On the dual nature of lichen‐induced rock surface weathering in contrasting micro‐environments

Abstract

Contradictory evidence from biogeomorphological studies has increased the debate on the extent of lichen contribution to differential rock surface weathering in both natural and cultural settings. This study, undertaken in Côa Valley Archaeological Park, aimed at evaluating the effect of rock surface orientation on the weathering ability of dominant lichens. Hyphal penetration and oxalate formation at the lichen-rock interface were evaluated as proxies of physical and chemical weathering, respectively. A new protocol of pixel-based supervised image classification for the analysis of periodic acid-Schiff stained cross-sections of colonized schist revealed that hyphal spread of individual species was not influenced by surface orientation. However, hyphal spread was significantly higher in species dominant on northwest facing surfaces. An apparently opposite effect was noticed in terms of calcium oxalate accumulation at the lichen-rock interface; it was detected by Raman spectroscopy and complementary X-ray microdiffraction on southeast facing surfaces only. These results suggest that lichen-induced physical weathering may be most severe on northwest facing surfaces by means of an indirect effect of surface orientation on species abundance, and thus dependent on the species, whereas lichen-induced chemical weathering is apparently higher on southeast facing surfaces and dependent on micro-environmental conditions, giving only weak support to the hypothesis that lichens are responsible for the currently observed pattern of rock-art distribution in Côa Valley. Assumptions about the drivers of open-air rock-art distribution patterns elsewhere should also consider the micro-environmental controls of lichen-induced weathering, to avoid biased measures of lichen contribution to rock-art deterioration.

Synthesis and Characterization of (Ca,Sr)[C2O4]∙nH2O Solid Solutions: Variations of Phase Composition, Crystal Morphologies and in Ionic Substitutions

“…These minerals also are found in salivary stones and other human pathogenic formations [3][4][5]. Besides, weddellite and whewellite often occur in crustose and foliose lichen thalli on the surface of Ca-bearing rocks and minerals [6][7][8][9]. Oxalate crystallization is a result of the interaction between metabolites of lichens and associated microscopic fungi with an underlying stone substrate in this case [10][11][12][13][14].…”

Deterioration processes affecting prehistoric rock art engravings in granite in NW Spain

“…Of the organisms implicated in rock deterioration, lichens are the most studied, given their morphological variety, their capacity for causing deterioration and, especially from a cultural heritage perspective, the fact that they are very difficult to remove. Their contribution to physical and chemical changes on rocks is indisputable, although the relative contribution of each weathering type is still being studied (Favero-Longo et al, 2005Aubry et al, 2012;Marques et al, 2016).…”

“…Aubry et al (2012) proposed that the different kinds of weathering experienced by rock art panels in the Côa compared with the Douro river valleys (in Portugal) is due to different solar radiation and humidity dynamics and to differential lichen and bryophyte colonization rates. Recently, Marques et al (2016) evaluated the ability of lichens to deteriorate phyllite in Côa Valley Archaeological Park in Portugal, identifying hyphal penetration and oxalate formation at the lichen-rock interface as the main physical and chemical weathering features, respectively.…”

A review of the nature, role and control of lithobionts on stone cultural heritage: weighing-up and managing biodeterioration and bioprotection

“…The first approach is based on the fact that the presence of lithobiontic communities, and, in particular, of certain biodeteriogenic species, depends on the availability of suitable microenvironmental conditions, allowing their establishment, expansion, and reproduction. At any one heritage site, different CHSS often represent diverse ecological microniches, hosting different lithobiontic communities which interact with the substrate and influence its conservation in different ways (Marques et al 2016;Tonon et al 2019). Knowledge and monitoring of (micro-)environmental parameters favouring or discouraging lithobiontic communities, and in particular, deteriogenic species, can help recognise and achieve the best conservation solutions (Caneva et al 2016;Schumacher and Gorbushina 2020).…”