Jökull - 01.12.1990, Blaðsíða 73

Table IV. Radiocarbon dates from Flateyjardalur —
Geislakolsaldursgreiningarfrá Flateyjardal.
L*- Depth, 14C-age 513C Pretreatm.
no- cm BP. (xO.1%)
Lu-
>996 522,5-527,5 8740± 95 -18.8 HCl (mildly)
Lu-
1433 555-560 9650 ± 120 -17.3 none
Figure 3. Plot of sediment accumulation rate. Depth
°f sediment used for each radiocarbon sample and one
standard deviation shown by vertical and horizontal
lines respectively. Fully drawn line is based on
mterpolation; stippled segment on extrapolation.
Tephra thickness is excluded in the sediment column,
us it is considered an instantaneous thickening event.
' Grafið sýnir setþykknun í fyrrum stöðuvatni,
Par sem Krosshólsmýri er nú. Grafið er byggt á
Seislakolsaldursgreiningum. Þykkt hvers sýnis er sýnd
með lóðréttum línum, láréttar línur eru staðalfrávik
Seislakolsmœlinganna. Reiknaður aldur á atburðum
1 gróðurfarssögunni er byggður á þessu grafi.
cm/l4C-year, before and after Lu-1996, gives 6900
B.P. for the youngest pollen sample analysed. Ac-
cording to the two dates the deposition time is 28,5 (1
a range 22-35) 14C-years per cm, i.e. the deposition
rate is 3,5 (1 a range 2,9—4,5> cm per 100 14C-years,
which is similar to that in the 14C-dated lake sediment
in the adjoining Skagafjörður district (the author’s un-
published data from lake Vatnskotsvatn, for location
see Fig. 6). In Fig. 3 a sediment accumulation rate
curve (time/depth curve) is given for Krosshólsmýri,
based on the two radiocarbon dates available.
POLLEN AND SPORE
STRATIGRAPHY
Twenty three pollen spectra represent the pollen
diagram from Krosshólsmýri in Fig. 4. This com-
prises the lowest part of the Krosshólsmýri core, or
the sequence between 450 cm and 560 cm below mire
surface. For ease of description, discussion and com-
parison of the diagram, the diagram has been divided
into Local Pollen Assemblage Zones (LPAZ), based
upon the fossil pollen and spore content. These are
given an abbreviation of three letters in the site name,
Krosshólsmýri — KHM —, followed by an Arabic
numeral, numbered from the oldest to the youngest.
KHM 1 (547,5-560 cm). Pollen samples number
1-4. This zone is characterized by high values of Salix
pollen, attaining 47 % in sample 1. The Cruciferae and
Ranunculus pollen are high too, reaching 20 % and
14 % respectively at their maxima within the zone.
Empetrum cf. E. nigrum pollen reach their highest
values in this zone, as do the Ericaceae undiff. pollen.
Cyperaceae pollen values rise throughout the zone to
about 24 %. The spores of Lycopodium annotinum, L.
alpinum and L. selago are all fairly well represented in
the zone. Betula pollen values are low, just reaching
4 %.
The aquatic vegetation is sparsely represented by
pollen of Myriophyllum alterniflorum, Hippuris vul-
garis and the group Eu-Potamogeton type. A single
spore of Isoetes echinospora appeared in only one
sample.
The zone is named the Salix - Ericales - Cruciferae
- Ranunculus - Lycopodium Local Pollen Assemblage
Zone.
KHM 2 (469,5-547,5 cm). Pollen samples number
5-20. This zone is characterized by an continuous
Juniperus curve (6,6-26,4 %), showing two maxima
in the subzones KHM 2a and KHM 2c. Betula pollen
JÖKULL, No. 40, 1990 71