Mechanisms of dust emission from Pleistocene loess deposits, Nebraska, USA

Abstract

AbstractSaltation bombardment is commonly believed to be the primary mechanism by which large quantities of dust can be generated. Direct entrainment of silt, on the other hand, is thought to generate minimal quantities of dust. If so, loess landscapes should rarely be major sources of dust unless disturbed by human activity. To test the mechanisms by which loess can be eroded by the wind, we used the Portable In‐Situ Wind Erosion Lab (PI‐SWERL). PI‐SWERL tests were conducted on benches that were carved into loess at field locations. Coarse to fine loess was also collected and tested in a controlled setting using (1) dry, unconsolidated loess and (2) crusted loess. Results indicate that in most cases, the threshold friction velocity for silt is less than that for sand and that most dust generation occurs by direct entrainment of dust. Saltation, even in coarse loess, was nonexistent to intermittent. In a few cases, saltation of sand or soil aggregates preceded dust generation or enhanced emissions after direct entrainment began. Emission fluxes of loess are potentially high in both proximal and distal settings, suggesting that loess is easily entrainable if vegetation density is low. We hypothesize that an arid and windy climate of the late Pleistocene, paired with lower vegetation density, facilitated large‐scale erosion of Peoria Loess in Nebraska to generate wind‐aligned ridges and troughs. More broadly, our results indicate that loess landscapes have the potential to be major sources as well as sinks of dust.