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1 Palaeoclimatic records from peat bogs Jeff Blackford Peat-bog stratigraphy, from blanket mires and raised bogs, has been used since the last century to make inferences about past climate. Age depth profiles show that 1 mm of peat can represent between five and 5 years of accumulation 1, with typical values for the past 2 years of 1 2 years per 1 mm. Vertical sampling intervals of 1 mm or fewer can therefore provide data on the scale of decades. Well decomposed peat layers, especially those containing tree remains, have been considered indicative of drier, warmer climates, whereas lighter-coloured, Sphagnum-rich horizons have been associated with wetter and/or cooler conditions 1,2. This basic division of peat types was used to subdivide the postglacial period in northwestern Europe (approximately the past 1 years) into climatic stratigraphic divisions (the Blytt Sernander scheme 2,3 ), which have since been shown to be overgeneralized and imprecise 3. Studies of recurrence surfaces (apparently synchronous stratigraphic changes marking wetter conditions at mire sites) were correlated across northwestern Europe, resulting in the identification of significant key dates of inferred climatic change; some of these occurrences have been verified by more recent, radiocarbon-dated studies 1,2. Most current research aims to derive continuous, proxy climate data for the whole peat profile. However, several other factors in addition to climate are involved in altering the surface wetness of peat bogs, including vegetative succession and human-induced changes through drainage, peat cutting, vegetation burning, airborne pollution and grazing animals. Despite these potential problems, the most likely cause of wide-ranging and apparently synchronous wet-shifts, as have been demonstrated for the European range of ombrotrophic mires, is a series of regional-scale climatic changes 4. Therefore, there is evidence that palaeoenvironmental evidence from watershedding mires can provide a proxy palaeoclimatic record of significant replicability and resolution. Detecting palaeoclimatic events in peat-bog records The fundamental principles used in previous peat palaeoclimatic studies remain at the centre of more recent work 1. Assumptions that underpin peat-based climatic reconstructions include: (1) Mire vegetation assemblages respond to changes in the water table, which is itself responsive to changing climate. (2) The subfossil remains (preserved but not mineralized) of vegetation, preserved in mires, are an accurate record of the original vegetation cover at the time of peat deposition. (3) More decomposition occurs when the mire surface is dry, thus resulting in more humified The palaeoclimatic record for the past 6 years, implemented from peat-bog stratigraphy, has been limited by imprecision in dating and climatic interpretation. Recently, dating problems have been addressed by wigglematched radiocarbon dates and by volcanic ash layers, promising much tighter correlation between records from different regions. Recent research shows key dates of significant climatic change and tentative evidence for periodicity. Application of time-series analysis, generalized linear modelling and transfer function models to the proxy climate data show how improved climatic reconstructions can be obtained. Peatderived palaeoclimatic data might explain, as well as describe, climatic changes over timescales of years. Jeff Blackford is at the Dept of Geography, Queen Mary and Westfield College, University of London, Mile End Road, London, UK E1 4NS (j.j.blackford@qmw.ac.uk). peat, a darker colour and fewer identifiable remains than in peat that accumulates when the mire surface is relatively wet. (4) Reliable age estimates of peat profiles, and each point within a profile, can be obtained. Based on these assumptions, research published in recent years has led to three significant developments. First, the use of climate-response models and spectral analysis tools to analyse continuous proxy climate data sets. Second, the identification of welldated key horizons showing significant climatic change. Third, the application of peat data in searches for the causes of Holocene (1 years ago to the present) climatic variability. The peat-bog archive The peat-bog archive includes pollen and other microfossils that have been used for palaeoecological and palaeoclimatic investigations, but the focus here is those studies that have used the properties of the peat fabric. Two main approaches have been used: the analysis of plant macrofossil remains, particularly of Sphagna, and measurement of the degree of peat decomposition. In most raised bogs and some blanket peats, Sphagnum moss species are the main peat-formers, occupying a range of water table-related habitats. Identification and quantification of the species in subfossil samples have been used to show changes in mire conditions. The proportion of peat composed of Sphagnum and the relative abundance of different species have been used to show wetter phases [marked by Sphagnum section (S. s.) Cuspidata and Subsecunda] and drier phases [shown by S. s. Acutifolia, ericaceous peat, monocotyledonous peat and decomposed, unidentifiable organic matter 5 (Box 1)]. However, there are times when Sphagnum remains are either absent or unidentifiable because of decomposition and where competition between species, rather than climatic factors, causes the assemblage to change. This is particularly true of Sphagnum imbricatum, once a major constituent of mires in places where it is now absent 6. Palaeoecological records from the UK show that in some situations S. imbricatum has been replaced by S. magellanicum or S. papillosum, in a relationship possibly independent of climate change, thus leaving S. imbricatum in a much diminished range on hummock tops 6. The interpretation is complicated further by the range of habitats that individual Sphagnum species can occupy 7. The degree of peat humification (the decomposition of plant material into amorphous humus) has been used widely to infer changes in the peat surface wetness, either by recording the field stratigraphy in terms of colour and preservation, or by a laboratory-based colorimetric method 4,8. Raised and blanket mires across the European TREE vol. 15, no. 5 May //$ see front matter 2 Elsevier Science Ltd. All rights reserved. PII: S ()

2 Box 1. Sphagnum remains as palaeohydrological indicators There are between 15 and 2 species of the genus Sphagnum 37. Sphagna are most abundant where acidic, solute-poor water prevails, typically on peat bogs, where each species occupies a habitat range determined largely by the depth of the water table and nutrient availability 7,37. For example, S. cuspidatum is most commonly found in permanently waterlogged (water table at the surface) pools and hollows, S. papillosum is most common in lawn environments and S. capillifolium is more abundant on drier hummocks 37. However, range overlaps and competition between species with similar tolerances can cause problems of interpretation. The figure (a) shows the comparative ranges of three types of Sphagnum commonly used in the subfossil record. Identification of the peat-forming material can present a problem of taxonomic resolution. For example, members of the section Acutifolia, which includes S. capillifolium, are usually identified only to section level (S. s. Acutifolia) on the basis of leaf form 38. (a) Low Water table High Low Sphagnum capillifolium var. rubellum S. cuspidatum S. magellanicum Water table Nutrient status High Where a range of Sphagnum remains are present in peat, the relative abundance of different species or groups can be used to estimate the relative depth of the prevailing water table. In addition, the presence or absence of Sphagna of any kind, relative to the abundance of plants indicative of drier conditions, such as Ericaceae, can be used as an indicator of relative surface wetness. This is illustrated by the data from Bolton Fell Moss in Cumbria, UK (b). The diagram represents a 6 cm peat section dominated initially by S. magellanicum, a mid-range species in terms of water-table preference. S. s. Cuspidata, showing wetter conditions, dominates between 2 and 35 cm, and S. s. Acutifolia peaks showing a drier phase in the top 1 cm (Ref. 38). Figures reproduced, with permission, from Refs 7 and 38. Depth (cm) (Online: Fig. I) (b) (Online: Fig. II) Ericales Monocots Id. Sphagnum S. s. Acutifolia Percentage macrofossils S. magellanicum S. s. Cuspidata range have been investigated, north south from northern Norway to England and Ireland, and east west from Poland to Ireland. Colorimetric analyses have been shown to be replicatable, applicable to all peat types and show more variability than seen in the visible stratigraphic record, although problems remain regarding the effect of species change on the rate of decomposition and on the optical density of the humic extract 9. In theory, humification data indicate changes in the time elapsed between the death of plant matter and their remains reaching the anaerobic catotelm (the body of peat beneath the water table). Thus, these data represent a proxy for the position of the water table at the time of deposition although this relationship remains unquantified. The range of palaeoenvironmental indicators used has broadened to include an expanded range of micro- and macrofossils, and peat chemical properties. The presence of spermatophores of Copepoda can indicate relatively wet phases of peat development 8, whereas remains of the moss Racomitrium lanuginosum have been shown to indicate drier conditions 1. Work on R. lanuginosum is significant because previously it has been assumed that dry phases are difficult to distinguish because of the natural tendency of bog-forming plants to accumulate above the water table and to leave a palaeoecological signal of more humified, drier-habitat species that is indistinguishable from that caused by a change to less humid climatic conditions 2. Fungal remains, abundant in peat, have been shown to have palaeohydrological affinities 11. Testate amoebae (amoebae partially enclosed in a shell or test) remains are also recoverable from peat samples 12 and occupy distinct ranges of water-table positions 13. Values of D/H and 18 O/ 16 O (the difference from the standard ratio of these isotopes) have been measured from peat samples 14,15, resulting in the first quantitative climatic reconstruction of mid-holocene changes from peat-bog records 15. However, work on Carbury Bog in Ireland proved less conclusive 16 and at present it is thought to be impossible to separate the climate signal from the effects of changing species composition 14,16. Attempts to distinguish palaeoenvironments using plant lipid biomarkers, including measurements of 13 C from isolated components, have produced mixed results 17. A limited agreement between lipid biomarkers and macrofossils was noted, but the variations cannot yet be used to assess palaeoclimatic changes. Peat deposits from equatorial Africa have been used to show changes in 13 C, interpreted as showing the relative abundance of C 3 and C 4 plant species, possibly caused by changes in moisture availability and atmospheric CO 2 concentrations 18. Limiting factors in mire-based palaeoclimatology Currently, there are several problems that limit the interpretation of otherwise high-quality data. First, there is the crucial problem of dating, which has relied almost entirely on conventional radiocarbon determinations with age-ranges, within two standard deviations (SD), of between 2 and 5 years 19. This level of precision is inadequate for accurate correlation of cores, estimation of rates of change, linkage with other proxy climatic records or the analysis of causal mechanisms. Linear interpolations, assuming a constant time depth relationship between dated points, have been the most common, although an exponential age depth profile has been predicted, caused by continued decomposition in the catotelm 2. Polynomial best-fit curves have also been used to interpolate between fixed points 21, but age estimates for events that are not directly dated are, inevitably, more error-prone than the estimates for dated horizons. The problems are exacerbated by the effects of calibrating the age estimates to produce calendar years, essential for comparison with historical climate records or dendro-climatological sequences and for estimating rates of change. Calibration can increase the error width and, in some cases, a given radiocarbon age can be obtained from material of more than 194 TREE vol. 15, no. 5 May 2

3 one calendar age, thus leading to further uncertainty. The exact pretreatment used has also been shown to significantly influence radiocarbon dates from peat 22. Furthermore, recent high-precision dating has shown an error resulting from a reservoir effect altering 14 C concentrations in peat 23, leading to a potential 2-year (approximate) overestimate of calendar age. Therefore, where studies have suggested a correlation between mire responses in different areas the precision of that correlation cannot always be verified. Improving age estimates Two significant advances have shown how the age-estimate problem might be addressed wiggle-matched radiocarbon dates and tephrochronology. For the wiggle-matched method, high-precision (one SD 3 years) radiocarbon dates are taken in a sequence through the horizon of interest, including samples above and below. The results of these determinations are then cross-matched with the decadal 14 C curve obtained from tree-ring sequences, resulting, where a match is possible, in a precise calendar-age estimate 23,24. Although commonplace in volcanic regions, including New Zealand 25 and Iceland 26, recent development of microscopic tephra (volcanic ash) stratigraphy for regions distant from the volcanic source has enabled tephrochronology (dating and correlation of sediment sequences using tephra layers) to be extended to mainland European mires 27. The main advantages are 28 : (1) precise correlation of fixed points in peat sequences over a spatial range of km; (2) precise dating of points within the peat sequence over the past millennium; and (3) accurate and precise dating of fixed points where tephra is identified, and where accurate dates for that tephra have been obtained 29. For example, the Icelandic eruptions of Hekla in AD 114 and AD 151 are precisely dated from historical records 26, and their ashes occur in the peats of northern Britain and Ireland 27,29. Even eruptions of relatively minor magnitude have the potential to leave ash horizons in distant mires 3, and the number of datable points varies from over 1 close to the volcanic region, to between two and 15 at distances in the order of magnitude of 1 3 km. Even a small number of precise correlation and dating points greatly improves the interpretative value of a peat-based palaeoclimatic reconstruction (Fig. 1). Currently, there is no better way of correlating peat cores and sections, and recent work has begun to incorporate tephra records alongside radiocarbon chronology 21. Correlation with ice-core palaeoclimatic records from Greenland, shown to have ice-acidity peaks attributable to Icelandic (and other) eruptions 31, is also possible. However, tephra records alone can only be a partial solution to the dating problem facing peat climate studies. This is because the dated horizons might not be those of greatest interest and thus gaps between them would necessarily have to be interpolated. Additionally, the tephra fallout from any given eruption is patchy and some mires are too distant from any eruptive source. Even within a single mire, tephra can be found in one core but not within others. The techniques of wiggle-matched radiocarbon dates and tephrochronology have been used in combination to obtain accurate and precise age estimates for prehistoric tephras, and hence for peat layers 29. Improving climatic reconstructions The second major problem currently associated with proxy climate records from mires is the lack of climatological precision. Ideally, climatic reconstructions should provide temperature or rainfall data or, in the case of peat records, quantified water-table reconstructions. However, at present this is not the case, and rather vague references to wetter and cooler conditions are often used. There is a need for more exact reconstruction of past climates, given current debates concerning the present global-scale climate change. Recently, several ways of improving the climatic interpretation of the peat record have been attempted. One development has been the use of a climateresponse model 5. Macrofossil data, mainly Sphagnum frequencies, were first compared with the best available century-scale climatic data for the period AD Using this series (wetness or dryness and mildness or severity indices), a multinomial logit model, a specific type of generalized linear model, was fitted to the macrofossil and climate data, and was then used to estimate the climatic indices for longer timescales. However, the resulting data set remains imprecise in terms of temperature or rainfall reconstruction 5 (Fig. 2). Recent work on testate amoebae has shown that their hydrological tendencies can make them useful palaeoenvironmental indicators 13,32. Their relative abundance has been used to create transfer functions based on modern distributions, quantitatively linking species counts in subfossil form from peat horizons to the height of the water table 32. Shetland and Orkney Caithness Sutherland W. Isles Highlands Grampian Synthesis Loch Station Hill Glen Na Kebister Hoy Slethill Leir Altnabreac Beiste Lairg H151 Loch Portain B AD151 c. 45 BP Loch Portain Beinn Eighe Beinn Eighe Glen Garry New Pitsligo H151 (AD 151) Loch Portain B (c. 45 BP) Glen Garry c. 21 BP Glen Garry (c. 45 BP) Hekla-4 Kebister c. 38 BP Kebister (c. 36 BP) Hekla-4 (c. 38 BP) Hoy Hoy (c. 53 BP) Lairg A + B Lairg A (c. 6 BP) Lairg B Fig. 1. Summary of the tephra (volcanic ash) deposits identified from north Scotland, UK, as known in Similar connections have been made in Ireland and in several tephras recorded from Scandinavia. Some tephras, for example Hekla-4, appear to span a large part of the range of northwestern European mires, whereas others, for example the Hoy tephra, have a more limited range. Reproduced, with permission, from Ref. 27. TREE vol. 15, no. 5 May 2 195

4 (a) 1 Time [Calibrated (cal.) BP] % Humification (b) Standard deviation units of detrended correspondence analysis scores Calibrated (cal.) AD Calibrated (cal.) BC Frequency Calibrated (cal.) years BP Cycles per sampling interval Spectral density Ordinate Fig. 2. Time series and time-series analyses of whole-profile, peat-derived proxy records. (a) Peat humification data from Talla Moss in southern Scotland. A smoothed curve below shows century-scale oscillations. Spectral analysis shows a peak in spectral density (measured as variance for each frequency) at.12 cycles per sampling interval, a periodicity of approximately 21 years. Reproduced, with permission, from Ref. 21. (b) Time-series analysis of detrended proxy climate records (wetness dryness index) from Bolton Fell Moss in northern England. Spectral analysis shows a clear peak at.6 cycles per sampling interval, a periodicity of approximately 8 years. The y-axis ordinate is a measure of the proportion of variance allocated to each frequency range. Reproduced, with permission, from Ref. 5. Although the modern distribution studies of testate amoebae are limited in spatial range and timescale, the transfer function approach could be the best way to convert subfossil peat data into more precise climatic data, especially if it can also be applied to macrofossils, degree of decomposition and microfossil data. Evidence for significant climatic change Several studies have used peat-based climatic reconstructions to highlight evidence for climatic changes at particular times. Evidence from western Scotland appears to show a shift to wetter and colder conditions, as shown by peat humification records and microfossil data, at BP (Ref. 8) (Box 2). Another change to cooler and wetter climates has been inferred for 265 BP (85 76 BC) 14. The Sphagnum macrofossil record from a raised mire in The Netherlands showed a change, also evident in the visible stratigraphy, from S. s. Acutifolia, first to S. cuspidatum and S. papillosum, before dominance by S. imbricatum. This change coincides with 18 O and D/H fluctuations, and a fall in pollen percentages of Corylus avellana. This suite of changes has also been linked to a 14 C rise in tree-ring records at this time (Box 2), leading to the interpretation that solar variability caused the recorded changes 14. A further change to cooler and/or wetter conditions has been inferred for the period around 14 BP (calibrated age AD 55 75), when blanket peats from Ireland, Wales and northern England became wetter 33 (Box 2). Several of these changes coincide within the limits of available age estimates with the original recurrence surfaces recorded by earlier authors, including some studies that pre-date radiocarbon dating. For example, the BP change might be equivalent to Granlund s recurrence surface no. 5 (Ref. 2), and the 265 BP change is close to the sub-boreal sub-atlantic transition of the Blytt Sernander scheme 3. The 14 BP change can be correlated approximately with changes in mires across Europe 33. However, there are also inconsistencies. Peat humification 196 TREE vol. 15, no. 5 May 2

5 Box 2. Peat-derived evidence of three episodes of climatic change (a) Radiocarbon age (years BP) Drier Smoothed data Raw data Wetter Percentage transmission of light through NaOH extract (a) Blanket mire humification data, indicative of the degree of peat decomposition, show a change to wetter conditions from two upland blanket mire sites [Harold s Bog (left panel) and Wood Moss (right panel)] in England, UK at approximately 14 BP. The data are produced by first extracting the products of peat decomposition using sodium hydroxide and then colorimetric analysis of the resulting solution. Changes to a higher percentage transmission are interpreted as changes to wetter surface conditions, thus resulting in lower rates of decomposition. Reproduced, with permission, from Ref. 33. (c) Calendar years BP Drier Wetter (Online: Fig. III) (Online: Fig. I) (b) Percentage by volume δ 14 C (per mil) Calendar years BC Sphagnum imbricatum Sphagnum papillosum Sphagnum cuspidatum Sphagnum s. Acutifolia (Online: Fig. II) (b) Sphagnum macrofossil data and 14 C (the relative abundance of 14 C compared with mean levels), from the raised bog Engbertsdijksveen (The Netherlands) for the period 1 to 5 BC. The timescale is based on 14 C wiggle matching, in which a sequence of high-precision (one SD 3 years) radiocarbon dates are taken. The results are then cross-matched with the decadal 14 C curve obtained from tree-ring sequences, resulting in a more precise calendar-age estimate than conventional 14 C dating 23,24. The increase in 14 C at 8 cal. (calibrated) BC coincides with a change first to Sphagnum papillosum and S. cuspidatum, and then to S. imbricatum, indicative of a wetter peat surface than S. s. (section) Acutifolia, which dominated before 8 BC. The 14 C signal is thought to be dependent on variations in solar output, therefore this correlation might demonstrate a link between solar variability and climate change 14. Reproduced, with permission, from Ref. 14. (c) The peat humification curve, combined from three sites in northern Scotland, is produced using 25-year interval units and based on radiocarbon correlation. The highlighted change at BP coincides with changes in the microfossil record indicative of wetter conditions. Reproduced, with permission, from Ref. 8. data 21 for the Talla Moss in Scotland (Fig. 2a) show no clear changes between AD nor do the most recently published macrofossil data from Bolton Fell Moss in northern England (UK) 5 (Fig. 2b), although earlier work from the same bog showed wet-shifts at approximately 14 radiocarbon years BP (Ref. 34). Both of these studies show significant perturbations at the approximate time of the inferred climatic change at 265 BP and the Bolton Fell Moss data also show changes around 35 BP. Evidence for long-term climate changes and periodicity Long-term trends and cycles over the mid-late Holocene period have been examined by analysing single profiles in detail (Fig. 2). A frequency of approximately 26 years was shown from humification data from a Danish raised mire over the past 55 years 35. The period was estimated by comparing likely intervals a 26-year frequency showed a closer correspondence to the times of change than longer TREE vol. 15, no. 5 May 2 197

6 or shorter intervals. Raised mire macrofossil data from northern England showed a possible 8-year signal, after the data had been summarized using detrended correspondence analysis (Fig. 2). Time-series analysis was conducted using Fourier transform spectral analysis techniques 5, assuming a linear growth rate for the majority of the peat profile the 8-year frequency was tentatively described as ocean-driven. Peat humification data from Talla Moss blanket mire were used to generate a 25-year interval time series, based on radiocarbon dates 21. Spectral analysis of the data showed a frequency of approximately 21 years (Fig. 2a). A change to wetter conditions at AD 141 has been correlated with the onset of the Sporer minimum a period of reduced sun-spot activity 21 a link previously suggested from studies of Irish bogs 36. Prospects Peat-derived proxy climate data have shown significant climatic changes and evidence of periodicity over the mid-late Holocene period. However, there has also been considerable inconsistency between studies. When comparing the records from different sites and from different regions, some variation would be expected because of regional differences in climate and climatic variability, and because of different sensitivities of peat-forming systems. An unknown proportion of the inconsistency might be caused by remaining errors in correlation and dating, and because of the different techniques applied in each study published so far. If these inconsistencies can be solved, then mire-based palaeoclimatology has the potential to explain, as well as describe, significant preindustrial climatic fluctuations. Recent improvements in the range of available data sources and the treatment of the data obtained suggest that peat bogs will contribute accurate palaeoclimatic information to the debates surrounding recent and current climatic variability. References 1 Barber, K.E. (1982) Peat bog stratigraphy as a proxy climate record. In Climatic Change in Later Prehistory (Harding, A., ed.), pp , Edinburgh University Press 2 Barber, K.E. (1985) Peat stratigraphy and climatic changes: some speculations. In The Climatic Scene: Essays in Honour of Gordon Manley (Tooley, M.J. and Sheail, G.M., eds), pp , Allen & Unwin 3 Smith, A.G. (1981) The Neolithic. In The Environment in British Prehistory (Simmons, I.G. and Tooley, M.J., eds), pp , Duckworth 4 Nilssen, E. and Vorren, K.D. (1991) Peat humification and climate history. Norsk Geologisk Tiddskrift 71, Barber, K.E. et al. (1994) A sensitive high-resolution record of late- Holocene climatic change from a raised bog in northern England. Holocene 4, Stoneman, R. et al. (1993) Present and past ecology of Sphagnum imbricatum and its significance in raised-peat-climate modelling. Quat. News. 7, Daniels, R.E. and Eddy, A. (199) A Handbook of European Sphagna, Natural Environment Research Council 8 Anderson, D.E. et al. (1998) Evidence for abrupt climate change in northern Scotland between 3,9 and 3,5 calendar years BP. Holocene 8, Blackford, J.J. and Chambers, F.M. (1993) Determining the degree of peat decomposition for peat-based palaeoclimatic studies. Int. Peat J. 5, Tallis, J.H. (1995) Climate and erosion signals in British blanket peats: the significance of Racomitrium lanuginosum remains. J. Ecol. 83, Van Geel, B. et al. (1995) The indicator value of fossil fungal remains, illustrated by the palaeoecological record of a Late Eemian/Early Weichselian deposit in The Netherlands. Meded. Rijks Geol. Dienst. 52, Hendon, D. and Charman, D.J. (1997) The preparation of testate amoebae (Protozoa: Rhizopoda) samples from peat. Holocene 7, Charman, D.J. and Warner, B.G. (1992) Relationship between testate amoebae (Protozoa: Rhizopoda) and microenvironmental parameters on a forested peatland in northeastern Ontario. Can. J. Zool. 7, Van Geel, B. et al. (1996) Archaeological and palaeoecological indications of an abrupt climate change in The Netherlands, and evidence for climatological teleconnections around 265 BP. J. Quat. Sci. 11, Brenninkmeijer, C.A.M. et al. (1982) Variations in the D/H and 18 O/ 16 O ratios in cellulose extracted from a peat bog core. Earth Planet. Sci. Lett. 61, Van Geel, B. and Middeldorp, A.A. (1988) Vegetation history of Carbury Bog (Co. Kildare, Ireland) during the last 85 years and a test of the temperature indicator value of 2 H/ 1 H measurements of peat samples in relation to historical sources and meteorological data. New Phytol. 19, Ficken, K.J. et al. (1998) Lipid biomarker, 13 C and plant macrofossil stratigraphy of a Scottish montane peat bog over the last two millennia. Org. Geochem. 28, Ancour, A-M. et al. (1994) Late Quaternary biomass changes from 13 C measurements in a highland peat bog from equatorial Africa (Burundi). Quat. Res. 41, Pilcher, J.R. (1993) Radiocarbon dating and the palynologist: a realistic approach towards precision and accuracy. In Climate Change and Human Impact on the Landscape (Chambers, F.M., ed.), pp , Chapman & Hall 2 Clymo, R.S. (1984) The limits to peat bog growth. Philos. Trans. R. Soc. London Ser. B 33, Chambers, F.M. et al. (1997) A 55-year proxy-climate and vegetation record from blanket mire at Talla Moss, Borders, Scotland. Holocene 7, Shore, J. et al. (1995) Problems encountered with the 14 C dating of peat. Quat. Sci. Rev. 14, Kilian, M.R. et al. (1995) Dating raised bogs: new aspects of AMS 14 C wiggle matching, a reservoir effect and climatic change. Quat. Sci. Rev. 14, Van Geel, B. and Mook, W.G. (1989) High-resolution 14 C dating of organic deposits using natural atmospheric 14 C variations. Radiocarbon 31, Newnham, R.M. et al. (1995) Holocene vegetation, climate and history of a raised bog complex, northern New Zealand based on palynology, plant macrofossils and tephrochronology. Holocene 5, Thorarinsson, S. (1981) Greetings from Iceland; ashfalls and volcanic aerosols in Scandinavia. Geogr. Ann. 63A, Dugmore, A.J. et al. (1995) Seven tephra isochrones in Scotland. Holocene 5, Blackford, J.J. (1997) Volcanic ash in peat. In Conserving Peatlands (Parkyn, L. et al., eds), pp , CAB International 29 Pilcher, J.R. et al. (1996) An outline tephrochronology for the Holocene of the north of Ireland. J. Quat. Sci. 11, Dugmore, A.J. et al. (1996) Long-distance marker horizons from smallscale eruptions: British tephra deposits from the AD 151 eruption of Hekla, Iceland. J. Quat. Sci. 11, Zeilinski, G.A. et al. (1994) Record of volcanism since 7, BC from the GISP2 Greenland ice core and implications for the volcano-climate system. Science 264, Warner, B.G. and Charman, D.J. (1994) Holocene soil moisture changes on a peatland in northwestern Ontario based on fossil testate amoebae (Protozoa) analysis. Boreas 23, Blackford, J.J. and Chambers, F.M. (1991) Proxy records of climate from blanket peat: evidence for a dark age (14 BP) climatic deterioration in the British Isles. Holocene 1, Barber, K.E. (1981) Peat Stratigraphy and Climatic Change: A Palaeoecological Test of the Theory of Cyclic Peat Bog Regeneration, Balkema 35 Aaby, B. (1976) Cyclic climatic variations in climate over the last 5,5 years reflected in raised bogs. Nature 263, Blackford, J.J. and Chambers, F.M. (1995) Proxy-climate record for the last 1, years from Irish blanket peat and a possible link to solar variability. Earth Planet. Sci. Lett. 13, Clymo, R.S. (1997) The roles of Sphagnum in peatlands. In Conserving Peatlands (Parkyn, L. et al., eds), pp , CAB International 38 Barber, K.E. et al. (1998) Replicability and variability of the recent macrofossil and proxy-climate record from raised bogs: field stratigraphy and macrofossil data from Bolton Fell Moss and Walton Moss, Cumbria, England. J. Quat. Sci. 13, TREE vol. 15, no. 5 May 2