The ocean is a noisy place. Ship propellers and whale songs reverberate at the lowest pitches, while at higher tones dolphins click and shrimp snap their claws. Between these frequencies are the sounds of the churning sea itself, generated as waves, wind, and rain roil its surface. Researchers have now used this ambient noise to probe the rising acidity of the ocean. The acoustic technique, published last week in the Journal of Geophysical Research: Oceans, could make it easier to measure this key parameter of ocean health across vast distances rather than relying on point measurements.
"This is a really cool idea," says Andone Lavery, an acoustical oceanographer at Oregon State University. "And they've shown that it is possible."
The carbon emissions that are warming the globe also acidify seawater. The ocean naturally absorbs about one-third of annual carbon dioxide emissions; as this gas dissolves and reacts, it creates bicarbonate and hydrogen ions. The hydrogen lowers the pH of seawater, increasing ocean acidity, which can harm sea life and slow future carbon uptake. Ship-based measurements in shallow parts of the ocean have found that, since 1985, pH has already dropped from 8.11 to 8.04.
The waters of the ocean are layered, however, and measurements at one depth may not apply to others. Adding pH sensors to the thousands of robotic Argo floats that patrol the seas, diving as deep as 2000 meters, is one way to get a broader picture of acidity. But David Barclay, an acoustical oceanographer at Dalhousie University, and his co-authors found a way to measure average pH across even greater depth ranges, by taking advantage of the intrinsic physics of sound.
The work originated in 2009, when Barclay led a cruise to the Philippine Sea, funded by the U.S. Navy, to test a new acoustic listening device, called Deep Sound. Barclay's team dropped the instrument -- two hydrophones and their associated electronics encased in reinforced glass spheres -- down 5000 meters. And then they listened.
One immediate surprise was that the abyss wasn't as quiet as they expected. They found that when the winds at the surface whipped up to more than 10 knots or so, the sounds of breaking waves reached all the way to Deep Sound, at frequencies of about 1 to 10 kilohertz (kHz). Barclay knew two compounds in the ocean, boric acid and magnesium sulfate, can dampen sounds at 1 kHz and 10 kHz, respectively, because of the particular way the molecules absorb a bit of the sound wave's energy.
Crucially, rising acidity decreases the abundance of boric acid, and its attenuating effect, while leaving magnesium sulfate unscathed. By comparing the two frequencies, they thought they could get at the integrated pH of the entire water column, much as the echoes of earthquakes rippling through the ocean can be used to measure overall ocean heat.
It took 15 years to be sure: five more ship deployments and several upgrades of the instrument that ultimately allowed it to survive at depths of more than 10,000 meters in the Challenger Deep, in the Mariana Trench near the island of Guam. It also took a lot of painstaking mathematical analysis, says Ernst Uzhansky, an acoustic physicist now at the Naval Postgraduate School. "I can't tell you how happy I was," says Uzhansky, who was skeptical they would find the signal. "Now it really opens up an opportunity to do large-scale pH monitoring."
Not everyone is convinced. Liqing Jiang, a chemical oceanographer at the University of Maryland, notes that the team did not compare its measurement with known observations. "This is a serious flaw," he says. The technique is not as accurate as measurements from the new biogeochemical (BGC) Argo probes, some 500 of which will finish being deployed by the United States by late next year. "The data shown in the paper is not the kind of quality we need yet," says Ken Johnson, a chemical oceanographer at the Monterey Bay Aquarium Research Institute and a leader of BGC-Argo. But it certainly seems possible to get there, he adds. "With a little signal processing, who knows?"
At a minimum, the new method could serve as a backup for BGC-Argo, whose future is in doubt. Several years ago, Honeywell stopped making the pH sensor used by the floats, only to reverse its decision when former President Joe Biden's administration intervened. And although the National Science Foundation funded the first large-scale BGC-Argo deployment, neither the U.S. nor other countries have committed the money needed to keep the program going past next year. At some point, this acoustic method "might be what we've got," Johnson says.
The next step for proving out the technique will be leaving one of these instruments on the sea floor for months at a time, Barclay says. A lower cost version of the instrument might also prove useful for monitoring geoengineering experiments that aim to draw down atmospheric carbon by increasing the alkalinity of coastal waters. It's also possible that the fiber optic cables increasingly being used to sense acoustic signals on the ocean floor could pick up these pH-sensitive frequencies, Lavery says. "If you could set something up for continuous measurements, then it would become pretty useful."