An underwater glider to explore the depths of Mayotte

As part of a CIFRE thesis directed by Louis Géli (Ifremer) and supervised by Emmanuel Rinnert, Carla Scalabrin, Pascale Lherminier (Ifremer) and Félix Margirier (ALSEAMAR), Alexandre Heumann is studying the dynamics of magmatic plumes in the water column off Mayotte. The fruit of a partnership between ALSEAMAR and the GEO-OCEAN research unit (Ifremer), this project is based on the development of an innovative underwater glider to track volcanic fluid emissions in the water column at depths of up to 1,250 m in an active volcanic zone.

The volcanic context of Mayotte.

Mayotte, a French overseas department located in the Comoros archipelago, was the victim of a seismo-volcanic crisis unprecedented in its intensity and duration in May 2018. More than 11,000 earthquakes were recorded, resulting in significant surface displacements. These events led to the birth of Fani Maoré, an 800-meter-high underwater volcano located 50 km off the coast of Mauritania at a depth of 3,500 meters. With an estimated magma volume of 6.6 km³, it is one of the largest undersea eruptions documented to date.

A strategic study site: the Fer à Cheval.

Since 2019, the Réseau de Surveillance Volcanologique et Sismologique de Mayotte (REVOSIMA) has been organizing oceanographic campaigns (MAYOBS) to better understand this underwater activity. These explorations have led to the discovery of the Fer à Cheval volcanic structure, an area rich in acoustic plumes revealing the presence of liquid gas droplets, as well as geochemical anomalies in the water column. This site quickly became a scientific priority, with a continuous need for observations.
 

An underwater glider for in-depth measurements.

To explore this complex area, an underwater glider developed by ALSEAMAR has been deployed in 2021. Capable of acquiring oceanographic data (conductivity, temperature, pressure) and information specific to the monitoring of volcanic emissions (carbon dioxide, dissolved methane), this underwater glider sails to a depth of 1,250 meters. Transects are carried out across the Fer à Cheval area, lasting an average of 9 hours. This radial navigation strategy enables intensive sampling of the zone of interest, with maximum measurement density in its center. The long-term series now totals over 2.5 years of data. Acquisitions will continue until 2025.
 

The scientific challenges of this CIFRE thesis.

Alexandre Heumann, a CIFRE doctoral student supervised by Louis Géli (Ifremer) and supervised by Emmanuel Rinnert, Carla Scalabrin, Pascale Lherminier (Ifremer) and Félix Margirier (ALSEAMAR), is working to understand the physico-chemical structure of magmatic plumes and their spatio-temporal dynamics. This work includes the development of on-board sensors on the glider and the extension of data acquisition capabilities beyond 1,250 meters depth.
 

Promising initial results.

Since 2021, data collected in the study area have been published on the SEANOE platform and a scientific article has been submitted for publication. A further step towards knowledge and understanding of the Fani Maoré volcanic system.