Biologically-initiated rock crust on sandstone: Mechanical and hydraulic properties and resistance to erosion Martin Slavík a , Jiří Bruthans a, ⁎, Michal Filippi b , Jana Schweigstillová c , Lukáš Falteisek a , Jaroslav Řihošek a a Faculty of Science, Charles University, Albertov 6, 128 43 Prague 2, Czech Republic b Institute of Geology, CAS, v.v.i., Rozvojová 269, 165 00 Prague 6, Czech Republic c Institute of Rock Structure and Mechanics, CAS, v.v.i., V Holešovičkách 41, 182 09 Prague 8, Czech Republic abstract article info Article history: Received 18 May 2016 Received in revised form 29 September 2016 Accepted 29 September 2016 Available online 23 November 2016 Biocolonization on sandstone surfaces is known to play an important role in rock disintegration, yet it sometimes also aids in the protection of the underlying materials from rapid erosion. There have been few studies comparing the mechanical and/or hydraulic properties of the BIRC (Biologically-Initiated Rock Crust) with its subsurface. As a result, the overall effects of the BIRC are not yet well understood. The objective of the present study was to brief- ly characterize the BIRC from both the mineralogical and biological points of view, and especially to quantify the effect of the BIRC upon the mechanical and hydraulic properties of friable sandstone. The mineralogical investi- gation of a well-developed BIRC showed that its surface is enriched in kaolinite and clay- to silt-sized quartz par- ticles. Total organic carbon increases with the age of the BIRC. Based on DNA sequencing and microscopy, the BIRC is formed by various fungi, including components of lichens and green algae. Using the method of drilling resistance, by measuring tensile strength, and based on water jet testing, it was determined that a BIRC is up to 12 times less erodible and has 3–35 times higher tensile strength than the subsurface friable sandstone. Satu- rated hydraulic conductivity of the studied BIRC is 15–300 times lower than the subsurface, and was measured to also decrease in capillary water absorption (2–33 times). Water-vapor diffusion is not significantly influenced by the presence of the BIRC. The BIRC thus forms a hardened surface which protects the underlying material from rain and flowing water erosion, and considerably modifies the sandstone's hydraulic properties. Exposing the material to calcination (550 °C), and experiments with the enzyme zymolyase indicated that a major contribu- tion to the surface hardening is provided by organic matter. In firmer sandstones, the BIRC may still considerably decrease the rate of weathering, as it is capable of providing cohesion to strongly weathered (and disintegrated) sandstone surfaces. However, only a near-surface zone of the sandstone is stabilized by the BIRC, and additional sources of stabilization (gravity-induced stress, inorganic cement, etc.) contribute to the resistance of the subsur- face zone of sandstone exposures. © 2016 Elsevier B.V. All rights reserved. Keywords: Biofilm Biocrust Biologically-initiated rock crust Sandstone protection Case hardening 1. Introduction Due to its complexity and contact with the atmosphere, a sandstone surface is a critical zone, with properties affecting the erosion rate. Along with abiotic weathering and erosion factors, biotic factors are considered to play an important role both in the deterioration and stabilization of sandstone surfaces (Gorbushina, 2007 and references therein). Biocolonization on sandstone surfaces is known to play an im- portant role in rock disintegration, yet it sometimes also aids in the pro- tection of the underlying materials from rapid erosion. Our study is concerned with the effect of biologically-initiated rock crust on the me- chanical and hydraulic properties of friable sandstone and its resistance to erosion. 1.1. Biocolonization of sandstone surfaces Various types of organisms and their communities can be found on rock surfaces (epilithic) as well as within their subsurface (endolithic) (Goloubic et al., 1981; Hirsch et al., 1995; Hallmann et al., 2013). Different terms are used for these organisms and/or for the whole bi- ologically-affected surface zone of sandstone, based on the range of the biological impacts, species, kinds of growth, or other characteristics; and are also based on the scientific field. The most common terms biofilm, Geomorphology 278 (2017) 298–313 Abbreviations: BSE, backscattered electrons; BIRC, biologically-initiated rock crust; B- surface, uneven non-tectonic surface with a developed BIRC; C-surface, exposed surface of a tectonic fracture with a developed BIRC; EPS, extracellular polymeric substances; F- surface, surface of tectonic fracture without a BIRC; K, hydraulic conductivity; PBS, phosphate buffered saline; REI, relative erodibility indicator; SEM, scanning electron microscope; TOC, total organic carbon; TS, tensile strength; TSP, tensile strength parallel with the surface; TSP dry , tensile strength parallel with the surface done on dried prisms; TSP wet , tensile strength parallel with the surface done on wet prisms; Tx, capillary water absorption rate; δ, diffusion coefficient. ⁎ Corresponding author. E-mail address: bruthans@natur.cuni.cz (J. Bruthans). http://dx.doi.org/10.1016/j.geomorph.2016.09.040 0169-555X/© 2016 Elsevier B.V. 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