Carbon-isotope variation in Sphagnum form hummock-hollow complexes: Implications for Holocene climate reconstruction

The interpretation of isotopic variation in Sphagnum from Holocene peat profiles rests upon several assumptions concerning physiological processes such as Variation in CO2 uptake and differences in the isotopic ratio of the source CO2. To examine such variability Sphagnum from hummock-hollow complexes from two raised bogs (Florencecourt and Clara) in Ireland was sampled for carbon isotope analysis. Samples were taken from relatively dry hummock tops, margins and submerged hollows. Carbon isotope measurements on cellulose obtained from the Sphagnum revealed a variation of similar to 6 parts per thousand across the bog surface. The mean values for Florencecourt and Clara Bog are -25.6 parts per thousand and -26.5 parts per thousand (PDB) respectively, which is consistent with the sphagna utilizing C-3 photosynthesis to take up atmospheric CO2. The hummock-top species from both sites yielded lighter isotopic values than the hummock-margin species which is consistent with the hummock-top species photosynthesizing under a lower water content and hence lower external diffusion resistance. A large range of partial derivative(13)C values was recorded for the hollow samples. The heaviest isotopic values result from high external resistance to CO2. in some situations, however, the carbon source contains a proportion of recycled CO2, depleted in C-13, which produces relatively light isotopic values. Such large isotopic variation within the wet hollows may lead to erroneous climatic interpretation.