P
eat-bog stratigraphy, from blanket mires and raised bogs, has been used since the last century to make in-ferences about past climate. Age– depth profiles show that 10 mm of peat can represent between five and 50 years of accumulation1,with typical values for the past 2000 years of 10–20 years per 10 mm. Vertical sampling intervals of 10 mm or fewer can therefore provide data on the scale of decades. Well decomposed peat layers, especially those containing tree remains, have been consid-ered indicative of drier, warmer cli-mates, whereas lighter-coloured, Sphagnum-rich horizons have been associated with wetter and/or cooler conditions1,2. This basic
division of peat types was used to subdivide the postglacial period in northwestern Europe (approxi-mately the past 10 000 years) into climatic–stratigraphic
divi-sions (the Blytt–Sernander scheme2,3), which have since
been shown to be overgeneralized and imprecise3. Studies
of recurrence surfaces (apparently synchronous strati-graphic changes marking wetter conditions at mire sites) were correlated across northwestern Europe, resulting in the identification of significant key dates of inferred cli-matic change; some of these occurrences have been veri-fied by more recent, radiocarbon-dated studies1,2. 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 burn-ing, airborne pollution and grazing animals. Despite these potential problems, the most likely cause of wide-ranging and apparently synchronous wet-shifts, as have been dem-onstrated for the European range of ombrotrophic mires, is a series of regional-scale climatic changes4. 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 palaeo-climatic studies remain at the centre of more recent work1.
Assumptions that underpin peat-based climatic reconstruc-tions 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
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 cli-mate-response models and spec-tral analysis tools to analyse con-tinuous proxy climate data sets. Second, the identification of well-dated key horizons showing sig-nificant climatic change. Third, the application of peat data in searches for the causes of Holo-cene (10 000 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 palaeoecologi-cal 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 quantifi-cation 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 Sphagnumsection (S. s.) Cuspidata and Subsecunda] and drier phases [shown by S. s. Acutifolia, ericaceous peat, monocotyledonous peat and decomposed, unidentifiable organic matter5(Box 1)]. However, there are
times when Sphagnum remains are either absent or unidenti-fiable 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 absent6. Palaeoecological records from the
UK show that in some situations S. imbricatum has been replaced by S. magellanicumor S. papillosum, in a relation-ship possibly independent of climate change, thus leaving S. imbricatum in a much diminished range on hummock tops6. The interpretation is complicated further by the range
of habitats that individual Sphagnumspecies can occupy7.
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 method4,8. Raised and blanket mires across the European
Palaeoclimatic records from peat bogs
Jeff Blackford
The palaeoclimatic record for the past 6000 years, implemented from peat-bog stratigraphy,
has been limited by imprecision in dating and climatic interpretation. Recently, dating problems have been addressed by ‘ wiggle-matched’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.
Peat-derived palaeoclimatic data might explain, as well as describe, climatic changes over
timescales of 102–103years.
Jeff Blackford is at the Dept of Geography, Queen Mary and Westfield College, University of
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 vari-ability 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 extract9. 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 development8, whereas remains of the
moss Racomitrium lanuginosum have been shown to indi-cate drier conditions10. Work on R. lanuginosumis
signifi-cant 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 con-ditions2. Fungal remains, abundant in peat, have been
shown to have palaeohydrological affinities11. Testate
amoebae (amoebae partially enclosed in a shell or test) remains are also recoverable from peat samples12 and
occupy distinct ranges of water-table positions13.
Values of dD/H and d18O/16O (the difference from the
stand-ard ratio of these isotopes) have been measured from peat samples14,15, resulting in the first quantitative climatic
recon-struction of mid-Holocene changes from peat-bog records15.
However, work on Carbury Bog in Ireland proved less con-clusive16and at present it is thought to be impossible to
sep-arate the climate signal from the effects of changing species composition14,16. Attempts to distinguish
palaeoenviron-ments using plant lipid biomarkers, including measurepalaeoenviron-ments of d13C from isolated components, have produced mixed
results17. 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 d13C,
interpreted as showing the relative abundance of C3 and C4plant species, possibly caused by changes in moisture availability and atmospheric CO2concentrations18.
Limiting factors in mire-based palaeoclimatology
Currently, there are several problems that limit the interpret-ation of otherwise high-quality data. First, there is the crucial problem of dating, which has relied almost entirely on con-ventional radiocarbon determinations with age-ranges, within two standard deviations (SD), of between 200 and 500
years19. 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 pro-file has been predicted, caused by continued decomposition in the catotelm20. Polynomial best-fit curves have also been
used to interpolate between fixed points21, but age estimates
for events that are not directly dated are, inevitably, more error-prone than the estimates for dated horizons. The prob-lems are exacerbated by the effects of calibrating the age estimates to produce calendar years, essential for compari-son 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
Box 1. Sphagnum remains as palaeohydrological indicators
There are between 150 and 200 species of the genus Sphagnum37. 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 availability7,37. 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’
environ-ments and S. capillifolium is more abundant on drier hummocks37. 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 includesS. capillifolium, are usually
identified only to section level (S. s. Acutifolia) on the basis of leaf form38.
Where a range of Sphagnum remains are present in peat, the relative abundance of different species or groups can be used to estimate the rela-tive 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 60 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 20 and 35 cm, and S. s. Acutifolia peaks showing a drier phase in the top 10 cm (Ref. 38). Figures reproduced, with permission, from Refs 7 and 38.
Trends in Ecology & Evolution Nutrient status
Water table
Low High
Water table
Low
High
S. magellanicum
S. cuspidatum
Sphagnum capillifolium var. rubellum
(a)
Trends in Ecology & Evolution 0
10
20
30
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60
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20 40 60 80 20 40 60 80 100 20 40 60 80 20 20 40 60 80
0 0 0 0 40 60 80 1000
EricalesMonocots Id. Sphagnum S. s.
Acutifolia
S. magellanicum S. s. Cuspidata
(b)
Percentage macrofossils
one calendar age, thus leading to further uncertainty. The exact pretreatment used has also been shown to significantly influence radiocarbon dates from peat22. Furthermore,
recent high-precision dating has shown an error resulting from a reservoir effect altering 14C concentrations in peat23,
leading to a potential 200-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’ radiocar-bon dates and tephrochronology. For the wiggle-matched method, high-precision (one SD, 630 years) radiocarbon
dates are taken in a sequence through the horizon of inter-est, including samples above and below. The results of these determinations are then cross-matched with the decadal 14C
curve obtained from tree-ring sequences, resulting, where a match is possible, in a precise calendar-age estimate23,24.
Although commonplace in volcanic regions, including New Zealand25and Iceland26, recent development of
micro-scopic 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 mires27. The
main advantages are28: (1) precise correlation of fixed points
in peat sequences over a spatial range of 1023–103 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 obtained29. For example, the
Ice-landic eruptions of Hekla in AD1104 and AD1510 are precisely
dated from historical records26, and their ashes occur in the
peats of northern Britain and Ireland27,29. Even eruptions of
relatively minor magnitude have the potential to leave ash horizons in distant mires30, and the number of datable
points varies from over 100 close to the volcanic region, to between two and 15 at distances in the order of magnitude of 103km. Even a small number of precise correlation and
dating points greatly improves the interpretative value of a peat-based palaeoclimatic reconstruction (Fig. 1). Cur-rently, there is no better way of correlating peat cores and sections, and recent work has begun to incorporate tephra records alongside radiocarbon chronology21. Correlation
with ice-core palaeoclimatic records from Greenland, shown to have ice-acidity peaks attributable to Icelandic (and other) eruptions31, 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 sin-gle 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 layers29.
Improving climatic reconstructions
The second major problem currently associated with proxy climate records from mires is the lack of climatological pre-cision. Ideally, climatic reconstructions should provide tem-perature 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 cur-rent 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 climate-response model5. Macrofossil data, mainly Sphagnum
fre-quencies, were first compared with the best available cen-tury-scale climatic data for the period AD1100–1850. Using
this series (wetness or dryness and mildness or severity indices), a multinomial logit model, a specific type of gen-eralized linear model, was fitted to the macrofossil and cli-mate data, and was then used to esticli-mate the climatic indices for longer timescales. However, the resulting data set remains imprecise in terms of temperature or rainfall reconstruction5 (Fig. 2).
Recent work on testate amoebae has shown that their hydrological tendencies can make them useful palaeoenvir-onmental indicators13,32. Their relative abundance has been
used to create transfer functions based on modern distri-butions, quantitatively linking species counts in subfossil form from peat horizons to the height of the water table32.
Fig. 1. Summary of the tephra (volcanic ash) deposits identified from north Scotland, UK, as known in 1995. 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 north-western European mires, whereas others, for example the Hoy tephra, have a more limited range. Reproduced, with permission, from Ref. 27.
Trends in Ecology & Evolution Shetland and
Orkney Caithness Sutherland W. Isles Highlands Grampian Synthesis
Kebister Hoy Slethill LochLeir Station HillAltnabreacGlen NaBeiste Lairg PortainLoch BeinnEighe GarryGlen PitsligoNew
AD1510 H1510
c. 450 BP
c. 2100 BP
c. 3800 BP
Lairg A + B Beinn Eighe Loch
Portain B
Garry
Hoy Kebister Hekla-4
H1510 (AD 1510) Loch Portain B (c. 450 BP)
Glen Garry (c. 450 BP)
Kebister (c. 3600 BP) Hekla-4 (c. 3800 BP)
Hoy (c. 5300 BP)
Lairg A (c. 6000 BP) Lairg B
Although the modern distribution studies of testate amoe-bae 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 reconstruc-tions to highlight evidence for climatic changes at particu-lar 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 3500–3900 BP
(Ref. 8) (Box 2). Another change to cooler and wetter cli-mates has been inferred for 2650 BP(850–760 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 d18O and dD/H fluctuations, and
a fall in pollen percentages of Corylus avellana. This suite of changes has also been linked to a d14C rise in tree-ring
records at this time (Box 2), leading to the interpretation that solar variability caused the recorded changes14. A
fur-ther change to cooler and/or wetter conditions has been inferred for the period around 1400 BP (calibrated age AD550–750), when blanket peats from Ireland, Wales and
northern England became wetter33(Box 2).
Several of these changes coincide – within the limits of available age estimates – with the original recurrence sur-faces recorded by earlier authors, including some studies that pre-date radiocarbon dating. For example, the 3900–3500 BP change might be equivalent to Granlund’s
recurrence surface no. 5 (Ref. 2), and the 2650 BPchange is
close to the sub-Boreal–sub-Atlantic transition of the Blytt– Sernander scheme3. The 1400 BPchange can be correlated
approximately with changes in mires across Europe33.
However, there are also inconsistencies. Peat humification 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 0.12 cycles per sampling interval, a periodicity of approximately 210 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 0.06 cycles per sampling interval, a periodicity of approximately 800 years. The y-axis ordinate is a measure of the proportion of variance allocated to each frequency range. Reproduced, with permission, from Ref. 5.
Trends in Ecology & Evolution
0
% Humification
100
80
60
40
20
0 –5 5
0 1000 2000 3000
1000 1950
1000 2000 3000 Time [Calibrated (cal.) BP]
Calibrated (cal.) AD Calibrated (cal.) BC
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–2 0
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–1 0 1
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Standard de
viation units of detrended
correspondence analysis scores
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(a)
data21for the Talla Moss in Scotland (Fig. 2a) show no clear
changes between AD550–750 nor do the most recently
pub-lished 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 1400 radio-carbon years BP(Ref. 34). Both of these studies show
significant perturbations at the approximate time of the inferred climatic change at 2650 BP and the Bolton Fell
Moss data also show changes around 3500 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 260 years was shown from humification data from a Danish raised mire over the past 5500 years35. The period was estimated by
comparing likely intervals – a 260-year frequency showed a closer correspondence to the times of change than longer
Box 2. Peat-derived evidence of three episodes of climatic change
T
rends in Ecology & Ev
olution
16 20 25 18 23 28
1300
Drier Wetter
Smoothed data 1400
1500
Radiocarbon age (y
ears
BP
)
Percentage transmission of light through NaOH extract (a)
Raw data
(Online: Fig. I)
T
rends in Ecology & Ev
olution
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ercentage b
y v
olume
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C (per mil)
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0 10 15 20
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um cuspidatum Sphagn
um papillosum Sphagn
um imbr icatum
500 600 700 800 900 1000
Calendar years BC
(b)
(Online: Fig. II)
T
rends in Ecology & Ev
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0
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2000
3000
3700
3900 4000
5000
6000
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Drier Wetter
(c)
(Online: Fig. III)
(b) Sphagnum macrofossil data and d14C (the relative abundance of 14C
compared with mean levels), from the raised bog Engbertsdijksveen
(The Netherlands) for the period 1000 to 500 BC. The timescale is
based on 14C wiggle matching, in which a sequence of high-precision
(one SD, 630 years) radiocarbon dates are taken. The results are
then cross-matched with the decadal 14C curve obtained from tree-ring
sequences, resulting in a more precise calendar-age estimate than
conventional 14C dating23,24. The increase in d14C at 800 cal. (calibrated) BC
coincides with a change first to Sphagnum papillosum and S. cuspidatum,
and then toS. imbricatum, indicative of a wetter peat surface than S. s.
(section) Acutifolia, which dominated before 800 BC. The 14C signal
is thought to be dependent on variations in solar output, therefore this correlation might demonstrate a link between solar variability and
climate change14. 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
radio-carbon correlation. The highlighted change at 3900–3700 BPcoincides
with changes in the microfossil record indicative of wetter conditions. Reproduced, with permission, from Ref. 8.
(a) Blanket mire humification data, indicative of the degree of peat decompo-sition, 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 1400 BP. The data are produced by first extracting the
or shorter intervals. Raised mire macrofossil data from northern England showed a possible 800-year signal, after the data had been summarized using detrended corres-pondence analysis (Fig. 2). Time-series analysis was con-ducted using Fourier transform spectral analysis tech-niques5, assuming a linear growth rate for the majority of
the peat profile – the 800-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 dates21.
Spec-tral analysis of the data showed a frequency of approxi-mately 210 years (Fig. 2a). A change to wetter conditions at
AD1410 has been correlated with the onset of the Sporer
minimum – a period of reduced sun-spot activity21– a link
previously suggested from studies of Irish bogs36.
Prospects
Peat-derived proxy climate data have shown significant cli-matic changes and evidence of periodicity over the mid-late Holocene period. However, there has also been consider-able 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 pro-portion 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 palaeocli-matology has the potential to explain, as well as describe, significant preindustrial climatic fluctuations. Recent im-provements 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 de-bates surrounding recent and current climatic variability.
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