High-Frequency Gas Monitoring Reveals Carbon Dioxide Precursor to Eruptions at Turrialba Volcano

Turrialba volcano has deposited ash on the capital city of Costa Rica and its 3 million inhabitants numerous times since 2014.

In a new article in the Journal of Geophysical Research and an online Earthchem database, a DCO-DECADE team led by Maarten de Moor (National University, Costa Rica) and Alessandro Aiuppa (Palermo University, Italy) tracked changes in gas composition and flux from 2014 to present [1,2]. The near-continuous and high-frequency gas monitoring time series (Multi-GAS and scanning DOAS stations) reveal a volcano in a state of extreme turmoil, posing an increasing threat to local lives and livelihoods.

During the monitoring time period the deployed instruments recorded large changes in gas composition and flux. Carbon dioxide to sulfur ratios show significant variations with notable peaks that occur weeks to days before eruptive episodes. The precursory spikes in carbon dioxide are the result of pulses of deep magma injected into the volcanic system. These rising magma bodies are directly responsible for the timing and magnitude of Turrialba’s eruptive periods. The hydrogen sulfide to sulfur dioxide ratio also displays remarkable changes over two orders of magnitude during the monitoring period, from constituting a major component of the bulk gas early in the time series to being undetectable by the Multi-GAS station. The disappearance of detectable H2S in the gas emissions indicates the progressive boiling off of what must be an enormous hydrothermal system as magma intrusions invade the volcanic edifice, and a transition to purely magmatic gas compositions.

Maarten de Moor sampling gases for sulfur isotopes in early 2014 from the 2012 vent (T ~800°C), before it was destroyed during the collapse of the western crater wall on 29 October 2014. Credit: Geoffroy Avard

Perhaps most interesting for understanding the deep carbon cycle, the authors’ estimations of CO2 flux from Turrialba suggest the most voluminous release of carbon dioxide did not occur at the same time as magma intrusion. Rather, they saw the release of huge amounts of CO2 well after the first magma intrusion event, an observation also associated with high H2S emissions. This suggests that a major proportion of CO2 released over the three-year period was actually hydrothermal in origin, with deeply derived CO2 stored in the hydrothermal system for an indeterminate amount of time and then released into the atmosphere during energetic explosions.

Decompression degassing modelling and analysis of the CO2-SO2-H2S-H2O system allowed the authors to estimate magma depth. The long time series of SO2 emission rate allowed them to calculate the total volume of magma intruded since the beginning of Turrialba’s unrest. Importantly, the large variations in both gas composition and CO2 flux highlight the need for continuous gas monitoring to define the carbon budget of convergent margins. These results once again show the potential power of high-frequency gas monitoring for forecasting eruptions.

Vladimir Conde (Chalmers University of Technology, Sweden) fine-tuning a scanning DOAS station during the eruption in May 2015. Credit: Maarten de Moor

A short demonstration of the changes in carbon dioxide to sulfur dioxide gas ratios that occur before, during and after eruptions. Note the footage under the data is for demonstration purposes only and is not from the volcano during the same time frame. Download this video (5.1 MB)

 

Contributed by Maarten de Moor (Observatorio Vulcanológico y Sismológico de Costa Rica, Universidad Nacional, Costa Rica) and Alessandro Aiuppa (Università degli Studi di Palermo, Italy)

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