Links between physical and chemical weathering inferred from a 65-m-deep borehole through Earth’s critical zone

Abstract

As bedrock weathers to regolith – defined here as weathered rock, saprolite, and soil – porosity grows, guides fluid flow, and liberates nutrients from minerals. Though vital to terrestrial life, the processes that transform bedrock into soil are poorly understood, especially in deep regolith, where direct observations are difficult. A 65-m-deep borehole in the Calhoun Critical Zone Observatory, South Carolina, provides unusual access to a complete weathering profile in an Appalachian granitoid. Co-located geophysical and geochemical datasets in the borehole show a remarkably consistent picture of linked chemical and physical weathering processes, acting over a 38-m-thick regolith divided into three layers: soil; porous, highly weathered saprolite; and weathered, fractured bedrock. The data document that major minerals (plagioclase and biotite) commence to weather at 38 m depth, 20 m below the base of saprolite, in deep, weathered rock where physical, chemical and optical properties abruptly change. The transition from saprolite to weathered bedrock is more gradational, over a depth range of 11–18 m. Chemical weathering increases steadily upward in the weathered bedrock, with intervals of more intense weathering along fractures, documenting the combined influence of time, reactive fluid transport, and the opening of fractures as rock is exhumed and transformed near Earth’s surface.

Soil production and the soil geomorphology legacy of Grove Karl Gilbert

“…Between 18 and 38 m, the bedrock is physically fractured and is geochemically most weathered within the fractures. The patterns of geophysical and geochemical data appear to be closely coincident throughout the weathering profile (Holbrook et al., ).…”

“…The figure at the left is a conceptual model of the weathering profile, illustrating weathering advance and transport losses, and thus regolith production (12). The horizons of the soil and weathering profile (6, 8) are given in the middle of the figure and the depth dependence of the feldspar weathering fronts are illustrated on the right, with nearly all plagioclase exhausted by weathering at 12 m in the C horizon saprolite (Holbrook et al., )…”

“…We are well informed about this regolith (5), which we have long considered to be weathered in place (Bacon et al., ; Holbrook et al., ; Richter & Markewitz, , ; St. Clair et al., ). For example, inventories of meteoric 10 Be within this weathering profile [2 × 10 12 atoms cm −2 (Bacon et al., )] help demonstrate its very great age, >2 to 3 million years.…”

“…In the Southern Piedmont, broad, low-curvature, and ancient interfluves (Brecheisen, 2018) have regolith (5) that is weathered to tens of meters in depth (Bacon, Richter, Bierman, & Rood, 2012;Holbrook et al, 2019;Pavich, 1989). The coring site for this deeply weathered regolith (5) was selected (Bacon et al, 2012) for its geomorphic stability and low geologic erosion rates ( Figure 6).…”

Characterizing the Critical Zone Using Borehole and Surface Nuclear Magnetic Resonance

“…7 in Flinchum et al, 2018b). Although we lack geochemical data, the link between porosity and chemical weathering is well known, as higher porosity is associated with more weathered materials (Brimhall and Dietrich, 1987; Bazilevskaya et al, 2015; Navarre‐Sitchler et al, 2015; Holbrook et al, 2019). Therefore, we speculate that the decrease in porosity at our geophysically interpreted saprolite–weathered bedrock boundary could be associated with a chemical reaction front.…”