Stable Carbon and Nitrogen Isotopes in a Peat Profile Are Influenced by Early Stage Diagenesis and Changes in Atmospheric CO2 and N Deposition

In this study, we test whether the delta C-13 and delta N-15 in a peat profile are, respectively, linked to the recent dilution of atmospheric delta(CO2)-C-13 caused by increased fossil fuel combustion and changes in atmospheric delta N-15 deposition. We analysed bulk peat and Sphagnum fuscum branch C and N concentrations and bulk peat, S. fuscum branch and Andromeda polifolia leaf delta C-13 and delta N-15 from a 30-cm hummock-like peat profile from an Aapa mire in northern Finland. Statistically significant correlations were found between the dilution of atmospheric delta(CO2)-C-13 and bulk peat delta C-13, as well as between historically increasing wet N deposition and bulk peat delta N-15. However, these correlations may be affected by early stage kinetic fractionation during decomposition and possibly other processes. We conclude that bulk peat stable carbon and nitrogen isotope ratios may reflect the dilution of atmospheric delta(CO2)-C-13 and the changes in delta N-15 deposition, but probably also reflect the effects of early stage kinetic fractionation during diagenesis. This needs to be taken into account when interpreting palaeodata. There is a need for further studies of delta N-15 profiles in sufficiently old dated cores from sites with different rates of decomposition: These would facilitate more reliable separation of depositional delta N-15 from patterns caused by other processes.