Abstract

In the late 20th century, a weathering-transportation hypothesis that can be traced to G.K. Gilbert (1877) that today is known as “soil production” or “mobile regolith production”, has circulated with vigor first through geomorphology and subsequently through pedology. In other words, bedrock weathers at the base of the critical zone where particles and solutes are liberated thanks to geophysical and biogeochemical reactions. Toward the upper boundary of the weathering profile at the Earth’s terrestrial surface, erosion and dissolution remove particles and solutes from the mineral conveyor that is the regolith. As mineral particles are conveyed from bedrock to soil, they are attacked by biogeochemical weathering agents and resultant solutes are removed via drainage and precipitated into secondary minerals. It was Gilbert’s idea that the Earth feeds soils from below as a result weathering gains nearly always outpace transport losses. More than 98% of the Earth’s terrestrial surface, from the tropics to the boreal zone, is covered in a blanket of soil because over time and space weathering gains exceed transport losses.

At the Calhoun Critical Zone Observatory in North America’s Southern Piedmont, a borehole 15-cm in diameter was cored 70 meters into protolith granitic gneiss in a geomorphically stable area that has been augured many dozens of times to 5 to 10 m depth over more than two decades. We have biogeochemically characterized in detail the upper 10 meters of this regolith in all three of its phases -- solids, liquids, and gases. The area has also been interrogated to many 10s of meters depth for its seismic velocity, sonic velocity, porosity, and a variety of other physical, chemical, and optical properties.

Total elemental concentrations, mineralogical observations, and geochemical modeling demonstrate that the protolith’s plagioclase weathers to kaolinite across a dissolution front that extends between 38 and 12 m depth and thus creates porosity. Between 20 and 12 m depth, porosity increased from about 10% to nearly 40% in bulk volume and dilute-salt pH has been reduced to well under 5.0. In surficial layers above 12 m, the soil-saprolite system is in an advanced state of weathering. The protolith’s orthoclase persists into the upper several meters of soil, and is largely absent by <0.5 m depth. Orthoclase and plagioclase dissolution by 3-m, are responsible for the dominant kaolinite mineralogy in the upper C horizon saprolite and lower Bt horizons.

Within Bt horizons however kaolinites in all particle-size fractions (sand, silt, and clay sized) increase from about 40% by mass at 3-m to nearly 75% at 0.8-m. More spectacularly, clay-sized mineral particles increase from about 5% to well over 50% from the upper C horizon to the Bt. Some of this increase is attributable to eluvial-illuvial processes that recycle clay-sized particles back into Bt horizons. We hypothesize however that there is yet-to-be described pedological process that geophysically and geochemically shifts fine silt to medium sand-sized kaolinites in upper C horizons into clay-sized kaolinites in Bt horizons. Nearly all soil mapping units in the Southern Piedmont uplands that are presumed to be derived from residuum, has been found to have a coarse-over-fine-over-coarse sized particle size pattern in their A-Bt-C horizons. We propose that in regions like the Southern Piedmont on which Ultisols and Alfisols can form, the regolith conveyor produces clay minerals from weathering rock to C horizon saprolite, but the regolith-Bt soil conveyor produces clay-sized minerals.

Citation

Richter, Daniel deB. (2019): The Soil and Critical Zone Clay Factory: How Rock Weathers to Clay and Clay-Sized Minerals. American Geophysical Union Fall Meeting, San Francisco, CA, December 9-13, 2019.

This Paper/Book acknowledges NSF CZO grant support.