Jökull - 01.12.1990, Blaðsíða 72

shown. At that time little attention was given to the
value ofól3C. Because of this most datings performed
in the fifties and sixties are strictly not comparable to
later ones. According to Olsson (1986) an age cor-
rection is to be added to a result not normalized for
the <513C deviation from —25 per mil; for a single peat
sample this can vary from —145 to 95 years (op.cit.
Table 14.1) The dates assumed to lack the 513C adjust-
ment are indicated by an asterisk (*).
BIOMETRY OF BETULA POLLEN
All Betula pollen were measured with the aid of a
micrometer having one ocular line of 2,1 /im. A size
frequency diagram was constructed for each LPAZ,
except for the uppermost one, where the same was
done for each sample. The resulting data may later be
subjected to further statistical analysis, as e.g. done
by Prentice (1981). This must however wait, as the
necessary reference data on the size distribution of
recent Betula pollen in Iceland is lacking.
SEDIMENT STRATIGRAPHY AND
RADIOCARBON ANALYSIS
The core from Krosshólsmýri taken for radiocar-
bon dating and pollen analysis is 560 cm long, repre-
senting the last 9650 14C years. The lowest 110 cm
is lake sediment, which could be separated into five
strata (Table II). In this lowest part of the core a total
of four tephra layers were visible (Table III). This part
of the core was represented by three core segments,
560-510, 550-500 and 500-450 cm below the mire
surface.
This lowest part of the Krosshólsmýri core goes
too far back in time (c/. radiocarbon dates below)
for making use of the tephrochronology (Einarsson,
1986), as the tephrochronology available for North
Iceland does not go further back than the tephra
layer Hekla-5 (H5) with an age of about 6100 B.P.
(Vilmundardóttir and Kaldal, 1982; c/. also the older
datings of H5 in Þórarinsson 1964, made before 1960).
Therefore the only use of the Krosshólsmýri tephra
stratigraphy (Table III) is for correlating purposes for
the time being.
Two radiocarbon dates have been obtained from
Table II. Lithostratigraphy for the sediment at
Krosshólsmýri — Jarðlagaskipan neðst í Krosshóls-
mýri
Depth below surface, cm Number of layer Description of sediments
(450)-485 5 Detritus, greyish yellowbrown. Compo- sition: Ld23, Dgl, Ag -}- . Lower bound- ary diffuse.
485-524 4 Detritus, greyish yellow, slightly firmer than layer 3. Composition: Ld23,Dgl,Ag + . Lowerboundarysharp.
524-531 3 Detritus, greyish brown, lighter than layer 2, rich in diatoms. Composition: Ld22, Lsol, Dgl, Ag + . Lower bound- ary sharp.
531-552 2 Detritus, greyish brown with bands of slightly different colour, rich in diatoms. Composition: Lso2,Ld21 ,Dg 1,Ag + Lower boundary sharp.
552-560 1 Detritus, greyish white, rich in diatoms. Showing fine lamellae. Composition: Lso3, Agl,Ld°+, Dg + . Base notseen.
Table III. Tephra stratigraphy at Krosshólsmýri. —
Gjóskulög neðst í Krosshólsmýrarkjarnanum.
Depth Thickn. Description Abb. Est. age
below surface, in mm of tephra years BP
cm
497,8- 499,7 19 Black fine-sandy tephra S3 8060
509,5- 510,5 10 Black tephra S2 8350
548 Light coloured, discontinuous tephra horizon L1 9400
553,4- 554 6 Black fine-sandy tephra S1 9550
the core (Table IV). The date Lu-1433 is the oldest
hitherto obtained for an organic sediment of Holocene
age in Iceland. It postdates the last glacier advance
in central North Iceland, and may be considered as a
minimum age for the glacier retreat in the Preboreal
Chronozone (Norðdahl, 1979; 1991).
Assuming the same sedimentation rate of 0,035
70 JÖKULL,No. 40, 1990