Bowing
In 2023, researchers with the International Ocean Discovery Program (IODP) returned from a drilling expedition in the Atlantic with a notable prize: a tube of rock more than three quarters of a mile (1.2 kilometers) long containing material from Earth’s mantle.
“I was really impressed with how deep they got, how much recovery they were able to yield.”
Described in a new paper in Science, the tube, a core drilled from the ocean floor, is a long cross section of mantle rocks that reveals the chemical and physical processes happening deep beneath an active hydrothermal field, where scientists have little ability to visit or study. Researchers expect the core to fuel years of research into how mantle rocks rise and change, as well as how their interactions with the environment feed processes that may have kick-started life itself.
Though other holes drilled through the crust have gone deeper, none have retrieved as much rock from the mantle.
“This recent core is really exciting,” said Matthew Schrenk, a geomicrobiologist at Michigan State University who wasn’t involved with the research. “I was really impressed with how deep they got, how much recovery they were able to yield.”
The core owes its existence, in part, to sheer luck. The research team was aboard the JOIDES Resolution, the IODP’s venerable ocean drilling ship, as part of a mission to deepen an existing borehole.
“We didn’t actually plan to drill this deep” into mantle rocks, said Andrew McCaig, a geologist at the University of Leeds and a study coauthor. In fact, the team almost didn’t drill at all. Their first drill bit got stuck in a shallow pilot hole, forcing them to sever the line connecting it to the surface. Another effort, this time to place a reinforced concrete casing in another hole, broke a different drilling system.
“We basically used what I call the dartboard method in the end,” McCaig said, dropping the casing down the drill string to land smack-dab in the hole, half a mile (800 meters) beneath the ocean’s surface. From there, the drilling proceeded shockingly smoothly, allowing them to reach 4,160 feet (1.3 kilometers) beneath the seafloor and retrieve long sections of rock. They ended up preserving 71% of the core, significantly more than other cores drilled from the region.
“We stopped when we had to go back to the Azores,” McCaig said. “If we’d had another week, we would have carried on drilling.”
A Rocky History
We cannot usually drill into Earth’s mantle. Overwhelming pressure and heat at those depths make it difficult for a bit to reach beneath the crust. Most of the physical evidence we have from this region comes from ophiolites—bits of the mantle thrust to the surface along the edges of continental plates—and pieces of mantle dredged from the ocean floor.
“It’s like a grab bag of rocks that come up and you don’t know what their context was,” said Jessica Warren, a geologist at the University of Delaware who wasn’t involved with the research. “Drilling is one of our main ways that you can go in, and with that drill core you can see how one piece of rock relates to the other.”
The IODP expedition was drilling atop the Mid-Atlantic Ridge, a section of ocean floor where two tectonic plates are pulling apart at about an inch (2.5 centimeters) per year. That spreading allows deeper rocks from the mantle to rise: It’s one of the few places on Earth where this material is close to the surface, albeit under 2,790 feet (850 meters) of water.
The core tells a story of millions of years of geologic activity, beginning when melted rock from deep within Earth began rising. As it did, it became depleted of certain elements such as calcium and potassium, whereas other elements such as magnesium and silica remained. The rising rock, which is largely peridotite, a dense and coarse-grained rock that makes up much of the solid part of Earth’s mantle, eventually cooled.
The core sheds new light on exactly how the process of melting and cooling occurs. The researchers saw, for example, how different minerals were removed from the mantle materials as they rose, creating different types of rocks at various depths.
Other once-theoretical processes can be seen clearly in the core as well, including one known as focused melt transport. Horizontal veins of dunite and a densely interwoven mesh of gabbro are visible—some fractions of an inch wide, other dozens of feet. They reveal how later surges of melted rock moved through porous areas of the peridotite that function as channels, allowing material from below to reach the surface and form basalt outcrops.
“We’ve got a brilliant section through a net vein complex here within peridotite,” McCaig said. “We can look at the scale of these things, we can look at the compositional variations in the gabbro.”
Later, as the cooled peridotite was exposed to seawater percolating in from above, it underwent a chemical reaction known as serpentinization, creating serpentine and magnetite, along with heat, hydrogen, and methane. Though the existence of this reaction was already known, the core is giving researchers the chance to dig deeper into the serpentinization process and how it contributes to other processes on the ocean floor.
For example, in the area the core came from, serpentinization reactions help circulate warm, hydrogen-rich water back to the seafloor, where it powers the Lost City hydrothermal vent system, an eerie collection of spindly carbonate towers hosting a rich ecosystem of microbes and other sea creatures, all fed by the reactions happening below.
Digging Into the Plumbing
The core’s proximity to Lost City—the drill site was half a mile (800 meters) away—makes it exciting for not just geologists but also microbiologists and others interested in the origins of life.
“What we’ve basically sampled is the best thing we’ll ever get for the substrate of Lost City,” McCaig said.
By drilling through the “plumbing” of the hydrothermal vent system, the expedition is finally giving researchers a glimpse into the processes that created and sustain Lost City.
“Being able to see that deeper part of that system is really unique.”
The core gives, for the first time, information about how deep below the ocean floor serpentinization occurs and, from the kinds and amounts of different minerals found, exactly what other kinds of chemical reactions are happening. That information will allow researchers to shape theories about how organisms have adapted to these and other similar vents, which some scientists suggest may have been where life first emerged on Earth.
“Being able to see that deeper part of that system is really unique,” Warren said.
The new drill core will likely be delivering new findings for years to come on everything from the chemical reactions happening deep underground, to the movement of melt from the mantle, to the life cycles of microbes.
And it may be one of the last like it, at least for some time. The JOIDES Resolution, recently back from its final journey as the Unites States’ dedicated scientific drilling vessel, will no longer be funded by the National Science Foundation, restricting scientists’ access to new materials from deep below the seabed. Thankfully, for now at least, researchers have plenty of new rock to sift through.
—Nathaniel Scharping (@nathanielscharp), Science Writer
