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Isotopic composition of carbon dioxide lost by evasion from surface water to the atmosphere: Methodological comparison of a direct and indirect approach
Author(s) -
Billett Michael F.,
Garnett Mark H.
Publication year - 2010
Publication title -
limnology and oceanography: methods
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.898
H-Index - 72
ISSN - 1541-5856
DOI - 10.4319/lom.2010.8.45
Subject(s) - peat , carbon dioxide , atmosphere (unit) , dissolved organic carbon , environmental science , flux (metallurgy) , supersaturation , carbon fibers , stable isotope ratio , streams , composition (language) , surface water , environmental chemistry , carbon cycle , chemistry , ecology , materials science , environmental engineering , ecosystem , meteorology , physics , computer network , linguistics , philosophy , organic chemistry , quantum mechanics , composite number , computer science , composite material , biology
The age and source of CO 2 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 CO 2 lost by evasion (degassing) to be either measured or estimated. 13 C‐CO 2 and 14 C‐CO 2 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 CO 2 . On six occasions, samples of evaded CO 2 were collected above the water surface for the determination of δ 13 C and 14 C. 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 CO 2 and DIC were similar, on others the measured 14 C‐DIC age was significantly older. Differences were also observed between the directly measured and calculated δ 13 C‐CO 2 values. We critically evaluate both methods, highlight the importance of making the appropriate isotopic corrections for evasion CO 2 , 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.