Variations of soil CO2 concentration and pCO2 in a cave stream on different time scales in subtropical climatic regime (2023)

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 (δ¹³C_DIC) and trace element ratios, were monitored from August 2018 to December 2020. The results showed seasonal variations in the δ¹³C_DIC, 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 δ¹³C_DIC were more sensitive to the recharge changes from extreme precipitation and drought events. Therefore, dripwater δ¹³C_DIC is a reliable indicator of the recorded hydrological signal in the southwest monsoon region.

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.

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 δ¹³C_DIC 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 δ¹³C_DIC as well as hydrochemical compositions of dripwaters in the Maomaotou Big Cave, Guilin, South China. Great spatiotemporal variations in the dripwater δ¹³C_DIC values were observed: (i) seepage flow-fed drips commonly had lower δ¹³C_DIC values, with smaller temporal variation; (ii) the lowest δ¹³C_DIC 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 δ¹³C_DIC; (iii) the δ¹³C_DIC exhibits remarkable season changes, particularly at fracture-fed drips where higher δ¹³C_DIC values were observed in winter seasons; and (iv) an increasing trend in mean value of δ¹³C_DIC at individual sites during 2015–2019 was found. Both hydrochemical and isotopic analyses revealed that the dripwater δ¹³C_DIC 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 δ¹³C_DIC of fracture-fed dripwaters in dry periods. Comparisons of the dripwater δ¹³C_DIC records with local climate showed that the change of summer monsoonal rainfall, particularly the August-September-October precipitation, significantly influences the annual mean δ¹³C_DIC value of dripwaters in caves of South China. Further comparison analyses of dripwater δ¹³C_DIC records from both southern and northern China suggested that variations of dripwater δ¹³C_DIC 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.

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, $P_{{\mathit{CO}}_2}$ 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 (δ¹³C_DIC) 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, δ¹³C_DIC value, and elements/ions to variations in vegetation. Overall, this study highlights the responses of the interannual changes in δ¹³C_DIC, δ¹⁸O, and elements/ions of drip water to vegetation restoration, which contributes critical insights into the paleoenvironmental interpretation of proxies of speleothems.

Results in Geochemistry, Volume 2, 2021, Article 100008

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.

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.

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 δ¹³C_DIC 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. δ¹³C_DIC showed that the metabolic process has a negligible influence on DIC, δ¹³C_DIC, and pCO₂. δ¹³C_DIC 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 δ¹³C_DOC (−30.43‰ to −24.05‰) and δ¹³C_POC (−29.87‰ to −23.37‰) values confirmed that organic carbon mainly derived from the degradation of organic materials in soil. δ¹³C_DIC 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.

Chemical Geology, Volume 504, 2019, pp. 96-104

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.