An aquatic view of magmatic CO2 degassing at Taal volcano, Philippines

 

On 9 April 2011, the Philippine Institute of Volcanology and Seismology (PHIVOLCS) raised the Alert Level status of Taal Volcano to Alert Level 2. The most alarming parameter detected was a spectacular increase in the rate of carbon dioxide emissions from the main crater lake (MCL) where CO2 fluxes have more than doubled between January and March 2011. The fact that lake temperatures did not increase suggest that the discharge rate of hydrothermal waters remains unchanged and that this CO2 increase is not related to a change in hydrodynamic processes within the hydrothermal system (i.e. opening of fracture) but has a deeper magmatic origin.

 

lake_taal_s taal_dem
The active vent complex (the volcano island) lies in the middle of a large (15X22 km) prehistoric caldera filled with Lake Taal. The main crater of the volcanic island contains a volcanic lake termed MCL with a volume of 41-45 millions m3. About 6,000 people are living on the volcano island.

 

MCL_sm
View of MCL lake from the East rim of the crater. The lake is slightly acidic with pH around 2.9 and a temperature (30°C) a few degrees warmer than the average ambient atmosphere. This lake seems to be very placid but look at the current underwater activity!! (scroll down to the next image).

 

 

 

 

echo_sample
Lake's floor degassing activity recorded on 24 January 2011. The vertical tracks above the bottom (brown-red) are trains of CO2 bubbles ("flares") rising to the surface. Colour scale in decibels (dB) reflects variable intensities of the backscattering of the sound wave by the gas bubbles which are roughly proportional to bubble density in the water column. Echograms recorded with a SIMRAD ES60 single beam sonar equipped with a dual frequency (50 and 200 kHz) transducer. Echograms processed with SonarData Echoview software. Horizontal grid spacing = 50 meters and vertical (depth) spacing = 10 meters. Field work carried by Alain Bernard (ULB), Corentin Caudron (ORB-ULB) and PHIVOLCS (ULB: Université Libre de Bruxelles, ORB: Observatoire Royal de Belgique).

 

 

co2all40

Evolution of CO2 fluxes (tons per day) as measured by PHIVOLCS in collaboration with ITER (Instituto Tecnolóo y de energí renovables, Spain), ULB (Université Libre de Bruxelles, Belgium) and EMSEV. Figure courtesy of Nemesio Perez (ITER). CO2 fluxes were measured with the "floating accumulation chamber" method (see Kelud for more details).

 

 

integration_s
Echosounder can be used to evaluate the activity of gas vents at the lake floor by measuring the backscattering intensity (Sv). Sv is roughly proportional to bubble density in the water column. In order to minimize the effect of dissolution of CO2 bubbles during their rise to the surface, Sv are measured in individual cells close to the lake's floor i.e. 2 meters above the bottom ( 50kHz data processed with Sonar 4 software).

 

 

dBandSv
Map of the degassing activity from the lake bottom obtained from the integration of 23 km of echosounder profiles covering the all lake (almost). Sv is the mean backscattering strength of the water column, (dB) are decibels. No degassing is observed for Sv < -70 dB. More than 50% of the lake's floor is degassing.

 

bathy_50 track50
Bathymetric map of MCL obtained from echosoundings, maximum deph = 71 meters. Echosounder tracks collected during January 26-30, 2011.

 

 

Examples of echosoundings. Vertical (depth) and horizontal grid spacings are respectively: 10m and 50m.
90_30 canevas90
87_30 87_15
98_6h10 98
101_2h30 101_can
92_2h16 92cane
96_3h06 96cane
91_7h04 91canev
91_6h54 91Bcane
90_6h31 90canev
80_3h40 80canevas
77_3h10 77canev

 

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