Sediments surrounding waterlogged archaeological wood: could arsenic be involved in preserving wooden pile dwelling from microbial decay? The Gran Carro case study (Lake Bolsena, central Italy)

Abstract

• WAW conservation depends on the properties of the environment surrounding it. • Climate change and anthropic impact may affect cultural heritage conservation. • Gran Carro sediments show elevated As concentration in stable chemical forms. • As is inversely related to sediment enzymes and directly to microbial stress index. • Monitoring sediments helps to preserve heritage from natural and anthropic pressures. Waterlogged archaeological wood (WAW) conservation depends on several abiotic properties related to water and/or sediments (temperature, pH, redox potential, salinity, heavy metals or pollutants) and biotic factors (amount and type of microorganisms and their metabolic activity). All these factors may be altered by anthropic activities and/or climate changes thus accelerating or decreasing wood decay rates. Regardless of its rate, the decomposition and alteration of submerged material can be expected to leave traces in the surrounding sediment properties. Gran Carro is an archaeological site, located at Lake Bolsena (central Italy) and characterised by the presence of wooden pile dwellings (WPD). According to the most recent archaeological evidence, the site dates back in the period spanning from Middle Bronze Age (XV century BC) to the Early Iron Age (IX century BC). The study of waterlogged WPD, particularly those located in Mediterranean areas, was the object of WOODPDLAKE project financed by JPI-CH19. Lake Bolsena sediments chemical, biochemical (enzymatic activities) and microbiological properties (EL-FAME profiles), discriminated in the portion in tight contact with WAW (WSed) and the sediment in the surrounding area (Sed), are reported in this article with the aim to highlight any “wood effect”. Most properties showed a clear “wood effect” indicating that WSed was strongly influenced by the processes occurring within WAW. The chemical properties of Gran Carro sediments showed a consistent amount of arsenic deriving from the volcanic lithological substrate whose fractionation showed, however, that it was mostly present in stable and immobilized forms. The presence of sulphate reducing bacteria suggested, in fact, the use of arsenic to form insoluble sulphide mineral-like phases that precipitated in the sediment and in wood structure. Arsenic was also highly concentrated within WAW, negatively impacting microbial biomass and activity, in terms of enzymatic activities, which were significantly decreased in WSed. The geochemistry and mobility of arsenic ion depend on environmental factors that may be impacted by climate changes. Therefore, it is advisable to monitor the current variations of environmental matrices such as soil, sediments, water in contact with archaeological material to understand the potential effects on the conservation of cultural heritage. A possible mechanism explaining the processes occurring in the vicinity of waterlogged archaeological wood. Arsenic is immobilized into wood porous structure thus acting as a likely «protection» factor from microbial decay.

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