Jökull


Jökull - 01.12.2007, Page 34

Jökull - 01.12.2007, Page 34
Maciej Dąbski DISCUSSION Bradwell (2004) and McKinzey et al. (2004) obta- ined lichenometric frequency gradients for moraines in SE Iceland using all thalli (also the largest ones) within each population. However, for one moraine at Hólárjökull, Bradwell (2004) obtained inaccurate age estimations due to the single largest lichen which sho- uld be, in his opinion, omitted. If the largest thalli at Fláajökull are included in the analysis, the obta- ined ages are obviously incorrect, since moraine III cannot be older then moraine II (Figures 3). Rela- tively good correlation between previously obtained dates (Dąbski, 2002) for Fláajökull moraine ridges I– IV and the lichenometric dates obtained after omitting the largest thalli shows the advantage of the described modification of input data into the lichenometric size- frequency method. A slight but steady decrease in the value of R2 in the direction of the oldest moraine is lo- gical, and can be explained by the growing age of the lichen populations. Bradwell (2004) andMcKinzey et al. (2004) found a similar regularity in R2 value chan- ges while surveying moraines of Fjallsjökull, Hólár- jökull, Skálafellsjökull and Heinabergsjökull, neigh- bours of Fláajökull (Figure 1). It must be emphasi- sed that if the largest thalli growing on Fláajökull mo- raines were included into the lichenometrical dating, there would not be a gradual and logical change in the frequency gradient and R2 value. The method used in this research is sound and based on the assumption that the population frequency gradient should be cal- culated only by omitting exceptionally large thalli that constitute the right outliers of the frequency distribu- tion graphs (not only the single largest thallus). Such a procedure produces the most reasonable dates for Fláajökull moraine ridges (Table 2). The lichen size-frequency gradient method used on moraine ridge V yields a date (AD 1944), which is 9-10 years younger than the date derived in pre- vious research (Dąbski, 2002), Table 2. This discre- pancy cannot be attributed to the described reduc- tion method since there are no exceptionally large li- chens in this population and the method was not im- plied on this ridge (Figure 3). Superficial material on ridge V was deposited in 1934/35 according to Dąbski (2002). This date is based on direct glacio- logical measurements (Sigurdsson, 2000) performed along the Hólmsárgarður profile situated in the south- western part of the Fláajökull moraines (Figure 1). Therefore, the date refers precisely to the run of the profile. The measurements revealed a transgression in 1941 of 59 m along the profile followed by further regression. Taking under consideration that 1) the te- sting field for lichenometrical measurements covered 300 m of the ridge length, 2) the fact that Fláajökull glacier front has many small-scale lobes, and 3) the mentioned short-term transgression, the several-year difference between glaciological measurement along the profile and the lichenometrical date for the whole testing field can be accepted. Dates derived with the use of Bradwell’s (2001) dating curve based on a single largest lichen (without the single exceptionally large thallus) suggest that the Fláajökull moraine ridges I, II, III and IV were pro- duced in the 19th century (Table 2) which is, in light of other data (Dąbski, 2002), impossible. Extrapola- tion of the frequency gradient trend line allowed to obtain the “1-in-1000” lichen thallus for each popu- lation, Table 2 (Locke et al. 1979). Dates calculated with the use of such thalli and the Bradwell’s (2001) dating curve show that the oldest Fláajökull end mo- raine was abandoned in the beginning of the 19th cen- tury, which would support the findings of McKinzey et al. (2004). Nevertheless, these dates cannot be ac- cepted because they disagree with the direct glacio- logical measurements, which is the case for moraine ridge V (Figure 1, Table 2). Discrepancies between Evans et al. (1999) ap- proach, based on the average of five largest thalli, the dates obtained in this study and the former da- ting (Dąbski, 2002), Table 2, result from the diffe- rent lichenometrical method implied. Possible insta- bility of the moraine surface due to differential mel- ting of buried glacial ice (Everest and Bradwell, 2003) can add to the differences obtained in dating, altho- ugh the buried ice was found only within the youn- gest moraine ridge. Other obstacles for use of liche- nometry in dating moraine ridges of Fláajökull can stem from uncertainty of colonisation lag time, un- 32 JÖKULL No. 57
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