Jökull - 01.12.1990, Blaðsíða 79

tundra, as indicated by the mean pollen influx ranging
between 74 and 106 pollen per cm2 and 14C-year (c/.
Birks and Birks, 1980).
At the beginning of this zone Eu-Potamogeton has
a hreakthrough, flourishing intensively. Pediastrum
kawraiskyi is new amongst the green algae. Otherwise
the lake vegetation is unchanged.
By means of interpolation and extrapolation of the
sedimentation rate, based on the two radiocarbon dates
available, the duration of this subzone is estimated to
be about one millennium, spanning the main part of
the Late Preboreal and the Early Boreal Chronozones.
KHM2b, Betula-Gramineae-Thalictrumalpinum-
Polypodiaceae undiff. Local Pollen Assemblage Sub-
zone. Estimated age ca 8400-7850 B.P. In this sub-
zone both the shrubs of Salix and Juniperus seem to
recede for a whileto the spreading birch copses, which
n°w form a shelter for Dryopteris linneana (Gymno-
carpium dryopteris) and other Polypodiaceae species.
Herbs and Graminids are still frequent, indicating an
°pen and mosaic character of the vegetation with peat-
lands, grass heaths and birch copses.
The lake vegetation seems to be suffering, as the
Pollen and spore production is minimal, but the green
algae show small changes. Isoetes echinospora reap-
pears.
The sedimentation rate places this subzone roughly
in the Late Boreal Chronozone.
KHM 2c, Juniperus-Cyperaceae-Rumex type Lo-
cal Pollen Assemblage Subzone. Estimated age ca
7850-7250 B.P. The shrub vegetation, particularly/M-
mperus, recovers some of its former place. Juniperus
ls generally assumed to be a poor ecological indica-
tor, having a wide ecological amplitude. However the
best climatic conditions for juniper are offered by hu-
mid, maritime climate (c/. discussion in Vasari and
^asari, 1968). According to the increasing values of
Cyperaceae the peatland seems to expand somewhat,
which suggests more oceanic climate, wetter and or
cooler with less evaporation. Maybe the relatively
rich soils of the eutrophic stage of the peat formation
favoured this vigorous growth of Juniperus. Later
°n as the peatland became more acid and reached the
mesotrophic stage, Betula was able to displace/;m/pe-
rus, at least on the more drier parts.
In the lake Isoetes echinospora flourishes and both
Myriophyllum alterniflorum and Eu-Potamogeton are
gaining. As Isoetes echinospora prefers a sandy bot-
tom, this might indicate some submergence of the for-
mer littoral zone of the lake. In the sediment core there
is no evidence of increased input of minerogenic ma-
terial at this level and therefore soil erosion can hardly
be the reason for this Isoetes expansion (c/. Vuorela,
1980), yet further analyses on the sediment are needed
to preclude this.
The sedimentation rate (Fig. 3) places this subzone
within the Early Atlantic Chronozone.
KHM 3, Betula undiff. Local Pollen Assemblage
Zone. Estimated age ca 7250-6900 B.P. In this zone
birch scrubs/trees seem to become the dominant com-
ponent of the tall and wooded elements of the vege-
tation, at least Betula pubescens type pollen has re-
placed both the shrub and the dwarf shrub pollen at
Krosshólsmýri. The AP/NAP ratio exceeds 1 for the
first time, which according to investigations on recent
material in Scotland (Birks, 1973) may indicate birch
woodland. By contradiction the pollen influx values
of ca 400 pollen grains per cm2 and 14C-year in this
zone indicate that tundra existed in Flateyjardalur at
this time (c/. table I in Hicks, 1986). At this point
it may be useful to stress that the pollen influx val-
ues for Krosshólsmýri are based on extrapolations of
the sedimentation rate, which may be unreliable. Sec-
ondly the pollen influx values used for comparison are
derived from continental sites, i.e. from Canada and
northem Scandinavia (Birks and Birks, 1980; Hicks,
1986), with widespread backland of boreal forests.
Therefore these values might not be suitable for an
ocean island far from any great pollen producers, as
the conifer forests. Also evidence from Greenland
(Fredskild, 1973; 1983) shows a marked difference
between the coast and inland regarding pollen influx.
At this time, i.e. about 7000 B.P., the seashore was
near Krosshólsmýri, as it still is today (Fig. 2). Coastal
influence could explain the low pollen influx values
for a birch woodland, suggested by the percentage
values of Betula pollen grains. Widespread bryophyte
and or lichen communities, prominent in the Icelandic
landscape today, would also lead to low pollen influx
values.
JÖKULL, No. 40, 1990 75