Paola Mameli

Fe concentration in palaeosols and in clayey marine sediments : two case studies in the Variscan basement of Sardinia (Italy)

Abstract Within the Variscan basement of Sardinia (Italy), two main Fe concentrations occur in the low-grade metamorphic tectonic units: (1) an uppermost-Ordovician oolitic ironstone of shallow anoxic water environment; and (2) a concentration of Fe oxyhydroxides lying on a palaeosurface. Two sets of samples were picked from the marine ironstone and from the continental Fe concentration following stratigraphic criteria. Chemical analysis, X-ray diffraction, scanning electron microscopy-energy dispersive X-ray analysis and thin-section studies were performed on 34 samples. Marine ironstones formed under a highly reducing, anoxic, non-sulphidic methanic environment, and their Fe phases are chamosite, siderite and magnetite. Detrital chlorite and illite, produced during physical weathering, were chamosite precursors. Using the V/Cr proxy, an emergence stage that caused a transition to an oxic environment is documented. In contrast, continental ironstones formed under oxic conditions and the dominant Fe phase is goethite, which can adsorb Zn2+ and U6+. Unexpected negative Ce anomalies occur in this set of samples, suggesting that the oxyhydroxides originated from Ce-depleted solutions. Although the ironstones of Sardinia formed in different environments (marine vs. continental) and under contrasting climatic conditions (sub-glacial vs. tropical) they share similar geochemical features. These dramatic palaeoenvironmental differences did not result in large differences between the geochemistry of the chemical sediments.

Messinian palaeoclimate and palaeo-environment in the western Mediterranean realm: insights from the geochemistry of continental deposits of NW Sardinia (Italy)

To evaluate the palaeo-environmental parameters of a portion of the Sardinia–Corsica microplate during the Messinian drop in sea level, we examined the chemistry and mineralogy of upper Tortonian–lower Messinian (late Miocene) clayey continental deposits from NW Sardinia. Differences exist between the uppermost part of the succession, which is devoid of carbonate phases, and the lower part, reflecting changes in provenance and climate. The carbonate-free samples were probably derived from quartzite of the metamorphic basement and were deposited under a climate characterized by alternating dry and relatively wet periods. The other samples were derived from basement phyllite and were deposited under a warm, dry climate that promoted the capillary rise of Ca2+ and bicarbonate from a shallow water table, and therefore, the precipitation of carbonate. This part of the succession contains both calcite and dolomite. The presence of barite indicates an important concentration of SO4 2− in the solution from which the CaMg(CO3)2 precipitated. The formation of dolomite under hypersaline conditions may be explained by bacterial degradation of organic matter, which produced CO2 and ammonia, thereby increasing the solution alkalinity. The succession formed in an oxic environment, except for a calcite-rich level that formed under relatively reducing conditions. For this level, the large amount of calcite and the lack of dolomite indicate an alkaline environment and a very low Mg2+/Ca2+ ratio in the soil solution. These observations, coupled with the reducing conditions, indicate the availability of large amounts of degraded organic matter, probably related to a period typified by a wetter climate.

Contrasting decay of historical building stone: relationships between petrophysical features and frontal polymerization treatment suitability on medieval buildings of north Sardinia, Italy

Textural, physical–mechanical, and mineralogical–chemical properties influence the degradation of building stone. These properties also control the efficacy of different preventative treatments to inhibit degradation. In this study, several historic buildings in northern Sardinia, Italy, constructed with a wide variety of building stones were examined in order to contrast degradation effects with and without frontal polymerization. Different types of degradation were observed in carbonate and volcanic lithologies, which compromise their durability and toughness. Among the different lithotypes tested, four revealed a good response to the polymerization treatment. Textural and physical–mechanical evidences show that open porosity and capillary absorption control the suitability of this treatment. Laboratory tests on both untreated and treated specimens revealed a strong reduction of parameters directly related to decay (e.g., open porosity and water absorption), as well as an improvement on crush strength. Moreover, the polymerization treatment retains a residual porosity sufficient to enable the rock to equilibrate with the ambient environmental humidity.

Il Castrum bizantino di Oschiri : un caso di diagnostica e conservazione

Nell’ambito dei programmi di valorizzazione dei Beni Culturali, entro le priorita di recupero e manutenzione del Patrimonio conosciuto, il Comune di Oschiri, in collaborazione con la Soprintendenza per i Beni Archeologici delle province di Sassari e Nuoro, ha realizzato un progetto per l’intervento di conservazione e restauro su una parte della struttura di fortificazione di un Castrum bizantino, ancora in fase di indagine e di studio.