Jökull - 01.12.1999, Side 14

ized by high Oxyria + Rumex values, and the peak in
this pollen taxon at the base of the zone was interpret-
ed by Rundgren (1995) as indicating a change to
milder and more humid conditions. Other important
herb-pollen taxa in this zone are, for example,
Caryophyllaceae, Koenigia islandica and Saxifraga,
while Betula nana is the most important dwarf-shrub
pollen taxon (Rundgren, 1995; Rundgren et al, 1997;
Rundgren, 1998). Empetrum nigrum is, however,
seen to expand already in RPAZ Skagi-4 in the Lake
Geitakarlsvötn sequence (Rundgren, 1998), in con-
trast to the other sites studied. Grasses were less dom-
inant on northernmost Skagi between 9900 and 9600
BP compared with earlier periods, and the vegetation
recorded in RPAZ Skagi-4 may be characterized as a
herb tundra.
Both Pediastrum concentration values and organic
carbon content reflect a higher limnic productivity in
Lake Torfadalsvatn and Lake Hraunsvatn after 9900
BP (Rundgren et al., 1997). Also the Lake
Kollusátursvatn and Lake Neðstavatn sequences show
raised organic carbon values at the Younger
Dryas/Holocene boundary, although these basins had
not been isolated by that time. The Lake
Geitakarlsvötn sequence does not reach back to RPAZ
Skagi-3.
The results from a detailed pollen and elemental
carbon analysis of the Lake Torfadalsvatn, Lake
Hraunsvatn and Lake Geitakarlsvötn sequences pre-
sented by Björck et al. (1997) show that there is clear
evidence of a short-lived cooling event, the Preboreal
oscillation (PBO), on northernmost Skagi around
9800-9700 BP. The onset of this event is registered as
a lithological change and coincidently decreased Pe-
diastrum concentration values and organic carbon
content. Also terrestrial vegetation responded to the
PBO cooling, as indicated, for example, by falling
Caryophyllaceae and Betula nana pollen concentra-
tion values. The end of this event was associated with
a general expansion of terrestrial vegetation. This ap-
plies particularly to Oxyria digyna, which seems to
have expanded in a similar way as recorded at the
Younger Dryas—Preboreal transition, suggesting a
change to milder conditions at the end of the PBO. It
was concluded by Björck et al. (1997) that the impact
of the Preboreal oscillation on terrestrial and limnic
environments on Skagi was similar to that recorded
during the Younger Dryas, but the PBO was found to
be a less severe event, which suggests less extensive
sea-ice conditions during the Preboreal oscillation.
The results presented by Rundgren et al. (1997)
show that a minor and short-lived transgression oc-
curred on northernmost Skagi within RPAZ Skagi-4
(Fig. 6). This transgression generated an almost con-
tinuous beach ridge (C) along the coast of northern-
most Skagi (Fig. 2) and a marine event (and probably
also a hiatus) in the Lake Kollusátursvatn succession.
The transgression is, however, not recorded in the
Lake Geitakarlsvötn basin, which is situated immedi-
ately landward of ridge C. This may be explained by
its more sheltered position. Rundgren et al. (1997) at-
tributed also this second transgression to glacier ex-
pansions in the vicinity of Skagi and elsewhere in Ice-
land, and they dated it to 9800-9700 BP. Since this
transgressional phase coincides with the early Prebo-
real cooling (PBO) discussed by Björck et al. (1997),
it is likely that this cold event induced glacier growth
and increased glacial loading that resulted in the sec-
ond transgression recorded on northemmost Skagi. A
palaeogeographical reconstraction of the situation at
9800-9700 BP is presented in Fig. 9.
9600-8800 BP
This episode is recorded in all five lake sequences
(Rundgren et al, 1997), but only that from Lake
Torfadalsvatn covers the period after deposition of the
Saksunarvatn ash at 9000 BP (Rundgren, 1998).
Since no distinct change in pollen assemblages was
detected at the 9000 BP level in the Lake Torfadals-
vatn sequence (Rundgren, 1998), the upper boundary
of RPAZ Skagi-5 is extended to 8800 BP (Fig. 5),
where a clear pollen-assemblage change was found
(Rundgren, 1998).
All five pollen records reflect a large expansion of
dwarf shrabs on northemmost Skagi around 9600 BP
(Rundgren et al., 1997), although Empetrum nigrum
started to expand earlier around Lake Geitakarlsvötn
(Rundgren, 1998). Salix was the most important dwarf
shrab after 9600 BP, and the macrofossil record from
Lake Geitakarlsvötn suggests that S. herbacea was the
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JÖKULL, No. 47, 1999