Deep Dive into Research
Scholars from the University of Tennessee, Knoxville, joined the University of Texas at Dallas, principal investigator on the project funded by the National Science Foundation, along with scholars from the University of California at San Diego, Woods Hole Oceanographic Institution, Queen’s University (Canada), the University of Lorraine (France), Kochi University (Japan), the Geological Survey of Japan, and the Japan Agency for Marine Earth Science at Nagoya University.
In late 2024, three geoscience graduate students and a professor sailed the Pacific Ocean to explore the origins of Earth. Rock samples they helped gather from the Mariana Trench as part of an international research voyage are being analyzed at the University of Tennessee, Knoxville.
The deepest part of the ocean, the Mariana Trench, was formed by the movement of one tectonic plate below another. “It’s one of these interfaces between the surface and the interior of the planet. We can actually sample the same material that goes all the way down to the core-mantle boundary,” explained Nick Dygert, the Larry and Dawn Taylor Associate Professor of Planetary Geosciences in the Department of Earth, Environmental, and Planetary Sciences (EEPS).
“To be able to study things like the flux of volatile elements into the interior in a place like this is really special,” he said. “It’s like a natural lab.”
Dygert was invited to join a proposal for National Science Foundation funding, which led to this collaboration with universities in Texas, California, Canada, France, and Japan.
The project directly relates to the doctoral work of Emily Etheridge, who received NSF support as a graduate research assistant, and with support from the EEPS Student Success Fund doctoral candidates Anah Bogdan and George Denny also were able to make the journey to a place most learn about in class but few have seen.
“We were exploring,” Denny said. “That’s a really special experience to still be somewhere where you can explore, where there’s still unknowns.”
“It almost feels like an alien world” said Bogdan, who had pictured in her mind a sheer wall and a clear image of the mantle cropping out of the crust. “It looked entirely different than what we expected.”
Art and Science
The Mariana Trench is more than 1,500 miles long, and the researchers hoped to sample 16 sites within the Challenger Deep section, which extends nearly seven miles below sea level.
They set off from Guam aboard the research vessel Thomas G. Thompson, with a remotely operated vehicle system called Jason and Medea. Every dive of Jason to the sea floor took about four hours each way, so the researchers served on two teams with 12-hour shifts.
The UT team members started their day at noon, working alongside a group that ranged from undergraduates to senior scientists. Controlling a remote camera system from aboard the ship, they would decide which samples to collect with a robotic arm, maximizing what they could gather with limited capacity to bring samples to the surface.
Because they could not cut into an outcrop, they would have to select samples that likely had fallen from it.
“Beyond that, there’s a little bit of art and science to just trying to figure out how many pieces we want, what looks the best. What if the sample is starting to crumble, do we bail and pick a new one?” Denny explained. With the movement of the boat, they also had to make decisions quickly.
After three weeks at sea, they returned to Knoxville in mid-December with approximately 150 fist-sized samples.
“We were specifically looking for gabbros and peridotites,” said Etheridge. They come from the lower crust and upper mantle, regions that are difficult to study directly. As part of the research, she will apply several geothermometers, a method for estimating the conditions under which rocks formed, including one she is developing.
“That can tell us more about the broader Mariana Trench as a whole, how that formed, and how that evolved over time,” she said.
Equipped for Analysis
UT has all the tools the researchers need for further analysis. A scanning electron microscope will map the elements within the rocks, and an electron microprobe will provide more quantitative major elemental information. They will also use the new EEPS laser ablation inductively coupled plasma mass spectrometry system.
Meanwhile Dygert is looking forward to applying an oxybarometer method he has been developing. “It looks at a specific element, europium, which can have two different oxidation states,” he explained. “By measuring this distribution, we can figure out what was the partial pressure of oxygen in the rock.”
They hope to present their first findings at the American Geophysical Union meeting in December.
One of the samples is a volcaniclastic rock, a sedimentary rock that looks like it was produced by explosive volcanism. “But these samples were collected at depths where the water pressure is so high, the bubbles shouldn’t form in the magma,” Dygert said. “So the question is, how did the rock fragment explosively at that depth?”
The chemical composition they find may provide the answer.
“It’s really cool to be active in that science and the collection process,to follow it all the way from where we got it on the ocean floor to understanding the broader impacts of what this means for the formation of Earth and other planets,” Etheridge said. “I’m very excited to see how it all turns out.”
Because the collection in 2024 was limited by mechanical delays and ocean swells during much of the voyage, Dygert already has submitted plans to return and explore shallower depths in the Mariana Trench, where they may see more exposed trench wall for collecting samples.
By Amy Beth Miller
