Jökull - 31.12.2001, Blaðsíða 24

Maciej Dqbski
Figure 5. Rhizocarpon geographicum agg. diameters measured on each of the moraine ridges and plotted against
age of the surface on which it grows. I, II, III, IV, V, VI - numbers of moraine ridges. Lichen growth rates
according to 1) Evans et al. (1999), 2) Snorrason (1984), 3) Gordon and Sharp (1983), and 4) Bradwell (2001).
- Þvermál flétta afgerðinni Rhizocarpon geographicum agg. á jökulgörðum af mismunandi aldri. Vaxtarhraði
fléttanna samkvœmt mœlingum 1) Evans og fl. (1999), 2) Sigfinns Snorrasonar (1984), 3) Gordon and Sharp
(1983), og 4) Bradwell (2001).
DISCUSSION
Figure 5 shows diameters of thalli collected on each
moraine ridge plotted against age of the moraines de-
rived from estimates of location of the glacier snout
and direct measurements. Differences between the
mean diameter of the 5 largest thalli and the single
largest thallus in each field are indicated. The new
lichenometric measurements seem to match curve 4
(Bradwell, 2001) quite well across younger moraines
whereas lichens on older moraines do not correlate
as well with curve 4. The most reliable dates (based
on former research) are those for ridges V and VI as
they are derived from direct glaciological measure-
ments (Figure 3). If the age of moraine ridges IV,
III and II was c. 10 years higher, the mean diame-
ter of the 5 largest lichens would match Bradwell’s
curve. This could be an argument for redefining the
age of the moraines and changing the glacier reces-
sion curve presented in Figure 3. However lichen di-
ameters collected on ridges IV and III seem to corre-
late with the curves of Snorrason (1984) and Gordon
and Sharp (1983). The curve constructed by Evans et
al. (1999) seems to be least proper for the Fláajökull
moraines.
Colonisation lag time can also cause discordance
in lichen growth estimates. Evans et al. (1999)
used a colonisation lag time of 6.5 years for all
glacier marginal zones south of Vatnajökull. Snorra-
son (1984) used a much longer lag period of 12 years.
Gordon and Sharp (1983) used an even longer lag
22 JÖKULL No. 51