Annual variability in the radiocarbon age and source of dissolved CO2 in a peatland stream

Radiocarbon dating has the capacity to significantly improve our understanding of the aquatic carbon cycle. In this study we used a new passive sampler to measure the radiocarbon (14C) and stable carbon (δ13C) isotopic composition of dissolved CO2 for the first time in a peatland stream throughout a complete year (May 2010–June 2011). The in-stream sampling system collected time-integrated samples of CO2 continuously over approximately 1 month periods. The rate of CO2 trapping was proportional to independently measured streamwater CO2 concentrations, demonstrating that passive samplers can be used to estimate the time-averaged dissolved CO2 concentration of streamwater. While there was little variation and no clear trend in δ13CO2 values (suggesting a consistent CO2 source), we found a clear temporal pattern in the 14C concentration of dissolved CO2. The 14C age of CO2 varied from 707 ± 35 to 1210 ± 39 years BP, with the youngest CO2 in the autumn and oldest in spring/early summer. Mean stream discharge and 14C content of dissolved CO2 were positively correlated. We suggest that the observed pattern in the 14C content of dissolved CO2 reflects changes in its origin, with older carbon derived from deeper parts of the peat profile contributing proportionally more gaseous carbon during periods of low stream flow. - Highlights: ► Dissolved CO2 was sampled from a peatland stream and radiocarbon dated. ► Samples collected using new passive sampler are suitable for integrated monthly samples. ► Age of CO2 ranged from 707 to 1210 years old and seasonal pattern is observed. ► Age correlated with discharge and reflected source of dissolved CO2. ► Study highlights the value of 14C analysis and potential of new method.