Reconstruction of the equilibrium line altitude

Fluctuations in the ELA provide an important indicator of glacier response to climate change which may allow reconstructions of palaeoclimate, but also of future glacier response to given climate change.

reconstruction of the equilibrium line altitude 59

Equilibrium Line AltitudeEquilibrium Line Altitude

Figure 4.6 The principle of calculating the depression of the equilibrium line altitude on a glacier based on the maximum elevation of lateral moraines. The previous extent (a) is compared with the modern extent (b) for an idealized glacier. Dashed lines indicate surface contours and arrows indicate ice flow direction. (Modified from Nesje, 1992)

The most common approaches in reconstructing palaeo-ELAs are to use:

(a) the maximum elevation of lateral moraines (MELM);

(b) the median elevation of glaciers (MEG);

(c) the toe-to-headwall altitude ratio (THAR);

(d) the ratio of the accumulation area to the total area (AAR); and

(e) the balance ratio method.

These will be considered in turn.

(a) Maximum elevation of lateral moraines. Due to the nature of glacier flow towards the centre and the margin of the glacier above and below the ELA, respectively, lateral moraines are theoretically only deposited in the ablation zone below the ELA. As a result, the maximum elevation of lateral moraines reflects the position of the corresponding ELA (Fig. 4.6).

Commonly, however, it is difficult to assess whether or not a lateral moraine is preserved entirely in the upper part or whether moraine deposition started immediately down-glacier of the ELA. Consequently, ELA estimates derived from eroded and/or non-deposited lateral moraines may be too low. In contrast, the assumption that the maximum altitude of lateral moraines is obtained during steady-state conditions can overestimate the ELA. If initial glacier retreat is slow, additional moraine material could be deposited in the prolongation of the former steady-state lateral moraine. A continuous supply of debris from the valley or cirque walls may lead to the same source of error in the ELA calculations.

(b) Median elevation of glaciers. The median elevation of glaciers (MEG) has been used to estimate ELAs. However, empirical evidence from modern glaciers suggests that the MEG overestimates the ELA. In addition, this method fails to take into account variations in valley morphology, which strongly affect the area-elevation distribution of a glacier. However, it works well for small glaciers with even area/altitude distributions. Still, the main problem is to define the headward limit of a former glacier.

(c) Toe-to-headwall altitude ratio. This ratio between the maximum and minimum altitude of a glacier has been used as a quick estimate to calculate the ELA. Ratios of 0.35-0.4 normally

MAP VIEW

MAP VIEW

Figure 4.7 An idealized glacier with straight valley sides at positions I (solid line) and II (punctuated line). The EL A difference between positions I and II, using the AAR approach at different slope angles, is shown. The figure illustrates the importance of surface slope angles of the underlying topography when applying the AAR approach for calculating former ELAs. (Adapted from Nesje, 1992)

Figure 4.7 An idealized glacier with straight valley sides at positions I (solid line) and II (punctuated line). The EL A difference between positions I and II, using the AAR approach at different slope angles, is shown. The figure illustrates the importance of surface slope angles of the underlying topography when applying the AAR approach for calculating former ELAs. (Adapted from Nesje, 1992)

give the most correct estimates. Again, a major problem is to define the headward limit of a former glacier.

(d) The ratio of the accumulation area to the total area (accumulation area ratio, AAR) is based on the assumption that the steady-state AAR of former glaciers is 0.6 ± 0.05 (Porter, 1975), a value derived from temperate glaciers from different regions of the world, mostly NW North America.

The AAR of a glacier varies mainly as a function of its mass balance; ratios below 0.5 indicate negative mass balance, 0.5-0.8 correspond to steady-state conditions, and values above 0.8 reflect positive mass balance regimes (Andrews, 1975). An AAR of 0.6 ± 0.05 is generally considered to characterize steady-state conditions of valley/cirque glaciers. Ice caps and piedmont glaciers may, however, differ significantly from this ratio. The largest source of inaccuracy related to the AAR method of determining the ELA on former glaciers is the reconstruction of the surface contours, especially if the glacier margins intersect valley-side topographic contours at small angles or coincide with them for some distance. However, this source of error is considered to be randomly distributed and is not considered to introduce major deviations from representative conditions. In addition, this method only requires glacier reconstruction as high as the former ELA. A theoretical evaluation of the AAR approach, using changing slope angles and valley morphology on idealized glaciers, shows that glaciers advancing into flat areas underestimate the ELA depression, while glaciers moving into areas of increasing slope angle overestimate the climatic ELA difference (Fig. 4.7) (Nesje, 1992). Consequently, topographical and morphological effects on calculated ELA depressions on glaciers must be carefully evaluated.

(e) Balance ratio method. As demonstrated above, one shortcoming of the AAR method, and also the MEG approach, is that they do not fully account for variations in hypsometry (distribution of glacier area over its altitudinal range). To overcome this problem, a balance ratio method was developed by Furbish and Andrews (1984). This approach takes account of both glacier hypsometry and the shape of the mass balance curve and is based on the fact that, for glaciers in equilibrium, the total annual accumulation above the ELA must balance the total annual ablation below the ELA. This can be expressed as the areas above and below the ELA multiplied by the average accumulation and ablation, respectively (for further details, see Furbish and Andrews, 1984; Benn and Evans, 1998: 84).

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