The Dust Bowl in the American West: A geochemical record of dust provenance and deposition in the Upper Fremont Glacier ice core

Mineral dust is an important component of earth surface processes, and influences the present-day climate through both direct and indirect means. Dust is also critical to biogeochemical cycles, providing limiting nutrients to both continental and marine ecosystems. Mineral dust generation, transport and deposition are detectable in paleoclimate records from land, ocean, and ice, providing valuable insight into earth surface conditions and cycles on a variety of timescales. Here, we have characterized dust from areas in the American West known for dust events and/or affected by increasing human settlement and livestock grazing during the last 150 years. Dust generation and uplift from these dust source areas depends on climate and land use practices; furthermore, the relative contribution of dust has likely changed since the expansion of industrialization and agriculture into the western United States. In tandem with the potential dust source characterization, we present elemental and isotopic analysis of dust preserved in the Upper Fremont Glacier (UFG) ice core, WY, USA through a series of 23 ice core samples. These ice core samples span 20.29-158.74 m in depth and cover a time period of 1715-1998 A.D. and were analyzed along with 28 potential dust source area samples using traditional Thermal Ionization Mass Spectrometry (TIMS) for 87Sr/86Sr and 143Nd/144Nd composition. Trace element concentrations were determined using Inductively Coupled Plasma Sector Field Mass Spectrometry (ICP-SFMS), and ice core dust concentration and size distribution by Coulter Counter. We find that the radiogenic isotope compositions of dust from the UFG ice core are variable over the past 150 years, and that the dust is most likely originating from a combination of source areas including the provinces of the Colorado Plateau, Mojavia, the Basin and Range, and an additional uncharacterized source. Ice core dust particle diameter increases with decreasing age, suggesting a source transition from far to near coincident with rising population and agricultural activity in the American West. The semi-continuous dust concentration data covering the entire ice core shows several significant peaks, and exceeds 8000 ppb during 1934 A.D., which may be the first evidence of the Dust Bowl in a North American ice core.