Formation of Wind-Erodible Aggregates for Salty Soils and Soils with Less Than 50% Sand Composition in Natural Terrestrial Environments

Abstract

Of nine possible mechanisms for the formation of non-sandy, wind-erodible aggregates from a more homogeneous soil or sediment, five are found to be widespread in arid and semiarid regions. In approximate order of significance, the most important mechanisms are tension and compression fracturing of shrinkable mud or soil during wet/dry cycles; tension fracturing and molding by compression during freeze/thaw cycles; direct abrasion (corrosion); fracturing and aggregation produced by salt efflorescence; and mechanical disturbance of surface materials by animals. A sixth mechanism (in soils) is by surface films, colloidal matting, or cements. Minor mechanisms are: floatation and lofting of foam and fracturing caused by hydration expansion or other chemical weathering of fine-grained bedrock. The flocculation of small particles in a water suspension or wet mud is a possible minor mechanism not yet observed. Surface soils and sediments with more than 28% clay and more than 2% organic material formed wind-erodible aggregates, but organic-poor materials did not. Calcareous loams, silt loams, silty clay loams, and clay loams formed wind-erodible aggregates, but non-calcareous materials of the same textures did not. Salt efflorescence was locally a major mechanism for production of wind-erodible aggregates. An experiment with expandible (high smectite content) clay shows that wet/dry cycles (such as might result from several summer rain showers on a dry lake bed) can produce wind-erodible aggregates without high temperatures or lengthy droughts and in the absence of salt efflorescence. For example, cold-climate clay dunes (now inactive) fringe ephemeral lakes in deflation basins in Montana at windy semiarid sites with short hot summers and intensely cold winters.