Holocene 17 (2)

The paper analyses a high-resolution record of reconstructed water-table changes based on testate amoebae analysis in relation to instrumental weather records since AD 1775. Replicate peat records are reconciled by multiple chronological techniques and tuning, and demonstrate that the reconstructions preserve many replicable high-frequency changes. Water-table variability is highly correlated with the total seasonal moisture deficit (precipitation--evapotranspiration, P-E) expressed as the sum of all months with negative P-E. The reconstructed water-table record reflects antecedent periods of 5 or 10 years (maximum 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 temperature and precipitation variability support the hypothesis that the summer deficit is determined by summer precipitation in midlatitude oceanic peatlands and that summer temperature plays a greater but still subsidiary role in higher latitude, continental settings. These relationships apply for all plausible past Holocene climate changes and future twenty-first 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 that combine peat accumulation, mire growth and hydrological processes are required to undertake this task.

The paper describes the LOVE (LOcal Vegetation Estimates) model for estimating local vegetation composition within the relevant source area of pollen. This model quantifies and then subtracts background pollen (ie, pollen coming from beyond the relevant source area) in order to arrive at a quantitative reconstruction of local vegetation. Parameters required for LOVE applications are pollen counts from target sites, the relevant source area of pollen of these sites, pollen productivity estimates and regional vegetation composition within 104--105 km2. Regional vegetation composition is obtained using fossil pollen from large sites (2 ha) with the REVEALS (Regional Estimates of VEgetation Abundance from Large Sites) model, the first step of the Landscape Reconstruction Algorithm (LRA) specifically designed for LOVE applications. POLLSCAPE simulations demonstrate that regional vegetation composition can be used to predict background pollen at and beyond the relevant source area of pollen for given-sized basins; that LOVE with the LRA framework provides a robust and accurate estimate of local vegetation composition much better than vegetation reconstruction using pollen percentages alone; and that, although the relevant source area of pollen is difficult to estimate particularly in past landscapes, a proposed method using backward modelling of LOVE is effective to estimate the relevant source area in landscapes of unknown vegetation patchiness and heterogeneity. Thus, the LRA will also be useful to estimate indirectly changes in spatial structure of past vegetation and landscape caused by natural and anthropogenic forcing. Includes

For interpreting past changes on a regional or global scale, the timings of proxy-inferred events are usually aligned with data from other locations. However it is argued that too often chronological uncertainties are ignored in proxy diagrams and multisite comparisons, making it possible for researchers to fall into the trap of sucking separate events into one illusionary event (or vice versa). In this paper the authors largely solve what is termed the `suck in and smear syndrome' for radiocarbon (14C) dated sequences. In a Bayesian framework, millions of plausible age-models are constructed to quantify the chronological uncertainties within and between proxy archives. The authors test the technique on replicated high-resolution 14C-dated peat cores deposited during the `Little Ice Age' (c. AD 1400--1900), a period characterized by abrupt climate changes and severe 14C calibration problems. Owing to internal variability in proxy data and uncertainties in age-models, these (and possibly many more) archives are not consistent in recording decadal climate change. Through explicit statistical tests of palaeoenvironmental hypotheses, it is possible to move forward to systematic interpretations of proxy data. However, chronological uncertainties of non-annually resolved palaeoclimate records are too large for answering decadal timescale questions.