Fróðskaparrit - 01.01.1998, Blaðsíða 188

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HOLOCENE TEPHRA LAYERS IN THE FAROEISLANDS
vironmental changes in Faroe at the close
of the lOth Millennium BP.
In addition to the physical records of
tephra found in Faroe, historical records
compiled by Thórarinsson (1981b) suggest
that traces of three ash-falls within the last
400 years may also be present, and, by ex-
tension into prehistory, that tephras may be
more numerous than the five identified by
Persson (1971) and Mangerud etal. (1986).
Tephras in historical time in Faroe
At least one silicic tephra has formed an
isochrone across Faroe in the early part of
historical time. On the basis of its silicic
character and radiocarbon age, Persson
(1968; 1971) suggested that a tephra at
Myrarnar may be either a part of the ninth
century AD Icelandic ‘Landnam Ash’ or
the 1104 AD Hekla tephra (Thórarinsson
1967). A silicic component of the ‘Land-
nam Ash’ was erupted from the Torfajokull
volcanic system c. 871 AD (Grónvold et
al., 1995), but the volume of this compo-
nent, at c. 0.1 km3, is comparatively small
(Larsen, 1984). As one alternative the
Faroese deposit may be a part of the much
larger-scale silicic tephra from the 1104 AD
eruption of Hekla or, as a third possibility,
the tephra from Hekla in 1158 AD. The
1158 AD tephra is smaller in volume than
the 1104 AD, but of a similar size to the
silicic component of the Landnam Tephra
(Larsen et al., submitted). Major element
geochemistry can resolve this question and
provide the identity of a key marker hori-
zon for the colonisation or early settlement
period in Faroe.
Other eruptions in Iceland during histor-
ical times in Faroe may have led to fall-out
across the islands. The Óræfajokull erup-
tion of 1362 AD was a very large, cata-
strophic event that produced a large volume
of tephra blown eastwards from Iceland
(Thórarinsson, 1958). Given the scale of
this tephra and its direction of fall-out, it
could be present in Faroe, although it was
not found by Persson (1968) at his four
sites. Traces are, however, present in the
British Isles (Dugmore et al., 1995a; Pil-
cher et al., 1996), and it has been identified
in the Greenland ice core (Palais et al.,
1991). Another tephra that may possibly be
present in Faroe is from the 1510 AD erup-
tion of Hekla. The tephra fall-out initially
spread south from the volcano, but has been
identifted at sites across Scotland and in the
north of Ireland, so it may have also
reached Faroe (Dugmore et al., 1995a;
1996; Pilcher etal., 1996).
Further additional possibilities are high-
lighted by Thórarinsson’s (1981 b) compi-
lation of historical records of dust falls on
both land and ships which suggests that
tephra from the Katla eruptions of 1625
AD, 1660 AD and 1755 AD could all be
present in Faroe.
In general the historical references to
dust falls in areas downwind of active vol-
canoes can be divided into two groups:
those where there is physical evidence of
fallout, and those where, at present, there is
none. Dust falls from a variety of sources
can coincide with either eruptions or the
passing of tephra clouds, yet may be totally
unrelated, so any such records have to be
treated with considerable caution. This is a
point stressed by Thórarinsson who drew