Comparing regional and supra-regional transfer functions for palaeohydrological reconstruction from Holocene peatlands

Testate amoebae-based transfer functions are commonly used in peatland palaeoclimate studies. These models have been developed in several regions of the world and are sometimes used for palaeohydrological reconstruction from fossil data in locations where no transfer functions exist. Limitations of this approach may include missing modern analogues and problems associated with site-specific or regional factors in testate amoebae ecology and biogeography. This study presents new testate amoebae-hydrology transfer functions based on data from six peatlands in Northern England. Transfer functions were generated for water table depth and moisture content using weighted averaging tolerance downweighted regression with inverse deshrinking and model performance was assessed using leave-one-out (jacknifing) cross-validation. To examine the robustness of applying transfer functions extra-regionally, we performed a number of spatially independent cross-validation (SICV) tests using contemporary testate amoebae and environmental data from established Northern Ireland and ACCROTELM transfer functions. Inferred water table depths were consistent with observed values in terms of position along the hydrological gradient, however magnitudes varied. We then applied the three independent transfer functions to fossil data from a peatland in Northern England to compare the reconstructions. The results show that the direction of the reconstructions is consistent in terms of wet/dry shifts. However, variation in the magnitudes of the reconstructed water tables is apparent. This probably reflects the sampling regime, including temporal/seasonal effects, and differences in testate amoebae ecology between regions and individual sites.