JOURNAL OF QUATERNARY SCIENCE (2007) 22(1) 77–84 Copyright ß 2006 John Wiley & Sons, Ltd. Published online 24 May 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jqs.1018
Separating climatic and possible human impacts in the early Holocene: biotic response around the time of the 8200 cal. yr BP event KEVIN J. EDWARDS,1* PETER G. LANGDON2 and HEATHER SUGDEN3 1 Department of Geography and Environment and Northern Studies Centre, University of Aberdeen, Aberdeen, UK 2 Department of Geography, University of Exeter, Exeter, UK 3 Department of Archaeology, University of Sheffield, Sheffield, UK Edwards, K. J., Langdon, P. G. and Sugden, H. 2006. Separating climatic and possible human impacts in the early Holocene: biotic response around the time of the 8200 cal. yr BP event. J. Quaternary Sci., Vol. 22 pp. 77-84. ISSN 0267–8179. Received 6 May 2005; Revised 12 February 2006; Accepted 15 February 2006
ABSTRACT: The early Holocene is characterised by rapid climate change events, which in the North Atlantic region are often associated with changes in thermohaline circulation. Superimposed on this in northwest Europe is localised evidence for human impact on the landscape, although separating climatic and anthropogenic mechanisms for environmental change is often difficult. Biotic and sedimentological evidence from a lacustrine sequence from the Inner Hebrides, Scotland, shows a considerable reduction in inferred local woodland centred upon 8250 cal. yr BP. These data correlate precisely with a distinctive rise in the charcoal:pollen ratio and hence suggest a possible Mesolithic human impact upon the vegetation around this time. A quantitative temperature reconstruction from chironomid analyses from the same sequence, supported by sedimentological data, indicates that the fall in arboreal pollen taxa occurred as climate warmed significantly during the early Holocene. This warming was followed by a significant cold event, with mean July temperatures reduced by 2 C, that lasted for at least 320 years ca. 7790–7470 cal. yr BP. Woodland recovered during this phase suggesting that the vegetation during the 8250 cal. yr BP interval was likely to have been responding to human activity, and not climate, and hence it is possible at specific sites to separate the influence of these key drivers of environmental change. Copyright ß 2006 John Wiley & Sons, Ltd. KEYWORDS: 8200 cal. yr BP climate event; Scotland; pollen analysis; chironomids; Mesolithic.
Introduction
sodes, notably the 8200 cal. yr BP event (Alley et al., 1997; Alley and A´gu´stsdo´ttir, 2005) or its longer-term manifestation (Rohling and Pa¨like, 2005), or whether they are a response to other drivers such as human impact. We focus on a pollen and charcoal sequence from the Inner Hebrides, Scotland, and utilise an independent quantitative temperature reconstruction that has been obtained from chironomid analysis using the same sequence. Chironomids have been shown to be extremely responsive to short-term Holocene climate change in the face of other forcing environmental variables, such as soil change and human impact (Brooks and Birks, 2001b; Heiri et al., 2003a,b; Langdon et al., 2004; Velle et al., 2005; Brooks et al., submitted), hence our approach allows us to consider the most likely roles of potential forcing mechanisms relating to the changes observed in the sequence.
The early Holocene in northwest Europe is characterised by the expansion of human populations into an essentially pristine environment, but coupled with a rapidly fluctuating climate that is thought to have been driven largely by freshwater discharges from Glacial Lake Agassiz (e.g. Clark et al., 2001; Teller et al., 2002) and/or possible variations in solar activity (e.g. Bond et al., 2001). Documenting the magnitude of human influence on the landscape is potentially difficult, as the signature of anthropogenic impacts could derive conceivably from climate change. High-resolution pollen studies in western Scotland provide strong evidence for terrestrial environmental changes during the early Holocene (Sugden, 1999), although identifying the forcing mechanism(s) behind these changes has remained problematic. The aim of this paper is to investigate whether these vegetation changes are most likely driven by climate, and are indeed coeval with specific climate epi-
Site, methods and presentation of results
* Correspondence to: K. J. Edwards, Department of Geography and Environment, University of Aberdeen, Elphinstone Road, Aberdeen AB24 3UF, UK. E-mail:
[email protected]
Loch an t’Suidhe (56 180 1000 N, 6 140 2700 W) is located in the southwest of the Island of Mull, part of the Inner Hebrides archipelago of Scotland (Fig. 1). The lake is approximately
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Figure 1 (a) Map of northeastern Atlantic Ocean and Europe. The rectangular area indicates the Scottish Hebrides. (b) Location of Loch an t’Suidhe (LAS), Isle of Mull, and the Isle of Ulva, in the Scottish Inner Hebrides
50 m in diameter and it lies less than 500 m from the northern coastline of the Ross of Mull in an area of flat bog surrounded by low hills (maximum altitude 85 m a.s.l.). The site is surrounded by a mire that is likely to represent an area of infilling of the original early Holocene lake. A core was collected from the southern edge of Loch an t’Suidhe with a Russian-type corer with chamber dimensions of 50 8 cm (Jowsey, 1966; Sugden, 1999), close to the location of one taken by Lowe and Walker (1986a,b). The core section presented here consisted of dark brown to black gyttja. Sedimentological analyses consisted of loss on ignition (LOI) for percentage organic content (Bengtsson and Enell, 1986). Pollen samples were processed at 2-cm intervals using standard KOH and acetolysis methods followed by embedding in silicone oil (Faegri and Iversen, 1989). Pollen counts were based on 500 total land pollen (TLP) with quantitative estimates of palynomorph concentrations and influx (accumulation rate). Plant names and pollen type nomenclature follow Stace (1997) and the catalogue of Bennett (2002a). Charcoal particles in pollen preparations were quantified by two-dimensional measurement (length and breadth) to produce values initially as total surface area per sample volume (cm2 cm3). In order to identify false charcoal peaks arising from variations in sediment accumulation rates (Swain, 1973; Patterson et al., 1987), the data are presented as charcoal to pollen ratios (Ch:P), although charcoal influx is also calculated. Preparation for chironomid analysis followed the standard methods of Walker et al. (1991) and Brooks (1997). Head capsule counts of >100 were obtained for most samples and a minimum count of 50 was maintained where concentrations were low (considered sufficient to permit statistically significant reconstructions (Heiri and Lotter, 2001; Quinlan and Smol, 2001)). Chironomid-inferred temperatures were reconstructed using a Norwegian chironomid temperature-inference model (Brooks and Birks, 2000, 2001b). This model was based on a training set of 153 lakes covering a temperature range of 3.5–16 C, and produced a weighted average, partial least 2 squares (WA-PLS) three-component model with an rjack of 0.91 and a RMSEP of 1.01 C (Brooks, unpublished data). Five AMS radiocarbon dates on bulk lake sediments (macrofossils, Copyright ß 2006 John Wiley & Sons, Ltd.
which can produce their own problems (e.g. Turney et al., 2000), were not available for dating) relate to the period of interest presented in this paper. Calibration was undertaken via the program CALIB v. 5.1 (Stuiver and Reimer, 2005) (Table 1). Dating estimates (highest probability interval at 2) for the age–depth profile (Fig. 2) and within the text are based on straight-line interpolations with the calibrated dates rounded to the nearest 10 years. The age–depth curve for the period of most interest here (bounded by the three dates centred on 7425 65 BP and taking the central points of the highest probability interval at 2) is markedly linear (r 2 ¼ 0.996). Selected pollen, chironomid and LOI data are shown in Figs 3–5.
Palynological patterns and interpretation The percentage palynological data are closely replicated by the patterns evident when the data are presented in concentration and influx forms (see Figs 3–5). A difference does appear in the basal section of the influx diagram and this is a reflection of the marked change in sediment accumulation after 7875 60 14C yr BP (the placement of that 14C date is, of course, the reason for the inflexion at this point), but has no bearing on the time interval that is the focus of this paper. Given the correspondence between the relative and absolute Table 1 Radiocarbon dates for the Loch an t’Suidhe core (see text for details) Laboratory no. AA-28153 AA-28152 AA-28151 AA-28150 AA-28149
Depth (cm) 648.5–647.5 688.5–687.5 708.5–707.5 726.5–725.5 740.5–739.5
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CLIMATIC AND POSSIBLE HUMAN IMPACTS IN THE EARLY HOLOCENE 640
Vaccinium-type. Arboreal pollen has recovered to 50% TLP by 8000 cal. yr BP, rising to 80% by ca. 7200 cal. yr BP. The reduced tree pollen component lasts for an estimated 560 years and is centred around 8250 cal. yr BP. Within the limits of the radiocarbon method (cf. Bennett, 2002b), this compares favourably with the event (8400–8000 cal. yr BP, centred on 8250 BP) proposed by Alley et al. (1997) and the timing is also close to a European sequence dated by annually laminated sediments in Estonia (8400–8080 BP, maximal cooling at 8250–8150 BP; Veski et al., 2004). Tinner and Lotter (2001) suggest that early Holocene vegetation around two Swiss lakes at a similar date was influenced directly by climate, as a collapse in Corylus was accompanied by the rapid extension in other tree taxa, including Pinus and Betula, and not positively correlated with increases in charcoal concentrations. It was suggested that Corylus was only able to survive prior to an 8200 cal. yr BP cooling as it is able to subsist better with the relatively arid conditions present in the early Holocene. The 8200 cal. yr BP event is thought to have decreased the drought stress in central Europe (Magny and Begeot, 2004), and hence allowed other taxa (including Betula) to out-compete Corylus (Tinner and Lotter, 2001). Many instances of the response levels of palynological and other proxies to climate cooling around 8200 cal. yr BP are reviewed in Alley and A´gu´stsdo´ttir (2005). Given the possible 560 calendar year span for the vegetational changes at Loch an t’Suidhe, the longevity of the event, whatever its cause, is consistent with the extended phases detailed in Rohling and Pa¨like (2005) rather than to an abrupt 8200 cal. yr BP event per se. At Loch an t’Suidhe, however, the decrease in Corylus is associated with a decrease in Betula and is positively correlated with a large increase in Ch:P (Fig. 3). Thus, while the change in vegetation may be associated with climatic change, it is also likely that Mesolithic human impact played a significant role in the woodland reduction (Edwards, 2000, 2004). These pollen data may thus be interpreted parsimoniously in at least three ways. Firstly, the changing fortunes of the pollen spectra may reflect the climatic deterioration of the broader scale 8200 cal. yr BP
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palynological data, the ensuing discussion largely makes reference to the percentage data. The early Holocene is marked by the high abundance of Corylus avellana-type (assumed to represent hazel rather than bog myrtle (Myrica gale), cf. Edwards (1981)) and Betula which dominate the pollen assemblages from ca. 10 470 cal. yr BP (Figs 3 and 4). There follows a rapid decline in these taxa, commencing around 8560 cal. yr BP, and commensurate with a slight increase in other tree taxa, notably Pinus, Ulmus and Quercus which, if local, are presumed to be minor constituents of the woodland. The period of reduced woodland (around 40% TLP) corresponds with a relatively open landscape (lasting until 8000 cal. yr BP) featuring substantial increases in Poaceae, Cyperaceae and Sphagnum (Fig. 5) and a rise in representation for Filipendula and the heaths Calluna vulgaris and
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event. Climate cooling would have been inimical to woodland during the period between ca. 8500–8000 cal. yr BP. The increased levels of charcoal, if climatically mediated, may be a response to greater aridity and incidence of regional lightning strikes that may find an echo in possible instances of enhanced burning during Loch Lomond times (cf. Edwards et al., 2000). Alternatively—and maintaining the climatic perspective—any deterioration may involve increased wetness as well as cooling, hence the rise in such taxa as Sphagnum, Cyperaceae, Calluna, Vaccinium-type and Filipendula which could be associated with the extension of mire and other wetland habitats. Wetness or cooling/wetness scenarios would force a reconsideration of the aridity/lightning argument (see below). Secondly, the reduced woodland and increased herbaceous and charcoal indicators may be reflecting Mesolithic age human impacts in which felled woodland (or woodland falling naturally as a result of senescence, wind-throw or lightning strikes) is burnt for domestic (heating, cooking) purposes (assuming that standing woodland communities are not being burned directly). The openings in woodland thus created or sustained would have the useful by-product of attracting game to the area—grazing would assist in maintaining openings (Buckland and Edwards, 1984)—and would provide a habitat for the expansion of a herb flora dominated by Poaceae and Calluna (Simmons and Innes, 1987). The spread of the latter along with Vaccinium may even have been facilitated by intentional burning of a dry ground layer (cf. Bennett et al., 1992; Edwards, 1996). Lithic artefacts of Mesolithic age are known J. Quaternary Sci., Vol. 22(1) 77–84 (2007)
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from the island of Ulva (Fig. 1), which is visible from the Ross of Mull (Bonsall et al., 1991), as well as from elsewhere in the Inner Hebrides and adjacent coastal areas (Edwards and Mithen, 1995; Macklin et al., 2000). Thirdly, it may be the case that both explanations apply collectively and that hunter-gatherer activity was taking place during a climatic revertence. This cannot be the place to elaborate on comparative palynological data from nearby or more distant sites. It might be mentioned, however, that relatively high-resolution pollen diagrams from A’Chrannaig, Ulva and Loch a’Bhogaidh, Islay (Sugden, 1999; Edwards, 2004) display similar patterns, though the Ulva site may reflect initial climatic effects sustained by human impacts. Other sites from the Inner and Outer Hebrides display reductions in woodland (especially Corylus) with, variously, expansions in herbaceous taxa through the period coeval with the 8200 cal. yr BP event (e.g. Andrews et al., 1987; Edwards et al., 2000).
Lacustrine and chironomid dynamics Similar major patterns of change are produced when the chironomid data are plotted in percentage, concentration and influx form (see Figs 3 and 4). As with the palynological data, however, there are differences in the basal samples; the slow sediment accumulation rates prior to 7875 60 14C yr BP might account for the high chironomid values and there may have been little impediment to high chironomid productivity, whereas the decreased palynomorph totals are probably a response to lower pollen productivity during the early Holocene. Only percentage and accumulation rate chironomid data are presented here (Figs 3 and 4). The majority of the discussion will therefore focus on the percentage data. The early Holocene between ca. 11 000–9000 cal. yr BP shows an increase in chironomid-inferred temperature (CI-T) from around 12–13 C that corresponds with a general expansion in woodland pollen and increase in LOI (the latter 20–47%) (Fig. 5), followed by a stabilisation of CI-T, and slight decline in LOI. The LOI increases significantly between 8400–8000 cal. yr BP (Figs 3–5), probably as a result of either localised deforestation and associated increased runoff high in dissolved organic matter content into the lake, or increased lake productivity under warmer conditions (see below), or as a combination of both processes. The pattern of change in LOI is also marked by rapid oscillations of between 10% and 20%, which is likely to indicate a relatively unstable catchment, as indicated by a drop in arboreal pollen and increases in Ch:P ratios. The chironomid fauna throughout this period is dominated by the warm stenotherms Stempellina and Tanytarsus pallidicornis-type (including Tanytarsus lactescens-type), and replaced the previous dominance of the cold stenotherms Stictochironomus and Tanytarsus lugens-type. The evidence from the chironomids therefore indicates that the vegetation change was occurring while the climate was warming, from around 13 C to over 15 C by 7910 cal. yr BP (Fig. 5). The chironomid community shows a dynamic change after this, with the warm stenotherms being replaced by the cold stenotherm Tanytarsus lugens-type. The CI-T displays a significant cold event lasting from at least ca. 7790–7470 cal. yr BP, with a duration of ca. 320 yr, and resulted in a prolonged cooling of ca. 2 C. A very similar change in chironomid-reconstructed temperatures and associated decline in woodland occurred in the Swiss Alps, centred around 7800 cal. yr BP (Heiri et al., 2004). The magnitude of change in CI-T at Loch an t’Suidhe is Copyright ß 2006 John Wiley & Sons, Ltd.
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large and occurs rapidly compared to the changes within the rest of the sequence. The cooling event is synchronous with a marked drop in LOI that also lasts for the same duration as the CI-T cold event, but also occurs while the percentage of tree pollen is recovering to a maximum of around 80% TLP by ca. 7200 cal. yr BP, and the Ch:P ratio is reduced compared with the preceding five centuries (Fig. 5). While the chironomid evidence indicates a clear pattern of rapid climate change during the early Holocene, and is corroborated to a large extent by the LOI data, it is possible that the biota were responding to changes in precipitation as well as temperature. If major changes in precipitation did occur, they are likely to be manifest in changes in lake levels. While the core location may be littoral at present, the current sample site is clearly an area of infilling and the gyttja of the examined core section indicates that sediment accumulation took place in open water. It would appear that there are clear shifts between littoral and profundal chironomid taxa through the sequence (e.g. Tanytarsus lugens-type is usually profundal whereas Tanytarsus pallidicornis-type is littoral; Figs 3 and 4), hence it is possible that the fauna were responding to past changes in lake level (cf. wetness) rather than temperature per se. Research from Hawes Water in northern England, however, suggests chironomid faunas were primarily driven by temperature during the early Holocene, and responded more to lake-level changes after ca. 7000 cal. yr BP (Lang, 2004). It is possible that the warming phase identified from ca. 8500–7910 cal. yr BP could, though, be due to lake shallowing as the predominant cold stenotherms, which are also profundal, are greatly reduced—this might be difficult to reconcile, however, with the palynological evidence for possible wetness posited above. The subsequent phase of re-expansion of cold stenotherms and an inferred colder climate (7790–7470 cal. yr BP) could relate to lake deepening, with a wetter climate. Although this phase shows a general increase in woodland, the majority of this is due to an increase in Alnus, typically a species associated with wetter substrates, and an associated decline in Corylus which survives better in drier habitats. It should be noted, however, that pollen and spore taxa which can respond positively to increased wetness (e.g. Cyperaceae, Filipendula and Sphagnum), are reduced in relative and absolute terms during this period. This would suggest that the expansion in Alnus pollen is a response to the migration of alder during a time of compliant, although not necessarily increasing, levels of precipitation. One other key feature of the CI-T reconstruction relates to the timing and magnitude of the Holocene thermal maximum (HTM). Few detailed, well dated, and quantitative (T C) reconstructions are available for comparison from northwest Europe for this period, although some other CI-T reconstructions have been undertaken. Sites from relatively high latitudes indicate that HTM conditions occurred from around 10 500–9400 cal. yr BP, with summer temperatures 3.5–3.0 C warmer than at present (Andreev et al., 2005) while a site in northeast Siberia also suggests the HTM occurred around 10 000 cal. yr BP (Porinchu and Cwynar, 2002). Sites in the Kola Peninsula (Ilyashuk et al., 2005) and northern Sweden (Larocque and Hall, 2003) suggest the HTM occurred in these regions between 10 000 and 9000 cal. yr BP, with temperatures around 3.0–2.5 C warmer than present in Sweden. Other sites in northern and central Fennoscandia suggest the HTM occurred a little later, between 8000 and 7000 cal. yr BP with reconstructed mean summer temperatures ca. 2.0–1.0 C warmer than present (e.g. Rose´n et al., 2001; Seppa¨ et al., 2002; Velle et al., 2005; Brooks et al., submitted), while sites in northern Iceland indicate the HTM was not reached until at least 8000 cal. yr BP (Caseldine et al., in press). Our data are clearly not out of phase with these results, especially if we assume J. Quaternary Sci., Vol. 22(1) 77–84 (2007)
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the HTM at Loch an t’Suidhe is not reached until at least 8000 cal. yr BP.
that increased precipitation would also lead to increased runoff and sedimentation (though this is only slight).
Synthesis
Conclusion
There is a clear synchronicity between increases in Ch:P and the decline in woodland pollen from Loch an t’Suidhe, and this event is clearly asynchronous with the subsequent decline in CI-T and LOI. It may well be the case that this is denoting at least a partial separation between human and climatic impact upon the landscape. Were it not for the relatively high resolution sampling at this site, it is likely that the cold reversal alone would be implicated in explaining the decline in woodland, especially as this drop in tree cover occurred either side of ca. 8250 cal. yr BP. The chironomidand LOI-inferred cold event persisted from ca. 7790–7470 cal. yr BP, and although it lasted approximately the same length of time as the 8200 cal. yr BP event, its estimated latest start date (by 7790 cal. yr BP) occurred some 460 years after an assumed central date of 8250 cal. yr BP for the 8200 event. While it is possible that the radiocarbon dates from Loch an t’Suidhe are consistently too young, it is more common for lacustrine dates to have a tendency to be too old as a consequence of the inwash of old detrital carbon (e.g. Edwards and Whittington, 2001). Furthermore, the dates for the rational tree pollen limits are not discrepant with those from the Inner Hebrides as a whole, though the relevant data from the area are not notably consistent in this regard (Sugden, 1999), perhaps a function of variable plant migration processes in island situations. The magnitude of the 8200 cal. yr BP event has been simulated by Renssen et al. (2001) who suggest a decrease in mean annual temperature of 1–2 C around the latitude of Mull. The suggestion is, therefore, that the magnitude and duration of the 8200 cal. yr BP event and the inferred cooling event from Mull are similar, although synchronicity between the two events remains problematic owing to apparent differences in timing. With regard to the inferred cooling episode at Loch an t’Suidhe, we are left with at least four different scenarios:
The palynological data from Loch an t’Suidhe are not inconsistent with anthropogenic changes in which Mesolithic huntergatherers were involved in causing and/or maintaining woodland openings, and in which animal grazing may have been a feature. Rather than indicating a proven period of local climatic cooling/wetness, these biotic changes are occurring during a period of chironomid- and sedimentary-inferred warming, and are followed by a cool/wet interval that may postdate the 8200 cal. yr BP event. If the dating evidence is reliable, the concurrence of palynologically based inferences of climate deterioration and anthropogenic change underscores the need for an awareness of equifinality when attempting to explain environmental data, and this provides a cautionary tale regarding the climatic inference potential of chironomids in some contexts (e.g. Kurek et al., 2004). The recovery of woodland during what might be expected to be a cooling phase suggests that the vegetation could be responding to the cessation of human and/ or faunal activity and not to a change in climate. If the palynological data are not reflecting a climate cooling ca. 8200 cal. yr BP, then any insensitivity may be a function of a muting effect on vegetation caused by the oceanic location of Mull (contra Baldini et al., 2002; Veski et al., 2004), which could be responsible for the inference of possible wetness at this stage. If a cooling event, as inferred from the chironomid and LOI evidence, is occurring post-8200 cal. yr BP, then this may be detecting a more major climatic event (e.g. Hughes et al., 2000; Heiri et al., 2004) than recognised hitherto. This may have been a period during which floristic change was apparently unresponsive or obscured following the restoration of a woodland-dominated vegetational cover subsequent to the removal of human or animal disturbance.
1 A cold reversal event, as possibly shown by the palynological data, is coeval with the 8200 cal. yr BP event, but there is a lag in the responsiveness of the chironomid and sedimentary (LOI) indicators within the catchment. This seems unlikely, however, as chironomids have been shown to respond rapidly to climate change (e.g. Brooks and Birks, 2001a; Larocque and Hall, 2003; Bedford et al., 2004). 2 The chironomid- and LOI-based Mull cold reversal is coeval with the 8200 cal. yr BP event, but the dating control is problematic (this may lead us into the fallacy of ‘suck in and smear’; cf. Baillie, 1991) or erroneous. (We have noted reasons why this may not be so, but some of the 13C values might be considered to be on the low side). 3 The dating control is accurate, and a cold event persisted from ca. 7790–7470 cal. yr BP. A wet/cool climate event ca. 7800 cal. yr BP has also been detected in Britain (Hughes et al., 2000) and further evidence for cooling at this time comes from a chironomid reconstruction in the Swiss Alps (Heiri et al., 2004). 4 Another possibility is that the 8200 cal. yr BP event in this part of Scotland, apart from being a period of cooling, was as importantly one of increased wetness. The chironomid data, however, argue for lake shallowing if anything, as profundal taxa are replaced by littoral fauna. It might be argued Copyright ß 2006 John Wiley & Sons, Ltd.
Acknowledgements Fieldwork and AMS radiocarbon dates were funded by grants from the Natural Environment Research Council with pretreatment carried out at the NERC Radiocarbon Laboratory, East Kilbride, under the supervision of Dr Charlotte Bryant, and measurements at the University of Arizona. We are very grateful to Steve Brooks and John Birks for allowing us to use their Norwegian chironomid-inferred temperature transfer function. Thanks are also due to the drawing office in the Geography Department at Exeter University for preparing the figures. We thank Chris Caseldine, Mike Walker and an anonymous referee for constructive comments on an earlier draft of the manuscript.
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