Mire-development pathways and palaeoclimatic ... - SAGE Journals

The Holocene 10,4 (2000) pp. 465–479

Mire-development pathways and palaeoclimatic records from a full Holocene peat archive at Walton Moss, Cumbria, England P.D.M. Hughes,1 D. Mauquoy,2 K.E. Barber1 and P.G. Langdon1 (1Palaeoecology Laboratory, Department of Geography, University of Southampton, Southampton SO17 1BJ, UK; 2Hugo de Vries Laboratorium, University of Amsterdam, Amsterdam, The Netherlands) Received 15 January 1999; revised manuscript accepted 27 September 1999

Abstract: Plant macrofossil data have been used to identify the successive mire communities occupying both central and marginal locations in the Walton Moss peatland complex, during the last 10 500 years. The reconstructed pathways of mire development indicate that early-Holocene fen and fen-carr communities were succeeded by species indicative of deep mire water tables and oligotrophic conditions. The character of the fen/bog transition (FBT) is compared with similar records of peatland development from Britain and Scandinavia and with independent climate data for the early Holocene. The ‘pseudohochmoor’ of central Europe is suggested as an approximate modern analogue for the dry pioneer oligotrophic mire type and alternative explanations for its presence are explored. The first major increase in ombrotrophic Sphagna occurred at c. 7800 cal. BP. Overlying Sphagnum peats provide a continuous record of climate change, inferred from fluctuations in raised mire surface wetness. The proxy palaeoclimate record, reconstructed using Detrended Correspondence Analysis, registers wet shifts commencing at c. 7800, c. 5300, 4410–3990 (2␴ range), c. 3500, 3170–2860 (2␴ range), 2320–2040 (2␴ range), c. 1750, c. 1450, c. 300 and c. 100 cal. BP. This climate record is compared with a similar one from Bolton Fell Moss and spectral analysis of the time-series gives periodicities of c. 1100 years and c. 600 years between wet shifts. Key words: Mire development, Holocene, palaeoclimate, macrofossils, peat stratigraphy, spectral analysis.

Introduction The identification of plant remains in stratified peats provides a detailed record of long-term vegetational change in wetland ecosystems (Grosse-Brauckmann, 1986; Barber, 1993). Until recently, few British studies have exploited the full potential of the technique. Early research into landscape history and mire development frequently included brief descriptions of macrofossils, but close sampling was rare and quantification of subfossil types limited or absent. Field stratigraphy was commonly used as the primary source of evidence for the development of peatlands. While macrofossil records were routinely included in vegetation history studies they usually accompanied palynological work to provide a context for the main pollen core, following the example of Godwin (1956). The recognition of raised mires as valuable archives of palaeoclimatic information (Aaby, 1976; Aaby and Tauber, 1974; Barber, 1981) has generated a significant body of research into raised peat stratigraphy (Haslam, 1987; Svensson, 1988; Foster et al.,  Arnold 2000

1988; Blackford and Chambers, 1991; 1995; Korhola, 1994; Barber et al., 1994b; 1998; Chambers et al., 1997; Stoneman, 1993; Mauquoy and Barber, 1999b). However, a greater understanding of the pathways of development leading to raised mire formation is required to underpin peat-based palaeoclimatic reconstructions. Furthermore, despite significant progress towards the refinement of the peat-based proxy climate archive, this source of evidence has not been fully accepted as a result of several misconceptions concerning the development of peat stratigraphy which have been addressed by Barber (1994) and Barber et al. (1998). One of the best ways of demonstrating the operation of a regional forcing factor, such as climate change, is to be able to find synchroneity of response in separate mire systems (Korhola, 1995; Mauquoy and Barber, 1999a). The simplest level of intersite comparison is to compare mires situated on similar geology, with a similar basin size and local climate. For this reason the upper 7 m of fully ombrotrophic peat at Walton Moss provide a valuable opportunity to compare the climate archive with the established record from Bolton Fell Moss (Barber et al., 1994b). This paper aims to 0959-6836(00)HL411RP

466 The Holocene 10 (2000)

Figure 1 Location of Walton Moss and Bolton Fell Moss in the British Isles.

reconstruct a detailed proxy-record of change in effective precipitation from the raised stratigraphy at Walton Moss, covering the mid- to late Holocene, for correlation with Bolton Fell Moss and other sites in northern Britain. The paper also aims to highlight the conservation importance of Walton Moss.

The study site Walton Moss (WLM; NY 504 667) offers the chance to study an intact large lowland raised mire complex containing a detailed archive of long-term vegetation change covering the entire Holocene. The peatland complex, which is located 2.5 km to the southeast of Bolton Fell Moss, extends over 150 ha (Figures 1 and 2) and has suffered only very minor peripheral hand-cutting

dating from the nineteenth century. In places a natural undisturbed rand is still visible. This makes Walton Moss almost unique among English lowland ombrotrophic bogs (Barber et al., 1994a). The mire has been described by English Nature (unpublished notes), Dumayne (1992), Stoneman (1993) and Barber et al. (1998). The vegetation of Walton Moss is classified as NVC M18 (sensu Rodwell, 1991), a Sphagnum-rich lowland raised mire type. The plant communities are dominated by Sphagnum cuspidatum, S. magellanicum, S. papillosum and S. capillifolium var. rubellum. In contrast to many modern raised bog surfaces, the microtopography is very subdued with a range of no more than 30 cm between hummock tops and hollows. Considerable areas of the moss comprise almost flat lawn, mainly composed of Sphagnum cuspidatum, with a water table lying at or just below the surface in spring. The vascular plant community is indicative of the relatively wet conditions, and includes Eriophorum angustifolium, Eriophorum vaginatum, Rhynchospora alba, Erica tetralix, Vaccinium oxycoccus, Narthecium ossifragum, Drosera rotundifolia and Andromeda polifolia. Higher hummock species, including Calluna vulgaris, are poorly represented over much of the peatland. The crown of the mire is punctuated by scattered Betula trees with further individuals occupying the damaged areas of the rand. The presence of tree Betula on the undamaged bog surface may be a response to atmospheric deposition of nutrients. Barkman (1992) has noted that Betula may tolerate significant levels of waterlogging if the supply of nitrogen is plentiful.

Methods

Figure 2 Local map of Walton Moss and Bolton Fell Moss, showing coring locations.

Stratigraphy and macrofossil analysis Two peat profiles of 10 m (WLM11) and 5.5 m depth (WLM15) were extracted from the centre of the main peatland basin and from a site 200 m from the rand, respectively. Coring locations were determined after studying the field stratigraphy of 16 4 cm bore Russian cores arranged along two intersecting transects (Barber et al., 1998). The uppermost 50 cm of peat at each location was sampled using a monolith tin, and lower levels were recovered using a 9-cm bore modified Russian corer (Barber, 1984). All samples were wrapped in airtight bags and stored in the laboratory at 4°C. Field stratigraphy was recorded using the Troëls-Smith (1955) system. Two crossed transects comprising 16

P.D.M. Hughes et al.: Mire-development pathways and palaeoclimatic records 467

Figure 3 Transect A: stratigraphy of Walton Moss.

boreholes were analysed and levelled using a Geodometer Total Station, and the results presented in Figures 3 and 4 using the TSPP plotting program (Duller, 1995). Initially macrofossil analyses were undertaken on both cores using 1 cm thick slices of peat (measuring 4 cm3) spaced at 16 cm intervals. Further levels were analysed at 8, 4 or 2 cm intervals across critical horizons. Samples were washed vigorously through a 125 ␮m sieve using a standard 5 litres of water. Large vegetative macrofossils forming the main body of the peat were quantified using the quadrat and leaf count (QLC) technique described by Barber et al. (1994b). Small discrete peat remains such as nutlets, seeds and charcoal fragments were assessed using a five-point scale of abundance (Walker and Walker, 1961; Barber, 1981). Nomenclature follows Daniels and Eddy (1990) for Sphagna, Smith (1978) for other bryophytes and Stace (1991) for vascular plants. Radiocarbon dating and pollen correlation Samples with a wet mass of 100–120 g and measuring 8 cm in vertical thickness were taken from the peat cores for conventional radiocarbon dating at the NERC East Kilbride facility. Obvious rootlets were extracted from the samples prior to submission. The

Figure 4 Transect B: stratigraphy of Walton Moss.

radiocarbon dates are listed in Table 1. Dates have been calibrated using the probability method and the bidecadal data set in the computer program CALIB version 3.0.3c (Stuiver and Reimer, 1993), using the calibration curves constructed by Linick et al. (1986), Pearson et al. (1993), Pearson and Stuiver (1993), Stuiver and Becker (1993) and Stuiver and Pearson (1993) from dendrochronological records. Radiocarbon dates are quoted as the 2 sigma (2 ␴) calibrated range BP. The lower seven radiocarbon dates have been transferred by pollen correlation from core BFML at Bolton Fell Moss, located 2.5 km northwest of Walton Moss. Core BFML has several similarities with core WLM11. Both are located at the deepest point in their respective mire basins and both measure 10 m deep. The main changes in tree pollen frequencies through the early and mid-Holocene have been used to cross-correlate between the two sites. Table 2 gives the interpolated dates of the pollen zone boundaries at Walton Moss and Table 1 includes the equivalent depths in core WLM11 for each of the dates sampled from Bolton Fell Moss. To account for the probable errors introduced into the dating framework by pollen correlation all interpolations based on transferred dates have been rounded to the nearest century and prefixed with ‘circa’ (c.)


Table 1 Radiocarbon dates from cores WLM11, BFML and WLM15. An asterisk (*) denotes a date transferred by pollen correlation from Bolton Fell Moss (BFML) to Walton Moss (WLM11) Lab. no.

Site code

Material

Depth below peat surface (cm), in core WLM11

SRR-5867 SRR-5868 SRR-5869 SRR-5870 SRR-5871 SRR-5872 SRR-4939* SRR-4940* SRR-4941* SRR-4942* SRR-4943* SRR-4944* SRR-4945* SRR-5638 SRR-5639 SRR-5640 SRR-5641

WLM11 WLM11 WLM11 WLM11 WLM11 WLM11 BFML BFML BFML BFML BFML BFML BFML WLM15 WLM15 WLM15 WLM15

Ombrotrophic Ombrotrophic Ombrotrophic Ombrotrophic Ombrotrophic Ombrotrophic Ombrotrophic Ombrotrophic Ombrotrophic Ombrotrophic Fen peat Fen peat Fen peat Ombrotrophic Ombrotrophic Ombrotrophic Fen peat

peat peat peat peat peat peat peat peat peat peat

peat peat peat

32–40 52–60 156–164 192–200 264–272 400–408 624 684 744 832 885 910 962 236–244 452–460 476–484 516–524

14

C date BP (uncal.) Calibrated range BP (2 ␴)

360 ± 40 630 ± 40 1790 ± 45 2145 ± 45 2860 ± 45 3775 ± 50 6335 ± 50 7005 ± 50 7660 ± 45 8470 ± 50 8840 ± 45 9165 ± 50 9305 ± 45 2235 ± 45 4555 ± 45 4910 ± 45 6645 ± 45

610–510 670–550 1840–1580 2320–2040 3170–2860 4410–3990 7400–7170 7940–7680 8550–8370 9510–9380 9970–9650 10 290–10 000 10 380–10 050 2340–2140 5320–5050 5730–5590 7540–7400

Table 2 Pollen correlation between cores BFML and WLM11 at Bolton Fell Moss and Walton Moss respectively (dates are rounded to the nearest century) Regional pollen zone

BFM(L) depth (cm)

WLM(11) depth (cm)

Main species in order of abundance

Flandrian zone (sensu Hibbert et al., 1971)

Interpolated date, upper boundary

RPZ-G RPZ-F RPZ-E RPZ-D RPZ-C RPZ-B RPZ-A

524–724 724–738 738–830 830–882 882–907 907–972 972–980

508–716 716–744 744–825 825–885 885–910 910–962 962–985

Alnus, Corylus, Quercus, Ulmus Corylus, Quercus, Betula, Pinus, Ulmus, Alnus Corylus, Betula, Quercus, Ulmus, Pinus, Alnus Corylus, Betula, Ulmus, Pinus Corylus, Betula, Pinus Betula, Salix, Pinus Betula, Pinus, Salix, Juniperus

Zone Zone Zone Zone Zone Zone Zone

c. 5800 cal. BP c. 8200 cal. BP c. 8500 cal. BP c. 9400 cal. BP c. 9800 cal. BP c. 10 100 cal. BP c. 10 200 cal. BP

The age-depth model Figure 5 gives the age-depth model for the ombrotrophic peats of core WLM11. The radiocarbon dates taken on the upper peats and the dates transferred by pollen correlation for the lower levels provide 10 reference points within the ombrotrophic peat. The undamaged peat surface gives an 11th point. The average rate of peat accumulation within the basal fen phase was significantly

FII FId FId FIc FIc FIb FIa

higher than in the overlying ombrotrophic peats at 6.4 years/cm compared with 10.3 years/cm. Therefore, the age-depth model, which provides the basis for time-series analysis on the proxy climate signals derived from the raised peats, excludes these lower levels. A linear regression model (r2 = 0.9978), fitted through the mid-points of the 2 ␴ calibrated ranges for each radiocarbon date, best describes the age-depth relationship for the ombrotrophic

Figure 5 Age-depth model for the raised bog deposits in core WLM11. Open circles denote directly dated samples from WLM11. Solid diamonds denote radiocarbon dates transferred by pollen correlation from core BFML.


peats. Dates for levels lying between the radiocarbon dated horizons have been calculated from the age-depth equation given in Figure 5 and appear in the text with the prefix ‘c.’. Detrended correspondence analysis Detrended correspondence analysis (DCA) was performed on the quadrat and leaf-count data for core WLM11 (i.e., excluding fivepoint scale abundance data such as propagules and bud scales), using the CANOCO computer program (ter Braak, 1987), to investigate the nature of environmental gradients present in the data set. All sample levels were included in the analysis initially, using options for downweighting of rare species. Subsequently, the ombrotrophic peats were studied separately to investigate possible gradients suitable for inferring proxy palaeoclimatic shifts. The procedure for reconstructing mire surface wetness as a proxy-climate record follows the methods described by Barber et al. (1994b). Time-series analysis DCA axis 1 scores for the ombrotrophic peat samples from core WLM11, covering the time interval from c. 9700 cal. BP to the present, were subjected to time-series analysis using the LOMB program (Lomb, 1976, Press et al., 1992), designed for unevenly spaced data. An interpolated calendar age was calculated for each sample point using the age/depth equation given in Figure 5, which was produced by fitting a linear regression through the midpoints of the 2 ␴ calibrated range of the radiocarbon dates. Prior to spectral analysis, the data were smoothed with a low pass filter (raw data were smoothed using a three-point moving average). The LOMB program has no capability for calculating confidence intervals, therefore the results, displayed as a periodogram (Figure 10), should be treated as an approximation to the actual amount of variance that each peak represents.

Results Field stratigraphy The 16 field stratigraphy cores (Figures 3 and 4) show that the Walton Moss peat complex is composed of a main asymmetrical basin to the north of the present geographical centre of the site and a lobe of shallower peat resting on gently dipping till deposits to the south and southeast. A maximum peat depth of 10 m was encountered within 300 m of the northern margin. The thin veil of peat fringing the mire and lying beyond the present limits of the rand has been damaged by localized hand-cutting. In places this extends into the rand but, remarkably, significant stretches of the rand to the south of the bog appear to be largely undamaged. The stratigraphy of Walton Moss may be simplified into three basic units: (1) fen/fen carr deposits; (2) highly humified Eriophorum/Calluna peat; (3) fresher Sphagnum-dominated peat with localized E. vaginatum tussocks. It is interesting to note that fen deposits are thinnest in the bottom of the main basin and marginally thicker on the slopes. Fen development on the slopes lying beyond the main basin was much more limited. All of the cores from the main basin record a protracted phase of Eriophorum vaginatum domination resulting in the formation of a highly humified lower ombrotophic peat bed. In cores 7, 8, 9 and 14 the contact between the humified Eriophorum peat and the upper Sphagnum-dominated peat occurs at approximately the same altitude at 99 m OD. Macrofossils Peat components quantified using the quadrat and leaf count method are presented in the macrofossil diagrams (Figures 6 and 7) as linked bar histograms, which give the percentage cover of the microscope graticule for each subfossil type. Macrofossils

assessed on the five-point scale of abundance (1 = rare; 2 = occasional; 3 = frequent; 4 = very frequent; 5 = abundant) are expressed as unlinked bar histograms to differentiate the two methods of quantification. The order of the taxa in the diagrams has been arranged so that components occurring in the same stratigraphic level are placed close together and approximately in the order of appearance. Where the macrofossil elements are not specified for a taxon the histogram relates to the total vegetative remains encountered (roots, rhizomes, leaves and stems). Local macrofossil zones have been defined with the aid of a stratigraphically constrained cluster analysis with no data transformation, using CONISS within TILIA v1.12; the resultant dendrograms are displayed on the macrofossil diagrams. The central core, WLM11 The basal peat in the main peatland basin is underlain by a finely laminated light grey silty clay which represents an unproductive low-energy pool probably of Lateglacial age. Organic accumulation began early in the Holocene between 10 380 and 10 050 cal. BP with the formation of a stiff, well-humified detritus peat containing abundant remains of Phragmites australis, Equisetum sp., Carex vesicaria and Potentilla palustris (Figure 6). This mesotrophic fen community was rapidly replaced by a tussocky swamp-carr dominated by Carex paniculata, Phragmites australis and Betula sp. at 975 cm depth (zone LMZ-WLM11B). The lifespan of the pioneer wetland stage was probably no more than c. 60–100 years. Local macrofossil zone C marks the colonization of the swamp carr by Sphagnum palustre and the disappearance of a range of fen plants including Menyanthes trifoliata, Phragmites australis and Equisetum sp. This phase was also short-lived, lasting no more than c. 250 years. The abundance of Sphagnum palustre peaks at the zone C/D boundary (885 cm depth) which also marks the abrupt departure of both Betula sp. and Carex paniculata from the mire surface, leaving an open poor fen composed of a variety of mesotrophic to oligotrophic moss taxa accompanied by infrequent oligotrophic vascular plants (Eriophorum vaginatum and Calluna vulgaris). An E.vaginatum/Aulacomnium palustre assemblage rapidly displaces the Sphagnum palustre fen peat in zone Ea marking the establishment of an open acidic bog with a flora broadly reminiscent of modern blanket mire vegetation. The significant depth of peat formed in this environment is very well humified, indicating that the mire surface was frequently exposed to desiccating conditions. The substantial increase in the frequency of soil fungi (Cenococcum sp.) also suggests aerated conditions during or after deposition. In the upper part of zone E at 750 cm, Calluna vulgaris replaces Eriophorum vaginatum as the dominant component of the flora marking the end of a 1000-year phase of dominance by E. vaginatum in which 125 cm of highly humified blue-black peat was laid down. Throughout the E. vaginatum zone the Sphagnum genus is a minor component and almost exclusively represented by the Acutifolia section (probably Sphagnum capillifolium or S. fuscum). The presence of numerous delicate branch leaves throughout the surviving E. vaginatum fibres suggests that the low overall abundance of Sphagnum is not primarily a function of differential preservation. The genus most likely formed small cushions in between the mass of E. vaginatum tussocks, producing a lowland mire type closer to the central European ‘pseudohochmoor’ of Rybnicek and Rybnickova (1968), and the Eriophorum vaginatum/Sphagnum fuscum-dominated raised bogs of Fennoscandia, than the modern oceanic raised mires that currently occupy the Cumbrian lowlands. A dramatic change in mire surface conditions is marked by the sudden rise in the abundance of Sphagna at c. 7800 cal. BP. Dominance by Sphagnum subnitens was followed by a major increase in the pool taxon Sphagnum sect. cuspidata at 625 cm

Figure 6 Core WLM11 macrofossil diagram.


Figure 7 Core WLM15 macrofossil diagram.



depth (c. 7300 cal. BP). This event marked the first point at which the mire maintained a permanently high water table. Thereafter various Sphagna dominated the majority of the 7800-year history of classic oceanic ombrotrophic mire development, with the exception of five relatively short-lived drier phases, represented in the macrofossil diagram as zones WLM11-K, -M, -O, and -Q, in which E. vaginatum and members of the Ericaceae increased in frequency. Within the ombrotrophic stratigraphy the behaviour of Sphagnum imbricatum is particularly noteworthy. During the early to mid-Holocene the species was an intermittent rare component of the macrofossil assemblages. At 450 cm depth (4410–3990 cal. BP) S. imbricatum suddenly assumed dominance on the mire. Thereafter, the species fluctuated in importance, becoming the main peat-former during at least four distinct phases. Above 54 cm depth, dated to between 670 and 550 cal. BP, the species disappeared completely from the macrofossil record to be replaced by pool peats characterized by Sphagnum sect. Cuspidata. In contrast to previous declines displayed by Sphagnum imbricatum the species never re-established itself, with Sphagnum magellanicum and Sphagnum papillosum invading to become the principal peatformers of the modern mire. Having dominated the flora of Walton Moss periodically over a 4000-year interval, Sphagnum imbricatum is now extinct on one of the least disturbed of all English raised mires. The question of what happened to Sphagnum imbricatum has been addressed in more detail by Stoneman et al. (1993) and Mauquoy and Barber (1999b). Marginal core WLM15 A macrofossil core located some 250 m from the edge of Walton Moss (WLM15; see Figure 7) records the developmental history of peat deposits lying beyond the rim of the main wetland basin on a plain of Devensian till dipping at approximately 10 degrees to the southeast. Peat accumulation commenced at WLM15 between 7540 and 7400 cal. BP with the establishment of Alnus glutinosa carr woodland in which both Phragmites australis and several Juncus species were prominent. Initial accumulation rates were very slow, averaging 36 years/cm, with the resultant peat containing between 60 and 80% unidentifiable organic matter, suggesting that conditions were frequently marginal for organic accumulation. In zone WLM15B the appearance of Betula sp., Luzula sylvatica and Sphagnum palustre signalled increasing levels of acidification. Towards the upper zone boundary and during zone C an open poor fen composed of Sphagnum palustre and S. recurvum rapidly established on the mire prior to the equally sudden displacement of fen species by strongly oligotrophic taxa (Eriophorum vaginatum and Calluna vulgaris) between 5320 and 5050 cal. BP (zone C/D boundary). Eriophorum vaginatum and Calluna vulgaris dominated the subsequent 2 m of mire development throughout zones D, Ea and Eb, accompanied by low frequencies of hummock-level mosses such as Polytrichum cf. alpestre, Aulacomnium palustre and Sphagnum sect. Acutifolia producing a peat on the sloping substratum that was remarkably similar in species composition and physical characteristics to the bed laid down over fen carr in the level part of the main basin 4000 years earlier in zone WLM11E. A major shift in the main peat-forming species from E. vaginatum to Calluna vulgaris in zone Ea, coupled with a coincident increase in the frequency of Aulacomnium palustre, may indicate a change towards prolonged deep-mire water tables. This phase which resumed in zone Ec was punctuated by a brief wet shift at 350 cm (c. 4000 cal. BP) This is indicated by the appearance of the mudbottom species Rhynchospora alba in zone Eb. In zone Fa Sphagnum imbricatum entered the mire at the sampling site some 2500 years after its first colonization at the site of core WLM11 in the main basin. In common with WLM11 the upper 2 m of the marginal peat sequence, which have not been analysed in detail in

Figure 7, are dominated by various Sphagna including S. imbricatum. Brief analysis of the field logs indicated that S. imbricatum disappeared from the marginal peat record at 40 cm depth. Detrended correspondence analysis (DCA) The results of the DCA analysis of core WLM11 are given in Figures 8 and 9. Figure 8 shows that DCA axis 1 represents a mire surface wetness gradient. The eigen value of 0.82 suggests a good separation of species along the axis, with pool and hummock species positioned at opposite ends. Figure 9 is a plot of the axis 1 scores versus depth and may be interpreted as a record of changing mire surface wetness conditions covering the last 9500 years.

Discussion Mire development Many of the raised bogs of Cumbria and the Scottish Borders that have their origins in shallow kettlehole depressions within Devensian till developed through very brief fen phases before the transition to oligotrophic conditions (Walker, 1966; Hughes, 1997). The authors have investigated the development of over a dozen such mires and Walton Moss is arguably one of the finest examples. Peat accumulation began in the deepest part of the main basin shortly before c. 10 200 cal. BP with the transition to oligotrophic conditions occurring just 300 years later at c. 9900 cal. BP. Similar basal stratigraphies have been recorded from Solway Moss, Glasson Moss and Bolton Fell Moss located between 2.5 km and 30 km northwest or west of Walton Moss (Hughes, 1997). The replacement of fen conditions by relatively dry Eriophorumdominated bog that produced highly humified dense peat is also a very familiar feature of raised bog stratigraphy. All eight sites studied by Hughes (1997) featured a fen-bog transition in which the first acidic communities contained species typical of deep water tables and prolonged surface aeration. The soil fungus Cenococcum sp., indicative of aerated conditions (Ferdinandsen and Winge, 1925), is frequent or abundant in much of the Eriophorum peat. There is also a significant increase in the frequency of macroscopic charcoal in the pioneer raised-bog phases, which suggests that the dry hummocky mire surface was frequently burnt. Studies of mire stratigraphy conducted by the North West Wetlands Survey have also identified Eriophorum/Calluna peat as a very common pioneer phase after the fen-bog transition in mires ranging from South Cheshire to Cumbria (Hall et al., 1995, Middleton et al., 1995, Leah et al., 1997; 1998). The widespread occurrence of the highly humified peat layer, which frequently contains only a minor Sphagnum component, represents a phase when the pioneer raised mires lacked stable water tables. Ingram (1982; 1983) has argued that raised mires maintain a stable, domed water table by impeded drainage resulting from the development of impermeable catotelmic peat. Brown et al. (1989) and Brown and Ovenden (1993) have suggested that the impermeability of the raised peat mass results from the entrapment of supersaturated methane bubbles in the pore spaces of degrading peat. Both of the hypotheses explain how the raised water mound may be maintained once catotelmic peat has developed; however, they do not deal with the question of how the water mound is created in the first instance. Given that catotelmic peat is considered to be responsible for maintaining the raised water mound, either by impeded drainage or methane entrapment, environmental conditions that favour the production of highly humified, finely structured peats with low permeability might be expected to provide the correct foundations for the raised water mound. The almost universal occurrence of a bed of highly degraded Eriophorum/ Calluna peat lying above the fen levels and below the upper fresher Sphagnum peats in raised bogs lends support to the hypothesis that one or more phases of peat surface desiccation


Figure 8 Biplot of axis 1 and axis 2 scores from the DCA of macrofossil results from core WLM11: diamonds denote sample scores and circles denote species scores.

Figure 9 Reconstruction of mire surface wetness: axis 1 scores from the DCA of macrofossil results from core WLM plotted against calibrated years BP.

and humification favour raised water-mound formation as a consequence of the production of a relatively impermeable, finely comminuted peat layer. Eriophorum vaginatum is adapted to tolerate deep-mire water tables (Kummerow et al., 1988) and usually dominates the mire vegetation under conditions of spring flooding, followed by desiccation of the peat surface in late summer (Gimingham, 1964; Wein, 1973). Additionally, it is able to survive deep winter freezing of the peat surface (Wein, 1973) and it is tolerant of burning. The growth rate of the species increases dramatically in response to increased phosphorus (P) which can be liberated in significant quantities during the humification of poor fen and raised-bog peats. All of these aspects of the ecology of Eriophorum vaginatum strongly suggest that the dominance of the species at the transition is a result of unstable water tables in the newly formed raised peat. The instability of the water table may be attributable to an insufficient depth of catotelmic peat and/or pronounced seasonality of the meteroric water supply. The physical characteristics of Eriophorum vaginatum macrofossil remains make them ideal for the accumulation of peat in conditions that would otherwise be marginal for organic accumu-

lation. The leaf and rhizome fibres are very resistant to decay and are often the last surviving macrofossils in highly degraded peat. The fibrous peat structure is also much more efficient than Sphagnum peat at retaining water during phases of desiccation. Having deposited a layer of highly degraded Eriophorum/ Calluna peat, the peatland complex at Walton Moss gained a suitable substrate for the entrapment of occluded methane in pore spaces and the maintenance of a high water table. The macrofossil remains, however, suggest that 2000 years elapsed before the first prolonged near-surface water tables were established as indicated by the arrival of Sphagnum subnitens at c. 7800 cal. BP, followed by Sphagnum sect. Cuspidata at c. 7300 cal. BP. This 2000-year interval may reflect either a prolonged phase of low effective precipitation in the early Holocene and a major switch to wetter climatic conditions at c. 7800 cal. BP, or the time taken for sufficient well-humified ombrotrophic, catotelmic peat to develop to maintain a high water table. A third possibility is that the climatically insensitive phase would naturally be shorter than 2000 years but, at the point at which the mire could potentially respond to wet shifts by forming pools, effective precipitation levels were low. Evidence from other sites in Cumbria (Solway Moss and


Glasson Moss; Hughes, 1997) and from the work of Korhola (1990) on peat initiation in Finland and Digerfeldt (1988) on lake levels in Scandinavia strongly suggests that the first wet shift at Walton Moss was a dramatic response to increased effective precipitation. Having established that Walton Moss had become sensitive to climatic changes by c. 7800 cal. BP the upper part of the peat sequence may be used to reconstruct a proxy curve for effective precipitation following the principles and methods of Barber et al. (1994b). This aspect of the data archive will be examined in the following section. Peat initiation at the WLM15 coring site began during the very wet interval recorded from the centre of the mire complex at between c. 7800 and c. 6800 cal. BP; a similar date for widespread peat initiation has been reported from Finland by Korhola (1994; 1995). In contrast to the pathway followed in the main basin, fen conditions prevailed for approximately 2500 years on the marginal sloping substrate possibly because of the continuous flow of water from the crown of the mire to the fringing lagg streams. After a Sphagnum palustre-dominated fen phase lasting approximately three times longer than the equivalent stage in the main basin, an Eriophorum/Calluna mire developed on the sloping fen surface. This relatively dry mire type prevailed on the margin of the bog for much of the mid-Holocene during a period when the main mire centre had developed a Sphagnum-rich flora indicative of pool, lawn and hummock microforms. The dry mire type lasted for approximately 2500 years on the margin of the moss indicating that this part of the mire experienced fluctuating mire water tables after the fen-bog transition for a similar period of time compared with the earlier transition registered in the centre of the main basin. The first appearance of the lawn species Sphagnum imbricatum at c. 2800 cal. BP, marking the point at which the water table stabilized at the marginal core site, correlates with a significant increase in surface wetness in the main mire basin. Although the marginal core crossed the FBT much later than the central core, both sites appear to have passed through a similar phase of insensitivity to changes in effective precipitation which came to an end once sufficient highly humified catotelmic peat had accumulated. The proxy palaeoclimatic record Table 3 gives the main wet shifts in the peat stratigraphy at Walton Moss and Bolton Fell Moss reconstructed using DCA to identify the principal mire surface wetness gradient following the procedures of Barber et al. (1994b). Taking into account the limitations of the radiocarbon chronology (Kilian et al., 1995; Pilcher, 1991a; 1991b; Dumayne et al., 1995) and the errors introduced by pollen correlation dating of the lower peats, there is

Table 3 Comparison of the timing of wet shifts from BFML and WLM11 Bolton Fell Moss (Barber, 1981)

Bolton Fell Moss (Stoneman, 1993)

c. 200 c. 500 c. 1000

c. 350

Bolton Fell Moss (core BFMJ) (Barber et al., 1994b)

c. 100 c. 300 to 350 c. 1300

c. 2400 c. 3100 c. 3550

Walton Moss (core WLM11)

c. 1900 c. 2650 c. 3300 c. 4000

to to to to

2200 2900 3600 4350

c. 1450 c. 1650 c. 2100 c. 2600 c. 3500 c. 3800 c. 4900 c. 6800

to 1750 to 2040–2320 to 2680–3170 to 3990–4410 to 5300 to 7800

a reasonable degree of correspondence between the two climate archives but also some significant differences. Both mires preserve records of four wet shifts between c. 4300 and c. 2200 cal. BP that appear to be broadly in phase with each other. This section of the climate reconstruction has been independently radiocarbon dated in each core (Table 1). The most significant differences in the two surface wetness records occurred in the early Holocene. For example, at c. 7800 cal. BP Walton Moss registered a major increase in the mire water table, marked by a change from dry Eriophorum vaginatum-dominated bog to fresh Sphagnum subnitens-rich peat. By contrast, at Bolton Fell Moss E. vaginatum continued to dominate until the major wet shift at 4410–3990 cal. BP. The absence of the early wet shifts at Bolton Fell Moss may indicate that the early-Holocene Eriophorum mire represented a phase when bog vegetation was insensitive to climatic change. Alternatively, the BFML core may have passed through a hummock area at this time. The timing of the c. 7800 cal. BP wet shift at Walton Moss must be regarded as tentative since the preceding mire phase with its deep water table may have taken some time to cross the threshold at which a response in the mire vegetation was triggered. Additionally, this wet shift is only dated by pollen correlation with BFML at present. Although the pollen stratigraphies from WLM11 and BFML are only separated by 2.5 km and appear to be very similar, significant differences in the timing of equivalent features in pollen spectra have been documented (Smith and Pilcher, 1973). In the upper part of the stratigraphy of the two mires there are further differences in the climate records. For example, a significant wet shift at Walton Moss at c. 1750 cal. BP is absent from Bolton Fell Moss. Barber et al. (1998) found that occasionally wet shifts were either poorly represented or missing in a set of multiple cores examined from the upper strata at Bolton Fell Moss, concluding that the registered wet shifts should be considered to be the minimum record of phase shifts. Walton Moss and Bolton Fell Moss register increases in mire surface wetness at c. 1460 and c. 1300 cal. BP respectively. At least some of the mismatch in the timing of this event may result from lags in species responses or radiocarbon dating errors. For example, rootlet penetration may introduce more recent carbon into a sample giving an age estimate that is too young (Olsson, 1986; Korhola, 1992). In the upper part of the stratigraphy at Walton Moss the climatic deterioration of the ‘Little Ice Age’ is apparent. A similar wet shift has been recorded by Barber (1981) and Stoneman (1993) from cores and peat sections at Bolton Fell Moss. Climatic teleconnections Recent research on proxy-records other than peat stratigraphy has challenged the conventional view that global climates have remained relatively stable throughout the Holocene (Lamb, 1995; O’Brien et al., 1995; Sejrup et al., 1995; Stager and Mayewski, 1997). O’Brien et al. (1995) interpret abrupt increases in the concentrations of soluble impurities in ice cores as phases of significant general cooling. At least five events are distinguished during the Holocene at c. 0–600, c. 2400–3100, c. 5000–6100, c. 7800– 8800 and c. 11 300 BP, with peaks at c. 300–400, c. 3000, c. 5600, c. 8200 and c. 11 300 calendar years BP. Evidence from North Atlantic deep-sea cores also indicates that the Holocene climate must have undergone several abrupt reorganizations. Bond et al. (1997) found that ice-rafted debris events (IRD), which represent phases when cold ocean waters were advected as far south as Britain, displayed a distinctive cyclicity with peaks at c. 1400, c. 2800, c. 4200, c. 5900, c. 8100, c. 9400, c. 10 300 and c. 11 100 calendar years BP. Table 4 provides a summary of radiocarbon dated wet shifts from selected mires across northwestern Europe. The dates


Table 4 Comparison of the timing of climatic deteriorations inferred from mire wet shifts at a selection of sites across Europe (years cal. BP) Walton Moss

This paper

c. 100 c. 350

Bolton Fell Moss

Barber, 1981; 1994

c. 210 c. 500

Abbeyknockmoy

Hughes, 1997

Talla Moss

Chambers et al., 1997

c. 540

Brecon Beacons Wood Moss Harold’s Bog Migneint Letterfrack

Blackford and Chambers, 1991; 1995

c. 490

Cross Lochs

Charman, 1990

Kirkpatrick Fleming Tipping, 1995 NW Europe

–

–

c. 1450 c. 1750 –

c. 1000 c. 1300 c. 1170

2320– 2040

3170– 2860

c. 2200 c. 2900

c. 3500 4410– 3990

c. 5300 c. 7800 c. 5900

c. 3600 c. 4350

c. 3500 c. 1100

c. 1700 c. 1930 c. 2270 c. 2600

8320–8020

c. 3460

c. 1910 c. 1150 c. 1310 c. 1330 c. 1310 2890–2750 c. 400

c. 1200

Haslam, 1987

c. 1150

Foulshaw Moss Wimble, 1986 Heslington Moss White Moss North White Moss South

c. 2050 c. 1850

c. 1350 c. 600 c. 800

Engbertsdijksveen

Van Geel, 1978; Van Geel et al., 1996

Isosuo Tremanskarr Kantosuo

Korhola, 1995

Draved Mose

Aaby, 1976

c. 450

c. 660 c. 860

20 bogs from N and C Norway

Nilssen and Vorren, 1991

c. 420

c. 720 c. 850

Bourtangersveen

Dupont, 1986

c. 1050

c. 1700 c. 1500

c. 4000 c. 5250 c. 2550 c. 3050

c. 4200

c. 2900 c. 2900 c. 2250 c. 2900 c. 2250

c. 3400 c. 4300 c. 3400 c. 3800 c. 3500

c. 2850 c. 3750 c. 4350 c. 5450 c. 6800 c. 3020 c. 5850 c. 7150 2750–2450 c. 6450 c. 4300

c. 1500 c. 1700

c. 2250 c. 3000

c. 3400 c. 4000 c. 4850 c. 4300 c. 5050 c. 4600 c. 5400

c. 1140 c. 1400 c. 1680 c. 1930 c. 2230 c. 2800 c. 2450 c. 3120

c. 3370 c. 3780 c. 5350 c. 4280 c. 4690

presented in the table represent the onset of wetter mire conditions as interpreted by individual authors. Where dates were quoted solely in radiocarbon years in the original text they have been calibrated using CALIB v3.0.3c using the bidecadal data set and rounded to the nearest ten years. (Note that most of the dates are derived by interpolation between radiocarbon dates and do not imply an age precision of ten years; age ranges in the table represent the 2 ␴ range of calibrated radiocarbon dates). It may be noted that the peat-based record has been indicating significant climatic variability for some years. Although comparisons of wet shifts, between sites, may be affected by lags in the responses of vegetation to climate change (Conway, 1948), the intrinsic errors in most dating techniques and the use of a variety of different chronologies in studies, the timings of major phase-shifts show some similarities between Walton Moss and other widely separated European mires. Some shifts are more universally represented, such as the events at 4410–3990 cal. BP, and c. 2750

c. 1950

c. 8000

c. 3300 c. 4450 c. 5300 c. 3650

cal. BP. Older significant events may be poorly represented because they predate the main phases of raised-mire initiation and development. The pattern of wet shifts discernible in Table 4 and evidence drawn from the wider literature on Holocene climate change suggest that several phase shifts in the Walton Moss stratigraphy may have been associated with wider-scale climate change. For example, climatic events with similar timings to the 4410–3990 cal. BP and c. 3500 cal. BP wet shifts at Walton Moss (which fall within the directly radiocarbon dated part of the WLM11 sequence) have been inferred from a variety of records including lake-level and sedimentation data (Pennington et al., 1972; Digerfeldt, 1988; Yu and Harrison, 1995), blanket mire humification records (Blackford, 1990; Nilsson and Vorren, 1991; Anderson et al., 1998), tree-line studies (Gear and Huntley, 1991) and raisedmire macrofossil surface wetness reconstructions (Barber et al., 1994b). Extensive radiocarbon dating of basal peat samples from


Finland also suggests that the period between c. 4300 and 3000 cal. BP represented a phase of intensive peat initiation which may reflect increased atmospheric humidity (Korhola, 1995). The wet shift commencing at 3170–2860 cal. BP and persisting until at least c. 2500 cal. BP at Walton Moss also falls within a time period highlighted by other palaeoclimatic records as a phase of increased effective precipitation (e.g., Barber, 1982; 1985, Charman, 1990; Magny, 1992). Barber (1985) concluded that a wet shift at c. 2700 cal. BP was one of the largest climatic events of the late Holocene. Similarly, van Geel et al. (1996) have presented extensive palaeoecological and archaeological evidence for a major climatic deterioration at c. 2750 cal. BP, supported by a literature study which indicates that similar climatic changes occurred globally between c. 2900 and c. 2500 cal. BP. The ‘Little Ice Age’ was the most recent phase of global climatic deterioration (Briffa et al., 1990; Feng and Epstein, 1994; Lamb, 1995; Keigwin, 1996), with particularly cold conditions occurring in the latter parts of the seventeenth and nineteenth centuries (Lamb, 1995). At Walton Moss pool formation increased from the seventeenth century following on from a period of very dry mire conditions consistent with the much-debated ‘Mediaeval Warm Period’. The timing of the first major wet shift at Walton Moss is less well constrained but may be tentatively correlated with the inferred climatic reorganization commencing between c. 8200 and 7800 cal. BP, termed the early- to mid-Holocene transition (EMHT; Stager and Mayewski, 1997). The EMHT is distinguishable in many terrestrial and aquatic records around the globe (Alley et al., 1997; Bond et al., 1997; Klitgaard-Kristensen et al., 1998; Digerfeldt, 1988; Korhola, 1994; 1995; Fisher et al., 1995; Dubois and Ferguson, 1985; Bridge et al., 1990; Von Grafenstein et al., 1998; Maley, 1982). Further work to improve the geochronology will be required to understand fully where this wet shift fits with respect to the time interval noted above. It would be an over-simplification to suggest that the Holocene proxy climate record is composed solely of a series of coherent, synchronous and far-reaching events, similar to those suggested by Blytt (1876) and Sernander (1908). Distinct regional palaeoclimatic gradients have been identified across Europe throughout the Holocene (Haslam, 1987; Starkel, 1991; Lamb, 1995). Furthermore, the relationship between climate and peat stratigraphy may be more complex than was previously recognized. Battarbee et al. (1996) and Barber et al. (1999) have suggested that mires may react in phase to a spatially coherent temperature signal and asyn-

chronously to a zonal precipitation signal. New goals for peatbased palaeoclimatic research must be to identify the relative importance of the temperature and precipitation elements of the effective precipitation signal and to couple these records to improved tephra-based chronologies.

Periodicity in the Walton Moss time-series Cyclic climatic changes with estimated periodicities of between 200 and 260 years have been inferred from mires in the Scottish Borders (Chambers et al., 1997) and from Denmark (Aaby, 1976). Equivalent spectral analyses of the WLM11 time-series reveal periodicities of c. 600 years and c. 1100. The c. 1100-year cycle which accounts for the greatest power in the periodogram (Figure 10) is very close to twice the length of the second cycle and may represent an amplification of this signal. The absence of a periodicity with a value close to 200 years may be an indication that mire vegetation acts as a relatively coarse low pass filter (Aaby, 1976), with the 600-year periodicity being the shortest climatic cycle revealed by changes in vegetation composition. Similarly, at Bolton Fell Moss, where an 800-year periodicity was identified (Barber et al., 1994b), there was no evidence of a shorter cycle. Peat humification analyses appear to be more amenable to the identification of the bicentennial periodicity (Aaby, 1976; Nilssen and Vorren, 1991; Chambers et al., 1997). In contrast to the ocean record, few millennial-scale climatic periodicities have been identified in terrestrial deposits at present. The apparent c. 1100-year pacing of wet shifts found at Walton Moss should be treated with caution because of the errors involved with dating of the lower peats by pollen correlation. However, the Walton Moss record is broadly comparable with the 1374 ± 502 year mean pacing of Holocene ice-rafted debris events in the North Atlantic (Bond et al., 1997), taking into account the sizeable error margins associated with both records. Further work is required to confirm the millennial-scale cycle at Walton Moss, but the correspondence of the mid-Holocene cycles, within the directly radiocarbon dated part of the WLM11 core, with the timing of Holocene IRD events adds support to the suggestion that climate changes recorded in northwest European mires are linked to changes in ocean circulation (Barber et al., 1994b; Anderson et al., 1998). Further research into long terrestrial records is required to evaluate the replicability of inferred millennial-scale cyclic climatic changes in peat strata.

Figure 10 Periodogram of the spectral analysis of DCA axis 1 scores from core WLM11.


Conclusions The mire development pathways at Walton Moss suggest that the first raised-mire communities to develop after the fen-bog transition existed in a regime of fluctuating water tables which stabilized at both the central and marginal core locations after approximately 2000 years of peat accumulation. During the Eriophorumdominated phase, the mire was complacent (sensu Lowe, 1993) to changes in effective precipitation. However, the deposition of highly humified catotelmic peat under conditions marginal to organic accumulation may be a precursor for the development of domed, permanently high water tables capable of supporting oceanic Sphagnum mosses. The recognition of a phase of mire insensitivity to climate change immediately after the FBT has important implications for the interpretation of surface wetness data in palaeoclimate research. The stratigraphy of the central core clearly registers the point at which the main mire basin was first able to support a near-surface raised water mound. Thereafter seven wet shifts are recorded in the peat profile, some of which support the inferred climatic shifts established from analyses at Bolton Fell Moss and blanket peat sites from the Pennines, the Scottish Borders and European mires. The more securely dated mid-Holocene wet shifts at Walton Moss show a striking level of agreement with the pacing of ice-rafted debris events found in ocean cores and may represent evidence of ocean-driven forcing of the regional climate.

Acknowledgements The authors would like to thank the University of Southampton and the Natural Environment Research Council for the support of two studentships from which the work presented in this paper is drawn. The authors are also indebted to Major Johnson for permission to work at Walton Moss, to the NERC Radiocarbon Laboratory for dating support, Dr L. Peaty for field assistance, Dr A. Korhola and an anonymous referee for their comments on an earlier draft of this paper, and the Southampton Cartographic Unit for the preparation of illustrations.

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