Megaripple mechanics: bimodal transport ingrained in bimodal sands

TL;DR

This paper presents a unified model for bimodal sand transport that explains the formation of megaripples with distinct bimodal grain compositions, supported by extensive terrestrial and extraterrestrial data.

Contribution

It introduces a simple quantitative model grounded in grain physics that predicts bimodal aeolian transport and megaripple morphodynamics, unifying diverse observations.

Findings

Quantitative signature of bimodal transport in grain size distributions

Unified phase diagram for bimodal aeolian processes

Robust model validated by terrestrial and extraterrestrial data

Abstract

Aeolian sand transport is a major process shaping landscapes on Earth and on diverse celestial bodies. Conditions favoring bimodal sand transport, with fine-grain saltation driving coarse-grain reptation, give rise to the evolution of megaripples with a characteristic bimodal sand composition. Here, we derive a unified phase diagram for this special aeolian process and the ensuing nonequilibrium megaripple morphodynamics by means of a conceptually simple quantitative model, grounded in the grain-scale physics. We establish a well-preserved quantitative signature of bimodal aeolian transport in the otherwise highly variable grain size distributions, namely, the log-scale width (Krumbein phi scale) of their coarse-grain peaks. A comprehensive collection of terrestrial and extraterrestrial data, covering a wide range of geographical sources and environmental conditions, supports the accuracy and robustness of this unexpected theoretical finding. It could help to resolve ambiguities in the classification of terrestrial and extraterrestrial sedimentary bedforms.