An assessment of chamber 14C methodologies for sampling aquatic CO2 evasion

The development of new methods to directly measure the radiocarbon age of dissolved and evaded aquatic carbon dioxide has enhanced our ability to understand carbon transport and cycling in the soil–water–atmosphere system. One of the methods involves collecting enough carbon dioxide for radiocarbon dating by allowing carbon dioxide to outgas from the water surface into an enclosed floating chamber, with the gas subsequently trapped onto a zeolite molecular sieve cartridge. There are, however, several different methodological approaches that can be used for the collection of floating chamber samples, and it is currently unknown whether these different approaches influence the isotopic (stable carbon and radiocarbon) composition of the measured sample. Here, we evaluate four different floating chamber approaches and compare the stable and radiocarbon composition of the evaded carbon dioxide. Chamber conditions varied considerably with the different methodologies, with for example, maximum chamber CO2 concentration ranging from approximately 400–6,300 ppm during sampling. Despite the varying chamber conditions, our results indicate no significant differences in the 14C age of evasion (range: 1,276–1,364 years before present) with any of the methodological approaches (in chambers where atmospheric carbon dioxide had been excluded). This confirms the methodologies are both robust and widely applicable.