Ce document présente les opérations menées et recense les prélèvements effectués durant les campagnes MoMARSAT 2024.

The MoMARSAT cruise series (https://doi.org/10.18142/130) ensures the annual maintenance of the EMSO-Azores observatory on the Lucky Strike vent field. This seabed observatory aims to acquire long time-series data (≥10 years) on hydrothermal, tectonic, volcanic processes and the associated ecosystems of an active hydrothermal field located on the Mid-Atlantic Ridge. EMSO-Azores is part of the European network EMSO ERIC (European Multidisciplinary Seafloor and water column Observatory - http://emso.eu/), supported in France by the Research Infrastructure (MESR) EMSO-FR which management is ensured by a collaboration Ifremer-CNRS.

The EMSO-Azores observatory at Lucky Strike has first been deployed in 2010. It is primarily directed at understanding a mid-ocean ridge black smoker hydrothermal system, from the magma chamber that supplies the heat, to the microhabitats that host the hydrothermal fauna, and to the hydrothermal plume in the water column. The maintenance procedure is performed every year and requires the use of a deep-sea submersible. It includes recovery of seafloor instruments and moorings, full onboard checkup and repairs of each component, and redeployment after reconditioning on board. In addition, we perform yearly sampling of fluids, rocks and fauna, and capture videos of vents and habitats that provide critical ground-truth information to instrumental time-series data. The Lucky Strike hydrothermal field is located in the center of one of the most volcanically active segment of the MAR.

In its actual configuration after the 2024 maintenance cruise, the observatory infrastructure is composed of two Sea Monitoring Nodes (SEAMON). The first node dedicated to large scale geophysical studies is moored in the center of the fossil lava lake characteristic of the Lucky Strike vent field. This node hosts an autonomous Ocean Bottom Seismometer (OBS) and a generic environmental measurement module (EGIM). The EGIM is equipped with a CTD, a pressure gauge (JPP), an optode, a turbidimeter, a hydrophone and an ADCP.

The coordination of maintenance operations and the initial data exploitation are handled by Pierre Marie Sarradin (Azores node manager) and Marjolaine Matabos. The management of on-board operations is coordinated this year by Marjolaine Matabos and Laurent Gautier. The data acquired during the Momarsat cruises are available on the portal http://www.emso-fr.org/fr/EMSO-Azores. The study area is part of Portugal's EEZ and is also a “Marine Protected Area” (OSPAR).

Bottom operations were carried out by the Remotely Operated Vehicle VICTOR6000 onboard the Research Vessel L’Atalante.

We took advantage of the ship transit from Horta to Lucky Strike and return to acquire MultiBeam Sonar data for the Instituto Hidrográfico (IH) in Portugal and contribute to the mapping effort around the Azores. The two mapping areas were defined in collaboration between Ifremer (PI Benoît Loubrieu), the IH Portugal mapping division and colleagues from the University of the Azores (contact Telmo Morato).

The second node, SEAMON East, is deployed at the base of the Tour Eiffel active edifice to study the links between faunal dynamics and physico-chemical factors. This node is composed of an Aanderaa optode and a Wetlab turbidimeter. An HD video camera equipped with 2 LED lights is located 10 meters away on the edifice. An array of 4 Ocean Bottom Hydrophones (Hydroctopus) is deployed south of the active edifice.

The MoMARSAT experimental design also includes autonomous instruments : 4 OBS, 7 autonomous current meters, a temperature probe array (29 autonomous probes deployed in smokers and diffusing areas), a sequential fluid sampler (DEAFS), 4 autonomous video cameras, 10 biological and microbiological colonization devices. The 2 autonomous pressure gauges (JPP) were retrieved this year for maintenance and replacement of their stand that appeared to be corroded. Site studies including video surveys, sampling (rocks, water and organisms), and monitoring of long-term colonization/perturbation experiments are also performed during the cruises. These measurements are also used to calibrate/validate the measurements made by the instrumental array.

This year, imagery surveys were conducted to map geological and biological features at the vent field scale as well as scientific gear and dead weights. These maps will be shared with the Azoreans government to inform marine protected area management (PROTECT project).