The fossilization of mammal bones at La Polledrara di Cecanibbio (Rome, Central Italy). Insights for in situ preservation

The identification of mineralogical and chemical composition of the fossil bones is essential for reconstructing the depositional paleo-environment and burial processes, and to highlight the mechanisms triggering the fossilization processes during diagenesis. At La Polle-drara di Cecanibbio, (the richest Middle Pleistocene paleontological and archaeological deposit in central Italy), a museum exhibits in situ thousands of fossils deposited into a fluvial and fluvio-palustrine environment during Marine Isotope Stage 9. The aim of this research is to disentangling between the chemical modifications undergone by the bone tissues of La Polledrara di Cecanibbio mammal remains exposed in situ to environmental agents on the excavated surface and those undergone by bones kept or displayed in museums. X-ray Diffraction (XRD), Raman Spectroscopy and SEM-EDS analyses were carried out to establish the chemical composition of fossil bones and the alteration occur-red over time. To estimate the trend of the environmental parameters inside the museum, the measures of the temperature, the relative humidity and the dew point were recorded during at least three seasons. Fluoroapatite is the principal mineral phase identified in the fossils, testifying that the fluoritisation of the hydroxylapatite is the main fossilization process at La Polledrara di Cecanibbio. Barite and gypsum are also present, in response to ground water qualities and geochemistry of the depositional and burial environment. In addition, efflorescences of gypsum crystals have been detected on the surface of fossil bones. The research performed at La Polledrara di Cecanibbio suggests that the crystallization of secondary minerals such as gypsum crystals depends on environmental conditions and the high solubility of this salt coupled with the facility of expansion upon its re-precipitation inside the bone tissues, constitute the main cause of deterioration. Therefore, the monitoring and stabilization of the indoor environmental parameters are essential for identifying the degradation potential factors and process of bone degradation. They represent the best protocol to follow for preserving in situ fossil bones.