Mineral weathering stabilizes global climate fluctuations over geologic timescales by consuming atmospheric acidity as carbon dioxide and supports the biosphere by supplying rock-derived nutrients to plants and microorganisms. A knowledge deficit exists in addressing how biological and physical processes interact to weather minerals, including mechanisms common to landscape surfaces, such as soil freeze-thaw. Our goal was to examine the effect of ice formation in conjunction with microbial activity on transforming granular basalt, granite, and quartz sand (250-53 µm). High resolution microscopy was employed to examine incipient weathering features and microbe-mineral interfaces of the granular substrates deployed along a semiarid to humid environmental gradient encompassed by the Catalina Critical Zone Observatory (CZO), AZ and the Calhoun CZO, SC. Ice formation experiments of pristine and deployed rocks were performed using an ice nucleation cell interfaced with a scanning electron microscope. In parallel, the composition of organic matter interacting with the substrates was identified using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Our work confirmed evidence for hyphal tunnelling, organic coatings, and biomineralization in samples after 3 years of field burial. FTICR-MS results suggest that the classes of biogeochemical compounds associated with the grains varied by ecosystem and mineral substrate composition. We observed that active sites for ice formation on grain surfaces differed by treatment type: unreacted grains and fungal-mineral interfaces displayed ice formation along edges and fractures whereas grains with extensive organic coatings exhibited water uptake and ice formation on the coatings themselves. These responses suggest that microbial activity influences the location of active sites for ice-driven weathering, generating the hypothesis that biological coatings impede physical freeze-fracture mechanisms at the submicron scale. Our findings indicate that interactive biophysical factors strongly influence microscale weathering, whereby mineral-biofilms provides a positive feedback to chemical weathering and a negative feedback to physical weathering.
Lybrand, Rebecca, George Dragos Zaharescu, Jason Austin, Swarup China, Rachel E. Gallery, Paul A. Schroeder, Daniel Veghte, and Odeta Qafoku (2019): Ice Formation and Microbes: A Microscale Perspective on Interactive Biophysical Weathering Agents in the Critical Zone. ASA-CSSA-SSSA International Annual Meeting, 10-13 Nov. 2019, San Antonio, Texas.
This Paper/Book acknowledges NSF CZO grant support.