『Abstract
This paper explores the role of five recurrent epilithic lichen
species (Aspicilia internutans (Nylk) Arnold, Xanthoparmelia
pulla (Ach) O. Blanco, A. Crespo, Elix, D. Hawlsw. & Lumdsch,
Rhizocarpon lecanorinum Anders, Tephromela atra
(Huds.) Hafellner and Lecanora bolcana (Pollini) Poelt),
which encrust granodiorite spheroidal boulders exposed in the
Sila uplands (Calabria, southern Italy), in weathering of plutonic
rocks in a typical mountainous Mediterranean environment. A detailed
investigation was carried out on the lichen-rock interface if
each species, by comparing them mutually and with lichen-free
granodiorite samples. For this purpose, the lichen species were
sampled together with the encrusted rock surface for detailed
mineral-petrographic analyses performed in thin and ultra-thin
sections. Optical and scanning electron microscopy of these sections
and of bulk samples permitted us to highlight the peculiar modes
of physical and chemical attacks of lichen thalli and hyphae on
and into the substratum for each species. Crack systems often
parallel to the outer rock surface appear often intruded by hyphae,
which cause rupture of primary minerals, with detachment and progressive
incorporation of their fragments into the thallus. In particular,
the species L. bolcana and T. atra revealed an unexpected,
partly endolithic behavior, presumably enhanced by the presence
of rock fractures earlier generated by other physical breakage
processes already affecting the spheroidal boulders in the Sila
mountains. Dissolution features often affect primary minerals
(even quartz), that may show very peculiar patterns which are
suggestive of a biologically-induced control. Various phyllosilicate
clay minerals were identified using SEM-EDS microprobe analyses
and FT-IR spectroscopy, which also enabled the identification
of possible amorphous silica (or quartz micrograins), rhizocarpic
acid and carotenoid at the encrusted granodiorite interface. In
contrast, neither oxalic acids nor oxalates were detected.
Keywords: Lichens; Granodiorite; Lichen-rock interface; Biomechanical
weathering; Biochemical weathering』
1. Introduction
2. Geological/geomorphological and climate setting
3. Materials and methods
4. Results
4.1. The five lichens studied
4.2. Petrographic analyses
4.2.1. Lichen-free granodiorite
4.2.2. Weathering features at the lichen-rock interface
4.2.2.1. A. intermutans
4.2.2.2. X. pulla
4.2.2.3. R. lecanorinum
4.2.2.4. T. atra
4.2.2.5. L. bolcana
4.3. FT-IR spectroscopy and SEM-EDS microprobe analysis
5. Discussion
5.1. Biophysical and biochemical weathering
5.2. Bioweathering products and organic acids
6. Conclusions
Acknowledgments
References