Spatial and temporal variations ofpCO2, dissolved inorganic carbon and stable isotopes along a temperate karstic watercourse

Abstract This study investigated CO 2 degassing and related carbon isotope fractionation effects in the Wiesent River that drains a catchment in the karst terrain of the Franconian Alb, Southern Germany. The river was investigated by physico‐chemical and stable isotope analyses of water and dissolved inorganic carbon during all seasons along 65‐km long downstream transects between source and mouth. Calculated p CO 2 values at the source were 21 400 ± 2400 µatm. The p CO 2 rapidly decreased in the river water and dropped to an average of 1240 ± 330 µatm near the mouth. About 90% of this decrease occurred within the first 6 km of the river. The river was supersaturated with respect to CO 2 over its entire course and must have acted as a continuous year‐round CO 2 source to the atmosphere. The average CO 2 flux from the karst river was estimated with 450 mmol m −2 day −1 with higher fluxes up to 5680 mmol m −2 day −1 at the source. At the source, δ 13 C DIC values showed no seasonal variations with an average of −14.2 ± 0.2‰. This indicated that groundwater retained high p CO 2 mainly from soil CO 2 . The contribution of soil CO 2 to dissolved inorganic carbon was estimated at 65% to 72%. The downstream CO 2 loss caused a positive shift in δ 13 C DIC values of 2‰ between source and mouth because of the preferential loss of the 12 C isotope during degassing. Considering the findings of this study and the fact that carbonate lithology covers a significant part of the earth's surface, CO 2 evasion from karst regions might contribute notably to the annual carbon dioxide release from global freshwater systems. Copyright © 2015 John Wiley & Sons, Ltd.