Jökull - 01.12.1999, Page 9

were described immediately after retrieval before the
sediments were wrapped in plastic film, put in plastic
tubes and transported to the laboratory in Lund.
Levelling of lake thresholds and beach ridges was
carried out using an instrumental level. The precision
in these measurements is estimated to ±0.2 m.
Laboratory methods
The collected sediment sequences were described
once again in the laboratory and then subsampled to
provide material for a range of analytical methods.
Mineral magnetic analysis
Saturation isothermal remanent magnetism
(SIRM) was measured on all sediment sequences in
order to produce continuous records that could sup-
port the grain-size determinations made by visual in-
spection of the cores. SIRM was also used to locate
minor tephra horizons. The measurements were per-
formed using a "Minispin" magnetometer on subsam-
ples previously magnetized in a field of 1 T.
Elemental carbon analysis
All five sequences were subjected to elemental
carbon analysis. Visible tephras were avoided during
subsampling in order to obtain representative records
of lake productivity. Combustion was effected in a
Leco RC-412 Multiphase Carbon Determinator from
100° to 950°C with a ramp rate of 50°Cmin ‘, en-
abling separation of carbonate from organic carbon.
Pollen analysis
All sequences were analysed for pollen, spores and
coenobia of the green algal genus Pediastrum. Pollen
analysis was the main method for reconstruction of
terrestrial and limnic vegetation, and it was also used
for correlation between sequences. The concentration
of Pediastrum coenobia was used as a measure of lake
productivity, independent of organic carbon content.
Preparations followed standard methods (Berglund
and Ralska-Jasiewiczowa, 1986), and Lycopodium-
tablets were added in order to estimate pollen concen-
tration and influx values (Stockmarr, 1971). Pollen
nomenclature follows Moore et al. (1991), with a few
exceptions (see Rundgren, 1998).
Plant macrofossil analysis
The sediment sequences from Lake Torfadalsvatn
and Lake Geitakarlsvötn were subjected to qualitative
plant macrofossil analysis. This method was used to
supplement pollen analysis in the reconstruction of
terrestrial vegetation. Samples were washed through
1.0 and 0.2 mm sieves, and all vascular plant remains
were picked out for identification.
Diatom analysis
All lake sequences were analysed for diatoms.
This was the main method used to reconstruct trans-
gressions and regressions of lake thresholds. Subsam-
ples were treated with 10% HCl, washed with distilled
water, and treated with 30% H202 (1-2 hours in a water
bath at 60°C). Taxonomy and ecological information
followed Hendey (1964), Hustedt (1930-1959), Kiam-
mer and Lange-Bertalot (1986-1991) and Patrick and
Reimer (1966-1975), and species were grouped with
regard to their salinity preferences according to the
halobian system of Hustedt (1957), originally pro-
posed by Kolbe (1927): Marine species = poly-
halobous (>30%o), brackish species = mesohalobous
(30-0.2%c), fresh-water species = oligohalobous
(halophile, indifferent) and halophobous (<0.2%o).
Radiocarbon dating
14C dating was the main method used to obtain ab-
solute chronologies for the sampled sequences. Björck
et al. (1992) published four radiocarbon dates from
another Lake Torfadalsvatn core (Al), and these were
lithostratigraphically correlated to the core (Bl) used
in this study. The dates of Björck etal. (1992) were ail
accelerator mass spectrometry (AMS) measurements,
three on bulk sediment samples and one on a moss
sample. In addition to these dates, one new bulk sedi-
ment sample was dated by the conventional method in
Lake Torfadalsvatn (core Bl). Three bulk sediment
samples were dated in the Lake Geitakarlsvötn se-
quence, two by the conventional method and one by
AMS. A moss sample from the same lake was also
dated by AMS. Plant macrofossils of definite terrestri-
al origin were too few at the sampled levels to allow
for AMS dating.
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