Patterns of mid-Holocene climate change: evidence from the peat archive

With growing concerns over future climate change, the need to understand past climate variability has become an important topic of research. In this thesis, a gap in knowledge of the transition to modern climatic conditions during the mid-Holocene is identified and addressed. Mid-Holocene atmospheric moisture availability has been reconstructed from five raised mires (Kortlandamossen, Tore Hill Moss, Raeburn Flow, Gällseredsmossen, and Misten Bog) located on geographical and climatological gradients across northwest Europe.In this thesis, a new testate amoebae-based palaeohydrological transfer function for south-central Sweden and an improved indicator-weighted Hydroclimatic Index (HCI) were created in order to establish univariate measures of mid-Holocene bog surface wetness (BSW). Together with detrended correspondence analysis, the HCI and testate amoebae transfer function were applied to palaeoecological data from the five bogs, in order to reconstruct climatic variability across northwest Europe from c. 9000 before present (BP) until c. 3500 BP. A series of wet and dry excursions of the bog surfaces were identified on all five bogs. These BSW changes were compared with palaeoclimatic evidence from the wider literature on a local, regional, continental and hemispheric scale. Throughout the early- to mid-Holocene, increased BSW on Misten Bog closely coincided with episodes of reduced SST in the North Atlantic. A similar link was established between SST and BSW on Raeburn Flow and Gällseredsmossen, following the end of the Holocene Thermal Maximum (HTM). The results from the analysis demonstrate that the HTM occurred asynchronously across northwest Europe. Reconstructions from all five bogs, except Misten Bog, showed a prolonged dry phase on the bog surface starting prior to c. 7000 BP. The end of the HTM occurred between c. 5600 BP and c. 4800 BP on all sites except Kortlandamossen, where regional environmental mechanisms delayed the end of the HTM. A climatic boundary at the end of the HTM was detected around 57ºN, with sites south of 57ºN experiencing the termination between c. 5600 BP and c. 5300, while it occurred between c. 4800 BP and c. 4400 BP on the sites north of this latitude. The only period when all five sites displayed similar BSW changes was between c. 4400 BP and c. 4000 BP, a period of global climatic change. A major wetting on the bogs during this time coincided with increased BSW and glacial advances across Europe as well as high lake levels in France and Switzerland

Development of a new pan-European testate amoeba transfer function for reconstructing peatland palaeohydrology

In the decade since the first pan-European testate amoeba-based transfer function for peatland palaeohydrological reconstruction was published, a vast amount of additional data collection has been undertaken by the research community. Here, we expand the pan-European dataset from 128 to 1799 samples, spanning 35° of latitude and 55° of longitude. After the development of a new taxonomic scheme to permit compilation of data from a wide range of contributors and the removal of samples with high pH values, we developed ecological transfer functions using a range of model types and a dataset of ∼1300 samples. We rigorously tested the efficacy of these models using both statistical validation and independent test sets with associated instrumental data. Model performance measured by statistical indicators was comparable to other published models. Comparison to test sets showed that taxonomic resolution did not impair model performance and that the new pan-European model can therefore be used as an effective tool for palaeohydrological reconstruction. Our results question the efficacy of relying on statistical validation of transfer functions alone and support a multi-faceted approach to the assessment of new models. We substantiated recent advice that model outputs should be standardised and presented as residual values in order to focus interpretation on secure directional shifts, avoiding potentially inaccurate conclusions relating to specific water-table depths. The extent and diversity of the dataset highlighted that, at the taxonomic resolution applied, a majority of taxa had broad geographic distributions, though some morphotypes appeared to have restricted ranges

Multi-proxy evidence of a mid-Holocene shift in the climatic system of Maritime Canada at ca. 6.8 ka BP

Persistent cyclical patterns of centennial- to millennial-scale changes in key climate drivers, such as North Atlantic Deep Water formation and the Atlantic Meridional Overturning Circulation, have frequently been identified in Holocene palaeorecords of the mid- and eastern-Atlantic region. Given its proximity to the major discharge routes of the Laurentide Ice Sheet and repeated meltwater discharge of varying magnitude, the western Atlantic region, however, experienced disruptions to such cyclical climate patterns during the early Holocene. Here we present evidence of a shift in the climate system of Maritime Canada coinciding with the end of meltwater discharge at ca. 6.8 ka BP. Petite Bog (45°8'43"N, 63°56'36"W), a large ombrotrophic plateau bog in central Nova Scotia, was cored and analysed as part of the PRECIP (Palaeo- REconstructions of ocean-atmosphere Coupling In Peat) project, which aims to reconstruct the spatial-temporal pattern of moisture balance changes on the eastern seaboard of North America. A multi-proxy approach using stable isotope (C, H, and O) from Sphagnum cellulose, testate amoebae and plant macrofossil analysis was used to reconstruct palaeoclimatic changes at Petite Bog. Peat accumulation of ca. 1 yr.mm-1 was consistent and near-linear throughout most of the Holocene with the exception of a clear 1,400 year long slowing to ca. 10 yr.mm-1 following the ‘8.2 ka BP cold event’. A robust chronology, constrained by 34 14C radiocarbon dates, 3 typed tephra layers and an uncut surface, provides a suitable platform for palaeoclimatic reconstructions and multi-centennial to millennial-scale time series analysis. Results show a 325-year cyclicity in bog surface wetness (BSW), and consequently atmospheric moisture availability, following the mid-Holocene slow down in accumulation. This multi-centennial cycle is not present prior to ca. 6.8 ka BP, suggesting that the end of the meltwater influence marks a distinct shift in the regional climatic regime. Underlying the entire Holocene record is a cycle with a periodicity of ca. 2,800 years. However, no evidence of the well-documented millennial-scale climate cycles was found, suggesting that Petite Bog was either insensitive to or outwith the reach of the forcing factor responsible for such climate variability in the North Atlantic region. This stands in stark contrast to the millennial-scale climate cycle identified in BSW reconstructions at Nordan’s Pond Bog, Newfoundland. Findings from Petite Bog show a clear shift to a multi-centennial cyclicity in the regional climate system following the end of meltwater influence and highlight the complexity of Holocene climate in Atlantic Canada

Statistical testing of a new testate amoeba-based transfer function for water-table depth reconstruction on ombrotrophic peatlands in north-eastern Canada and Maine, United States

Proxy reconstructions of climatic parameters developed using transfer functions are central to the testing of many palaeoclimatic hypotheses on Holocene timescales. However, recent work shows that the mathematical models underpinning many existing transfer functions are susceptible to spatial autocorrelation, clustered training set design and the uneven sampling of environmental gradients. This may result in over-optimistic performance statistics or, in extreme cases, a lack of predictive power. A new testate amoeba-based transfer function is presented that fully incorporates the new recommended statistical tests to address these issues. Leave-one-out cross-validation, the most commonly applied method in recent studies to assess model performance, produced over-optimistic performance statistics for all models tested. However, the preferred model, developed using weighted averaging with tolerance downweighting, retained a predictive capacity equivalent to other published models even when less optimistic performance statistics were chosen. Application of the new statistical tests in the development of transfer functions provides a more thorough assessment of performance and greater confidence in reconstructions based on them. Only when the wider research community have sufficient confidence in transfer function-based proxy reconstructions will they be commonly used in data comparison and palaeoclimate modelling studies of broader scientific relevance