LETTERSPUBLISHED ONLINE: 23 MAY 2010 | DOI: 10.1038/NGEO871

Increased fire activity at the Triassic/Jurassicboundary in Greenland due to climate-drivenfloral changeClaire M. Belcher1*, Luke Mander1, Guillermo Rein2, Freddy X. Jervis2, Matthew Haworth1,Stephen P. Hesselbo3, Ian J. Glasspool4 and Jennifer C. McElwain1

One of the largest mass extinctions of the past 600 millionyears (Myr) occurred 200 Myr ago, at the Triassic/Jurassicboundary. The major floral and faunal turnovers1 have beenlinked to a marked increase in atmospheric carbon dioxidelevels2, probably resulting from massive volcanism in theCentral Atlantic Magmatic Province3,4. Future climate changepredictions suggest that fire activity may increase5, in partbecause higher global temperatures are thought to increasestorminess6,7. Here we use palaeontological reconstructionsof the fossil flora from East Greenland to assess forestflammability along with records of fossil charcoal preservedin the rocks to show that fire activity increased markedlyacross the Triassic/Jurassic boundary. We find a fivefoldincrease in the abundance of fossil charcoal in the earliestJurassic, which we attribute to a climate-driven shift from aprevalence of broad-leaved taxa to a predominantly narrow-leaved assemblage. Our fire calorimetry experiments show thatnarrow leaf morphologies are more flammable than broad-leaved morphologies. We suggest that the warming associatedwith increased atmospheric carbon dioxide levels favoured adominance of narrow-leaved plants, which, coupled with morefrequent lightening strikes, led to an increase in fire activity atthe Triassic/Jurassic boundary.

Global climate has shifted markedly throughout Earth history.However, the effects of climate change on past fire activity arenot well known. The Triassic/Jurassic Boundary (TJB) was a timeof major environmental change, where ‘CO2 increased from 600to 2,100–2,400 ppmv’ (ref. 2; p. 1,387). Both climate models andmodern observations have shown that higher temperatures increaseupper-tropospheric water vapour6. Moreover, it is known thatupper-tropospheric water-vapour variability and global lightningactivity are linked, where a change in average global temperaturesof 1 ◦C is predicted to result in a 40% increase in lightning activity7.The main ignition source of natural fires is lightning, whereover eightmillion strikes occur a day under modern atmosphericconditions8. Global warming across the TJB is therefore likelyto have increased storm activity and ignition of fires. The TJBin East Greenland is characterized by a regional turnover in theecologically dominant plant genera from the Triassic to Jurassic1.Vegetation composition has been shown to explain the occurrenceof different fire activity through time9. Here we evaluate thepalaeofire record across the TJB at Astartekløft, East Greenland,a location where the floras have been intensively studied in a

1School of Biology and Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland, 2BRE Centre for Fire Safety Engineering, School ofEngineering, University of Edinburgh, Edinburgh EH9 3JL, UK, 3Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK,4Geology Department, Field Museum of Natural History, 1400 S. Lake Shore Drive, Chicago, Illinois 60605, USA. *e-mail: belchercm@gmail.com.

stratigraphic context, and test the hypothesis that global warmingled to increased storm-induced fire activity and that climate-drivenvegetation changes provided a positive feedback on fire potential.

We have developed a record of fire activity across the TJBfor East Greenland by assessing the abundance of fossil charcoalfrom eight fossil plant beds from Astartekløft (comprising 53subsamples taken every 10 cm throughout these plant beds,totalling n= 15,449 charcoal particles) and field observations fromSouth Tancrediakløft. We have correlated the Astartekløft sectionusing carbon isotope stratigraphy (Fig. 1) and biostratigraphy(Figs 1 and 2) (from 53 subsamples, 40 were productive forsporomorphs, representing n= 14,579 sporomorphs) to the Eibergbasin (Austria), which contains the candidate Global BoundaryStratotype Sections and Points section for TJB, and the St AudriesBay (UK) section, placing the Greenland sections in a stratigraphiccontext (Fig. 1). The fire-activity data set is compared with apalaeoecological record of vegetation change from the Astartekløftsection, based on >3,000 fossil leaves1 (Supplementary Table S1)and complementedwith newly collected sporomorph data.

A fivefold increase in charcoal abundance is observed (comparedwith average Triassic values). This is coincident with peak negativecarbon isotope values and the highest atmospheric CO2 levelsrecorded across the TJB (Fig. 2). The charcoal record at Astartekløftindicates relatively low fire activity in the latest Triassic (Rhaetian),followed by a sharp rise in fire activity across the TJB transition(Fig. 1). Increased fire activity is sustained through∼14mof sectionfollowing the TJB but returns to typical background Triassic levels26m above the boundary in bed 7 where carbon isotopes returnto pre-excursion values. The increase in fire activity probablyaffected the entire Jameson land region, as abundant charcoalis also found in the earliest Jurassic at the South Tancrediakløftlocality (S.P.H. and I.J.G. field observations). The Triassic and veryearliest Jurassic plant beds represent the same depositional setting(crevasse splay deposits1; Fig. 2); therefore, variations in charcoalabundance are unlikely to be driven by variations in depositionalenvironment. This pattern probably scales up to a supra-regionalor even global scale as a similar pattern has been reported inLate Triassic and Early Jurassic sections in Poland10. In the LateTriassic of Poland, fire activity was sporadic and characterizedby low-temperature wildfires10. Large charcoal pieces and highertemperature fires coupled with high concentrations of polycyclicaromatic hydrocarbons (chemicalmarkers of fire activity) are foundin the earliest Jurassic (Early Hettangian) in two sections in Poland,

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NATURE GEOSCIENCE DOI: 10.1038/NGEO871 LETTERS

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Figure 1 | Correlation of Astartekløft to TJB sections in the UK (St Audries Bay) and Austria (Hochalplgraben). Correlation based on C-isotope recordsand the first occurrence (FO) of the pollen morphospecies Cerebropollenites thiergartii following refs 21,22. C-isotopes at Astartekløft and St Audries Bayfrom ref. 23. First occurrence of Cerebropollenites thiergartii at St Audries Bay from ref. 21. Hochalplgraben data from ref. 22. pCO2 from ref. 2. Likelyreworked sporomorphs are marked with open circles.

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Figure 2 | Stratigraphy, proportions of broad versus narrow leaves and fire activity at Astartekløft. Data from plant beds reported at×3 verticalexaggeration. The grey shadow highlights plant-bed 5. Lithology after ref. 1. Plant-beds 1–5: crevasse splays, plant-bed 6: poorly developed coal swamp,plant-bed 7: abandoned channel1. The Late Rhaetian at Astartekløft is characterized by a declining relative abundance of Ricciisporites tuberculatus and thepresence of Rhaetipollis germanicus, similar to German TJB sections3. The first occurrence of Cerebropollenites thiergartii marks the base of the Hettangianfollowing refs 21,22. Single occurrences of Rhaetipollis germanicus in plant-beds 6–7 likely reworked (marked with open circles). The macrofossil turnoverwithin plant-bed 5 is highlighted to the right (after ref. 16). The macrofossils are mostly broad-leaved in bed 5A and mostly narrow-leaved in bed 5B.

where both factors decline upwards through the Jurassic.Moreover,a similar increase in fire activity is reported across the TJB from theKamien Pomorski IG-1 borehole in northern Poland10.

Fuel accumulation, vegetation distribution, leaf arrangementand connectivity (that is, packed or fragmented) and vegetationquality affect fire risk9. Therefore, changing vegetation compositioncan drive changes in fire activity. It is suggested that narrowleaves have a lower surface area (per unit volume) and are moreflammable than broad-leaved forms, which have a large surfacearea11. Flammable plants in modern ecosystems are characterizedby relatively lowmoisture contents and narrow plant parts that leadto a density and surface area per unit volume optimized for fire

spread12. Less-flammable species have high moisture contents andrelatively coarse dimensions (that is, broad leaves)12. We therefore,should expect to be able to estimate flammability traits in fossilfloras by assuming that narrow leaves will ignite faster and thus leadto increased fire spread rates13. On the basis of these observationswe suggest that leaf morphology may be useful in estimating leafflammability in the fossil record.

The leaves of the dominant taxa across the TJB at Astartekløftwere classified as either ‘flammable’ (narrow-leaved forms) or ‘lessflammable’ (broad-leaved forms) on the basis of their leaf mor-phology (see the Methods section for classification). The flora wasfurther subdivided by habit into canopy and subcanopy/understory,

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LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO871

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Figure 3 | Box and whisker plots of flammability data produced by the FPA. For each group, the 25–75% quartiles are indicated by the box. The median isrepresented by a horizontal line inside the box. The minimum and maximum values are shown with short horizontal lines (‘whiskers’). a, Time to ignition.b, Average heat of combustion. c, Total hydrocarbon flux (THC) rate at the point of ignition for the different sized leaf groups. Replicates: narrow= 5;slim-broad=6; broad=6. Asterisks denote statistically significant differences; see main text for details.

where Ginkgoales and conifers have been classified as canopyand ferns, cycads and bennettites as subcanopy/understory (seeref. 1). Figure 2 shows the changing dominance of flammableversus less-flammable leaf morphologies across the TJB comparedto the fire record at Astartekløft. It can be seen that the Triassicsubcanopy/understory habitat is dominated by less-flammable leafmorphologies. This pattern is broadly repeated in the canopy vege-tation. Plant-bed 1.5 shows an apparent dominance in flammablecanopy forms. This is probably a sampling artefact driven bythe very low number (six) of leaf macrofossils found in thisbed representing canopy vegetation. We also note that this bedpredominantly samples in situ open flood plain taxa, whereasall other Late Triassic beds reflect both closed forest vegetationof the levee’s and the more open floodplain associations1. Lowmacrofossil abundance also occurs for canopy vegetation in bed4 and for understory vegetation in beds 3 and 5a, all of whichhave less than 10macrofossils representing these groups. We notethat the average total number of leaf macrofossils (canopy andsubcanopy/understory) present per bed is 282 ranging between 58(bed 1.5) and 819 (bed 7). Above the TJB, understory vegetationbecomes more flammable with the flora exhibiting increased abun-dance, followed by dominance of narrow-leaved forms. This patternis repeated in the canopy vegetation, although the change towardsmore flammable leafmorphologies occurs just before the TJB.

To elucidate whether the observed changes in dominant leafforms led to the inferred change in fire activity, modern analoguesrepresenting the change in form across the TJB were testedfor their flammability using a fire propagation apparatus (FPA)calorimeter14. Extant conifer species that share morphological andecological similarity to the fossil collections from East Greenlandwere selected. Branches of the plants were collected from theRoyal Botanic Gardens Edinburgh. Six species were selected:Metasequoia glyptostroboides and Glyptostrobus pensilis (narrow-leaved), Wollemia nobilis and Afrocarpus sp. (slim-broad-leaved)and Agathis australis and Nageia nagi (broad-leaved). Each taxonwas tested in triplicate in the FPA. Time to ignition, effective heatof combustion and total hydrocarbon emission were measured intandem during each run using the FPA. We observed that the timetaken for a plant sample to ignite was lower for narrow leaves thanfor broad leaves (Fig. 3a) (KW = 5.208, p = 0.02248), indicatingthat narrow leaves ignite more rapidly than broad leaves. Time toignition is inversely proportional to flame spread rate, which is themost important variable to quantify the intensity of a real-scalefire15; thus, the faster a fuel ignites the higher the flame spread rate13.In general, the average heat of combustion was greater for narrowleaves than for broad leaves (Fig. 3b), implying that narrow leavesburn hotter and quicker. Narrow leaves were also found to have ahigher total hydrocarbon flux rate at the point of ignition (Fig. 3c)compared with broad leaves (KW=6, p=0.01431), suggesting thatthe pyrolysate flux was larger. This is amajor factor in rapid ignition

and flame spread as predicted by ignition theory13. These datasupport the potential use of leaf morphology towards estimation ofleaf flammability in the fossil record.

The Triassic succession at Astartekløft is characterized bylittle charcoal. We therefore suggest that fire played little-to-nosignificant role in ecosystem processes at this time, consistentwith the dominance of the less-flammable (broad) leaf forms.The sudden increase in abundance of charcoal at and above theTJB follows a change in flammability (increase in narrow-leavedmorphologies) of the vegetation into the Jurassic. This observationindicates an intimate relationship between the change in fireactivity and vegetation flammability at this location. Flammablemorphologies (narrow-leaved forms) begin to become abundant inbed 5a (Fig. 2) (0–30 cm below the TJB) at Astartekløft before theincrease in fire activity, with Podozamites (broad-leaved conifer—less-flammable) declining throughout bed 5a and disappearingat the TJB and Stachyotaxus (narrow-leaved—flammable form)doubling in abundance before the increase in fire activity at theboundary (Fig. 2). Fire activity increased markedly from 30 cmabove the TJB. Ecosystem richness and evenness decreased towardsthe TJB, suggesting that resilience to environmental change wasreduced1,16. Under such conditions, a major fire could compoundor trigger further ecological change. From the relative stratigraphicpositions of the observed vegetation changes versus those in fireactivity (Fig. 2), it can be seen that vegetation changed ahead of theincrease in fire activity, implying that vegetation change was notdriven by increased fire activity.

The rise in CO2 across the TJB is suggested to have increasedglobal temperatures by up to 4 ◦C, but the rise in local temperaturesin East Greenland was probably much higher17. This furtherclimatic warming is suggested to have impaired leaf photosyntheticfunction2. Heat loss is maximized in smaller/narrower/moredissected leaves2, suggesting that the change from less-flammable(broad) to flammable (narrow) leaf dominance might be drivenby extinction of large-leaved (broad) species across the TJB,and thus have influenced the survival and apparent subsequentdominance of narrow-leaved forms. We note that bed 7, which seesthe return to pre-boundary carbon isotopic values and decliningCO2, reveals a significantly lower abundance of charcoal thanbeds 5b and 6 (Fig. 2), yet the vegetation remains dominatedby flammable (narrow) forms. Global warming across the TJBis likely to have enhanced storm tracks and convective stormsowing to increased water vapour loadings in the upper troposphere,leading to a likely increase in the frequency of lightning strikesand subsequent ignition of wildfires. The fact that the observedincrease in fire activity is constrained to the TJB carbon isotopicexcursion supports this interpretation. We suggest that increasedvegetation flammability provided a positive feedback on fire activityduring this period. Increased fire activity may also have provided apositive feedback towards increasing the negative isotope excursion

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NATURE GEOSCIENCE DOI: 10.1038/NGEO871 LETTERSand increasing CO2 levels by releasing isotopically light carbon andCO2 through combustion of biomass18. We conclude that globalwarming probably led to increased storm activity and this coupledwith a climate-driven increase in vegetation flammability led to asignificant rise in fire activity at the TJB.

MethodsMacrofossil, fossil charcoal and sporomorph quantification. Rock samples andplant macrofossils from Astartekløft were collected by J.C.M., M.H., M. Popa, SPHand F. Surlyk in 2002 (see Supplementary S1 for location map). Macrofossil plantabundance was quantified as described in ref. 1. For the extraction of charcoal,53 bulk sediment samples (taken at 10 cm intervals throughout the eight plantbeds; ∼10 g) were demineralized to release resistant plant meso- and microfossilparticles. Treatment with cold hydrochloric acid (10%–37.5% HCl) for 72 h andcold concentrated hydrofluoric acid (38%) for a further 72 h removed carbonatesand silicates, respectively. The process was completed with a final treatment incold concentrated HCl overnight to avoid calcium fluoride precipitation. Sampleswere then rinsed with water until a neutral pH was achieved. The remainingorganic material was sieved through a 150 µm mesh and both fractions collected.The >150 µm residues of all of the 53 samples were examined using a low-powerbinocular microscope and the numbers of charcoal particles observed werequantified. Selected samples of charcoal were also analysed by means of scanningelectron microscopy to confirm their identification as charcoal (SupplementaryFig. S2). For extraction of sporomorphs, between 15 and 20 g of each sample waswashed and crushed, then dried for 24 h at 60 ◦C. Two Lycopodium spore tabletswere added to each sample, and the dry weight recorded. Each sample was treatedtwice alternately with cold HCl (30%) to remove carbonate minerals, and withcold hydrofluoric acid (38%) to remove silicate minerals. The residue from eachsample was washed with water until pH neutral, then sieved with 250 and 15 µmmesh. Finally, organic and inorganic residues were separated using ZnCl2. Twoslide preparations were made in glycerine jelly. All 53 samples were analysed forbiostratigraphic markers.

Analysis of vegetation flammability. ‘Flammable’ and ‘less flammable’ groupingsin the fossil flora are as follows. (1) Ginkgoales: dissected leaf forms (narrow-leaved;coming under the ‘flammable’ leaves grouping) are classified as those forms havingleaf portions where each dissected length is 10 times its width. ‘Less-flammable’forms are those with limited dissection of lengths <10 times their width.(2) Conifers: we have used Harris’s19,20 classification of the conifers as to whetherleaves are ‘broad leaved’ (‘less-flammable’) or ‘needle like’ (narrow-leaved;‘flammable’). (3) Ferns/cycads and bennettites: ‘flammable’ leaved forms arethose having pinnules <0.7 cm wide (‘less-flammable’ >0.7 cm wide). (SeeSupplementary Table S1 for information and data set.)

The FM global fire propagation apparatus is a calorimeter for advancedflammability measurements. Its operation for synthetic materials is detailed inthe Standard ASTM-E 2058. Gases were collected at 150 l s−1 rate. Plant sampleswere placed into a basket of 400 cm3 volume, open at the top with highly porouswalls (66% openings). Leaves were left on the stems, which were not compressedin the container, so that the plant architecture was maintained. Four infraredheaters each with six halogen lamps, set at 120–144V provided a radiative flux of40 kWm−2 of thermal radiation to the samples. This dries the sample and causesit to pyrolyse, generating flammable gases. The thermal radiation acts as if a firefront were approaching, which heats the vegetation in advance of ignition. A smallpilot flame, serving as the igniter, is positioned above the sample; once the pyrolysisgases mix with fresh air to form a flammable mixture, the sample ignites13. Theflame ignition of the gases leads to the steady burning of the fuel until burn out.Mass loss is measured by a load cell, with an accuracy of 0.1 g. A quartz cylinderaround the sample allows unperturbed supply of air, while enabling radiant energyfrom the infrared heating system to reach the sample surface. The flow of theexhaust combustion gases and concentration of oxygen, carbon dioxide and carbonmonoxide weremeasured. Each plant species was tested three times.

Received 2 March 2010; accepted 22 April 2010; published online23 May 2010

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AcknowledgementsWe thank J. Torero for providing access to test facilities and institutional supportand R. Hadden for help in the FireLab at the University of Edinburgh. Thanks toP. Thomas (Royal Botanic Gardens, Edinburgh) for providing modern vegetationsamples. L.M. thanks W. M. Kürschner for palynological guidance. We thank M. Popafor sample collection and floral taxonomy of the East Greenland samples, F. Surlykand N. Nøe-Nygaard for their aid in C.M.B.’s and L.M.’s 2009 Greenland field seasonand also D. Sunderlin’s and F. Surlyk’s contributions during a 2004 field season.Technical assistance from B. Moran (UCD) and N. Welters and J. van Tongeren(Utrecht University) is gratefully acknowledged, as is that of P. Ditchfield (Universityof Oxford). We thank A. McGowan for comments that we feel improved the quality ofthe manuscript. We acknowledge financial support through a European Union MarieCurie Excellence Grant (MEXT-CT-2006-042531) and the National Geographic Society(7038-01) for funding the 2002 expedition.

Author contributionsC.M.B. conducted charcoal and fossil flora flammability analysis. L.M. conducted thepalynological analyses. C.M.B., F.X.J. and G.R. conducted flammability tests on modernplants. S.P.H. conducted carbon isotope analyses. M.H. provided information onmodernequivalents of the TJB flora. J.C.M. provided plant macrofossil abundance data and bulksediment samples from Astartekløft. I.J.G. and S.P.H. provided field observational datafrom S. Tancrediakløft. C.M.B. analysed data and wrote the manuscript. C.M.B. and L.M.drafted the figures. All other authors contributed to editing themanuscript.

Additional informationThe authors declare no competing financial interests. Supplementary informationaccompanies this paper on www.nature.com/naturegeoscience. Reprints and permissionsinformation is available online at http://npg.nature.com/reprintsandpermissions.Correspondence and requests formaterials should be addressed to C.M.B.

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