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Pyroclastic density currents (PDCs) can have devastating impacts on urban settlements, due to their dynamic pressure and high temperatures. Our degree of understanding of the interplay between these hot currents and the affected infrastructures is thus fundamental not only to implement our strategies for risk reduction, but also to better understand PDC dynamics. We studied the temperature of emplacement of PDC deposits that destroyed and buried the Villa dei Papiri, an aristocratic Roman edifice located just outside the Herculaneum city, during the AD79 plinian eruption of Mt Vesuvius (Italy) by using the thermal remanent magnetization of embedded lithic clasts. The PDC deposits around and inside the Villa show substantial internal thermal disequilibrium. In areas affected by convective mixing with surface water or with collapsed walls, temperatures average at around 270 °C (min 190 °C, max 300 °C). Where the deposits show no evidence of mixing with external material, the temperature is much higher, averaging at 350 °C (min 300 °C; max 440 °C). Numerical simulations and comparison with temperatures retrieved at the very same sites from the reflectance of charcoal fragments indicate that such thermal disequilibrium can be maintained inside the PDC deposit for time-scales well over 24 hours, i.e. the acquisition time of deposit temperatures for common proxies. We reconstructed in detail the history of the progressive destruction and burial of Villa dei Papiri and infer that the rather homogeneous highest deposit temperatures (average 350 °C) were carried by the ash-sized fraction in thermal equilibrium with the fluid phase of the incoming PDCs. These temperatures can be lowered on short time- (less than hours) and length-scales (meters to tens of meters) only where convective mixing with external materials or fluids occurs. By contrast, where the Villa walls remained standing the thermal exchange was only conductive and very slow, i.e. negligible at 50 cm distance from contact after 24 hours. We then argue that the state of conservation of materials buried by PDC deposits largely depends on the style of the thermal interactions. Here we also suggest that PDC deposit temperatures are excellent proxies for the temperatures of basal parts of PDCs close to their depositional boundary layer. This general conclusion stresses the importance of mapping of deposit temperatures for the understanding of thermal processes associated with PDC flow dynamics and during their interaction with the affected environment. [ABSTRACT FROM AUTHOR]

Determining the deposit temperature of pyroclastic density currents (PDCs) is a first order priority in the assessment of volcanic hazard. For past eruptions, in which no direct measurements are possible, PDC deposit temperatures have been calculated mainly using magnetic proxies. A more recent and innovative approach consists of the determination of the degree of charring of preserved wood fragments by optical analysis. In this study, we perform optical analyses on wooden beams belonging to the Villa dei Papiri at the archeological site of Herculaneum, which was destroyed by PDCs during the 79 AD eruption of Vesuvius. The new temperature data from this study range between 240 °C and 370 °C, and are in good agreement with literature data for this eruption. The large T variation measured can be interpreted as due to the interaction of the PDCs with the local environment, i.e. the impacted buildings of the villa and the sea shoreline. The study of in situ charred wood allows to constrain the charring kinetics and to evaluate the sensitivity of the reflectance proxy in recording PDC depositional features. We demonstrate that the charcoal reflectance is an excellent proxy for the determination of the PDC deposit temperatures even at a local scale, and it can be used in conjunction with, or in alternative to other more traditional methodologies, especially in case of pre-heating effects which may alter the T determinations by thermoremanent magnetization. [ABSTRACT FROM AUTHOR]