EXPLORING RELATIONSHIPS BETWEEN POROSITY, MINERALOGY, AND EXPOSURE AGE OF ROCKS SAMPLED FROM THE MULLINS GLACIER, MCMURDO DRY VALLEYS, ANTARCTICA

The McMurdo Dry Valleys located in the Transantarctic Mountains experience an arid polar-desert climate. In an ongoing investigation studying the weathering characteristics of rocks found in this extreme environment, samples of sandstone boulders - thought to be Beacon Heights Orthoquartzite - exposed from the head of the Mullins Glacier to adjacent Beacon Valley were collected for petrographic analysis. Because 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 and porosity were determined using thin sections from 18 boulders sampled down 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 750 points were identified in each thin section. When the cross-hairs landed on blue stained epoxy, the point was classified as pore. Results indicate that the samples are subarkose in composition. Pores range in size from approximately ~.05mm to 1mm. We hypothesize that a portion of the porosity arises from mechanical weathering of the sandstones. Lamp et al. (2017) hypothesized that a primary cause of mechanical weathering in the Dry Valleys is thermal stress. We also hypothesize that mineralogy would influence porosity, given that thermal stresses are influenced by the relative proportion of minerals with differences in thermal expansion coefficients (e.g. Eppes and Keanini, 2017). To test these hypotheses, we compared porosity with relative age and mineralogy of the sandstones. We observed a positive trend between porosity and relative age as determined by distance from the head of the glacier, supporting the idea that the porosity is related to time-dependent weathering. There is also a positive relationship between increasing clay matrix content and increasing porosity. There is a negative correlation between high quartz and potassium feldspar content with porosity, suggesting that the proportion of matrix exerts the most control on weathering characteristics. This work suggests that small variations in mineralogy may influence weathering characteristics in one of the coldest, driest places on Earth.