Comment on esurf-2022-9

Abstract

Mineral specific surface area (SSA) is generated as primary minerals weather and restructure into secondary phyllosilicate, oxide, and oxyhydroxide minerals. SSA is a measurable property that captures cumulative effects of many physical and chemical weathering processes in a single measurement and has meaningful implications to many soil processes, including water holding capacity and nutrient availability. Here we report our measurements of SSA and mineralogy of two 21-meter deep SSA profiles at two landscape positions, in which the emergence of a very small mass percent (0.8–2.7 %) of secondary oxide generated 36–81 % of the total SSA at both landscape positions. The SSA transition occurred at 3 meters and did not coincide with the morphological boundaries of soil to weathered rock or with the water table. The 3-meter boundaries coincide with the depth extent of secondary iron minerals and secondary phyllosilicates. Although elemental depletions in both profiles extend to 7 and 10 meters, secondary minerals were not detected below 3 meters. The 3-meter depth marks the emergence of secondary oxide minerals, and this boundary appears to be the depth extent of oxidation weathering reactions. Our results suggest that oxidation weathering reactions may be the primary limitation in the co-evolution of both secondary silicate and secondary oxide minerals.