Charcoal reflectance as a proxy for the emplacement temperature of pyroclastic flow deposits

Abstract

Hot pyroclastic flow deposits often entomb and preserve vegetation as charcoal. When studied in polished section, this charcoal is highly reflective. Novel data from experimentally charred woods demonstrate that reflectance increases with both temperature and time. At temperatures above 250 °C, reflectance rises rapidly for the first hour, effectively stabilizing after 4 h for temperatures below 400 °C. However, for higher temperatures, reflectance only stabilizes after ∼24 h. Charcoalified woods from block and ash flow deposits generated by the Soufriere Hills volcano in Montserrat have yielded interpreted deposit temperatures, based on reflectance data, of 325–525 °C. These temperatures compare favorably with directly measured data (365–640 °C) from the same deposits, indicating that charcoal reflectance is useful as a temperature proxy for ancient pyroclastic flow deposits, particularly where magnetic or mineral data are absent for this purpose. However, caution should be exercised, as only a minimum temperature can be inferred where the heating duration of the deposit is unknown. Unlike magnetic or mineral data, temperature data from charcoalified woods can be obtained from reworked deposits, providing a valuable means of validating observations made about the style of eruption of volcanoes in ancient settings.