Variable source and age of different forms of carbon released from natural peatland pipes

We used the carbon isotope composition (C-14 and delta C-13) to measure the source and age of DOC, POC, dissolved CO2 and CH4 (delta C-13 only) released from three natural peat pipes and the downstream catchment outlet of a small peatland in northern England. Sampling under different hydrological extremes (high flows associated with storm events and low flows before or after storms) was used to explore variability in C sources as flow paths change over short periods of time. The d13C composition of organic C differed (delta C-13-DOC -28.6 parts per thousand to -27.6 parts per thousand; delta C-13-POC -28.1 parts per thousand to -26.1 parts per thousand) from that of the dissolved gases (delta C-13-CO2 -20.5 parts per thousand to +1.1 parts per thousand; delta C-13-CH4 -67.7 parts per thousand to -42.0 parts per thousand) and showed that C leaving the catchment was a mixture of shallow/deep pipe and non-pipe sources. The isotopic composition of the dissolved gases was more variable than DOC and POC, with individual pipes either showing C-13 enrichment or depletion during a storm event. The C-14 age of DOC was consistently modern at all sites; POC varied from modern to 653 years BP and evasion CO2 from modern to 996 years BP. Differences in the isotopic composition of evasion CO2 at pipe outlets do not explain the variability in delta C-13 and C-14 at the catchment outlet and suggest that overland flow is likely to be an important source of CO2. Our results also show that the sources of CO2 and CH4 are significantly more variable and dynamic than DOC and POC and that natural pipes vent old, deep peat CO2 and POC (but not DOC) to the atmosphere.