Abstract

Rock weathering replenishes nutrients in soil, shapes geochemical carbon sequestration, and drives physical, chemical, and biological processes at a multitude of scales. Our research integrates bioclimatic and topographic factors into a cross-scale examination of mineral weathering in granitic sites selected from two Critical Zone Observatories. The Catalina CZO (Arizona) encompasses a desert to conifer bioclimatic gradient that spans significant range in mean annual temperature (24-10ºC) and precipitation (45-95 cm) whereas the Calhoun CZO (S. Carolina) represents the wet, humid end member with 127 cm precipitation/year and a mean annual temperature of 16°C.

Soil and parent rock samples were examined from each site using X-ray fluorescence and X-ray diffraction to quantify elemental and mineral abundance. We assessed incipient weathering by deploying mesh bags filled with granite, basalt, and quartz sand (53-250 µm) in surface soils of divergent summit and convergent footslope positions at both CZOs. The exposed material was analyzed after one year using helium ion and scanning electron microscopies to identify biota-grain interactions and associated weathering features.

Bulk soil and grain scale analyses of the Catalina CZO samples indicate a tight coupling among mineral weathering, topography, and climate. Dust deposition also plays a critical role in nutrient cycling and soil accretion in the Catalina CZO, with dust fraction estimates up to 20% of the total soil mass in the desert and conifer systems. Calhoun CZO soils contain weathering products dominated by discrete kaolinite and mixed layered kaolinite-biotite, hydroxy-interlayered-vermiculite, gibbsite and goethite. These secondary minerals result from intense weathering and erosion in the Calhoun that were further altered by land use histories in the area. Chemical indices of alteration show clear trends related to climate and land use history within and across the Catalina and Calhoun CZOs. Results from the mesh bag experiment confirm the presence of fungi and bacteria on mineral surfaces and suggest that biota contribute to early stages of mineral transformation. Our findings advance understanding of soil geochemistry and biotic-mineral interactions in field systems that are needed to quantify controls on weathering in the critical zone.

Citation

Lybrand, Rebecca, Jason Austin, Paul A. Schroeder, Dragos Zaharescu, Rachel Gallery (2017): Cross-scale perspectives on mineral weathering in the Critical Zone (Invited Presentation). Geological Society of America Annual Meeting, Seattle, WA, 22-25 October, 2017.

This Paper/Book acknowledges NSF CZO grant support.