Jökull - 01.01.2005, Page 62

Bergrún Arna Óladóttir et al.
Figure 4b. Simplified soil logs showing the thickest tephra layers, characteristic grain sizes and the connections
between subsections and outcrops. Grain size is indicated by tick marks (<0.2, 1, 2, 10, 20, >20 mm) at the
top of the logs. – b) Einfaldað snið sem sýnir þykkustu gjóskulögin, einkennandi kornastærð og tengingar milli
sniða. Kornastærð (<0.2, 1, 2, 10, 20, >20 mm) er sýnd með hökum efst á hverri sniðasúlu.
microprobe session and all results are relative to that
standard. At least five different glass fragments were
analysed and the mean value used to characterise each
tephra layer for further discussion (Óladóttir, 2004).
Dating of tephra layers using soil accumulation
rate
The age of the seven prehistoric marker layers in the
region has been determined by the 14C-method, and
age of the Settlement layer has been determined in
the Greenland ice cores (Table 1a). These tephra
layers cover the period from ∼870 AD to ∼6380
BC whereas the time between successive dated lay-
ers ranges from 210 to 3100 years (Table 3). The
mean soil accumulation rate (SAR) is obtained for
time periods between two successively dated tephra
layers. The procedure is straightforward, because the
SAR for the period between two dated layers is sim-
ply the total soil thickness divided by age difference
of the tephra layers. The approximate age of individ-
ual tephra layers can then be calculated by assuming
constant SAR between the dated layers.
RESULTS AND DISCUSSION
The SAR age model
The results of the SAR calculations are presented in
Table 3. On the whole, the SAR varies from 0.027
cm/year to 0.081 cm/year in the composite section,
with a mean value of 0.054 cm/year. In the AT out-
crop, Atley, the first 2700 years of prehistoric time
show a rather steady SAR value, ranging from 0.034–
0.047 cm/year. Then the rate jumps to 0.081 cm/year
before dropping to its lowest observed value, 0.027
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