Lichen Weathering

One organism's weathering potential has been studied for over 100 years: lichens, a symbiosis of fungi (the mycobiont) with a phototrophic partner

(the photobiont), algae or blue green algae, more precisely cyanobacteria. For two recent surveys of lichen biology and ecology see Ahmadjian (1993) and Nash (1996). Early work focused on the identification of calcium oxa-late, derived from oxalic acid reactions with calcium-bearing substrates, as well as the description of hyphae breaking up and partially decomposing silicate and carbonate mineral grains (e.g., Bachmann 19o4; Stahlecker 19o6; Fry 1924 1927). The role of lichens in weathering of exposed rock is well established (see reviews by Syers and Iskandar 1973; Jones and Wilson 1985; Wilson 1995). Lichens apparently accelerate physical and chemical weathering oftheir substrate by rhizine penetration with thallus expansion and contraction, and by the production of carbonic and organic acids (e.g., oxalic) and chelating agents. Lichen weathering effects include etching of substrate grains and synthesis of secondary minerals such as iron oxides, silica, and possibly clay (see reviews by Jones 1988; Wilson and Jones 1983; Wilson 1995) (figure 4-2).

A pioneering study compared weathering rates of lichen-covered and lichen-free basalt lava flows of recent age in Hawaii (Jackson and Keller 197oa). Lichens appeared to have increased weathering rates by one to two orders of magnitude compared with adjacent bare rock, based on measurements ofrelative weathering rind thickness and chemistry. We will return to a discussion of this study in chapter 5, in our consideration of estimates of the biotic enhancement of weathering.

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