INVESTIGATING DUST PRODUCTION THROUGH LAB-SIMULATED GLACIAL GRINDING

Understanding production mechanisms of fine sediment capable of eolian transport is important because these sediments act as climate proxies in the rock record and can affect climate through both direct radiative forcing in the atmosphere and fertilization of ecosystems. While production of fines from processes such as glacial grinding, eolian abrasion, and fluvial comminution is recognized, the genesis of fines is often inferred on the basis of their proximity to, e.g. a glacial, eolian dune, or fluvial environment. Yet this “place-based” interpretation is complicated by the potentially long distances of eolian transport, that commonly result in deposition of fines far from the site(s) of their production. Experiments replicating abrasion processes can provide insight into potential end-member particle-size characteristics of fine sediments before they are transported and altered via winnowing.

Previous abrasion experiments investigating glacial grinding failed to produce particle size distributions consistent with those found in glacial sediments (till, outwash, and loess), possibly owing to the use of unnaturally smooth grinding wheels or grinding of sand in a ring shear. In contrast, our experimental design replicates the grinding of surfaces with asperities, analogous to the natural environment.

In our experimental device a rock stylus is cut to mimic an angular plucked stone. The stylus is weighted and pulled across a rock slab, replicating bedrock or a subglacial boulder. The end of the stylus mimics asperities of an angular boulder or gravel clast and thus breaks into finer pieces as it progresses across the basal slab. Preliminary results indicate that this process produces a range of sediment from sand to clay, similar to the range found in glacial sediments. The < 63 µm fractions have modes and distributions consistent with loess deposits.

Results from continued experiments with a variety of lithologies will improve our understanding of fine sediment distributions produced by glacial grinding. Additionally, given that grain size and composition of mineral dusts in the atmosphere determine their effect on the climate, this work will illuminate the potential influence of glacially produced fines on radiative forcing and nutrient dispersal to the biosphere.