The JOIDES Resolution ship wanted to say goodbye in style. The vessel, launched in 1978 and which over the past four decades has become one of the most celebrated scientific drilling vessels, will cease to be supported by the National Science Foundation (NSF) in 2024. Prior to that retirement has nevertheless participated in a historical geological expedition. Not so much because of how deep she has drilled, but because of what she has achieved: a valuable sample of Earth’s mantle that geologists have dreamed of for decades.
The finding is relevant for as long as we’ve been chasing it.
And what it can tell us about our planet.
What has happened? That the researchers traveling aboard the JOIDES Resolution have achieved an exceptional sample of the Earth’s mantle. Drilling into the seabed of the Atlantic Ocean, they have drilled tubes and tubes of material from the upper mantle, mostly peridotite, an igneous plutonic rock. “It’s really unusual: more than a kilometer of heavily altered lower crust and/or upper mantle. I’d say a mix,” Andrew Fisher of the University of California Santa Cruz explains to The Washington Post. In the scientific community there are those who already see the extraction as a valuable “record”.
Is it a Guinness piercing? The truth is that scientists have made more impressive holes. The Kola, the result of a Soviet experiment, reached a depth of 12.2km. And in 2021, a group of scientists announced a well that went 8 km below the surface of the Pacific — yes, taking advantage of a trench — and allowed them to take samples at 8,060 mbs (meters below sea level). If the JOIDES Resolution team has achieved such success now, it is not because of their expertise in drilling the seabed. What has really allowed them to win the jackpot is their accuracy when it comes to choosing where to work.
Their efforts have focused near the Mid-Atlantic Ridge, a mountain range that extends mainly under the waters of the Atlantic Ocean reaching a height of three kilometers above the ocean floor.
Is that important? Yes. What the scientists were looking for is a “tectonic window” in the North Atlantic where drills would not have to drill as far to reach what they were looking for. And the reason is simple: due to the movement of the oceanic plates, the mantle rocks have been pushed towards the surface.
If usually the Mohorovičić (Moho) discontinuity, the boundary between the earth’s crust and the mantle, is usually located around 35 kilometers below the thick continental crust, when we talk about the oceanic crust that distance is reduced to seven kilometers and it is still shallower if we refer specifically to the Mid-Atlantic Ridge.
And where has the prize been obtained? The JOIDES Resolution team collected their sample of Earth’s mantle rock about a mile deep on the North Atlantic ocean floor, at a seamount known as the Atlantis Massif, located in a “tectonic window” of the Mid-Atlantic Ridge. “On Earth, mantle rock is normally very difficult to access. The Atlantis Massif offers a rare advantage in accessing it, as it is composed of mantle rocks that have been moved closer to the surface through the ultra-slow expansion process of the seabed”, the experts explain.
The most curious thing is that the objective of the expedition was not to obtain a huge sample of mantle. As Science magazine details, the team was actually exploring Atlantis for clues to the origin of life, but relocated to a spot where shallow probes in 2015 had turned up what appeared to be heavily sea-altered mantle rocks. . “The drilling went magically well,” admits Andrew McCaig, a geologist at the University of Leeds.
And the reward has been…? A treasure in the form of rocks. Thanks to their aim and a drilling of 4,156 feet (1,267 m) the scientists achieved a sample of the Earth’s mantle, a core 3,280 feet long, which is equivalent to about one kilometer. “It’s an amazing section,” Fisher says. Technically they did not get to drill the mantle and their well was not a record, but there are those who already point out that the sample they obtained, more than a kilometer long arranged in cylinders, is. This is pointed out, for example, by the biogeochemist Susan Lang.
The achievement is not minor. After all, six decades ago, in 1961, a group of geologists launched an ocean expedition, known as Project Mohole, which aimed precisely to drill through the thinnest crust of the ocean floor to reach the mantle. What he actually did was inaugurate a series of frustrated projects that have coveted the “jackpot” that, some claim, is now held by the JOIDES crew. “We have achieved an ambition that has been fueling science communication for decades,” McCaig says.
How was the process? “Unusual”, in the words of the expedition itself, which acknowledges that although it had hoped to access the mantle rock “with relative ease” due to its location on the Atlantic ridge, the reality far exceeded any of its expectations. Even the most optimistic.
“Typical expeditions into similar rock formations produce cores about every three hours and typically have less than 50% recovery. This mission was unusual: cores were coming out every hour and recovery was in excess of 100% on many occasions. The drill was going through the rocks serpentinized from the mantle like butter, and the team reached a final depth of 1,267.8 mbsf (meters below the seabed) after about a month of drilling,” he says. Margot Godard goes further: “It is the dream place for geologists”.
What’s up now? How the sample has been reached or if it is more or less large is not what is really important about the finding. What is crucial is what it can tell us about the depths of our planet, how the oceanic crust differs in composition from the upper mantle or how magma melts, flows and separates. That’s what scientists hope to unravel with the JOIDES cylinders.
Another challenge, and no less, will also be to determine to what extent the rocks are representative of the mantle. At the end of the day —as The Washington Post slips— most of this crucial layer of the planet is under the crust, not exposed to the ocean and questions remain on the table about whether the material obtained by JOIDES will allow us to know the mantle or the lower crust. In any case, Lang stresses, “these are the types of rock we’ve been waiting for for a long time.”
Imágenes: Polar Fox (Flickr) y ANZIC – IODP (Twitter), JOIDES Resolution (Lesley anderson)
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