentrations were higher with lower d13CDIC at Chenqi, reflecting greater DIC passing from soil into the karst water (Fig. 3). Note that SIc during these rainy seasons was negative at Chenqi, showing that the karst water was unsaturated. In the dry seasons of 2007–2008 and 2008–2009, rainfall was low and soil activity was weak, so that input of DIC from carbonate dissolution became dominant. The HCO 3 concentrations at Chenqi and Dengzhanhe were close to each other. However, the d13CDIC of Chenqi was significantly lower than Dengzhanhe. Again, this implies that the lower pH and higher pCO2 at Chenqi site may be attributed to more input of soil CO2. Therefore, the land use with thick soil cover and strong farm activity is one of the factors controlling DIC behavior and d13CDIC of the karst water. Comparison of mean values of pH, EC, HCO 3 concentration, d13CDIC and calculated pCO2 and SIc between the Banzhai site and Dengzhanhe over the 2007–2012 period can be found in Table 2. The mean pH and HCO 3 concentration at Banzhai were slightly higher but the mean EC was significantly lower than at Dengzhanhe where there was gypsum dissolution as well as carbonate. The mean values of calculated pCO2 for the Banzhai and Dengzhanhe sites were 330 Pa and 455 Pa, respectively. d13CDIC values ranged from 14.73& to 9.11& with a mean value of 12.78& at Banzhai and ranged from 12.19& to 2.85& with an average of 8.22& at Dengzhanhe (Table 2). The d13CDIC values in Banzhai underground river were much lower than those in Dengzhanhe karst spring, reflecting the difference in carbon sources from organic matter decomposition. Although vegetation can produce CO2 through root respiration and decomposition, the sparse and thin soil cover at Banzhai does not easily store it. With low CO2 in the soil, the pH of karst water is higher so that its capacity to dissolve the
rock is relatively weak. Low d13CDIC, low EC, low cation and anion concentrations support this notion. In contrast, at Dengzhanhe in response to strong bedrock dissolution, limited vegetation cover and intensive farming, karst water has higher d13CDIC and pCO2 (Tables 1 and 2). In summary, DIC behavior and d13CDIC in the karst water are influenced by many factors, including (1) soil CO2 input controlled by vegetation and soil coverage, land use activity, and soil thickness; (2) carbonate bedrock dissolution related to water residence time which is controlled by rates of groundwater flow, geomorphology, and initial pH of surface water; (3) climatic condition. At the three study sites, Banzhai has dense vegetation but thin soil with less land use activity. DIC in the karst water is mainly from root respiration and organic carbon decomposition with weak carbonate bedrock dissolution, resulting in very low d13CDIC, low pCO2 and EC, low cation and anion concentrations. Chenqi has large soil CO2 input due to thick soil and strong farming activity, and very weak carbonate bedrock dissolution because of short groundwater flowpath and steep hydraulic gradients. Its karst water has high pCO2 and low pH, SIc and d13CDIC. At Dengzhanhe, sparse and thin soil coverage, strong carbonate bedrock dissolution caused by the long groundwater flowpath in a long synclinal valley, and the presence of interbedded gypsum, account for high EC, cation and anion concentrations, and higher d13CDIC (gypsum dissolution in a carbonate system shows the classic ‘common ion’ effect. This effect increased the saturation state of the water with respect to calcite, leading to enhanced 13C-depleted CO2 degassing from groundwater and thus higher d13CDIC). 5.4. Seasonal variations of hydrochemistry and d13CDIC in the karst waters - effects of dilution, soil CO2 and carbonate dissolution Figs. 2 and 3 show that there are strong seasonal varia13 tions in measured pH, EC, HCO 3 and d CDIC at all three sites following changes in rainfall and water temperature, so that the calculated pCO2 and SIc changed accordingly. In general, lower pH, EC, HCO concentrations and 3 d13CDIC result from the high rainfall and water temperatures of the rainy season, and vice versa during the dry season. During the rainy season, fresh inflows of precipitation at low pH dilutes the karst water and dissolves further soil CO2 to it (Liu et al., 2004, 2007). The higher temperatures in the rainy season lead to increased biomass productivity, strong root respiration and organic decomposition, providing more soil CO2 with low d13C (Atkin et al., 2000). During the dry seasons, low rainfall leads to longer groundwater residence times, resulting in increased carbonate solute concentrations per mL, which raises pH, EC and HCO 3 concentrations and introduces DIC with high d13C. On the other hand, cooler temperatures increase the rate of carbonate dissolution in the karst water and reduces biomass productivity and biological activity, so that increased inorganic carbon and decreased organic carbon coincide in time. Thus, higher cation and anion concentrations appear in the dry season, especially at Dengzhanhe (Table 1). Higher pH, ion concentrations and EC resulting
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in low pCO2 provide supersaturation of calcite (SIc > 0) in dry seasons (Figs. 2 and 3; Tables 1 and 2). Matsuoka et al. (2001) found that groundwater degassing might be one of the causes of seasonal variation of d13C in karst water at their study site in SW Japan where winter is also the dry season and summer the rainy season. They attributed the stronger degassing effect in winter to the subsurface temperature being warmer (less dense) than the atmosphere, resulting in increased d13CDIC due to degassing of 13C-depleted CO2 from the groundwater (Matsuoka et al., 2001). However, the seasonal variation of d13CDIC may be controlled by at least four factors: (1) high soil CO2 with low d13C due to biological activity (more biomass, root respiration and organic decomposition) in the rainy and warm season, and vice versa during the dry and cold season; (2) higher soil CO2 input due to more rainwater input; (3) lesser concentration of dissolved carbonates per mL owing to shorter groundwater residence times and the enhanced dilution in the rainy conditions, and the reverse during the dry and cold season; (4) strong degassing of 13C-depleted CO2 from the karst water during the dry and cold season leading to d13CDIC increase. Our records agree that CO2 degassing in winter season is one of the factors to elevate d13CDIC. However, as d13CDIC chiefly followed the pCO2 changes controlled by pH
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(Figs. 6 and 7), soil CO2 input and carbonate bedrock dissolution corresponding to the seasonal climatic conditions also play important roles in the seasonal cyclic variation of d13CDIC. The seasonal variations in hydrochemistry and d13CDIC of the karst water indicate that dilution and enhanced soil CO2 input are dominant in rainy seasons, whereas higher concentrations of dissolved rock and CO2 degassing are dominant factors during dry seasons. Therefore, a negative change in d13CDIC corresponds to wet and warm climatic conditions; a strong increase in d13CDIC may reflect dry and cold climatic conditions (Fig. 8). In addition to these general features, the three study sites show some detailed exceptions due to local differences in conditions. For example, HCO 3 concentrations at Chenqi were not decreasing when rainfall increased in the summer months (Fig. 3). As noted, the Chenqi karst spring was featured by greater soil cover and paddy land in the catchment area, both of which could produce and keep more CO2 in the soil-aquifer system (Zhao et al., 2010). The paddy land can generate plenty of CO2 in the soil during the summer months when farming activity is at its maximum. The abundant soil CO2 was washed out with the rainwater to increase HCO 3 concentrations and reduce pH. The soil CO2 effect rather than the dilution effect was dominant in this case.
Fig. 6. Correlation between d13CDIC and pCO2 at Banzhai and Dengzhanhe.
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Fig. 7. Correlation between pCO2 and pH at the study sites. BZ: Banzhai; DZH: Dengzhanhe; CQ: Chenqi.
Another feature is that the seasonal d13CDIC variation at Banzhai was different from that at Dengzhanhe (Fig. 9), even though there was clear seasonal variation at both sites. At Dengzhanhe, d13CDIC values increased quickly after the rainy season ended in September, while they remained relatively low for a few months at Banzhai after the rains (shaded bars in Fig. 9). The explanation for such a discrepancy may be the strong carbonate dissolution at Dengzhanhe. When the soil CO2 contribution became less after the rainy season due to reduced vegetation growth and biological activity, the carbonate rock tended to make the dominant contribution to DIC in the water (Reardon et al., 1979; Baldini et al., 2005; Maunoury et al., 2007). Consequently d13CDIC should increase, which occurred at Dengzhanhe. Banzhai contains dense virgin evergreen forests but thin soil cover, so that bedrock dissolution was not strong. With relatively warm temperature and dense vegetation, the carbon source for DIC in the karst water was still mainly from root respiration and organic decomposition at
the site. Fig. 9 shows that the entire d13CDIC record at Banzhai is below 9&. Development of rocky desertification (with soil and vegetation loss) due to human activity at Dengzhanhe elevated the d13CDIC of the karst water. 5.5. Influence of the 2011 drought on d13CDIC trend potential of travertine d13C records as paleoclimate proxies On top of the seasonal d13CDIC variations, the d13CDIC records at both Banzhai and Dengzhanhe appear to display multi-year trends of increase from 2007 to 2011 (Figs. 2 and 3). The strong enrichment of d13CDIC in 2011 corresponds to less rainfall in summer and lower water temperatures in winter (Fig. 8). The d13CDIC values were highest in both the summer and winter of 2011 compared with other years, reflecting changes in soil CO2, carbon sources and hydrochemical responses to climatic conditions. Table 3 shows that the annual rainfall in 2011 was the lowest during the five years caused by the summer monsoon failure.
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Fig. 8. Comparison of d13CDIC records with weather records at Banzhai and Dengzhanhe. BZ: Banzhai; DZH: Dengzhanhe.
Fig. 9. Comparison of d13CDIC records between Banzhai and Dengzhanhe. BZ: Banzhai; DZH: Dengzhanhe.
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Table 3 Annual rainfall (mm) at Banzhai and Puding (Chenqi and Dengzhanhe sites). Site
Banzhai Puding
Year 2008
2009
2010
2011
2012
1965 1563
1514 997
1931 1198
1178 744
1270 1157
According to Atekwana and Krishnamurthy (1998), discharges lower than the annual means will result in decrease in DIC concentrations, based on a plot of the discharge-DIC relationship. Yang et al. (2012) found that pCO2 in karst water changed synchronously with soil CO2 (i.e., higher in summer and lower in winter), because higher soil moisture in the rainy season provided more dissolved CO2 migrating into groundwater through quick flow. Tsypin and Macpherson (2012) also reported that transport of dissolved total inorganic carbon to DIC pool was also limited by the low soil moisture during the season. Normally, biological activity becomes the dominant factor controlling the DIC pool during wet and warm summers through CO2 removal by photosynthesis and release via respiration and decay of plants (Pawellek and Veizer, 1994; Yang et al., 1996; Atekwana and Krishnamurthy, 1998). Rainfall determines the soil water content available for biological respiration and the air-filled pore spaces available for CO2 flux (Ouyang and Zheng, 2000), and plays an important role in the generation of soil CO2 by regulating soil moisture, in turn shifting the d13CDIC values (Deng et al., 2011). Thus, during the drought summer of 2011, reduced biomass productivity, weakened biological activity and decreased soil CO2 transportation resulted in less soil CO2 contribution to d13CDIC. On the other hand, less rainfall during the rainy season in 2011 increased the groundwater residence times and enhanced solute carbonate concentrations, which brought up the contribution of inorganic carbon to the DIC and elevated the d13CDIC. In fact, the HCO 3 concentrations at Banzhai and Dengzhanhe displayed inverse seasonal changes in 2011 compared to the other years, i.e., high HCO 3 concentrations in summer, low HCO 3 concentrations in winter (Figs. 2 and 3). In addition, during the 2011 drought dilution effects became minimal and degassing increased strongly with SIc > 0 all the time (Figs. 2 and 3). These factors led to high d13CDIC in the rainy season of 2011, and may have continued to influence d13CDIC during the following winter season. In cold and dry winter seasons, d13CDIC was usually high. In the winters of 2010–2011 and 2011–2012, the d13CDIC reached maxima, being at least 2& higher than in other winters (Fig. 9). Note that the air temperatures in the winters of 2010–2011 and 2011–2012 were colder than in other winters, but the water temperatures did not show the same response (Fig. 4). Air temperature affects vegetation via photosynthesis, root respiration and decay of plants, while water temperature influences carbonate dissolution. With colder air temperature, production of soil CO2 decreases, resulting in lesser contributions of soil CO2 to DIC in karst water and d13CDIC increase. However, the decreased biological activity takes some time
to exert its effect on d13CDIC, while the impact of carbonate bedrock dissolution on d13CDIC can occur quickly. This may explain why the d13CDIC record at Banzhai has a 1– 2 months lag behind the air temperature, while the lag at Dengzhanhe is much shorter. As discussed above, the effect of carbonate bedrock dissolution on d13CDIC at Banzhai is much weaker than at Dengzhanhe site. According to Yang et al. (2012), the pCO2 in spring water varied frequently in the rainy season and remained stable in the dry season. Since the winter of 2010, pCO2 has remained stable and SIc maintained positive values. As a result, degassing has increased d13CDIC since the winter of 2010. Nevertheless, with less soil CO2 input during cold winter and more contribution from bedrock dissolution, the d13CDIC values during 2010–2011 and 2011–2012 winters became the highest. Combining the effects of the summer drought and cold winter of 2011, the mean d13CDIC value from winter 2010–2011 to winter 2011–2012 was at least 3& higher than in other years, which should be recorded strongly in travertine deposited from these karst waters (Liu et al., 2006, 2011). 6. CONCLUSIONS The variations of water temperature, pH, EC, HCO 3, major cations and anions, d13CDIC measured between September 2007 and October 2012 at three different field sites (Banzhai, Chenqi and Dengzhanhe) in the monsoonal karst region of Guizhou, SW China, allow us to calculate pCO2 and SIc, and to evaluate the factors controlling the hydrochemical behavior and d13CDIC. Soil CO2 input related to vegetation cover and land use, carbonate bedrock dissolution controlled by lithology, groundwater passage and geomorphology, as well as CO2 degassing under different climatic conditions at the study sites are the major factors to affect the DIC behavior and d13CDIC. At Banzhai where there is dense vegetation coverage but thin soil layer with less land use activity, root respiration and organic carbon decomposition with weak carbonate bedrock dissolution result in the lowest d13CDIC, low pCO2 and EC, low cation and anion concentrations. Chenqi has large soil CO2 input due to a thick soil layer and strong farming activity, and very weak carbonate bedrock dissolution because of a short groundwater flowpath and steep surface slope; consequently, the karst water has low pH and d13CDIC, high pCO2 and low SIc. At Dengzhanhe, strong gypsum bedrock dissolution and lesser soil CO2 input due to more karst rocky desertification contribute to higher d13CDIC, high EC and high cation and anion concentrations. Thus, in order to estimate detailed carbon sources, sinks and budgets in karst regions, all such factors need to be considered. There are clear seasonal variations in hydrochemical parameters and d13CDIC in the karst waters of the study areas, with high pCO2 and low pH, EC, SIc and d13CDIC in the warm and rainy seasons, and vice versa during the cold and dry seasons. Higher soil CO2 input controlled by biological activity and available soil moisture and dilution effects are dominant during the rainy season. Carbonate bedrock dissolution influenced by groundwater residence times and carbonate solubility, as well as degassing, are
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the major factors to control hydrochemistry and d13CDIC. The seasonal variation in d13CDIC demonstrates that a negative change in d13CDIC corresponds to wet and warm climatic conditions, while a strong increase in d13CDIC may reflect dry and colder climatic conditions. During the drought year, 2011, the d13CDIC of the study area exhibited anomalously high values. Extremely low rainfall and cold winter temperatures can be recorded strongly in an anomalously high d13CDIC signal. The d13C records of travertine deposited from these karst waters may permit the reconstruction of high-resolution records of paleoclimate change and extreme events. ACKNOWLEDGEMENTS This work was supported by the 973 program (2013CB956700), the National Natural Science Foundation of China (41430753, 41103084), Doctoral Foundation of Guizhou (2011GZ62743) and the Fund of the State Key Laboratory of Environmental Geochemistry (SKLEG2013205). Grants from NSC 102-2811-M-002-177 and MOST 103-2116-M-002 -001 to H. Li supported the postdoc study of Min Zhao at NTU. Special thanks are given to Derek Ford for his thoughtful academic and editorial suggestions on the manuscript.
REFERENCES Amiotte-Suchet P., Aubert D., Probst J. L., Lafaye-Gauthier F., Probst A., Andreus F. and Viville D. (1999) d13C pattern of dissolved inorganic carbon in a small granitic catchment: the Strenbach case study (Vosges mountains, France). Chem. Geol. 159, 129–145. Atekwana E. A. and Krishnamurthy R. (1998) Seasonal variations of dissolved inorganic carbon and d13CDIC of surface waters: application of a modified gas evolution technique. J. Hydrol. 205, 265–278. Atkin O. K., Edwards E. J. and Loveys B. R. (2000) Response of root respiration to changes in temperature and its relevance to global warming. New. Phytol. 147, 141–154. Aucour A. M., Sheppard S. M. F., Guyomar O. and Wattelet J. (1999) Use of 13C to trace origin and cycling of inorganic carbon in the Rhoˆne river system. Chem. Geol. 159, 87–105. Baldini J. F., McDermott F., Baker A., Baldini L. M., Mattey D. P. and Railsback L. B. (2005) Biomass effects on stalagmite growth and isotope ratios: a 20th century analogue from Wiltshire, England. Earth Planet. Sci. Lett. 240, 486–494. Cane G. and Clark I. D. (1999) Tracing ground water recharge in an agricultural watershed with isotopes. Ground Water 37, 133– 139. Cerling T. E. (1984) The stable isotopic composition of soil carbonate and its relationship to climate. Earth Planet. Sci. Lett. 71, 229–240. Clark I. D. and Fritz P. (1997) Environmental isotopes in hydrogeology. CRC. Das A., Krishnaswami S. and Bhattacharya S. K. (2005) Carbon isotope ratio of dissolved inorganic carbon (DIC) in rivers draining the Deccan Traps, India: sources of DIC and their magnitudes. Earth Planet. Sci. Lett. 2361, 419–429. Deng Q., Zhou G., Liu S., Chu G. and Zhang D. (2011) Responses of soil CO2 efflux to precipitation pulses in two subtropical forests in southern China. Environ. Manage. 48, 1182–1188. Fritz P., Fontes J. C., Frape S. K., Louvant D. and Michelot J. L. (1989) The isotope geochemistry of carbon in groundwater at Stripa. Geochim. Cosmochim. Acta 53, 1765–1775.
135
Guo H., Jiao J. and Weeks E. P. (2008) Rain-induced subsurface airflow and Lisse effect. Water Resour. Res. 44, W07409. http:// dx.doi.org/10.1029/2007WR006294. Han G., Tang Y. and Wu Q. (2010) Hydrogeochemistry and dissolved inorganic carbon isotopic composition on karst groundwater in Maolan, southwest China. Environ. Earth. Sci. 60, 893–899. Harris G. P. (1999) Comparison of the biogeochemistry of lakes and estuaries: ecosystem processes, functional groups, hysteresis effects and interactions between macro-and microbiology. Mar. Freshwater Res. 50, 791–811. He´lie J. F., Marcel C. H. and Rondeau B. (2002) Seasonal changes in the sources and fluxes of dissolved inorganic carbon through the St. Lawrence River – isotopic and chemical constraint. Chem. Geol. 186, 117–138. Kanducˇ T., Mori N., Kocaman D., Stibilj V. and Grassa F. (2012) Hydrogeochemistry of Alpine springs from North Slovenia: insights from stable isotopes. Chem. Geol. 300, 40–54. Li S., Liu C., Li J., Lang Y., Ding H. and Li L. (2010) Geochemistry of dissolved inorganic carbon and carbonate weathering in a small typical karstic catchment of Southwest China: isotopic and chemical constraints. Chem. Geol. 277, 301– 309. Liu Z., Zhang M., Li Q. and You S. (2003) Hydrochemical and isotope characteristics of spring water and travertine in the Baishuitai area (SW China) and their meaning for paleoenvironmental reconstruction. Environ. Geol. 44, 698–704. Liu Z., Groves C., Yuan D., Meiman J., Jiang G., He S. and Li Q. (2004) Hydrochemical variations during flood pulses in the south-west China peak cluster karst: impacts of CaCO3–H2O– CO2 interactions. Hydrol. Proc. 18, 2423–2437. Liu Z., Li H., You C., Wan N. and Sun H. (2006) Thickness and stable isotopic characteristics of modern seasonal climatecontrolled sub-annual travertine laminas in a travertine-depositing stream at Baishuitai, SW China: implications for paleoclimate reconstruction. Environ. Geol. 51, 257–265. Liu Z., Li Q., Sun H. and Wang J. (2007) Seasonal, diurnal and storm-scale hydrochemical variations of typical epikarst springs in subtropical karst areas of SW China: soil CO2 and dilution effects. J. Hydrol. 337, 207–223. Liu Z., Dreybrodt W. and Wang H. (2010) A new direction in effective accounting for the atmospheric CO2 budget: considering the combined action of carbonate dissolution, the global water cycle and photosynthetic uptake of DIC by aquatic organisms. Earth Sci. Rev. 99, 162–172. Liu Z., Sun H., Li H. and Wan N. (2011) d13C, d18O and deposition rate of tufa in Xiangshui River, SW China: implications for land-cover change caused by climate and human impact during the late Holocene. Geol. Soc. Lond. Spec. Pub. 352, 85–96. Matsuoka J., Kano A., Oba T., Watanabe T., Sakai S. and Seto K. (2001) Seasonal variation of stable isotopic compositions recorded in a laminated tufa, SW Japan. Earth Planet. Sci. Lett. 192, 31–44. Maunoury F., Berveiller D., Lelarge C., Pontailler J. Y., Laurent V. and Damesin C. (2007) Seasonal, daily and diurnal variations in the stable carbon isotope composition of carbon dioxide respired by tree trunks in a deciduous oak forest. Oecologia 151, 268–279. Ouyang Y. and Zheng C. (2000) Surficial processes and CO2 in soil ecosystem. J. Hydrol. 234, 54–70. Pawellek F. and Veizer J. (1994) Carbon cycle in the upper Danube and its tributaries: d13CDIC constraints. Israel. J. Earth Sci. 43, 187–194. Reardon E. J., Allison G. B. and Fritz P. (1979) Seasonal chemical and isotopic variations of soil CO2 at Trout Creek, Ontario. J. Hydrol. 43, 355–371.
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M. Zhao et al. / Geochimica et Cosmochimica Acta 165 (2015) 123–136
Sun H., Liu Z. and Yan H. (2014) Oxygen isotope fractionation in travertine-depositing pools at Baishuitai, Yunnan, SW China: effects of deposition rates. Geochim. Cosmochim. Acta 133, 340–350. Taylor C. B. and Fox V. J. (1996) An isotopic study of dissolved inorganic carbon in the catchment of the Waimakariri River and deep ground water of the North Canterbury Plains, New Zealand. J. Hydrol. 186, 161–190. Tsypin M. and Macpherson G. L. (2012) The effect of precipitation events on inorganic carbon in soil and shallow groundwater, Konza Prairie LTER Site, NE Kansas, USA. Appl. Geochem. 27, 2356–2369. Wigley T. M. L. (1977) WATSPEC: a computer program for determining equilibrium speciation of aqueous solutions. Br. Geomorphol. Res. Group Tech. Bull. 20, 1–46. Yang C., Telmer K. and Ja´n V. (1996) Chemical dynamics of the “St. Lawrence” riverine system: dDH2O, d18OH2O, d13CDIC,
d34Ssulfate, and dissolved 87Sr/86Sr. Geochim. Cosmochim. Acta 60, 851–866. Yang R., Liu Z., Zeng C. and Zhao M. (2012) Response of epikarst hydrochemical changed to soil CO2 and weather conditions at Chenqi, Puding, SW China. J. Hydrol. 468–469, 151–158. Zhao M., Zeng C. and Liu Z. (2009) Influence of land use change on dissolved inorganic carbon and stable isotopic compositions of karst groundwater. Geochimica 38, 565–572 (in Chinese with English abstract). Zhao M., Zeng C., Liu Z. and Wang S. (2010) Effect of different land use/land cover on karst hydrogeochemistry: a paired catchment study of Chenqi and Dengzhanhe, Puding, Guizhou, SW China. J. Hydrol. 388, 121–130. Associate editor: F. McDermott
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Response of dissolved inorganic carbon (DIC) and d13CDIC to changes in climate and land cover in SW China karst catchments Min Zhao a,b,c, Zaihua Liu a,b,⇑, Hong-Chun Li c,⇑, Cheng Zeng a,b, Rui Yang a,b, Bo Chen a,b, Hao Yan a,b b
a State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, CAS, Guiyang 550002, China Puding Comprehensive Karst Research and Experimental Station, Institute of Geochemistry, CAS and Science and Technology Department of Guizhou Province, Puding 562100, China c Department of Geosciences, National Taiwan University, Taipei 10617, Taiwan
Received 9 September 2014; accepted in revised form 26 May 2015; available online 1 June 2015
Abstract Monthly hydrochemical data and d13C of dissolved inorganic carbon (DIC) in karst water samples from September 2007 to October 2012 were obtained to reveal the controlling mechanisms on DIC geochemistry and d13CDIC under different conditions of climate and land cover in three karst catchments: Banzhai, Dengzhanhe and Chenqi, in Guizhou Province, SW China. DIC of karst water at the Banzhai site comes mainly from carbonate dissolution under open system conditions with soil CO2 produced by root respiration and organic carbon decomposition with lowest d13C values under its dense virgin forest coverage. Weaker carbonate bedrock dissolution due to sparse and thin soil cover results in lower d13CDIC, pCO2, DIC and EC, and lower cation and anion concentrations. At the Chenqi site, larger soil CO2 input from a thick layer of soil results in high pCO2 and DIC, and low pH, SIc and d13CDIC in the karst water. At the Dengzhanhe site, a lesser soil CO2 input due to stronger karst rock desertification and strong gypsum dissolution contribute to higher d13CDIC, high EC and high cation and anion concentrations. Soil CO2 inputs, controlled by biological activity and available soil moisture, carbonate bedrock dissolution, dilution and degassing effects, vary seasonally following rainfall and temperature changes. Consequently, there are seasonal cycles in hydrochemistry and d13CDIC of the karst water, with high pCO2 and low pH, EC, SIc, and d13CDIC values in the warm and rainy seasons, and vice versa during the cold and dry seasons. A strongly positive shift (>3&) in d13CDIC occurred in the drought year, 2011, indicating that d13CDIC in groundwater systems can be an effective indicator of environmental and/or climate changes. Ó 2015 Elsevier Ltd. All rights reserved.
1. INTRODUCTION ⇑ Corresponding authors at: State Key Laboratory of Environ-
mental Geochemistry, Institute of Geochemistry, CAS, Guiyang 550002, China. Tel.: +86 851 85895263 (Z. Liu), Department of Geosciences, National Taiwan University, Taipei 10617, Taiwan. Tel.: +886 2 33662929 (H.-C. Li). E-mail addresses: [email protected] (Z. Liu), [email protected] (H.-C. Li). http://dx.doi.org/10.1016/j.gca.2015.05.041 0016-7037/Ó 2015 Elsevier Ltd. All rights reserved.
Recent debate on the contribution of karst processes to the global carbon budget is stimulating discussion upon dissolved inorganic carbon (DIC) behavior in karst aquatic systems (Liu et al., 2010). Utilization of DIC in photosynthesis in karst waters depends on hydrochemical and biological conditions in a system. In order to study the carbon budget and its controlling factors in a karst area, monitoring
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spatiotemporal hydrochemical and isotopic changes of the system is essential to investigate sources and timing of groundwater recharge, water-rock interaction along flow paths, and mixing of distinct groundwater sources (Kanducˇ et al., 2012). In addition, d18O and d13C of travertine deposits are considered as high-resolution paleoclimatic records because of their high depositional rates (Sun et al., 2014). Interpretation of travertine d13C records requires more fundamental study of the factors controlling DIC behavior and d13CDIC change in the systems. Using DIC concentrations and their d13C values together with other chemical measurements, previous studies have evaluated the sources, sinks and fluxes of carbon in natural waters (Taylor and Fox, 1996; Yang et al., 1996; Atekwana and Krishnamurthy, 1998). Many studies have shown that the DIC in surface waters has three main sources: soil CO2, dissolution of carbonate minerals, and atmospheric CO2 exchanged through the air–water interface (Yang et al., 1996; Atekwana and Krishnamurthy, 1998; Amiotte-Suchet et al., 1999; Aucour et al., 1999; Das et al., 2005). The processes not only vary seasonally but also control carbon input and output in the aquatic system, creating variations in DIC and d13CDIC. Although investigations of DIC concentration and d13CDIC in freshwater environments have been carried out recently by many researchers (Han et al., 2010; Zhao et al., 2010; Kanducˇ et al., 2012; Tsypin and Macpherson, 2012), studies on variations of inorganic carbon systems in the karst catchments on multiple time scales (e.g., the annual, seasonal and storm-event scale) are still very limited (He´lie et al., 2002; Liu et al., 2004, 2007; Guo et al., 2008; Li et al., 2010; Zhao et al., 2010). Thus, our understanding of the DIC and d13CDIC variations and their controlling factors at seasonal to long term scales in karst aquatic systems needs further investigation, especially under different land covers in different karst areas due to changes in biological activities, nutrient loads and the processes of mutual interaction that may occur (Harris, 1999). This study presents our measurements of temperature, rainfall, pH, conductivity, DIC concentration and d13CDIC in the underground karst rivers and springs of three small catchments with different land covers between September 2007 and October 2012. The inter-annual and seasonal variations over five hydrological cycles enable us to better understand the behavior of DIC geochemistry in small karst catchments. Our ultimate goal is to reveal the source and behavior of DIC and the factors controlling d13CDIC shifts in the catchments. The results will be used to estimate the effects of different land covers and climate changes on carbon budgets. This study also provides basic information for analysis of d13C records in travertine that can be used as good indicators of abnormal climatic conditions (e.g., annual precipitation and rainfall intensity) and land cover changes in the past (Matsuoka et al., 2001; Liu et al., 2011; Sun et al., 2014). 2. STUDY AREAS AND MONITORING SITES The study areas are located in central and south Guizhou Province, the center of the well-known karst
regions of SW China (Fig. 1). Under the typical monsoonal climates in the area the numerous karst aquatic systems may have a significant influence on the global carbon budget. One of the three study sites, Banzhai (25°120 –25°150 N, 107°550 –108°050 E), is situated in Maolan National Nature Reserved Park in Libo County, southeastern Guizhou, containing dense evergreen virgin forests growing on rugged peak-cluster karst topography (Fig. 1). The drainage area of Banzhai is 19 km2 with >90% forest coverage. The mean annual air temperature is 18.3 °C and annual precipitation is 1750 mm with 80% summer monsoonal rain. The lithology is chiefly dolomitic limestone of Middle and Lower Carboniferous age (Han et al., 2010). The monitoring station is built on the exit of an underground river (Fig. 1). Although this area has abundant vegetation coverage, the soil cover overlying carbonate rocks is sparse and thin with small farmlands (5% in area) in the bottoms of karst dolines. Thus, the Banzhai site represents an environment with abundant vegetation but barren soil. Chenqi and Dengzhanhe karst spring catchments are close to each other and located in Puding County, central Guizhou (Fig. 1). The main lithology is limestone and dolomite of the Guanlin Formation, middle Triassic (Zhao et al., 2009). Both catchments are characterized by a humid subtropical monsoon climate, with annual precipitation of 1400 mm/yr and mean annual air temperature of 16.5 °C (Zhao et al., 2010). Morphologically the two catchments are karst valleys with a variety of farmlands, including dry farming fields, rice paddies, shrubs and fruit orchards. However, there are large differences in the distribution and proportion of land use types in the two catchments (Zhao et al., 2009, 2010). The monitoring station at the Chenqi site was built on the small outlet of a karst spring (diameter of 0.5 m) surrounded by dense farmland before October 2009 (Fig. 1). The measured drainage area of this karst spring was about 1.31 km2 at that time. However, the station was enlarged and connected with other surface water sources via a canal after October 2009 (Fig. 1). Therefore, the hydrochemical record and d13CDIC values for Chenqi are used only from September 2007 to October 2009. The monitoring station at Dengzhanhe site is built on a karst spring pool about 3 m2 in size (Fig. 1). The catchment of this spring is 2.82 km2. The environment around this station is sparse and thin in soil and vegetation. In summary, the ecological environments represented by the three study sites can be classified as (1) abundant virgin forest vegetation but with barren soil at Banzhai; (2) thick soil with intensive land usage (i.e., strong farming activity) at Chenqi; and (3) less vegetation and thin soil coverage at Dengzhanhe. 3. MONITORING, SAMPLING AND LABORATORY ANALYSIS 3.1. Continuous monitoring and measurement of hydrochemistry Monthly water temperature (T), pH, and electrical conductivity (EC) with resolution of 0.01 °C, 0.01 pH and
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Fig. 1. The location of the study areas in Guizhou Province, China (modified after Zhao et al., 2010).
0.01 ls/cm, respectively were measured by hand-held water quality meter (WTW MultiLine P3 pH/LF-SET: Germany) from September 2007 to October 2012. A CTDP300 multi-parameter data logger (Greenspan Co., Australia) was used at each site to record water temperature and rainfall every 15 min with resolutions of 0.01 °C and 0.05 mm respectively. In situ titration with the Aquamerck Alkalinity Test and Hardness Test was carried out monthly to measure [HCO 3]
2+ and [Ca2+] of water. The resolution for [HCO 3 ] and [Ca ] is 6 and 1 mg/L respectively. Two sets of 60 ml samples were transferred in acid-washed hydroplastic bottles for chemical analysis of cations and anions after filtering through 0.45 lm Millipore filters. The samples for cation analysis were acidified to pH <2.0 using concentrated nitric acid. Concentrations of Na+, K+, Ca2+, and Mg2+ were determined by inductively coupled plasma optical emission
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spectrometer (ICP-OES). Concentrations of SO2 4 , Cl , and NO were analyzed by a Dionex ICS-90 ion chromatogra3 phy (IC).
3.2. d13CDIC measurement Water samples for d13CDIC analysis were collected in 600 ml hydroplastic bottles which contained a saturated BaCl2 solution. By injecting 2 N NaOH into the sealed bottles to increase pH to >12, all dissolved inorganic carbon was precipitated as BaCO3 (Liu et al., 2003; Zhao et al., 2010). During the process the bottles were filled completely to avoid CO2 exchange with ambient air, and then stored in a refrigerator before analysis. The BaCO3 precipitate was filtered, washed and dried, then analyzed in the laboratory with a GV IsoPrim IRMS. The carbon isotope data are reported on VPDB (&) scale, with a standard deviation (1r) of 0.15&.
4. CALCULATING CO2 PARTIAL PRESSURE AND THE CALCITE SATURATION INDEX CO2 partial pressure (pCO2) and the calcite saturation index (SIc) are calculated from a geochemical model with pH, temperature and concentrations of seven major ions (Liu et al., 2004, 2007; Zhao et al., 2010). Since the host rocks in the regions are limestone and dolomite, interca2 lated with gypsum strata, Ca2+, Mg2+, HCO 3 and SO4 are the major ions. With the recorded temperature and pH, as well as the measured ion concentrations, calculations of pCO2 and SIc were processed through the program WATSPEC (Wigley, 1977). 5. RESULTS AND DISCUSSIONS Time series data of rainfall, T, pH, EC, HCO 3 , pCO2, SIc and d13CDIC for Banzhai are shown in Fig. 2 and the
Fig. 2. Variations in hydrochemistry and d13CDIC of the Banzhai underground river in relation to changes in rainfall and water temperature. RS: rainy season; DS: dry season.
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Fig. 3. Variations in hydrochemistry and d13CDIC of the Chenqi and Dengzhanhe karst springs in relation to changes in rainfall and water temperature. RS: rainy season; DS: dry season; DZH: Dengzhanhe; CQ: Chenqi.
data sets for both Chenqi and Dengzhanhe sites are plotted in Fig. 3. Seasonal variations appear in all parameters in the figures. On top of the seasonal variations there are also longer-term trends in some parameters. For instance, increasing SIc and d13CDIC trends and decreasing pCO2 trends from 2007 to 2012 can be seen at Banzhai and Dengzhanhe sites (Figs. 2 and 3). The seasonal variations and multi-year trends shed the light on our understanding of DIC behavior in the karst water and provide fundamental information on interpretation of travertine d13C paleo-proxy records. Table 1 lists the minimum, maximum and mean values of the measured major cations and anions in the karst waters at the three study sites. Table 2 shows the minimum, maximum and mean values of the measured 13 water temperature, pH, EC, HCO 3 , and d CDIC, as well as the calculated pCO2 and SIc values at each site. Besides the seasonal variations and multi-year trends, the hydrochemical and d13CDIC data sets show some different features among these sites. We shall discuss them below.
5.1. Comparison of climate records and the 2011 drought Fig. 4 presents the comparison of rainfall records at Banzhai, Chenqi and Dengzhanhe with the rainfall records from the nearest weather stations – Dushan for Banzhai and Anshun for Chenqi and Dengzhanhe, indicating that the study sites reflect local climatic conditions very well. The rainfall records in Fig. 4 shows a severe drought in 2011 due to a summer monsoon failure. The drought caused serious damage in southwestern China, especially Guizhou and Yunnan provinces. Fig. 5 compares monthly air temperatures at the Dushan and Anshun weather stations with the water temperature recorded at the three study sites. Note first that the water temperature (measured only once each month) follows the air temperature, showing strong seasonal variations but with much smaller range. Second, the average annual air temperature at Banzhai is 1.5 °C higher than at Dengzhanhe. This difference is reflected in the water temperature except the April 2010
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Table 1 Concentrations of major cations and anions in the karst water samples at the three study sites. Site
Item
Ca2+ mg/L
Mg2+ mg/L
K+ mg/L
Na+ mg/L
HCO 3 mg/L
Cl mg/L
SO2 4 mg/L
NO 3 mg/L
Banzhai
Min Max Mean RS DS
54 102 74 73 75
7.9 21.7 16.7 17.3 16.7
0.18 0.75 0.44 0.43 0.44
0.21 0.78 0.57 0.60 0.51
176.9 268.4 227.8 225.7 233.8
0.31 2.25 1.15 1.14 1.17
5.2 21.6 14.6 14.3 15.3
0.5 13.0 3.8 3.8 3.4
Chenqi
Min Max Mean RS DS
84 140 106 103 111
9.5 26.0 15.6 14.3 17.0
0.71 8.20 1.70 1.35 2.09
0.87 6.50 1.94 1.28 2.66
170.8 311.1 232.6 234.6 230.0
3.41 9.39 6.52 6.45 6.60
38.0 211.4 98.1 70.2 128.3
2.6 26.9 11.3 12.4 10.0
Dengzhanhe
Min Max Mean RS DS
62 250 159 138 189
0.6 56.3 26.4 23.1 31.0
0.56 3.44 1.73 1.66 1.85
0.28 3.41 1.71 1.47 2.08
158.6 268.4 213.4 205.5 225.7
2.47 9.09 5.60 6.37 4.59
33.9 840.8 367.5 249.4 527.3
0.8 39.3 14.6 15.7 12.5
RS = rainy season from May to October; DS = dry season from November to April.
Table 2 Minimum, maximum and annual mean values of water temperature (water T), pH, electrical conductivity (EC), saturation index (SIC), CO2 partial pressure (pCO2) and d13CDIC at the three study sites. HCO 3 (mg/L)
SIC
340.0 409.0 373.9 41 375.6 373.6
176.9 268.4 227.8 41 225.7 233.8
6.94 8.15 7.36 24 7.23 7.48
418.0 743.0 571.5 24 553.3 589.8
7.25 8.14 7.65 44 7.53 7.78
374.0 1393.0 882.0 44 735.4 1042.6
Site
Item
Water T °C
pH
Banzhai
Min Max Mean N RS mean DS mean
16.2 19.5 18.2 41 18.6 17.7
7.37 8.36 7.82 41 7.73 7.99
Chenqi
Min Max Mean N RS mean DS mean
11.4 18.7 16.2 24 17.6 14.8
Dengzhanhe
Min Max Mean N RS mean DS mean
14.9 21.2 17.1 44 17.5 16.6
EC (ls/cm)
pCO2a (pa)
d13CDIC &
0.13 1.00 0.50 38 0.38 0.66
86 881 330 38 413 227
14.73 9.11 12.78 44 13.18 12.26
170.8 311.1 232.6 23 234.6 230.0
0.27 0.92 0.14 22 0.01 0.30
144 2799 1045 22 1291 749
13.74 9.85 11.54 21 11.78 11.28
158.6 268.4 213.4 44 206.1 221.3
0.28 1.00 0.43 44 0.27 0.60
140 1194 455 44 550 350
12.19 2.85 8.22 42 9.33 7.11
N = number of samples; RS = rainy season; DS = dry season. a Calculated CO2 partial pressure of water by WATSPEC (Wigley, 1977).
and August 2010 measurements at Dengzhanhe, which might be influenced by some anomalously warm water temperatures occurring on a daily scale. Third, the water temperatures had 1–2 months time lags behind the air temperatures (Fig. 5). Finally, the water temperature at Chenqi was a few degrees colder than that at Dengzhanhe site while the air temperature at the two sites should be the same because the two sites are only 1.2 km apart and at the same elevation. The colder water temperature at Chenqi may be due to higher mean height of catchment area. Although the elevations (1312 m a.s.l.) of the two
sites at the monitoring spots are similar, the area of lower elevation (1325–1375 m a.s.l.) is much smaller than that of higher elevation (1375–1525 m a.s.l.) in Chenqi catchment area, whereas the lower elevation area in Dengzhanhe catchment area is similar to the higher elevation area. These comparisons indicate that the karst waters at the three sites come chiefly from surface runoff and shallow groundwater. The discrepancies between air temperature and water temperature in terms of timing and variation affect pCO2 and SIc though the seasonal trends are similar.
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Fig. 4. Comparison of monitored rainfall records in the study sites with the rainfall records from nearby weather stations. BZ: Banzhai; DZH: Dengzhanhe; CQ: Chenqi.
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dolomite (Table 1). The major anion at Banzhai is 2 HCO 3 , but SO4 becomes dominant at Dengzhanhe and Chenqi due to interbedded gypsum strata. Karst waters at Banzhai thus are the Ca–HCO3 type, whereas the waters at Dengzhanhe are Ca–HCO3–SO4 (Table 1). The dissolution of gypsum provides abundant Ca2+ and SO2 ions, 4 which in turn will influence SIc, carbonate precipitation. Thus, the hydrochemical conditions of Banzhai and Dengzhanhe site are distinctly different. The concentrations of Ca2+, Mg2+ and SO2 at 4 Dengzhanhe are significantly higher than at Chenqi despite having the same bedrock lithology and climatic conditions. The cause of the hydrochemical differences may be manifold, such as soil pCO2, carbonate dissolution and precipitation, CO2 degassing and water residence time, etc. With higher soil pCO2, lower water temperature and pH at Chenqi, the host rock dissolution would be stronger (Fig. 3). But, this is not the case, meaning that the effect of soil pCO2 on host rock dissolution is not the reason for lower ion concentrations at Chenqi. As bedrock lithology and climatic conditions are the same for both Chenqi and Dengzhanhe, the situation of dissolution, CaCO3 precipitation and CO2 degassing are unlikely the reasons for lower ion concentrations at Chenqi. Instead, water residence time that is related to geomorphology and size of catchment may be the reason. The main course of groundwater flow of Dengzhanhe site is 2.2 km in length while that at Chenqi is only 400 m. The morphology of Chenqi is shovel-like in shape with steep slopes on three sides (Zhao et al., 2010). With shorter groundwater passage and fast water flow on the steep slopes, groundwater residence time at Chenqi site is short. This is why the concentrations of Ca2+, Mg2+ and SO2 4 and pH, EC and SIc in the Chenqi karst spring are lower than those in the Dengzhanhe karst spring (Fig. 3, Tables 1 and 2). Concentrations of K+, Na+, Cl and NO 3 at Chenqi and Dengzhanhe are higher than those at Banzhai (Table 1), indicating that the farming impacts at the former are stronger. Because these ions are not chiefly from limestone dissolution, they may be introduced by stronger farming activities (e.g., fertilizing). 5.3. pCO2 and d13CDIC in the karst water - effects of soil CO2 and carbonate dissolution
Fig. 5. Comparison of measured water temperature records at the study sites with the air temperature records from nearby weather stations. BZ: Banzhai; DZH: Dengzhanhe; CQ: Chenqi.
In addition, note that the winter air temperature of 2010– 2011 was anomalously cold, which might be one reason for the 2011 drought. 5.2. Hydrochemistry of the karst water The major cations in the study areas are Ca2+ and significant amounts of Mg2+ from dissolution of limestone and
Since the karst waters at the three sites have pH ranging from 6.94 to 8.36 with an average value of 7.64 (Table 2), HCO 3 is the main inorganic carbon species in water (Clark and Fritz, 1997). As it is dominant, HCO 3 can be used to represent DIC in the karst catchments (He´lie et al., 2002; Das et al., 2005). In Table 2, concentrations of HCO 3 at the three sites are similar, but the calculated pCO2 and measured d13CDIC are quite different. pCO2 is mainly determined by HCO 3 concentration and pH, with lower pH indicating higher pCO2. The HCO 3 concentrations at both Chenqi and Dengzhanhe sites are similar, thus calculated pCO2 depends mainly on pH. The measured pH values at Chenqi site were lower than those at Dengzhanhe except for the months of January 2009– March 2009. Therefore, the calculated pCO2 values at
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Chenqi were much higher than at Dengzhanhe during the most period, September 2007–October 2009. For the dry months of January 2009–March 2009, pCO2 was similar (Fig. 3). The higher calculated pCO2 in Chenqi karst spring was caused by lower pH which can be attributed either to higher soil CO2 input, or to less carbonate dissolution in the groundwater. d13CDIC may tell us the dominant factor. DIC in the karst water derives mainly from soil CO2 and dissolution of carbonate bedrock. Because the d13C of soil CO2 (24& in general) is much lower than that of carbonate bedrock (0–2&) (Cerling, 1984; Fritz et al., 1989; Cane and Clark, 1999), the d13CDIC in a karst water depends on the mixing ratio of the two sources. In Fig. 3, d13CDIC values in the Chenqi record were lower than those of Dengzhanhe regardless of pH and HCO 3 concentration, even though when pH was higher during January 2009–March 2009 and HCO 3 concentration was higher during the rainy seasons of 2008 and 2009. This indicates that more CO2 derived from soil CO2 at Chenqi. The thick soil there maintains higher CO2 with lower d13C, so that greater soil CO2 always produces lower d13CDIC in the karst water. For instance, during the rainy seasons of 2008 and 2009, EC values at both Chenqi and Dengzhanhe were similar, indicating the same amount of carbonate dissolution but the HCO 3 concentrations were higher with lower d13CDIC at Chenqi, reflecting greater DIC passing from soil into the karst water (Fig. 3). Note that SIc during these rainy seasons was negative at Chenqi, showing that the karst water was unsaturated. In the dry seasons of 2007–2008 and 2008–2009, rainfall was low and soil activity was weak, so that input of DIC from carbonate dissolution became dominant. The HCO 3 concentrations at Chenqi and Dengzhanhe were close to each other. However, the d13CDIC of Chenqi was significantly lower than Dengzhanhe. Again, this implies that the lower pH and higher pCO2 at Chenqi site may be attributed to more input of soil CO2. Therefore, the land use with thick soil cover and strong farm activity is one of the factors controlling DIC behavior and d13CDIC of the karst water. Comparison of mean values of pH, EC, HCO 3 concentration, d13CDIC and calculated pCO2 and SIc between the Banzhai site and Dengzhanhe over the 2007–2012 period can be found in Table 2. The mean pH and HCO 3 concentration at Banzhai were slightly higher but the mean EC was significantly lower than at Dengzhanhe where there was gypsum dissolution as well as carbonate. The mean values of calculated pCO2 for the Banzhai and Dengzhanhe sites were 330 Pa and 455 Pa, respectively. d13CDIC values ranged from 14.73& to 9.11& with a mean value of 12.78& at Banzhai and ranged from 12.19& to 2.85& with an average of 8.22& at Dengzhanhe (Table 2). The d13CDIC values in Banzhai underground river were much lower than those in Dengzhanhe karst spring, reflecting the difference in carbon sources from organic matter decomposition. Although vegetation can produce CO2 through root respiration and decomposition, the sparse and thin soil cover at Banzhai does not easily store it. With low CO2 in the soil, the pH of karst water is higher so that its capacity to dissolve the
rock is relatively weak. Low d13CDIC, low EC, low cation and anion concentrations support this notion. In contrast, at Dengzhanhe in response to strong bedrock dissolution, limited vegetation cover and intensive farming, karst water has higher d13CDIC and pCO2 (Tables 1 and 2). In summary, DIC behavior and d13CDIC in the karst water are influenced by many factors, including (1) soil CO2 input controlled by vegetation and soil coverage, land use activity, and soil thickness; (2) carbonate bedrock dissolution related to water residence time which is controlled by rates of groundwater flow, geomorphology, and initial pH of surface water; (3) climatic condition. At the three study sites, Banzhai has dense vegetation but thin soil with less land use activity. DIC in the karst water is mainly from root respiration and organic carbon decomposition with weak carbonate bedrock dissolution, resulting in very low d13CDIC, low pCO2 and EC, low cation and anion concentrations. Chenqi has large soil CO2 input due to thick soil and strong farming activity, and very weak carbonate bedrock dissolution because of short groundwater flowpath and steep hydraulic gradients. Its karst water has high pCO2 and low pH, SIc and d13CDIC. At Dengzhanhe, sparse and thin soil coverage, strong carbonate bedrock dissolution caused by the long groundwater flowpath in a long synclinal valley, and the presence of interbedded gypsum, account for high EC, cation and anion concentrations, and higher d13CDIC (gypsum dissolution in a carbonate system shows the classic ‘common ion’ effect. This effect increased the saturation state of the water with respect to calcite, leading to enhanced 13C-depleted CO2 degassing from groundwater and thus higher d13CDIC). 5.4. Seasonal variations of hydrochemistry and d13CDIC in the karst waters - effects of dilution, soil CO2 and carbonate dissolution Figs. 2 and 3 show that there are strong seasonal varia13 tions in measured pH, EC, HCO 3 and d CDIC at all three sites following changes in rainfall and water temperature, so that the calculated pCO2 and SIc changed accordingly. In general, lower pH, EC, HCO concentrations and 3 d13CDIC result from the high rainfall and water temperatures of the rainy season, and vice versa during the dry season. During the rainy season, fresh inflows of precipitation at low pH dilutes the karst water and dissolves further soil CO2 to it (Liu et al., 2004, 2007). The higher temperatures in the rainy season lead to increased biomass productivity, strong root respiration and organic decomposition, providing more soil CO2 with low d13C (Atkin et al., 2000). During the dry seasons, low rainfall leads to longer groundwater residence times, resulting in increased carbonate solute concentrations per mL, which raises pH, EC and HCO 3 concentrations and introduces DIC with high d13C. On the other hand, cooler temperatures increase the rate of carbonate dissolution in the karst water and reduces biomass productivity and biological activity, so that increased inorganic carbon and decreased organic carbon coincide in time. Thus, higher cation and anion concentrations appear in the dry season, especially at Dengzhanhe (Table 1). Higher pH, ion concentrations and EC resulting
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in low pCO2 provide supersaturation of calcite (SIc > 0) in dry seasons (Figs. 2 and 3; Tables 1 and 2). Matsuoka et al. (2001) found that groundwater degassing might be one of the causes of seasonal variation of d13C in karst water at their study site in SW Japan where winter is also the dry season and summer the rainy season. They attributed the stronger degassing effect in winter to the subsurface temperature being warmer (less dense) than the atmosphere, resulting in increased d13CDIC due to degassing of 13C-depleted CO2 from the groundwater (Matsuoka et al., 2001). However, the seasonal variation of d13CDIC may be controlled by at least four factors: (1) high soil CO2 with low d13C due to biological activity (more biomass, root respiration and organic decomposition) in the rainy and warm season, and vice versa during the dry and cold season; (2) higher soil CO2 input due to more rainwater input; (3) lesser concentration of dissolved carbonates per mL owing to shorter groundwater residence times and the enhanced dilution in the rainy conditions, and the reverse during the dry and cold season; (4) strong degassing of 13C-depleted CO2 from the karst water during the dry and cold season leading to d13CDIC increase. Our records agree that CO2 degassing in winter season is one of the factors to elevate d13CDIC. However, as d13CDIC chiefly followed the pCO2 changes controlled by pH
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(Figs. 6 and 7), soil CO2 input and carbonate bedrock dissolution corresponding to the seasonal climatic conditions also play important roles in the seasonal cyclic variation of d13CDIC. The seasonal variations in hydrochemistry and d13CDIC of the karst water indicate that dilution and enhanced soil CO2 input are dominant in rainy seasons, whereas higher concentrations of dissolved rock and CO2 degassing are dominant factors during dry seasons. Therefore, a negative change in d13CDIC corresponds to wet and warm climatic conditions; a strong increase in d13CDIC may reflect dry and cold climatic conditions (Fig. 8). In addition to these general features, the three study sites show some detailed exceptions due to local differences in conditions. For example, HCO 3 concentrations at Chenqi were not decreasing when rainfall increased in the summer months (Fig. 3). As noted, the Chenqi karst spring was featured by greater soil cover and paddy land in the catchment area, both of which could produce and keep more CO2 in the soil-aquifer system (Zhao et al., 2010). The paddy land can generate plenty of CO2 in the soil during the summer months when farming activity is at its maximum. The abundant soil CO2 was washed out with the rainwater to increase HCO 3 concentrations and reduce pH. The soil CO2 effect rather than the dilution effect was dominant in this case.
Fig. 6. Correlation between d13CDIC and pCO2 at Banzhai and Dengzhanhe.
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Fig. 7. Correlation between pCO2 and pH at the study sites. BZ: Banzhai; DZH: Dengzhanhe; CQ: Chenqi.
Another feature is that the seasonal d13CDIC variation at Banzhai was different from that at Dengzhanhe (Fig. 9), even though there was clear seasonal variation at both sites. At Dengzhanhe, d13CDIC values increased quickly after the rainy season ended in September, while they remained relatively low for a few months at Banzhai after the rains (shaded bars in Fig. 9). The explanation for such a discrepancy may be the strong carbonate dissolution at Dengzhanhe. When the soil CO2 contribution became less after the rainy season due to reduced vegetation growth and biological activity, the carbonate rock tended to make the dominant contribution to DIC in the water (Reardon et al., 1979; Baldini et al., 2005; Maunoury et al., 2007). Consequently d13CDIC should increase, which occurred at Dengzhanhe. Banzhai contains dense virgin evergreen forests but thin soil cover, so that bedrock dissolution was not strong. With relatively warm temperature and dense vegetation, the carbon source for DIC in the karst water was still mainly from root respiration and organic decomposition at
the site. Fig. 9 shows that the entire d13CDIC record at Banzhai is below 9&. Development of rocky desertification (with soil and vegetation loss) due to human activity at Dengzhanhe elevated the d13CDIC of the karst water. 5.5. Influence of the 2011 drought on d13CDIC trend potential of travertine d13C records as paleoclimate proxies On top of the seasonal d13CDIC variations, the d13CDIC records at both Banzhai and Dengzhanhe appear to display multi-year trends of increase from 2007 to 2011 (Figs. 2 and 3). The strong enrichment of d13CDIC in 2011 corresponds to less rainfall in summer and lower water temperatures in winter (Fig. 8). The d13CDIC values were highest in both the summer and winter of 2011 compared with other years, reflecting changes in soil CO2, carbon sources and hydrochemical responses to climatic conditions. Table 3 shows that the annual rainfall in 2011 was the lowest during the five years caused by the summer monsoon failure.
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Fig. 8. Comparison of d13CDIC records with weather records at Banzhai and Dengzhanhe. BZ: Banzhai; DZH: Dengzhanhe.
Fig. 9. Comparison of d13CDIC records between Banzhai and Dengzhanhe. BZ: Banzhai; DZH: Dengzhanhe.
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Table 3 Annual rainfall (mm) at Banzhai and Puding (Chenqi and Dengzhanhe sites). Site
Banzhai Puding
Year 2008
2009
2010
2011
2012
1965 1563
1514 997
1931 1198
1178 744
1270 1157
According to Atekwana and Krishnamurthy (1998), discharges lower than the annual means will result in decrease in DIC concentrations, based on a plot of the discharge-DIC relationship. Yang et al. (2012) found that pCO2 in karst water changed synchronously with soil CO2 (i.e., higher in summer and lower in winter), because higher soil moisture in the rainy season provided more dissolved CO2 migrating into groundwater through quick flow. Tsypin and Macpherson (2012) also reported that transport of dissolved total inorganic carbon to DIC pool was also limited by the low soil moisture during the season. Normally, biological activity becomes the dominant factor controlling the DIC pool during wet and warm summers through CO2 removal by photosynthesis and release via respiration and decay of plants (Pawellek and Veizer, 1994; Yang et al., 1996; Atekwana and Krishnamurthy, 1998). Rainfall determines the soil water content available for biological respiration and the air-filled pore spaces available for CO2 flux (Ouyang and Zheng, 2000), and plays an important role in the generation of soil CO2 by regulating soil moisture, in turn shifting the d13CDIC values (Deng et al., 2011). Thus, during the drought summer of 2011, reduced biomass productivity, weakened biological activity and decreased soil CO2 transportation resulted in less soil CO2 contribution to d13CDIC. On the other hand, less rainfall during the rainy season in 2011 increased the groundwater residence times and enhanced solute carbonate concentrations, which brought up the contribution of inorganic carbon to the DIC and elevated the d13CDIC. In fact, the HCO 3 concentrations at Banzhai and Dengzhanhe displayed inverse seasonal changes in 2011 compared to the other years, i.e., high HCO 3 concentrations in summer, low HCO 3 concentrations in winter (Figs. 2 and 3). In addition, during the 2011 drought dilution effects became minimal and degassing increased strongly with SIc > 0 all the time (Figs. 2 and 3). These factors led to high d13CDIC in the rainy season of 2011, and may have continued to influence d13CDIC during the following winter season. In cold and dry winter seasons, d13CDIC was usually high. In the winters of 2010–2011 and 2011–2012, the d13CDIC reached maxima, being at least 2& higher than in other winters (Fig. 9). Note that the air temperatures in the winters of 2010–2011 and 2011–2012 were colder than in other winters, but the water temperatures did not show the same response (Fig. 4). Air temperature affects vegetation via photosynthesis, root respiration and decay of plants, while water temperature influences carbonate dissolution. With colder air temperature, production of soil CO2 decreases, resulting in lesser contributions of soil CO2 to DIC in karst water and d13CDIC increase. However, the decreased biological activity takes some time
to exert its effect on d13CDIC, while the impact of carbonate bedrock dissolution on d13CDIC can occur quickly. This may explain why the d13CDIC record at Banzhai has a 1– 2 months lag behind the air temperature, while the lag at Dengzhanhe is much shorter. As discussed above, the effect of carbonate bedrock dissolution on d13CDIC at Banzhai is much weaker than at Dengzhanhe site. According to Yang et al. (2012), the pCO2 in spring water varied frequently in the rainy season and remained stable in the dry season. Since the winter of 2010, pCO2 has remained stable and SIc maintained positive values. As a result, degassing has increased d13CDIC since the winter of 2010. Nevertheless, with less soil CO2 input during cold winter and more contribution from bedrock dissolution, the d13CDIC values during 2010–2011 and 2011–2012 winters became the highest. Combining the effects of the summer drought and cold winter of 2011, the mean d13CDIC value from winter 2010–2011 to winter 2011–2012 was at least 3& higher than in other years, which should be recorded strongly in travertine deposited from these karst waters (Liu et al., 2006, 2011). 6. CONCLUSIONS The variations of water temperature, pH, EC, HCO 3, major cations and anions, d13CDIC measured between September 2007 and October 2012 at three different field sites (Banzhai, Chenqi and Dengzhanhe) in the monsoonal karst region of Guizhou, SW China, allow us to calculate pCO2 and SIc, and to evaluate the factors controlling the hydrochemical behavior and d13CDIC. Soil CO2 input related to vegetation cover and land use, carbonate bedrock dissolution controlled by lithology, groundwater passage and geomorphology, as well as CO2 degassing under different climatic conditions at the study sites are the major factors to affect the DIC behavior and d13CDIC. At Banzhai where there is dense vegetation coverage but thin soil layer with less land use activity, root respiration and organic carbon decomposition with weak carbonate bedrock dissolution result in the lowest d13CDIC, low pCO2 and EC, low cation and anion concentrations. Chenqi has large soil CO2 input due to a thick soil layer and strong farming activity, and very weak carbonate bedrock dissolution because of a short groundwater flowpath and steep surface slope; consequently, the karst water has low pH and d13CDIC, high pCO2 and low SIc. At Dengzhanhe, strong gypsum bedrock dissolution and lesser soil CO2 input due to more karst rocky desertification contribute to higher d13CDIC, high EC and high cation and anion concentrations. Thus, in order to estimate detailed carbon sources, sinks and budgets in karst regions, all such factors need to be considered. There are clear seasonal variations in hydrochemical parameters and d13CDIC in the karst waters of the study areas, with high pCO2 and low pH, EC, SIc and d13CDIC in the warm and rainy seasons, and vice versa during the cold and dry seasons. Higher soil CO2 input controlled by biological activity and available soil moisture and dilution effects are dominant during the rainy season. Carbonate bedrock dissolution influenced by groundwater residence times and carbonate solubility, as well as degassing, are
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the major factors to control hydrochemistry and d13CDIC. The seasonal variation in d13CDIC demonstrates that a negative change in d13CDIC corresponds to wet and warm climatic conditions, while a strong increase in d13CDIC may reflect dry and colder climatic conditions. During the drought year, 2011, the d13CDIC of the study area exhibited anomalously high values. Extremely low rainfall and cold winter temperatures can be recorded strongly in an anomalously high d13CDIC signal. The d13C records of travertine deposited from these karst waters may permit the reconstruction of high-resolution records of paleoclimate change and extreme events. ACKNOWLEDGEMENTS This work was supported by the 973 program (2013CB956700), the National Natural Science Foundation of China (41430753, 41103084), Doctoral Foundation of Guizhou (2011GZ62743) and the Fund of the State Key Laboratory of Environmental Geochemistry (SKLEG2013205). Grants from NSC 102-2811-M-002-177 and MOST 103-2116-M-002 -001 to H. Li supported the postdoc study of Min Zhao at NTU. Special thanks are given to Derek Ford for his thoughtful academic and editorial suggestions on the manuscript.
REFERENCES Amiotte-Suchet P., Aubert D., Probst J. L., Lafaye-Gauthier F., Probst A., Andreus F. and Viville D. (1999) d13C pattern of dissolved inorganic carbon in a small granitic catchment: the Strenbach case study (Vosges mountains, France). Chem. Geol. 159, 129–145. Atekwana E. A. and Krishnamurthy R. (1998) Seasonal variations of dissolved inorganic carbon and d13CDIC of surface waters: application of a modified gas evolution technique. J. Hydrol. 205, 265–278. Atkin O. K., Edwards E. J. and Loveys B. R. (2000) Response of root respiration to changes in temperature and its relevance to global warming. New. Phytol. 147, 141–154. Aucour A. M., Sheppard S. M. F., Guyomar O. and Wattelet J. (1999) Use of 13C to trace origin and cycling of inorganic carbon in the Rhoˆne river system. Chem. Geol. 159, 87–105. Baldini J. F., McDermott F., Baker A., Baldini L. M., Mattey D. P. and Railsback L. B. (2005) Biomass effects on stalagmite growth and isotope ratios: a 20th century analogue from Wiltshire, England. Earth Planet. Sci. Lett. 240, 486–494. Cane G. and Clark I. D. (1999) Tracing ground water recharge in an agricultural watershed with isotopes. Ground Water 37, 133– 139. Cerling T. E. (1984) The stable isotopic composition of soil carbonate and its relationship to climate. Earth Planet. Sci. Lett. 71, 229–240. Clark I. D. and Fritz P. (1997) Environmental isotopes in hydrogeology. CRC. Das A., Krishnaswami S. and Bhattacharya S. K. (2005) Carbon isotope ratio of dissolved inorganic carbon (DIC) in rivers draining the Deccan Traps, India: sources of DIC and their magnitudes. Earth Planet. Sci. Lett. 2361, 419–429. Deng Q., Zhou G., Liu S., Chu G. and Zhang D. (2011) Responses of soil CO2 efflux to precipitation pulses in two subtropical forests in southern China. Environ. Manage. 48, 1182–1188. Fritz P., Fontes J. C., Frape S. K., Louvant D. and Michelot J. L. (1989) The isotope geochemistry of carbon in groundwater at Stripa. Geochim. Cosmochim. Acta 53, 1765–1775.
135
Guo H., Jiao J. and Weeks E. P. (2008) Rain-induced subsurface airflow and Lisse effect. Water Resour. Res. 44, W07409. http:// dx.doi.org/10.1029/2007WR006294. Han G., Tang Y. and Wu Q. (2010) Hydrogeochemistry and dissolved inorganic carbon isotopic composition on karst groundwater in Maolan, southwest China. Environ. Earth. Sci. 60, 893–899. Harris G. P. (1999) Comparison of the biogeochemistry of lakes and estuaries: ecosystem processes, functional groups, hysteresis effects and interactions between macro-and microbiology. Mar. Freshwater Res. 50, 791–811. He´lie J. F., Marcel C. H. and Rondeau B. (2002) Seasonal changes in the sources and fluxes of dissolved inorganic carbon through the St. Lawrence River – isotopic and chemical constraint. Chem. Geol. 186, 117–138. Kanducˇ T., Mori N., Kocaman D., Stibilj V. and Grassa F. (2012) Hydrogeochemistry of Alpine springs from North Slovenia: insights from stable isotopes. Chem. Geol. 300, 40–54. Li S., Liu C., Li J., Lang Y., Ding H. and Li L. (2010) Geochemistry of dissolved inorganic carbon and carbonate weathering in a small typical karstic catchment of Southwest China: isotopic and chemical constraints. Chem. Geol. 277, 301– 309. Liu Z., Zhang M., Li Q. and You S. (2003) Hydrochemical and isotope characteristics of spring water and travertine in the Baishuitai area (SW China) and their meaning for paleoenvironmental reconstruction. Environ. Geol. 44, 698–704. Liu Z., Groves C., Yuan D., Meiman J., Jiang G., He S. and Li Q. (2004) Hydrochemical variations during flood pulses in the south-west China peak cluster karst: impacts of CaCO3–H2O– CO2 interactions. Hydrol. Proc. 18, 2423–2437. Liu Z., Li H., You C., Wan N. and Sun H. (2006) Thickness and stable isotopic characteristics of modern seasonal climatecontrolled sub-annual travertine laminas in a travertine-depositing stream at Baishuitai, SW China: implications for paleoclimate reconstruction. Environ. Geol. 51, 257–265. Liu Z., Li Q., Sun H. and Wang J. (2007) Seasonal, diurnal and storm-scale hydrochemical variations of typical epikarst springs in subtropical karst areas of SW China: soil CO2 and dilution effects. J. Hydrol. 337, 207–223. Liu Z., Dreybrodt W. and Wang H. (2010) A new direction in effective accounting for the atmospheric CO2 budget: considering the combined action of carbonate dissolution, the global water cycle and photosynthetic uptake of DIC by aquatic organisms. Earth Sci. Rev. 99, 162–172. Liu Z., Sun H., Li H. and Wan N. (2011) d13C, d18O and deposition rate of tufa in Xiangshui River, SW China: implications for land-cover change caused by climate and human impact during the late Holocene. Geol. Soc. Lond. Spec. Pub. 352, 85–96. Matsuoka J., Kano A., Oba T., Watanabe T., Sakai S. and Seto K. (2001) Seasonal variation of stable isotopic compositions recorded in a laminated tufa, SW Japan. Earth Planet. Sci. Lett. 192, 31–44. Maunoury F., Berveiller D., Lelarge C., Pontailler J. Y., Laurent V. and Damesin C. (2007) Seasonal, daily and diurnal variations in the stable carbon isotope composition of carbon dioxide respired by tree trunks in a deciduous oak forest. Oecologia 151, 268–279. Ouyang Y. and Zheng C. (2000) Surficial processes and CO2 in soil ecosystem. J. Hydrol. 234, 54–70. Pawellek F. and Veizer J. (1994) Carbon cycle in the upper Danube and its tributaries: d13CDIC constraints. Israel. J. Earth Sci. 43, 187–194. Reardon E. J., Allison G. B. and Fritz P. (1979) Seasonal chemical and isotopic variations of soil CO2 at Trout Creek, Ontario. J. Hydrol. 43, 355–371.
136
M. Zhao et al. / Geochimica et Cosmochimica Acta 165 (2015) 123–136
Sun H., Liu Z. and Yan H. (2014) Oxygen isotope fractionation in travertine-depositing pools at Baishuitai, Yunnan, SW China: effects of deposition rates. Geochim. Cosmochim. Acta 133, 340–350. Taylor C. B. and Fox V. J. (1996) An isotopic study of dissolved inorganic carbon in the catchment of the Waimakariri River and deep ground water of the North Canterbury Plains, New Zealand. J. Hydrol. 186, 161–190. Tsypin M. and Macpherson G. L. (2012) The effect of precipitation events on inorganic carbon in soil and shallow groundwater, Konza Prairie LTER Site, NE Kansas, USA. Appl. Geochem. 27, 2356–2369. Wigley T. M. L. (1977) WATSPEC: a computer program for determining equilibrium speciation of aqueous solutions. Br. Geomorphol. Res. Group Tech. Bull. 20, 1–46. Yang C., Telmer K. and Ja´n V. (1996) Chemical dynamics of the “St. Lawrence” riverine system: dDH2O, d18OH2O, d13CDIC,
d34Ssulfate, and dissolved 87Sr/86Sr. Geochim. Cosmochim. Acta 60, 851–866. Yang R., Liu Z., Zeng C. and Zhao M. (2012) Response of epikarst hydrochemical changed to soil CO2 and weather conditions at Chenqi, Puding, SW China. J. Hydrol. 468–469, 151–158. Zhao M., Zeng C. and Liu Z. (2009) Influence of land use change on dissolved inorganic carbon and stable isotopic compositions of karst groundwater. Geochimica 38, 565–572 (in Chinese with English abstract). Zhao M., Zeng C., Liu Z. and Wang S. (2010) Effect of different land use/land cover on karst hydrogeochemistry: a paired catchment study of Chenqi and Dengzhanhe, Puding, Guizhou, SW China. J. Hydrol. 388, 121–130. Associate editor: F. McDermott