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Chronicle at sea: Exploring the ridges off the Azores at depths of over 2,000 metres

By Javier Escartin and Muriel Andreani

The bottom of the Earth’s oceans is less well known than the surface of the continents or Mars. However, it is 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, heat or magma among other things, which can ultimately have repercussions on the atmosphere.

These exchanges affect the chemistry of the oceans and can be associated with different types of ecosystems that are still being discovered today.

To better understand the topography of these submarine zones and their magmatic, tectonic and hydrothermal processes, we take you on an oceanographic campaign along the Rainbow submarine massif, on the Mid-Atlantic Ridge, south of the Azores.

36 scientists and 30 sailors, an autonomous submarine and a remotely operated robot are going to sea for a month to search for traces of low-temperature, diffuse hydrothermal activity over the whole massif, which would co-exist with the well-known high-temperature hydrothermal vents.

Ocean ridges, places of exchange between the deep Earth, the oceans and the atmosphere

Long considered uneventful steppes, the ocean floor attracted growing interest in the mid-20th century when the ridges, which cross the middle of the oceans over 65 000 kilometres, were identified as active volcanic zones, where life can develop in extreme conditions. The tectonic plates move apart at the ridges, causing magma or deep rocks to rise to the seafloor and form faults and topography. Seawater can then seep deep into the rock and interact chemically before rising to the surface under the effect of heat. Ridges are therefore an extraordinary place for the exchange of heat and matter between the deep Earth and the oceans, and then the atmosphere.

The most spectacular manifestations of these interactions are the hydrothermal vents that release high-temperature liquids and gases (up to 400°C) into the ocean. But other types of emanations, colder and more discreet, can also take place. They are thought to represent a non-negligible, although so far unestimated, part of thermal and elemental exchanges. Hydrothermal fluids, for example, are a source of iron to the ocean, an essential element for many organisms. In the form of methane or dihydrogen, the circulation of these fluid is a source of carbon and energy for various ecosystems. Identifying the nature, duration and spatial extent of these exchanges is fundamental to a better understanding of their impact on ocean chemistry.

How to explore at depths of over 2000 metres

To understand such a system, as geologists, we need a good knowledge of the relief (bathymetry), direct observations in the field, and rock and fluid samples. This implies going to sea in an oceanographic campaign, with a dedicated scientific vessel from the French oceanographic fleet and a whole series of on-board devices that provide direct or indirect access to the ocean floor.

Unlike the planetary surface, where the topography is known with a few centimetres of resolution – including on other planetary bodies such as Mars or the Moon – the topography of submerged land is difficult to map, as the water column prevents direct measurement of heights.

An initial estimate can be obtained using satellites (bathymetry by satellite altimetry) and models, but the resolution of around 100 metres is insufficient for a good analysis of geological processes. Direct measurement by sonar installed under the boats provides a better resolution (a few tens of metres) but such mapping represents less than 25% of the seabed and remains ineffective for the detection of specific structures of smaller dimensions, such as hydrothermal vents.

A first step in the mission to explore this terra incognita is therefore the deployment of an autonomous vehicle – our AUV “IdefX” – which travels around the study area to provide sonar mapping closer to the seabed (at around 70 metres) with 2 to 3 metres of resolution. It is also equipped with sensors to measure, water turbidity or seabed magnetism for example.

This mapping will then be used to select areas suitable for “underwater fieldwork” using a remote-controlled robot, “Victor”, which will be able to film and photograph the seabed, take rock or fluid samples and measure their temperature using its articulated arms controlled from the surface.

This campaign at sea is above all the result of a collaboration that we have been conducting for years, with two different and complementary approaches to underwater geological processes. Muriel is a mineralogist, interested in the chemical reactions between fluids and rocks based on the study of natural samples and laboratory experiments. Javier deciphers tectonic processes from the fine morphology of underwater reliefs. We submitted the scientific project in 2018. It was selected the same year and here we are, ready to leave, in May 2022.

Our main aim is to test the hypothesis of low-temperature hydrothermal activity, which we believe to be diffuse over the whole massif, and which would develop in parallel with the very localised high-temperature activity at the level of the known hydrothermal vents.

On board the Pourquoi Pas ?

On 5 May, our team of 24 scientists and 12 engineers joined the ship Pourquoi Pas ? and its 30 crew members from the French oceanic fleet. Between us, as co-mission leaders, we cover all the scientific needs on board, which allows for a more efficient management of the campaign, with the vagaries of the weather, technical problems, and the need to achieve the scientific objectives. Strategic choices also must be made, and our exchanges facilitate decision-making.

In human terms, we have tested our understanding and complementarities during previous missions. They are essential to lead all the participants towards a collective project and to constitute an involved and motivated team. The entire scientific team supports the definition of scientific strategies and objectives, in collaboration with the technical teams of the underwater vehicles and the ship’s staff, who make it all possible.

The transit to the “Rainbow” study area lasted two days. The weather was good, making it easier for the participants to get used to life on the water. Each crew member has left behind a slightly different personal and professional situation, with courses or exams to move and family lives to organise. Despite these complexities, the human and scientific adventure of a campaign at sea is such, since it leads to the discovery of terrain never before explored, that there are many repeat offenders among us. Some of us have been on more than fifteen campaigns at sea. Those work alongside colleagues who are discovering on-board operations for the first time with pleasure and astonishment. The more experienced crew members have been able to provide valuable advice to the novices during the preparation stages, 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 deck.

On arrival at the site, the team stands united and has organised the labs and scientific operations on board: measuring instruments are ready (battery recharging, calibration, parameterisation), the first dives based on the bathymetric maps are planned, the description of the rocks and their archiving are organized, images of the ocean floor already available for geolocalised 2D and 3D visualisation are processed.

We have been able to start the dives immediately, during the day for the autonomous vehicle, all night for the remote-controlled robot Victor. Within a few days, we have already been able to make some beautiful and unexpected discoveries. For example, we have detected the presence of an ancient high-temperature hydrothermal site: it is now extinct but very similar to one that is active today. We also observed spectacular fault planes and their association with fossil bivalve clusters. But the highlight of the first few days was the sighting of living bivalves around active zones from which low temperature fluids are escaping, which was unknown in this area.

Unfortunately, after a week and for two days, the vagaries of the weather forced us to suspend operations!

But it is impossible to get bored on board: we need to process the data we acquired, plan for the next steps, have scientific and strategic discussions… all in all, we are very busy while waiting for operations to resume – which you can follow in the next episode of our Chronicle at Sea.