INVESTIGATING THE DIRECTIONALITY OF EOLIAN DUST DELIVERY TO THE ALPINE ZONE OF THE UINTA MOUNTAINS, UTAH, USA

The Uinta Mountains of northeastern Utah were extensively modified by alpine-style glaciation during the Quaternary Period. Periglacial uplands along the ridgecrest of the Uintas, however, were above the influence of alpine glaciers, and soils on this upland have been extensively influenced by dust deposition over time. Passive samplers reveal contemporary dust accumulation rates of 3-5 g/m²/year in the alpine zone, and modern dust is strongly enriched in elements indicative of upwind anthropogenic activity. Passive samplers alone, however, cannot identify the direction(s) from which components of the modern dust are delivered. Instead, to evaluate the directionality of the modern dust flux into the Uinta alpine zone, a specially designed active sampler was deployed in June 2013 at the location of one of the existing passive samplers. A remote weather station installed at this location (3700 m asl) in 1998 reveals a strongly bimodal wind regime, with wind approaching either from the NNW or SSE. The active sampler uses solar-powered fans to pull air through a filter of glass beads. A wind-actuated switch activates one fan when the wind is from the NNW, and another when the wind is from the SSE, allowing the collection of separate samples for the two primary wind directions. During the first four months of deployment (June through October, 2013) the SSE fan ran 1065 hours, while the NNW fan ran 657 hours. Dust collection rates were 37 μg/hr from the NNW, and 81 μg/hr from the SSE. Grain size analysis reveals that both samples are dominated by medium-fine silt (~15 µm), although the sample from the NNW contains a significant secondary mode of coarse silt (~45 µm). XRD analysis reveals that dust samples from both directions contain amphibole and plagioclase feldspar, neither of which are present in Uinta bedrock. The sample from the NNW also contains a poorly ordered mineral with wide d-spacing, perhaps smectite or hydobiotite. Geochemical analysis with ICP-MS reveals that Cr, Mo, Ni, and Sn are more abundant in the NNW sample. In contrast, most other trace and rare earth elements are considerably more abundant in the SSE sample. Most notably, the concentration of Cu is nearly 3x greater in samples delivered by wind from the SSE.