Jökull - 01.01.2010, Blaðsíða 188

M. Da̧bski and P. Angiel
METHODS
Geological and geomorphological mapping was per-
formed with a GPS Garmin 60CSX, a topographic
map from AD 1937–1938 with 20 m contour lines
(Landmælingar Íslands) and an aerial photograph
from AD 1945. Figures 4 and 5 were drawn partly
from satellite images from Spot-5 in scale 1:30 000
taken in AD 2004. Information on glacier variation
published in Jökull issues: 1 (1951), 25 (1975), 28
(1978), 40 (1990) and 56 (2006) provided additional
information about the general timing and rate of the
Skálabjörg deglaciation. The last two issues include
useful photographs documenting glaciation levels in
AD 1951, 1994 and 2006.
100
80
60
40
20
0
4 3 2 1 0 -1
%
phi
AB
non frost-susceptible
Figure 3. Granulometric cumulative curves of the
black uppermost sediment, presumably volcanic ash
(A) and regolith (B) plotted against the frost suscepti-
bility limit of Beskow (1935). – Hlutfallsleg korna-
stærð rofefna á yfirborði með tilliti til frostveðrunar.
Lichenometry was applied in order to estimate the
timing of the highest lateral moraine ridge depositi-
on. Largest diameters (enveloping circles) of 417
relatively circular thalli of Subgenus Rhizocarpon
were measured with use of a plastic ruler (Da̧bski,
2007). The analysed lichen populations were samp-
led on proximal, upper and distal surfaces of the
moraine ridges shown on Figure 5. The method of
data processing followed that of Evans et al. (1999),
namely: the mean of 5 largest thalli per surface
was considered. Maximum and minimum ages were
calculated using a constant growth rates of 0.5 mm/yr
with colonisation lag time of 16 years (maximum age)
and 0.8 mm/yr with colonisation lag time of 6.5 years
(minimum age) (Table 1).
1 km
mountain ridges
nunatak area in 1945
rocks exposed between 1945 and 2004
glacier surface
ice-dammed lake developed between 1994 and 2004
terminal moraine ridge of Fossadalur glacier in 1945
N
Figure 4. Nunatak area exposure since 1945 and an
ice-dammed lake in Fossadalur. – Kort af jökulhörfun
í Vesturbjörgum og Skálabjörgum, 1945–2004, ásamt
lóni og jökulgarði við Fossadal.
No growth curve of Rhizocarpon thalli exists for
Esjufjöll. The lower rate (0.5 mm/yr and colonisati-
on lag time 16 years) was used by Evans et al. (1999)
for the marginal zone of Brúarjökull (northern outlet
glacier of Vatnajökull), where the mean annual air
temperature is from 0 to -1◦C (similar to that of Esju-
fjöll), but precipitation is about 10 times lower, which
must slow down the Rhizocarpon growth rate. On the
other hand 0.8 mm/yr growth rate (and colonisation
lag time 6.5 years) was used by Evans et al. (1999) for
humid southernmost marginal zones of Vatnajökull,
where mean annual air temperature is 2–3◦C higher
than at Esjufjöll and climatic conditions for lichen
growth thus better. Therefore, the lichen growth rate
at the nunatak should be between 0.5 and 0.8 mm/yr
and colonisation lag time between 16 and 6.5 years,
respectively.
The growth rate of Rhizocarpon is actually not
linear (Bradwell and Armstrong, 2007), and a size-
frequency approach could probably provide better
results (Bradwell, 2004), but linear growth rate seems
188 JÖKULL No. 60