[LUM#18] Beneath the Oceans, the Earth
What if the secrets of Earth’s formation lay beneath the sea? Last May, geologist Benoît Ildefonse took part in the Arc-en-Sub expedition organized by the French oceanographic fleet. Their destination was Rainbow, an underwater dome formed on the Mid-Atlantic Ridge, the dynamics of which still elude researchers.
“In marine geology, we study the oceans—but without the water—because what interests us is what lies beneath.” And to delve once againinto“what lies beneath,” Benoît Ildefonse took part in the Arc-en-Sub expedition last May. Twenty-six days at sea aboard the Pourquoi pas?, one of the four ships in the French oceanographic fleet.“There were about twenty of us scientists on board: geophysicists, petrologists, people interested in rocks, others in earthquakes—but only geologists,” says the director of Géoscience Montpellier, who has lost count of the number of ocean expeditions he’s undertaken since the very first one in 1997.
And for this mission, we’re heading to the Rainbow Ridge, an underwater dome rising 1,500 meters below sea level, located on the Mid-Atlantic Ridge two days’ sail west of the Azores. “What we call oceanic ridges are essentially volcanic mountain ranges 60,000 kilometers long, located at depths of approximately 1,000 to 4,000 meters, and corresponding to the boundaries of tectonic plates , ” explains the geologist. These plates can collide or drift apart at speeds ranging from one centimeter per year for the slowest, such as the Mid-Atlantic Ridge, to fifteen centimeters per year for the fastest, in the eastern Pacific.
A factory for the Earth's crust
Acting as veritable factories for creating the Earth’s crust thanks to the magma escaping from their faults,mid-ocean ridges are also zones of intense hydrothermal activity.“Cold ocean water seeps into the faults and, upon contact with the magma, heats up to several hundred degrees, ”continues Benoît Ildefonse. “When this water—which, incidentally, has become enriched with a number of chemical elements—reaches supercritical conditions, it rises and emerges in the form of plumes. These plumes, upon contact with colder seawater of a different chemical composition, precipitate minerals such as manganese or iron sulfides.”
In addition to the unique biodiversity they support and the deposits of precious minerals they contain, these mid-ocean ridges play a vital role in cooling the planet.“It is estimated that the entire volume of the ocean is recycled through this circulation more than 100 times over 80 million years. In a single day, that amounts to more than 1 million Olympic-sized swimming pools or 45 Lake Salagous.”
Although the Rainbow site has been known for 25 years, scientists have not yet fully understood how it works, which is why this latest mission was launched.“The site’s topography is more typical of a volcanically inactive area, yet we observe hydrothermal activity that implies the presence of localized pockets of magma beneath the surface; however, we do not know exactly where they are located,”the researcher summarizes. Another factor: scientists discovered, near this high-temperature site, an older, low-temperature site with different types of rock. “This indicates geographical and temporal variability within this system, and this expedition aimed to explore that complexity.”
Change of scale
To achieve this, geologists retrieved more than 320 rock samples from the depths of the sea and recorded nearly 200 hours of video. Victor, a remotely operated submersible, and IdefX, an autonomous underwater vehicle, were also used to create a micro-bathymetric map of the area with centimeter-level resolution, as well as a systematic photographic record of the seafloor. “This detailed mapping has revealed extraordinary things to us. Analyzing the data and samples will now allow us to place all of this within a comprehensive integrated framework linking magmatism, hydrothermal activity, and tectonics,” says Benoît Ildefonse.
Since returning from their expedition, the scientists have been analyzing the samples through the lens of their respective specialties: chemical composition and variability, mineralogical changes linked to reactions with hydrothermal fluids… The geologist from Montpellier, for his part, is focusing on their deformation by measuring the orientation of the crystals using an electron microscope on 30-micron-thick slides.“These microstructures provide clues about the temperature at which these rocks were deformed,” explains Benoît Ildefonse, before concluding: “In geology, we very often work on a very small scale to understand dynamics that operate on a very large scale.”
At Sea with the FOF
“ “The French Oceanographic Fleet (FOF) is a tool I know well, ” explains Benoît Ildefonse, director of Géoscience Montpellier. “In the French scientific landscape, it’s what we call a major research infrastructure, like the Soleil synchrotron or certain telescopes.” It was in 2000 that he first boarded the Atalante, one of the FOF’s four ocean-going vessels, which operate under the direction ofIfremer, the CNRS, theIRD, and the network of French marine universities to which the University of Montpellier belongs. Benoît Ildefonse has chaired the FOF’s National Commission for the Offshore Fleet for the past four years and, in this capacity, participates in the selection and evaluation of the various research campaigns conducted. “I love being at sea. I love the routine that settles in, working in shifts, especially the 4/8 shift (4 a.m.–8 a.m./4 p.m.–8 p.m.) because you get to see the sunrises and sunsets. Sea campaigns are always extraordinary human adventures.”
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