Jökull - 01.01.2012, Page 76

S. Wastegård and J. Boygle
1966; 1967a; 1968). These papers were reviewed and
compiled in his doctoral thesis (Persson, 1971). Al-
though geochemical analyses were not possible, he
could make some tentative correlations with known
tephra layers, for example with Askja-1875, Hekla-
3 and Hekla-4, using refractive indices of the glass
and radiocarbon dates. Later geochemical analyses
have confirmed many of Persson’s results from Swe-
den and the Faroe Islands (Wastegård et al., 2001;
Boygle, 1998, 2004).
The interest in tephrochronology grew during the
1980s partly because new analytical methods became
available, and partly because some new key findings
were made, for example the first terrestrial findings of
the Vedde Ash, (ca. 12.1 ka BP) and the Saksunarvatn
Ash, (ca. 10.3 ka BP) in lake sediment sequences in
western Norway and on the Faroe Islands (Mangerud
et al., 1984; 1986). Later in the 1980s the first record
of distal Icelandic tephra was made on the British
Isles (Dugmore, 1989), and during the 1990s sev-
eral records of Holocene tephras of Icelandic origin
were reported from bogs on the British Isles, north-
ern Germany and Scandinavia (e.g. Pilcher and Hall,
1992; van den Bogaard et al., 1994; Dugmore et al.,
1995; Boygle, 1998). The vast majority of the tephras
found in distal sites have a high silica content (SiO2
>63 wt%) and some of the layers, e.g. Hekla-1 (AD
1104), Glen Garry (ca. 2100 BP), Hekla-3 (ca. 3000
BP), Hekla-S/Kebister (ca. 3720 BP), Hekla-4 (ca.
4260 BP) and Lairg-A (ca. 6900 BP) have emerged
as widespread and valuable isochrones. Since the
late 1990s new methods for extracting microscopic
tephra (cryptotephra) from minerogenic deposits have
been developed (Turney, 1998) which has added
a large number of tephra horizons to the growing
tephrochronology networks of NW Europe, especially
for the Last Glacial-Interglacial transition (LGIT, ca.
15–9 ka BP). This has led to a spatial expansion of
several tephra layers and especially the distribution of
the Vedde Ash has been extended enormously by this
technique. Its dispersal as a cryptotephra has now ex-
panded into the Mediterranean region and to western
Russia (Wastegård et al., 2000b; Lane et al., 2011).
In this paper we briefly describe the distal
tephrochronology of Scandinavia, with a special focus
on Sweden. This has previously been described by
Wastegård (2005), but some recent findings are also
discussed here. No tephra layers found so far in Swe-
den are visible to the naked eye, and the size of the
shards usually ranges between 10 and 100 µm. Most
tephra layers detected so far consist of rhyolitic or in-
termediate glass with SiO2 contents between 57% and
74%. All ages are reported as calendar years BP, un-
less indicated. All reported tephra layers and sites are
listed in Tables 1 and 2.
THE LAST GLACIAL-INTERGLACIAL,
LGIT (ca. 15–9 ka BP)
The only record of a tephra layer from the LGIT be-
fore the late 1990s was made by Påhlsson and Bergh
Alm (1985) who tentatively identified the Laacher
See Tephra (LST; ca. 12.9 ka BP) in the marine core
14103-3 taken NW of the island of Gotland in the
Baltic Sea (Figure 1). A few shards from the core
were analysed by van den Bogaard and Schmincke
(1985) and seem to confirm the correlation with the
LST, although the results are shown in plots and are
not tabulated. Efforts have been made to find the LST
in terrestrial settings in Sweden, including Gotland,
but with little success. The only positive indication
so far is from the Hässeldala port site in SE Swe-
den (Figure 1) where a cryptotephra was found in the
right stratigraphical position (the Late Allerød pollen
zone) and with morphological characteristics of the
LST (Davies et al., 2003; Wohlfarth et al., 2006). No
compositional data are available, however.
The Vedde Ash (ca. 12.1 ka BP) is probably the
best example of the success made with the density
separation technique described by Turney (1998). Al-
though density separation techniques have been used
earlier for extracting tephra from minerogenic sedi-
ments (e.g. Merkt et al., 1993; Eden et al., 1994),
this new and simple technique has revolutionized the
search for cryptotephra in distal settings. Originally
using solutions of sodium polytungstate with densities
between 2.40 and 2.50 g cm−3, several tephrochro-
nologists now use wider density ranges in order to
74 JÖKULL No. 62, 2012