Dating prehistoric bog‐fires in northern England to calendar years by long‐distance cross‐matching of pine chronologies

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  Dating prehistoric bog‐fires in northern England to calendar years by long‐distance cross‐matching of pine chronologies
   JOURNAL OF QUATERNARY SCIENCE (1997)  12  (3) 253–256 CCC 0267-8179/97/030253–04$17.50 ©  1997 by John Wiley & Sons, Ltd. Rapid CommunicationDating prehistoric bog-fires in northernEngland to calendar years by long-distancecross-matching of pine chronologies FRANK M. CHAMBERS 1 *, JONATHAN G. A. LAGEARD 2 , GRETEL BOSWIJK 3 , PETER A. THOMAS 4 , KEVIN J. EDWARDS 5 and JENNIFER HILLAM 51 Centre for Environmental Change and Quaternary Research, Department of Geography and Geology, Cheltenham andGloucester College of Higher Education, Francis Close Hall, Swindon Road, Cheltenham GL50 4AZ, England 2 Department of Environmental and Leisure Studies, Manchester Metropolitan University, Crewe Campus, Crewe GreenRoad, Crewe, Cheshire CW1 5DU, England 3 Research School of Archaeology and Archaeological Science, University of Sheffield, West Court, 2 Mappin Street,Sheffield S1 4DT, England 4 Department of Biological Sciences, Keele University, Keele, Staffs ST5 5BG, England 5 The Department of Archaeology and Prehistory, University of Sheffield, Sheffield S10 2TN, England Chambers, F. M., Lageard, J. G. A., Boswijk, G., Thomas, P. A., Edwards, K. J. and Hillam, J. 1997. Dating prehistoric bog-fires in northern England tocalendar years by long-distance cross-matching of pine chronologies.  J. Quaternary Sci. ,  Vol. 12 , pp. 253–256. ISSN 0267-8179 (No. of Figures: 3. No. of Tables: 0. No. of References: 26)Received 10 March 1997 Revised 5 May 1997 Accepted 6 May 1997 ABSTRACT: The ages of prehistoric fires can be approximated by radiocarbon dating of charcoalor associated material, but such dating is often inaccurate and at best imprecise. Pine trunkspreserved in British and Irish peats occasionally show firescars, which might be dated throughdendrochronology to yield calendar-year dates. However, unlike oak, there is no master pinechronology to provide absolute dates, so dating is dependent on interspecies cross-matching;for sites in the British Isles with no dated oaks, calendar-year dating of prehistoric pines hashitherto proved impossible. We present a first success in dating, accurately and precisely,prehistoric fire events recorded in subfossil bog-pine trunks, using long-distance cross-matchingof pine chronologies between White Moss, Cheshire, and the Humberhead Levels, England.Results demonstrate a bog-fire in Cheshire in spring 2800  bc , and again in 2710  bc , betweenspring and summer. Further successful long-distance cross-matching of pine would permitinternational climatological comparisons.  ©  1997 by John Wiley & Sons, Ltd. KEYWORDS: fire; prehistory; Scots pine; dendrochronology; absolute dating. half millennium in which they fall (Pilcher, 1993). Improved Introduction methods, such as AMS dating of charred cereal grain, maystill be inaccurate by more than a century; and the ‘wiggle-matching’ (van Geel and Mook, 1989) of contiguous peatCharcoal is often found in Holocene sediments (Tolonen,1986; Patterson  et al. , 1987), and testifies to past natural or layers containing a band of charcoal, cannot pinpoint itsage to better than a half-century. Furthermore, use of suchhuman-induced fires in prehistory and more recent times.The most commonly applied method of dating prehistoric methods is potentially erroneous: radiocarbon dating of char-coal does not date the fire itself, but merely the age of fires is radiocarbon dating, but this is often inaccurate andis at best imprecise (Baillie, 1990; Pilcher, 1991). Individual, growth-years that yielded the charcoal. For charcoal derivedfrom the heartwood of a 400-year-old tree, or from reusedconventional radiocarbon dates may be no better than thetimber, its radiocarbon age may pre-date the fire by hundredsof years (Baillie, 1995). *Correspondence to: F. M. Chambers, Centre for Environmental Change andQuaternary Research, Department of Geography and Geology, CGCHE,Francis Close Hall, Swindon Road, Cheltenham GL50 4AZ, England  Dating prehistoric fires to calendar years More tightly constrained dating is conceivable if twig char- Contact grant sponsor: NERC (UK)Contact grant sponsor: Cheshire County Council  coal were contained within annual laminations, such as lake  254 JOURNAL OF QUATERNARY SCIENCE varves. However, charcoal in varves is allochthonous; it White Moss, Cheshire (research by JGAL) and at Thorne andHatfield Moors in the Humberhead Levels (research by GB).could have washed in from the locus of a catchment firethat took place decades previously, or it could have been Research on bog-pines at White Moss commenced in 1988,based initially from Keele University; whereas work on theredeposited within the lake (Bradbury, 1996). So, althoughhistorical records at particular sites may attest to a historical buried forest at Thorne and Hatfield Moors was initiatedfrom Sheffield University in 1993 as part of an archaeologicalfire in a calendar year, and in particular circumstances thismight be matched with charcoal records in adjacent sedi- and environmental survey of the moors. Current work,guided by the Sheffield Dendrochronology Laboratory, hasments, dating a prehistoric fire to a calendar year is very dif-ficult. matched a 477-year pine chronology (PISY) from the Humb-erhead Levels with prehistoric oak chronologies fromBetter dating might be achieved for the firescars on subfos-sil pine-wood entombed in peat bogs. However, surface England, and with the Garry Bog pine chronology. Thisprovides the first absolutely dated pine chronology fromcharring may represent fire that scorched the tree-trunk, longafter its death; to date a fire event accurately, the firescar England, and the first long-distance cross-match betweenpine chronologies from England and Northern Ireland. Nowmust be encapsulated in the trunk. Encapsulated firescarscan be seen in the cross-sections of felled modern pine trees we can also report that subfossil pines from Cheshire, westof the Pennines, match with PISY, providing the first ever(Lehtonen and Huttunen, 1997); by counting back the ringsfrom present day, a fire event can be dated to the calendar link between pines from different raised mire sites inEngland. The cross-match between pine chronologies PISYyear, perhaps even to its season. Major encapsulated firescarsare encountered occasionally in subfossil bog-pine timbers, (from Thorne Moors, Humberhead Levels) and WM4 (fromWhite Moss, Cheshire) gives  t   =  10.01 — a very significantsuch as those preserved in British and Irish peats, and intheory these timbers might be dated through dendrochronol- match. The Cheshire pine chronology includes trees scarredby fire. Absolute dating of this chronology has enabled theogy (Schweingruber, 1988) to provide calendar-year ages forthe fires (Baillie, 1995). fire events to be dated to calendar years.For such dating to work, the tree-ring sequence incorporat-ing the firescar must be matched with an absolutely datedtree-ring chronology. Unfortunately, unlike for oak (Pilcher et al. , 1984), there is no master pine chronology for the  Methods and results British Isles. Here, absolute dating of Scots pine ( Pinus sylvestris   L.) is dependent on interspecies cross-matchingwith oak ( Quercus  ). A successful cross-match for pine was Pine samples from each site were air-dried and sandedbefore being measured. Ring-width sequences were pro-made in Northern Ireland at Garry Bog (Brown, 1991; Brownand Baillie, 1992), where prehistoric populations of pine duced for each tree, and samples with over 50 rings werecompared using cross-matching programs (Baillie andand oak coexisted, but absolute dating of pines at siteswhere there are no contemporaneous and dated oak timbers Pilcher, 1973; Munro, 1984) and verified by visual compari-son of graphs. Samples that cross-matched with each othernearby has hitherto proved impossible.were combined to form site chronologies. At White Moss,mean tree ring-width measurements were produced for eachof 136 samples. These initially yielded five chronologiesranging from 71 to 261 years (Lageard, 1992). Further sam- Current research pling and analysis has created a 323-year chronology(WM4). One tree (D2.9) showed a major firescar betweenchronology years 82 and 83 (Fig. 2).Research on bog-pines in northern England, conducted inde-pendently in former raised mires either side of the Pennines Established methods (Arno and Sneck, 1977; Sheppard  et al. , 1988) were used to analyse the firescar rigorously for(Fig. 1), has focused on subfossil bog-pines preserved at Figure 1  Map showing location of White Moss, Cheshire; Thorne and Hatfield Moors (Humberhead Levels), South Yorkshire andHumberside. Map also shows location of Garry Bog, Northern Ireland.  J. Quaternary Sci., Vol. 12(3) 253–256 (1997)  ©  1997 by John Wiley & Sons, Ltd.  255DATING PREHISTORIC BOG-FIRES Discussion and conclusions In the initial stages of research on the dendrochronology of bog-pines, there were doubts as to whether meaningful bog-pine chronologies could be compiled satisfactorily from sites.These doubts were assuaged by studies in Scotland (Bridge et al. , 1990; Gear and Huntley, 1991), which demonstratedsuccess in local cross-matching. However, early pine studiesdid not achieve the same success in regional and nationalcross-matching as was possible for oak. Indeed, hitherto,pine growth on mires in the British Isles has been consideredidiosyncratic and perhaps only capable of local (intrasite)cross-matching.The data presented here represent the first long-distancecross-matching of bog-pine between regions in mainlandBritain, and a first success in dating, accurately and precisely,prehistoric fire events recorded as firescars in subfossil bog- Figure 2  Scots pine ( Pinus sylvestris   L.) wood-disc sample D2.9,from White Moss, with major firescar, absolutely dated to the  pine trunks, using long-distance cross-matching of pine spring of 2800 BC. Note the disruption (caused by the fire) to the  chronologies. Results demonstrate a bog-fire in Cheshire previously concentric growth pattern, and the subsequent growth in spring 2800  bc , and again in 2710  bc , between spring release, in the tree ring immediately afterwards. Photograph: and summer. Gerald Burgess. The replication of this first long-distance match betweenEnglish pines was checked by testing WM4 against the Irishpines: WM4 was found to match well against the Irish pinethe season of the year in which scarring had taken place;chronologies from both Garry Bog ( t   =  4.77) and Sharavoguesthe first half of the early wood was affected, indicating a( t   =  4.76). There is now the prospect of success in futurespring fire. This tree, and 10 contemporaneous unscarredattempts at both interregional and international long-distancetrees, exhibited significant growth release in the tree-ringscross-matching of bog-pines — for example, of prehistoricimmediately after the fire; two others suffered severe stress,pines in bogs in Scotland and Ireland. There is the prospectin the form of narrow rings; and two trees appear to havetoo that for significant periods of the Holocene, such asdied as a direct consequence of this fire. (The spatial andthose highlighted by Pilcher  et al.  (1995), pine tree-ringsecological aspects of this will be considered elsewherefrom a range of sites in the British Isles might eventually be(Lageard  et al. , in preparation).) Although the tree (D2.9)tied to calendar years. If calendrical dates can be applied,survived the fire and continued to add tree rings, they neverthen both fire-frequency and year-by-year dendroclimatolog-subsequently completed the trunk’s circumference. A laterical comparisons might be made with pine chronologiesfirescar event, recorded in another sample, took placeelsewhere (Fritts, 1991), for example, in Fennoscandia, wherebetween tree-rings 172 and 173 of the WM4 chronology —there are long (albeit incomplete) chronologies (Briffa  et al. ,this time between the late spring and early summer wood.1990; Briffa, 1994; Zetterberg  et al. , 1996), compiled largelyThe best estimates for the ages of these fires had beenfrom lake sites. Eventually, this would allow comparison of the calibrated radiocarbon ages of subfossil pine wood frominternationally matched palaeo-datasets with modern fire-the site, which at 2-sigma gave a range from 3520 tofrequency data and climate dynamics (Larsen, 1996).2462 cal.  bc  (Lageard, 1992). Now, a significant advancehas produced actual calendar years for the fires. The cross- Acknowledgements   Initial work at White Moss was funded by a match of chronology WM4 against the absolutely dated PISY NERC studentship to JGAL, and by grants from Cheshire County chronology has enabled the fire events at White Moss to be Council. Radiocarbon dates on White Moss pines were funded by assigned to spring 2800  bc  and to between the spring and NERC Scientific Services. Research at Thorne and Hatfield Moors summer of 2710  bc  (Fig. 3).  was initiated by ARCUS, University of Sheffield. GB thanks DavidBrown (Queen’s, Belfast) for comparative pine data and advice. References ARNO, S. F. and SNECK, K. M. 1977.  A Method for Determining Fire History in Coniferous Forests in the Mountain West  . USDAForest Service General Technical Report INT-42, US Departmentof Agriculture, Washington, DC.BAILLIE, M. G. L. 1990. Checking back on an assemblage of published radiocarbon dates.  Radiocarbon  ,  32 , 361–366. Figure 3  Tree-ring graph showing the cross-match between pinechronologies PISY (from Thorne Moors, Humberhead Levels) and BAILLIE, M. G. L. 1995.  A Slice Through Time: Dendrochronology and Precision Dating  . Batsford, London.WM4 (from White Moss, Cheshire), which gives  t   =  10.01 — avery significant match. The fire events at White Moss (identified BAILLIE, M. and PILCHER, J. R. 1973. A simple cross-dating programfor tree-ring research.  Tree-ring Bulletin  ,  33 , 7–14.from firescars) are highlighted. The Thorne Moors chronology,PISY (2921 to 2445 BC), contains 68 trees. The White Moss BRADBURY, J. P. 1996. 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