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. 2006 (November): First discovery of cryptotephra in Holocene
peat 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, 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. 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 from
both sites for the study. Colourless tephra shards were identified at 312 316 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. 4260 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 266 270 cm in the Mustjärve sequence indicate that the Kebister tephra (c. 3750 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 (e-mail: 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-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, 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 10.1080/03009480600827272 # 2006 Taylor & Francis
Holocene cryptotephra in Estonia
BOREAS 35 (2006)
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 5000 14C yr BP (c. 2000 5500 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
Depth, cm
0 20
20 100
100 200
200 510
510 660
600 660
660 685
685 734
734 743
743 760
760 770
Parika bog
Description
Sphagnum peat, low humification, light green
Depth, cm
0 100
Description
600 880
Sphagnum peat with some Eriophorum , low
humification, greenish-brown
Sphagnum Eriophorum peat, low to medium
humification, brown
Woody Phragmites peat, medium humification,
dark brown
Woody Carex Phragmites peat
880 930
Silt with organic matter
Sphagnum peat with some Eriophorum
100 560
Sphagnum Eriophorum peat, low humification,
brown
Sphagnum Eriophorum peat, with changing
humification level
Woody Carex Phragmites peat, medium to
low humification, brown
Woody Carex Phragmites peat
Coarse detritus gyttja/woody
Carex Phragmites peat
Coarse detritus gyttja, greyish
Fine to medium sand
Silty clay with black FeS bands
Gravel
560 600
646
Tiit Hang et al.
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 95909/120 14C yr BP (Veski 1998),
c. 10 900 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. Age depth plots for the Mustjärve bog and the Parika bog.
Investigated sediment interval in both sequences is indicated by a
rectangle with raster. The age depth curves follow the midpoints of
calibrated age intervals connected with straight line segments. Error
bars show the 2s range of each calibrated date.
BOREAS 35 (2006)
central Estonia (Fig. 1). The total area of the
ombrotrophic Parika bog is 34 km2 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 10 200 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 266 270 cm and 312316 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/cm3. Electron
microprobe analysis was attempted on samples from
these two levels from Mustjärve; however, only a few
shards from 312316 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 FeOtot/TiO2 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. 7000 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. 4800 5000 cal. yr BP
for the tephra at 312 316 cm. This is several hundred
years older than the generally accepted age for the
Hekla-4 (c. 4260 cal. yr BP; Pilcher et al. 1995), but
since the age depth 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
Holocene cryptotephra in Estonia
BOREAS 35 (2006)
647
Table 2. Geochemical composition of analysed volcanic glass from Mustjärve 312 316 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 P2O5
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
1
2
3
4
5
Mean
1s
SiO2
TiO2
Al2O3
FeOtot
MnO
MgO
CaO
Na2O
K2O
P2O5
Total
71.576
0.098
13.005
1.985
0.108
0.045
1.301
3.937
2.600
0.000
94.655
73.501
0.058
12.948
1.984
0.087
0.031
1.314
3.756
2.827
0.022
96.528
73.155
0.072
13.406
1.935
0.124
0.018
1.291
3.859
2.641
0.000
96.501
70.099
0.071
12.479
2.084
0.124
0.000
1.283
4.774
2.614
0.009
93.537
68.895
0.097
12.117
2.006
0.090
0.052
1.345
4.278
2.755
0.000
91.645
71.442
0.079
12.791
1.999
0.107
0.029
1.307
4.121
2.687
0.006
94.571
1.967
0.018
0.500
0.054
0.018
0.021
0.024
0.414
0.099
0.010
that are too old. The unidentified tephra at 266 270 cm has an age around 4000 cal. yr BP according
to the agedepth model (Fig. 2). This age matches the
Kebister 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),
1.2
Mustjärve bog, 312-316 cm
MgO (%)
1.0
Discussion and conclusions
0.8
0.6
0.4
Hekla-3
ter
is
Keb
0.2
Hekla-4
0.0
0.0
0.5
1.0
1.5
2.0
2.5 3.0
CaO (%)
3.5
4.0
4.5
5.0
11
12
Mustjärve bog, 312-316 cm
1.5
TiO2 (%)
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 266270 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.
Kebisterr
1.0
3
Hekla-3
0.5
la-4
Hek
H-3
0.0
0
1
2
3
4
5
6
7
FeOtot (%)
8
9
10
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; http://www.
tephrabase.org), presented as CaO wt% vs. MgO wt% and FeOtot
wt% vs. TiO2 wt%.
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 3.12 3.16 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 266 270 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
648
Tiit Hang et al.
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 0182530s03 and 0331758s01, Estonian Science Foundation
Grants 5681 and 4963. S.W.’s contribution was funded by the
Swedish Research Council.
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