Carbon-isotope variation in Sphagnum from hummock-hollow complexes: implications for Holocene climate reconstruction

Abstract

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 ∼6‰ across the bog surface. The mean values for Florence court and Clara Bog are -25.6‰ and -26.5‰ (PDB) respectively, which is consistent with the sphagna utilizing C3 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 photosyn thesizing under a lower water content and hence lower external diffusion resistance. A large range of ∂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 ¹³C, which produces relatively light isotopic values. Such large isotopic variation within the wet hollows may lead to erroneous climatic interpretation.