LINKING WEATHERING TO THE MINERALOGY AND GRAIN SIZE CHARACTERISTICS OF SUPRAGLACIAL BOULDERS OF THE ANTARCTICA DRY VALLEYS

Weathering plays a key overall role in long-term landscape evolution. In an ongoing investigation studying weathering characteristics of rocks found in extreme environments, we collected boulders of the Ferrar Dolerite along the Mullins Glacier in Beacon Valley, Antarctica. Boulders were on the surface of the glacier in locations ranging from the head of the Mullins Glacier to its terminus in Beacon Valley. Boulders were collected and then subsampled for surface exposure dating as well as for mechanical and petrographic analysis. Since boulders are believed to primarily travel supraglacially after deposition by headwall rockfall events or via short durations of englacial transport, the distance down-glacier from the headwall represents a rough approximation of the boulders’ relative exposure age (Mackay, et al., 2017). In this study mineralogy, porosity and grain size were determined using thin sections from a series of boulders of fine, medium and coarse grain size, sampled from the headwall to a maximum distance of 5 km. To avoid sampling from sidewall erosion, we sampled along the centerline far from the sidewalls. Troughs on the sides of the glacier catch debris from sidewall rockfall events, preventing significant clast deposition on the glacier surface. Thin sections were stained blue, and >500 points were identified in each thin section, with maximum crystal size measured for >100 points. Samples are comprised primarily of plagioclase, pyroxene, olivine, glass and opaque minerals, with common alteration in some thin sections. When the crosshairs landed on blue-stained epoxy, the point was classified as pore. We hypothesize that a portion of the porosity arises from mechanical weathering. Lamp et al. (2017) hypothesized that a primary cause of mechanical weathering in the Dry Valleys is thermal stress. We therefore also hypothesize that mineralogy and grain size will influence porosity, given that thermal stresses are influenced by the relative proportion of minerals with differences in thermal expansion coefficients, and by the size of minerals (Eppes & Keanini, 2017). To test these hypotheses, we will compare porosity with relative exposure age and mineralogy. This work may determine how small variations in mineralogy may influence weathering characteristics in one of the coldest, driest places on Earth.