Mechanisms of loess-sized quartz silt production and their relative effectiveness: laboratory simulations

The production of loess-sized quartz siltis frequently ascribed to glacial grinding mechanisms. In this study a series of laboratory simulations was set up, the purpose of which was to demonstrate that a range of geomorphic mechanisms are capable of producing siltunder laboratory conditions. The geomorphic mechanisms investigated were: aeolianabrasion, fluvial comminution, glacial grinding, salt weathering and frost weathering. The results confirmed that under laboratory conditions all of these mechanisms were capable of producing silt. The relative effectiveness of these mechanisms at producing siltunder laboratory conditions was then calculated in order to provide a starting point for a consideration of their potential effectiveness within the natural environment. These results indicate that fluvialand aeolianactivity are highly effective at producing quartz siltover short time periods. In a dust stormof four days duration 287 g of siltcould potentially be produced from every kilogramof sand entrained, while fluvial comminutionin a turbulent fluvialenvironment could potentially produce 900 g of siltfrom every kilogramof sand entrained within 32 h. Over longer periods of time the glacial grinding and weathering mechanisms represent more continuous and hence more effective silt-producing mechanisms. Although it is realised that the relative importance of each silt-producing mechanism will differ spatially and temporally due to differences in climate, relief, geology and geomorphic history, the findings from this work do have implications for the genesis of loess deposits. They demonstrate that glacial and cold weathering processes are not solely responsible for fine-particle formation and that efficient silt-producing mechanisms can operate within a variety of environments. Under suitable conditions, weathering, together with episodic contributions from aeolianand fluvialmechanisms, could be responsible for a considerable proportion of global quartz siltproduction.