Abstract. Mineral specific surface area (SSA) increases 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 for many soil processes,
including water-holding capacity and nutrient availability. Here we report
our measurements of SSA and mineralogy of two 21 m deep SSA profiles at
two landscape positions, in which the emergence of a very small mass percent
(<0.1 %) of secondary oxide generated 36 %–81 % of the total SSA
in both drill cores. The SSA transition occurred near 3 m at both
locations and did not coincide with the boundary of soil to weathered rock. The
3 m boundary in each weathering profile coincides with the depth extent
of secondary iron oxide minerals and secondary phyllosilicates. Although
elemental depletions in both profiles extend to 7 and 10 m depth, the
mineralogical changes did not result in SSA increase until 3 m depth. The
emergence of secondary oxide minerals at 3 m suggests that this boundary may be
the depth extent of oxidation weathering reactions. Our results suggest that
oxidation weathering reactions may be the primary limitation in the
coevolution of both secondary silicate and secondary oxide minerals. We
value element depletion profiles to understand weathering, but our finding
of nested weathering fronts driven by different chemical processes (e.g.,
oxidation to 3 m and acid dissolution to 10 m) warrants the recognition that
element depletion profiles are not able to identify the full set of
processes that occur in weathering profiles.