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Arc-en-Sub oceanographic mission to the Azores

To the well-known tune of the Beatles' "Yellow Submarine", an international team of 21 scientists, including six from IPGP, set sail for the Azores aboard the oceanographic vessel "Pourquoi Pas?" as part of the Arc-en-Sub mission, from May 5 to June 2, 2022.

Arc-en-Sub oceanographic mission to the Azores

Publication date: 27/06/2022

Press, Research

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Marine Geosciences

The Arc-en-Sub mission, which runs from May 5th to June 2nd, 2022, is led by Javier Escartín (ENS Paris) and Muriel Andreani (Université Claude Bernard Lyon 1), and involves several members of IPGP’s marine geosciences team: Simon Besançon, Jérôme Dyment, Antoine Demont, Alex Hughes, Milena Marjanović and Valentine Puzenat.

The aim of the oceanographic campaign is to map the Rainbow hydrothermal massif using high-resolution bathymetry data, in order to identify active and fossil hydrothermal fields, characterize the nature of the volcanic and tectonic structures in the area, and understand the organization and evolution of hydrothermal activity on different time scales.

The many and varied operations carried out on board include the acquisition of bathymetry and magnetism data on the seafloor, in situ sampling and observation to better understand the regional geology, temperature measurements of hydrothermal fluids combined with autonomous pressure sensors and current meters, and the study of seafloor compliance to identify subsurface materials and provide information on the presence of fluids (e.g. hydrothermal systems, molten bodies).

Rainbow Massif Diagram

The Rainbow Massif

Nestled in a non-transforming discontinuity of the slow Mid-Atlantic Ridge, at 36°14’N, the Rainbow Massif is located near a tectonic peak that develops between the terminations of two ridge segments, where mantle peridotites outcrop along a large detachment fault. The massif hosts two types of hydrothermal systems: one active (Rainbow), and two inactive (Clamstone and Ghost City). A recent seismic survey revealed the presence of vertically superimposed reflectors, which have been interpreted as magmatic lenses located between 3 and 8 km below the seafloor.

Compliance measures

Compliance is the study of seabed movements under the effect of oceanic gravity waves. The amplitude of these movements is related to the mechanical properties of the subsurface, and enables us to characterize the materials that make up the oceanic crust. Compliance studies on the Rainbow Massif should constrain the presence of subsurface molten bodies, which could feed hydrothermal circulation and thus influence the overall alteration of the massif. These measurements require highly sensitive broadband seismometers (BBOBS) and pressure sensors available from the IPGP’s OBS park. During the expedition, five BBOBSs will be deployed, some once, others twice.

5 BBOS ready to be launched

Autonomous underwater vehicles

Autonomous Underwater Vehicles (AUVs) are underwater drones used to acquire high-resolution bathymetric data (~1m per pixel) from the seabed. During Arc-en-Sub, the IdefX AUV (IFREMER) is used to map the general morphology of the Rainbow Massif and identify major tectonic, volcanic and hydrothermal structures down to a water depth of 2850m. The bathymetric maps thus produced are also used to identify dive zones for the ROV Victor 6000 (IFREMER). IdefX is also equipped with a nephelometer that measures turbidity in the water column, helping the team to identify new sites of hydrothermal activity.

Remotely operated underwater robots

Remotely Operated Vehicles (ROVs) are directly controlled by engineers on board the ship. ROVs are equipped with a high-definition camera to transmit live video of the seabed, enabling scientists to identify seabed morphologies and collect samples using the robot’s arms. Arc-en-Sub uses IFREMER’s Victor 6000 ROV, which can reach a maximum depth of 6,000 m. In addition to cameras, the ROV can also be fitted with a range of geophysical equipment. For Arc-en-Sub, the ROV carries a vertical static camera for seabed photomosaics, and a magnetometer to measure magnetic anomalies and help locate new hydrothermal sites.

ROV Victor 6000 (IFREMER).
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