We know less well the bottom of the terrestrial oceans than the surface of the continents or that of the planet Mars. It is however at the bottom of the seas, at the level of the ridges – where the tectonic plates move apart – that the most important exchanges take place between the deep Earth and the oceans, of heat or magma among others, which can ultimately affect the atmosphere.
These exchanges affect the chemistry of the oceans and can be associated with different types of ecosystems whose discovery continues to progress.
To better understand the topography of these underwater areas and their magmatic, tectonic and hydrothermal processes, we take you on an oceanographic campaign along the underwater massif. Rainbowon the Mid-Atlantic Ridge, south of the Azores.
36 scientists and 30 sailors, an autonomous submarine and a remote-controlled robot set off for a month at sea to look for traces of low-temperature hydrothermal activity diffused throughout the massif, which would co-exist with high-temperature hydrothermal vents which are well known.
The oceanic ridges, places of exchange between the deep Earth, the oceans, then the atmosphere
Long considered dreary steppes, the ocean floor aroused growing interest in the middle of the 20th century.e century when the ridges, these reliefs that crisscross the middle of the oceans for more than 65,000 kilometers, have been identified such as active volcanic zones, where oases of life develop in extreme conditions. The tectonic plates move apart at the level of the ridges, which causes a rise of magma or deep rocks on the ocean floor, and the formation of faults and reliefs. Seawater can then infiltrate deep into the rock and interact there chemically before rising to the surface under the effect of heat. The ridges are therefore an extraordinary place of exchange of heat and matter between the deep Earth and the oceans, then the atmosphere.
Ifremer, Provided by the author
The most spectacular manifestations of these interactions are hydrothermal vents which release high temperature liquids and gases (up to 400°C) into the ocean. But other types of emanations, more discreet, colder, can also take place and represent a non-negligible, still inestimated part of thermal and elementary exchanges. Hydrothermal fluids are, for example, a source of iron to the ocean, an essential element for many organisms. In the form of methane or dihydrogen, their circulation is a source of carbon and energy for various ecosystems. Identifying the nature, duration and spatial extent of these exchanges is fundamental for a better understanding of their impact on ocean chemistry.
How to explore more than 2000 meters deep
To apprehend such a system, as geologists, we need a good knowledge of the reliefs (bathymetry), direct observations on the ground, samples of rocks and fluids. Access to the land therefore requires an oceanographic campaign, with the provision of a dedicated vessel of the French oceanographic fleet and a whole series of on-board devices that provide direct or indirect access to the ocean floor.
Unlike the emerged planetary surfaces, where the topography is known with a few centimeters of resolution – including on other planetary bodies like Mars or the Moon, the topography of the submerged lands is difficult to map, the water column preventing a direct measurement heights.
Kevin Gill/GEBCO/Flickr, CC BY
A first estimate can be obtained using satellites (satellite altimetry bathymetry) and models but the resolution of regarding 100 meters is insufficient for a good analysis of geological processes. The direct sonar measurement installed under boats provides better resolution (a few tens of meters) but such mapping represents less than 25% of the seabed and remains ineffective in detecting specific structures such as hydrothermal vents.
A first step in the mission of exploring this terra incognita is therefore the deployment of an autonomous vehicle – our “AUV” IdefX – which scans the study area to provide sonar mapping closer to the bottom (at regarding 70 meters) with 2 to 3 meters of resolution. It can also be equipped with other sensors to measure, for example, the turbidity of the water or the magnetism of the ocean floor.
Benoît Ildefonse, Arc en Sub Campaign, Provided by the author
This mapping then makes it possible to select the areas suitable for “underwater terrain” via a remote-controlled robot, « Victor »which will be able to film and photograph the bottom, take samples of rocks or fluids while measuring their temperature thanks to its articulated arms controlled from the surface.
This countryside by the sea was above all born of a collaboration that we have been carrying out for years, with two different and complementary approaches to underwater geological processes. Muriel is a mineralogist, she is interested in the chemical reactions between fluids and rocks from the study of natural samples and laboratory experiments. Javier, for his part, deciphers the tectonic processes from the fine morphology of the underwater reliefs. We submitted the scientific project in 2018. It was selected the same year and here we are, ready to depart, in May 2022.
We are mainly looking to test the hypothesis of low-temperature hydrothermal activity, which we believe to be diffuse over the entire massif, and which would develop in parallel with the very localized high-temperature activity at the level of known hydrothermal vents.
On board of Why not ?
On May 5, our team of 24 scientists and 12 engineers joined the ship Why not ? and its 30 crew members from the French ocean fleet. Between us, co-heads of mission, we cover all the scientific needs on board, which allows more efficient management during the campaign, with the vagaries of the weather, the technical problems, and the need to achieve the objectives. scientists. Strategic choices must also be made, and our discussions facilitate decision-making.
Alexandre Schubnel, Arc en Sub Campaign, Provided by the author
Humanly, we tested our agreement and our complementarities during previous missions. They are essential to lead all the participants towards a collective project and to constitute an involved and motivated team. This entire scientific team, invited to participate during the preparatory phase, is of course in support of the organization and definition of scientific strategies and objectives, in collaboration with the technical teams of the underwater vehicles and the ship’s personnel, who make it all possible.
The transit to the “Rainbow” study area takes two days. The weather is mild, which allows participants to get “amarinated” more easily. Each crew member left a slightly different personal and professional situation on land, between lessons or exams to be moved and family life to be organised. Despite these complexities, the human and scientific adventure of a campaign at sea is such, since it leads to the discovery of land never explored to date, that there are many repeat offenders among us. Some have to date accumulated more than fifteen campaigns at sea, and rub shoulders with colleagues who discover the operations on board for the first time, with pleasure and astonishment. The most experienced were able to provide, from the preparation stages, valuable advice to these novices, such as the need to bring enough chocolates for the Victor’s exploration evenings or waterproof clothing for the use of the rock saw. on the bridge.
Benoît Ildefonse, Arc en Sub Campaign, Provided by the author
On arrival at the site, the team is now united and has organized the labs and scientific operations on board: preparation of the measuring instruments (recharging the batteries, calibration, configuration), planning of the first dives from the bathymetric maps , organization of rock description and archiving, processing of ocean floor images already available for geolocated 2D and 3D visualizations.
The dives were able to start immediately, during the day for the AUV, all night for the remote-controlled robot Victor. With them, we have already made beautiful unexpected discoveries. For example, we have detected the presence of an ancient high temperature hydrothermal site: it is now extinct but very similar to the one that is active today. We also observed spectacular fault planes and their association with clusters of fossil bivalves. But the great moment of discovery of these first days was the observation of living bivalves, around active zones from which escape fluids at low temperature, which was unknown in this zone.
Alas, following a week and for 2 days, the vagaries of the weather forced us to suspend operations!
But we are not lacking in activity on board: between the processing of the data already acquired, the planning of the suite, the scientific and strategic discussions, we are very busy while waiting for the resumption of operations – which you will be able to follow in the next episode. of our Chronicle at Sea.
