The disappearance of Sphagnum imbricatum from Butterburn Flow, UK

The disappearance of the previously abundant moss species Sphagnum imbricatum has been investigated at Butterburn Flow, northern England, using organic geochemical, elemental, macrofossil, pollen and testate amoebae analyses. Variations in the assemblage of peat-forming plants were tracked using the macrofossil distributions as well as the relative chain lengths of n-alkanes and concentrations of 5-n-alkylresorcinols and triterpenols. No significant changes to the vegetation assemblage could be detected prior to the loss of S. imbricatum. Variations in water depth were reconstructed using a testate amoebae transfer function and inferred qualitatively using bulk elemental composition and biomarkers for changing redox conditions in the bog subsurface: the degree of isomerization in the C-31 hopanes, and the concentrations of bishomohopanol and archaeol. Pollen analysis reconstructed the landscape surrounding the mire and revealed evidence for human disturbance. The results suggest that bog surface wetness increased with the transition from Sphagnum imbricatum to Sphagnum magellanicum, but the increase was not large and S. imbricatum had previously survived similar periods of wetness. However, the loss of S. imbricatum coincides with increasing human disturbance surrounding the bog, which may have altered nutrient inputs to the bog surface from agriculturally derived dust, to the detriment of S. imbricatum but to the benefit of S. magellanicum and Eriophorum vaginatum. It is proposed here that the stresses imposed by the combination of changing nutrient inputs and a rapidly rising water-table drove the disappearance of S. imbricatum from Butterburn Flow at c. cal. AD 1300.