Late holocene peat stratigraphy and climatic change : a macrofossil investigation from the raised mires of north western Europe

Abstract

The present study combines palaeoecological, radiocarbon and chemical analyses of Holocene peat sequences to yield detailed information on the nature and causal mechanisms responsible for the formation of the main humification change (MHC) in north west European raised mires. A suite of 18 sites between western Ireland and north eastern Poland have been considered. The work has focused directly on the measurement, analysis and interpretation of close-interval macrofossil data. A semi-quantitative technique to assess macrofossil abundance has been developed, facilitating more accurate statements regarding palaeovegetation dynamics. From the palaeoecological data obtained, a series of semi-quantitative curves for the moisture conditions at each site have been calculated using simple weighted averages ordination. A pronounced increase in surface wetness, associated with an expansion of local wet vegetations, may be recognised in the immediate pre-MHC matrix. Short, sharp dry-wet fluctuations characterise each sequence. In the pre-boundary peats of several western maritime profiles, these oscillations are marked by an unusual Sphagnum sect. Acutifolia (cf. S. capillifolium)-Sphagnum sect. Cuspidata (cf. S. cuspidatum) association. It is suggested that the frequency and severity of these dry-wet shifts excluded other competing Sphagna. DECORANA ordination of macrofossil data has given the opportunity to discriminate between the various mechanisms involved in MHC formation. A shift to more maritime conditions appears important in western maritime areas whilst increased continentality is associated with western continental profiles. The absence of an MHC-type feature in north eastern Poland confirms that whilst changes in peat formation were being initiated in western maritime and western continental areas, the climatic shift was of insufficient magnitude to trigger unhumified peat formation in more continental regions. Radiocarbon dates indicate greater diachroneity than was formerly assumed. Based upon these and previously published dates, five periods favouring recurrence surface formation are identified between 4500-500 B.P. Some correspondence with phase shifts recorded elsewhere in Europe is suggested. Antecedent moisture conditions appear to play an important role in determining the date at which mire stability thresholds were transgressed. Hydrological modelling indicates that variations in surface area, the front of effective efflux, topographic situation and effective precipitation modify the climatic `sensitivity' of mire systems. Based upon DECORANA sample scores and available radiocarbon dates for the MHC, a proxy climatic curve has been constructed for the period 4500-500 B.P. in north western Europe.

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