Carbon Dioxide in Taal Volcanic Lake: A Simple Gasometer for Volcano Monitoring

We report here an increase in the amount of carbon dioxide in Taal lake during the year preceding the January 2020 eruption. Starting in February 2019, the CO2 emissions from the lake increased from background value (700 t day−1) to a flux close to 2,400 t day−1 at the time of the eruption. We show that the lake acts as a highly sensitive gasometer where CO2 (aq) reflects the balance between CO2 supplied to the lake (by hot springs) and CO2 lost by diffusion at air‐water interface. The lake waters are extremely enriched in dissolved carbon dioxide with pCO2 values as high as 0.35 atm (350,000 ppmV) equivalent to a CO2 (aq) of 9.32 mmol l−1. The residence time of CO2 in the lake is around 1 week which allows for fast detection of change in magma degassing and makes carbon dioxide a very promising tool for volcano monitoring. Plain Language Summary Volcanic gas fuels volcanic eruptions. Among the dominant species, sulfur dioxide is easily monitored using remote sensing and ground‐based instruments. Carbon dioxide remains however challenging to detect and quantify due to its high background in the atmosphere. We have developed a new methodology for monitoring CO2 by using a volcanic lake in an innovative way. It allows to quantify CO2 emissions by the lake, thereby providing critical insights into the change in degassing magma at depth. Using this new methodology, we were able to detect the earlier precursory signs of the January 2020 eruption of Taal volcano in the Philippines which affected over 380,000 people from local communities. The Taal volcanic lake is a unique location to study CO2 emission with spectacular degassing from more than 500 subaqueous vents that were active for many years. The lake waters are extremely enriched in carbon dioxide, to our knowledge, the highest value ever recorded in a lake. Our method also provides the longest time series to date of continuous CO2 emission from a volcano. Key Points pCO2 in a volcanic lake reflects the balance between magmatic CO2 supplied by hot springs and CO2 lost by diffusion to the atmosphere The continuous monitoring of CO2 flux by the volcanic lake is achieved using in situ measurement of lake pCO2 with a unique sensor The increase in pCO2 in Taal volcanic lake is a precursory sign of the January 2020 eruption