Biodeterioration in marble stones in an underwater archaeological site

Trabajo presentado en el IX Congresso Nazionale AIAR, celebrado en Arcavacata di Rende (Italia) del 9 al 11 de marzo de 2016The decay of materials in underwater environments has been poorly understood for its complexity, since a multitude of factors are involved including hydrodynamic variables, hydrochemical and geochemical features, sedimentary dynamics and biological communities [Fernández-Montblanc et al. 2014, La Russa et al. 2015]. This work is focused on the study of the deterioration processes derived from the biological colonization in seawater conditions, in geomaterials broadly used in cultural heritage. To this end, two types of marble (the Italian Carrara and the Spanish Macael), broadly used in cultural heritage, were subjected to an environment exposition test. Samples were submerged for one and a half year in a submarine archaeological area in the Bay of Cadiz (southern Spain), Bajo del Chapitel, considered as Cultural Interest and recognized as Archaeological Zone in the General Catalogue of the Andalusian Historical Heritage. This area includes remains of the shipwreck Bucentaure, sunken during the Trafalgar Battle (1805), along with remains of Roman and Punic ships. Three types of conditions were considered simulating the original position in which archaeological objects could be exposed in the sea bottom (Type I-fully exposed to seawater, Type II-half-buried and Type III-covered, simulating burial). The characterization of Carrara and Macael marbles by polarized optical microscopy and fluorescence and diffraction X-ray techniques showed differences in grain size and mineral composition. The biological colonization and the interaction with the substrate was investigated under stereomicroscope and scanning electron microscopy revealing that the extension of the colonization and the decay associated was more related to the type of exposure underwater condition than to the intrinsic characteristics of the materials. The largest bio-cover was observed in those samples subjected to the condition Type I (30-35%). This colonization consisted of superficial deposits and encrustations mainly attributable to remains of serpulids tubes and barnacle shells, that cause aesthetic damage and micro-fractures subparallel to the marble surface. In addition, the bioerosion phenomena in the form of microperforations (micro-pitting), was also detected in the three types of conditions, but with more intensity in Macael and Carrara samples fully exposed and half-buried. The origin of these microperforations is probably attributable to the action of endolithic microorganisms. This study constitutes an ideal assessment of biodeterioration processes in underwater cultural heritage, which could contribute to improve conservation measures aimed to the in situ preservation (Rule 1 of the Annex of the UNESCO 2001 Convention for the Protection of the Underwater Cultural Heritage) and musealization of underwater archaeological sites.Community of Madrid for financing Geomateriales2 program (P2013/MIT2914), CEI-Moncloa UCM-UPM, Applied Petrology for Heritage Stone Materials Conservation Research Group.Peer reviewe