中国地质科学院岩溶地质研究所, 自然资源部/广西岩溶动力学重点实验室, 广西 桂林 541004
Study on Carbon Dioxide Outgassing in a Karst Spring-fed Surface Stream
Key Laboratory of Karst Dynamics, Ministry of Natural Resources & Guangxi; Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin 541004, China
摘要：碳酸盐岩风化作用（即岩溶作用）能够吸收大气二氧化碳（CO2），形成溶解无机碳（DIC，dissolved inorganic carbon），被认为是一种重要的陆地碳汇，其在全球碳收支平衡和未来陆地增汇中可能会有重要贡献。然而，目前对岩溶碳汇的稳定性还存在争议，一些学者认为岩溶地下水出露地表后会发生CO2脱气，对岩溶碳汇通量估算带来不确定性。本文以广西桂林长流水表层岩溶泉补给的溪流（约2.7km长）为例，利用水化学和同位素质谱仪测试技术，研究了溪水水化学指标和溶解无机碳同位素（δ13CDIC）沿流程变化，探讨了溪流CO2脱气过程、通量及其影响因素，以更好地了解岩溶碳汇的稳定性。结果表明：从泉口向下游，在陡坡地段（C1~C14段，长约270m，坡度约10°），溪水pH值、方解石饱和指数和δ13CDIC沿流程分别升高了0.9、0.9和1.8‰，而CO2分压、电导率、Ca2+浓度和DIC浓度分别下降了85%、34μS/cm、0.2mmol/L和0.7mmol/L，说明溪水发生了显著的CO2脱气，并伴随碳酸钙沉淀。而在平缓地段（C18~C26段，长约2.1km，坡度 < 1°），溪水各水化学指标和δ13CDIC变化较小，指示CO2脱气作用较弱。这些发现表明溪流CO2脱气受到了地形决定的水动力条件控制。另外，在下游渠段，受支流汇入影响，溪水pH值和方解石饱和指数有所降低，在一定程度上抑制了CO2脱气。溪流CO2脱气能够抵消部分岩溶作用固定的大气CO2量，但是在长流水这一高地势、低流量且有碳酸钙沉积的环境下，其抵消的量也仅占29%。对于在低缓地区受流量很大的岩溶泉/地下河补给的河流，其CO2脱气作用对岩溶碳汇的影响有限，加之受可能增强的水生光合生物固碳效应影响，岩溶碳汇应具有很高的稳定性。
Study on Carbon Dioxide Outgassing in a Karst Spring-fed Surface Stream
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.
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.
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.
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.
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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