Contemporary Rates of Dust Deposition in the Alpine Zone of the Uinta Mountains, Utah, USA

Abstract:

Modern dust deposition in the Uinta Mountains (Utah, USA) has been studied intensively since 2011. Eight passive dust samplers are deployed at elevations >3300 m asl and form a key component of the recently launched DUST² Critical Zone Thematic Cluster. Dry masses of mineral dust collected by these samplers were normalized to 365-day years spanning July 1 through June 30, from 2011-2017. These masses were then converted to dust fluxes taking into account the area of each sampler (0.32 m²). The overall average flux is 3.5 g/m²/yr, with a maximum of 4.3 g/m²/yr in 2011-12, and a minimum of 2.7 g/m²/yr in 2012-13. Deposition rates vary from sampler to sampler, and from year to year, however it is notable that the two samplers on the north slope of the range collect less dust (~1.5 g/m²/yr) than those on the main ridge line and the south slope (~3.6 g/m²/yr). This difference also held for a single collection spanning summer 2017 through fall 2019 (2.9 g/m²/yr north vs. 4.3 g/m²/yr south). Measurements reveal that the dust flux at each sampler is positively correlated with the duration of snow cover. For the winter of 2016-17, each additional day of snowcover corresponded to an increment of 7.5 mg in the annual dust flux (r²=0.9075). During the 2017-19 deployment, each additional day of snowcover corresponded to a smaller increment in the annual dust flux (4.1 mg), and snowcover duration and dust flux were less strongly correlated with one another (r²=0.6724). However, this deployment spanned a much wider range of total snowcover durations (18 to 474 days compared with 17 to 235 days in 2016-17), making it difficult to directly compare the two sets of observations. Overall, this relationship suggests that dust contained within the snowpack is accumulating in the samplers during snowmelt. On the other hand, there is no consistent correlation between annual dust flux and the April 1 or May 1 snowpack anomaly at lower elevation, forested measuring sites near each sampler. This discrepancy suggests that the duration of snowcover is more important than the amount of snow, or that measurements at these lower sites are not representative of the snow situation on the alpine tundra where the samplers are located.