SOIL PRODUCTION FROM ABOVE AND BELOW: A UNIFICATION THEORY FROM A GRANULAR PHYSICS PERSPECTIVE

In many landscapes, soil blankets chemically altered saprolite. The production of soil from saprolite occurs via chemical and physical processes working on porous rocks under shear, flow, and confinement. Empirical findings from field studies in California, Oregon, Australia, New Zealand and beyond suggest that there is an exponential relationship between the rate of soil production and soil depth. The observation of an exponential soil production function has been associated with diffusive processes, like the penetration of oxygen or fluid into pore space, decreasing with depth. However, a physical understanding of the mechanism for soil production variation with depth remains elusive. On the other hand, the exponential decay of flux with depth is a ubiquitous observation in creep and slow deformation of disordered, amorphous and granular materials under gravity and shear. The decay rate of velocity (mass flux/mass density) in granular materials, is called the granular shear rate. The granular shear rate feeds into a non-dimensional number, known as the Inertial number, together with grain size, local normal effective stress, and mass density, thus relating grain size with soil depth and deformation (transport and creep) dynamics. Despite these similarities, creep phenomena in granular materials hasn't yet been connected to soil flux and soil production rate functions.

Here we combine computational physics models of amorphous and disordered granular materials, with field observations of soil production obtained from in situ-produced cosmogenic ¹⁰Be in saprolite sampled under soils of different ages. We present new data and a compilation of previously published soil production rates and compare to calculations of soil flux from granular shear and creep, and the resulting vertical mixing and diffusivity. The computational model uses grains of different size and size distributions to accommodate size variability that is often related to the environment and geology. In addition to obtaining a physics-based relationship for variation of soil flux with depth in different environments, our findings could have important implications for understanding feedbacks between chemical and physical erosion, and roles of these processes in driving landscape evolution, nutrient transport and the global carbon cycle.