First discovery of cryptotephra in Holocene peat deposits of Estonia,eastern Baltic

TIIT HANG, STEFAN WASTEGARD, SIIM VESKI AND ATKO HEINSALU

BOREAS Hang, T., Wastegard, S., Veski, S. & Heinsalu, A. 2006 (November): First discovery of cryptotephra in Holocenepeat deposits of Estonia, eastern Baltic. Boreas, Vol. 35, pp. 644�649. Oslo. ISSN 0300-9483.

This article reports the first discovery of middle Holocene cryptotephra from a peat sequence in Estonia, easternBaltic. Two sequences, Mustjarve and Parika (located 110 km apart), were chosen for a pilot study aimed atfinding traces of tephra fallout during the middle Holocene. Peat accumulation at both sites started in the earlyHolocene (c. 9500�9000 14C yr BP; c. 11 000�10 000 cal. yr BP) and continued throughout the whole Holocene.The radiocarbon-dated intervals between c. 2000 and 5000 14C yr BP (c. 2000�5500 cal. yr BP) were chosen fromboth sites for the study. Colourless tephra shards were identified at 312�316 cm below the peat surface in theMustjarve peat sequence, while no tephra was found in peat of the same age at Parika. Electron microprobeanalyses suggest a correlation with the initial phase of the Hekla-4 eruption (c. 4260 cal. yr BP), although the age�depth model indicated an age around 4900 cal. yr BP. Small concentrations of colourless to light brown tephrashards at 266�270 cm in the Mustjarve sequence indicate that the Kebister tephra (c. 3750 cal. yr BP) might alsobe present, but geochemical analyses were not possible. The low concentration and small size of the tephraparticles indicate that Estonian bogs are probably on the verge of where tephrochronology is possible innorthwestern Europe. Further studies of full Holocene sequences are required in order to discover traces of otherash plumes reaching as far east as the eastern Baltic area.

Tiit Hang (e-mail: Tiit.Hang@ut.ee), Institute of Geology, Tartu University, Vanemuise 46, Tartu 51014, Estonia;Stefan Wastegard, Department of Physical Geography and Quaternary Geology, Stockholm University, SE-106 91,Stockholm, Sweden; Siim Veski, Institute of Geology at Tallinn University of Technology, Estonia 7, Tallinn 10143,Estonia; Atko Heinsalu, Institute of Geology at Tallinn University of Technology, Estonia 7, Tallinn 10143, Estonia;received 22nd April 2005, accepted 10th March 2006.

Horizons of volcanic ash, or tephra layers, formexcellent isochrones because of their widespread dis-tribution, rapid dispersal and distinct chemical signa-tures (e.g. Westgate & Gorton 1981; Einarsson 1986).Tephrochronology has been recognized as an excellentcorrelation tool for sediment and peat sequencesspanning the Last Termination and the Holocene innorthwestern Europe (e.g. Dugmore 1989; Pilcher &Hall 1992, 1996; Dugmore et al. 1995; Birks et al. 1996;Turney et al. 1997; Wastegard et al. 2000a, b; Langdon& Barber 2001, 2004; Zillen et al. 2002; Davies et al.2003; Bergman et al. 2004; Pilcher et al. 2005). Recentadvances in the detection of cryptotephra horizons(horizons of very low concentration of shards, usuallyinvisible to the naked eye) (cf. Lowe & Hunt 2001;Turney et al. 2004) have led to the discovery ofpreviously undetected tephra horizons (e.g. theBorrobol Tephra; Turney et al. 1997) and haveextended the known limits of individual Icelandic ashplumes into areas much more distal to the source thanwas previously known. This includes areas such as theBritish Isles (Turney et al. 1997; Mackie et al. 2002),southern and central Sweden (Wastegard et al. 2000a;Davies et al. 2003), western Russia (Wastegard et al.2000b), northern Germany (van den Bogaard &Schmincke 2002) and The Netherlands (Davies et al.2005). As the relatively small-scale 1947 eruption ofHekla deposited tephra along the south coast of

Finland (Salmi 1948) and the stratigraphically impor-tant Vedde Ash, from the last glacial�interglacialtransition, has been reported in two lakes on KarelianIsthmus, northwestern Russia (Wastegard et al. 2000b),it is not unreasonable that ash plumes from otherIcelandic eruptions may have reached the easternBaltic Sea area. The aim of this article was to reportthe results of the first pilot study into detection ofthe Holocene cryptotephra from peat sequences inEstonia.

Methods and material

Two sequences, one from the Mustjarve bog and onefrom the Parika bog (Fig. 1), were collected fromthe Holocene peat record. The manually operatedRussian type peat-corer (length 1 m, diameter 7 cm)was used. Tephra was detected using the methoddescribed by Pilcher & Hall (1992). Continuous10-cm samples were initially combusted at 5508Cfor 4 h and treated in HCl in order to dissolve thecarbonates. For those samples from which tephrashards were detected as a result of this initialapplication, the equivalent sediment interval was re-sampled using 2-cm slices in order to determine thetephra distribution more precisely. Geochemicalanalyses were performed on a WDS (Wavelength

DOI 10.1080/03009480600827272 # 2006 Taylor & Francis

Dispersive Spectrometer) Cameca SX100 electronmicroprobe at the Department of Geology andGeophysics, University of Edinburgh (Table 2).

Only samples dated to the time interval between2200 and 5000 14C yr BP (c. 2000�5500 cal. yr BP)were chosen for analyses within the current study.Several widespread tephra horizons from this timeinterval (Hekla-3, Microlite, Kebister, Hekla-4) have

been found in Sweden and northern Germany (Boygle1998, 2004; van den Bogaard & Schmincke 2002; Zillenet al. 2002; Bergman et al. 2004; Wastegard 2005). Thesource of these, with the exception of the Microlitetephra, is the Hekla volcano in southwest Iceland; it isreasonable to speculate that ash clouds from theseexplosive eruptions could also have reached the easternBaltic area.

Table 1. Lithology of investigated cores from the Mustjarve and Parika bogs. For location of the sites, see Fig. 1. For exact location of theboreholes, see Veski (1998) and Niinemets et al . (2002).

Mustjarve bog Parika bog

Depth, cm Description Depth, cm Description

0�20 Sphagnum peat, low humification, light green 0�100 Sphagnum peat with some Eriophorum , lowhumification, greenish-brown

20�100 Sphagnum peat with some Eriophorum 100�560 Sphagnum�Eriophorum peat, low to mediumhumification, brown

100�200 Sphagnum �Eriophorum peat, low humification,brown

560�600 Woody Phragmites peat, medium humification,dark brown

200�510 Sphagnum�Eriophorum peat, with changinghumification level

600�880 Woody Carex�Phragmites peat

510�660 Woody Carex �Phragmites peat, medium tolow humification, brown

880�930 Silt with organic matter

600�660 Woody Carex�Phragmites peat660�685 Coarse detritus gyttja�/woody

Carex�Phragmites peat685�734 Coarse detritus gyttja, greyish734�743 Fine to medium sand743�760 Silty clay with black FeS bands760�770 Gravel

Fig. 1. Location of the studyarea and investigated sites at theMustjarve and Parika bogs.

BOREAS 35 (2006) Holocene cryptotephra in Estonia 645

The coring site at Mustjarve bog (N59804?41ƒ,E24806?31ƒ) is situated in northwest Estonia, onthe coastal plain of the Baltic Sea at an altitude of39 m a.s.l. (Fig. 1). The ombrotrophic bog is 2 km indiameter and the thickness of organic deposits in thecentral and deepest part reaches 7.5 m (Veski 1998).The basin was a shallow coastal lake after deglaciationand was finally isolated during the Ancylus Lakestage of the Baltic Sea and later infilled by peat. Atthe coring site, the beginning of organic sedimentationwas dated to 95909/120 14C yr BP (Veski 1998),c. 10 900 cal. yr BP.

The age�depth model from the Mustjarve sequenceis based on eight radiocarbon dates of bulk peat andgyttja (Fig. 2). These were calibrated using the OxCalprogram, version 3.9 (Bronk Ramsey 2001) andthe IntCal98 calibration curve (Stuiver et al. 1998).The peat accumulation is relatively uniform through-out the Holocene (0.7 to 2.26 mm/yr; Fig. 2).

The coring site at Parika bog (N58828?48ƒ,E25846?11ƒ) is located in the Lake Vortsjarv depression,

central Estonia (Fig. 1). The total area of theombrotrophic Parika bog is 34 km2 and the altitudeof the peat surface varies between 45 and 50 m a.s.l.(Niinemets et al. 2002). The thickness of organicdeposits is 8.80 m in the deepest parts and the onsetof organic sedimentation has been dated to 90009/7514C yr BP (Niinemets et al. 2002) or 10 200 cal. yr BP.

The age�depth model from the Parika sequence isbased on uncalibrated radiocarbon dates of bulk peat(Fig. 2) calibrated using the same method as the datesfrom Mustjarve. Also, in Parika bog the peat accumu-lation has been relatively uniform throughout theHolocene (0.40 to 1.60 mm/yr) (Niinemets et al. 2002).

Results

Colourless and light brownish glass particles wereobserved in two 10-cm samples from the Mustjarvesequence. Subsequent analyses of 2-cm samples showedthat tephra occurred at 266�270 cm and 312�316 cm.No calculation of the concentration of tephra particleswas attempted, but the extremely low number of shardsidentified on the microscope slides suggests a very lowconcentration, probably below 25 shards/cm3. Electronmicroprobe analysis was attempted on samples fromthese two levels from Mustjarve; however, only a fewshards from 312�316 cm survived the preparation.This is not uncommon when dealing with smallconcentrations of tephra from peat. The results of theanalyses (Table 2; Fig. 3) show a correlation with theHekla volcanic system on southwest Iceland (e.g.Larsen & Thorarinsson 1977; Dugmore et al. 1995).The CaO/MgO and FeOtot/TiO2 ratios show thattephra from the initial, most evolved Plinian phaseof the Hekla-4 eruption is present (e.g. Larsen& Thorarinsson 1977; Boygle 1999; Kirkbride &Dugmore 2001; van den Bogaard & Schmincke 2002).This distribution is similar to most analyses of theHekla-4 tephra from bogs in Sweden, Norway andnorthern Germany (van den Bogaard & Schmincke2002; Bergman et al. 2004; Boygle 2004; Pilcher et al.2005; Wastegard 2005). This also excludes the possibi-lity that the tephra identified in Mustjarve is theHekla-3 or the Kebister tephra (Fig. 3). However, theLairg A tephra (c. 7000 cal. yr BP; Hall & Pilcher 2002;Bergman et al. 2004) has a similar geochemistry, butthis can be excluded owing to its much higher age. Notephra was found in the samples from the Parika bog.

The crude age�depth model for Mustjarve bog(Fig. 2) indicates an age of c. 4800�5000 cal. yr BPfor the tephra at 312�316 cm. This is several hundredyears older than the generally accepted age for theHekla-4 (c. 4260 cal. yr BP; Pilcher et al. 1995), butsince the age�depth relationship in this part of the coreis based on only a few radiocarbon dates, an exact ageof the tephra cannot be anticipated. The fact that bulkpeat was used for dating may also be a source of dates

Fig. 2. Age�depth plots for the Mustjarve bog and the Parika bog.Investigated sediment interval in both sequences is indicated by arectangle with raster. The age�depth curves follow the midpoints ofcalibrated age intervals connected with straight line segments. Errorbars show the 2s range of each calibrated date.

646 Tiit Hang et al. BOREAS 35 (2006)

that are too old. The unidentified tephra at 266�270 cm has an age around 4000 cal. yr BP accordingto the age�depth model (Fig. 2). This age matches theKebister tephra (c. 3750 cal. yr BP; Gunnarson et al.2003), rather than the much younger Hekla-3 tephra(c. 3000 cal. yr BP; van den Bogaard et al. 2002),

especially since the age�depth model suggests a slightlyhigher than expected age for the Hekla-4 tephra. TheKebister tephra is widespread in Sweden (e.g. Boygle1998; Zillen et al. 2002; Bergman et al. 2004; Wastegard2005). Some of the particles at 266�270 cm had a lightyellowish to brownish colour. This has also beenobserved in occurrences of the Kebister tephra inSweden, but only occasionally in the Hekla-3 andHekla-4 tephras.

Discussion and conclusions

The first discovery of the middle Holocene Hekla-4tephra of Icelandic origin in Estonia is reported. In one(out of two) studied peat sequence, namely Mustjarve,at the depth interval 3.12�3.16 m, a low concentrationof tephra shards was detected. The microprobe analysisdemonstrated that tephra from the explosive initialphase of the Hekla-4 eruption reached as far east asnorthwest Estonia. An unidentified tephra at 266�270 cm in the same site is tentatively correlated withthe Kebister tephra. Thus, the finding of the Hekla-4tephra in Mustjarve confirms that this tephra isprobably the most widespread known Holocene tephralayer in the North Atlantic region, so far detected inScotland, England, Ireland, northern Germany, FaroeIslands, Norway and Sweden (e.g. Persson 1971;Dugmore 1989; Pilcher & Hall 1992, 1996; Wastegardet al. 2001; van den Bogaard & Schmincke 2002;Boygle 2004; Wastegard 2005).

These new findings are important in developing andextending the tephrochronology framework of north-west Europe and they open up new perspectives intostudies of the late Quaternary chronology in theeastern Baltic Sea area. They also open new possibi-lities for exact comparisons between climate shifts

Table 2. Geochemical composition of analysed volcanic glass from Mustjarve 312�316 cm analysed on a Cameca SX100 electron microprobeequipped with five vertical WD spectrometers. Ten major elements were measured with a counting time of 10 s. All elements are expressed asweight%. An accelerating voltage of 20 kV and a beam strength of 10 nA determined by a Faraday cup were used, with a rastered beam over anarea of 10�/10 mm to reduce instability of the glass and subsequent sodium loss. An empirical correction was applied to reduce to the P2O5

figure due to overlap with CaO counts (P. Hill, pers. comm. 2004). Calibration was undertaken using a combination of standards of puremetals, simple silicate minerals and synthetic oxides, including andradite. These were used regularly between analyses to correct for any drift inthe readings. A PAP correction was applied for the effects of X-ray absorption (Pouchou & Pichoir 1991). All analyses with totals above 91%were accepted due to the small number and low concentration of shards.

Analysis 1 2 3 4 5 Mean 1s

SiO2 71.576 73.501 73.155 70.099 68.895 71.442 1.967TiO2 0.098 0.058 0.072 0.071 0.097 0.079 0.018Al2O3 13.005 12.948 13.406 12.479 12.117 12.791 0.500FeOtot 1.985 1.984 1.935 2.084 2.006 1.999 0.054MnO 0.108 0.087 0.124 0.124 0.090 0.107 0.018MgO 0.045 0.031 0.018 0.000 0.052 0.029 0.021CaO 1.301 1.314 1.291 1.283 1.345 1.307 0.024Na2O 3.937 3.756 3.859 4.774 4.278 4.121 0.414K2O 2.600 2.827 2.641 2.614 2.755 2.687 0.099P2O5 0.000 0.022 0.000 0.009 0.000 0.006 0.010Total 94.655 96.528 96.501 93.537 91.645 94.571

Hekla-4

Kebister Hekla-3

Hekla-3

Kebister

3

r

FeOtot (%)0 1 2 3 4 5 6 7 8 9 10 11 12

TiO

2 (%

)

0.0

0.5

1.0

1.5

Hekla-4

H-3

CaO (%)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

MgO

(%

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Mustjärve bog, 312-316 cm

Mustjärve bog, 312-316 cm

Fig. 3. Geochemical characterization of the Hekla-4 tephra fromMustjarve bog (diamonds) compared with the main geochemicaldistribution of the Hekla-3 (dotted line), Kebister (solid line) andHekla-4 (shaded area) tephras in Iceland, the Faroe Islands,the British Isles and Sweden (data from Tephrabase; http://www.tephrabase.org), presented as CaO wt% vs. MgO wt% and FeOtot

wt% vs. TiO2 wt%.

BOREAS 35 (2006) Holocene cryptotephra in Estonia 647

recorded in peat bogs in an area extending fromIceland to the British Isles and Scandinavia and nowinto the eastern Baltic area. For example, expansion ofpeatlands and/or a shift to wetter conditions hasbeen observed in many bogs in northwest Europe ataround 4200 cal. yr BP, i.e. close in time to theHekla-4 eruption (e.g. Nilssen & Vorren 1991; Korhola1995; Hughes et al. 2000; Langdon & Barber 2004;Borgmark 2005). Whether this is coincidental or isrelated to the eruption remains to be proven (cf.Blackford et al. 1992; Edwards et al. 1996), but thepresence of the Hekla-4 tephra in northwest Europeanbogs and lake sediments opens up the possibility to testif this climate deterioration was synchronous orasynchronous throughout northwest Europe.

Acknowledgements. � Journal reviews by J. R. Pilcher and ananonymous reviewer were much appreciated. Pete Hill and AnthonyNewton, University of Edinburgh, were helpful with the geochemicalanalyses of tephra. The study was funded by the Estonian AcademicFoundation for International Exchanges, Estonian target fundingprojects 0182530s03 and 0331758s01, Estonian Science FoundationGrants 5681 and 4963. S.W.’s contribution was funded by theSwedish Research Council.

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