Disparate Sandstone Weathering Beneath Lichens, Red Mountain, Arizona

Though the ability of lichens to disaggregate and dissolve the substrate is understood, zones of disparate weathering mechanisms examined in this study were previously unobserved. On a steep (60°) northern (340°N) slope on Red Mountain, Arizona, a 5×5 m study plot was chosen for its maximum lichen coverage (33%) of the same lichen genus Xanthoparmelia in varying growth stages, on a sandstone substrate of consistent lithology. From twenty-two (22) representative lichen thalli, portions were removed to examine sub-thallic rhizine density, and others were resin-inbedded for optical, scanning and backscatter electron microscopy.
In every sample, sandstone beneath and adjacent to the lichen thallus displayed disparate weathering mechanisms. Beneath the center of the lichen cortex, where rhizine density was observed to be the greatest, rhizine penetration disaggregated the sand-stone clasts from the matrix, but little chemical dissolution was apparent. This study found physical weathering predominates beneath the lichen cortex, and chemical weathering predominates at the thallus fringe and beyond the thallus boundary. It was generally found that physical weathering decreased (disaggregation) and chemical weathering increased (dissolution) from the cortex center to the thallus edge. Toward the thallus fringe, minimal clast disaggregation was observed, and substrate dissolution was obvious.