Modelling of Quaternary Glacier Extent and Climate in Tasmania, Australia

Abstract

<p>The aim of this study was to improve our understanding of Quaternary glaciations in Tasmania, and to assess their climatic significance. During the Quaternary, Tasmania experienced ice cap development in the West Coast Range and Central Plateau, with smaller cirque and valley glaciers developing on surrounding mountains. Geomorphic evidence indicates at least four glacial advances occurred. A 500 m resolution, three-dimensional thermomechanical ice-sheet model was used to reconstruct and simulate these glacier fluctuations. The model, while a simplification of reality, provided a framework in which to understand the genesis of contemporary landforms and former glacial climate conditions in Tasmania. Input specifications of basal topography, temperature and precipitation were required, with the latter two interpolated from present-day datasets. Numerous experiments were carried out, involving systematic alterations to temperature and precipitation, basal sliding and atmospheric temperature lapse-rates. Modelled output was compared to empirical evidence with the aim of optimizing the parameters (temperature, precipitation) from which model mismatch was minimised. An annual temperature reduction of 6' C is required for glaciers to form in Tasmania. When temperatures cool beyond this threshold, an ice mass develops over the Central Highlands, Central Plateau, West Coast Range and other mountains. In all but the most extensive glaciations, the Central Plateau ice field consists of a relatively thin carapace of ice, and the thickest and most dynamic glaciers develop in the deep valleys of the Central Highlands. The model shows that ice readily flows down the west-facing valleys from the Central Highlands to join with West Coast Range glaciers. A temperature reduction between 7 and 9' C, with corresponding regional precipitation changes of between +/-50% and an increase in orographic precipitation is required to simulate the Last Glacial Maximum (LGM) ice cover. Earlier glacial advances require larger coolings of up to 11.25' C depending on the regional precipitation conditions prescribed. Mismatches between geomorphic evidence and modelled reconstructions probably result from model grid size issues and parameters not accommodated by the model such as wind-blown snow redistribution. This means that temperature reductions derived from the model may have been overestimated. Despite these limitations, reconstructed temperatures compare well to some multi-proxy palaeo-temperature records from Tasmania, although the cooling identified was larger than that recorded in nearby ocean sediments.</p>