Estimating the growth rate in desert biological rock crusts by integrating archaeological and geological records

Abstract. Biological rock crusts (BRCs) are ubiquitous features of rock surfaces in drylands composed of slow-growing microbial assemblages. BRC presence is often correlated with rock weathering, soiling effect, or with mitigating geomorphic processes. However, their development rate has not been quantified. In this work, we characterised and dated BRCs in an arid environment, under natural conditions, by integrating archaeological, microbiological and geological methods. To this end, we sampled rocks from a well-documented Byzantine archaeological site, and the surrounding area located in the Central Negev Desert, Israel. The archaeological, which is dated to the 4th–7th centuries CE, was constructed from two lithologies, limestone and chalk. BRC started developing on the rocks after being carved, and its age should match that of the site. The BRC samples showed mild differences in the microbial community assemblages between the site and its surrounding, irrespective of lithology, and were dominated by Actinobacteria, Cyanobacteria and Proteobacteria. We further measured the BRC thickness, valued at 0.1–0.6 mm thick BRC on the surface of 1700 years old building stone block of about 0.1 square metres. Therefore, a BRC growth rate was estimated, for the first time, to be 0.06–0.35 mm 1000 yr⁻¹. We propose that BRC growth rates could be used as an affordable yet robust dating tool in archaeological sites in arid environments.