Yankuang ceshi (Sep 2021)
Study on Carbon Dioxide Outgassing in a Karst Spring-fed Surface Stream
Abstract
BACKGROUND Chemical weathering of carbonates (i.e.karstification) involves considerable uptake of atmospheric CO2 which is converted to dissolve inorganic carbon (DIC), thereby acting as one of the important terrestrial carbon sinks. This karst-related carbon sink could contribute greatly to the global carbon budget and have the potential to be an increasing carbon sink on land. However, its stability has long been debated because CO2 sequestered by the dissolution of carbonate could return to the atmosphere through CO2 outgassing from groundwater-feeding surface waters, which can cause uncertainties for the estimation of the karst-related carbon sink. OBJECTIVES In order to better understand the processes responsible for CO2 outgassing and the flux and influencing factors of CO2 outgassing, and provide more insights into the stability of the karst-related carbon sink. METHODS Hydrochemical and isotopic techniques were used to monitor the change of water chemistry and carbon isotopic composition of dissolved inorganic δ13CDIC along the flow path. Based on the downstream variations of hydrochemical indicators and δ13CDIC, the flux and influencing factors of CO2 outgassing along the stream were analyzed. RESULTS From the spring (C1) to site C14, the stream channel (270m-long) had a steep gradient of~10°, and pH, calcite saturation index and δ13CDIC of stream water increased by 0.9, 0.9 and 1.8‰, respectively, whereas CO2 partial pressure, electrical conductivity, Ca2+ and DIC concentrations decreased by 85%, 34μS/cm, 0.2mmol/L and 0.7mmol/L, respectively. These observations indicated the occurrence of significant CO2 degassing and calcium carbonate precipitation in the channel. In contrast, less downstream variations in water chemistry and δ13CDIC of stream water occurred along C18-C26 segment (about 2.1km long, slope gradient < 1°) in the plain area, suggesting weak CO2 outgassing and very limited calcite precipitation. Furthermore, the hydrochemical and isotopic compositions of stream water were likely to be affected by tributary mixing and dilution in the downstream area, and consequently the pH value of the stream and calcite saturation index decreased to some degrees, which inhibited the occurrence of CO2 degassing. CONCLUSIONS The downstream variation in hydrochemical and isotopic compositions suggest that the stream CO2 degassing is chiefly affected by topographically controlled hydrological conditions. At Changliushui, the CO2 degassing in streams partly counteracts the atmospheric CO2 sequestered by carbonate weathering, but causes 29% of the total amount of CO2 sequestered in DIC of the feeding spring water to be released back to the atmosphere. For streams/rivers from low-relief areas fed by karst springs/underground rivers that have a large discharge rate, the CO2 degassing should have limited impact on the stability of karst-related carbon sinks. In addition, the possibly enhanced "carbon pump" effects of aquatic phototrophs would make the karst-related carbon sink more stable.
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