Jökull - 01.12.2007, Blaðsíða 25

Testing the size-frequency-based lichenometric dating
The aim of this paper is to present the size-
frequency approach on new lichenometric measure-
ments taken on the Fláajökull moraines in 2005 and
to show diversification in the lichen population growth
on proximal, distal and upper surfaces of stones due
to environmental factors.
STUDY AREA
Fláajökull flows south out of the Vatnajökull ice-cap
and its marginal zone is at about 50–80 m a.s.l. (Fi-
gure 1). The eastern lobe of the glacier has formed
easily discernible moraine ridges, which to the west
are bordered by the moraines of Heinabergsjökull and
Skálafellsjökull. Together with Fláajökull, the gla-
ciers formed a single piedmont lobe between the years
1860–1870 (Evans et al., 1999). Figure 1 shows the
situations after the opening of a new subglacial tunnel
in the spring of 2001, the erection of a new dam and
the inundation of large parts of the marginal zone.
METHODOLOGY
Lichenometric measurements were performed in the
summer 2005 following certain criteria:
1. only the longest axis (enveloping circle) of rela-
tively circular isolated Rhizocarpon agg. thalli
were measured;
2. measurements were made with a plastic ruler to
the nearest millimetre;
3. testing fields were delimited along 5 moraine
ridges of different ages (the oldest is number I),
each having an area of 6000 m2 (20x300 m);
4. only the relatively largest thalli were measured
and the aspect of the overgrown side of each
stone (proximal, distal or upper) was noted, see
Figure 2;
5. measurements were performed on all types of
rocks, mostly basalts, dolerites and diabases.
Between 497 and 942 thalli were measured on
each testing field, with the total amounting to
3877.
Figure 2. Sides of stones on which lichens were me-
asured. Skilgreining á mælistöðum fléttna með tilliti
til afstöðu þeirra til jökuljaðarsins.
The procedure of data processing was the same as
used by Bradwell (2004) and McKinzey et al. (2004).
Lichens were divided into classes with an interval of 3
mm, and size frequency graphs were constructed. The
logarithm (base 10) of the frequency (%), excluding
lichens smaller then the modal class, was calculated
and multiplied by -1. The slope of the regression li-
nes (the frequency gradient) was calculated using the
least squares method. According to this method, the
steeper the slope of the regression line, the younger is
the lichen population. Such a procedure was used for
the entire population in each testing field and also se-
parately for populations growing on proximal, distal
and upper stone surfaces.
The lichenometric dates obtained were verified by
comparing them with the age of the moraines previo-
usly derived by Dąbski (2002), Figure 1, who ana-
lysed several sources of information, most important
of which were: direct glaciological measurements
(published annually by Icelandic Glaciological So-
ciety in Jökull), a topographic map 1:50 000 based
on aerial photographs from 1946, aerial photography
from 1989, historical data published by Ahlmann and
Thorarinsson (1937), Thorarinsson (1943), Denton
(1975) and studies of neighbouring glaciers’ margi-
nal zones (Bradwell, 2001; Jaksch, 1975; Gordon and
Sharp, 1983; Kirkbridge and Dugmore, 2001; Snor-
rason, 1984; Maizels and Dugmore, 1985; Thompson
and Jones, 1986; Thomson, 1988; Evans et al., 1999).
JÖKULL No. 57 23