Presenter: Manon Mastin (IFREMER)

Description:
Submarine volcanic activity releases large amounts of gas and metals in the water column, affecting biogeochemical cycles and ecosystems at a regional and local scale. In 2018, a submarine volcano erupted ~50 km offshore Mayotte Island and an active eruptive plume was observed in May 2019 at the summit with acoustic plumes rising 2 km into the water column coupled to strong geochemical anomalies. Between May 2019 and October 2020, 3 cruises monitored the eruptive activity above the volcano, focusing on water column geochemistry using noble gas, volatile species and dissolved metals concentrations. Beginning of the eruption, concentrations of >800 nM in CH₄ and H₂ were coupled with TdMn and TdFe concentrations above 500 nM, and CO₂ values of 35 µM. In addition, strong water column acidification (1upH) was measured compared to the regional background. From May 2019 to October 2020, we observed a general decrease in gas and metals concentrations, and an evolution of the TdMn:TdFe ratios consistent with a decrease in the eruptive activity at the volcano, similar to literature values observed in other volcanic contexts. In October 2020, high H₂ concentrations were consistent with ship-echo sounder surveys showing new lava flow installations and pictures of active lava flows at the seafloor. Correlation between He, CO₂ and CH₄ concentrations demonstrates magmatic degassing origin, but a d¹³C-CH₄ value of -34‰ suggests a biogenic contribution likely due to magma/sediments interaction during its ascent. Weak correlations between H₂ and He suggest complex processes of H₂ from magmatic degassing, lava/seawater interaction, and oxidation processes in water column. Water column acidification linked to the eruption seems to be associated with a release of silicates in the water column suggesting rock water interaction. After a year, the carbonate system seems to be back to steady-state highlighting the buffer capacity and resilience of the seawater column environment.