Calibrating Carbonization Temperatures of Wood Fragments Embedded within Pyroclastic Density Currents through Raman Spectroscopy

Abstract

The study of the structural order of charcoals embedded in pyroclastic density currents provides information on their emplacement temperature during volcanic eruptions. In the present work, a set of charcoals from three distinct pyroclastic density currents deposits whose temperatures have been previously estimated by charcoal reflectance analyses to lie between 250 °C and 550 °C, was studied by means of Raman spectroscopy. The analyses reveal a very disordered structural ordering of the charcoals, similar to kerogen matured under diagenetic conditions. Changes in Raman spectra at increasing temperatures reflect depolymerization and an increase of aromaticity and can be expressed by parameters derived from a simplified fitting method. Based on this approach, a second order polynomial regression with a high degree of correlation and a minimum error was derived to predict paleotemperatures of pyroclastic deposits. Our results show that Raman spectroscopy can provide a reliable and powerful tool for volcanological studies and volcanic hazard assessment given its advantage of minimum samples preparation, rapid acquisition processes and high precision.

Towards a kerogen-to-graphite kinetic model by means of Raman spectroscopy

“…Raman spectroscopy on wood-derived charcoals has been used to assess charring temperatures with implications in paleo-wildfires for paleoecological and volcanological studies (Ascough et al, 2010;Mauquoy et al, 2020;Theurer et al, 2021Theurer et al, , 2022Schito et al, 2022), or to evaluate coal and wood reactivity for metallurgical application (Urban et al, 2003;Paris et al, 2005;Zickler et al, 2006;Chabalala et al, 2011;Morga, 2011;Surup et al, 2019) (Table 4). Studies dealing with other fuel types (i.e., biomass char) or on the effect of catalysts on char reactivity at very high temperatures (from 1500 to 2700 • C) are not considered.…”

“…Samples from Schito et al (2022) derive from charred plant material entombed in pyroclastic sediments and the maximum temperatures experienced are calculated from charcoal reflectance (Scott and Glasspool, 2005;Pensa et al, 2018) or from partial thermal remanent magnetization (pTRM; Pensa et al, 2015).…”

“…The application to charcoal geothermometry is probably the most recent advance, and has shown attractive potential application in archaeological, paleo-environmental and geological studies (Deldicque and Rouzaud, 2020;Mauquoy et al, 2020;Theurer et al, 2021Theurer et al, , 2022Schito et al, 2022) as well as for industrial application (Surup et al, 2019). Such large spectrum of applications results in a wide range of temperatures used in calibration experiments, up to almost 2000 • C. Most of the results shown in Fig.…”

“…In other words, the research has been mainly focused on the evolution of Raman data under low heating rates and is not clear how different heating (and pressure) conditions can affect it. Some studies point out a mismatch between Raman parameters and vitrinite reflectance evolution under natural and artificial maturation conditions (Hackley and Lünsdorf, 2018;Khatibi et al, 2019;Hao et al, 2019;Hackley et al, 2022;Sanders et al, 2022), but this has not been verified in natural settings or under extreme conditions such as frictional heating or wildfires (Kitamura et al, 2012;Hirono et al, 2015;Deldicque et al, 2016;Kaneki et al, 2016;Ito et al, 2017;Theurer et al, 2021;Schito et al, 2022).…”

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

“…The measure of the re ectance is a very robust and experimentally well constrained proxy for temperature at which the charcoal formed 38,39,50,52 . This method has been widely applied as geothermometer in different carboni cation contexts: burial diagenetic environment, where organic material carbonizes slowly 44,53,54 , wild bush res 49,55,56 , natural and anthropogenic res 42,57 volcanic deposits 10,22,23,[36][37][38]40,41,58 .…”