Regional controls on dust generation and accumulation in the southwestern US

Background/Question/Methods

The generation, transport, and deposition of eolian dust comprise an important set of geomorphic and ecologic processes in drylands of the southwestern U.S.  However, their significance has been underplayed compared to the recognition of global effects of Saharan and Asian dust sources. Dust generation in the Southwest is a complex response to the interaction of weather cycles (ENSO), changes in annual and perennial plant cover, and hydrologic status of dust-source areas. In contrast, dust accumulation rates are largely governed by local topographic factors, as well as distance from dust sources. We use annual and seasonal vertical-deposition rates (dust flux), measured by marble traps, as a proxy for dust generation and deposition.

Results/Conclusions

Peaks in dust flux correspond with both La Nina (dry) conditions and strong El Nino (wet) periods in the Mojave and southern Great Basin deserts. Such response is a seemingly counterintuitive result caused by regional differences in the response of rainfall and vegetation growth to ENSO influence. Major contributing factors are the different responses of source types (alluvial plains, dry playas, and wet playas) and the hydrologic condition of surface sediments. The silt-clay and soluble-salt fluxes increase during El Nino events at sites close to wet playas with shallow depths to groundwater, due to rapid evaporative concentration of salts that disrupts physical surface crusts and increases susceptibility to deflation. The silt-clay flux also increases during drought periods (consecutive La Nina years) at sites downwind of alluvial sources and dry playas with deeper groundwater, due to the die-off of vegetation, and in some cases, to local runoff events that deliver fresh sediment to playa margins and distal portions of alluvial fans. Consistent with these rainfall responses, wet playas tend to produce more dust in the winter and spring, whereas alluvial plains and dry playas produce more dust in the summer and fall. Dust generated from the Mojave and southern Great Basin is transported at least as far as the eastern Colorado Plateau, where it contributes 25-60% of the eolian fines (silt and clay) deposited on, and incorporated into, surface soils. Deposition fluxes on the Plateau are strongly seasonal, being about twice as high in the summer and fall than in winter and spring. Combined with locally generated dusts, deposition rates of fines are actually larger on the Colorado Plateau than in the drier deserts farther to the southwest, and likely contribute significant amounts of plant nutrients to soils.