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

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1 First discovery of cryptotephra in Holocene peat deposits of Estonia, eastern Baltic TIIT HANG, STEFAN WASTEGÅRD, SIIM VESKI AND ATKO HEINSALU BOREAS Hang, T., Wastegård, S., Veski, S. & Heinsalu, A (November): First discovery of cryptotephra in Holocene peat deposits of Estonia, eastern Baltic. Boreas, Vol. 35, pp Oslo. ISSN This article reports the first discovery of middle Holocene cryptotephra from a peat sequence in Estonia, eastern Baltic. Two sequences, Mustjärve and Parika (located 110 km apart), were chosen for a pilot study aimed at finding traces of tephra fallout during the middle Holocene. Peat accumulation at both sites started in the early Holocene (c C yr BP; c cal. yr BP) and continued throughout the whole Holocene. The radiocarbon-dated intervals between c and CyrBP(c cal. yr BP) were chosen from both sites for the study. Colourless tephra shards were identified at cm below the peat surface in the Mustjärve peat sequence, while no tephra was found in peat of the same age at Parika. Electron microprobe analyses suggest a correlation with the initial phase of the Hekla-4 eruption (c cal. yr BP), although the age depth model indicated an age around 4900 cal. yr BP. Small concentrations of colourless to light brown tephra shards at cm in the Mustjärve sequence indicate that the Kebister tephra (c cal. yr BP) might also be present, but geochemical analyses were not possible. The low concentration and small size of the tephra particles indicate that Estonian bogs are probably on the verge of where tephrochronology is possible in northwestern Europe. Further studies of full Holocene sequences are required in order to discover traces of other ash plumes reaching as far east as the eastern Baltic area. Tiit Hang ( Tiit.Hang@ut.ee), Institute of Geology, Tartu University, Vanemuise 46, Tartu 51014, Estonia; Stefan Wastegård, Department of Physical Geography and Quaternary Geology, Stockholm University, SE , 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 Horizons of volcanic ash, or tephra layers, form excellent isochrones because of their widespread distribution, rapid dispersal and distinct chemical signatures (e.g. Westgate & Gorton 1981; Einarsson 1986). Tephrochronology has been recognized as an excellent correlation tool for sediment and peat sequences spanning the Last Termination and the Holocene in northwestern Europe (e.g. Dugmore 1989; Pilcher & Hall 1992, 1996; Dugmore et al. 1995; Birks et al. 1996; Turney et al. 1997; Wastegård et al. 2000a, b; Langdon & Barber 2001, 2004; Zillén et al. 2002; Davies et al. 2003; Bergman et al. 2004; Pilcher et al. 2005). Recent advances in the detection of cryptotephra horizons (horizons of very low concentration of shards, usually invisible to the naked eye) (cf. Lowe & Hunt 2001; Turney et al. 2004) have led to the discovery of previously undetected tephra horizons (e.g. the Borrobol Tephra; Turney et al. 1997) and have extended the known limits of individual Icelandic ash plumes into areas much more distal to the source than was previously known. This includes areas such as the British Isles (Turney et al. 1997; Mackie et al. 2002), southern and central Sweden (Wastegård et al. 2000a; Davies et al. 2003), western Russia (Wastegård et al. 2000b), northern Germany (van den Bogaard & Schmincke 2002) and The Netherlands (Davies et al. 2005). As the relatively small-scale 1947 eruption of Hekla deposited tephra along the south coast of Finland (Salmi 1948) and the stratigraphically important Vedde Ash, from the last glacial interglacial transition, has been reported in two lakes on Karelian Isthmus, northwestern Russia (Wastegård et al. 2000b), it is not unreasonable that ash plumes from other Icelandic eruptions may have reached the eastern Baltic Sea area. The aim of this article was to report the results of the first pilot study into detection of the Holocene cryptotephra from peat sequences in Estonia. Methods and material Two sequences, one from the Mustjärve bog and one from the Parika bog (Fig. 1), were collected from the Holocene peat record. The manually operated Russian type peat-corer (length 1 m, diameter 7 cm) was used. Tephra was detected using the method described by Pilcher & Hall (1992). Continuous 10-cm samples were initially combusted at 5508C for 4 h and treated in HCl in order to dissolve the carbonates. For those samples from which tephra shards were detected as a result of this initial application, the equivalent sediment interval was resampled using 2-cm slices in order to determine the tephra distribution more precisely. Geochemical analyses were performed on a WDS (Wavelength DOI / # 2006 Taylor & Francis

2 BOREAS 35 (2006) Holocene cryptotephra in Estonia 645 Fig. 1. Location of the study area and investigated sites at the Mustjärve and Parika bogs. Dispersive Spectrometer) Cameca SX100 electron microprobe at the Department of Geology and Geophysics, University of Edinburgh (Table 2). Only samples dated to the time interval between 2200 and CyrBP(c cal. yr BP) were chosen for analyses within the current study. Several widespread tephra horizons from this time interval (Hekla-3, Microlite, Kebister, Hekla-4) have been found in Sweden and northern Germany (Boygle 1998, 2004; van den Bogaard & Schmincke 2002; Zillén et al. 2002; Bergman et al. 2004; Wastegård 2005). The source of these, with the exception of the Microlite tephra, is the Hekla volcano in southwest Iceland; it is reasonable to speculate that ash clouds from these explosive eruptions could also have reached the eastern Baltic area. Table 1. Lithology of investigated cores from the Mustjärve and Parika bogs. For location of the sites, see Fig. 1. For exact location of the boreholes, see Veski (1998) and Niinemets et al. (2002). Mustjärve bog Parika bog Depth, cm Description Depth, cm Description 020 Sphagnum peat, low humification, light green Sphagnum peat with some Eriophorum, low humification, greenish-brown Sphagnum peat with some Eriophorum SphagnumEriophorum peat, low to medium humification, brown Sphagnum Eriophorum peat, low humification, brown Woody Phragmites peat, medium humification, dark brown SphagnumEriophorum peat, with changing Woody CarexPhragmites peat humification level Woody Carex Phragmites peat, medium to Silt with organic matter low humification, brown Woody CarexPhragmites peat Coarse detritus gyttja/woody CarexPhragmites peat Coarse detritus gyttja, greyish Fine to medium sand Silty clay with black FeS bands Gravel

3 646 Tiit Hang et al. BOREAS 35 (2006) The coring site at Mustjärve bog (N59804?41ƒ, E24806?31ƒ) is situated in northwest Estonia, on the coastal plain of the Baltic Sea at an altitude of 39 m a.s.l. (Fig. 1). The ombrotrophic bog is 2 km in diameter and the thickness of organic deposits in the central and deepest part reaches 7.5 m (Veski 1998). The basin was a shallow coastal lake after deglaciation and was finally isolated during the Ancylus Lake stage of the Baltic Sea and later infilled by peat. At the coring site, the beginning of organic sedimentation was dated to 95909/ C yr BP (Veski 1998), c cal. yr BP. The age depth model from the Mustjärve sequence is based on eight radiocarbon dates of bulk peat and gyttja (Fig. 2). These were calibrated using the OxCal program, version 3.9 (Bronk Ramsey 2001) and the IntCal98 calibration curve (Stuiver et al. 1998). The peat accumulation is relatively uniform throughout 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 Võrtsjärv depression, Fig. 2. Agedepth plots for the Mustjärve bog and the Parika bog. Investigated sediment interval in both sequences is indicated by a rectangle with raster. The agedepth curves follow the midpoints of calibrated age intervals connected with straight line segments. Error bars show the 2s range of each calibrated date. central Estonia (Fig. 1). The total area of the ombrotrophic Parika bog is 34 km 2 and the altitude of the peat surface varies between 45 and 50 m a.s.l. (Niinemets et al. 2002). The thickness of organic deposits is 8.80 m in the deepest parts and the onset of organic sedimentation has been dated to 90009/75 14 C yr BP (Niinemets et al. 2002) or cal. yr BP. The agedepth model from the Parika sequence is based on uncalibrated radiocarbon dates of bulk peat (Fig. 2) calibrated using the same method as the dates from Mustjärve. Also, in Parika bog the peat accumulation has been relatively uniform throughout the Holocene (0.40 to 1.60 mm/yr) (Niinemets et al. 2002). Results Colourless and light brownish glass particles were observed in two 10-cm samples from the Mustjärve sequence. Subsequent analyses of 2-cm samples showed that tephra occurred at cm and cm. No calculation of the concentration of tephra particles was attempted, but the extremely low number of shards identified on the microscope slides suggests a very low concentration, probably below 25 shards/cm 3. Electron microprobe analysis was attempted on samples from these two levels from Mustjärve; however, only a few shards from cm survived the preparation. This is not uncommon when dealing with small concentrations of tephra from peat. The results of the analyses (Table 2; Fig. 3) show a correlation with the Hekla volcanic system on southwest Iceland (e.g. Larsen & Thorarinsson 1977; Dugmore et al. 1995). The CaO/MgO and FeO tot /TiO 2 ratios show that tephra from the initial, most evolved Plinian phase of 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 the Hekla-4 tephra from bogs in Sweden, Norway and northern Germany (van den Bogaard & Schmincke 2002; Bergman et al. 2004; Boygle 2004; Pilcher et al. 2005; Wastegård 2005). This also excludes the possibility that the tephra identified in Mustjärve is the Hekla-3 or the Kebister tephra (Fig. 3). However, the Lairg A tephra (c cal. yr BP; Hall & Pilcher 2002; Bergman et al. 2004) has a similar geochemistry, but this can be excluded owing to its much higher age. No tephra was found in the samples from the Parika bog. The crude age depth model for Mustjärve bog (Fig. 2) indicates an age of c cal. yr BP for the tephra at cm. This is several hundred years older than the generally accepted age for the Hekla-4 (c cal. yr BP; Pilcher et al. 1995), but since the agedepth relationship in this part of the core is based on only a few radiocarbon dates, an exact age of the tephra cannot be anticipated. The fact that bulk peat was used for dating may also be a source of dates

4 BOREAS 35 (2006) Holocene cryptotephra in Estonia 647 Table 2. Geochemical composition of analysed volcanic glass from Mustjärve cm analysed on a Cameca SX100 electron microprobe equipped with five vertical WD spectrometers. Ten major elements were measured with a counting time of 10 s. All elements are expressed as weight%. 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 an area of 10/10 mm to reduce instability of the glass and subsequent sodium loss. An empirical correction was applied to reduce to the P 2 O 5 figure due to overlap with CaO counts (P. Hill, pers. comm. 2004). Calibration was undertaken using a combination of standards of pure metals, simple silicate minerals and synthetic oxides, including andradite. These were used regularly between analyses to correct for any drift in the 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 Mean 1s SiO TiO Al 2 O FeO tot MnO MgO CaO Na 2 O K 2 O P 2 O Total that are too old. The unidentified tephra at cm has an age around 4000 cal. yr BP according to the agedepth model (Fig. 2). This age matches the Kebister tephra (c cal. yr BP; Gunnarson et al. 2003), rather than the much younger Hekla-3 tephra (c cal. yr BP; van den Bogaard et al. 2002), MgO (%) TiO 2 (%) Mustjärve bog, cm H-3 Kebister Hekla-3 Hekla Hekla CaO (%) Mustjärve bog, cm Hekla-4 Kebister FeO tot (%) Fig. 3. Geochemical characterization of the Hekla-4 tephra from Mustjärve bog (diamonds) compared with the main geochemical distribution of the Hekla-3 (dotted line), Kebister (solid line) and Hekla-4 (shaded area) tephras in Iceland, the Faroe Islands, the British Isles and Sweden (data from Tephrabase; tephrabase.org), presented as CaO wt% vs. MgO wt% and FeO tot wt% vs. TiO 2 wt%. especially since the agedepth model suggests a slightly higher than expected age for the Hekla-4 tephra. The Kebister tephra is widespread in Sweden (e.g. Boygle 1998; Zillén et al. 2002; Bergman et al. 2004; Wastegård 2005). Some of the particles at cm had a light yellowish to brownish colour. This has also been observed in occurrences of the Kebister tephra in Sweden, but only occasionally in the Hekla-3 and Hekla-4 tephras. Discussion and conclusions The first discovery of the middle Holocene Hekla-4 tephra of Icelandic origin in Estonia is reported. In one (out of two) studied peat sequence, namely Mustjärve, at the depth interval m, a low concentration of tephra shards was detected. The microprobe analysis demonstrated that tephra from the explosive initial phase of the Hekla-4 eruption reached as far east as northwest Estonia. An unidentified tephra at cm in the same site is tentatively correlated with the Kebister tephra. Thus, the finding of the Hekla-4 tephra in Mustjärve confirms that this tephra is probably the most widespread known Holocene tephra layer in the North Atlantic region, so far detected in Scotland, England, Ireland, northern Germany, Faroe Islands, Norway and Sweden (e.g. Persson 1971; Dugmore 1989; Pilcher & Hall 1992, 1996; Wastegård et al. 2001; van den Bogaard & Schmincke 2002; Boygle 2004; Wastegård 2005). These new findings are important in developing and extending the tephrochronology framework of northwest Europe and they open up new perspectives into studies of the late Quaternary chronology in the eastern Baltic Sea area. They also open new possibilities for exact comparisons between climate shifts

5 648 Tiit Hang et al. BOREAS 35 (2006) recorded in peat bogs in an area extending from Iceland to the British Isles and Scandinavia and now into the eastern Baltic area. For example, expansion of peatlands and/or a shift to wetter conditions has been observed in many bogs in northwest Europe at around 4200 cal. yr BP, i.e. close in time to the Hekla-4 eruption (e.g. Nilssen & Vorren 1991; Korhola 1995; Hughes et al. 2000; Langdon & Barber 2004; Borgmark 2005). Whether this is coincidental or is related to the eruption remains to be proven (cf. Blackford et al. 1992; Edwards et al. 1996), but the presence of the Hekla-4 tephra in northwest European bogs and lake sediments opens up the possibility to test if this climate deterioration was synchronous or asynchronous throughout northwest Europe. Acknowledgements. Journal reviews by J. R. Pilcher and an anonymous reviewer were much appreciated. Pete Hill and Anthony Newton, University of Edinburgh, were helpful with the geochemical analyses of tephra. The study was funded by the Estonian Academic Foundation for International Exchanges, Estonian target funding projects s03 and s01, Estonian Science Foundation Grants 5681 and S.W. s contribution was funded by the Swedish Research Council. References Bergman, J., Wastegård, S., Hammarlund, D. & Wohlfarth, B. 2004: Tephras of the Klocka mire, mid-central Sweden, extending the known distribution of several important Holocene tephra isochrones. Journal of Quaternary Science 19, Birks, H. H., Gulliksen, S., Hafliðason, H., Mangerud, J. & Possnert, G. 1996: New radiocarbon dates for the Vedde Ash and the Saksunarvatn ash from Western Norway. Quaternary Research 45, Blackford, J. J., Edwards, K. J., Dugmore, A. J., Cook, G. T. & Buckland, P. C. 1992: Icelandic volcanic ash and the mid-holocene Scots pine (Pinus sylvestris) pollen decline in northern Scotland. 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