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Dendrochemical analysis of a tree-ring growth anomaly associated with the Late Bronze Age eruption of Thera

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Dendrochemical analysis of a tree-ring growth anomaly associated with the Late Bronze Age eruption of Thera
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  Dendrochemical analysis of a tree-ring growth anomaly associated with theLate Bronze Age eruption of Thera Charlotte L. Pearson a , * , Darren S. Dale b , Peter W. Brewer a , Peter I. Kuniholm a , Jeffrey Lipton b , Sturt W. Manning a a Cornell Tree-Ring Laboratory, B-48 Goldwin Smith Hall, Cornell University, Ithaca NY 14853-3201, USA b Cornell High Energy Synchrotron Source, Wilson Laboratory, Cornell University, Ithaca NY 14853-8001, USA a r t i c l e i n f o  Article history: Received 9 December 2008Received in revised form23 December 2008Accepted 6 January 2009 Keywords: TheraPorsukDendrochemistryAegeanTree-rings a b s t r a c t The most marked tree-ring growth anomalyin the Aegean dendrochronological record over the last 9000years occurs in the mid 17th century BC, and has been speculatively correlated with the impact of theLate Bronze Age eruption of Thera (Santorini). If such a connection could be proved it would be of majorinterdisciplinary significance. It would open up the possibility of a precise date for a key archaeological,geological and environmental marker horizon, and offer a direct tie between tree-ring and ice-corerecords some 3600 years ago. A volcanic explanation for the anomaly is highly plausible, yet, in theabsence of a scientifically proven causal connection, the value of the proposed correlation is limited. Inorder to test the hypothesis, dendrochemical analysis via Synchrotron Radiation Scanning X-ray Fluo-rescence Microscopy (SXFM), Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) andInductively Coupled Plasma Mass Spectroscopy (ICP-MS) was carried out on growth-ring series from fourtrees displaying the anomaly. Increases of sulfur, calcium, and rare earth elements following the onset of altered growth, plus concentration spikes of zinc and hafnium in the first affected growth-ring providepromising new evidence in support of a volcanic causal factor. Although a volcanic association is implied,the new data are not sufficient to prove a link to the exact eruption source.   2009 Elsevier Ltd. All rights reserved. 1. Introduction During the Late Bronze Age a major explosive eruption of theAegean volcanic island of Thera (Santorini) propelled a layer of ashand pumice across the eastern Mediterranean (Guichard et al.,1993). The known and speculated impact of this event ranges fromthe burial of local towns and villages (Doumas, 1992, 1983), totsunamis and the collapse of Minoan civilization on Crete (Bruinset al., 2008; McCoy and Heiken, 2000b), to a range of climaticdisturbances around the globe (Grudd et al., 2000; Baillie andMunro, 1988). While many such assertions are themselves thesubject of dispute (Eastwood et al., 2002; Pyle,1997), it is the exactdate of the eruption which continues to raise major controversy(Wiener, 2007; Bietak, 2003; Warren,1999), as the volcanic depo-sition layer provides a unique stratigraphic time marker withpotential to resolve the chronology of the eastern Mediterraneanregion.Radiocarbon evidence puts the eruption between 1660 and1600 BC (Manning et al., 2006; Friedrich et al., 2006). Whilst thisevidence, produced by two independent research teams, iscompelling, an  absolute  date, the actual year in which the eruptiontook place, would provide the most useful and appropriate startingpoint for any analysis of subsequent impact on society and climate.The dendrochronological record offers the potential for suchabsolute (precise and accurate) dating to a specific annual growthseason.Over the years, tree-ring records around the globe have yieldedshort-lived anomalous changes in growth connected via modernproxy to the impact of short-term perturbations in climate occa-sioned by Plinian volcanic eruptions (Robock, 2002; Robock andMao, 1995). For the early Late Bronze Age period, two notableclusters of such growth anomalies occur.In 1627/1628 BC narrow growth or frost events are found inNorth European (Grudd et al., 2000; Baillie and Munro, 1988) andNorth American (Salzer and Hughes, 2007; LaMarche and Hirsch-boeck, 1984) dendrochronological records. These have been corre-lated with acidity horizons in the GISP2 (Zielinski et al.,1994) andDye 3 (Clausen et al., 1997) ice cores, and are supported by the *  Corresponding author. E-mail address:  c.pearson@cornell.edu (C.L. Pearson). Contents lists available at ScienceDirect  Journal of Archaeological Science journal homepage: http://www.elsevier.com/locate/jas 0305-4403/$ – see front matter    2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.jas.2009.01.009  Journal of Archaeological Science 36 (2009) 1206–1214  1627–1600 BC date range (Friedrich et al., 2006) derived from anolive tree buried by eruption debris. The accuracy of this narrowdate range has been questioned, however, due to major difficultiesinherent in identifying annual growth increments in olive wood(Vinther et al., 2008).Around 1645 BC, (within the broader radiocarbon range of Friedrich et al., 2006 and Manning et al., 2006), a second cluster ofanomalousgrowtheventsincludesNorthAmerican,SiberianandFinnish ring width minima in 1652 BC and 1648/1649 BC, and afrostdamaged bristlecone pine ring in 1653 BC (Salzer and Hughes,2007; Hantemirov and Shiyatov, 2002; Eronen et al., 2002). Again,possible volcanic forcing is evidenced by correlation with a signifi-cant acidity horizon around 1642  5 BC in the Greenland ice corerecord (Vinther et al., 2006, 2008).While the relevance of any of these specific dates in relation toThera will continue to be argued (e.g. Denton and Pearce, 2008),what can be agreed upon is that there is evidence to place bothvolcanic eruptions and climateperturbations at these twopoints inthe early Late Bronze Age.Within the 16th or early 15th century BC time range favored forthe eruption by proponents of conventional archaeological dating(Wiener, 2003, 2007; Warren, 1999), such evidence is moresparselydistributed.Whilstabristleconepineringwidthminimumin 1597 BC may link with a minor ice-core signal (Salzer andHughes, 2007), this is not well substantiated. Minima in 1544 BC,1524 BC and 1418 BC (with frost damage in 1419 BC) have no otherproxycorrelations,andthelatterinparticularisfaroutsidethedaterange suggested by all radiocarbon evidence.Irrespective of the actual date of the Thera eruption, for anabsolute date for any volcanic eruption to ever be proven via thetree-ring width archive, a direct causal connection (as with sulfurand tephra in ice cores) is required between eruption and growth-ring. Dendrochemical analysis offers a potential means by which toinvestigate the feasibility of proving such a link. The principle thatannual tree-rings can retain a sub-sample of the chemistry of thecontemporarygrowthenvironmentiswellestablished(Watmough,1999; Watmough and Hutchinson,1996,1999; Shortle et al.,1997;Robitaille,1981) though controversial (Hagemeyer,1993; Lo¨vestamet al.,1990; Watmough,1999; Zayed et al.,1992; Smith and Shortle,1996). Whilst the various pathways and physiological controlsdetermining element distribution in the xylem are highly complexand vary between species and with location, dendrochemistry can,and has, proved useful in reconstructing histories of environmentalelemental change (Baes and McLaughlin,1984; Symeonides, 1979;Bondietti et al., 1990; Guyette et al., 1989). Moreover, volcaniceruptions of known date have been traced in previous den-drochemical studies (Hall et al.,1990; Padilla and Anderson, 2002;Pearson et al., 2006, 2005), and a model formalizing the impact of sulfur dioxide-rich volcanic aerosol on metal uptake in trees hasbeen suggested (U¨ nlu¨  et al., 2005). The potential success of suchstudies relies upon a wide range of site and tree specific variables,not least of which is proximity to the erupting volcano. Ideally,therefore, the best chance of establishinga dendrochemical link fortheTheraeruptionwouldcomefromanalysisofcontemporarytree-rings, growing in sufficientlyclose proximity tothe volcano to havereceived some portion of direct fallout from that event.Unfortunately, as yet, there is no Aegean equivalent to the long,continuous, absolutely dated tree-ring series for the second millen-nium BC derived from German and Irish oaks, Swedish pines, or thebristlecone pines of the American southwest. Samples from theAegeanforthiscriticaltimeperiodarelimitedtorarefindspreservedat archaeological sites,cross-dated to build floating chronologies andthen anchored in time by wiggle-matched radiocarbon dates, ratherthan by more conventional dendrochronological procedures. Never-theless, The Malcolmand Carolyn Wiener Laboratory for Aegean andNearEasternDendrochronologyatCornellUniversityhassuchuniquesamplesfromtheearlyLateBronzeAgeinitscollection.Ofthese,61of at least 64 individual trees, represented in over 200 samples fromPorsuk,SETurkey(Fig.1),displayanexceptionalshort-lived(c.7year)growthspikewhichisbyfarthemostsignificantgrowthanomalytobefound in over 9000 years of Aegean tree-rings. Although the exactprovenance of the trees from Porsuk is unknown, it is almost certainthat they grew c. 5 miles to the south of the site, in the TaurusMountains, approximately 840km downwind of Thera (Kuniholmet al.,1992), during the time frame in which, according to the radio-carbon evidence, the Minoan eruption of Thera took place.The samples, retrieved from the foundations of a Hittite posterngate, preserved in a partially carbonized state in anaerobic condi-tions (Kuniholm et al., 1992, 2005), include  Juniperus ,  Pinus  and Cedrus  sp. from two separate building phases. The trees repre-sented range from 19 to 237 years old at the time of the growthspike indicating that the cause was wide ranging, impacting alltrees in a mature forest ecosystem at the same point in time. Theincreaseingrowthischaracterizedbyarapidsurgetobetween200and 700% of normal growth over the first 3 years, tailing back toprevious levels in the next 3–4 years (Fig. 2).Dendroecologically speaking, the most logical cause for ananomaly such as this would be a sudden, short-term improvementingrowthconditionsforthesetrees,specificallytoacooler,moisterenvironment, possibly with increased cloud cover. Although thedegree of climatic impact of the Thera eruption has been muchargued in the literature (Pyle, 1997; Eastwood et al., 2002), evenconservative estimates of sulfur dioxide emissions, the key volca-nogenicfactorinfluencingsurfacetemperatures(RampinoandSelf,1982), are predicted to have resulted in 2 or 3 years of slightlywarmer winters and cooler summers in high latitudes (Pyle,1997).Such conditions could have extended the growth season, resultingin the observed short-term improvement in growth. This makesplausible the suggestion of  Kuniholm et al. (1996) that this may bethe Aegean equivalent to other European growth-ring anomaliesattributed to the Thera eruption, especially in light of recentevidence suggesting the eruption may have been larger thanpreviously estimated (Sigurdsson et al., 2006). Indeed, it is difficultto come up with an alternative hypothesis for the cause, given thevery unique nature of the anomalous pattern. Short-term anthro-pogenic alteration of the water table, or thinning of smaller treesand shrubs (facilitating growth of the more mature trees) arepossibilities, but are impossible to test further given the lack of exact provenance for the wood samples. The onset of the anomaly,starting with relatively dated ring 854, is currently radiocarbondated to 1650 BC  þ 4/  7 (Manning et al., 2003, 2001). Given the proximity to Thera, the nature of the growth pattern, and thecoincidence of the date, a Theran causal hypothesis appears highlypossible. Yet, in the absence of a provable causal connection, suchconjecture is of somewhat limited use.In this paper, high-resolution multi-elemental mapping viaSynchrotron Radiation Scanning X-Ray Fluorescence Microscopy(SXFM) in combination with Inductively Coupled Plasma AtomicEmission Spectroscopy (ICP-AES) and Mass Spectrometry (ICP-MS)isused fora dendrochemical investigationof the tree-ring anomalyfrom Porsuk. The volcanic causal hypothesis is tested via aninvestigation to establish prospects for isolating an elementalmarker signature which could be used to positively link the erup-tion of Thera, or some other event, with the change in growthpattern at this point in time. 2. Methods Samples of   Juniperus  sp. were taken from the archive of theAegean Dendrochronology Project where theyhad previously been C.L. Pearson et al. / Journal of Archaeological Science 36 (2009) 1206–1214  1207  dated both by dendrochronological methods and radiocarbonwiggle-match dating (Manning et al., 2001; Kuniholm et al.,1996).Sections of rings covering the growth anomaly at relative year 854were extracted with a sterile steel scalpel. Prior to chemical anal-ysis, these subsamples were re-measured and re-surfaced witha sledge microtome to produce a flat, uncontaminated samplingsurface. Intensity variations of a suite of elements were measuredvia SXFM at the F3 bending-magnet beamline at the Cornell HighEnergy Synchrotron Source. Incident X-ray energies between 10and 19 keV were selected to maximize the fluorescence of thewidest possible range of elements. A 0.3% bandpass mono-chromatic X-ray beam, using a 15 Å d-spacing Mo/B 4 C multilayerdouble-bounce monochromator was used. The beam was colli-mated to 1 mm and then focused to 30  m m using a single-bouncecapillary focusing optic (Bilderback et al., 2007). To preserve spec-tral integrity, shields of molybdenum metal were placed aroundeach sample to attenuate fluorescence from anything but thesample. The energy spectra were recorded using a Vortex-90EXsingle-element energy-resolving silicon-drift detector and multi-channel analyzer. Each energy spectrum was collected andanalyzed using PyMCA (Sole´ et al., 2007), a purpose-designed,open-source software package which deconvolves overlappingpeaks, determines peak areas and mass fractions for individualelements, and provides an interface for interpretive analysis.Area scans for four different  Juniperus  sp. trees (C-TU-POR-1, 3,26and167)weremadetoinvestigatethephysiologicaldistributionof elements within the wood structure, and the spatial/temporaldistribution of elements in the growth-rings in and around theanomaly. In addition, individual tree-rings from two of the Porsuksamples were prepared and analyzed via ICP-AES (C-TU-POR-3 and26) and ICP-MS (C-TU-POR-26) according to methodologiesand calibration procedures described in Pearson et al. (2006) andPearson (2006), in order to detect the widest possible range of elements for interpretation.Aims were to investigate the temporal/spatial elemental distri-bution of the wood to assess, first: if the sample appeared to becontaminated by the chemistry of the burial environment; andsecond: assuming the samples were suitable for further analysis, tocompare any elemental change detected at the anomaly betweeneach sample. 3. Results The combination of the three analytical methods resulted indetection of a range of over 30 elements, with some detectable byonly one method (e.g. sulfur (S) via ICP-AES or the rare earthelements(REE)viaICP-MS)andothers(e.g.calcium(Ca))replicatedby all methods allowing for intercomparison of data series for thesame element. Fig. 1.  Distance from Thera to Porsuk with known tephra deposition and approximate tephra spread based on data from Eastwood et al. (2002), Sullivan (1988), Guichard et al.(1993), McCoy and Heiken (2000a), and Momigliano (2000). At Sofular Cave, Frisia et al. (2008) report a large spike in sulfur from a speleothem record, which they attribute to the Bronze Age eruption of Thera. Fig. 2.  Example of the Porsuk growth-ring anomaly in tree C-TU-POR-3 (191-yearsequence). C.L. Pearson et al. / Journal of Archaeological Science 36 (2009) 1206–1214 1208  SXFM elemental maps were produced for potassium (K), Ca,manganese (Mn), iron (Fe), copper (Cu), bromine (Br), rubidium(Rb)andstrontium(Sr).Otherkeyelements(e.g.hafnium(Hf),REE)are present in insufficient levels in the wood to be detected by thistechnique. Observable correlations between element distributionand macro/microscopic characteristics of the wood samples madeit possible to discern where elemental changes were most clearlyreflecting the chemistry of the cell structure rather than possiblecontamination in the burial environment. For example, higherintensity Ca, Br, Rb and Sr in the latewood of the growth-ringsshowsphysiologicaldominanceratherthansomesortofpostburialcontaminant. In contrast, correlation of these same elements withdramatically higher intensities towards the outer edge of some of the samples showed the deleterious impact of carbonization andexternal contamination on any potential elemental record withinthe tree-rings (see C-TU-POR-1, Fig. 3).For each of the four samples from Porsuk, increases and/ordecreases in intensity for a range of elements occur at the ring 854growth anomaly. For C-TU-POR-26 and C-TU-POR-3 additionalincreases, decreases or spikes in concentration were detected viaICP-AES and ICP-MS.C-TU-POR-1 showed the least discernible elemental response,largely due to high levels of external contamination. This is shownin Fig. 3 for Ca. Significantly higher Ca intensity towards the exte-rior of the sample makes it difficult to discern a more subtleincrease in the latewood of ring 854 and in ring 855.Ca was the most responsive element detectable by SXFM at thegrowth-ring anomaly. In all samples a slight increase (c.10%) can beobserved at or immediately following the anomaly. This was bestdefinedinC-TU-POR-3(Fig.3)wherebothearlywoodandlatewoodof ring 854 showa 22% increase in Ca intensity preceded bya seriesof uncontaminated, elementally unresponsive rings. In C-TU-POR-26 both K and Ca show an increase following the onset of thegrowth anomaly which corresponds with a depletion of Cu, Br, Rb,and Sr. The response for Ca is shown in Fig. 3. Although there areotherringsinthesequencepriortoring854wherehigherintensityCa can be observed, following ring 854 all latewood Ca shows overa10%increase.Asimilarincreasecanbeobservedintheearlywood,though this is not consistent for all rings following 854. C-TU-POR-167showsasimilarresponseforKandCa,howeverBr,Rb,andSr are also increased in this sample.ICP-AES analysis of C-TU-POR-3 and 26 replicated the observedincreases/decreases in essential elements detected by SXFM. Inaddition, a short-term increase in zinc (Zn) and more sustainedincrease in sulfur (S) were shown following ring 854 in bothsamples (Fig. 4). ICP-MS analysis of C-TU-POR-26 replicated theobserved increases/decreases in essential elements detected byICP-AES. It also showed increases of rare earth elements (REE) inrings 856 and 857, and unusual elements such as selenium (Se) andyttrium (Y) from ring 855 onwards. Of particular note is a spike of Hf in ring 854 (Fig. 5). 4. Discussion The elemental changes observed in the four trees from Porsukfrom the onset of the growth-ring anomaly are not entirelyconsistent. Whilst the majorityof elements appear to increase atorfollowingring854,thatchangeisnotequallystronginalltrees,nordoesitaffectallelementsinthesamewayatexactlythesametime.In the absence of any means to prove the exact growth provenanceof the material it is not possible to address these inconsistenciesfurther as they are likely to reflect localized, micro-site specificvariables.Thekeypointis,whilsttheexacttypeoftheelementalchangeisnot replicated in each tree, some degree of change occurs for mostelements, in all the trees, from the onset of the growth anomaly.This was replicated via three separate analytical methods. The factthat the change is slightly different in individual trees indicatesa response to external elemental variability, impacted by a widerange of micro-site/tree specific factors (e.g. soil depth, chemistry,or the ageof the tree), rather than some type of physiological effectcommon to  Juniperus  sp. in response to more favorable growthconditions. This is supported by the fact that where longersequenceswereanalyzed(e.g.Fig.4),thechangecanbeobservedtoextend beyond the physical anomaly into the resumed regulargrowth. The implication of all this is that, when considering theprobable cause of the ring 854 growth anomaly, we are looking notonly for a change in environmental conditions which causeda short-term improvement in growth, but also an event whichcaused a significant change in environmental chemistry.The demonstrated increase in S provides a working hypothesisfor increases and decreases in other elements at the anomaly.Studies of the impact of anthropogenically induced acid rain onforest ecosystems (Bondietti et al., 1989; DeWalle et al., 1991;Shortle and Bondietti, 1992) have shown that depending on theconcentration of sulfuric deposition and the buffering capacity of the soil at any particular site, other elements (in particular Ca orMn) can be mobilized and more freely incorporated in the xylem.Where acidity levels are particularly high or sustained, Al can bemobilized, binding to the fine root tips and blocking out otherelements (Shortle et al.,1997). Fig. 3.  Maps of Ca intensity for sample C-TU-POR-1, 3, and 26. Relative year 854 is currently dated 1650 BC  þ 4/  7. C.L. Pearson et al. / Journal of Archaeological Science 36 (2009) 1206–1214  1209  So increases and decreases in elements observed in the Porsuktrees can be explained by a sudden influx of acid and its effect onmicro-site specific variables for each of the trees. The distinctincrease in S concentrations shown in Fig. 4 indicates a large influxof Sto the forest system, while increased Ca, Mn, and Sr (e.g. Fig. 5)may indicate mobilization of these elements in response to thatincreasedacidity.Whilstaforestfiremightresultinsimilarchangesinenvironmentalchemistry(Bondiettietal.,1989),thegrowth-ringpatternisnotconsistentwiththeimpactofsuchanevent,norwereany fire scars found in the Porsuk samples. Given the approximatedate of the anomaly, the most logical source of sulfuric depositionwould be volcanic, and given its proximity, Thera would seem themost likelycandidate. Yet S alone does notprove a connectionwiththe Minoan eruption. Likewise, the spikes of Zn (see Fig. 4) and Zr,whilst indicating an influx of new chemistry to the growth envi-ronment (or rapid mobilization of these elements in the soil),cannot be directly linked to a particular source. The only potentialindicators detected for which a specific volcanic srcin might bemore directly hypothesized are the Hf spike (e.g. see Fig. 5), whichoccurs in ring 854, and the increase in REE, Se and Y following theonset of the anomaly. Hf occurs with Zr in zircon crystals in silicaterich igneous rocks. The bedrock of the Taurus Mountains is Fig. 4.  S and Zn in samples C-TU-POR-3 and 26 via ICP-AES. For both samples S rises after ring 854 (marked by vertical line), and a large spike of Zn occurs in ring 854/855. C.L. Pearson et al. / Journal of Archaeological Science 36 (2009) 1206–1214 1210
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