The carbon isotopic signature of CO2 gas is a useful tracer for magmatic sources in volcanic areas. Emplacement of a new body of magma at depth can cause changes in the CO2 isotope composition of volcanic gases. Traditionally, carbon isotopes have been measured at fumaroles, volcanic vents, and in soil gases to gauge these changes. A volcanic plume represents the bulk source of volcanic CO2 emitted from these sources, and is the best indication of the bulk isotope composition of an active volcanic system. However, plume measurements are difficult to make because of their low CO2 concentrations and extensive mixing with the atmosphere. With the development of portable cavity ring‑down spectrometers (CRDS), scientists are now able to make isotope measurements of low concentration gases in the field and in near real time to characterize the bulk isotope composition of a volcano. The relatively high concentration of CO2 in the plume at Turrialba makes it an ideal location for isotope measurements of CO2 in the field.
From 31 March to 6 April 2014, a team from McGill University accompanied by Maarten DeMoor from the National University (OVSICORI-UNA; Costa Rica) sampled plume gases from Turrialba volcano, Costa Rica, in plastic gas bags. The team brought a portable CRDS for isotopic CO2 to the field to sample and analyze isotopic CO2 the same day as collection. They sampled the volcanic plume from the west rim of the west crater, where the multi-gas instrument was installed as part of the DECADE project. CO2 concentrations in the plume ranged from 400 to 1000 ppm, and showed increased dilution farther from the crater rim.
The team also characterized other sources of CO2 for their isotopic composition, including fumaroles, the high temperature 2012 vent, soil gases, and the background atmosphere. These sources all contribute to the carbon budget of a volcano and influence the isotopic composition of the volcanic plume.
Carbon isotope measurements of the plume at Turrialba will lay the foundation for future work. These will be among the first measurements of a volcanic plume, serving to establish a method for determining the bulk isotope composition of a volcanic system. This work will also help establish procedures for measuring volcanic CO2 from plumes in real time, which will be advantageous for monitoring programs of active volcanic systems.
Report and images contributed by Kalina Malowany (McGill University)