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A mid to late Holocene cryptotephra framework from eastern North America

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内容提示: A mid to late Holocene cryptotephra framework from eastern NorthAmericaHelen Mackaya , * , Paul D.M. Hughes a , Britta J.L. Jensen b , 1 , Pete G. Langdon a ,Sean D.F. Pyne-O'Donnellb , Gill Plunkett b , Duane G. Froese c , Sarah Coulter d ,James E. Gardnerea Geography and Environment, University of Southampton, Southampton, SO17 1BJ, UKb School of Geography, Archaeology and Palaeoecology, Queen's University Belfast, Belfast, BT7 1NN, UKc Department of Earth and Atmospheric Sciences, University of Alberta,...

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A mid to late Holocene cryptotephra framework from eastern NorthAmericaHelen Mackaya , * , Paul D.M. Hughes a , Britta J.L. Jensen b , 1 , Pete G. Langdon a ,Sean D.F. Pyne-O'Donnellb , Gill Plunkett b , Duane G. Froese c , Sarah Coulter d ,James E. Gardnerea Geography and Environment, University of Southampton, Southampton, SO17 1BJ, UKb School of Geography, Archaeology and Palaeoecology, Queen's University Belfast, Belfast, BT7 1NN, UKc Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, T6G 2E3, Canadad Omagh Minerals Ltd, 56 Botera Upper Rd., Omagh, BT78 5LH, UKe Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, TX, 78705, USAa r t i c l e i n f oArticle history:Received 18 August 2015Received in revised form29 October 2015Accepted 14 November 2015Available online 10 December 2015Keywords:TephraCryptotephraVolcanic ashPeatlandsLate HoloceneNorth Americaa b s t r a c tHolocene cryptotephras of Alaskan and Pacific Northwestern origin have recently been detected ca.7000 km away on the east coast of North America. This study extends the emerging North Americantephrochronological framework by geochemically characterising seventeen cryptotephra layers from fournewly explored peatlands. All detected tephras were deposited during the late Holocene, with no ho-rizons present in the peat between ca. 3000e5000 years ago. The prevalence of the Alaskan White RiverAsh eastern lobe (AD 847 ± 1) is confirmed across the eastern seaboard from Newfoundland to Maineand a regional depositional pattern from Mount St Helens Set W (AD 1479e1482) is presented. The firstoccurrences of four additional cryptotephras in eastern North America are described, three of which mayoriginate from source regions in Mexico, Kamchatka (Russia) and Hokkaido (Japan). The possibility ofsuch tephras reaching eastern North America presents the opportunity to link palaeo-archives from thetropics and eastern Asia with those from the western Atlantic seaboard, aiding inter-regional compari-sons of proxy-climatic records.© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license(http://creativecommons.org/licenses/by/4.0/).1. IntroductionPrecise palaeoclimatic comparisons between sites and regionsare essential for understanding past climate dynamics. However,inter-site correlation is often limited by poor chronological control.Tephrochronology provides an age-equivalent dating method byusing volcanic ash layers with unique geochemical signatures astime-specific marker horizons (isochrons) to connect and syn-chronise archives (e.g. Alloway et al., 2013; Lowe, 2011). Theseisochrons are used to create high-resolution records of palae-oenvironmental or archaeological events, the relative timing ofwhich can be compared across sites and regions (e.g. Hall et al.,1993; Lane et al., 2013a, b; Lowe et al., 2012; Plunkett andSwindles, 2008). An air-fall ash layer from a volcanic eruption canbe regarded as having been deposited instantaneously in geologicaltime and can thus adopt the eruption's agewherever it is found as awell-defined primary horizon. If the eruption history is unknownor poorly constrained then archives can still be correlated if thesame tephra horizons are present and these fixed tie-points can beused to create a common timescale (Lowe, 2011).Investigations of far-travelled microscopic volcanic glass shards(cryptotephra e with dimensions typically <~125 m m; sensu Loweand Hunt, 2001) in sediments allow for the detection of previ-ously unrecognised ash horizons and sometimes unknown erup-tions. These cryptotephras provide the opportunity to obtainprecise chronologies in areas that were thought to be outside therange of tephrochronology, thus greatly expanding the datingframework and increasing the number of regions that can be linkedtogether. Cryptotephra studies originally focused on Icelandictephras in Western Europe, but the potential for North Americancryptotephra studies is rapidly emerging (Payne et al., 2008; Pyne-* Corresponding author. School of Geography, Politics and Sociology, University ofNewcastle, Newcastle upon Tyne, NE1 7RU, UK.E-mail address: Helen.Mackay@newcastle.ac.uk (H. Mackay).1Current address: Royal Alberta Museum, Edmonton, AB, T5N 0M6, Canada.Contents lists available at ScienceDirectQuaternary Science Reviewsjournal homepage: www.elsevier.com/locate/quascirevhttp://dx.doi.org/10.1016/j.quascirev.2015.11.0110277-3791/© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Quaternary Science Reviews 132 (2016) 101e113 O'Donnell et al., 2012).The first crypto-tephrostratigraphy for the eastern seaboard ofNorth America in Newfoundland was recently developed from onepeatland, Nordan's Pond Bog (Pyne-O'Donnell et al., 2012). Seventephras in this ca. 9000-yr-long sequence were correlated tosources in Alaska and the Cascade Range, four of which occurredduring the late Holocene: tephra from Mount St Helens (MSH) setW, AD 1479e1482 (Fiacco et al.,1993; Yamaguchi,1985; Yamaguchiand Hoblitt, 1995); White River Ash, eastern lobe (WRAe), ~AD847 ± 1 (Jensen et al., 2014a); Newberry Pumice ca. 1460 cal yr BP(Kuehn and Foit, 2006); and Aniakchak, Greenland Ice Core Chro-nology (GICC05) age 3590 ± 1 BP (Coulter et al., 2012). A fifth lateHolocene tephra was tentatively correlated with Mount AugustineG, ca. 2100 cal yr BP (Tappen et al., 2009). However, subsequentgeochemical comparisons with reference materials suggest thatalthough this tephra shares characteristics with material sourcedfrom Augustine, it is unlikely to be Augustine G (Kristi Wallace,pers. comm.). All correlated tephras in Nordan's Pond Bog were ofNorth American origin but the detection of horizons from othersource regions cannot be precluded. Ash from the 2010 eruption ofEyjafjallaj € okull, Iceland, approached Newfoundland (Davies et al.,2010) and tephra from Changbaishan, China, has been identifiedin Greenland ice cores (Sun et al., 2013); therefore, sources evenfurther afield cannot be discounted.This study builds on the existing eastern seaboard record bycontributing four newly developed peatland tephrostratigraphiesfrom the region. Undisturbed peatlands are excellent archives forpreserving tephrostratigraphies since cryptotephra horizons areoftenpresent in discrete layers that have been subjected tominimalpost-depositional movement (Dugmore and Newton, 1992;Dugmore et al., 1996; Payne et al., 2005). The primary air-falltephra deposit may be reworked in some peatlands, particularly ifthe site has been disturbed (e.g. Swindles et al., 2013). However, themajority of deposited shards are usually confined within a narrowstratigraphic layer of no more than a few centimetres depth(Swindles and Plunkett, 2011). The findings from this study extendthe known regional spatial distribution of previously identifiedtephras, add several newly characterised tephras, and demonstratethe increased potential of this technique in obtaining late Holocenehigh-precision chronologies. The major and minor element chem-istry of several newly characterised tephras in this study suggeststhat there is potential for delivery of tephra from more distal, andpreviously unconsidered source regions, to eastern North America.2. MethodsThe study sites, Saco Heath (SCH10: 43 ? 33 0 05 00 N; 70 ? 2 0 03 00 W),Villagedale Bog (VDB12: 43 ? 31 0 09 00 N; 65 ? 31 0 54 00 W), Framboise Bog(FBB12: 45 ? 43 0 14: N; 60 ? 33 0 09 00 W) and Jeffrey's Bog (JRB12:48 ? 12 0 46 00 N; 58 ? 49 0 06 00 W) are ombrotrophic plateau bogs locatedalong a south-west to north-east transect across Maine, NovaScotia, and Newfoundland (Fig.1). The cores weresampled from thecentre of each bog using an 11-cm-diameter Russian pattern corer,following a full stratigraphic investigation based on the Troels-Smith (1955) system.The stratigraphic position and shard concentration of thecryptotephra layers were established by the standard method ofashing the peat at 5 cm contiguous intervals (Pilcher and Hall,1992). The ashed residues were mounted in glycerol and countedunder a high power microscope. Guided by these counts, thestratigraphic depth of cryptotephra layers were refined to 1 cmresolution. Samples containing less than fifteen shards in the 5 cmresolution counts were not investigated further since they wereunlikely to yield sufficient shard concentrations to be compre-hensively geochemically characterised. If two consecutive samplescontained similar elevated concentrations of cryptotephra shardsthen both wereinvestigated at 1 cm resolution. Depthscontaining afurther local rise in shard concentration within sections of suc-cessive elevated cryptotephra concentrations were also analysed at1 cm resolution. Samples containing peak tephra concentrationswere selected for geochemical analysis (Fig. 2; Appendix A,Supplementary Information), based on the assumption that theyare representative of the primary air-fall deposition (cf. Payne andGehrels, 2010).Glass shards for electron probe microanalyses were extractedfrom the peat matrix using the heavy liquid flotation method(Blockley et al., 2005), modified with additional cleaning floats andgentle stirring to improve shard extraction yields. The flotationmethod was chosen to avoid any possible chemical alteration thatmay arise from the alternative acid digestion technique (Blockleyet al., 2005). Whilst other peat studies have obtained consistentresults using acid digestion (e.g. Roland et al., 2015), flotation wasdeemed an important precaution since the shards were small witha high surface to volume ratio, characteristics which may makeshards prone to chemical alteration (Blockley et al., 2005; Dugmoreet al., 1992). Extracted shards were mounted on epoxy resin discsand exposed at the surface by careful grinding and polishing.Major and minor element compositions of single glass shards ofunknown cryptotephra horizons were determined by electronprobe microanalysis (EPMA) with wavelength dispersive spec-trometry (WDS-EPMA) at the Tephra Analytical Unit, University ofEdinburgh, using a 3 m m beam (Appendix B.1, SupplementaryInformation). This beam size was used because shard sizes weretypically very small (25e102.5 m m) with many vesicles (Hayward,2012). Ksudach 1 (KS1) proximal tephras were analysed at theUniversity of Edinburgh using the same parameters and at Queen'sUniversity Belfast (analytical set up outlined in Appendix B.2,Supplementary Information). All Mount St. Helen's, Jala Pumiceand White River Ash glass analyses were analysed at the Universityof Alberta on a JEOL 8900 using a 10 m m beam, 6 nA current, and15 keV voltage. Analyses at all laboratories used a similar suite ofminerals and glass for calibration, and a Lipari obsidian as a sec-ondary standard to track the quality of calibration and assurerepeatable analyses (e.g. Kuehn et al., 2011). The results of thestandard analyses were consistent, predominantly remainingwithin the accepted analytical range (Appendix B, SupplementaryInformation). Therefore, the datasets are comparable among labo-ratories. All results are normalised to 100% on a water and volatilefree basis (e.g. Froggatt, 1983; Lowe, 2011) to further assist com-parisons. Correlations were identified by searching the Universityof Alberta tephra database (containing North American geochem-ical data with some Russian and Icelandic data), the Queen's Uni-versity Belfast dataset (containing Icelandic and Russiangeochemical data), Tephrabase (Newton et al., 1997; containingIcelandic/Europe and Mexican geochemical data) and publishedliterature. Potential correlations to the unknown cryptotephralayers were visually examined using biplots of selected elements,with correlation strengths indicated by similarity coefficients(Borchardt et al., 1972; Appendix C, Supplementary Information).Age-depth models were constructed using14 C measurementson Sphagnum stems with the exception of the basal dates, whichwere obtained from bulk peat or brown mosses (Appendix D,Supplementary Information).14 C measurements were convertedto calendar age distributions using the IntCal13 calibration curve(Reimer et al., 2013) and the weighted averages of the date rangedistribution (2 s ) are referred to throughout. Bayesian age-depthmodels were constructed using the R package “BACON” (Blaauwand Christen, 2011) assuming piece-wise linear accumulation(Fig. 2; Appendix D, Supplementary Information). The age-depthmodels were constructed using BACON's default prior settingsH. Mackay et al. / Quaternary Science Reviews 132 (2016) 101e113 102 (based on Goring et al., 2012), with the exception of the accumu-lation mean which was reduced to 10 years cm ?1 to reflect averageaccumulation rates of oceanic raised peatlands.3. Results and discussionCryptotephras extracted from seventeen layers across the fourpeat cores were geochemically analysed (Fig. 2), with all isolatedhorizons lying within the upper 2 m of peat. Attempts were madeto geochemically characterise other layers of tephra containing ca.15e20 shards/5 cm 3 ; however, too few shards were successfullyisolated from the peat matrix despite using large volumes of peat(up to 10 cm 3 ). These cores may therefore contain more low con-centration cryptotephra horizons that have not been characterisedwithin this study. Background shard concentrations are generallylow throughout the cores (average shards/5 cm 3 : SCH10 ¼ 1;VDB12 ¼ 3.5; FBB12 ¼ 2.6; JRB12 ¼ 3).Each crypto-tephrostratigraphy contains one prominent erup-tion occurring at 118 cm, 100 cm, 70 cm and 131 cm from thewesternmost to easternmost sites, respectively. The glass compo-sitions are rhyolitic, with some dacitic shards present in SCH10-42and VDB12-42 (Appendix E, Supplementary Information). Eachpeak appears to represent a geochemically discrete tephra, with theexception of SCH10-42, which consists of two populations. Asdescribed below, eight of the cryptotephras investigated correlatewith two known eruptions, four have possible correlations (FBB12-31, FBB12-162, VDB12-90, VDB12-176) and five have not beencorrelated with known late Holocene eruptions from North Amer-ica, Iceland, Mexico, Kamchatka or Japan on account of geochemicaldifferences or insufficient geochemical data (SCH10-42, SCH10-57,VDB12-42, VDB12-53, FBB12-47; Tables 1 and 2). All potentialcorrelations are supported by core chronologies, major elementgeochemistry and shard morphology where reference material wasavailable.3.1. Correlated eruptions3.1.1. Mount St Helens (MSH)JRB12-71 (AD 1404e1578) correlates to the MSH set W erup-tions of AD 1479 (MSH-Wn) to 1482 (MSH-We) (Fig. 3). The tephrais characterised by colourless pumiceous shards, similar to prox-imal samples of Mount St. Helens W. Distinguishing between thedominant ash layers from this set, Wn (which extended north-eastwards) and We (which extended eastwards), is difficultbecause of the chemical similarities and short time interval be-tween these ash layers (ca. 2e3 years; Fiacco et al., 1993;Mullineaux, 1996; Yamaguchi, 1985). However, when plottingJRB12-71 with proximal data from both eruptions, this tephrademonstrates a clear affinity with the We layer (Fig. 3). This isconsistent with conclusions from Nordan's Pond Bog (Pyne-O'Donnell et al., 2012). The MSH set W ash is restricted to thenorthern study site, a pattern that could either be indicative of apatchy tephra fall-out distribution (e.g. Davies et al., 2010) or it mayshow that the southernmost limit of detectable cryptotephradeposition for this eruption lies close to Newfoundland. Analysesfromfurther sites will be requiredtodistinguish between these twopossibilities.3.1.2. The White River Ash (WRA)All four sites contain horizons of highly pumiceous shards thatgeochemically correlate with the eastern lobe of the WRA (WRAe)from Mount Bona-Churchill, Alaska (Jensen et al., 2014a; Fig. 4;Appendix C.2, Supplementary Information). This tephra has beenrecently correlated to the European cryptotephra, “AD860B”(Jensen et al., 2014a) which has been dated by GICC05 toAD 847 ± 1(Coulter et al., 2012). Three tephra layers characterised withinSCH10 and two characterised within FBB12 geochemically correlatewith the WRA (discussed in Section 3.4). The upper WRA horizonsin both sites contain shard counts that are an order of magnitudehigher than the other WRA layers within these cores; therefore,they have been assumed to represent the primary deposition of theFig. 1. The location of study sites Saco Heath (SCH10), Villagedale Bog (VDB12), Framboise Bog (FBB12) and Jeffrey's Bog (JRB12) in relation to the previous Newfoundland recordfrom Nordan's Pond Bog (Pyne-O'Donnell et al., 2012).H. Mackay et al. / Quaternary Science Reviews 132 (2016) 101e113 103 WRAe eruption. All core chronologies within this study (based onmodelled14 C measurements) provide age estimates that encom-pass the GICC05 date and further support the assumption that theupper WRA layers in SCH10 and FBB12 do represent the primarydeposition of WRAe. The detection of this cryptotephra at all foursites builds on the previous identification of the WRAe inNewfoundland (Pyne-O'Donnell et al., 2012) and shows its preva-lence across the eastern seaboard of North America. The largestWRAe horizon is located in Maine (SCH10-118), with an order ofmagnitude more shards (2120 shards/5 cm 3 ) than the closest studyFig. 2. Tephrostratigraphies from the four study sites. Shard concentrations are rhyolitic shards/5 cm 3 and layers extracted for EPMA analyses are highlighted in red. Higherresolution shard concentration data are provided in Appendix A, Supplementary Information. The radiocarbon age-depth models indicate results from Markov Chain Monte Carloiterations completed in Bacon (Appendix D, Supplementary Information). The darker areas represent the most likely age ranges. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)H. Mackay et al. / Quaternary Science Reviews 132 (2016) 101e113 104 site Villagedale Bog, Nova Scotia. Whilst local conditions generatevariable intra-peatland shard concentrations (Watson et al., 2015),the observed distribution of the WRAe within this study suggeststhat the southernmost limit of this ash has not yet been located.This may indicate that there is an opportunity to extend the tephradistribution map in North America and connect archives acrossgreater distances.3.2. Potential correlationsThe following potential correlations are based on major-elementgeochemistry, age data, and glass morphology where referencematerial was available. Additional access to proximal referencematerials and/or data, and trace-element geochemistry could helpconfirm these correlations.Table 1Means and standard deviations of single-grain glass compositions from cryptotephra layers. All data are normalised. n ¼ number of analyses.Sample SiO 2 TiO 2 Al 2 O 3 FeOt MnO MgO CaO Na 2 O K 2 O H 2 O diff nSCH10-42 Mean 70.12 0.46 15.63 2.89 0.07 0.82 3.07 5.07 1.86 0.78 6Pop. 1 StDev 1.74 0.05 1.21 0.23 0.01 0.16 0.40 0.32 0.14 2.16SCH10-42 Mean 75.89 0.27 13.19 1.44 0.04 0.18 1.22 4.20 3.57 3.04 3Pop. 2 StDev 1.62 0.24 1.93 0.59 0.01 0.08 0.66 0.37 0.17 1.86SCH10-57 Mean 70.74 0.39 14.94 2.73 0.11 0.65 2.75 4.80 2.88 1.21 4StDev 0.41 0.01 0.18 0.14 0.02 0.02 0.16 0.29 0.07 1.65SCH10-118 Mean 74.06 0.19 14.22 1.54 0.05 0.40 1.93 4.35 3.27 3.51 26StDev 0.51 0.02 0.48 0.16 0.01 0.04 0.11 0.19 0.09 1.81SCH10-131 Mean 74.06 0.18 14.44 1.45 0.05 0.37 1.87 4.35 3.23 2.87 15StDev 0.84 0.03 0.56 0.20 0.01 0.06 0.17 0.21 0.12 1.87SCH10-150 Mean 73.79 0.20 14.70 1.54 0.05 0.39 1.89 4.29 3.15 2.56 13StDev 0.23 0.01 0.40 0.12 0.01 0.02 0.08 0.20 0.07 1.94VDB12-42 Mean 70.44 0.48 15.26 2.96 0.06 0.80 3.13 5.03 1.83 2.31 11StDev 0.58 0.04 0.24 0.34 0.01 0.19 0.18 0.29 0.16 1.31VDB12-53 Mean 75.67 0.38 12.72 2.26 0.06 0.33 2.60 3.85 2.12 0.50 2StDev 0.65 0.01 0.34 0.07 0.01 0.03 0.72 0.26 0.08 0.67VDB12-90 Mean 71.64 0.26 15.46 1.94 0.10 0.37 1.36 5.37 3.49 2.25 23StDev 0.50 0.02 0.37 0.12 0.01 0.04 0.11 0.22 0.11 1.39VDB12-100 Mean 74.38 0.17 14.26 1.42 0.04 0.35 1.81 4.27 3.30 3.05 15StDev 0.95 0.03 0.61 0.19 0.00 0.08 0.18 0.25 0.15 1.89VDB12-176 Mean 73.76 0.36 13.96 2.64 0.12 0.49 2.23 5.03 1.40 2.16 9StDev 0.63 0.03 0.62 0.34 0.01 0.08 0.16 0.22 0.07 2.15FBB12-31 Mean 72.52 0.37 14.62 2.37 0.05 0.61 2.47 4.81 2.17 2.72 12StDev 1.63 0.07 0.77 0.37 0.01 0.16 0.45 0.24 0.20 2.57FBB12-47 Mean 76.58 0.21 12.95 1.53 0.03 0.25 1.45 4.44 2.55 2.37 2StDev 0.23 0.01 0.07 0.07 0.00 0.02 0.14 0.22 0.15 0.28FBB12-70 Mean 74.90 0.17 13.76 1.47 0.04 0.34 1.84 4.22 3.24 4.14 23StDev 0.97 0.06 0.71 0.34 0.01 0.10 0.29 0.32 0.38 1.44FBB12-105 Mean 74.84 0.19 14.08 1.44 0.04 0.35 1.79 4.18 3.10 2.50 14StDev 0.79 0.03 0.55 0.20 0.01 0.06 0.24 0.37 0.30 2.25FBB12-162 Mean 76.72 0.34 11.86 2.21 0.06 0.44 2.40 3.64 2.34 3.10 5StDev 0.34 0.03 0.24 0.07 0.01 0.02 0.19 0.16 0.04 1.61JRB12-71 Mean 76.29 0.22 13.29 1.45 0.03 0.23 1.39 4.55 2.55 1.55 18StDev 0.79 0.02 0.66 0.11 0.01 0.04 0.23 0.27 0.15 1.66JRB12-131 Mean 74.32 0.19 14.25 1.49 0.05 0.36 1.87 4.20 3.28 2.56 27StDev 0.49 0.03 0.38 0.13 0.01 0.05 0.14 0.18 0.10 1.91Table 2Means and standard deviations of single-grain glass compositions from reference material of potential correlatives, all data are normalised, n ¼ number of analyses. Datasources: MSH-Wn and MSH-We: Pyne-O'Donnell et al. (2012) and Jensen et al. (2014b); MSH Layer T: Jensen et al. (2014b) and this study; Ceboruco P1 and Ksudach KS1: thisstudy; Tarumai Ta-c2: Nanayama et al. (2003); WRAe: Pyne-O'Donnell et al. (2012) and Jensen et al. (2014a); WRAn: Jensen (2007).Sample SiO 2 TiO 2 Al 2 O 3 FeOt MnO MgO CaO Na 2 O K 2 O Cl H 2 O diff nMSH-Wn Mean 74.83 0.19 14.32 1.61 0.04 0.30 1.70 4.63 2.32 0.08 2.75 83StDev 0.42 0.03 0.44 0.10 0.03 0.03 0.08 0.25 0.11 0.03 2.45MSH-We Mean 75.89 0.25 13.51 1.50 0.04 0.26 1.44 4.50 2.50 0.10 2.61 66StDev 0.50 0.04 0.27 0.08 0.03 0.03 0.12 0.19 0.09 0.03 1.94MSH Mean 70.80 0.44 15.69 2.88 0.06 0.72 2.89 4.69 1.93 0.12 1.80 62Layer T StDev 1.52 0.08 0.73 0.43 0.03 0.17 0.54 0.31 0.21 0.04 1.78Ceboruco Mean 71.44 0.28 15.94 1.96 0.09 0.34 1.34 5.13 3.39 0.102 1.89 25P1 StDev 0.21 0.05 0.14 0.07 0.03 0.03 0.04 0.16 0.09 0.03 1.05Ksudach Mean 73.32 0.40 13.97 2.67 0.16 0.45 2.22 5.22 1.35 0.17 2.54 28KS1 StDev 1.14 0.09 0.75 0.53 0.06 0.14 0.31 0.53 0.10 0.02 1.38Tarumai Mean 76.47 0.34 11.94 2.15 0.04 0.46 2.27 3.97 2.36 e 2.20 15Ta-c2 StDev 0.82 0.04 0.22 0.29 0.02 0.14 0.26 0.08 0.12 e 1.04WRA east Mean 73.89 0.21 14.49 1.52 0.05 0.36 1.83 4.13 3.19 0.34 2.73 107(WRAe) StDev 0.63 0.04 0.32 0.20 0.02 0.07 0.18 0.16 0.17 0.04 1.48WRA north Mean 73.96 0.21 14.41 1.61 0.06 0.33 1.77 4.10 3.23 0.32 2.69 50(WRAn) StDev 1.16 0.07 0.53 0.25 0.03 0.08 0.30 0.18 0.15 0.05 1.54H. Mackay et al. / Quaternary Science Reviews 132 (2016) 101e113 105 3.2.1. FBB12-31 (AD 1565e1830)Major and minor element geochemistry of FBB12-31 match wellwith data collected on both proximal and more distal referencesamples of MSH layer T (Fig. 5). This tephra was deposited in AD1799e1800 (Yamaguchi, 1983) and it is associated with the MountSt. Helens Goat Rocks eruptive period (Mullineaux, 1996). Layer Tformed a narrow lobe limited to the northeast of the volcano andalthough thickness and grain-size rapidly decrease away fromsource, it has been reported as a visible unit up to 500 km from thesource (Mullineaux, 1996). The glass morphology (pumiceousshards with the occasional bubble-walled shards) and the 2 smodelled age range of FBB12-31 confirm the similarity of this ho-rizon to layer T. The correlation is also supported by the lack ofother Mount St. Helens eruptions that fall into the required agerange (e.g. MSH set X, ash bed Z or set W) with this specificcomposition (Mullineaux, 1996; Jensen et al., unpublished data).3.2.2. FBB12-162 (3604e2643 cal yr BP)This layer is characterised by pumiceous shards. Only sevenanalyses were successfully completed, of which two appear to bepotentially unrelated to the main population. The remaining ana-lyses characterise a high-SiO 2 rhyolite (~76e77 wt%) that hasrelatively low Al 2 O 3 , but high FeO t and CaO for this SiO 2 concen-tration in comparison with available glass compositional data fromAlaska, Cascades (Appendix F, Supplementary Information), Iceland(Tephrabase, Newton et al., 1997) and Kamchatka (Vera Ponomar-eva, pers. comm., 2014) in this age range. Comparisons with Japa-nese glass chemistries (Hughes et al., 2013; Nanayama et al., 2003)indicate that Hokkaido is a possible source region for this tephra(Fig. 6; Appendix C.3, Supplementary Information). Biplots of K 2 Oand TiO 2 concentrations have proved useful for distinguishing be-tween Hokkaido regional volcanoes (Aoki and Machida, 2006;Furukawa et al., 1997; Tokui, 1989); however, comparisons be-tween the FBB12-162 and proximal ash are limited both by thenumber of cryptotephra shards analysed from FBB12 and the lack ofpublished individual proximal shard chemistries. FBB12-162 hasgreater geochemical similarities with the proximal ash fromTarumai than that from Komagatake. This potential correlation issupported bythe ageof the Tarumai tephra Ta-c2 (3000-2000 cal yrBP; Sato, 1971; Nanayama et al., 2003) that is comparable to themodelled age of FBB12-162. However, the lack of reference dataprecludes the firm correlation with Tarumai. The geochemicalcharacteristics of two shards from VDB12-53 also show similaritieswith Japanese tephra (Table 1; Appendix C.3, SupplementaryInformation); however, the number of successfully typed shardsand the exhaustion of core material prevent a firm correlation.3.2.3. VDB12-176 (2055e1771 cal yr BP)This horizon is characterised by pumiceous, sometimes blockyshards. The glass composition of this unknown tephra is strikinglysimilar to Ksudach 1 from the eastern volcanic front of the Kam-chatka Peninsula (KS1; Fig. 7AeB). Ksudach is a large shield volcanowith five overlapping calderas (Volynets et al., 1999). The eruptionof KS1 in ca. 1800 cal yr BP was Kamchatka's second largest Holo-cene eruption (Braitseva et al.,1997), containing three proximal fallunits of white or yellow pumice, density current deposits and a fallunit of grey pumice (Andrews et al., 2007; Braitseva et al., 1996).Initially, ash was deposited to the north but during the early phasesof the eruption it travelled eastward until the final stages when itshifted westward (Melekestsev et al., 1996). Fall units can beidentified over 1000 km from the source covering an area of 2e3million km 2 (Braitseva et al.,1996). Glass morphology, compositionand similar age estimates strongly support this correlation.3.2.4. VDB12-90 (AD 889e1130)The geochemistry in this horizon does not match any knowneruptions of this age from Alaska, the Cascade Range (Appendix F,Supplementary Information), Kamchatka (Vera Ponomareva, pers.comm., 2014), Iceland (Tephrabase, Newton et al., 1997) or Japan(Appendix C.3, Supplementary Information). However, examina-tion of published geochemical data available from Mexican sourcesshowed that the chemistry closely resembled the main Plinian falldeposit (P1) of the Jala Pumice, which was deposited by thecaldera-forming eruption of Volc ? an Ceboruco, AD 990e1020Fig. 3. Harker diagrams comparing the major element glass compositions of MSH-Wn, MSH-We and JRB12-71. JRB12-71's composition mirrors the geochemical trend seen in MSH-We.H. Mackay et al. / Quaternary Science Reviews 132 (2016) 101e113 106 (Chertkoff and Gardner, 2004; Gardner and Tait, 2000; Sieron andSiebe, 2008). Volc ? an Ceboruco is located ca. 4400 km from thepeatland in southeastern Nova Scotia, making the distance of ashtransport ca. 800 km less than that between Mount Bona-Churchill(WRAe) and VDB12. Re-analysis of lapilli from the upper part of theP1 proximal deposit, collected at Locality 18 in Fig.1 of Gardner andTait (2000), confirm that VDB12-90 and P1 chemistry share strikingsimilarities (Fig. 7CeF). The geochemical data are supported by theVDB12 core chronology. Therefore, it seems likely that VDB12-90represents a distal deposit of the Jala Pumice fromVolc ? an Ceboruco.The potential for eastward dispersion of ash from Volc ? anCeboruco was demonstrated using the UK Met Office's NumericalAtmospheric-dispersion Modelling Environment (NAME; Joneset al., 2007). To test the variability in ash transport directions, alarge (up to 30 km in plume height) ten-day constant eruption ofVolc ? an Ceboruco was assumed and a small random meteorologicalsample (current 10 days of meteorology) was used to track theparticle dispersion through themodelled loweratmosphere. Whilstthe AD 990e1020 eruption of Volc ? an Ceboruco may have beensmaller than the modelled eruption and likely occurred duringdifferent meteorological conditions than those of today, themodelling results reveal the potential for the main ash plume toextend eastwards into Africa whilst a secondary plume splits fromthe main plume over the North Atlantic Ocean, extendingnortheastwards across Nova Scotia (Appendix G, SupplementaryInformation). Tephra transportation from Mexico to Nova Scotiamayalso occur when anticyclonic advection merges with a stronglymeridonal polar jet stream, as modelled using weather and climateobservations from NASA's MERRA dataset (NASA, 2012). Further-more, there are precedents for Mexican tephras reaching the NorthAtlantic region, since ash from the ca. AD 1250e1400 (Palais et al.,1992) and AD 1982 (Zielinski et al., 1997) eruptions of El Chich ? onhave been detected in the Summit region of Greenland.3.3. Unidentified tephra horizon3.3.1. VDB12-42 (AD 1517e1750) and SCH10-42 (AD 1572e1762)The shard morphology of horizons VDB12-42 and SCH10-42 aredominated by pumiceous shards accompanied byoccasional browncoloured shards. Although these rhyo-dacitic tephras (Fig. 8;Appendix E, Supplementary Information) fall within the composi-tional range of known Icelandic and Aleutian tephra and sharesimilar glass morphological characteristics with them, they havenot been correlated and have been provisionally termed here as the‘Villagedale tephra’. The Villagedale tephra and the WRAe are theonly two tephras characterised within this study that have beendetected in more than one study site.Fig. 4. Selected major element glass geochemical plots of proximal White River Ash (east and north lobe) and correlated cryptotephras. Most samples tend to cluster with the mainWRAe geochemical population of ~73e74.5 SiO 2 wt%.H. Mackay et al. / Quaternary Science Reviews 132 (2016) 101e113 107 3.4. Multiple within-core White River Ash horizonsOne caveat related tothe use of isochrons is demonstrated in thestratigraphy of SCH10 and FBB12, which register multiple layers ofelevated tephra concentrations that geochemically correlate withthe WRA (Fig. 4; Appendix C.2, Supplementary Information). Theoverlapping geochemistry, representing a single volcanic centre,complicates both the assignment of an age within stratigraphiesand the degree of precision when correlating with other records.Therefore, under these conditions the isochron may represent a‘passage in time’ rather than a ‘moment in time’ (sensu Dugmoreet al., 2004). SCH10 contains three horizons that are distinct atboth 5 cm and 1 cm resolutions and FBB12 contains two horizonsthat are separated by 35 cm of peat accumulation with low or zerobackground shard counts (Fig. 2; Appendix A, SupplementaryInformation). Since the upper WRA tephra horizons of SCH10-118and FBB12-70 are an order of magnitude larger than the otherwithin-core WRA horizons, they have been assumed to representthe main WRAe eruption.Two main factors may have contributed to the multiple WRAhorizons: firstly, there may be several eruption events/stages fromthe same volcanic centre (Preece et al., 2014) and secondly, post-depositional movement of the shards may have occurred (e.g.Payne and Gehrels, 2010; Swindles et al., 2013; Watson et al., 2015).Although atmospheric conditions also influence site-specific de-livery of ash layers (since circulation and localised precipitationpatterns can result in intermittent shard deposition (Davies et al.,2010; Dugmore and Newton, 1997; Payne et al., 2013; Pyne-O'Donnell, 2011)), delayed atmospheric transport of shardscannot explain this multiple peak feature. Peat accumulation ratesin oceanic settings are usually around 10 yr/cm, whereas themaximum atmospheric retention time of ash clouds is in the orderof 1e2 years (Cole-Dai et al., 2000, 2009; Fiacco et al.,1994; Robock,2002; Zielinski et al., 1994).Multiple eruptions are a...

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