[Variations of CO2 Exchange Fluxes Across Water-air Interface and Environmental Meaning in a Surface Stream in Subtropical Karst Area, SW China]

Abstract

CO₂ cycle process or sources/sinks are not only the basis of understanding and responding to global climate change, but also the core of the current global climate change research. Gas exchange across water-air interface in terrestrial surface water is an important way of nutrient elements (carbon, nitrogen) exchange between aquatic ecosystems and ambient air. Escaping CO₂ gas from surface water is also actively involved in the modern carbon cycle. In the material cycle in karst regions, CO₂ plays a key role in karst processes, driving the formation of karst features. Karst groundwater with high water CO₂ partial pressure (pCO₂) often shows highly positive CO₂ concentration gradient to atmosphere after it is discharged to surface, so the evaluation of CO₂ exchange fluxes across karst water-air interface is important for karst carbon cycle research. This paper researched CO₂ exchange fluxes across water-air interface in the karst surface stream in detail which was fed by Guancun subterranean stream in Liuzhou city, Guangxi province. Closed static chamber method and portable hand-holding CO₂ sensor (GM70) were both employed in CO₂ exchange fluxes monitoring. The results showed that CO₂ degassing was the mainly form of CO₂ exchange across the steam water-air interface. CO₂ degassing flux in subterranean stream outlet (G1 site) ranged from 139.48 to 890.84 mg·(m²·h)^-1 with an average of 445.72 mg·(m²·h)^-1. CO₂ degassing flux in stream downstream site (G2 site) ranged from 16.54 to 844.18 mg·(m²·h)^-1 with an average of 159.81 mg·(m²·h)^-1. The CO₂ degassing flux in G1site was higher than that in G2 site. CO₂ degassing fluxes in rainy season in both G1 and G2 site were higher than those in dry season. Stable carbon analysis of CO₂ gas (δ¹³C-CO₂) found that CO₂ degassing from karst stream might influence air CO₂ carbon isotope near water surface, which resulted in the more negative δ¹³C-CO₂ value with the increase of CO₂ degassing flux. Significant spatio-temporal variations of δ¹³C-CO₂ were found, and the δ¹³C-CO₂ in the rainy season was more negative than that in dry season and δ¹³C-CO₂ in G1 site was more negative than that in G2 site. As a result of stream CO₂ degassing, the hydrochemical characteristics of steam varied along stream running, which resulted in decrease of HCO₃^-, EC and pCO₂ and increase of pH, SIc and δ¹³C-DIC in the stream.

Keywords: CO2 degassing; Guancun karst subterranean stream; fluxes; influencing factors; karst surface stream.