Fróðskaparrit - 01.01.1998, Side 292

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DISTRIBUTION OF LATEGLACIAL TEPHRA IN SCANDINAVIA
complicated to prove by using radiocarbon
dating. Comparisons of marine and terres-
trial records are hampered by different
reservoir ages and by the fact that the ma-
rine reservoir age was not constant and
seems to have increased during the
YoungerDryas (e.g. Bard etal., 1994). Fur-
thermore, the time period in question is
characterised by a number of radiocarbon
plateaux with near-constant radiocaibon
ages, e.g. at 10,400 - 10,300 14C years BP
and 10,000 14C years BP (e.g. Wohlfarth,
1996; Hughen et al.. 1998).
The use of tephra layers as time-syn-
chronous marker horizons in Late Quater-
nary deposits has increased considerably
during the last decades. Tephra horizons
enable high-precision comparisons be-
tween sequences and sediments/deposits of
different origin. The volcanic activity dur-
ing the Lateglacial period was extremely
high, possibly due to increased crustal
stresses that occur with changing volumes
of continental ice sheets (Zielinski et al.,
1997). This infers that many time-equiva-
lent tephra horizons may occur in sedi-
ments deposited during this time period.
Volcanic eruptions on Iceland has fre-
quently dispersed tephra over north-west
Europe in the Late Quaternary. Recent
identification of tephra in the ice cores from
Greenland have increased the value of these
tephra horizons as dating tools, as these of-
ten can be dated with high precision. Sev-
eral significant sulphate peaks in the ice
cores also suggest that many tephra hori-
zons remain to be discovered.
The first tephrochronological investiga-
tions in Scandinavia were made by Pers-
son, who studied Holocene tephra in peat
deposits in Sweden, Norway and on the
Faroe Islands (e.g. Persson, 1971). Persson
described many distinct tephra horizons
which he correlated with well-known erup-
tions, such as the Hekla 4, Hekla 3 and the
Óræfajokull eruption at 1362 AD. There
are also well-known records of tephra fall-
out in Scandinavia and Finland from histor-
ical eruptions on Iceland Askja 1875 AD
and Hekla 1947 AD; see e.g. Einarsson,
1986).
Distribution of Lateglacial tephra
in Scandinavia
So far, three tephra horizons from the LGIT
have been discovered in Scandinavia. Of
these, two are of Icelandic origin, the Ved-
de Ash (c. 10,300 14C years BP) and the
Saksunarvatn Ash (c. 9000 14C years BP)
and one of Central European origin, the
Laacher See Tephra (LST, c. 11,200 l4C
years BP). The recent discovery of the pre-
viously unreported Borrobol Tephra (c.
12,400 14C years BP) in Scotland (Turney
et al., 1997), implies that other Lateglacial
tephra horizons of Icelandic origin also
may be searched for in Scandinavia.
The distribution of these three tephra
horizons is shown in Fig. 1. The dispersal
fans are drawn according to known terres-
trial sites with geochemically confirmed
occurrence of tephra.
The Laacher See Tephra is a widespread
tephra horizon which was the result of the
first and only eruption of the Laacher See
volcano in the Eifel area in westem Ger-
many (Fig. 1). The distribution of this
phonolitic tephra is mostly confmed to