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.
Highlights
Pyroclastic density currents (PDCs) are among the most devastating and least predictable volcanic phenomena on Earth, as shown by recent catastrophic events like the 2006 and 2010 eruptions of Merapi volcano, Indonesia [1,2,3], the 1999 and 2005 eruptions of Volcàn de Colima, Mexico [4,5,6,7] and the 2015 eruption of Calbuco [8,9]
Structural changes in charcoals can be detected by different chemical and physical methods including: elemental analysis, molecular markers, optical analyses, pyrolysis–gas chromatography-mass spectrometry (Py–GC–MS), solid state 13C nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, Raman spectroscopy, near edge X-ray absorption fine-structure spectroscopy (NEXAFS), X-ray diffraction or high resolution transmission electron microscopy (HRTEM)
Wood samples from Colima and Calbuco volcanoes are mainly constituted by gymnosperm group, while both angiosperms and gymnosperms families have been recognized in samples from Fogo Volcano. This is consistent with the presence of pine and spruce trees in Colima and Calbuco volcanoes, while at Fogo volcano it is likely that conifer trees were embedded in the PDCs together with laurel trees [21]
Summary
Pyroclastic density currents (PDCs) are among the most devastating and least predictable volcanic phenomena on Earth, as shown by recent catastrophic events like the 2006 and 2010 eruptions of Merapi volcano, Indonesia [1,2,3], the 1999 and 2005 eruptions of Volcàn de Colima, Mexico [4,5,6,7] and the 2015 eruption of Calbuco [8,9]. Studying the changes of the lignocellulosic structures in the original raw wood into the highly stable, condensed, polyaromatic configurations of the charcoal can provide information on the maximum temperatures attained during combustion (e.g., wildfire) or pyrolysis. Only Scott and Glaspool [19] planned calorimeter experiments to reproduce pyrolysis conditions of wood fragments embedded in pyroclastic flow deposits and were able to correlate Rc% with emplacement temperatures. Charcoal reflectance analysis proved to be an effective, comparable and accurate geothermometer in numerous case studies and different volcanic deposit types [3,6,7,19,20,21], but despite its wide application, this technique implies time consumption in sample’s preparation and analyses processing. To have an appropriate and representative statistical analysis, prolonged time and a skilled operator are required, since the procedure is not automatic but linked to the operator’s data interpretation
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