Jökull


Jökull - 01.01.2010, Page 188

Jökull - 01.01.2010, Page 188
M. Da̧bski and P. Angiel METHODS Geological and geomorphological mapping was per- formed with a GPS Garmin 60CSX, a topographic map from AD 1937–1938 with 20 m contour lines (Landmælingar Íslands) and an aerial photograph from AD 1945. Figures 4 and 5 were drawn partly from satellite images from Spot-5 in scale 1:30 000 taken in AD 2004. Information on glacier variation published in Jökull issues: 1 (1951), 25 (1975), 28 (1978), 40 (1990) and 56 (2006) provided additional information about the general timing and rate of the Skálabjörg deglaciation. The last two issues include useful photographs documenting glaciation levels in AD 1951, 1994 and 2006. 100 80 60 40 20 0 4 3 2 1 0 -1 % phi AB non frost-susceptible Figure 3. Granulometric cumulative curves of the black uppermost sediment, presumably volcanic ash (A) and regolith (B) plotted against the frost suscepti- bility limit of Beskow (1935). – Hlutfallsleg korna- stærð rofefna á yfirborði með tilliti til frostveðrunar. Lichenometry was applied in order to estimate the timing of the highest lateral moraine ridge depositi- on. Largest diameters (enveloping circles) of 417 relatively circular thalli of Subgenus Rhizocarpon were measured with use of a plastic ruler (Da̧bski, 2007). The analysed lichen populations were samp- led on proximal, upper and distal surfaces of the moraine ridges shown on Figure 5. The method of data processing followed that of Evans et al. (1999), namely: the mean of 5 largest thalli per surface was considered. Maximum and minimum ages were calculated using a constant growth rates of 0.5 mm/yr with colonisation lag time of 16 years (maximum age) and 0.8 mm/yr with colonisation lag time of 6.5 years (minimum age) (Table 1). 1 km mountain ridges nunatak area in 1945 rocks exposed between 1945 and 2004 glacier surface ice-dammed lake developed between 1994 and 2004 terminal moraine ridge of Fossadalur glacier in 1945 N Figure 4. Nunatak area exposure since 1945 and an ice-dammed lake in Fossadalur. – Kort af jökulhörfun í Vesturbjörgum og Skálabjörgum, 1945–2004, ásamt lóni og jökulgarði við Fossadal. No growth curve of Rhizocarpon thalli exists for Esjufjöll. The lower rate (0.5 mm/yr and colonisati- on lag time 16 years) was used by Evans et al. (1999) for the marginal zone of Brúarjökull (northern outlet glacier of Vatnajökull), where the mean annual air temperature is from 0 to -1◦C (similar to that of Esju- fjöll), but precipitation is about 10 times lower, which must slow down the Rhizocarpon growth rate. On the other hand 0.8 mm/yr growth rate (and colonisation lag time 6.5 years) was used by Evans et al. (1999) for humid southernmost marginal zones of Vatnajökull, where mean annual air temperature is 2–3◦C higher than at Esjufjöll and climatic conditions for lichen growth thus better. Therefore, the lichen growth rate at the nunatak should be between 0.5 and 0.8 mm/yr and colonisation lag time between 16 and 6.5 years, respectively. The growth rate of Rhizocarpon is actually not linear (Bradwell and Armstrong, 2007), and a size- frequency approach could probably provide better results (Bradwell, 2004), but linear growth rate seems 188 JÖKULL No. 60
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