North European last glacial–interglacial transition (LGIT; 15–9 ka) tephrochronology: extended limits and new events

JOURNAL OF QUATERNARY SCIENCE (2006) 21(4) 335–345Copyright � 2006 John Wiley & Sons, Ltd.Published online 14 March 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jqs.990

North European last glacial–interglacial transition(LGIT; 15–9 ka) tephrochronology: extended limitsand new eventsC. S. M. TURNEY,1* K. VAN DEN BURG,2 S. WASTEGARD,3 S. M. DAVIES,4 N. J. WHITEHOUSE,2

J. R. PILCHER2 and C. CALLAGHAN2

1 GeoQuEST Research Centre, School of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW, Australia2 School of Archaeology and Palaeoecology, Queen’s University, Belfast, UK3 Department of Physical Geography and Quaternary Geology, Stockholm University, Stockholm, Sweden4 Department of Geography, University of Wales Swansea, Swansea, UK

Turney, C. S. M., Van Den Burg, K., Wastegard, S., Davies, S. M., Whitehouse, N. J., Pilcher, J. R. and Callaghan, C. 2006. North European last glacial–interglacialtransition (LGIT; 15–9 ka) tephrochronology: extended limits and new events. J. Quaternary Sci., Vol. 21 pp. 335–345. ISSN 0267-8179.

Received 10 May 2005; Revised 16 September 2005; Accepted 20 October 2005

ABSTRACT: High-precision correlation of palaeoclimatic and palaeoenvironmental records iscrucial for testing hypotheses of synchronous change. Although radiocarbon is the traditional methodfor dating late Quaternary sedimentary sequences, particularly during the last glacial–interglacialtransition (LGIT; 15–9 ka), there are inherent problems with the method, particularly during periodsof climate change which are often accompanied by major perturbations in atmospheric radiocarboncontent. An alternative method is the use of tephras that act as time-parallel marker horizons. WithinEurope, numerous volcanic centres are known to have erupted during the LGIT, providing consider-able potential for high-precision correlation independent of past radiocarbon fluctuations. Here wereport the first identification of the Vedde Ash and Askja Tephra in Ireland, significantly extending theknown provenance of these events. We have also identified two new horizons (the Roddans PortTephras A and B) and tentatively recognise an additional horizon from Vallensgard Mose (Denmark)that provide crucial additional chronological control for the LGIT. Two phases of the Laacher SeeTephra (LST) are reported, the lower Laacher See Tephra (LLST) and probably the C2 phase of theMiddle Laacher See Tephra (MLST-C2) indicating a more northeasterly distribution of this fan thanreported previously. Copyright � 2006 John Wiley & Sons, Ltd.

KEYWORDS: Lateglacial rapid climate change; Askja Tephra; Laacher See Tephra; Roddans Port Tephras; Vallensgard Mose Tephra; Vedde Ash.

Introduction

The last glacial–interglacial transition (LGIT, 15–9 ka BP) was aperiod characterised by extreme and rapid climate change, ofwhich rapid warming at 14.7 ka GRIP ice-core yr BP (the startof the Lateglacial Interstadial, or GI-1 in the Greenland ice-coreisotope stratigraphy) and the well-known period of severe cool-ing referred to as the Younger Dryas Stadial (GS-1) dated tobetween 12.8 and 11.5 ka GRIP ice-core yr BP are the mostpronounced (Lowe et al., 2001). A key question in Quaternaryclimatic research is whether such changes were synchronous ata regional level. Testing this hypothesis using traditional meth-ods has proved problematic, however. Radiocarbon dating isthe most widely employed method for dating terrestrial andmarine sequences spanning this period, though this approachhas several problems. These include: contamination and strati-

graphical disturbance of sedimentary layers and their con-tained fossils; the lack of a precise radiocarbon calibrationmodel for the Last Termination; and reservoir uncertainties thatinfluence the radiocarbon activity of marine organisms, themagnitude of which appear to have varied both spatially andtemporally (Lowe and Walker, 2000; Bondevik et al., 2001;Siani et al., 2001; Turney et al., 2000; Waelbroeck et al.,2001; Bjorck et al., 2003).

An alternative approach to radiocarbon dating is the use oftephrochronology. The virtually instantaneous atmosphericdeposition of tephra following an eruption can form time-parallel marker horizons that allow high-precision correlationbetween ice, marine and terrestrial sequences, independent ofother dating uncertainties, including fluctuations in the atmo-spheric radiocarbon content over time (Turney and Lowe,2001). Europe is particularly well suited for the application ofthis method, having several major centres of volcanic activityincluding Jan Mayen, Iceland, the Massif Central (France), theEifel region (Germany), Campania (Italy) and the Hellenic Arc(Greece), most if not all of which were active at some timethrough the LGIT (Davies et al., 2002).

* Correspondence to: C. S. M. Turney, GeoQuEST Research Centre, School ofEarth and Environmental Sciences, University of Wollongong, Wollongong,NSW 2522, Australia. E-mail: [email protected]

Originally the application of tephrochronology was restrictedto areas proximal to volcanic sources or where taphonomicprocesses concentrated the number of shards sufficiently tobe visible to the naked eye (Persson, 1971; Mangerud et al.,1984, 1986; van den Bogaard and Schmincke, 1985). Table 1lists all published tephra layers from the LGIT reported to datein terrestrial deposits in Scandinavia (including the FaroeIslands) and the British Isles. Of particular note, the LaacherSee Tephra (LST) which originated from the Eifel region(12.9 k cal. yr BP), generated three principal ash fans that forma major visible horizon across central Europe (van den Bogaard

and Schmincke, 1985; Harms and Schmincke, 2000) (Fig. 1).Furthermore, three distinct phases of the LST have been identi-fied (Lower, Middle and Upper LST) of which the MLST isdivided into three further components (A, B and C) on the basisof composition, and physical and chemical properties of thetephra units (van den Bogaard and Schmincke, 1985). Notably,there is a general increase to higher SiO2 values from the LLSTup to MLST-C but variable values above the MLST-C to ULST,with similar trends for FeOtot, K2O and CaO. Of these, thePlinian LLST, MLST-B and MLST-C have previously been iden-tified to have composed the northeasterly fans from the Eifel

Table 1 Tephra horizons reported from the LGIT (ca. 15–9k cal. yr BP) found in terrestrial deposits in Scandinavia (including the Faroe Islands) andthe British Isles prior to this study. Ages are given as approximate cal. yr BP

Tephra Age (cal. yr BP) Source volcano Composition Reference Area

QUB-608 9500 Snæfellsjokull Rhyolitic Pilcher et al., 2005 N NorwayAn Druim 9560 Torfajokull? Rhyolitic Ranner et al., 2005 ScotlandHogstorpsmossen 10 200 Snæfellsjokull? Rhyolitic Bjorck et al. (2002) E SwedenSaksunarvatn 10 240 Grımsvotn Basaltic Mangerud et al. (1986) Faroe Islands, W Norway, ScotlandHovsdalur 10 500 Snæfellsjokull? Rhyolitic Wastegard (2002) Faroe IslandsL3574 10 800?a ? Dacitic Dugmore and Newton (1998) Faroe IslandsAskja 10-ka 11 100b Askja Rhyolitic Davies et al. (2003) SE Sweden, N NorwayHasseldalen 11 400b Snæfellsjokull? Rhyolitic Davies et al. (2003) SE SwedenVedde 12 000 Katla Rhyol./Bas. Mangerud et al. (1984) Norway, Sweden, ScotlandSluggan B 12 800 ? Rhyolitic Lowe et al. (2004) IrelandLaacher See 12 900 Laacher See Phonolitic Usinger (1977) BornholmBorrobolc 14 400 ? Rhyolitic Turney et al. (1997) Scotland, SE SwedenSluggan A 14 500 ? Rhyolitic Lowe et al. (2004) Ireland

aNo age given, reported as ‘older than Saksunarvatn tephra’.bAge based on wiggle-matching of AMS dates on terrestrial macrofossils (Wohlfarth et al., submitted).cRecent results from Sweden, yielding an age of ca. 13.9k cal. yr BP (Davies et al., 2004) indicate either that the Borrobol Tephra in Sweden andScotland represents two separate eruptions from the same volcanic system, or that the British age estimate is slightly too old.

Figure 1 Location of sites (A: Lough Nadourcan; B: Long Lough; C: Roddans Port; D: Vallensgard Mose) and the known provenance of Vedde Ash(light grey; Wastegard et al., 2000; Turney et al., 2004; Pilcher et al., 2005; Davies et al., 2005) and Laacher See Tephra (LST; dark grey; van denBogaard and Schmincke, 1985). The different phases of the LST eruption are given as 1 (Lower LST), 2 (MLST) and 3 (ULST) of which A, B and C refer tothe different phases of the MLST

336 JOURNAL OF QUATERNARY SCIENCE

Copyright � 2006 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 21(4) 335–345 (2006)

source, while the MLST-A and the phreatomagmatic ULSTformed the southerly fan, and the southwesterly fan comprisessolely tephra from the ULST phase of the eruption (Fig. 1) (vanden Bogaard and Schmincke, 1985; Juvigne et al., 1995; Harmsand Schmincke, 2000). Relatively recent methodologicaldevelopments using flotation (Turney, 1998) and magneticseparation (Mackie et al., 2002) have now allowed the identi-fication of tephra horizons invisible to the naked eye withinminerogenic sediments (termed ‘cryptotephra’; Lowe and Hunt(2001) and Turney et al. (2004)). Using these methods has pro-vided the potential to identify previously unrecognised hori-zons and significantly extend the provenance of knownlayers. As a result, the Icelandic Vedde Ash (12.0 k cal. yr BP;Mangerud et al., 1984; Bjorck et al., 1992; Lowe and Turney,1997; Wastegard et al., 1998, 2000; Fig. 1) and the BorrobolTephra (14.4 k cal. yr BP; Turney et al., 1997; Eirıksson et al.,2000; Davies et al., 2003) have now been identified acrosslarge parts of the North Atlantic region and form critical hori-zons for correlating within the Younger Dryas Stadial andLateglacial Interstadial respectively. At present, however, nopre-Holocene North European sequence has been reported tocontain tephras originating from two different volcanicsources, thereby preventing the combination of local schemesinto a European-wide tephrochronological framework (Turneyet al., 2004).

To resolve further the debate concerning the synchroneity ofclimate change across Europe it is crucial that more LGITtephras are identified in sequences containing horizons thatoriginate from different volcanic sources. Here we report theidentification and extension of tephra horizons within LGITsequences across northern Europe.

Site locations and stratigraphiccontexts

Lough Nadourcan (latitude 55 �030N,longitude 7 �540W)

Lough Nadourcan is a small lake measuring approximately160 m by 55 m and is located approximately 16 km northwestof Letterkenny in northwest Ireland and lies within theGlenveagh National Park, Co. Donegal (Fig. 1). The local bed-rock consists of granite and the immediate area is today char-acterised by widespread blanket bog. Previous palynologicalwork at the site (Watts, 1977) has demonstrated the existenceof a ‘classic’ LGIT tripartite sequence (Lowe and Walker,1986), supported by the identification of a Juniperus–Empe-trum pollen assemblage interpreted to represent the warmestpart of the Lateglacial Interstadial (referred to in Ireland asthe Woodgrange Interstadial; Singh, 1970) and an Artemisiaphase believed to represent the Younger Dryas Stadial (locallyreferred to as the Nahanagan Stadial; Colhoun and Synge,1980). Ongoing work is investigating the fossil chironomidand coleopteran record ( J. Watson, in progress).

Long Lough (latitude 54 �260N, longitude 5 �550W)

Long Lough is a lake to the immediate east of Saintfield, Co.Down, Northern Ireland (Fig. 1). The area is characterised byextensive drumlin formation. Long Lough has formed withinan interdrumlin hollow of which the basal sediments arecharacterised by a tripartite sequence consisting of silty-clay

lake muds, overlain by highly organic lake muds and silty-claylake muds (the latter lithostratigraphically considered theequivalent of the Younger Dryas Stadial). No previous workhas been reported for the LGIT component of Long Lough,though extensive palynological work has been undertaken onthe Holocene part of the sequence (Hall, 1990).

Roddans Port (latitude 54 �310N, longitude 5 �300W)

Roddans Port is an intertidal site on the east coast of the ArdsPeninsula, 3 km northwest of Ballyhalbert, Co. Down, North-ern Ireland (Fig. 1), originally formed in an inter-drumlinhollow. Investigation of the site was undertaken by Morrisonand Stephens (1965) who identified a tripartite sequence.Extensive radiocarbon dating of the sequence (Morrison andStephens, 1965) and multiproxy analyses (including Coleop-tera, Chironomidae, pollen and plant macrofossils; White-house et al., in preparation) confirm the sequence is LGIT inage. Unfortunately, however, recent sea erosion has destroyedthe upper part of the Younger Dryas Stadial and early Holocenesediments, precluding tephra analyses of sediments from thelatter period of the LGIT at the main beach exposure. Theselatter sediments, however, still exist at a separate location,buried partially beneath the current sea wall, and are nowunder investigation. Here, only results from the beach exposureare reported.

Vallensgard Mose (latitude 55 �050N,longitude 14 �530E)

Vallensgard Mose is part of an extensive Holocene bog com-plex approximately 4 km northwest of Aakirkeby, BornholmIsland, Denmark (Fig. 1). The Holocene peat and lake sedi-ments have partly been excavated but underlying the Holocenesequence are extensive LGIT sediments, as indicated by a tri-partite sequence, a characteristic Betula and Juniperus pollenassemblage and the presence of a tephra correlated to theLST (Usinger, 1977; van den Bogaard and Schmincke, 1985).As a result of the latter and its strategic location effectively‘downwind’ of Iceland, the site was selected as having thepotential for containing tephras from different volcanic regions.Two coring locations at the margin and towards the centre ofthe site were investigated for tephra (Cores 1 and 3 respec-tively). A tephra horizon was visible in the calcareous sedi-ments of both cores representing the upper part of theAllerød pollen zone. Core 1 coincides with the coring locationof Usinger (1977). A hiatus between the Allerød and YoungerDryas was identified in the pollen stratigraphy several centi-metres above the tephra in Core 1 (Usinger, 1977). The geo-chemistry of the tephra was later confirmed to represent theLLST (Lower Laacher See Tephra), erupted during the Plinianphase of the eruption (van den Bogaard and Schmincke, 1985).

Methodology

The sequences were investigated at contiguous 1-cm intervalsto minimise the risk of missing any cryptotephra horizons. Sam-ples were prepared following the methodology of Turney(1998). Samples were combusted at 550 �C for 2 hr in a mufflefurnace (which also provided loss-on-ignition values). The

LAST GLACIAL–INTERGLACIAL TRANSITION TEPHROCHRONOLOGY 337


remaining material was left overnight in 10% HCl to removeany carbonate material and then sieved through 80 andretained on 15 mm mesh. The sediments within the 80–15 mmfraction were floated twice at 2.5 g cm�3 using sodiumpolytungstate. The samples were cleaned thoroughly in dis-tilled water and mounted onto microscope slides for inspectionunder an optical microscope, where the total number of glassshards was counted. Identification of tephra was achievedusing a range of morphological characteristics and optical testsincluding the degree of vesicularity, the use of polarised lightand the Becke line.

Significant tephra horizons in the sequences were preparedfor major oxide geochemical analysis (expressed as weight percent) using wavelength dispersive spectrometry (WDS) oneither the Jeol 733 Superprobe at the Electron Microscope Unit,Queen’s University, Belfast, or the Cameca SX100 ElectronMicroprobe at the University of Edinburgh. To avoid migrationof some major oxides as a result of heating, the material wasacid-digested, following the procedure outlined by Dugmore(1989) and the size fraction 70 to 15 mm isolated for analysis.The analyses at Belfast were undertaken with an acceleratingvoltage of 15 kV, a beam current of 10 nA and a slightly defo-cused beam diameter of approximately 8 mm. A Lipari standardwas analysed at regular intervals during the analysis period. AZAF correction was applied to all analyses to correct for atomicnumber, absorption and fluorescence effects (Sweatman andLong, 1969). The analyses at Edinburgh were undertaken usingslightly different analytical conditions. Here an acceleratingvoltage of 20 kV and a beam strength of 10 nA, determinedby a Faraday cup were used, with a rastered beam over an areaof 10� 10 mm to reduce instability of the glass and subsequentsodium loss. Calibration was undertaken using a combinationof standards of pure metals, simple silicate minerals and syn-thetic oxides, including andradite. These were used regularlybetween analyses to correct for any drift in the readings. APAP correction was applied for the effects of X-ray absorption(Pouchou and Pichoir, 1991). Counter dead-time was also cor-rected for. Virtually all analyses exceeded 95% totals (Huntand Hill, 1993) and the geochemical results were statisticallyindistinguishable between the Belfast and Edinburgh operatingsystems, allowing direct comparison.

Results

Six tephra horizons have been geochemically identified in thefour sites investigated during the course of this study (Fig. 2).These are situated at 741 cm at Lough Nadourcan, 311 cm atLong Lough, 0 cm and 63 cm at Roddans Port, and 136 cmand 174 cm at Vallensgard Mose (Core 3 depths). The concen-tration of the shards varied significantly between the sites, ran-ging from a visible horizon at Vallensgard Mose at 174 cm(>50 000 shards cm� 3) to numerous cryptotephra horizonsthat had concentrations as low as 22 shards per cm3 (63 cmat Roddans Port). An additional colourless tephra horizonwas also identified at 748 cm in Lough Nadourcan (54 shardscm� 3). Despite several attempts, no geochemical analyses ofshards from this horizon were obtained.

Roddans Port Tephras A and B

Within the middle part of the Lateglacial Interstadial, a geo-chemically distinct horizon was identified in Roddans Port at63 cm. The timing of this event is currently uncertain but onthe basis of the radiocarbon ages reported by Morrison andStephens (1965), an age of approximately 12 k 14C yr BP canbe proposed. The colourless glass shards have a geochemicallyvariable FeOtot, CaO and K2O content (Figs 3 and 4 andAppendix) with SiO2 values ranging between 67.2% and75.7%. The geochemistry of the shards, however, is not consis-tent with the Vedde Ash, Borrobol Tephra (Davies et al., 2002)or the Sluggan Tephras (Turney et al., 2001; Walker et al.,in press). For example, CaO values (0.9–1.7%) are in excessof those for the Borrobol Tephra (0.6–0.8%; Turney et al.,1997; Davies et al., 2003). The analyses appear to consist oftwo distinct populations (Figs 3 and 4) which we identify hereas Roddans Port Tephras A and B. Roddans Port Tephra Avalues are internally consistent, with a SiO2 range of 67% to70%, and are tightly clustered in both the FeO vs. TiO2

(Fig. 3) and SiO2 vs. Na2OþK2O (Fig. 4) plots. In contrast,Roddans Port Tephra B has a considerably higher SiO2 content

Figure 2 Lithostratigraphy, loss-on-ignition (represented by single line) and tephra counts (represented by filled curve) for the sites investigated(A: Lough Nadourcan; B: Long Lough; C: Roddans Port; D: Vallensgard Mose). The stratigraphic zones defining the Lateglacial Interstadial, YoungerDryas Stadial and the Holocene are also shown



(73–76%) with a distinct overall geochemistry (Figs 3 and 4).Both tephras clearly fall within the Icelandic geochemicalenvelope (Fig. 4) but the source of these eruptions is currentlyuncertain. At present, it is unclear whether these horizonsrepresent two synchronous eruptive events or one or more thatare reworked. The ‘peak’ from which the sample was takenrepresents a broad zone of relatively high tephra concentrationspanning 60 to 66 cm (Fig. 2), potentially indicating two eventsthat erupted relatively closely in time.

Laacher See Tephra

Within the latter part of the Lateglacial Interstadial sediments atVallensgard Mose, a visible tephra horizon was identified,stratigraphically consistent with the Laacher See Tephra repor-ted by Usinger (1977) and originally geochemically analysedby van den Bogaard and Schmincke (1985). Analysis of thecolourless, vesicular and fluted shards from two separate corestaken across the basin indicate a population of phonolitic

Figure 3 (A) Binary plot of FeOtot and TiO2 of tephras analysed in this study and compared to known events during the LGIT. Geochemical envelopesof the Askja Tephra, Hasseldalen Tephra (HDT), Vedde Ash, Borrobol Tephra and Sluggan A and B are as reported by Davies et al. (2002, 2003),Turney et al. (1997, 2001), Sigvaldason (2002), Pilcher et al. (2005) and Walker et al. (in press). (B) FeOtot and TiO2 and (C) Na2O and K2O of theLaacher See Tephra in the Vallensgard Mose sequence showing two distinct LST populations (A and B). Geochemical envelopes of the different com-ponents of the LST are compiled from glass analyses from Worner (1982), van den Bogaard and Schmincke (1985), Harms and Schmincke (2000),Berndt et al. (2001) and Johansson (2005). Averages for analyses from NW Poland (Juvigne et al., 1995) are shown as means with 1� in Fig. 3(B).Envelopes from Harms and Schmincke (2000) were used for the different components of the LST in Fig. 3(C). Data are normalised



composition (Fig. 4 and Appendix). Geochemical analysis ofthe shards confirms the event recorded in Bornholm Islandreflects that of the Laacher See Tephra. Compared to Icelandicmaterial, the Laacher See Tephra has characteristically

low SiO2 (55–60%), and high Al2O3 (19–23%), Na2O (6.8–11.2%) and K2O (4.2–7.5%) values (Fig. 4 and Appendix).The binary plots of FeOtot and TiO2 (Fig. 3(B)) and Na2O andK2O (Fig. 3(C)) indicates many of the shards fall within the LST

Figure 3 Continued

Figure 4 Binary plot of SiO2 and Na2OþK2O (normalised to 100%) of the tephras analysed in this study and compared to known events during theLGIT. Geochemical envelopes of Jan Mayen, Iceland and the individual volcanic events of the Askja Tephra, Hasseldalen Tephra, Vedde Ash, LaacherSee Tephra, Sluggan Tephras A and B, and the Borrobol Tephra are as reported by Davies et al. (2002, 2003), Turney et al. (1997, 2001), Sigvaldason(2002), van den Bogaard and Schmincke (1985), van den Bogaard and Schmincke (2002) and Walker et al., (in press)



envelopes but the highly variable titanium and sodium contentof the shards identifies two or possibly three different phases ofthe eruption preserved in the Vallensgard Mose sequence.

The new analyses undertaken from the site recognise twodistinct LST populations in the Vallensgard Mose sequence(A and B) (Appendix). These analyses indicate that populationA represents the Plinian phase LLST (Figs 3(B) and 3(C);Worner, 1982; van den Bogaard and Schmincke, 1985; Harmsand Schmincke, 2000; Berndt et al., 2001; Johansson, 2005).This glass has the lowest TiO2 and highest Na2O of all phasesof the eruption. In contrast, the geochemically distinct popu-lation B has relatively high TiO2 (0.40–0.55%) and CaO(1.4–2.2%) values with lower Al2O3 (19.7–20.8%) comparedto the LLST component (Fig. 3 and Appendix). The FeOtot vs.TiO2 (Fig. 3(B)) plot indicates that population B can be corre-lated either with one phase of the Middle LST (the MLST-C2) orthe phreatomagmatic ULST. The Na2O vs. K2O diagram sup-ports a correlation to the MLST-C for most shards (Fig. 3(C)).In contrast to previous analyses that have suggested theMLST-C2 component only reached 200 km from the volcano(van den Bogaard and Schmincke, 1985), our results indicatethat this phase of the eruption also reached Scandinavia as partof the northeasterly fan.

Vedde Ash

Within the uppermost sediments of Roddans Port (which arelithostratigraphically equivalent to the Younger Dryas Stadial),a single tephra horizon of 189 colourless rhyolitic shardscm� 3 was identified at 0 cm. The binary plots of FeOtot andTiO2 (Fig. 3) and SiO2 and Na2OþK2O (Fig. 4) indicate atightly clustered population centred on the rhyolitic VeddeAsh which originated from the Katla system in Iceland(Mangerud et al., 1984; Lacasse et al., 1995; Turney et al.,1997), which is clearly differentiated from the other significantevents known to have occurred through this period.

Vallensgard Mose Tephra

Within the Younger Dryas Stadial sediments a further tephrahas been tentatively identified. In Vallensgard Mose, a geo-chemically distinct population of 114 rhyolitic shards cm� 3

at 134 cm were identified in Core 3 and are apparently of Ice-landic origin (Figs 3 and 4). Only a total of four shards weregeochemically analysed, but these have a significantly differentgeochemical composition from the Vedde Ash, although theyappear to lie in approximately the same stratigraphic position(midway through the Younger Dryas Stadial sediments). Insome respects, the shards are comparable to the compositionof the Roddans Port Tephras A and B, although the latter wasdeposited during the middle part of the Lateglacial Interstadial.It seems unlikely that this horizon marks a reworked event asthe visible Laacher See Tephra is stratigraphically below themid-Stadial and would have dominated the sediments if sucha mechanism were to be invoked. If a bimodal compositionis proved to be representative of this horizon, it is possible thatthe Roddans Port Tephras A and B may also represent a singleevent from the same source. No shards were identified in Core1, possibly due to the occurrence of a hiatus between theAllerød and mid-Younger Dryas. This horizon is tentativelynamed the Vallensgard Mose Tephra though further analyseswill be required to confirm whether this is a significant tephrahorizon.

Askja Tephra

The geochemical composition of the colourless rhyolitic shardsobtained from Lough Nadourcan and Long Long are statisti-cally indistinguishable from one another (Figs 3 and 4 andAppendix) and are identical to the Askja Tephra, originatingfrom the Dyngjufjoll centre, Iceland (Sigvaldason, 2002). Inboth sequences the horizon is identified in the lowermostorganic sediments following the Younger Dryas Stadial (Naha-naghan Stadial) and reflect deposition during warming early inthe Holocene, consistent with the 10.0k 14C yr BP age reportedfor this event (Sigvaldason, 2002). The geochemistry is suffi-ciently different to indicate this is a distinct horizon and notreworked. The geochemical signature of the Askja horizon issignificantly different from that of the Vedde Ash and BorrobolTephra (Turney et al., 1997), with higher SiO2 content com-pared to the former (72.5% and 70.5% respectively) and higherFeOtot content compared to the latter (2.5% and 1.5% respec-tively) (Fig. 3 and 4). This horizon is also distinct from theslightly older Hasseldalen tephra (HDT; Davies et al., 2003),found in southeast Sweden. The Askja Tephra has higher FeOtot

(2.5 % compared to 1.1% for the HDT) and CaO contents(1.5% compared with 0.5%) (Figs 3 and 4).

Determination of the origin of a further tephra recognised atLough Nadourcan (748 cm) is problematic in the absence ofany firm geochemical evidence. The early Holocene natureof this event and its stratigraphic position below the AskjaTephra raises the possibility that this horizon might be that ofthe Icelandic HDT. At present, however, this eruption has onlybeen identified within southern Sweden. In the absence of geo-chemical analyses, we are not able to make a correlation and itis equally possible that this horizon may be reworked fromolder sediments.

Discussion

The identification of the Vedde Ash and the Askja Tephra bothsignificantly increase the known provenance of these events.The westernmost limit of Vedde Ash has until now beenrestricted to west Scotland (Turney et al., 1997) and raises thepotential of high-precision correlation of Irish sequences withinother north European terrestrial sequences stretching acrossScotland (Lowe and Turney, 1997), the Netherlands (Davieset al., 2005), Norway (Mangerud et al., 1984; Birks et al.,1996; Pilcher et al., 2005), southern Sweden (Wastegardet al., 1998) and western Russia (Wastegard et al., 2000) as wellas the North Atlantic marine (Kvamme et al., 1989; Austinet al., 1995; Lacasse et al., 1995; Bondevik et al., 2001) andthe Greenland ice core records (Gronvold et al., 1995;Mortensen et al., 2005). The Askja Tephra has previously beenidentified in Iceland (Sigvaldason, 2002), the Lofoten Islands,northern Norway (Pilcher et al., 2005) and southern Sweden(Davies et al., 2003). The identification of this horizon withinIreland indicates that this event might be found across most ofnorthern Europe and will provide a critical early Holocenemarker horizon, independent of known fluctuations in atmo-spheric 14C content at this time (Stuiver et al., 1998). Further-more, the identification of the MLST-C2 in Vallensgard Moseindicates this component is considerably more widespreadthan hitherto believed (van den Bogaard and Schmincke,1985; Juvigne et al., 1995) and may also play a crucial rolein extending the known limits of the LST.

The two new tephra horizons within the early to mid-LGIT (the Roddans Port Tephras A and B at 12 k 14C yr BP)



and potentially within the mid-Younger Dryas Stadial (theVallensgard Mose Tephra) are in similar stratigraphic positionsto other geochemically distinct eruption events, such as theVedde Ash and Borrobol Tephra (Turney et al., 1997, 2001;Lowe et al., 2004; Walker et al., in press). At present it isunclear what the chronological relationship is between thesedifferent events. Clearly the Roddans Port and VallensgardMose Tephras were deposited in areas where the concentrationof these other horizons is low or even absent, such as theBorrobol Tephra which has not yet been identified in Irelandor Denmark. The relatively high geochemical variability inthese new tephra populations raises the possibility that theymay be a product of multiple events, one of which may havebeen reworked from older deposits. We consider this unlikelywith regard to the Vallensgard Mose Tephra owing to its strati-graphic location above the visible Laacher See Tephra whichwould be the most likely source of subsequent reworked mate-rial during the Younger Dryas Stadial. Owing to the similargeochemical composition of the Roddans Port Tephras to theVallensgard Mose Tephra it is possible that these horizonsrepresent a single Icelandic source that erupted twice throughthe LGIT, though reworking within the former cannot be dis-counted. Future work is now needed to better define the geo-chemical variability of these events, their source and timing.

Regardless of the above, the potential geochemical identifi-cation of Icelandic tephra shards on Bornholm Island within thesame sequence that contains the LST provides an indicationthat sites in this region hold considerable promise for bringingtogether the different European tephrochronological frame-works. Other tephras in different sites must now be identifiedto strengthen and expand the European tephrochronologicalframework if a more comprehensive scheme is to be estab-lished for robustly testing hypotheses of synchronous climatechange across Europe during the LGIT (Turney et al., 2004).

Acknowledgements CSMT gratefully acknowledges holding anAustralian Research Council Queen Elizabeth II Fellowship. KVDBcompleted elements of this work as part of her undergraduate disserta-tion at Queen’s University, Belfast. CSMT would also like to thankThe Leverhulme Trust (Grant number F/07537/C) for supporting asignificant portion of this work. SW’s participation is funded by theSwedish Research Council. Many thanks to Stephen McFarland(Queen’s University, Belfast) and Peter Hill (University of Edinburgh)for their help while analysing the tephra samples. We are grateful toGlenveagh National Park and the landowner of Vallensgard Mose,Ancher Muller, for access to the sites. Anthony Newton and SvanteBjorck kindly assisted in improving an earlier draft of the manuscript.Magnus Johansson (Hannover) was very helpful with discussions on theLaacher See Tephra.

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Appendix

Sample SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Total

LoughNadourcan741–742 cmBelfast1 74.88 0.36 12.5 2.65 — 0.26 1.47 3.86 2.47 98.442 72.75 0.38 12.06 2.59 — 0.27 1.45 3.84 2.54 95.863 72.24 0.35 12.25 2.64 — 0.27 1.5 3.5 2.35 95.094 72.69 0.29 12.21 2.56 — 0.24 1.49 3.81 2.44 95.725 74.02 0.33 12.45 2.6 — 0.26 1.49 3.92 2.61 97.676 71.96 0.35 11.99 2.62 — 0.24 1.34 3.62 2.38 94.57 71.81 0.32 12.18 2.54 — 0.27 1.5 3.8 2.47 94.98 70.4 0.31 11.88 2.51 — 0.27 1.51 3.88 2.46 93.229 71.14 0.38 11.99 2.61 — 0.26 1.53 3.37 2.43 93.71Mean 72.43 0.34 12.17 2.59 — 0.26 1.48 3.73 2.46 95.46St. dev. 1.38 0.03 0.21 0.05 — 0.01 0.06 0.19 0.08 1.71LongLough311–312 cmBelfast1 72.85 0.35 12.29 2.53 — 0.24 1.75 3.83 2.50 96.342 72.56 0.27 12.53 2.50 — 0.26 1.59 3.93 2.33 95.973 71.66 0.29 12.21 2.49 — 0.25 1.51 3.70 2.30 94.404 71.86 0.26 12.44 2.40 — 0.23 1.39 3.74 1.92 94.245 71.68 0.31 12.32 2.51 — 0.24 1.34 3.95 2.24 94.586 74.32 0.33 12.65 2.56 — 0.24 1.62 3.80 2.42 97.947 72.58 0.30 12.34 2.48 — 0.25 1.57 3.85 2.49 95.868 71.38 0.36 13.42 2.51 — 0.23 1.55 3.75 2.51 95.719 72.25 0.38 12.35 2.47 — 0.26 1.63 3.60 2.42 95.3710 72.98 0.31 12.65 2.53 — 0.28 1.53 3.93 2.41 96.6211 72.58 0.35 12.32 2.49 — 0.24 1.56 3.70 2.34 95.58Mean 72.43 0.32 12.50 2.50 — 0.25 1.55 3.80 2.35 95.69St. dev. 0.82 0.04 0.34 0.04 — 0.01 0.11 0.11 0.17 1.08RoddansPort0–1 cmBelfast1 70.95 0.29 13.78 3.69 — 0.23 1.2 5.28 3.52 98.962 69.29 0.33 13.2 3.7 — 0.22 1.14 4.75 3.57 96.193 68.76 0.3 13.59 3.63 — 0.2 1.24 4.99 3.43 96.124 67.77 0.3 12.87 3.51 — 0.23 1.11 6.04 3.81 95.65 66.89 0.34 13 3.59 — 0.21 1.1 5.01 3.44 93.566 69 0.3 13.42 3.61 — 0.21 1.15 5.74 3.8 97.227 69.18 0.29 13.24 3.67 — 0.22 1.11 4.93 3.46 96.098 69.12 0.29 13.47 3.67 — 0.2 1.17 4.59 3.43 95.94Mean 68.87 0.31 13.32 3.63 — 0.22 1.15 5.17 3.56 96.21St. dev. 1.18 0.02 0.30 0.06 — 0.01 0.05 0.50 0.16 1.52RoddansPort63–64 cmBelfastTephra A1 69.00 0.70 15.69 2.98 — 0.66 1.62 5.01 3.55 99.202 69.59 0.76 15.68 2.87 — 0.65 1.61 4.92 3.55 99.633 69.18 0.69 15.8 3.05 — 0.70 1.74 4.75 3.5 99.584 67.17 0.72 17.22 2.83 — 0.61 1.52 5.15 3.46 98.675 70.22 0.73 15.63 2.97 — 0.67 1.59 5.24 3.49 100.556 69.31 0.70 15.57 3.00 — 0.66 1.71 4.40 3.53 98.877 70.50 0.71 15.68 2.82 — 0.60 1.41 5.09 3.64 100.458 67.18 0.71 18.07 2.78 — 0.64 1.53 5.03 3.47 99.429 69.20 0.75 16.23 2.99 — 0.66 1.53 4.95 3.61 99.9310 68.51 0.70 15.95 2.96 — 0.65 1.57 5.19 3.56 99.09Mean 68.99 0.72 16.15 2.93 — 0.65 1.58 4.97 3.54 99.54St. dev. 1.11 0.02 0.83 0.09 — 0.03 0.10 0.25 0.06 0.63Tephra B1 75.41 0.21 12.18 0.57 — 0.04 0.85 2.70 5.10 97.062 76.66 0.26 12.94 1.36 — 0.34 1.64 4.64 1.62 99.45

Continues



Appendix (Continued)

Sample SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Total

3 75.74 0.16 12.00 0.93 — 0.17 0.96 2.95 3.95 96.864 74.51 0.18 12.10 0.87 — 0.16 1.03 3.04 3.98 95.875 73.08 0.26 13.12 1.50 — 0.17 1.13 2.86 2.79 94.92Mean 75.08 0.21 12.47 1.05 — 0.18 1.12 3.24 3.49 96.83St. dev. 1.36 0.05 0.52 0.38 — 0.11 0.31 0.79 1.33 1.69VallensgardMose Core3134–135 cmBelfast1 69.53 0.73 15.36 3.09 — 0.64 1.71 3.44 2.81 97.292 69.59 0.65 15.32 2.61 — 0.55 1.46 3.60 5.59 99.363 75.44 0.11 12.10 0.93 — 0.07 0.48 3.75 3.95 96.824 74.84 0.18 12.05 1.15 — 0.06 0.67 3.24 4.16 96.35VallensgardMose Core1110–115 cmEdinburgh1 (B) 59.36 0.43 20.77 1.92 0.21 0.21 2.15 6.77 6.72 98.612 (B) 59.24 0.44 20.44 2.18 0.18 0.24 1.54 7.05 7.08 98.473 (B) 58.33 0.46 20.76 2.19 0.17 0.28 2.03 7.73 6.50 98.524 (B) 58.23 0.40 20.37 2.32 0.21 0.22 1.49 6.84 7.14 97.325 (B) 58.15 0.55 19.76 2.35 0.14 0.27 1.53 7.44 6.77 97.016 (B) 58.03 0.42 19.86 2.05 0.20 0.27 1.55 6.78 7.46 96.687 (A) 55.76 0.10 22.45 1.54 0.65 0.07 0.35 10.43 4.88 96.238 (A) 55.66 0.08 22.27 1.50 0.55 0.04 0.37 10.26 4.97 95.699 (A) 55.60 0.11 22.97 1.53 0.52 0.06 0.37 11.18 4.86 97.2210 (A) 55.50 0.12 22.53 1.65 0.46 0.04 0.37 10.44 4.72 95.83VallensgardMose Core3174–176 cmBelfast1 (B) 59.26 0.48 20.24 2.20 — 0.25 1.55 7.34 6.84 98.152 (B) 61.02 0.46 20.59 1.84 — 0.18 2.22 6.94 5.98 99.223 (A) 56.12 0.17 22.47 1.64 — 0.05 0.32 10.45 4.24 95.454 (B) 60.19 0.54 20.56 2.29 — 0.22 1.44 7.53 6.91 99.675 (B) 60.17 0.46 20.50 1.90 — 0.24 1.78 7.40 6.43 98.886 (B) 60.27 0.45 20.65 2.23 — 0.19 1.37 7.73 6.82 99.717 (B) 59.64 0.50 20.30 2.26 — 0.24 1.37 7.29 6.91 98.528 (B) 59.58 0.41 21.51 2.12 — 0.18 1.15 8.70 6.16 99.819 (B) 59.52 0.55 20.12 2.17 — 0.25 1.38 7.50 6.84 98.3210 (B) 60.05 0.42 20.40 1.77 — 0.19 1.70 7.03 6.48 98.04MeanTephra A(LLST) 55.73 0.12 22.54 1.57 0.55 0.05 0.36 10.55 4.73 96.08St. dev. 0.24 0.03 0.26 0.07 0.08 0.01 0.02 0.36 0.29 0.70MeanTephra B(MLST-C2) 59.40 0.46 20.46 2.12 0.19 0.23 1.62 7.34 6.74 98.46St. dev. 0.89 0.05 0.41 0.18 0.03 0.03 0.31 0.50 0.38 0.94



Documents

North European last glacial–interglacial transition (LGIT; 15–9 ka) tephrochronology: extended limits and new events