Soil drips and droplets: Solifluction patterns analogous to classic fluid instabilities

Abstract:

Arctic soil movement, accumulation and stability exert a first order control on the fate of permafrost carbon in the shallow subsurface and landscape response to climate change. A major component of periglacial soil motion is solifluction, in which soil moves as a result of frost heave and mysterious, flow-like gelifluction. Soliflucting soil is a complex granular-fluid-ice mixture, and its rheology and other material properties are largely unknown. However, solifluction terraces and lobes are organized into striking large-scale patterns that may be used to better understand solifluction dynamics. Common fluids—such as paint dripping down walls—produce markedly similar finger instabilities resulting from competition between surface tension and viscous and gravitational forces. Inspired by fluid and granular mechanics, we develop a formulation for bulk effective surface tension and effective viscosity of soil and find that the wavelengths of large-scale solifluction patterns on Earth (and likely Mars) are quantitatively analogous to small-scale fluid patterns. Relationships between solifluction wavelengths, morphology and topography measured from high resolution digital elevation models across Norway generally agree with our theoretical predictions, while correlations with freeze-thaw cycles point to climatic controls on lobe morphology. Our work demonstrates that grain-scale cohesion not only slows down soil motion, but can result in a system state change that promotes large-scale heterogeneity in topography and soil velocities.