Fróðskaparrit - 01.01.1998, Side 190

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HOLOCENE TEPHRA LAYERS IN THE FAROEISLANDS
traction of tephra from peat profiles, but
followed by grain-discrete geochemical
analysis (Larsen, 1981; Dugmore, 1989a),
and comparison with reference material
from Iceland (Imslands 1978; Jakobsson
1979; Dugmore etal., 1992; 1995a; Larsen
et al., submitted).
Refining the palaeoenvironmental
records of tephra
A basic framework of a prehistorical Holo-
cene tephrochronology of Faroe is indicat-
ed by the work of Persson (1968; 1971) and
Mangerud et al. (1986). While the status of
Persson’s putative ‘Landnam Ash’/Hekla
1104 AD and the c. 365 AD Saksunmyren
tephra continues to be uncertain, recent
work at Skaelingsvatn does seem to have
resolved the status of the two older silicic
tephras reported by Persson (1968), and
linked to H-3 and either H-4 or H-S (‘H-2’
of Persson, 1968; 1971). At Skaelingsvatn
palaeoecological studies have been under-
taken on the lake sediments (Edwards et al.,
1994), the full results of which will be re-
ported elsewhere (Sadler et al., ms). Two
visible tephra layers were found, and their
geochemistry characterised. Comparison to
Icelandic reference material shows these
tephras to be the c. 3500 BP H-S and c.
3800 BP H-4 layers (Kjartansson et al.,
1964; Larsen and Thórarinsson, 1977;
Dugmoreeía/., 1995b; and Table 1, Fig. 1).
The implication is that upper of the two vis-
ible silicic tephra found elsewhere in Faroe
by Persson (1968) and attributed to H-3 on
the basis of radiocarbon dating is also part
of somewhat older H-S tephra, and the low-
er silicic tephra is H-4. This also supports
the conclusion of Waagstein and Jóhansen
(1968) who identified the presence of ‘H-2’
(now known as H-S) elsewhere in Faroe on
the basis of the radiocarbon dating. The H-
S tephra may also be present in Shetland
(the ‘Kebister tephra’ of Dugmore et al.,
1995a, and Dugmore and Newton, 1998),
and in central Sweden (Boygle, in press).
In this paper we also present additional
data on the Saksunarvatn tephra from the
reference core in Saksunarvatn (Table 2).
These analyses have been conducted under
the same conditions as all other analyses of
Holocene Icelandic tephra undertaken at
Edinburgh (e.g. Dugmore and Newton,
1992; Dugmoreeía/., 1992; l995a;)andso
are directly comparable with a growing
North Atlantic tephra database (Newton et
al., 1997; and Tephrabase, 1998 at http://
www.geo.ed.ac.uk/tephra/tbasehom.html).
The spread of the data on the Saksunarvatn
tephra (which probably reflects analytical
precision as well as sample variability), is
significantly less on this new data set than
those reported by Mangerud et al. (1986)
for Faroe, Kvamme et al. (1989) for the
North Atlantic, and Bennett et al. (1992)
for Shetland. Despite this improved resolu-
tion it is still not possible to use major ele-
ment data alone to effectively discriminate
between separate eruptions of Grimsvotn
(Larsen, 1982). Resolving this question
will still rely on the use of additional lies of
evidence such as stratigraphic position, as-
sociated biostratigraphy and radiometric
dating.
Jóhannes Jóhansen also provided a sam-
ple for us to analyse of a silicic tephra from
below the Saksunarvatn tephra in the refer-