Grain-size heterogeneity controls non-equilibrium aeolian transport across large spatial scales

In natural sandy environments, such as gobi deserts, inland dune fields, and coastal zones, surface sediments typically consist of mixed grain-size compositions. While extensive research has addressed aeolian transport over uniform sand beds, the spatiotemporal dynamics of sediment transport under large-scale, mixed-grain conditions remain poorly understood, particularly with respect to surface sorting and its feedbacks on transport efficiency. Here, we introduce a two-grain-size simulation framework, based on the Aeolis model, to quantify how wind velocity, sediment supply, grain-size combinations, and relative abundance influence sand flux and bed surface sorting. Results show that non-equilibrium transport is strongly modulated by the evolving accessibility of mobilizable grains, with threshold contrasts between fine and coarse particles producing pronounced shifts in relative flux contributions. These threshold differences not only reshape the spatial distribution of surface grains but also feed back into the pattern of surface shear stress, further amplifying the sorting effect. The results advance the physics of non-equilibrium, multi-grain aeolian transport, and provide a quantitative foundation for predicting landscape evolution on Earth and other planetary surfaces.