Jökull - 31.12.2001, Blaðsíða 25

Dating ofthe Fláajökull moraine rídges
of 15 years for the Skálafellsjökull marginal zone,
due to melting out of buried ice. It is possible that
the time prior to colonisation should be even greater
for the Fláajökull moraines as they include much
buried ice. Comparing the lichen sizes on ridges II
and I to the curves of Snorrason (1984) and Gor-
don and Sharp (1983) it is possible to explain the ab-
normal small thalli with argumentation provided by
Maizels and Dugmore (1985). Having analysed sands
by Sólheimajökull (southern Iceland) they noted that:
1) The lichens may already be of a second genera-
tion - earlier colonisers having undergone natural de-
cay. 2) Lichens may develop on stones that emerged
to the surface some time after sedimentation due to
the removal of the surface layer of the material. 3)
The rate of growth may not have been stable due to
climatic fluctuations. 4) Lichen growth could have
been arrested by processes such as chemical and frost
disintegration, frost heaving or aeolian abrasion. 5)
Lack of large lichens could be the result of a lack of
clasts large enough to provide optimal conditions for
lichen development. The first argument put forward
by Maizels and Dugmore (1985) has been emphasized
by McCarthy (1999). He argues that until lichen pop-
ulation dynamics are fully understood, it is unrealis-
tic to expect accurate age predictions based simply on
normality assumption. Lichenometric dates obtained
by Evans et al. (1999) for moraines at Heinabergs-
jökull formed after 1946 did not match with other
data. This they explained by a lack of large boulders
on the surface. Snorrason (1984) used a similar ex-
planation for the lower growth rate of Rhizocarpon on
the fine-grained surface of Fláajökull moraines, how-
ever the actual lower rate is not given. Jaksch (1975)
also pointed out active solifluction and frost weath-
ering in the marginal zone of Fláajökull as factors
limiting lichen growth. In the author’s opinion, all
above stated reasons could play an important role in
distorting a linear outcome of lichenometrical mea-
surements.
CONCLUSIONS
The results of lichenometric dating of the surface ma-
terial of moraine ridges from the Fláajökull glacier
foreland should be considered with care. The data has
a considerable margin of error due to uncertain growth
rates and environmental conditions which may have
significantly distorted the assumed linear growth rate.
There is also the problem of assessing the colonisa-
tion lag time. Lichenometry thus does not provide un-
equivocal argumentation for changing the dates of the
Fláajökull moraine ridges derived from previous re-
search. However, lichenometrical investigation can
help in assessing the age of stabilization of stone-
filled furrows and other periglacial phenomena in the
area (D^bski and Gryglewicz, 1998). Only with other
data (cartographic, historical and glaciological) can
the moraines be dated properly.
Fláajökull retreated over 1500m between the end
of the \9th century and 2000. The retreat was punc-
tuated by short (usually annual) advances occurring
every 3 to 7 years except during three periods, 1903-
1925, 1948-1952, 1980-2000 when the glacier ad-
vanced or stood still. Moraine ridges I, II, III and
IV were formed during the 20í/l century. The largest
ridges V and VI must, however, have formed earlier
and only overridden with little remodelling during the
LIA. Snorrason (1984) states that the ridges V, VI and
even VII could have formed during late Pleistocene.
A clear correlation is observed between the over-
all rate of the retreat of Fláajökull and climatic fluc-
tuations over the 20th century. Periods of lower rnean
annual air temperature in Iceland were: 1901-1925,
1947-1952 and 1965-1990 (Einarsson, 1993). Dur-
ing each of these periods Fláajökull advanced, was
at a still-stand or the rate of its retreat slowed down
markedly.
Acknowledgements
I would like to thank Sigfinnur Snorrason for render-
ing accessible his Cand. Real. thesis, Professor Adam
Kotarba for consultation on lichenometry and Pro-
fessor Elzbieta Mycielska-Dowgiatto for useful com-
ments. This work is a part of a larger project con-
cerning periglacial phenomena. I wish to express my
gratitude to the Icelandic Institute of Natural History
for the permission to export glacial till samples.
JÖKULLNo. 51 23