Climatic Controls on Ombrotrophic Peatland Surface Wetness

Interpretation of proxy-climate records depends on a thorough understanding of the proxy-climate relationship. Hydroclimatic reconstructions are often loosely interpreted as `water balance', but it is not always clear whether these are forced primarily by temperature or precipitation, or whether a seasonal or annual signal is represented. In Europe, peatland surface wetness records have been interpreted as reflecting changes between cool and/or wet conditions and warm and/or dry conditions and generalised patterns of hydrological change are now becoming better established from multi-site and multi-proxy compilations of peatland data. However, to understand fully the nature of past hydrological variability, and to begin compilations of multi- archive data over larger geographical scales, it is necessary to determine the climatic drivers of decadal to centennial scale hydrological change on peatlands. This paper determines the relationship between peatland water balance and climate variables by; 1) analysing a high resolution record of reconstructed water table changes based on testate amoebae analysis in relation to monthly weather data since AD 1775 in northern England, 2) modelling responses of water deficit to precipitation and temperature changes at different geographical locations. Replicate peat records from northern England are reconciled by multiple chronological techniques and tuning, and demonstrate that the reconstructions preserve many reproducible high frequency changes. Monthly potential evapotranspiration is estimated from temperature data using the Thornthwaite equation. Water table variability is highly correlated with the total seasonal moisture deficit (precipitation-evapotranspiration) expressed as the sum of all months with negative P-E. The reconstructed water table record reflects antecedent periods of 5 or 10 years (r2 = 52.4 %) and proxy bog surface wetness records can therefore be interpreted as reflecting the length and intensity of the summer water deficit period. Response surfaces of the summer deficit in relation to a range of temperature and precipitation changes support the hypothesis that the summer deficit is determined by summer precipitation in mid-latitude oceanic peatlands and that summer temperature plays a greater but still subsidiary role in higher latitude and continental settings. These relationships apply for all plausible past Holocene climate changes and future 21st century climate scenarios. Non-linear responses to longer-term climate states prevent the direct application of a calibration of the reconstructed water table records to infer quantitative estimates of climate variables. Models which combine peat accumulation, mire growth and hydrological processes are required to undertake this task.