New evidence for the provenance and formation of loess deposits in the Ili River Basin, Arid Central Asia

Loess deposits are thick and widespread along the piedmonts of Arid Central Asia (ACA), however the source (provenance) and processes of formation of these fine-grained aeolian deposits are poorly understood. Here we investigate the provenance and possible distribution mechanisms for loess along the slopes of the Ili River basin, located in northwest China and southeastern Kazakhstan, using a grain-size mixture model and an elemental geochemistry-based source fingerprinting technique. Our results indicate that the Ili loess experiences low rates of sedimentary recycling downstream within the basin piedmont, and are strongly dependent on local geomorphic context. Loess deposits are dominated by proximal sources, indicating short-distance aeolian transport from the Ili River alluvial plains and local proluvial fans. Local sourcing dominated regardless of location within the catchment, although the proportion of fluvial input increases proportionally with increasing distance downstream. Our results suggest that the Central Asian deserts did not act as significant interim storage reservoirs for the loess deposits in the Ili River basin, which contrasts with the popular model for piedmont loess formation across Central Asia. Most likely the relatively enclosed and highly variable basin topography precluded transport from the open desert steppe into the upper Ili River valley. Our study provides the first clear evidence for a genetic link between the Asian high mountains and the loess of the adjacent piedmonts, based on geochemical and grain-size data, with the caveat that the high degree of topographic variability along the Tianshan piedmont likely results in a strongly localized influence on loess formation and accumulation.