Implications of colonizing biofilms and microclimate on west stucco masks at North Acropolis, Tikal, Guatemala

Abstract

Introduction: The Mayan archaeological sites belong to the World cultural heritage. The porous nature of limestone and stucco (calcareous coating) along with the high humidity and temperature typical of Southern Mexico and Central America, make these monuments prone to stone biodeterioration. The Mayan masks and stelae of the North Acropolis Complex (Tikal, Guatemala) are one of the most emblematic and valuable items at this site. As a common practice to keep these items from weathering, archaeologists and restorers build palm roofs over them. A field survey undertaken in august 2006 has shown that the West Mask (WM) semi-protected under a roof was heavily colonized by biofilms and display decayed feature. The East Mask (EM) located in a vault and kept from the exterior environment appears dry and sound. The apparent correlation of biofilm coverage and substratum deterioration led us to hypothesize that biofilm coating the WM had deteriogenic activity. Results: The purpose of this study was to characterize the biofilms colonizing WM, determine efflorescence chemistry and monitor microclimate under the palm roof in order to shed light on the ongoing deterioration phenomena. Contrasting microclimates were observed associated with WM and EM, with varying levels of relative humidity, sunlight and temperature being linked to the former. These conditions allowed the development of biofilm communities dominated by cyanobacteria only on the WM. Some of the detected taxa are recognized endolithic organisms (i.e. Gloecapsa). Fourier-transformed infrared spectroscopy showed that polysaccharides dominated the chemistry of biofilms. X-ray diffraction analyses (XRD) confirmed the presence of halite, gypsum and weddellite in efflorescences associated with the WM. Our results suggest that the joint impact of a varying microclimatic regime and the presence of biofilms promoted the deterioration. Conclusions: The use of palm roofs as protective practice in the Mayan area should be reassessed. Although they prevent deterioration stucco masks and stelae from direct exposure to the environment, they also induce a microclimate suitable for phototrophic biofilms capable of deteriorating directly and indirectly stucco materials. Management of microclimate and biofilm control should be further investigated as additional means to conserve these valuable items.