Jökull - 01.12.2007, Page 35

Testing the size-frequency-based lichenometric dating
Figure 10. Frequency gradients of lichens’ populations on proximal, distal and upper sides of stones. – Hlutföll
fléttna eftir vaxtarstað.
certain growth rates, changeable environmental con-
ditions and periglacial processes (Dąbski and Grygle-
wicz, 1998; Dąbski, 2002, 2005).
The late 19th century maximum of the Little Ice
Age for glaciers flowing south of Vatnajökull is ad-
vocated by Ahlmann and Thorarinsson (1937), Tho-
rarinsson (1943), Guðmundsson (1997) and Evans et
al. (1999). Older ages for terminal moraines in S
Iceland are postulated by Kirkbridge and Dugmore
(2001), Bradwell (2001, 2004) and McKinzey et al.
(2004). However, the latter write: “... Skálafells-
jökull’s LIA maximum occurred by the early 19th
century, while Heinabergsjökull advanced to its LIA
maximum extent slightly later.” Fláajökull is the next
one to the east of Heinabergsjökull and could have
advanced later than its western neighbour. This can
be explained by a possible shift in the north-east di-
rection of the maximum snow accumulation on the
Vatnajökull firn field.
The small number of lichen found on the proximal
sides of stones (Figures 5–9) can be attributed to mi-
croclimatic conditions (NNW exposure, shaded area),
which may impede lichen growth. According to Pen-
tecost (1979), shaded sides of boulders in NorthWales
are also relatively free of Rhizocarpon thalli. Howe-
ver, on young moraines, the proximal populations of
lichens have the smallest frequency gradient (Figure
10), with the least R2 value, which testifies to an older
age of the populations compared with distal or upper
ones. It is therefore inferred that pioneer lichens co-
lonise shaded and damp areas, but later they are out-
numbered by lichens growing on the sunny sides of
stones. This is supported by Armstrong (2002), who
proved that young thalli of Rhizocarpon geographi-
cum grow faster on north-west rock surfaces compa-
red with south-east facing surfaces, but also there is a
higher mortality rate on northern surfaces. With an in-
crease of substratum age, the differences in lichen po-
pulations on Fláajökull moraines decrease. Explana-
tion of the reversal of the frequency gradient tendency
on the oldest moraine and probable “rejuvenation” of
the proximal population (Figure 10) is difficult, and
calls for further research.
CONCLUSIONS
Bradwell’s (2004) lichenometric dating curve for SE
Iceland based on the gradient of size-frequency distri-
bution has been tested on previously dated moraines
of Fláajökull (southeast outlet of Vatnajökull). The
following conclusions emerge from this study:
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