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In karst regions, soil CO2 is a major chemical driving force for the karst processes and finally has a significant impact on the hydrochemical processes of karst underground river. Hydrochemical features, soil and climatic parameters with a high-temporal resolution have been monitored on different scales (daily scale, seasonal scale and interannual scale, storm scale) in the Xueyu Cave watershed from 2009 to 2015. The aim of this study is to understand how cave stream pCO2 and hydrochemistry respond to overlying soil carbon recharge on different time scales. The results show that the variational amplitudes of the hydrochemistry in Xueyu Cave underground river (XUR) tend to be in the order: storm-scale > seasonal > interannual > daily scale. Soil CO2, pCO2 (CO2 partial pressure in the XUR) and Spc (specific conductivity) were higher in summer and autumn than those in winter and spring. The synchronous variations of XUR pCO2 with soil CO2 concentrations in the same order of magnitude confirm the “soil CO2 effects” on the formation of XUR hydrochemical features. The storm-scale fluctuations of hydrochemistry in XUR water are especially depending on the intensities of rainfalls that determine whether the “dilution effects” or the “CO2 effects” are predominant in the stream during rainfall events. At the same time, soil moisture and soil CO2 work as important factors for controlling pCO2 variations in the XUR. The identified relationship of soil-XUR pCO2 suggests a temperature control on carbonate weathering on daily and seasonal scale but a rainfall/soil moisture control on storm and annual scale. The combined effect of point and diffused infiltration that delays the arrival of storm flows determines the fluctuations of the discharge and pCO2 variation. Defining relationships between CO2 from overlying soils and groundwater offers the chance to explore the processes at different time scales, potentially increasing our ability to understanding the carbon dynamics in karst systems.
Change and increasing concentrations of atmospheric greenhouse gases, not only lead to gradual mean global warming but may also change the frequency, the severity and even the nature of extreme events (IPCC, 2013). Diverse climate-dependent processes occurring on different timescales are involved in ecosystems carbon cycling (Berner, 2003). Karst landscapes provide a natural laboratory to investigate C cycling as C is present in various reservoirs, where the cycling drives development of conduits via calcite dissolution from carbon dioxide (CO2) (Dreybrodt, 1988, Gulley et al., 2013). The subterranean CO2 pool could represent more than half of the total CO2 content of the atmosphere as the non-negligible role of cavities as a temporal depot of CO2 coming from different processes (Serrano-Ortiz et al., 2010). Carbonate weathering and underground CO2 storage are important parts of the terrestrial flux of carbon at different scales (daily to annually) (Liu and Zhao, 2000, Serrano-Ortiz et al., 2010). As CO2 dissolves in water, H2CO3 that can dissolve any carbonate substrate (e.g. calcite and dolomite) is formed. Soil CO2 serves as a major chemical driving force for carbonate dissolution and has a significant influence on hydrochemical features of karst spring water (Morse and Arvidson, 2002, Ford and Willams, 2007, Liu et al., 2007, Zhao et al., 2010, Yang et al., 2012). For example, variations of air temperature could cause the changes of soil CO2 concentrations by altering the intensity of photosynthesis and respiration of soil organisms, resulting in hydrochemical variations of karst spring water (Liu et al., 2007).
Subsurface caves in the vadose zone always present higher concentration of CO2 than outdoor air. Part of these CO2 fluxes takes place in the overlying soil where CO2 soil diffusion depends on the soil properties and water content (Pu et al., 2014, Pla et al., 2017) and/or the vadose zone, e.g. recent studies have identified an important source of CO2 in caves in the decay of soil organic matter washed down into the unsaturated zone (Mattey etal., 2016). The CO2 stored in caves commonly presents CO2 variations strongly driven by ventilation that is regulated by synoptic weather conditions (Kowalczk and Froelich, 2010, Fernandez-Cortes et al., 2011, Frisia et al., 2011, Yang et al., 2012). Moreover, advection movement that is determined by air density and cave geometry has been observed to be an important mechanism of CO2 transport throughout some underground environments (Frisia et al., 2011, Faimon et al., 2012, Mattey et al., 2016). Rainfall and the relative humidity of air regulate the water content in soil and host rock porous media controlling gas exchange between the surface and underground (Cuezva etal., 2011). The arrival of freshly filtrated rainwater at a karst spring can thus be recognized by changing water temperature and decreasing specific electric conductivity (Ford and Willams, 2007). Karst springs and karst aquifers typically show marked and rapid reactions to precipitation events in both water quantity and quality variables. Therefore, monitoring at high temporal resolutions, ideally continuous monitoring, is required to characterize the dynamic behavior and variability of karst systems (Hartmann et al., 2014).
It has been found that hydrochemical features of karst springs show variations on different time scales, including diurnal (de Montety et al., 2011, Jiang et al., 2013, Pu et al., 2014), seasonal and storm-scale variations (Liu et al., 2007, Pu et al., 2014, Li et al., 2016, Cholet et al., 2017). Abrupt changes in the hydrochemistry of the karst spring water in response to rainfall events are well documented by previous studies (Hess and White, 1988, Quinlan and Alexander, 1987, Ryan and Meiman, 1996, Vesper and White, 2004, Liu et al., 2007; Li et al., 2016). “Soil CO2 effect” with an increase in pCO2 and Spc, and “dilution effect” with a decrease in pH and Spc are alternatively dominant in the karst underground flow (Quinlan and Alexander, 1987, Yang et al., 2012). CO2 concentrations in karstic cavities show significant seasonal (Spötl et al., 2005, Fernandez-Cortes et al., 2011, Pla et al., 2016) and even daily variations (Baldini et al., 2008, Kowalczk and Froelich, 2010). After rainfall events, infiltrating water dissolves the soil CO2, acting as a geochemical CO2 sink by reducing the CO2 emissions and percolates downward (Serrano-Ortiz et al., 2010).
In this study, an extensive study of soil temperature, soil water content, soil CO2 and the pH, water temperature and Spc with high-temporal resolution (every 15 min) monitoring has been conducted at Xueyu Cave, a typical karst watershed, Chongqing, SW China (Fig. 1), to understand CO2 variations of soil and underlying cave stream at different scales in a detailed field study and to find out major controlling factors that are responsible for the pCO2 variations in the XUR, to understand the biogeochemical processes that regulate the soil CO2 flux. Overall, the aim of the present research is to characterize processes involved in CO2 exchange in the underground stream-soil system at the diel/seasonal/annual and rainfall scale.
Cave karst underground river (XUR) system (latitude 29°47′00″ N, longitude 107°47′13″ E; altitude 233 m a.s.l.) with an area of 13 km2 is located on the left bank of Long River (a tributary of the Yangtze River), SE of Fengdu county, Chongqing, Southwest China (Fig. 1). The XUR is developed in the Triassic Feixianguan Formation (T1f), which consists of limestone with a thickness of 150–250 m (Huang et al., 2008) with sedimentary environment of evaporate-carbonate platform (Zharkov and Chumakov,
Automatic data logging
A MS-5 multi-channel water quality multiprobe (made by Hach Corporation, U.S.A) was placed at the outlet of the Xueyu Cave XUR to obtain continuous hydrochemical variations from January 2009 to December 2015. The information of specific equipment and related precision were listed in Table 1. Also, a GMP22 carbon dioxide probe of VALSALA with waterproof films was placed to measure pCO2 from October 2014. The soil temperature and soil CO2 concentrations were obtained from May 2013 by a composite
Soil-Cave physicochemical parameters outside of the cave from daily to annual scales
The air temperature showed significant daily and seasonal changes in the Xueyu Cave watershed (Fig. 4, Fig. 5, Fig. 6). Besides, the magnitudes of daily variations in air temperature were higher in spring/autumn (10.7/11.9 °C) than in summer/winter (8.1/4.6 °C). The seasonal air temperature ranged from 10.5 to 25.6 °C with a mean value of 18.8 ± 6.4 °C. The seasonal soil temperature ranged from 13.5 to 22.3 °C with a mean value of 17.9 ± 4.1 °C, which is very close to the air temperature range (
Factors controlling XUR pCO2 variations at different time scales
In Table 5 it has been shown the controlling factors of XUR pCO2 variations through PCA analysis on different scales. On a diurnal scale, PCA of the remaining variables showed that 78.2% of the variance was explained by principal component PC1 (39.1%), PC2 (21.6%) and PC3 (17.5%). Not surprisingly, soil temperature, soil moisture, soil CO2 and Spc, pH had strong loadings on PC1, air temperature and discharge on PC2, indicating that soil parameters are the main factors for the diurnal
The results based on seven-year continuous monitoring show that Xueyu Cave watershed is a dynamic and varied system in terms of hydrochemistry. Interannual, seasonal, daily and storm-scale variations were continuously observed for pH, Spc, pCO2 and SIc in the cave stream. The variability of these hydrochemical features tended to be in the order of storm-scale > seasonal > interannual > daily scale.
The seasonal and daily variations of these features varied with soil temperature which influenced
This research was financed by the National Key Research and Developmental program of China (2016YFC0502306), the National Natural Science Foundation of China (NSF Grant no. 41472321) and the open project from Chongqing Key Laboratory of Karst Environment (Cqk201701). Thanks to Ze Zeng, XianFu Lv, Jiaqi Lei, Ge Hu and Sibo Zeng who helped with the field work. Thanks to all other students who made contributions to the cave monitoring.
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Cited by (9)
Seasonal variations of cave dripwater hydrogeochemical parameters and δ<sup>13</sup>C<inf>DIC</inf> in the subtropical monsoon region and links to regional hydroclimate
2023, Science of the Total Environment
Stalagmites are considered natural archives of climate proxies. However, under the combined effects of atmospheric circulation patterns, precipitation, and karst environments, drip hydrogeochemical processes can be coupled and linked to each other to control cave sediment record information. Therefore, the evolution of chemistry and factors controlling the isotopic composition of the dripwater during regional precipitation migration from the surface to caves need to be evaluated. In this study, hydrogeochemical characteristics and the isotopic composition of the dripwater in the Mahuang Cave in Guizhou Province, Southwest China, including stable isotope (δ13CDIC) and trace element ratios, were monitored from August 2018 to December 2020. The results showed seasonal variations in the δ13CDIC, Mg/Ca, and Sr/Ca values of the dripwater in dry and wet seasons under the control of water-gas-rock reactions, such as soil CO2 concentrations and carbonate rock dissolution. In addition, the five monitored dripwater points in the Mahuang Cave showed fast and slow seepage due to the complex cave fractures and stratigraphy, reflecting the effects of precipitation variations to different degrees. Indeed, the δ13CDIC were more sensitive to the recharge changes from extreme precipitation and drought events. Therefore, dripwater δ13CDIC is a reliable indicator of the recorded hydrological signal in the southwest monsoon region.
CO<inf>2</inf> dynamics in a small and old subtropical reservoir in East Asia: Environmental controls driving seasonal and spatial variability
2023, Science of the Total Environment
Citation Excerpt :
Both FCO2 and pCO2 were highest at this site during the September 2020 sampling campaign (Fig. 7), which was conducted after a heavy rainfall event with >170 mm of precipitation recorded up to two days before sampling (Fig. 7, Table S1 in supplementary information). Therefore, pCO2 in the reservoir may be elevated during heavy rainfall owing to the inflow of soil CO2 into the streams and eventually the reservoir (Zeng and Masiello, 2010; Macklin et al., 2018; Cao et al., 2020), as well as the increased contribution of runoff from the upper catchment with elevated pCO2 (Dinsmore et al., 2013). These effects appeared to have outweighed the “dilution effect” on CO2 that can result from increased stream discharge (Zeng and Masiello, 2010; Cao et al., 2020).
Inland waters have been increasingly viewed as hotspots for greenhouse gas (GHG) emissions owing to their strong capability to intercept and mineralize carbon from the terrestrial environment. Although small waterbodies in humid subtropical climates have the potential to emit considerable amounts of GHG, their emission patterns have remained understudied. This study involved intensive measurements of carbon dioxide (CO2) emissions from a small reservoir and its upstream and downstream reaches located in subtropical Hong Kong. Our results revealed that a variety of metabolic, hydrological, and hydrochemical processes play a critical role in regulating its CO2 dynamics. The reservoir was an overall source of CO2 to the atmosphere with an average areal flux of 24.6 mmol m−2 d−1, and it occasionally functioned as a sink for atmospheric CO2 under intense solar radiation when primary productivity was high. This flux is on the low side relative to that of global (sub)tropical reservoirs, which was likely attributable to the prolonged history of the reservoir (>150 years) and the occasional undersaturation of CO2 in the water column. We also noticed pronounced differences in the underlying controls of CO2 dynamics between the reservoir and its upstream and downstream reaches, emphasizing the importance of taking into account the distinct characteristics of both lentic and lotic waters when evaluating catchment-scale CO2 fluxes.
Overview on the impacts of CO<inf>2</inf> acidification in a very sensible and complex system: The cenotes, Yucatan, Mexico
2022, CO2 Acidification in Aquatic Ecosystems: An Integrative Approach to Risk Assessment
Carbonate minerals represent Earth’s largest carbon (C) reservoir but have long been considered inconsequential to the global C cycle over long periods of time (e.g., >106years). However, carbonate mineral reactions may impact the carbon cycle at short time scales (102–105years) particularly when atmospheric pCO2 changes by several hundred ppm (Martin, 2017).
The cenotes system is a labyrinth of freshwater sources considered one of the main groundwater reservoirs in the world. It is located in the Yucatan Peninsula, Mexico, and is mainly composed of calcium carbonate-based ores.
As they are easily dissolved, an increase in CO2 concentrations would result in acidification and hence promote the potential dissolution of carbonate systems such as the cenotes in Yucatan’s karstic system. Dissolution of carbonate minerals increases the overall vulnerability of the system and poses a serious threat to the health of groundwater-dependent ecosystems, which include the cenotes, mangroves, coral reefs, among others.
We present a brief but in-depth analysis on the current status of the cenotes system in the Yucatan Peninsula and some of the main impacts that could be expected under increasing global CO2 concentrations.
Deciphering the hydroclimatic significance of dripwater δ<sup>13</sup>C<inf>DIC</inf> variations in monsoonal China based on modern cave monitoring
2021, Journal of Hydrology
Citation Excerpt :
The water-CO2-rock interactions within karst aquifers can be temporally regulated by vegetation and soil activities which control the soil CO2 production (i.e. soil pCO2). In the monsoonal region of China, higher soil pCO2 are produced between June and October as the increase in both temperature and moisture enhances biological productivity and thus root respiration and microbial activities in soil zone, while lower soil pCO2 is observed in winter seasons when the soil respiration rate is low (Cao et al., 2020; Liu et al., 2007, 2010; Wang et al., 2020; Zhao et al., 2015; Zhong et al., 2018, 2021). Accordingly, larger supply of soil CO2 is available and thus carbonate dissolution occurs under more open system conditions during the summer season.
Stable carbon isotopic composition (δ13C) of speleothems has often been recognized as a proxy of vegetation and soil processes in many climatic regimes. In monsoonal regions, the speleothem δ13C records are thought to be able to document changes in local and/or regional precipitation as well. Due to the complexity of carbon isotopic evolution within heterogeneous karst aquifers, accurate interpretations of this proxy are challenging. It is essential to carry out detailed monitoring of cave dripwater δ13CDIC to disentangle various processes governing carbon isotopic evolution and decipher the hydroclimatic constraints. Here, we reported the results of a five-year monitoring of the δ13CDIC as well as hydrochemical compositions of dripwaters in the Maomaotou Big Cave, Guilin, South China. Great spatiotemporal variations in the dripwater δ13CDIC values were observed: (i) seepage flow-fed drips commonly had lower δ13CDIC values, with smaller temporal variation; (ii) the lowest δ13CDIC values were found at fracture-fed drips that had a large supply of soil CO2, whereas the fracture-fed drips with less, episodic CO2 recharge had much more positive δ13CDIC; (iii) the δ13CDIC exhibits remarkable season changes, particularly at fracture-fed drips where higher δ13CDIC values were observed in winter seasons; and (iv) an increasing trend in mean value of δ13CDIC at individual sites during 2015–2019 was found. Both hydrochemical and isotopic analyses revealed that the dripwater δ13CDIC is mainly controlled by spatiotemporal variations in water-CO2-rock interactions in association with hydrological processes, coupled with soil CO2 dynamics, which are closely linked to changes in local rainfall at monthly to annual timescales. Prior calcite precipitation (PCP) along the flow path above the cave could also affect the δ13CDIC of fracture-fed dripwaters in dry periods. Comparisons of the dripwater δ13CDIC records with local climate showed that the change of summer monsoonal rainfall, particularly the August-September-October precipitation, significantly influences the annual mean δ13CDIC value of dripwaters in caves of South China. Further comparison analyses of dripwater δ13CDIC records from both southern and northern China suggested that variations of dripwater δ13CDIC could reflect changes of regional monsoonal precipitation over inter-annual (and maybe decadal) timescale. This demonstrates that the δ13C of speleothems, if precipitated at isotopic equilibrium and not greatly influenced by the CO2 degassing, is likely to be a valuable proxy of monsoon rainfall variability across East Asia.
Geochemical responses of cave drip water to vegetation restoration
2020, Journal of Hydrology
Citation Excerpt :
It is possible that the soil moisture content above Shawan Cave in 2017 was comparatively lower, whereas the soil moisture content of 2018 and 2019 may be a moderate condition. The moderate soil moisture content and air temperature in 2018 and 2019 could have promoted soil respiration, whereas low soil moisture content and high air temperature in 2017 would have limited soil respiration (Cao et al., 2020). Accordingly, soil respiration in 2017 may have been weaker than that in 2018 and 2019.
The transmission of stable isotopes and elements/ions from the outside to the inside of a cave and their incorporation into drip water can involve numerous biogeochemical processes. To understand how the original signals of stable isotopes and elements/ions are modified by these processes, integrated studies of the interactions between vegetation, soil, epikarst, and caves are required. We conducted a multi-year monitoring study of the vegetation biomass, tree breast-height diameter, in soil air and cave air, δ13C in soil air, stable oxygen isotope in rainwater and drip water, and stable carbon isotope and elements/ions concentrations in drip water in Shawan Cave system, southwest China. The main results were as follows: (1) The evaporation effect weakened and the transpiration effect strengthened outsides the cave as vegetation improved, thus leading to a year-by-year increasing trend in the δ18O value of drip water. This indicates that changes in vegetation may have been another potential factor influencing the interannual variation of the δ18O value of drip water. (2) The CO2 concentration and δ13C value in soil air increased and decreased, respectively, with vegetation restoration, which caused the interannual variation in the dissolved inorganic carbon isotope (δ13CDIC) value of drip water during the autumn and winter to exhibit a year-by-year decreasing trend. (3) The variations in the elements/ions concentrations of drip water were affected by vegetation uptake, vegetation transpiration, and water–rock interactions. It is inferred that the interannual variation in the elements/ions concentrations of drip water responded to vegetation restoration. (4) A conceptual model demonstrated that the three response modes of drip water δ18O value, δ13CDIC value, and elements/ions to variations in vegetation. Overall, this study highlights the responses of the interannual changes in δ13CDIC, δ18O, and elements/ions of drip water to vegetation restoration, which contributes critical insights into the paleoenvironmental interpretation of proxies of speleothems.
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Condensation corrosion alters the oxygen and carbon isotope ratios of speleothem and limestone surfaces
Results in Geochemistry, Volume 2, 2021, Article 100008
Condensation corrosion is a natural process which enhances the chemical weathering of limestone cave chambers and speleothems. We evaluated the use of carbonate tablets for detecting condensation corrosion in Glowworm Cave, New Zealand, using local limestone and speleothem as experimental substrates (herein tablets). Evidence for condensation corrosion was assessed via three methods: gravimetric (mass wasting), microscopic (surface pitting, recrystallization) and isotopic (δ13C and δ18O changes). Our results show little evidence of tablet mass loss throughout a 6-month deployment period. However, SEM imaging and isotope analysis (δ13C and δ18O) of the upper ~50μm layer of the tablets, suggest that condensation corrosion operates in the cave, especially in sectors affected by large diurnal microclimate variations.
Most notably, condensation water altered the tablet surface δ13C and δ18O values. Small, positive shifts in surface δ13C and δ18O values are considered to reflect pure dissolution (where dissolution favours the removal of lighter isotopologues). In contrast, tablets that exhibited large positive shifts in δ13C in tandem with large negative shifts in δ18O values, are interpreted as showing calcite recrystallization and the inheritance of higher DIC δ13C values (13C fractionation by CO2 degassing), lighter water δ18O values and/or kinetic fractionation of δ18O. This study therefore demonstrates that stable isotopes could be applied to detect paleoclimatic episodes of condensation corrosion in speleothems.
Factors controlling the growth rate, carbon and oxygen isotope variation in modern calcite precipitation in a subtropical cave, Southwest China
Journal of Asian Earth Sciences, Volume 119, 2016, pp. 167-178
A prerequisite for using cave speleothems to reconstruct palaeoenvironmental conditions is an accurate understanding of specific factors controlling calcite growth, in particular the isotopic partitioning of oxygen (δ18O) and carbon (δ13C) which are the most commonly used proxies. An in situ monitoring study from April 2008 to September 2009 at Xueyu Cave, Chongqing, SW China, provides insight into the controls on calcite growth rates, drip water composition, cave air parameters and δ18O and δ13C isotopic values of modern calcite precipitation. Both cave air PCO2 and drip water hydrochemical characteristics show obvious seasonality driven by seasonal changes in the external environment. Calcite growth rates also display clear intra-annual variation, with the lowest values occurring during wet season and peak values during the dry season. Seasonal variations of calcite growth rate are primarily controlled by variations of cave air PCO2 and drip water rate. Seasonal δ18O-VPDB and δ13C-VPDB in modern calcite precipitates vary, with more negative values in the wet season than in the dry season. Strong positive correlation of δ18O-VPDB vs. δ13C-VPDB is due to simultaneous enrichment of both isotopes in the calcite. This correlation indicates that kinetic fractionation occurs between parent drip water and depositing calcite, likely caused by the variations of cave air PCO2 and drip rate influenced by seasonal cave ventilation. Kinetic fractionation amplifies the equilibrium fractionation value of calcite δ18O (by ∼1.5‰) and δ13C (by ∼1.7‰), which quantitatively reflects surface conditions during the cave ventilation season. These results indicate that the cave monitoring of growth rate and δ18O and δ13C of modern calcite precipitation are necessary in order to use a speleothem to reconstruct palaeoenvironment.
The impact of heterotrophic bacteria on recalcitrant dissolved organic carbon formation in a typical karstic river
Science of The Total Environment, Volume 815, 2022, Article 152576
Recalcitrant dissolved organic carbon (RDOC) resulting from microbial carbon (MCPs) holds promise as a relatively long-term natural carbon sink in marine environments. However, the RDOC formation mechanism remains uncertain in terrestrial aquatic systems. To determine the microbial impacts on autochthonous dissolved organic carbon (DOC), RDOC formation, and the critical influencing bacteria species, spatial changes in hydrochemistry, carbon isotopes, and microbial diversity were investigated in water samples from the karstic Lijiang River, southwest China. Samples were collected at various locations along the river system in May and July 2017. The biodegradable DOC (BDOC), RDOC, soil sourced DOC (SDOC), submerged aquatic vascular plant sourced DOC (PDOC) and microbial sourced DOC (MDOC) were calculated using the in-situ microbial incubation method, stable carbon isotopes and C/N ratio. RDOC accounted for 67% to 93% of DOC concentrations, measuring 1.3mg/L and 1.2mg/L in May and July, respectively. In May, BDOC concentrations increased by 0.05mg/L from 0.18mg/L to 0.23mg/L, but decreased by 0.43mg/L from 0.66mg/L to 0.23mg/L in July. The spatiotemporal variation of BDOC indicated photosynthesis was the main BDOC source and induced high autochthonous DOC formation, especially in May. However, RDOC was the dominant accumulation component in Lijiang River. MDOC increased by 0.86mg/L from 0 to 0.86mg/L in May and 0.78mg/L from 0.10mg/L to 0.88mg/L in July, which was the dominant accumulated DOC and RDOC component. The abundance of Sporichthyaceae accounted for 3.4%–22.6% in May and Novosphingobium accounted for 3.5%–34.0% in July. These were the critical bacteria species induced MDOC formation, which were confirmed by their abundances in KEGG pathway modules determined by PICRUAST2. These results demonstrate that heterotrophic bacteria dominate autochthonous DOC and RDOC formation in the karst surface river, which is valuable for understanding organic carbon cycling in karstic aquatic systems.
Dynamic of riverine pCO2, biogeochemical characteristics, and carbon sources inferred from δ13C in a subtropical river system
Science of The Total Environment, Volume 821, 2022, Article 153296
Rivers significantly contribute to the global carbon budget, but data limitations and uncertainty are hampered by CO2 quantification in the global rivers. Thus, this study estimated riverine pCO2 by employing the pH-alkalinity-temperature method, and dissolved inorganic (DIC), dissolved organic (DOC), particulate organic (POC) carbon, and their isotopes (δ13C) with Chlorophyll-a (Chl a) were measured in river water samples from 26 sampling sites for characterization and source identification in the Yangtze River system. The estimated pCO2 varies from (120 ppm) to (3400 ppm) with an average (1085 ppm) across the Yangtze River and pCO2 is almost three times oversaturated than the ambient air (380 ppm). The downstream sites pronounced elevated pCO2 than the upstream sites. The relationship of δ13CDIC and pCO2 indicated that pCO2 control is seasonally independent. The significant correlations between DOC, POC, and pCO2 revealed that organic carbon influenced pCO2 in the river. The seasonal fluctuations of pCO2 were observed with an average of (762.23 ppm) and (1407.35 ppm) in winter and summer, respectively. δ13CDIC showed that the metabolic process has a negligible influence on DIC, δ13CDIC, and pCO2. δ13CDIC values increased from −8.95‰ to −4.91‰ during summer, whereas winter increased from −19.76‰ to −1.97‰ suggesting that DIC derived from carbonate weathering, dissolution of atmospheric and soil CO2. The δ13CDOC (−30.43‰ to −24.05‰) and δ13CPOC (−29.87‰ to −23.37‰) values confirmed that organic carbon mainly derived from the degradation of organic materials in soil. δ13CDIC revealed that anthropogenic sewage discharge slightly modified DIC composition. Overall, this study provides new insight into recent seasonal fluctuations of the pCO2, DOC, POC, DIC, δ13C, and their inputs. Thus, these variations and inputs of carbon transported by the Yangtze River could have a significant influence not only on the biogeochemical cycle and ecosystem process but also on the global carbon budget.
Stable isotopes and trace elements of drip waters at DeSoto Caverns during rainfall-contrasting years
Chemical Geology, Volume 504, 2019, pp. 96-104
A monitoring study at DeSoto Caverns during two years (2012−2013) of rainfall-contrasting variability presents the opportunity to test the response of the hydroclimate proxies in drips and active speleothems to forcing factors on intrannual and interannual time scales.
The weighted monthly mean rainwater δ18O and δ2H range from −1.2 to −6.4 (‰ V-SMOW) and −4 to −41.6 (‰ V-SMOW), respectively, and show modest interannual variation. D-excess values exhibit a large intrannual contrast suggesting a primary control by sub-cloud evaporation processes. Coeval drip-water δ18O and δ2H vary from −3.1 to −5.3 (‰ V-SMOW) and −9.9 to −30.5 (‰ V-SMOW), respectively, and exhibit interannual negative trends from the 2012 dry/warm year to the 2013 relatively wet/cool year. Substantial attenuation of drip-water isotope amplitudes, relative to its rainwater source, is likely caused by mixing of fresh with residual evaporated-water in the epikarst zone.
Drip-water Ca, Mg, Sr and Mg/Ca and Sr/Ca ratios exhibit an inverse relation with respect to the contrasting hydroclimate years such that lower values and higher ratios occur during the dry/warm year and higher values and lower ratios occur during the wet/cool year. We assert that interannual rainfall variability exerts a dominant control on the elemental concentrations and their ratios of the drips through changes of biomass productivity in the soils overlying the cave, and prior aragonite precipitation in the epikarst. The distribution coefficients of Mg (DMg = 3.49 × 10−3 ± 1.06 × 10−3) and Sr (DSr = 1.12 ± 0.041) between drips and aragonite speleothems estimated in this study are in broad agreement with aragonite-solute experimental values. Coeval changes of trace elements and δ18O in response to interannual rainfall variability confirm their usefulness to better constrain the controlling hydroclimate drivers.
Excitation-emission matrix fluorescence spectra of chromophoric dissolved organic matter reflected the composition and origination of dissolved organic carbon in Lijiang River, Southwest China
Journal of Hydrology, Volume 598, 2021, Article 126240
Aquatic photosynthesis transforms inorganic carbon to organic carbon (OC), which contributes to autochthonous organic carbon (AOC) in sediment and particulate organic carbon in surface karst aquatic systems. Aquatic plant and microbes are participated in autochthonous dissolved organic carbon (ADOC) formation in surface karst aquatic systems, but the composition and formation of ADOC remains little known, which leaves problem on calculation of organic carbon flux. In this study, the Lijiang River was chosen as typical surface karst river to identify the DOC composition and its origin, and to explore the environmental influencing factors. Samples were collected seasonally from the upper to lower reaches of the river to analyze hydrochemical parameters and the excitation-emission matrix (EEM) spectrum of chromophoricdissolved organic matter (CDOM). Three CDOM components were calculated by parallel factor analysis (PARAFAC) from EEM spectra, which were allochthonous soil-sourced DOC (SDOC), autochthonous aquatic plant-sourced DOC (APDOC), and microbial-sourced DOC (MDOC). Based on the DOC component concentrations, SDOC is induced by large amounts of precipitation causing soil erosion in summer. APDOC formation is encouraged by moderate water temperatures in spring and fall restricted by high water turbidity in summer. The significant positive linear correlations between APDOC and dissolved inorganic carbon (DIC) and MDOC indicate DIC fertilization effect in karst aquatic systems directly promotes APDOC formation and indirectly promotes MDOC. Our study develops a relatively simple way to calculate composition of karst aquatic DOC, which demonstrates the participation of aquatic plants and microbes in APDOC production and reiterate that autochthonous DOC should be considered when calculating the carbon sink in surface karst aquatic systems.
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