Fróðskaparrit - 01.01.1998, Side 186

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HOLOCENE TEPHRA LAYERS IN THE FAROE ISLANDS
duced by Icelandic volcanoes (Thórarins-
son, 1981a; Sigurdsson and Loebner, 1981;
Dugmore et al., 1995a). Over the last 50
years detailed proximal chronologies have
been developed in Iceland, (Thórarinsson,
1944; 1974; Larsen et al., submitted) and,
more recently, long-distance connections
have been made to distal sequences on the
ocean floor (Sigurdsson, 1982; Ruddiman
and Mclntyre, 1981), Scandinavia (Pers-
son, 1971; Mangerud et al., 1984; Birks et
al., 1996), the British Isles (Dugmore,
1989a; Pilcher and Hall, 1992; 1996), NW
Europe (Merkt et al., 1993; Bogaard et al.,
1994), and the Greenland icecores (Grón-
vold et al., 1995). The key to recent devel-
opments has been the attention paid to mi-
croscopic tephra horizons which are impor-
tant for two main reasons; firstly, they can
enhance temporal resolution in areas with
well-established tephrochronologies based
on a sequence of visible layers. Secondly
they can be used to greatly extend the spa-
tial coverage of the technique.
Once established tephrochronology can
be used to tackle a number of problems in
palaeoenvironmental studies (e.g. Brown et
al., 1992; Blackford et al., 1992; Froggatt
and Rogers, 1990; Austin et al., 1995).
Most importantly tephras can be used to for
the precise correlation of stratigraphic se-
quences, such as the connection of soil,
peat and lake sediments, some of which
may otherwise be undatable (Dugmore
1989b; Haflidason etal., 1992). The identi-
fícation of known tephras can permit the
application of the best available absolute
dating on each isochrone and associated
sediments. For example, the identification
of H-4 in the Faroes would enable precise
absolute dates determined elsewhere to be
applied within the islands. These would in-
clude the composite radiocarbon date 3,833
+/- 11 14 C yrs BP based on sites in Iceland
and the UK (Dugmore et al., 1995b) or the
equivalent calendrical ‘wiggle match’ date
of 2310+/- 20 BC determined in Ireland
(Pilcher et al., 1995). This use of precise
and accurate absolute dates is particularly
important for key periods of rapid change,
such as deglaciation and the first human
colonisation. Finally, the stratigraphic dis-
tribution of a known tephra can be used to
identify both sediment movement within a
profile, and the wholesale regional rework-
ing of sediments (Boygle, 1994).
Tephra layers and tephrochronology
could make a key contribution to studies of
environmental change in Faroe. The pur-
pose of this paper is to assess the current
status of tephrochronology in Faroe and
consider future developments.
Holocene tephra layers
in the Faroe Islands
Pioneering work on Holocene tephra layers
in Faroe was undertaken by Persson (1968)
as part of a wide ranging study that showed
Icelandic tephra was present as discrete
horizons in the peat bogs of both Faroe and
mainland Scandinavia (Persson, 1966;
1967). In Faroe, Persson studied four bogs
in detail, and showed that in the last 7,000
years silicic tephras have been deposited on
at least four occasions. The tephra deposits
are all younger than 4000 BP, with the re-
fractive indices of the glass showing little
variation between 1.497 - 1.521, and grain