Jökull


Jökull - 01.12.2007, Page 26

Jökull - 01.12.2007, Page 26
Maciej Dąbski RESULTS Lichen populations (Rhizocarpon agg.) growing on moraines I, II, III and IV are composite and are cha- racterised by the presence of some exceptionally large thalli which make the frequency gradient as well as the correlation (R2) quite low (Figure 3, solid lines, Table 1 A). Exceptionally large thallus on moraine III with a 150 mm diameter, suggests that this sur- face is older than the surface on moraine II, which is obviously impossible (Dąbski, 2002). Also, the sin- gle largest thallus (almost two times larger then the second largest thallus) on moraine I suggests that it began growing prior to the moraine ridge formation. The extra large thallus found on ridge I may be inhe- rited from a supraglacial period of growth, although Fláajökull does not have a distinct medial moraine, which would allow for relevant substrate stabilisation (Griffey, 1978). This lichen may have been incorpora- ted into the end moraine from an older deposit, distal to the moraine. The extra large size of a single thallus found on ridge III is difficult to explain. Table 1. Regression lines parameters: A) whole li- chen populations, B) populations without exceptio- nally large lichens. – Stærðarhlutföll mældra fléttna á jökulgörðum I–V. A moraine grad. (-1) R2 n I 0.0219 0.5264 497 II 0.0502 0.9400 942 III 0.0195 0.4909 702 IV 0.0618 0.8918 846 V 0.0950 0.9856 890 B moraine grad. (-1) R2 n I 0.0505 0.9030 496 II 0.0554 0.9441 940 III 0.0748 0.9487 701 IV 0.0871 0.9612 842 V 0.0950 0.9856 890 It was concluded that the largest lichens should be omitted in the following way: if there was a gap in the frequency distribution of at least 2 size-classes (6 mm), all thalli falling to the right of such gaps were excluded. After making this correction, subsequent regression lines were constructed (Figure 3, dashed li- nes). Lichen population frequency gradients calcula- ted for the moraines of Fláajökull show a logical sequ- ence (Figure 3, Table 1 B) only after implementation of the above described correction. It is worth empha- sising that the correlation parameter (R2) also gradu- ally increases from the oldest to the youngest moraine (Table 1B). The lichen population measured on mora- ine ridge V appears to be single, does not exhibit any extraordinary large thalli, and therefore, it is not ne- cessary to implement the above described correction. The frequency gradients (after corrections) were plotted on the Bradwell’s (2004) dating curve for SE Iceland and the age of moraine surfaces was obtained (Figure 4). The new dates match quite closely the da- tes previously derived by Dąbski (2002), see Figure 1, for older moraines, but there are some discrepan- cies concerning younger ridges (Table 2). Results of the frequency gradient analysis for li- chen populations with a distinction between proximal, distal and upper surfaces of stones are shown on Figu- res 5–9. The following patterns are seen: 1. Lichens growing on the proximal (northern) si- des of stones constitute the smallest measured population on all moraines. However, their share of the total measured population of li- chens seems to gradually increase from 16– 19% to 26% as moraine age increases. Lichen populations on the upper surfaces of stones are usually the largest. 2. With only one exception from the oldest mora- ine ridge, the largest thalli grow on the upper or distal (southern) stone surfaces. 3. Proximal populations exhibit the smallest fre- quency gradient on young moraines and the hi- ghest on old ones, but there is a gradual dif- ferentiation between the moraines (Figure 10), which suggest that it is time dependent. 4. Proximal populations on each moraine are cha- racterised by the least value of R2. 24 JÖKULL No. 57
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