Regolith transport quantified by braking block, McMurdo Dry Valleys, Antarctica

The McMurdo Dry Valleys of Antarctica are today a hyper-arid, polar desert. Prior work has identified several in situ volcanic ashes (6–11 Myr old) resting on the surface regolith that suggests the persistent stability of the regolith surfaces and climate. However, our field observations of characteristic regolith bulges above and cavities below boulders that are lodged in the hillslope (braking blocks) contradict the apparent preservation of the regolith surfaces. To quantify the regolith mobility we modeled the downslope regolith transport around large boulders in the Dry Valleys using a finite difference regolith-transport model. To guide our modeling effort, we surveyed the detailed topography around one large boulder in the field. Model results fit the observed topography well and allow for the calculation of the minimum volume of regolith per unit width of slope that was mobilized (> 4 m3/1 m width). To assess the general applicability of the braking block analysis on random boulders on hillslope we surveyed the topographic characteristics adjacent to 997 boulders on 10 separate hillslopes. The bulge-cavity development appears to be sensitive to: 1) the adjacent obstructions that restrict the free movement of the regolith around the given boulder, and 2) the inherent surface roughness. Even though, theoretically boulders of all sizes should have a cavity-bulge pair surrounding them, the signal to noise ratio prevented us from extracting such measurements reliably on boulders whose diameter was smaller than about 1 m, on boulders that sat on bedrock covered by a thin veneer of regolith, and on boulders that were part of an actively aggrading talus. The limiting minimum topographic diffusivity was found to be 10− 6 m2/yr. Over time periods of millions of years this is enough to rework the surfaces and highlights the intriguing contradiction between the preserved ashes and the observed regolith transport.