Differential mechanical weathering and the evolution of topographic asymmetry in the Antarctic Dry Valleys

Abstract:

Topographic asymmetry between polar and equator facing hillslopes (north-south) is widespread in various climatic regions. In the absence of structural or stratigraphic forcing, under invariable precipitation regime, topographic asymmetry is commonly associated with microclimatic variability, driven by differential solar irradiation and its effect on surface energy balance. A range of explanations are suggested for the mechanistic link between the evolution of topographic asymmetry and solar irradiation, including differences in soil moisture and its influence on vegetation cover and resistance to erosion, fluvial channel migration and frequency of freeze-thaw cycles.

Here, we investigated potential drivers of topographic asymmetry in the extreme cold hyper-arid environment of the Antarctic Dry Valleys (ADV). Topographic analyses revealed a north-south systematic valley asymmetry with significantly steeper south (polar) facing hillslopes. East-west facing hillslopes in the ADV do not display significant asymmetry. Considering that the ADV has been mostly ice-free since the Miocene and that processes related to vegetation and/or fluvial processes are less applicable to this environment, we examined possible causes for higher weathering rates on sun-facing slopes.

Field observations revealed effective extraction of atmospheric humidity and its delivery in liquid phase into cracks within surface rocks. Remote sensing data and numerical modeling of rock-air thermal fluxes suggest that with respect to the polar facing slopes - the warmer, sun-facing slopes of the ADV are more conducive to environmental conditions in which atmospheric humidity is delivered into rocks. Considering the role of water in accelerating subcritical cracking and rock breakdown rates, we hypothesize that effective condensation of atmospheric humidity is a significant factor driving the formation of topographic asymmetry in the ADV. We also explore the potential aspect controlled role of salts in delivering liquid water to the rock surface and cracks by adsorbing atmospheric moisture and by lowering the freezing temperature. Understanding possible drivers for topographic asymmetry in the ADV may also contribute to our understanding of landscape evolution in other cold and arid environments, such as on Mars.