Younger Dryas Equilibrium Line Altitudes and Rockglaciers in the Ferwall-Group (Western Tyrol, Austria)
by
Rudolf Sailer
Institut fuer Geographie, Universitaet Innsbruck, Austria
Coauthors: Hanns Kerschner (Institut fuer Geographie, Universitaet Innsbruck, Austria)

The abundant glacial and permafrost-related landforms of the Ferwall Group (western Tyrol, Austria) can be used for paleoclimatic reconstruction. Especially the various active and relict rockglaciers in comparison with presently and formerly glaciated regions of the Ferwall Group give us information about annual temperature and precipitation changes during the Younger Dryas (Sailer, Kerschner 1999).

The reconstruction of the ELA (equilibrium line altitude) is based on numerous well documented end- and lateral moraines of the Younger Dryas "Egesen Stadial", which was distinctly three-phased. Relative to the Little Ice Age ELA, the ELA depression of the Egesen maximum glaciers is approximately 300 m. The second and third phase of the Egesen Stadial found in Ferwall-Group have an ELA depression of 190 m (Egesen II) and 120 m (Egesen III) relative to the LIA. The ELA depression of the final advance, which was clearly larger than LIA (Kromer / Kartell, Preboreal Oscillation ?) is 90 m.

Additional to changes in ELA, the fluctuations of the lower boundary of permafrost can be used to obtain more information about precipitation and temperature changes. In that case, the lower boundary of permafrost is indicated by active or relict rockglaciers. Presently active rock glaciers in the Ferwall group reach down to approximately 2500 m. In some cirques, areas which were glacierized during the Egesen maximum (early Younger Dryas) have been covered subsequently by large rockglaciers down to 1900 m after deglaciation. These rock glaciers started to develop at approximately the same time as the second advance of the Egesen Stadial (Egesen II). They can be found in the north-western part (Mardusa cirque) as well as in the south-eastern part (Vergöss and Sesslad cirques) of the investigation area. This succession of glacial and periglacial processes is primarily caused by the interaction of temperature and precipitation changes.

The depression of mean annual air temperature can be obtained from the altitudinal difference of active and relict rockglaciers. If a temperature lapse rate of -0.7 K / 100 m is used, it amounts to -4 - -4.5 K, which is less than the depression of Younger Dryas mean annual air temperatures as they can be derived from other proxy data. Precipitation change can then be estimated by the difference between rockglacier altitudes and the ELA with a cryosphere-model (Haberli 1982, 1983, Kerschner 1983).

Preliminary results show that precipitation during the maximum Egesen advance (early Younger Dryas) was about 80 -100 % of modern precipitation rates in the Ferwall Group, which is typical for areas closer to the western fringe of the Austrian Alps (Kerschner et al. 2000). The subsequent advances are characterised by a significant rise of the ELA, whereas the rockglacier altitude did not change significantly. This can be interpreted as a result of constantly cool and increasingly drier conditions during the later parts of the Younger Dryas cooling event. During later phases of the Younger Dryas, precipitation seems to have been reduced to about 50 - 60% of the present day values.

References: Haeberli, W. 1982. Basler Beitr. Physiogeographie 4, 9 - 17. Haeberli, W. 1983. Permafrost, 4th Int. Conference, Proceedings, 415-420. Kerschner, H. 1983. Permafrost, 4th Int. Conference, Proceedings, 589-594. Kerschner, H., G. Kaser, R. Sailer. 2000. Ann. Glaciol. 31, 80-84. Sailer, R., H. Kerschner. 1999. Ann. Glaciol. 28, 141-145

Date received: May 14, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cahi-92.