Jökull - 01.11.1998, Blaðsíða 26

is the shear stress, p is density, g is acceleration due to
gravity and h, L and x are in metres. This equation is
based on a flat glacier bed where the ice profile is as-
sumed to be a parabola. For x0 = 0.1 M Pa, p = 920
kg/m3 and g = 9.82 m/s2 we can write (Paterson, 1981)
h = 4.7 V(L - x)
Sigmundsson (1990) has calculated a new yield
stress for the Vatnajökull ice cap by using 910 m as the
maximum thickness of the ice cap. Sigmundsson's re-
sult gives a slightly lower value of 0.072 M Pa com-
pared with 0.1 M PA from Paterson (1981). Thus, Sig-
mundsson (1990) has modified the latter equation (Pa-
terson, 1981) for the Vatnajökull ice cap hence writing
h = 4.0 V(L - x)
where h, L and x are in metres. This formula would
give a maximum ice thickness of about 220 m in the
case of the Eiríksjökull ice cap by using L=3200 m
(Icelandic Geodetic Survey, 1988).
CLIMATIC IMPLICATIONS
The snowline at the end of the summer roughly
corresponds to the equilibrium line on glaciers in tem-
perate regions that are in equilibrium with the climate
(Paterson, 1981). The Accumulation/Ablation Ratio
(AAR) for glaciers in southern Iceland is typically
about 70/30, respectively (Björnsson, 1979). Given
the same AAR for the Eiríksjökull ice cap, the eleva-
tion of the Equilibrium Line Altitude (ELA) would be
about 1450 m a.s.l. at present. This is in good agree-
ment with observation on recent aerial photographs.
If Eiríksjökull was about 49 km2 during the LIA max-
imum, a calculated ELA value on the western side
might have been about 1200 m, or ca. 250 m lower
than at present. This would suggest a 1.5°C drop in
the mean annual temperature given the lapse rate of
0.6°C/100 m in moist air (Einarsson, 1975) and uni-
form precipitation. This calculated assessment of the
temperature is in broad agreement with temperature
estimation from sea ice data from Bergthórsson
(1969) during the LIA in Iceland.
DISCUSSION
Most of the glaciers in Iceland retreated from their
maximum LIA positions until approximately 1960,
after which they either readvanced, or were at a still-
stand to present time (Björnsson, 1979; Sigurðsson,
1993). This oscillation pattern is broadly consistent
with the LIA glacier fluctuation of the Eiríksjökull ice
cap. In most glaciers a small readvance occurred be-
tween 1967 and 1969 when the mean temperature
dropped considerably in Iceland. The late 1960s ad-
vance could not be detected in the study area due to the
lack of aerial photographs from that time. The results
of this study indicates that the LIA advance of the
Eiríksjökull ice cap occurred in the 1880s, which is
very close in time to similar advances around Iceland
(e.g. Thórarinsson, 1943; Jaksch, 1970, 1975; Gordon
and Sharp, 1983; Snorrason, 1984; Caseldine, 1983,
1985, 1987; Maizels and Dugmore, 1985; Thompson
and Jones, 1986; Kugelman, 1990; Guðmundsson,
1997). The dating evidence of the LIA advances in Ice-
land comes from lichenometry, a relative method based
on exposure dates of the substrate. This is an important
caveat because different growth rates occur between
species depending on the environment they live in. In
Iceland, environmental factors seem to inhibit lichen
growth (Maizels and Dugmore, 1985; Guðmundsson,
1992; 1997) which suggest that lichens are only useful
for approximately 150-200 years old substrates in Ice-
land. Any earlier LIA glacier advances are thus not eas-
ily distinguished with the lichen dating method. This
could imply that the dates obtained by lichenometry in
Iceland are more representative of the limitations of the
dating method itself rather than the actual age of the
landform. Therefore, lichenometry should be used with
other dating methods including tephrochronology or
historical annals to obtain a better and more accurate
record of glacier fluctuations in Iceland.
Only one of the outlet glaciers studied, namely
Klofajökull, reveals an older advance than the LIA
maximum in the late 19th century. Other outlet
glaciers showed a broadly similar record although
their recession timing varied slightly. The Klofajökull
deposit is morphologically different from the other
glacial landforms and can be interpreted as a rock
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JOKULL, No. 46, 1998