Soil Production and the Soil Clay Factory.

In 1877, Gilbert stated with wonder, “Over nearly the whole of the earth's surface, there is a soil, and wherever this exists we know that conditions are more favorable to weathering than to transportation.” Gilbert’s ideas about soil, weathering, and transportation would go a century before circulating vigorously in the Earth sciences. Today, Gilbert’s simple and elegant difference expression of weathering and transportation not only motivates work on “soil-production functions” but perhaps more importantly, it serves as a contemporary paradigm for critical zone formation and evolution. The soil-production system allows us to re-think old Earth science problems. Here as one example, we explore processes of particle-size fractionations as particles ride the soil production treadmill from bedrock to the soil’s surface. Coarse-over-fine textured soils (COF) are widely studied as their texture-dependent properties and behavior are important and they form from many bedrocks and in many climates. They are obvious examples of particle size fractionations that soil scientists who focus mainly on A and B horizons attribute to lessivage, i.e., clays moving from A to B horizons. We use the ancient and deep weathering profiles at the Calhoun CZO to explore how bedrock is weathered into the particles of the critical zone. In terms of mass, feldspars weather to sand- and silt-sized kaolinites in C horizon saprolite, and sand- and silt-sized kaolinites are comminuted into clay-size micelles in the very upper layers of saprolite. Clay-sized kaolinites are further concentrated by lessivage in B horizons which remarkably can peak at >70% kaolinite by mass. Sand- and silt-sized quartz grains continue on the treadmill and dominate surficial A and E horizons where sand-sized quartz grains are about 80% by mass. Given the broad perspective provided by the soil production paradigm and the critical zone, we suggest that most COF soils may actually by COFOC (coarse over fine over coarse) in their A, B, and C horizons.