A comparative study of the role of lichens and "inorganic" processes in the chemical weathering of Recent Hawaiian lava flows

Abstract

The lichen Stereocaulon vulcani, which grows abundantly on recent lava flows on the Island of Hawaii, greatly accelerates the chemical weathering of its substrate. The importance of the lichen (or its associated microflora, or both) in chemical weathering is indicated by the following evidence: (1) The weathering crust of lichen-covered rock is thicker by some orders of magnitude than the weathering crust of bare rock; (2) the lichen-covered weathering crust is considerably enriched in Fe and impoverished in Si, Ti, and Ca with respect to the lichen-free weathering crust, whose chemical composition is much closer to that of the unaltered rock; (3) the lichen-covered weathering crust; and (4) the iron oxide in the lichen-covered weathering crust is mineralogically different from the iron oxide in the lichen-free weathering crust. Biological acceleration of chemical weathering can be explained by the action of respiratory CO 2 and the H+ ions of organic acids, but the data also suggest the action of organic complexing agents. It is shown, however, that the biogenic "primary laterite" could not have been formed by differential accumulation of rock-forming elements by the lichen. Thus, a considerable proportion of the Si and Al solubilized by the organisms must be leached into the subsurface by rain water.