79AD Vesuvius PDC deposits' temperatures inferred from optical analysis on woods charred in-situ in the Villa dei Papiri at Herculaneum (Italy)

Abstract

Determining the deposit temperature of pyroclastic density currents (PDCs) is a first order priority in the assessment of volcanic hazard. For past eruptions, inwhich 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 effectswhich may alter the T determinations by thermoremanent magnetization.