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Polycyclic aromatic hydrocarbons (PAHs) in lake sediments record historic fire

events: Validation using HPLC-fluorescence detection

Elizabeth H. Denis a,1, Jaime L. Toney a,2, Rafael Tarozo a, R. Scott Anderson b, Lydia D. Roach c,Yongsong Huang a,⇑

a Brown University, Department of Geological Sciences, 324 Brook St., Box 1846, Providence, RI 02912, USAb Northern Arizona University, School of Earth Sciences & Environmental Sustainability, Box 5694, Flagstaff, AZ 86011, USAc Scripps Institution of Oceanography – UCSD, Department of Geosciences, Mail Code 0208, 9500 Gilman Dr., La Jolla, CA 92093-0208, USA

a r t i c l e i n f o

 Article history:

Received 29 April 2011Received in revised form 11 January 2012Accepted 13 January 2012Available online 24 January 2012

a b s t r a c t

Understanding the natural mechanisms that control fire occurrence in terrigenous ecosystems requireslong and continuous records of past fires. Proxies, such as sedimentary charcoal and tree-ring fire scars,have temporal or spatial limitations and do not directly detect fire intensity. We show in this study thatpolycyclic aromatic hydrocarbons (PAHs) produced during wildfires record local fire events and fireintensity. We demonstrate that high performance liquid chromatography with fluorescence detector(HPLC-FLD) is superior to gas chromatography–mass spectrometry (GC–MS) for detecting the low con-centrations of sedimentary PAHs derived from natural fires. The HPLC-FLD is at least twice as sensitiveas the GC–MS in selective ion monitoring (SIM) mode for parent PAHs and five times as sensitive forretene. The annual samples extracted from varved sediments from Swamp Lake in Yosemite NationalPark, California are compared with the observational fire history record and show that PAH fluxes recordfires within 0.5 km of the lake. The low molecular weight (LMW) PAHs (e.g., fluoranthene, pyrene andbenz[a]anthracene) are the best recorders of fire, whereas the high molecular weight (HMW) PAHs likelyrecord fire intensity. PAHs appear to resolve some of the issues inherent to other fire proxies, such as sec-

ondary deposition of charcoal. This study advances our understanding of how PAHs can be used as mark-ers for fire events and poses new questions regarding the distribution of these compounds in theenvironment.

 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Understanding wildfires is exceedingly important in the contextof global climate change because fires have direct effects on globalcarbon storage, atmospheric chemistry (input of CO2, CH4, NO x)and ecosystem diversity (Gill and Bradstock, 1995; van der Werf et al., 2004). Even in the best-case global warming scenarios, firefrequency is expected to increase (Scholze et al., 2006; Westerlinget al., 2006). While wildfires are important globally, increases in

spatial extent, damage and management costs associated with fireshave already been observed in the US over the past decade. In par-ticular, 2006 set the record for the largest area burned in US historyand cost the US Federal Agency over $2 billion (Costs of WildfireSuppression, 2007; National Interagency Coordination Center,

2008). Understanding the natural mechanisms (e.g., climate) thatcontrol fire occurrence in terrigenous ecosystems requires contin-uous records of past fires over hundreds and thousands of years.Anthropogenic activities such as fire suppression have left few nat-ural fire ecosystems in many regions of the US, so land managersrely on reconstructions of fire history to restore natural ecosystemsand to understand how climate influences natural fire regimes.

The most common methods for reconstructing fire history in-clude sedimentary charcoal counts and tree-ring fire scar analysis.

These methods have advanced the understanding of fire frequency,fire extent and the timing of past fires in relation to mechanismsthat control fire, such as climate (Clark, 1990; Niklasson andDrakenberg, 2001; Whitlock and Larsen, 2001; Whitlock andAnderson, 2003; Prichard et al., 2009; Margolis and Balmat,2009). Each method, however, has spatial and temporal limitationsand does not directly detect fire intensity. Sedimentary charcoalanalysis is a time intensive procedure that can require up to 5 ccof sediment collected consecutively along the core depending onthe charcoal concentration (Whitlock and Larsen, 2001). For tradi-tional lake coring techniques with a square rod piston corer (5 cmdiameter), this necessitates using up to a quarter of the core mate-rial. Physical processes of charcoal deposition and degradation can

0146-6380/$ - see front matter    2012 Elsevier Ltd. All rights reserved.doi:10.1016/j.orggeochem.2012.01.005

⇑ Corresponding author. Tel.: +1 401 863 3822; fax: +1 401 863 2058.

E-mail addresses: [email protected] (E.H. Denis), [email protected] (J.L.Toney),  [email protected]   (R. Tarozo),   [email protected]   (R. ScottAnderson), [email protected] (L.D. Roach), [email protected] (Y. Huang).

1 Present address: Department of Geosciences, The Pennsylvania State University,

Deike Building, University Park, PA 16802, USA.2 Co-first author.

Organic Geochemistry 45 (2012) 7–17

Contents lists available at SciVerse ScienceDirect

Organic Geochemistry

j o u r n a l h o m e p a g e :   w w w . e l s e v i e r . c o m / l o c a t e / o r g g e o c h e m

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affect fire history interpretations. Secondary deposition of charcoalcan lead to ambiguous results, as it is possible for charcoal from asingle event to be transported into the lake up to several years later(Whitlock and Larsen, 2001; Whitlock and Anderson, 2003). Tree-ring fire scar analysis, on the other hand, requires a large popula-tion of trees to ensure continuity and to capture all events. Tempo-rally, fire scars are limited to the age of the trees, or in some casesto stumps or downed logs.

Pyrogenic polycyclic aromatic hydrocarbons (PAHs) are pro-duced during the combustion of plant material, such as fossil fuelsor organic material during forest fires, and are predominantly un-branched, mostly 3–6 ring PAHs (Ramdahl, 1983; Page et al., 1999;Yunker et al., 2002; McGrath et al., 2003; Yang et al., 2007). Preli-

minary work shows that the concentration of pyrogenic PAHs in-creases in sediments following forest fires and pyrogenic PAHsare distinguishable from petrogenic PAHs, which are petroleumbased and often have a branched or substituted structure (Pageet al., 1999). A few extant studies use PAHs as indicators of firein the paleorecord in Triassic, Jurassic or Cretaceous age sediments,often in conjunction with charcoal or pollen analysis (Venkatesanand Dahl, 1989; Killops and Massoud, 1992; Arinobu et al., 1999;Marynowki et al., 2007; Marynowski and Simoneit, 2009; van deSchootbrugge et al., 2009; Marynowski et al., 2011). While somestudies show that the ratios of specific PAHs, such as anthraceneto anthracene plus phenanthrene (An/(An + Phe)), indicatewhether the source of PAHs is petrogenic or pyrogenic (Yunkeret al., 2002; van de Schootbrugge et al., 2009), validation of PAHs

as a fire marker through comparison with historical fire eventshas not been undertaken. Therefore, additional studies are neces-sary to determine the relationship between sedimentary PAHsand known fire events to evaluate how PAHs record the fire events.

Sedimentary PAHs have several characteristics that make themattractive fire indicators, including: (1) a high resistance to diagen-esis ( Johnsen et al., 2005), which may help improve the temporalconstraints on tree-ring analysis; (2) a structure related to the tem-perature of the burn event with a more condensed structure(increasing number of rings) related to a higher burn temperature(McGrath et al., 2003), which could provide information on fireintensity; and (3) production during a broader temperature rangethan charcoal (charcoal: 200–600 C, PAHs: 200–900 C or hotter)(Conedera et al., 2009; Lu et al., 2009), which could help to record

higher temperature events than those recorded by charcoal. Inaddition to these important characteristics, PAHs can be quantified

accurately with modern, automated analytical instruments, whichmeans that PAHs possibly could be correlated with fire parameterssuch as fire proximity to depositional basin, total area burned andfire intensity. As aerosols, PAHs potentially can provide a more re-gional record of fire while individual PAH distributions can provideinformation about the type of material that was combusted (Burnset al., 1997; Yang et al., 2007; Lu et al., 2009).

The main concern with using pyrogenic PAHs as fire markers isthe limit of detection on instruments because PAHs are typicallystudied as pollutants in much higher concentrations than thoseproduced naturally by forest fires (Notar et al., 2001; Liu et al.,2007; Oros et al., 2007). In high concentrations, PAHs are easily de-tected using gas chromatography–mass spectrometry (GC–MS)

(Burns et al., 1997; Gabos et al., 2001). However, high performanceliquid chromatography with fluorescence detector (HPLC-FLD) se-lects for and detects PAHs at much greater sensitivities. TheHPLC-FLD’s selectivity and sensitivity is particularly important fordetectingand quantifying PAHs fromhighresolution records whereonly a small volume of sedimentis available. Vergnoux et al. (2011)report PAH analysis by HPLC-FLD from near surface soils. However,there is so far noreport of using HPLC-FLD to analyze PAHs in aqua-tic sediment cores to reconstruct natural fires at longer time scales.

In this study, we assess the use of known pyrogenic PAHs as anindicator of local and regional fire events and as a recorder of fireintensity using freeze core samples that were collected fromSwamp Lake, Yosemite National Park (YOSE), California, US(Fig. 1). Because we have found PAHs that are below the detection

limit of GC–MS with our sampling sizes, we develop a new methodfor PAH measurement by HPLC-FLD. Our comparison of the sedi-mentary PAH patterns with the well documented fire history re-cord (since 1930) of YOSE indicates high fidelity of PAHs as firemarkers. We apply the newly developed PAH marker to a secondcore that extends from 1325 to 1432, and use the PAH concentra-tions to infer fire events adjacent to the lake.

2. Methods

 2.1. Site description

Swamp Lake is located at 1554 m elevation in the northwest

corner of Yosemite National Park, California (37.57N, 119.49W)(Fig. 1). This site was chosen for its isolated location, which reduces

Fig. 1.  (A) GIS map of Yosemite National Park (YOSE) boundary. The white dot is Swamp Lake. Dark gray marks the area burned by 1996 fires and light gray by 1968 fires,2 years with fires within 1 km of the lake. (B) United States Map with YOSE marked in California. Adapted from the web image on the John A. Dutton e-Education Institutewebsite, (https://www.e-education.psu.edu/files/geosc10/image/Textbook%20images/Unit%207/map_Yosemite.gif , accessed 2 March 2010).

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the risk of PAH input from sources other than forest fires. The sur-face area of Swamp Lake is about 8 hectares and its maximumdepth is 20 m with no inlet and only a shallow, marshy outlet(Smith and Anderson, 1992). The forest surrounding the lake isdominated by white fir ( Abies concolor ), black oak (Quercus kel-

loggii), incense cedar (Calocedrus decurrens) and ponderosa (Pinus

 ponderosa), Jeffrey (Pinus jeffreyi) and sugar (Pinus lambertiana)pines (Smith and Anderson, 1992).

From 1891 until 1968, fire management at YOSE consisted of total fire suppression. After 1968, the park performed prescribedburns to restore near natural conditions (van Wagtendonkand Lutz,2007). YOSE is an advantageous site for this study because the  Na-tional Park Service (NPS) has kept detailed geographic informationsystem (GIS) records including area burned and location on all firesthat have occurred in the park since 1930(http://www.nps.gov/gis/data_info/park_gisdata/ca.htm, accessed 18 September 2009).

 2.2. Sample collection

Two freeze cores were collected by Lydia Roach in October 2006(Core SL0601T) and September 2007 (Core SL0708) from SwampLake. The 22 cm SL0601T core contains the surface sediments.The 62 cm SL0708 core was collected from about 49 cm belowthe lake floor as inferred from visual correlation with other freezecores. Both cores are varved (Roach and Cayan, 2007; Cayan andCharles, 2010).

 2.3. Sediment sampling and age control

The SL0601T core and the lower portion of the SL0708 core(36.5–62 cm) were sampled in a   20 C cold room with a razorblade at 1–3 mm intervals, with care taken to follow annual lami-nations. For age control, the presence of annual laminations in theSL0601T core was confirmed through microscopic inspection of thin sections and counting of layers following a distinct charcoalband deposited in 1996 during the massive Ackerson Fire in YOSE.

Light–dark couplets were consistent with a characteristic seasonalcycle of lakes in this region, where diatoms blooms (light couplet)occur in the spring and fall, while terrigenous material (dark cou-plet) is washed into the lake by winter and early spring runoff. TheSL0601T core was dated by varve counting on high resolution dig-ital images backwards from the sediment water interface. Thechronology for the SL0708 core was calculated by determiningthe sedimentation rate through linear interpolation between1872 and 1350 (based on fixed dates for an earthquake and a te-phra layer). Comparison of tephra geochemistry with that from alibrary of previously analyzed tephra samples confirmed that thetephra layer was likely generated from the eruption of the MonoCraters in the mid 1300s. Sieh and Bursik (1986) used radiocarbondating, dendrochronology and stratigraphy to determine an upper

limit of 1368 for this eruption, which they constrained to have oc-curred within a few months to 2 years after the latest eruption of the Inyo craters. This Inyo eruption was dated at 1350 using den-drochronology and death dates (Millar et al., 2006). The SL0708chronology was independently verified by varve counting.SL0601T core samples date from 1898–2006, while the estimatedchronology for SL0708 is 1325–1432.

 2.4. Organic geochemical sample preparation

Freeze dried sediment samples used for extraction range from0.01–0.3 g (weighed accurately to the fourth decimal point or±0.1 mg). Samples were transferred into Teflon extraction vesselsand extracted in 20 ml of 9:1 (v:v) dichloromethane(DCM):metha-

nol at 100 C in a Mars Express microwave assisted, solvent extrac-tion system (Mars 5, CEM corp.) for 30 min with stirring. Samples

were separated by Lydia Roach in the laboratory of Alex Sessions,California Institute of Technology, using the solid phase extractionprocedure and elution schedule described in Sessions et al. (1999).Briefly, 8 ml glass syringe barrels were packed with 0.5 g of sta-tionary phase bulk packing material (Supelco Discovery DSC-NH2). The elution scheme was: F1 –   n-hexanes (4 ml), F2 – 4:1n-hexanes:DCM (7 ml), F3 – 9:1 DCM:acetone (7 ml), F4 – 2% for-mic acid in DCM (8 ml). PAHs were captured in the F1 and F2 frac-tions. Samples were blown down with caution under a gentlestream of nitrogen, just until samples were dry.

F1 and F2 fractions from 71 depth levels from SL0601T and F2fractions from 37 depth levels from SL0708 were prepared forand analyzed with reverse phase, high performance liquid chroma-tography. Each sample was dissolved in 1.0 ml of acetonitrile(ACN), shaken vigorously by hand and sonicated for 40 s. Therewas no change in chromatography when a sample was dissolvedin 20% water and 80% ACN, so 100% ACN was used to allow for eas-ier evaporation. Each sample was filtered (NIOSH, 1998; Ramalh-osa et al., 2009) with a syringe filter (Whatman 4 mm Single UseFilter, 0.45lm PTFE membrane) to a 2 ml vial. Samples wererefrigerated at 5 C and brought to room temperature before anypreparation or analysis. Prior to injection on GC–MS samples wereblown down with caution under a gentle stream of nitrogen, justuntil dry, before being dissolved in DCM for GC–MS analysis.

 2.5. HPLC method development and sample analysis

The HPLC-FLD analyses were performed with an Agilent 1200series chromatograph with a binary pump interfaced to a diode ar-ray detector (DAD) and fluorescence detector in series and an auto-sampler. A method for DAD was developed with a 24 PAH standardmix (Quebec Ministry of Environment PAH Mix, H-QME-01) to as-sess the separation and elution order of the PAH compounds. Oncethe compounds were identified on DAD, the method was developedonFLDto optimizethe signalfor each compoundwhile maintaining

a suitable chromatogram. Note that the PAH acenaphthylene doesnot fluoresce, but could be quantified by DAD in future studies.The original HPLC conditions were developed based on the Agilentmethod (Henderson et al., 2008). The FLD configuration was devel-oped based on previously published studies of PAHs in sedimentsusing HPLC-FLD (Liu et al., 2007; Meire et al., 2008; Salgueiro-Reyet al., 2009). For the FLD, if too many excitation and emittancewavelengths were applied in a short time period, a staircase base-line tended to develop. The peaks and their retention times wereidentified with the standard. The PMT (photomultiplier tube) gainon the FLD settings was adjusted to obtain the highest peak signal,while maintaining a Gaussian shape. Retention time for each PAHbetween runs was very consistent (<2% deviation) and was usedto identify individual PAH peaks for the samples. Two methods

were developed because the original column was later replacedwith a new column with different specifications (Table 1).

The data acquisition and analysis on the HPLC-FLD was donewith ChemStation for LC 3Dsystems software (Rev. B.04.01SP1[647]). An external calibration curve was developed for each com-pound, using the 16 PAH standard mixture solution from Accu-Standard, M-610-QC. The external calibration for retene wasbased on an individual compound standard (Sigma–Aldrich, Tech-nical grade) since retene was not a part of the mixed PAH standard.The calibration curves for all compounds were constructed induplicate with relative standard deviation <3%. The abundancesof the selected compounds were calculated by comparisons of peakareas on the calibration curve with the peak areas of the individualPAHs obtained from the HPLC-FLD chromatograms. Peak identifi-

cation was carried out by comparison of retention times withstandards.

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Analyses for the F2 samples of SL0601T and SL0708 were per-formed according to ‘Method 1’ conditions in   Table 1. TheSL0601T F1 samples were analyzed using ‘Method 2’, which wasdeveloped subsequently with different specifications (Table 1).The two methods were similar and optimized the signal for each

compound while maintaining a suitable chromatogram (Fig. 2).Coelution of PAH isomers on HPLC is possible in real samples de-spite our use of authentic standards, though such coelution doesnot affect our interpretations. Although the F1 fraction for SL0708was not available for analysis, the analysis of the F1 samples from

 Table 1

HPLC-FLD method conditions. Analyses for the F2 samples of SL0601T and SL0708 were performed according to ‘Method 1’ conditions. The SL0601T F1 samples were analyzed

using ‘Method 2’, which was developed subsequently using a new column with different specifications.

Method 1 Method 2

Column: Zorbax Eclipse PAH 2.1 100mm, 3.5 lm Column: Zorbax Eclipse PAH 2.1 50 mm, 1.8 lmFLD settings: Time Ex Em PMT FLD settings: Time Ex Em PMTZero all baselines 4 224 330 12 Zero all baselines 0 224 330 12

8 276 330 12 3.4 276 330 12

9.7 250 368 12 4.7 250 368 1211.55 270 470 15 5.8 270 470 1512.75 240 386 14 6.26 240 386 1414.5 270 390 15 7.5 270 390 1522 300 470 15 9.1 255 420 15

Flow: 0.420 ml/min 11.7 300 470 15Mobile phase: A = Water; B = Acetonitrile Flow: 0.417 ml/minGradient: Time (min) %B Flow Mobile phase: A = Water; B = Acetonitrile

0 40 0.42 Gradient: Time (min) %B Flow0.53 40 0.42 0 45 0.41719.85 100 0.42 10 100 0.41721 100 0.5 15 100 0.41726 100 0.5 16 100 0.327 100 0.42 21 100 0.328 40 0.42 22 45 0.417Stop time = 30.00 Stop time = 22.00

Temp.: 25.0 C Temp.: 25.0 C

Injection: 5.00 ll Injection: 5.00 llSamples: SL0601T and SL0708 F2s Samples: SL0601T F1s

A

-20

20

60

100

140

180

220

260

300

2 4 6 8 10 12

   L   U

Time (min)

1

2

34

5

6

7

8

9

10

1112

13

14

15

B

Fig. 2.  Chromatograms of 16-PAH standard on HPLC-FLD. (A) Method 1 for SL0601T and SL0708 F2 samples. (B) Method 2 for SL0601T F1 samples. A different column wasused in each method (see Table 1), but the two methods are similar and equally appropriate for PAH analysis. The 16 commonly analyzed PAHs are: (1) naphthalene, (2)acenaphthene, (3) fluorene, (4) phenanthrene, (5) anthracene, (6) fluoranthene, (7) pyrene, (8) benz[a]anthracene, (9) chrysene, (10) benzo[b]fluoranthene, (11)benzo[k]fluoranthene, (12) benzo[a]pyrene, (13) dibenz[a,h]anthracene, (14) benzo[ ghi]perylene, (15) indeno[1,2,3-cd]pyrene; acenaphthylene does not fluoresce, but elutes

after naphthalene. Retene, which elutes after chrysene, was not part of this 16-PAH standard mixture solution, but was quantified in the samples based on a separate retenestandard.

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this time period wouldlikely provide a more complete picture of to-tal PAH concentration, especially for the LMW (low molecularweight) three and four ring PAHs, which were found in this fractionfrom the SL0601T core. We can, however, make robust conclusionsbased on F2 only, because (1) the three PAHs we summed for theHMW (high molecular weight) total did not elute in the F1 fractionand the F1 fraction only provided additional LMW data and (2) theF1 fraction helped to slightly accentuate how LMW PAHs recordfires near the lake. Thus, the F2 fraction alone provides sufficientinformation to make assertions of past fires. Note that because in-deno[1,2,3-cd]pyrene (IP) and benzo[ ghi]perylene (Bghi) were only

found in the F2 fractionand not the F1 fraction, the IP/(IP + Bghi) ra-tio can still be utilized to indicate biomass and petroleum combus-tion even if only F2 samples are analyzed.

 2.6. GC–MS analyses

In order to confirm the main compounds identified by HPLC-FLD, GC–MS analyses were performed using an Agilent 6890 seriesgas chromatograph with a split/splitless injector and an autosam-pler interfaced to an Agilent 5973N (quadrupole mass analyzer)mass spectrometer on selected samples that contain relatively highconcentrations of PAHs. Prior to injection on GC–MS samples atroom temperature were blown down with caution under a gentlestream of nitrogen, just until dry, before being dissolved in DCM

for GC–MS analysis. Separation of the compounds was achievedusing a fused silica capillary column (HP-5MS, 30 m 0.25 mm

i.d. 0.25lm film thickness) and injecting 1 ll. The oven temper-ature was programmed from 60 C (isothermal for 1 min) to 100 Cat 15 C/min, then to 310 C a t 6 C/min. The final temperature washeld for 15 min. The carrier gas was helium at 1.0 ml/min. The MSDtransfer line heater temperature was kept at 300 C. The ion sourcewas kept at 230 C and quadrupole mass analyzer at 150 C. Themass spectrometer was operated in the electron impact mode at70 eV ionization energy. Mass spectra were recorded in full scanmode at 50–500 Da. Also, mass spectra were obtained in selectiveion monitoring (SIM) mode to improve instrument sensitivity forlow concentration compounds (mass/charge ratio   m/ z   165 and

166 for fluorene,  m/ z  178 for phenanthrene and anthracene,   m/ z 202 for fluoranthene and pyrene,  m/ z  219 and 234 for retene,  m/ z 

228 for benz[a]anthracene and chrysene,   m/ z   252 and 253 forbenzo[b]fluoranthene, benzo[k]fluoranthene, and benzo[a]pyrene,m/ z  276 and 277 for benzo[ ghi]perylene,  m/ z  276 and 278 for inde-no[1,2,3-cd]pyrene, and   m/ z   278 and 279 for dibenz[a,h]anthra-cene. MSD ChemStation software (ver. E. 02.00.493) was used fordata acquisition and analysis. The PAHs were identified by match-ing the retention times and fragmentation profiles against corre-sponding standards and those in the NIST/EPA/NIH Mass SpectraLibrary version 2.0d.

 2.7. Climate data

The NPS Fire History for YOSE from 1930–2008 was acquiredfrom the NPS Data Store online (http://www.nps.gov/gis/data_in-

Fig. 3.   RLMW and RHMW PAH flux through time compared with charcoal, fire, and precipitation for core SL0601T. Note that PAH flux is on a logarithmic scale. The lowmolecular weight PAHs summed were fluoranthene, pyrene and benz[a]anthracene. The high molecular weight PAHs summed were dibenz[a,h]anthracene,benzo[ ghi]perylene and indeno[1,2,3-cd]pyrene. The National Park Service fire records extend from 1930–2005 AD and there is no observational fire history before 1930

AD. The charcoal record is from R. Scott Anderson (unpublished results).

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fo/park_gisdata/ca.htm, accessed 18 September 2009). The recordsinclude total area burned, longitude and latitude, fire dates andother relevant information for all fires in that time span, althoughnot all fires have equally detailed records. Using ArcGIS, a 200 celldistance raster was created to calculate the distance from the cen-ter of a fire to the Swamp Lake centroid (37.57N, 119.49W). An-nual data were manually separated and viewed as yearly GIS mapsfor analysis of fire characteristics and occurrences. From these datathe number of fires per year and the total area burned per yearwere determined. The maximum, minimum and average distancefrom Swamp Lake and area burned were calculated for each year(Table S1). A year in which a fire occurred within 2 km of the lakewas dubbed a ‘fire year’. Precipitation data for YOSE was retrievedfrom the   NOAA.gov site   (http://www.wrh.noaa.gov/hnx/coop/ynphdqtr.htm, accessed 4 December 2009).

 2.8. Charcoal analyses

The charcoal count record for Swamp Lake (Fig. 3) was based onmacroscopic analyses of charcoal (Anderson et al., unpublished re-sults). Samples for charcoal analysis were taken at 0.5 cm intervals.Each sample was deflocculated in a solution of 10% Na(PO 3)3 (so-dium hexametaphosphate) for 2–5 days, then wet sieved through250 and 125 lm screens. Charcoal particles >100lm reflect occur-rence of local fires because particles of this size do not travel farfrom their source (Whitlock and Anderson, 2003). Charcoal parti-cles were counted and tallied using a dissecting microscope at50 magnification. Charcoal identification was determined basedon particle color, texture and size as follows: (1) particle was com-pletely black; (2) particle exhibited cellular structure; and (3) par-ticle size was >125lm. Incompletely burned particles, notexclusively black in color, were not tallied.

3. Results

 3.1. Fire history data

From 1930–2005, as recorded by the NPS, there were five fireswithin 0.5 km of Swamp Lake, six fires between 0.5–1 km and 11fires between 1–2 km. In 1996 there was a very large fire thatburned around the full perimeter of Swamp Lake (Fig. 1). Basedon the NPS records, the number of fires/year in YOSE has increasedfrom 1930–2005. The total area burned per year in YOSE has in-creased since 1930, with an average area burned per year of 14.7 km2 between 1930 and 2005 and 28.3 km2 since 1970 (Fig. 3).

 3.2. HPLC-FLD versus GC–MS for PAH analyses

To optimize identification and quantification of sedimentaryPAHs, we compared the use of HPLC-FLD to that of GC–MS. The

HPLC-FLD was more sensitive to PAHs than the GC–MS in full scanmode or SIM mode. For example, for the GC–MS in full scan mode,only retene was detected and no other PAHs for the 2001 sample.On the HPLC-FLD, the retene peak was very high (500 LU) and 10other known PAHs were detected and quantified. By comparing

injection mass and the signal/noise ratio, the HPLC-FLD was about60 times more sensitive to retene than GC–MS in full scan modeand about five times more sensitive than GC–MS in SIM mode (lim-it of detection (LOD) for retene: GC–MS full scan – 250 pg/ll, GC–MS SIM – 40 pg/ll and HPLC-FLD – 8 pg/ll). The concentrations of other PAHs in the samples were much lower than retene. For otherPAHs the HPLC-FLD tended to be at least twice as sensitive as theGC–MS in SIM mode (e.g., Table 2).

 3.3. Sediment samples

PAHs were detected only in the first (F1 –  n-hexanes) and sec-ond fractions (F2 – 4:1  n-hexanes:DCM) of the four fractions col-lected. Pyrene, chrysene, retene and benzo[a]pyrene weredetected in almost all of the F1 fractions; phenanthrene, anthra-cene and fluoranthene were detected in most of the F1 fractions,while fluorene was only detected in some and naphthalene andacenaphthene were only detected in a few samples. Benz[a]anthra-cene, dibenz[a,h]anthracene, benzo[ ghi]perylene and indeno[1,2,3-cd]pyrene were not detected in the first fraction.

In the second fraction, phenanthrene, fluoranthene, pyrene,chrysene, retene, benzo[b]fluoranthene, benzo[k]fluoranthene,benzo[ ghi]perylene and indeno[1,2,3-cd]pyrene were detected inalmost all of the samples, benz[a]anthracene, benzo[a]pyrene anddibenz[a,h]anthracene were detected in most samples and naph-thalene, acenaphthene, fluorene and anthracene were only de-tected in some of the samples (Table S2). Approximately 38–45%of the concentrations of LMW PAHs (e.g., phenanthrene, fluoranth-ene, pyrene and chrysene) are found in the F1 fraction and the restin the F2 fraction.

 3.4. PAH data analysis

Unless otherwise noted, further data discussion of the SL0601Tsamples combines the concentrations of compounds found in theF1 and F2 fractions as determined by HPLC-FLD. The PAH fluxesin the SL0601T samples varied in relation to fire events ( Fig. 3).All PAHs had a distinct, high peak in flux in 1967 (Table S2). TheLMW PAHs had a distinct, high peak in 1996, as well. Less distinctpeaks in PAHs occurred in 1953 and 1988. In these 4 years fires oc-curred within 0.5 km of the lake. Some years with fires within 2 kmof the lake are recorded with peaks in PAH flux (e.g., 1958). Individ-ual PAH concentrations have varied over time in relation to fireparameters such as fire proximity, area burned, and number of fires per year and are discussed below.

The core SL0708 samples had several peaks in PAH concentra-tion through time (Fig. 4). Because a constant sedimentation ratewas estimated for this time period, the concentrations were notconverted to flux. At 1394, both fluoranthene and pyrene had high

 Table 2

Limit of detection (LOD) instrument comparison.

Compound HPLC-FLD (pg/ll) GC–MS SIM modea (pg/ll)

Fluoranthene 0.5 1Pyrene 0.5 1Benz[a]anthracene 0.5 5Dibenz[a,h]anthracene 2.5 5Benzo[ ghi]perylene 0.5 1Indeno[1,2,3-cd]pyrene 2.5 1

a Forsberg et al. (2011).

Fig. 4.   RLMW and  RHMW PAH concentrations from core SL0708 through time.

Note that PAH concentrations are on a logarithmic scale and from the F2 fractiononly. The low and high molecular weight PAHs summed are the same as in Fig. 3.

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peaks in concentration. Other periods with peaks in PAH concen-trations occurred at about 1341, 1359, 1378, 1401, 1420 and1432. The SL0601T F2 samples were three to five times more con-centrated than the SL0708 F2 samples for all PAHs except forbenzo[k]fluoranthene. Retene, however, was on average 17 timesmore concentrated in the SL0601T F2 samples than the SL0708F2 samples.

 3.5. PAH ratios indicate a pyrogenic source

Previous studies have used ratios between different PAHs to as-sess whether the PAH source was petrogenic or pyrogenic and todistinguish between combustion source types.   Yunker et al.(2002) show that PAH samples with an An/(An + Phe) ratio >0.10are associated with pyrogenic sources. This ratio was >0.10 for allSL0601T samples in which both PAHs were detected and for allbut one sample for SL0708 (Fig. 5). The indeno[1,2,3-cd]pyrene toindeno[1,2,3-cd]pyrene plus benzo[ ghi]perylene (IP/(IP + Bghi)) ra-tio is used to further distinguish combustion sources: high ratios(>0.50) indicate grass, wood or coal combustion, intermediate ra-tios (0.20–0.50) indicate liquid fossil fuel combustion and low ra-tios (<0.20) indicate petroleum sources (Yunker et al., 2002)(Fig. 5). All SL0708 samples and all but one sample for SL0601Thad IP/(IP + Bghi) ratios >0.50, whichis characteristic of wood com-bustion. In addition, Yan et al. (2005) and Kuo et al. (2011) use theretene to retene plus chrysene (Ret/(Ret + Chr)) ratio to distinguishbetween petroleum/coal combustion (0.15–0.50) and softwoodcombustion (>0.80). All SL0601T samples and all but one samplefor SL0708 had Ret/(Ret + Chr) ratios >0.75. All of these ratios

support that the core samples have a pyrogenic signature and indi-cates that the PAHs were from plant combustion (i.e. forest fires).

4. Discussion

4.1. HPLC-FLD analysis

The HPLC-FLD is more sensitive and has greater selectivity fordetection of sedimentary pyrogenic PAHs than the GC–MS. Basedon the signal/noise ratio for retene, the sensitivity of HPLC-FLD isabout 60 times greater than GC–MS in full scan mode and 5 timesgreater than GC–MS in SIM mode. Due to low concentrations otherPAHs were not detected by GC–MS in full scan mode; however, theHPLC-FLD was at least twice as sensitive as the GC–MS in SIMmode for most other PAHs (Table 2). The HPLC-FLD method devel-oped in this study allowed us to detect and quantify 16 knownPAHs in a single run (Table 1 and Fig. 2). Other PAHs were presentin these samples, which once identified, could be quantified andprovide more information in the future. The HPLC-FLD is moreselective than GC–MS, even in SIM mode, because the HPLC-FLDonly detects compounds that fluoresce (in sediments, those are pri-marily PAHs).

The high sensitivity and selectivity of the HPLC-FLD is impor-tant for assessing how PAHs record fire because pyrogenic PAHconcentrations tend to be much lower than pollutant PAHs. Inaddition, constructing a nearly annual record required using verylittle sediment per sample. In the SL0601T core, for example, yearlyhorizons were  0.3 cm thick, and on average, 80 mg dry mass of sediment was extracted per sample. Thus, it was not possible to

0

0.1

0.2

0.3

0.4

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0.7

0.8

0.9

1

1895 1915 1935 1955 1975 1995

   A  n   /   A  n

   +   P   h  e

 Year 

0

0.1

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1895 1915 1935 1955 1975 1995

   I   P   /   I   P

   +   B  g   h   i

 Year (SL0601T Core)

SL0601T Core

SL0708 Core

1952 Petroleum

Combustion

Petroleum

Grass, Wood 

& CoalCombustion

1325 1345 1365 1385 1405

 Year (SL0708 Core)

Petroleum

Combustion

A

B

1415

Fig. 5.   PAH ratios for cores through time. (A) Anthracene to anthracene plus phenanthrene (An/(An + Phe)) ratio, (B) Indeno[1,2,3-cd]pyrene to indeno[1,2,3-cd]pyrene plusbenzo[ ghi]perylene (IP/(IP + Bghi)) ratio. Source thresholds are based on Yunkeret al. (2002). All SL0708 samples, except one, had An/(An + Phe) ratios >0.10, whichindicate apyrogenic source. Core SL0708 was not included on the plot, because the F1 fraction for SL0708 was not available for analysis and the ratios may not be comparable to

SL0601T. Note that because IP and Bghi were only found in the F2 fraction and not the F1 fraction, the IP/(IP + Bghi) ratio can still be utilized to indicate biomass andpetroleum combustion even if only F2 samples are analyzed.

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use a greater injection volume because the total injection was lim-ited by the amount of sample available.

4.2. PAH relationship to known fire events

The 16 PAHs detected and quantified in the SL0601T samplesfrom 1930–2005 show that PAHs do record fires within 0.5 km of the lake with increased LMW PAH fluxes (Fig. 3). During data anal-ysis we assessed the correlation of individual PAHs and severalPAH ratios with the known fire events and found that the LMWPAH flux best captured the known fires. We chose to use the fluxrather than concentration of PAHs because flux takes into accountthe annual sedimentation rate and a high sedimentation rate mightotherwise dilute the apparent concentration of PAHs in a givensample. Increased concentrations of PAHs do occur during knownfires events, however, the PAH flux provides a more accurate rela-tionship to known fire events.

The LMW and HMW PAHs show different signals. Of the LMWPAHs, fluoranthene, pyrene and benz[a]anthracene showed themost distinct peaks during known fire events, especially thosenearest the lake (wihtin 0.5 km) as indicated by high peaks in fluxfor the 1996 and 1967 fire events (Table S2). We chose to sumthese three LMW PAHs to represent LMW PAHs based on empiricalevidence of correspondence to known fire events and becausethese PAHs were quantifiable in the majority of samples. Thesum of these three PAHs correlated the best with known fireevents, as opposed to a sum of all the LMW PAHs studied, forexample. However, because previous studies show that combus-tion of different organic material (e.g. wood types) can have differ-ent PAH profiles (Yang et al., 2007), we suggest that more researchis necessary from individual sites to determine what pyrogenicPAHs are the best indicators of fire in different forest regimes.Although these three PAHs were the best recorders for fires adja-cent to the lake, the other PAHs provide useful information byamplifying key fire events. For example, the 1967 fire is recordedby a distinct peak in flux for all the PAHs (Table S2). The pervasive

pattern amongmany PAHs highlights the occurrence of a single fireevent and may be more accurate than relying on a single com-pound. We chose to sum three HMW PAHs, dibenz[a,h]anthracene,benzo[ ghi]perylene and indeno[1,2,3-cd]pyrene, based on empiri-cal evidence of correspondence to known fire events and becausethese three PAHs were quantifiable in the majority of samples, inorder to compare LMW and HMW PAH signals (Fig. 3).

Of the five fire events within 0.5 km of Swamp Lake since 1930,the 1996 and 1967 fires were recorded by distinct peaks in LMWPAH flux. The HMW PAHs recorded the 1967 fire, but did not detectthe 1996 fire. All of the individual PAHs had a distinct peak in fluxin 1967. We hypothesize that the different response of LMW andHMW PAHs may be attributed to differences in burn temperature.HMW PAHs with a greater number of rings require higher burn

temperatures (McGrath et al., 2003); therefore, the lack of HMWPAHs in the sedimentary record during the 1996 fire may indicatethat the event was not hot enough to produce as many HMW PAHs.The peaks in LMW PAH flux associated with the 1996 and 1967fires are distinct and suggest that PAHs do not have secondarydeposition in the years following these fires. If there was secondaryPAH deposition, we would expect to see broad peaks in flux or tail-ing of peaks. Therefore, based on these fire years, it is possible thatPAHs could resolve ambiguity resulting from secondary depositionof charcoal particles that can occur up to several years following afire (Whitlock and Larsen, 2001).

The three other fire events within 0.5 km of the lake were in1988, 1953 and 1944; in 1953 there were actually two fires thatburned within this distance. The peaks in flux for 1988 and 1953

were not as high as those for the 1967 and 1996 (Fig. 3). The1944 fire is only evident in retene (Table S2). This fire may not have

been recorded with a high amplitude peak if the transport mecha-nisms reduced the amount of PAHs reaching the lake sediments(e.g., unfavorable wind directionor transport of the PAHs in the fireplume up and away from the burn area).

Several fires occurred within 1 and 2 km of the lake in the late1930s, early 1960s, and late 1970s. Peaks in LMW or HMW PAHflux are not as discernable during these times, which suggests thatPAHs primarily detect fires that are within 0.5 km of the lake site.We evaluated whether variations in individual and combined PAHfluxes through time may relate to other fire parameters, such as to-tal area burned, the number of fires burned per year, or annual pre-cipitation. On average, 51 fires occurred annually from 1930 to2005, with a maximum of 146 fires during 1987. The PAH fluxesbest detect those fires within 0.5 km and show little relationshipto more distant fire parameters.

Notably, increased annual precipitation values occur duringmost fire years except 1988 and 1944. The converse is not true;not all increased annual precipitation years have fires within0.5 km of the lake. Rainfall associated with fire events may helpscavenge PAH particles from the atmosphere and increase the like-lihood of deposition in the lake. Although runoff is a possiblemeans of transport, it is not the sole or main means of transportof PAHs to the lake because otherwise we would expect both an in-crease in PAHs after every high rainfall year and ‘secondary’ depo-sition of PAHs, similar to charcoal (Whitlock and Larsen, 2001),several years after a fire event, both of which are not the case.Rather, there is a distinct and sharp peak in PAHs for both 1967and 1996 fire years, which suggests stream transport is not a majorcontributor. If stream contributed to the majority of PAHs, wewould expect there to be a hump or large tail after the 1996 and1967 fire events. If enhanced rainfall increases the likelihood of deposition in the lake, the low rainfall in 1944 may explain the lackof increase in PAH flux for the 1944 fire.

4.3. Comparison of PAH record to charcoal

PAH fluxes were analyzed from 1898–1929, years that predatethe NPS fire records for YOSE. A charcoal inferred fire record fromSwamp Lake shows that between 1910 and 1925 there were twopeaks in sedimentary charcoal (Anderson et al., unpublished re-sults; Fig. 3). Two peaks also occur in LMW PAH flux during thistime, specifically at 1908 and 1917. In addition, the period fromabout 1920–1930, which predates the NPS fire records, had consis-tently high LMW PAH fluxes that correspond to elevated charcoallevels, as well and may reflect increased fires within a 0.5 km of the lake. The precipitation record for YOSE does not have annualresolution at this time, so it is difficult to assess the impact of rain-fall on charcoal and PAH deposition. A small peak in the charcoalrecord also recorded the 1953 fire, but there was no charcoal peakassociated with the 1967 fire, which was a prominent peak in the

LMW PAH flux. One possibility is that the 1967 fire did not occur inthe drainage basin of Swamp Lake and thus charcoal was notdeposited in the lake, but the PAHs as aerosols did reach the lake.The charcoal record at least partially corroborates the PAH record;however, because the PAHs did record the 1967 fire that did notappear in the charcoal record, PAHs could complement existing firereconstructions and help provide a more complete picture of firehistory for a given site. These data demonstrate that PAH flux re-cords local fire events (within 0.5 km) and demonstrate that PAHscan serve as indicators of fire within the paleorecord.

4.4. Detection of fires from the fourteenth and fifteenth centuries

In this section, we build upon the relationship established

above to suggest the occurrence of unknown fires in the 14thand 15th centuries. We analyzed the LMW and HMW PAHs in a

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core from Swamp Lake that extends from 1325–1432. We interpretthe peaks in LMW PAH concentration at 1341, 1359, 1378, 1394,1401, 1420 and 1432 as fire events within 0.5 km of Swamp Lake(Fig. 4). While the fires we interpret as occurring in 1341, 1359,1378, 1401, 1420, and 1432 were accompanied by a peak inHMW PAH concentration, the interpreted fire event peak at 1394has no corresponding peak in HMW PAHs. Like the 1996 fire event,the 1394 fire event appears to have burned at a lower temperatureand produced few or no HMW PAHs. The fire years inferred fromthe LMW PAH concentrations occurred at  20 year intervals, ex-cept the 1401 fire. This is consistent with the fire return intervalfor the dominant montane vegetation type, inferred from pollenaround Swamp Lake at the time (Smith and Anderson, 1992). Thefire return interval for montane forests ranges from 15–40 years,with an average of 27 years, based on a study conducted in SequoiaNational Park, CA (Knapp et al., 2005; Knapp and Keeley, 2006).This fire return interval is consistent with the interval between in-ferred fire events from our PAH record from about 1325–1432.

4.5. Source of PAHs

The An/(An + Phe), IP/(IP + Bghi), and Ret/(Ret + Chr) ratios indi-cate that for both cores the dominant PAH source was from plantcombustion and PAHs are recording forest fires. Samples with aAn/(An + Phe) ratio >0.10 are associated with pyrogenic sources(Yunker et al., 2002). For SL0601T, all samples in which both phen-anthrene and anthracene were detected have a pyrogenic signature(Fig. 5). For SL0708, only one sample had a ratio lower than the0.10 threshold, which may be due to incomplete data since theF1 samples were not analyzed. Anthracene and phenanthrene wereboth found in abundance in the F1 samples of SL0601T.

For IP/(IP + Bghi), high ratios (>0.50) indicate grass, wood, orcoal combustion, intermediate ratios (0.20–0.50) indicate liquidfossil fuel combustion, and low ratios (<0.20) indicate petroleumsources (Yunker et al., 2002) (Fig. 5). All SL0708 samples and allbut one sample for SL0601T (sample year 1954 ratio was 0.46)had IP/(IP + Bghi) ratios >0.50, whichis characteristic of wood com-bustion. IP and Bghi were only found in F2 fraction and not F1 frac-tion, so the IP/(IP + Bghi) ratio supports wood combustion for bothcores, regardless of F1 data.

Yan et al. (2005) and Kuo et al. (2011) use the retene to reteneplus chrysene (Ret/(Ret + Chr)) ratio to distinguish between petro-leum/coal combustion (0.15–0.50) and softwood combustion(>0.80). The Ret/(Ret + Chr) ratios were >0.75 for all SL0601T sam-ples and all but one sample from SL0708, which may be due toincomplete data since the F1 samples were not analyzed. TheRet/(Ret + Chr) ratios, as well as the high flux of retene comparedto other PAHs, support soft wood combustion as the dominantsource of PAHs to the lake. All three of these ratios are consistentwith the core samples having a pyrogenic signature and indicatingthat the PAHs were from plant combustion (i.e. forest fires).

Retene recorded the 1967 fire very clearly and was much moreabundant throughout both cores compared to other PAHs(Table S2). Because retene is related to the combustion of conifer-ous wood (Ramdahl, 1983), in future work retene should be inves-tigated in more detail, especially for regions with conifers as animportant part of the vegetation history. Comparison of PAH pro-files in sediments after wildfires in a variety of vegetation settingscouldhelpto discern how theburning of different vegetation affectsthe relative distribution of PAHs (Burns et al., 1997; Lu et al., 2009).

5. Conclusions

Previous studies of PAHs in sediments often used GC–MS (e.g.,Burns et al., 1997; Gabos et al., 2001; Notar et al., 2001; Kuo

et al., 2011), but the HPLC-FLD’s selectivity for and sensitivity toPAHs make it an ideal detector for analyzing low concentrationsof natural PAHs produced by regional fires, especially at nearly an-nual resolution where sampling material is limited. In the SL0601Tcore, for example, on average 80 mg dry mass of sediment was ex-tracted per sample. The sensitivity of the HPLC-FLD to retene wasabout 60 times greater than the GC–MS in full scan mode and fivetimes greater than the GC–MS in SIM mode.

Based on a comparison of the YOSE fire records with the PAHsamples from 1930–2005, LMW PAH flux records fires within0.5 km of the lake. Of the five fire events within 0.5 km of the lakein YOSE since 1930, the 1967 and 1996 fires were recorded the bestwith distinct peaks in PAH flux for all PAHs in 1967 and all LMWPAHs in 1996. That only LMW PAHs recorded the 1996 fire sug-gests that this fire burned at a lower temperature, so it did not pro-duce as many HMW PAHs. Fluoranthene, pyrene andbenz[a]anthracene were the best recorders of fire, but patternsfrom many PAHs in combination emphasized the occurrence of afire event. The relationship among rainfall, fire years and PAH fluxsuggest that rainfall during the fire season may help scavengePAHs from the atmosphere and aid in their depositioninto the lake.

Two peaks in PAH flux occurred at about 1908 and 1917 andwere likely two fire events that contributed to the high charcoalcounts during that time. The 1953 fire was also recorded slightlyin the charcoal record, but the 1967 fire, which was very well re-corded by PAHs was not recorded by this charcoal record. Thatthe 1996 and 1967 fires were recorded with such distinct peaksin PAH flux suggests that PAHs do not have secondary depositionin the years following a fire. Future work should include pairedstudies of sedimentary PAHs and sedimentary charcoal to deter-mine if PAHs can resolve the issue of secondary deposition of char-coal particles. The An/(An + Phe), IP/(IP + Bghi), and Ret/(Ret + Chr)ratios indicate that for both cores the dominant PAH source wasfrom plant combustion and that PAHs are recording forest fires.Thus, PAHs can serve as indicators of fires farther in the past.

We infer that several fires occurred within 0.5 km of Swamp

Lake between 1325 and 1432. Around 1394, both fluorantheneand pyrene had high peaks in concentration, which strongly sug-gests a fire nearby the lake, possibly a low temperature burn asindicated by the lack of peak in HMW PAHs. In addition, theremay have been nearby fires around 1341, 1359, 1378, 1401, 1420and 1432. This interval between fire events is consistent with thefire return interval for montane forests, which was the dominantvegetation type at this time (Smith and Anderson, 1992; Knappet al., 2005; Knapp and Keeley, 2006; Anderson et al., unpublishedresults).

By capturing distinct events, PAHs may provide details for morecomplete paleofire reconstructions. Further analysis of PAH fluxesin relation to fire parameters such as fire proximity, burn tempera-ture, vegetation type burned, area burned and the number of fires

per year is necessary to facilitate the correlation of these parame-ters with PAHs. By comparing PAH fluxes with known fire eventswe candetermine how PAHs recordregionalfire in the paleorecord.

 Acknowledgements

This work could not have been achieved without the supportand help of many individuals. Reviews by A. Vergnoux and ananonymous referee substantially improved this manuscript. Wethank Dan Cayan and Jane Teranes for assistance with core collec-tion, Alex Sessions for use of his laboratory for sample preparation,the Limnological Research Center at the University of Minnesotawhere the cores were processed, Lynn Carlson for GIS assistance,and Jan and Kent van Wagtendonk for directions to National Park

Service fire records. This research was supported by NSF062325and EAR-0902805 to Yongsong Huang, and a grant from the

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Yosemite Association to R. Scott Anderson. Laboratory of Paleoecol-ogy Contribution 137.

 Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.orggeochem.2012.01.005.

 Associate Editor  –  Mark Yunker 

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