Wetting-driven formation of present-day loess structure

Abstract

Present-day loess, especially Malan loess formed in Later Quaternary, has a characteristic structure composed of vertically aligned strong units and weak segments. Hypotheses describing how this structure forms inside original loess deposits commonly relate it to wetting-drying process. We tested this causal relationship by conducting unique experiments on synthetic samples of initial loess deposits fabricated by free-fall of loess particles. These samples were subjected to a wetting-drying cycle, and their structural evolutions were documented by close-up photography and CT scanning. Analysis of these records revealed three key stages of structural evolution: initiation (evenly distributed cracks appear due to wetting); inhomogeneitization (some cracks grow, forming large polygons); and development (polygon-forming cracks grow further - cracks within polygons narrow down or heal up). These experiments successfully reproduced the characteristic structure of present-day loess, and led to a discovery that it is the wetting of initial loess that initiates and drives the structural evolution, while drying preserves and expands resulting features.