MIDDLE EARTH - A SPECULATIVE (SUB)CRITICAL RECONSIDERATION OF THE SOIL PRODUCTION FUNCTION (Invited Presentation)

Earth surface scientists have long posited what controls bedrock to soil conversion rates, which we can now test using cosmogenic nuclides (presuming steady state). Additionally, near-surface geophysics allows us to image the near-surface with increasing fidelity, such that weathering states from ‘fresh’ bedrock to weathered rock to soil can be inferred over hillslope scales. Current soil production functions do not always match CRN-derived field data, and we are yet unable to predict soil thickness - hampering Critical Zone past, present and future earthcasting. Weathering mechanisms (e.g., thermal, ice segregation, wind-driven tree sway, and plant water uptake) are cyclic over brief (seconds to minutes), diurnal, or seasonal cycles. Almost all bring water to the crack network. All near-surface rock is to some degree broken by tectonics, the journey to the surface, or contraction cooling; however, a threshold density of cracks is necessary for cracks to intersect significantly. Importantly, this crack intersection threshold is a necessary condition for significant physical weathering and associated chemical weathering. Because crack growth rate is a function of the crack length and eventually, degree of stress accommodation due to increasing porosity, crack growth in non-uniform over time and thus we suggest physical weathering can be non-uniform even if conditions remain constant.

Additionally, unlike traditional rock physics laboratory experiment conditions, surface rock is often buffered by a soil layer and is subject to disturbance agents that can remove loose fragments - thus modifying the stress state and the crack network. Geomorphologists and soil scientists have generally ignored factors governing fracture propagation (soil production from below), and rock physicists, focused on index properties and detailed process understanding, have not simulated relevant field conditions (fracture propagation from above). Here we ask if a 'middle earth' perspective that combines rate controls from both above and below may lead to a more mechanistic form of a soil production function. We test our preliminary ideas by returning to field sites in the Henry Mountains, the Oregon Coast Range, and the Southern Alps of New Zealand.