Linking soil element-mass-transfer to microscale mineral weathering across a semiarid environmental gradient

- We examine plagioclase weathering in desert scrub and mixed conifer ecosystems.
- Elemental loss is quantified in bulk soils and at the microscale.
- A microscale classification technique was developed to categorize mineral transformation.
- Na and Si elemental loss increases from unaltered to edge to altered grain sections.
- Interactions among climate, vegetation, and landscape position drive feldspar weathering.

Understanding controls on silicate weathering is critical to characterizing critical zone evolution. The objective of this study was to investigate how climate, vegetation, and landscape position control feldspar transformations across a semiarid environmental gradient. Granitic surface soil and saprock samples were collected from desert scrub and mixed conifer sites within the Santa Catalina Mountain Critical Zone Observatory where mean annual temperature ranges from 24 °C to 10 °C and mean annual precipitation from 25 to 85 cm. Quantitative X-ray diffraction, X-ray fluorescence, and electron microprobe analyses were employed to quantify elemental changes in bulk soils and across plagioclase grains. The chemical depletion of Na in bulk soils ranged from 5.4 - 15% in the desert scrub sites relative to 16-33% in the mixed conifer sites. Plagioclase grain alteration was classified into unaltered, edge, and altered sections to compare microscale weathering and elemental variation. The Na/Al and Si/Al ratios decreased from unaltered, to edge, to altered grain sections in the mixed conifer sites, whereas the element ratios of the desert scrub system were similar between unaltered and edge grain sections, and only exhibited significant decreases in Na/Al and Si/Al ratios between edge and altered materials. The microscale depletion of Na and Si suggested increased silicate weathering in the cooler, wetter, and more biologically productive mixed conifer system compared to the hot, dry desert scrub system. The results also demonstrated a topographic control on mineral transformation where increased plagioclase weathering occurred in convergent footslope landscapes with little change in elemental depletion of soils in divergent summit sites.