The sources and biogeochemical cycling of carbon in the Wudalianchi UNESCO Geopark volcanic system in Northeast China
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The biogeochemical cycling and response mechanisms of carbon within the Wudalianchi UNESCO Global Geopark were characterized by the isotopic compositions of dissolved inorganic carbon (δ13CDIC) and dissolved organic carbon in ground and surface (lake) waters and their relating carbon isotopic composition of soil (δ13CSOC) and sediment organic carbon (δ13Corg). In addition to mantle-derived CO2, the oxidation of organic matter was prevalent in shallow groundwater during the summer. Their associated degassing of CO2 produced higher pCO2 values than in autumn or winter and elevated δ13CDIC values. In summer, DIC in the epilimnion showed a wide range of δ13CDIC from − 8.4 to 2.6‰. Waters in open-lake areas with relatively positive δ13CDIC values and the low levels of pCO2 were primarily influenced by CO2 degassing. Photosynthesis elevated the δ13CDIC values and led to minimal pCO2 levels in closed lake areas. Isotopically, δ13Corg was found to be positively related to δ13CSOC. In addition, lake bed sediments generally had lower concentrations and larger δ13C values of organic carbon than the surrounding soils. These results suggest that 12CO2 derived from the degradation of sediment was preferentially utilized by phytoplankton in the epilimnion during photosynthesis. The remaining 13C-rich organic matter was retained in the sediment. Since 2000, δ13Corg increased in lake 3 over time, reflecting the input of sewage and land use changes associated with a resort used for tourism. The values of δ13Corg in lake 5, distant from the resort, did not change substantially, indicating minimal human impacts.
KeywordsCarbon isotopes Carbon cycling Biogeochemistry Lake–groundwater system Wudalianchi
Carbon is an essential nutrient and a major source of energy in aquatic ecosystems (Raymond et al. 2013; Galy et al. 2015). It strongly influences acid buffering and the transport and availability of ions, nutrients, heavy metals, and organic pollutants. It is closely related to the evolution of water quality (Regnier et al. 2013; Evans et al. 2005; Dawson et al. 2008). Traditional hydrogeological research in the Wudalianchi region (WR) of China has primarily focused on the geochemical processes of leaching, mixing, and cation exchange, as well as on the effects of adsorption of major and trace ions through the use of multivariate statistical methods and PHREEQC modeling (Zhang et al. 2018a). Such studies have largely ignored the importance of biochemical processes to mineral dissolution and the geochemistry of surface and groundwater systems. Biogenic CO2 in water usually originates from soil CO2, produced by the biogeochemical processes of mineralization/decomposition induced by microbiological activity, root respiration, and the precipitation of carbonates. Soil CO2 is consumed during carbonate and silicate weathering reactions that contribute to the formation of dissolved inorganic carbon (DIC) (Li et al. 2008). During weathering processes, carbonate acidity is buffered, pH rises, and the distribution of dissolved inorganic carbon (DIC) species shifts toward HCO3−- and CO32−-dominated systems. Within the epilimnion of lakes where water is exposed to sunlight, photosynthesis can consume DIC, increase pH, and cause variations in CO2 at spatial and seasonal scales (Herczeg and Fairbanks 1987; Herczeg 1987).
In this study, the seasonal variation of pCO2 in the waters calculated for shallow mineral springs, deep mineral springs, shallow wells, and lake waters was compared to emphasize contribution of biogenic CO2 in summer. And then, the results of the assessment of the multiple carbon isotopes were used to (1) characterize the predominant biogeochemical processes in the region and their influences on δ13CDIC and pCO2 and (2) analyze the cycling of carbon within and between surface and groundwaters in this complex volcanic area. This analysis provides a theoretical basis for deciphering the role of springs in the carbon cycling among deep, subsurface, and surface environments.
Wudalianchi UNESCO Global Geopark is in the northeastern part of China (Fig. 1). Wudalianchi Lake is the second largest volcanic barrier lake in China. There are approximately 14 volcanoes and 6 cold mineral springs in the area, which stretches more than 1400 km2 area. Five interconnected lava-dammed lakes were formed by lava flow pouring into Shillong River after a volcanic eruption in 1710 AD. The average annual temperature is 0–0.5 °C; the minimum monthly temperature is in January and averages − 24 °C. Average annual rainfall is 467.8 mm, with 65% of the total falling between June and August. Average annual evaporation is 1256.6 mm (Gui et al. 2012). From north to south, lakes 5 to 1 (from upstream to downstream), five barrier lakes with total surface area of 18.36 km2 and water storage capacity of 21.57 × 108 m3 (Wang et al. 1996) can be found along a valley. All five are hydrologically connected. Lake 3 has the largest area (8.2 km2) and greatest depth 10 m; lake 5 has intermediate area (5.3 km2) with a maximum depth of 6 m, and lake 1 is smallest (0.25 km2) (Gui et al. 2012). Phytoplankton is the dominant aquatic plant associated with accelerated eutrophication, and algae blooms occur in the summer (Gui et al. 2012). The Wudalianchi lakes are situated above Pleistocene basalt and Holocene pillow lava, as well as Cretaceous sand–mud stones and Quaternary deposits (Du et al. 1999). In addition to these lakes, the Yaoquan volcanic area (YQVA) in the southern portion of the Wudalianchi region has world-famous cold mineral springs. Most of the springs around Yaoquan volcano are rich in carbon dioxide (Mao et al. 2009) and are located at crossing points of deep faults that connect groundwater in various water-bearing formations. Deep groundwater is supplied by water from the north of Yaoquan volcano that flows toward a CO2 degassing zone, a N–E to S–W trending fault. CO2 gas is derived from magma and dissolves in the groundwater to form carbonated primary mineral water (Mao et al. 2009; Sun and Du 1998). This water is derived from Cambrian metamorphic rock and Indosinian granite deep in the subsurface (Zhang et al. 2018a). The Erlongyan and Fanhua shallow mineral springs are derived from the mixing of infiltrating meteoric waters and Quaternary phreatic waters that emerge at the junction of E–W and N–E faults east of the volcano. In addition, phreatic water flowing toward S–N extensional faults mixes with deep confined water, forming the south and north springs (Sun and Du 1998), which have substantial pCO2. These mineral springs have typical secondary mineral water. The development of commercial bottling plants and spa resorts has centered on these springs, and Wudalianchi has become a UNESCO Geopark and a popular destination for both domestic and overseas tourists.
Waters from four types of sources, three shallow mineral springs, two deep mineral springs, three wells, and one lake, were collected in the YQVA during March 2016 (winter), October 2016 (autumn), and July 2017 (summer) to assess the seasonal variation in major ions and geochemical index (pH, T, and pCO2). The July 2017 (summer) sampling campaign (during 13th–18th) also included 18 water samples in the epilimnion to allow us to characterize the DIC and DOC concentrations and their isotopes (δ13CDIC and δ13CDOC) (Fig. 1; Table S1). Soil and sediment samples were also collected at the lake water sampling locations in the Wudalianchi lakes region (WLR) in July. Because of the long ice cover period and extremely low temperature in the WR, no water samples were collected for testing isotopes during the dry period, but we determined the relevant geochemical parameters. Water temperature (T), pH, DO, and electrical conductivity (EC) were measured at the sites using a portable multiparameter water quality meter. HCO3− concentrations were determined in the field by titration with HCl before filtration. Water samples filtered through 0.45-μm cellulose acetate filter paper were collected and preserved in polyethylene bottles with HgCl2 to prevent biological activity and allow determination of the carbon isotopic composition of DIC. Some filtrates were acidified with ultra-purified HCl to pH < 2 for measuring major cations. Water samples for analyses of DOC concentration and δ13CDOC values were filtered through 0.7-μm pore size Whatman glass fiber filter papers that had been pre-combusted at 550 °C for 6 h and preserved with HgCl2 to prevent biological activity and stored in a brown glass bottle. Prior to use, the polyethylene and brown glass bottles were soaked in an acid solution for 24 h and rinsed with ultrapure water, after which they were rinsed two or three times with filtered river water.
Three solid samples including soil, lake sediment, and mineral mud were air-dried and then sufficiently disaggregated by passage through a 2-mm sieve. Solid samples were treated with 0.5 mol/L HCl at 25 °C for 24 h to remove carbonates (Midwood and Boutton 1998) before washing to neutrality with distilled water, centrifuging, and drying at 60 °C. Next, the samples were pulverized and saved for carbon and isotopic analyses.
Major cations were measured by ion chromatography using a Dionex ICS-90 (Thermo Fisher, USA) (with error of 5%; Table S1). DOC concentrations were determined using an Aurora 1030-W TOC (total organic carbon) analyzer (IO) (precision 0.01 mg/L). Organic carbon concentrations were determined by reference to a sulfanilamide standard consisting of 41.81% C using an Elementar Vario MICRO cube (precision < ± 0.5%; Table S1). The concentration of chlorophyll (Chla) was measured with a YSI 6600.
Carbon isotopic analyses of DIC and DOC were conducted using a GasBench online high-precision gas headspace sample coupled with a MAT-253 isotope ratio mass spectrometer (Thermo Fisher Scientific, Bremen, Germany; with a precision of 0.03‰) (Zhou et al. 2015). Carbon isotopic analysis of the solid samples (soil, sediment, and mineral mud) was done using a iTOC-CRDS (Picarro, USA) coupled with Picarro 13C combustion module (analytical error ≤ ± 0.3%).
pCO2 values were determined based on measured alkalinity, pH, and water temperature using the CO2SYS program, with the constants K1 and K2 dependent on the temperature from Zhang et al. (2009). Calcite saturation (SIc) indexes were calculated using the thermodynamic constants at a given temperature (Li et al. 2010).
Results and discussion
Spatial variation and controlling factors of carbon characteristics
Spatial variation of carbon characteristics
From upper to the lower reaches of the WLR, there were no readily visible laws to the spatial variation in DIC concentration (1.07–5.76 mmol/L), δ13CDIC (− 8.4–2.6‰), and pCO2 (15.95–22,450 μatm) in the waters collected from lakes 5 to 1 found in this study. DIC concentration, δ13CDIC, and pCO2 of the waters in lake 5 showed large difference. In contrast, the waters of lakes 4 to 1 showed a similar range of changes (Table S1). The biggest difference in DIC concentration and pCO2 between lake 5 and others was the waters from wetland in lake 5 (nos. 1, 2, and 7; Table S1). These three bodies of water are fed by wetland waters, and they generally showed higher DIC concentration (5.52–5.76 mmol/L) and pCO2 levels (9105–22,450 μatm), which suggested that wetland inflow might influence the spatial variation of carbon characteristics. However, no similar pattern was observed in the variation of their δ13CDIC values. These phenomena indicated that the controlling factors of δ13CDIC in the Wudalianchi lakes were complex and multivariate. In the YQVA, the DIC concentration (1.98–2.83 mmol/L), δ13CDIC (− 5.8–1.8‰), and pCO2 (13310–47,710 μatm) in the shallow groundwater (shallow mineral spring and shallow well) showed big difference from deep mineral springs (DIC concentration 18.06–26.85 mmol/L, δ13CDIC − 1.2 to − 0.5‰, pCO2 333800–443,300 μatm). However, these values of geochemical parameters varied within the range of Wudalianchi lake and YqL waters. It seems that the waters from surface, subsurface, and deep sources were geochemically different in WR. Shallow groundwaters generally had lower δ13CDIC values than water from lakes and much higher pCO2 levels. Photosynthesis may have an influence on geochemistry of the waters exposed to sunlight in the epilimnion. This merits further investigation on controlling factors of δ13CDIC, especially in summer.
Controlling factors of δ13CDIC
δ13CDIC is mainly influenced by different sources and several biogeochemical processes: (1) influx of soil water (baseflow) and interflow; (2) in situ respiration of CO2 (organic matter oxidation); (3) dissolution of carbonate minerals in soil, aquifers, and surface waters; (4) exchange with atmospheric CO2; (5) and kinetic effects at the water–atmosphere interface due to CO2 outgassing; and (6) photosynthesis (Aucour et al. 1999; Bade et al. 2004; Brunet et al. 2005; Spence and Telmer 2005; Barth and Veizer 1999). The first two processes involve chemical weathering and produce CO2 characterized by depleted δ13C values, which usually decreases δ13CDIC. In contrast, the latter four processes raise δ13C values.
In addition to such biogenic CO2, deep-sourced CO2, such as magmatic CO2 from the Lesser Antilles volcanic arc, metamorphic CO2 from the eastern Tibetan Plateau, and CO2 from mineral spring waters derived from the magma chamber of the upper mantle in the WR, magmatic CO2 from the Changbaishan volcanic area in northeastern China may also heavily influence the geochemical evolution of surface and subsurface waters (Mao et al. 2009; Li et al. 2014; Rive et al. 2013; Bai et al. 2017).
Regarding biogenic CO2, the δ13C of soil CO2 depends on both the relative proportion of C3 and C4 plants and the diffusion rate of CO2 (Clark and Fritz 1997). The δ13CSOC and the δ13Corg values were generally close to − 25‰ (Table S1). No carbon isotopic fractionation occurs when CO2 is produced by the oxidation of soil organic matter and during root respiration (Park and Epstein 1961); therefore, soil CO2 produced from respiration is expected to be from − 27 to − 23.9‰. Molecular diffusion of CO2 would cause a slight enrichment of (4.4‰) 13C in the soil relative to organic matter (Cerling et al. 1991). When CO2 from soil reacts with carbonate rocks during subsurface weathering, δ13CDIC usually derived from soil CO2 and carbonate minerals. The dissolution of carbonates in the region shifts δ13CDIC toward less 13C-depleted values because Paleozoic limestones exhibit a “marine” isotopic signature of 0‰. The observed δ13C values roughly reflect the 1:1 mixing of soil CO2 dissolved during recharge and calcite dissolved in the subsurface in a closed system (Clark and Fritz 1997). This should result in intermediate δ13CDIC values between the two sources of carbon, around − 9.5‰. However, the value would depend on the amount of CO2 recharge and limestone dissolution and showed a variation between − 8.25 and − 13‰ (Spence and Telmer 2005; Karim et al. 2011; Li et al. 2008).
Mantle-derived CO2 continuously migrates along, and is released from, fault zones and dissolves in groundwater, forming carbonated water. These carbonated waters, characterized by high pCO2 levels, have the potential to cause significant chemical erosion (dissolution). Mantle CO2 reacts with carbonate during the water–rock interactions and then DIC could form as an open-system (continuous equilibrium with a gas phase of a given pCO2 and continuous isotopic exchange between CO2 and the solution) (Doctor et al. 2008; Li et al. 2008). As the chemical reaction proceeds, the carbonated water and the DIC produced are constantly separated from the open system along conduits in the subsurface flow system that lead to the surface. This process is similar to a closed system condition. Both conditions assume water residence times are sufficiently long to achieve substantial isotopic exchange, an assumption that generally holds true in groundwater because of the fast reaction kinetics associated with carbonate systems (Deines et al. 1974). Carbonated water collected from mineral springs had δ13CCO2 values ranging between − 8.6 and − 4‰ in the YQVA (Mao et al. 2009). The DIC produced had δ13C between − 4.3 and − 2‰. Assuming the system is closed (isotopic equilibrium with a gas phase and then isolation from that reservoir before carbonate dissolution), the carbonated water appears to have reacted with the surrounding carbonate rock (average 0‰), while silicate weathering by carbonic acid produced DIC with δ13C values close to the initial isotopic values of δ13CCO2. Continuous isotopic exchange with CO2 under open-system conditions drove δ13CDIC toward more negative values of CO2. The δ13CDIC of shallow groundwater (− 5.8 to − 2.5‰, except FhS with 1.8‰; Table S1) varied within the overlapping values of carbonate dissolution by both soil CO2 and mantle-sourced CO2.
Water–rock interactions during the water flow and formation, including dissolution of dolomite, calcite, halite, k-feldspar, and Fe-bearing mineral, govern the water chemistry (Zhang et al. 2018a; Zou et al. 2018). Ion exchange, mixing, and anthropogenic activity (fertilizer) are also important factors for the water dynamics, particularly in the shallow systems during hydrochemical evolution (Zhang et al. 2018b; Zou et al. 2018). However, the δ13CDIC values of surface water (lake) and groundwater were generally bigger than the predicted values (rock weathering by carbonic acid) especially for the lakes. Biogeochemical activities were the most common. However, no previous work has fully deciphered the such processes in water sources of this kind. In other words, the measured δ13CDIC is obtained as a result of the transformation of biogeochemical activities based on known endmembers (sources). The endmembers here include soil carbon, carbonate mineral, magmatic CO2, and sewage.
Biogeochemical characteristics in a shallow groundwater system
Zhang et al. (2018b) found that shallow groundwaters exhibited higher oxygen isotope values than Wudalianchi lake water, and the nitrates in both surface water and groundwater were derived from the mixing of fertilizers and precipitation. The waters from shallow and deep mineral springs exhibited similar saturation indexes (Zhang et al. 2018a) and uniform distribution above the local meteoric water line (Zhang et al. 2018b). The supply of infiltrating meteoric water and deep upwelling of primary water contributed to the shallow groundwater. Shallow mineral springs were derived from upwelling of deep mineral springs and had a composition as an endmember of shallow wells and lakes according to fingerprint of Sr isotopes (Zou et al. 2018). The infiltration from precipitation mixed with the upwardly migrating groundwater as it moved toward the lakes. These results suggested that the shallow groundwater was supplied by deep groundwater characterized by large pCO2 and δ13C values from the deep subsurface and diluted by soil CO2, characterized by lower pCO2 and δ13C values.
Moreover, the SIc of shallow groundwater was < 0, and > 0 in the lakes (Fig. 3e). The pCO2 levels were negatively correlated with SIc and much higher than those of the lakes (average 4647 μatm) and three orders of magnitude higher than the air (380 μatm). These findings demonstrate that during upward migration, substantial dissolved CO2 escaped from the shallow mineral springs and produced high pCO2 levels (average 21,520 μatm) in the water–air interface. SIc was < 0 and was subject to dilution by rain infiltration. The much larger δ13CDIC value at site FhS reflects the fact that CO2 continuously degassed from the water surface (Tamooh et al. 2013). It is seen that CO2 degassing influenced δ13CDIC at different levels. The larger 13CDIC value within the deep-mineral spring waters at SS and NS than shallow groundwater may have been caused by maximum pCO2 levels (average 388,550 μatm) and the continuous escape of dissolved mantle-derived CO2 from the water column under low pressure, before δ13CDIC was determined.
During summer, the biogeochemical processes such as the degradation of organic matter and root respiration are active in the subsurface environment, resulting in strong CO2 input to the shallow groundwater during organic carbon degradation, causing the summer pCO2 levels to be much higher than in autumn and winter (Fig. 3c; Table S2). This reflects complicated carbon dynamics and biogeochemical processes during the summer wet season. High discharge will usually accelerate material transport, and high temperatures will increase primary production in the catchment, both of which should be responsible for the shift of δ13CDIC values in the high-flow season (Zhong et al. 2018). This needs to be further studied accordingly.
δ13C characteristics and its influencing factors in lakes
Closed basin lakes tend to have long hydraulic residence times and have less CO2 than open-basin lakes in summer. This is because photosynthetic demand commonly exceeds hydrologic inputs of CO2 and bioavailable DOC (Striegl et al. 2001). In the WLR, waters from the epilimnion of sampling sites 5Lake2, 4Lake, 5LakeE, and YqL were nearly still, remaining in a relatively closed water environment, and these were affected by eutrophication. These lake waters generally had higher Chla (13.2–30.7 μg/L), DO% (116.2–240.6%), more alkaline pH values (9.03–9.6), lower DIC concentrations (1.07–1.85 mmol/L), and relatively depleted δ13CDOC values (< − 30‰), similar to phytoplankton (− 29.8 to − 36.1‰) and larger δ13CDIC values (− 4.3 to + 2.6‰). The effect of photosynthesis (phytoplankton) on DIC concentrations and pCO2 levels in the epilimnion was obvious. Photosynthesis preferential uses 12C in the aquatic system, resulting in a relative enrichment of 13C within the residual DIC (Baird et al. 2001). The magnitude of this enrichment depends on the amount of CO2 available to photosynthesizing organisms, which produces fractionation factors from 0 to 20‰ (Bade et al. 2004; Cole et al. 2002). Photosynthetic activities enhanced by eutrophication influenced the carbon isotopic composition of waters in the epilimnion of closed lake areas in the region. HCO3− served as a source of inorganic C under CO2-limited conditions during intense photosynthesis. This weakens the discrimination of 13C and enriches organic compounds with 13C (Han et al. 2018). However, the pCO2 levels (15.95–110.2 μatm) in the closed lakes were lower than those of the air (380 μatm). Lakes and dams remain in one place and so facilitate exchange with atmospheric CO2, generating high δ13CDIC values in the surface water (Brunet et al. 2005). Atmospheric CO2 exchange with the lakes in relatively closed lakes is relatively intense, as is replenishment photosynthesis. For this reason, we believe HCO3− is not the contributor of inorganic carbon in this system. The epilimnion in relatively closed lakes was characterized by pH levels higher than 9, and they help to create a relatively alkaline environment.
In contrast, water from the epilimnion in relatively open-lake areas that had direct hydraulic connections or inflow from tributaries was minimally influenced by photosynthesizes. Most water in the epilimnion had larger values than pCO2air. In addition, the inflow tributaries and runoff samples (5Y1, 5Y2, 23River; 3LakeY1, 2Lakerunoff, and 3LakeY2) showed much larger pCO2 values (5007–22,450 μatm) than waters in the open-lake area (306.4–9105 μatm). Carbonate precipitation is unlikely because the SIc of epilimnion is general similar to or lower than 0 in the inflow and runoff (− 0.59–0.07; Table S1). CO2 degassing is the most plausible mechanism, which allows the fractionation of carbon isotopes. This effect can be seen in the δ13CDIC values of inflow and runoff samples (− 8.4–1.7‰), which generally display lower isotopic values than those of relatively open lakes (− 6–2.6‰) fed by inflow and runoff. Collectively, the open lake waters showed higher Chla (average 6.8 μg/L), DO%, and pH values (108.96%; 8.22), and DIC concentrations (average 2.92 mmol/L) were lower than those of waters collected from the inflow and runoff (average 6.5 μg/L, 92.13%; 7.28; 4.17 mmol/L) which suggests that photosynthesis had a greater effect on the epilimnion in relatively open lakes than inflow and runoff did. In relatively open-lake areas, phytoplankton digests CO2 during photosynthesis. Then, terrestrial HCO3− was retained and caused the lake to become alkaline.
In addition, most water samples from the Wudalianchi in the epilimnion had larger δ13CDIC and DOC values than groundwater samples. Photosynthesis has been proven to have an influence on waters in the epilimnion and to obscure the influence of organic matter degradation resulting in an increase in DOC concentrations and a positive shift in δ13CDIC. Based on the multiproxies, the sediments recorded the change of the aquatic plant community from macrophyte dominant to algae-dominant in lake 3 and to a mixed macrophyte and algae community in lake 5 (Gui et al. 2012). Lake 3 connects to lake 4 through a river which receives drainage from farmland. Some sampling sites on lake 5 are surrounded by arable land and wetland, and this area is prone to terrestrial material input (Zou et al. 2018). It also showed high DOC concentrations (11–20.8 mg/L) and variable δ13CDIC values (Fig. 2). Consequently, the lake water data do not fall along the positive trend line of shallow groundwater (Fig. 2).
Characteristics of organic carbon in the sediment of the WLR
Isotope analysis of organic carbon in soil (δ13CSOC) can be used to trace the sources and fate (degradation, migration, and transformation) of SOC and provides a means to distinguish both natural and anthropogenic contributions of carbon to soils, such as coal, rocks, and plants (C3 and C4 plants, − 32 to − 24‰ and − 22 to − 10‰, respectively) (Guo et al. 2013; Wang et al. 2017). The δ13CSOC (− 27 to − 23.9‰; average − 25.5‰) of the Wudalianchi lakes fell in the range of C3 plants. However, soils adjacent to downstream lakes (lakes 3 to 1 and YqL) had larger δ13Csoc values (average − 25.1‰) and lower SOC concentrations (average 3.04%) than upstream (lakes 5 and 4; − 25.9‰ and 5.3%). This might indicate anthropogenic activity near Wudalianchi, a city famous for tourism in China in the lower reaches of the WLR. Accordingly, the higher SOC concentrations and similar δ13Csoc of sewage (3LakeY1 and 2Lakerunoff) and farmland water (3LakeY2) downstream of Wudalianchi relative to related soils may reflect the migration of SOC and storage in nearby lake sediment.
Changes of δ13C in sediment (δ13Corg) are closely linked with the succession of aquatic and terrestrial plants (Hodell and Schelske 1998). δ13Corg values of the lake sediments showed positive correlation with δ13Csoc of nearby terrestrial soil (R2 = 0.6489). Gui et al. (2012) found that changes of precipitation were most likely the main influence on sediment accumulation rates in the WLR. In summer, water is abundant and SOC washes into the lakes and is buried in the form of lake sediment. In this way, sediment is created from soil through the process of alluviation and sediments produced during in rainy seasons tend to have origins similar to each other. However, sediment and soil generally showed similar δ13C values for organic matter, but lower organic carbon concentrations were observed in sediment than in the surrounding soils were found (Table S1). Since 2000, phytoplankton has increased with accelerated eutrophication of the Wudalianchi lakes (Gui et al. 2012). Lower organic carbon concentrations reflect strong microbial respiration in aquatic environment within the water column. The process of DIC production in respiration has no large isotope fractionation, and the decomposition of organic C preferentially releases 12C (Han et al. 2018). Partially respired 13C-depleted CO2 is preferentially taken up by phytoplankton. The remaining 13C-rich organic matter deposited in the sediment resulted in sight difference between the δ13C values of organic carbon in the sediment and soils (Table S1). Excess respired CO2 derived from sediment degasses to the atmosphere. Accordingly, photosynthesis and CO2 degassing from the epilimnion may have positively shifted δ13CDIC, whereas deep waters had negative δ13CDIC values (Herczeg and Fairbanks 1987; Herczeg 1987; Quay et al. 1986; Wachniew and Różański 1997).
Data collected 10 years ago shows that the δ13Corg of lake 3 has risen (− 26.8 to − 25.8‰ as compared to − 25.6 to − 24.7‰). In contrast, past and present δ13Corg data from lake 5 (− 26.8 to − 25.8‰) were similar (− 25.8 to − 24.9‰). The increased δ13Corg values of lake 3 in our study are consistent with the slight positive increase of δ13Corg values that have been reported since 2000 in previous research (Gui et al. 2012). Enhancement from human activities, including the input of domestic sewage and land use changes around the lake 3, is likely responsible for elevation of 13Corg over the last 20 years (Gui et al. 2012). In contrast, the trophic status of lake 5 during the past 20 years has not significantly changed.
Effect of deep carbon cycling
Biogeochemical activities elevated pCO2 values of deep mineral springs and shallow groundwaters in summer; pCO2 values during the other seasons were constant and, in general, much higher than in water from the Wudalianchi lakes. The observed variations may be closely related to the effects of deep carbon cycling on the groundwater and degassing of CO2 to the atmosphere (as discussed earlier). The magmatism in Wudalianchi is due to an upwelling of a hydrous mantle plume from the hydrated mantle transitional zone (Zhang et al. 2018a). An asthenosphere–lithosphere interaction model proposed for the origin of the Wudalianchi (Tian et al. 2016) area suggests that C is transported into the mantle by a subducted slab and returned to the surface by degassing of volcanoes (Li et al. 2017). The gases in the deep mineral waters are mainly CO2 and, to a lesser degree, N2, O2, Ar, and He. In contrast, the mineral water close to the surface exhibits a gas content similar to air (such as at ElyS; Zhang et al. 2018a). Mantle-derived CO2 dissolves and accumulates in the deep groundwaters where it forms primary carbonate mineral water. Such carbonated water migrates upwards along fractures and faults. During its migration, the water continuously interacts with the surrounding rock, during which it is involved with a series of processes including dilution, leaching, mixing, cation exchange, and anthropogenic disturbance. Finally, at the junction of the fault zone, the groundwater emerges in the form of shallow mineral springs. This transformation from deep carbon cycling to surface carbon cycling results in the observed geochemical and isotopic difference between groundwater in the different aquifers and lake waters (Zou et al. 2018).
The evasion of CO2 from inland waters (rivers, lakes, and wetlands) plays a major in the atmospheric carbon budget (Wehrli 2013). However, such high mantle-derived CO2 associated with deep carbon cycling showed minor seasonal variation in pCO2 in mineral springs (Fig. 3c). Especially in summer, multiple biogeochemical processes lead to complexity in the isotopic composition of carbon. The problem of CO2 sources and sinks in spring water, and the series of associated biogeochemical processes, makes it worth further investigating the carbon cycling in dammed lake–groundwater systems, such as in the Wudalianchi region. These uncertainties will make it difficult to accurately estimate the contributions of CO2 from the region under mid to cool temperatures and monsoon-controlled climate to the atmospheric carbon budget. Future studies should focus on the investigation of CO2 sources and sinks in different climate zones and associated biogeochemical activities on the local and regional carbon cycle. Moreover, the influence of deep carbon cycling on the evolution of long-term climate at regional and global scales requires accurate and systematic research and should be taken into account when evaluating carbon budgets in the aquatic systems.
Biogeochemical cycles and carbon sources and sinks within the area were characterized using carbon isotope techniques. The mechanisms responsible for changes in the carbon isotopic composition of the aquatic environment were also deciphered. Biogenic CO2 is usually important to pCO2 dynamics and δ13CDIC in the surface–ground water system. It was deduced from the correlation between isotopes and geochemical parameters that biogenic CO2, along with mantle-derived CO2, influenced the δ13CDIC of shallow groundwater. In contrast, DIC in the Wudalianchi lakes was supplied by shallow groundwater and surface runoff during the summer. The epilimnion was affected spatially by photosynthetic activities and CO2 degassing.
Organic matter degradation in lake sediment appears to be the main source of CO2 used by phytoplankton. The 13C-rich organic carbon produced at the lake bottom appears responsible for higher δ13Corg values and lower organic carbon concentration than terrestrial soil. The downstream reaches of the Wudalianchi lake system showed higher δ13CSOC values and lower SOC than the upper reaches. The δ13Corg values in lake 3 are consistent with an increasing trend since 2000. δ13Corg values of upper stream lakes changed within the 20-year period, especially for lake 5, which was removed from significant human activities.
Our findings demonstrate processes by which lake–groundwater interaction and environmental problem complicate carbon metabolism and sedimentary environments. Possible management strategies with respect to rare cold-water mineral springs should focus on the development of high quantity of mantle-derived CO2 at local scales. At regional to global scales, the estimation of CO2 source and sink in springs should be incorporated into the global carbon budget in consideration of different climate zones and associated biogeochemical activities during different seasons.
We gratefully acknowledge Prof. Philippe Garrigues and anonymous reviewers for their thoughtful and constructive comments.
This work was supported financially by China Postdoctoral Science Foundation (2018M641774), Jilin University Postdoctoral Research Start-up Funds (801171050425), the National Natural Science Foundation of China (41472237), and Liaoning Innovation Team Project (LT2015017).
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