A new hazard scenario at Vesuvius: deadly thermal impact of detached ash cloud surges in 79CE at Herculaneum.

Diluted pyroclastic density currents are capable to cause huge devastation and mortality around volcanoes, and temperature is a crucial parameter in assessing their lethal power. Reflectance analysis on carbonized wood from ancient Herculaneum allowed a new reconstruction of the thermal events that affected buildings and humans during the 79CE Vesuvius eruption. Here we show that the first PDC entered the town was a short-lived, ash cloud surge, with temperatures of 555-495 °C, capable of causing instant death of people, while leaving only a few decimeters of ash on ground, which we interpret as detached from high concentration currents. The subsequent pyroclastic currents that progressively buried the town were mostly higher concentration PDCs at lower temperatures, between 465 and 390 and 350-315 °C. Charcoal proved to be the only proxy capable of recording multiple, ephemeral extreme thermal events, thus revealing for the first time the real thermal impact of the 79CE eruption. The lethal impact documented for diluted PDC produced during ancient and recent volcanic eruptions suggests that such hazard deserves greater consideration at Vesuvius and elsewhere, especially the underestimated hazard associated with hot detached ash cloud surges, which, though short lived, may expose buildings to severe heat damages and people to death.

Preservation of neurons in an AD 79 vitrified human brain.

Detecting the ultrastructure of brain tissue in human archaeological remains is a rare event that can offer unique insights into the structure of the ancient central nervous system (CNS). Yet ancient brains reported in the literature show only poor preservation of neuronal structures. Using scanning electron microscopy (SEM) and advanced image processing tools, we describe the direct visualization of neuronal tissue in vitrified brain and spinal cord remains which we discovered in a male victim of the AD 79 eruption in Herculaneum. We show exceptionally well preserved ancient neurons from different regions of the human CNS at unprecedented resolution. This tissue typically consists of organic matter, as detected using energy-dispersive X-ray spectroscopy. By means of a self-developed neural image processing network, we also show specific details of the neuronal nanomorphology, like the typical myelin periodicity evidenced in the brain axons. The perfect state of preservation of these structures is due to the unique process of vitrification which occurred at Herculaneum. The discovery of proteins whose genes are expressed in the different region of the human adult brain further agree with the neuronal origin of the unusual archaeological find. The conversion of human tissue into glass is the result of sudden exposure to scorching volcanic ash and the concomitant rapid drop in temperature. The eruptive-induced process of natural vitrification, locking the cellular structure of the CNS, allowed us to study possibly the best known example in archaeology of extraordinarily well-preserved human neuronal tissue from the brain and spinal cord.