Fróðskaparrit - 01.01.1998, Síða 316

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CLIMATE INDUCES TWENTIETH-CENTURY GLACIER FLUCTUATIONS
IN SOUTHEASTICELAND
moraine ridge maximum lichen diameter (mm) age (years) date (AD)
i 65 116 1880
2 43 78 1918
3 38 68 1928
4 36 62 1934
5 29 52 1944
6 24 42 1954
7 17 30 1966
Table 1: Lichenometric data collected from the
proglacial foreland at Virkisjdkull-Falljókull.
Talva 1: Likenometriskt tilfar savnað úr tí proglasiala
strandarlendinum við Virkisjokull-Falljokull.
ployed. Rhizocarpon geographicum agg.
species was chosen to calibrate the age of
the various substrates. The maximum di-
ameter of the twenty largest lichens on the
ice-proximal slope of each discrete
morainic feature was measured to an accu-
racy of +/- 0.5mm. These values were then
averaged, to eradicate possible sampling
errors, and applied to a linear growth curve.
Similar lichenometric studies carried out in
this area of Iceland (Gordon and Sharp,
1983; Thompson and Jones, 1986; Thomp-
son, 1988) have proved relatively success-
ful. This study aims to show the feasibility
of lichenometric dating in decoding a com-
plex glacial history over the last century.
Glaciers have been monitored in Iceland
since the first settlers arrived in the tenth
century. This record of glacier fluctuations
makes Iceland an ideal location to test the
validity of dating techniques. Observed
limits supported by lichen measurements
can be used to calibrate relative growth
curves for a specific environmental regime.
Assuming growth rates to be linear allows
ages of various substrates to be interpolat-
ed. Several studies have reported a linear
relationship between lichen size and age, at
least over the first two centuries of growth
(Anderson and Sollid, 1971; Burrows and
Orwin, 1971; Gordon and Sharp, 1983;
Kugelmann, 1991). This would correspond
with the ‘Great Period’ of growth observed
by Beschel (1950) and supported by later
workers (Benedict, 1967; Calkin and Ellis,
1980; Caseldine, 1991). However, some ar-
gument surrounds precisely the validity of
this relationship, particularly in the first 15
years following colonization (Beschel,
1961; Mottershead and White, 1972;
Jochimsen, 1973). Any discrepancies be-
tween the calculated and subsequently ob-
served landform ages would give an indica-
tion of the shortcomings of this particular
method.
Virkisjokull-Falljokull has been ob-
served only sporadically since 1880, with
annual observations beginning in 1932. Pri-
or to this time, documentation is rather
scarce and therefore less reliable. However,
a good impression can be gathered from the
data compiled and published by Thorarins-
son (1943), Eythorsson (1963, 1966), Rist
(1970-1987) and Sigurdsson (1989, 1994).
Further lines of evidence can be drawn
from neighbouring valley glaciers whose
chronologies have been documented and in
some instances extended by various dating
techniques (Thompson, 1988).