Impact Of Plateau Ice On Reconstructed Equilibrium Line Altitudes

Once the presence of glaciers has been confirmed by the application of a landsystems approach, palaeo-climate reconstructions can be undertaken based upon the limits and interpreted dynamics of the glacier coverage. Where plateau icefields existed but left little or no evidence of their presence they have often been ignored and reconstructions have been confined to valley glaciers only (e.g. Sissons, 1980; Ballantyne, 1990). In the following subsections the potential impact of overlooking plateau icefield contributions on climate interpretations (reconstructed ELAs) is assessed. One example is based on the LIA maximum glaciers in the southern Lyngen Peninsula centred on the peak of Jiek'kevarri, where ice supply from icefield to valley was mainly by avalanching. The other is the Younger Dryas glacier coverage of Oksfjordjokelen where the icefield was physically connected to the valley glaciers via steep valley-head ice falls.

16.6.1 Lyngen

The reconstructed LIA (1700—1750) valley glaciers and plateau icefields centred on Jiek'kevarri in Lyngen are presented in Fig. 16.17. Rea et al. (1998) calculated ELAs for the area using the AAR method (Meier and Post, 1962; Porter, 1975; Kuhle, 1988) and these are presented in Table 16.1. The results highlight the potential impact on ELA calculations and thus inferences of palaeo-climate in such landscapes if contributions from plateau icefields are ignored.

Figure 16.18 shows the icefield area as a percentage of the combined glacier (the valley glacier plus the contributing plateau icefield area) plotted against the measured ELA shifts. This supports the intuitive prediction that the larger the plateau contributing area, the greater the

Figure 16.17 Little Ice Age maximum reconstruction of glaciers and plateau ice centred on Jiek'kevarri. Note the radial drainage pattern of the valleys. (Adapted from Rea et al. (1998).)

resulting vertical shift in the ELA. Excessive shifts in ELA occur when the ELA moves above the main valley glacier body and up a narrow, steep gully connection. Rea et al. (1998) compared the plateau-corrected ELAs with previous climate reconstructions (Ballantyne, 1990) and found that, for the minimum interpreted temperature depression during the LIA maximum (1700—1750), winter accumulation rates up to 20 per cent lower than those calculated by Ballantyne (1990) were possible.

Glacier

Glacier area (km2)

Plateau area (km2)

Total area (km2)

Per cent area plateau

Fornesbreen

7.97

0.39

8.36

4.67

1

1.02

0.19

1.21

15.69

Fugledalsbreen

2.75

1.00

3.75

26.54

2

1.13

0.18

1.31

13.98

3

1.68

1.29

2.97

43.49

Vestbreen

5.64

0.96

6.60

14.48

Fauldalsbreen

2.03

2.05

4.08

50.23

Sydbreen-Midtbreen

10.56

3.44

14.00

24.54

4

0.40

0.11

0.51

20.75

Blâisen

1.38

0.54

1.92

28.03

Goverdalsbreen

1.52

1.37

2.89

47.44

5

0.52

0.31

0.84

37.47

N. Veidalsbreen

0.66

0.27

0.93

29.27

Table 16.1 Little Ice Age maximum reconstructed plateau icefield and valley glacier area distributions for 13 glaciers in the southern Lyngen Alps.

Table 16.1 Little Ice Age maximum reconstructed plateau icefield and valley glacier area distributions for 13 glaciers in the southern Lyngen Alps.

16.6.2 0ksfjordj0kelen

Evans et al. (2002) reconstructed the deglaciation chronology of the Bergsfjord Peninsula, and the Younger Dryas extent of 0ksfjordj0kelen. Figure 16.19 shows the Younger Dryas icefield, and Table 16.2 presents the statistics on each outlet glacier of the icefield. All but one glacier has a significant proportion of its area above the 800 m contour (chosen as a representative altitude for the plateau edge), and so the implications for reconstructing ELAs using the AAR method are obvious. The data are plotted on Fig. 16.20 and show again the same trend as Lyngen. In comparison with the Lyngen data, there are more glaciers with plateau-contributing areas in excess of 40 per cent of the combined glacier area, resulting in greater ELA shifts. It is obvious from Figs 16.18 and 16.20 that any climate reconstructions based on ELAs that are calculated without accounting for the contributions of plateau ice will be erroneous.

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