Isotopic composition of carbon dioxide lost by evasion from surface water to the atmosphere: Methodological comparison of a direct and indirect approach

Abstract

The age and source of CO2 released from water surfaces to the atmosphere provides significant insight into the processes that control this important flux in key areas of the biosphere, such as peatlands and tropical river systems. Two approaches currently exist that enable the isotopic composition of CO2 lost by evasion (degassing) to be either measured or estimated. 13C-CO2 and 14C-CO2 can be measured directly using a molecular sieve trapping system attached to a floating chamber, or indirectly by calculation from DIC. We present a comparison of the two methods from samples collected from low-productivity UK peatland streams supersaturated in CO2. On six occasions, samples of evaded CO2 were collected above the water surface for the determination of δ13C and 14C. Simultaneously, samples of streamwater were collected for the determination of the isotopic composition of DIC. Our results suggest that whereas on some occasions the radiocarbon ages of evaded CO2 and DIC were similar, on others the measured 14C-DIC age was significantly older. Differences were also observed between the directly measured and calculated δ13C-CO2 values. We critically evaluate both methods, highlight the importance of making the appropriate isotopic corrections for evasion CO2, and conclude that care needs to be taken in applying the indirect approach, as some of the inherent assumptions associated with equilibrium conditions used in the underlying calculation may not hold in certain aquatic systems.