Fróðskaparrit - 01.01.1998, Síða 294

300
DISTRIBUTION OF LATEGLACIAL TEPHRA IN SCANDINAVIA
Central Europe, but the north-eastern fan of
ash dispersal reached at least as far as the
southern Baltic area, including the Born-
holm and Riigen islands (Fig. 1; Usinger,
1977; van den Bogaard and Schmincke,
1985; Juvigne et al., 1995). It is possible
that at least the south-eastern part of Swe-
den also may have received tephra from this
eruption. The northernmost record of LST
was made in a core with laminated clay
from the Baltic, north-west of Gotland, and
was dated by pollen analysis to the late
Allerød (Páhlsson and Bergh Alm, 1985).
This record has not been geochemically
conftrmed. and it would therefore be im-
portant to re-sample the Lateglacial clays in
the Central Baltic Sea, especially as the
LST can be used as a marker in the Swedish
clay-varve chronology. LST is dated to the
later part of the Allerød, c. 11,200 14C years
BP (e.g. Hajdas et al., 1995), and it may be
used as an important marker horizon for the
Killarney/Gerzensee oscillation, which is
the last cold event during the Lateglacial
Interstadial. LST has not yet been identi-
fted in the ice-core records, but some sul-
phate peaks around 13,000 ice-core years
BP may be possible candidates (cf. Zielins-
ki etal., 1996; 1997).
The Vedde Ash is the most important
Lateglacial ash horizon in the North At-
lantic region and is also the main compo-
nent of the so called North Atlantic Ash
Zone 1 (e.g. Kvamme et al., 1989; Lacasse
et al., 1995). It has been found in a large
area around in the North Atlantic and con-
sists of two geochemically distinct popula-
tions: one rhyolitic and one basaltic (e.g.
Mangerud et al., 1984). A new technique
for extracting the rhyolitic part of this bi-
modal tephra from minerogenic deposits
(Turney, 1998) has extended the area of de-
tection to the British mainland and south
Sweden (Fig. 1). The technique relies upon
a difference between the specific gravity of
the rhyolitic shards (2.4 - 2.5 g/cm3 for the
Vedde Ash) and that of the host sediment,
and uses sodium polytungstate as the flota-
tion medium (Turney, 1998). This has led
to the identification of the rhyolitic part of
the Vedde Ash in several sites in Scotland
and south Sweden (e.g. Roberts, 1997; Tur-
ney et ai, 1997; Wastegárd et ai, 1998; J.
Bjorck and Wastegárd, submitted), even at
sites where the peak concentration of
shards is extremely low (<100 shards/cm3
sediment).
The Vedde Ash is radiocarbon dated by
the AMS technique to a plateau around
10,400 - 10,300 14C years BP (Birks et ai,
1996; Wastegárd et ai, 1998), and is pre-
sent in the GRIP core where it is dated to
11,980 ± 80 ice-core years (Gronvold et ai,
1995). The source of the Vedde Ash is
probably the Katla volcano on south-west
Iceland (Lacasse et al., 1995).
The Saksunarvatn Ash was first found on
the Faroe Islands where it can be found as a
several cm thick horizon in many places
(Mangerud et al., 1986; Edwards and
Craigie 1998). Its terrestrial distribution
also includes northern Germany (Merkt et
al., 1993) and western Norway (the
Krákenes site; Birks et al., 1996). It is pos-
sible that this ash has a much wider distrib-
ution, especially in Scandinavia, and fur-
ther investigations are needed to investigate
its distribution. The source of the Saksunar-