Oceans

Sound waves traveling thousands of kilometers through the ocean may help scientists monitor climate change.
As greenhouse gas emissions warm the planet, the ocean is absorbing vast amounts of that heat. To monitor the change, a global fleet of about 4,000 devices called Argo floats is collecting temperature data from the ocean’s upper 2,000 meters. But that data collection is scanty in some regions, including deeper reaches of the ocean and areas under sea ice.
So Wenbo Wu, a seismologist at Caltech, and colleagues are resurfacing a decades-old idea: using the speed of sound in seawater to estimate ocean temperatures. In a new study, Wu’s team developed and tested a way to use earthquake-generated sound waves traveling across the East Indian Ocean to estimate temperature changes in those waters from 2005 to 2016.
Comparing that data with similar information from Argo floats and computer models showed that the new results matched well. That finding suggests that the technique, dubbed seismic ocean thermometry, holds promise for tracking the impact of climate change on less well-studied ocean regions, the researchers report in the Sept. 18 Science.

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Sound waves are carried through water by the vibration of water molecules, and at higher temperatures, those molecules vibrate more easily. As a result, the waves travel a bit faster when the water is warmer. But those changes are so small that, to be measurable, researchers need to track the waves over very long distances.
Fortunately, sound waves can travel great distances through the ocean, thanks to a curious phenomenon known as the SOFAR Channel, short for Sound Fixing and Ranging. Formed by different salinity and temperature layers within the water, the SOFAR channel is a horizontal layer that acts as a wave guide, guiding sound waves in much the same way that optical fibers guide light waves, Wu says. The waves bounce back and forth against the upper and lower boundaries of the channel, but can continue on their way, virtually unaltered, for tens of thousands of kilometers (SN: 7/16/60).
In 1979, physical oceanographers Walter Munk, then at the Scripps Institution of Oceanography in La Jolla, Calif., and Carl Wunsch, now an emeritus professor at both MIT and Harvard University, came up with a plan to use these ocean properties to measure water temperatures from surface to seafloor, a technique they called “ocean acoustic tomography.” They would transmit sound signals through the SOFAR Channel and measure the time that it took for the waves to arrive at receivers located 10,000 kilometers away. In this way, the researchers hoped to compile a global database of ocean temperatures (SN: 1/26/1991).
But environmental groups lobbied against and ultimately halted the experiment, stating that the human-made signals might have adverse effects on marine mammals, as Wunsch notes in a commentary in the same issue of Science.
Forty years later, scientists have determined that the ocean is in fact a very noisy place, and that the proposed human-made signals would have been faint compared with the rumbles of quakes, the belches of undersea volcanoes and the groans of colliding icebergs, says seismologist Emile Okal of Northwestern University in Evanston, Ill., who was not involved in the new study.
Still, Wu and colleagues have devised a work-around that sidesteps any environmental concerns: Rather than use human-made signals, they employ earthquakes. When an undersea earthquake rumbles, it releases energy as seismic waves known as P waves and S waves that vibrate through the seafloor. Some of that energy enters the water, and when it does, the seismic waves slow down, becoming T waves.
Those T waves can also travel along the SOFAR Channel. So, to track changes in ocean temperature, Wu and colleagues identified “repeaters” — earthquakes that the team determined to originate from the same location, but occurring at different times. The East Indian Ocean, Wu says, was chosen for this proof-of-concept study largely because it’s very seismically active, offering an abundance of such earthquakes. After identifying over 2,000 repeaters from 2005 to 2016, the team then measured differences in the sound waves’ travel time across the East Indian Ocean, a span of some 3,000 kilometers. 
The data revealed a slight warming trend in the waters, of about 0.044 degrees Celsius per decade. That trend is similar to, though a bit faster than, the one indicated by real-time temperatures collected by Argo floats. Wu says the team next plans to test the technique with receivers that are farther away, including off of Australia’s west coast.
That extra distance will be important to prove that the new method works, Okal says. “It’s a fascinating study,” he says, but the distances involved are very short as far as T waves go, and the temperature changes being estimated are very small. That means that any uncertainty in matching the precise origins of two repeater quakes could translate to uncertainty in the travel times, and thus the temperature changes. But future studies over greater distances could help mitigate this concern, he says.
The new study is “really breaking new ground,” says Frederik Simons, a geophysicist at Princeton University, who was not involved in the research. “They’ve really worked out a good way to tease out very subtle, slow temporal changes. It’s technically really savvy.”
And, Simons adds, in many locations seismic records are decades older than the temperature records collected by Argo floats. That means that scientists may be able to use seismic ocean thermometry to come up with new estimates of past ocean temperatures. “The hunt will be on for high-quality archival records.” More

When an intense heat wave strikes a patch of ocean, overheated marine animals may have to swim thousands of kilometers to find cooler waters, researchers report August 5 in Nature.
Such displacement, whether among fish, whales or turtles, can hinder both conservation efforts and fishery operations. “To properly manage those species, we need to understand where they are,” says Michael Jacox, a physical oceanographer with the National Oceanographic and Atmospheric Administration based in Monterey, Calif.
Marine heat waves —  defined as at least five consecutive days of unusually hot water for a given patch of ocean — have become increasingly common over the past century (SN: 4/10/18). Climate change has amped up the intensity of some of the most famous marine heat waves of recent years, such as the Pacific Ocean Blob from 2015 to 2016 and scorching waters in the Tasman Sea in 2017 (SN: 12/14/17; SN: 12/11/18).
“We know that these marine heat waves are having lots of effects on the ecosystem,” Jacox says. For example, researchers have documented how the sweltering waters can bleach corals and wreak havoc on kelp forests. But the impacts on mobile species such as fish are only beginning to be studied (SN: 1/15/20).

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“We have seen species appearing far north of where we expect them,” Jacox says. For example, in 2015, the Blob drove hammerhead sharks — which normally stay close to the tropics, near Baja California in Mexico — to shift their range at least hundreds of kilometers north, where they were observed off the coast of Southern California.
To see how far a mobile ocean dweller would need to flee to escape the heat, Jacox and colleagues compared ocean temperatures around the globe. First, they examined surface ocean temperatures from 1982 to 2019 compiled by NOAA from satellites, buoys and shipboard measurements. Then, for the same period, they identified marine heat waves occurring around the world, where water temperatures for a region lingered in the highest 10 percent ever recorded for that place and that time of year. Finally, they calculated how far a swimmer in an area with a heat wave has had to go to reach cooler waters, a distance the team dubs “thermal displacement.”

In higher-latitude regions, such as the Tasman Sea, relief tended to be much closer, within a few tens of kilometers of the overheated patch, the researchers found. So while ocean heat waves in that region might spell doom for firmly rooted corals and kelp, mobile species might fare better. “We were surprised that the displacements were so small,” Jacox says.
But in the tropics, where ocean temperatures are more uniform, species may have had to travel thousands of kilometers to escape the heat.  
Projecting how species might move around in the future due to marine heat waves gets increasingly complicated, the researchers found. That’s because over the next few decades, climate change is anticipated to cause not just an increase in frequency and intensity of marine heat waves, but also warming of all of Earth’s ocean waters (SN: 9/25/19). Furthermore, that rate of warming will vary from place to place. As a result, future thermal displacement could increase in some parts of the ocean relative to today, and decrease in others, writes marine ecologist Mark Payne of the Technical University of Denmark in Copenhagen, in a commentary in the same issue of Nature.
That complexity highlights the task ahead for researchers trying to anticipate changes across ocean ecosystems as the waters warm, says Lewis Barnett, a Seattle-based NOAA fish biologist, who was not involved in the study. The new work provides important context for data being collected on fish stocks. For example, surveys of the Gulf of Alaska in 2017 noted a large decline in the abundance of valuable Pacific cod, now known to be linked to the Blob heatwave that had ended the year before.
But there’s a lot more work to be done, Barnett says.
The study focuses on surface ocean temperatures, but ocean conditions and dynamics are different in the deep ocean, he notes. Some species, too, move more easily between water depths than others. And heat tolerance also varies from species to species. Biologists are racing to understand these differences, and how hot waters can affect the life cycles and distributions of many different animals.
The effects of marine heat waves might be ephemeral compared with the impacts of long-term climate change. But these extreme events offer a peek into the future, says Malin Pinsky, a marine ecologist at Rutgers University in New Brunswick, N.J., who was not involved in the study. “We can use these heat waves as lessons for how we’ll need to adapt.” More

Even after 100 million years buried in the seafloor, some microbes can wake up. And they’re hungry.
An analysis of seafloor sediments dating from 13 million to nearly 102 million years ago found that nearly all of the microbes in the sediments were only dormant, not dead. When given food, even the most ancient microbes revived themselves and multiplied, researchers report July 28 in Nature Communications.
Scientists have pondered how long energy-starved microbes might survive within the seafloor. That such ancient microbes can still be metabolically active, the researchers say, just goes to show that scientists are still fathoming the most extreme limits to life on Earth.
The microbes’ patch of seafloor lies beneath a kind of ocean desert, part of a vast abyssal plain about 3,700 to 5,700 meters below sea level. Researchers, led by microbiologist Yuki Morono of the Japan Agency for Marine-Earth Science and Technology in Kochi, examined sediments collected in 2010 from part of the abyssal plain beneath the South Pacific Gyre. That region of the Pacific Ocean contains few nutrients that might fuel phytoplankton blooms and thereby support a cascade of ocean life. As a result, very little organic matter makes its way down through the water to settle on the seafloor.
The extremely slow accumulation of organic material and other sediments in this region does allow oxygen in the water to seep deep into the sediments. So Morono and colleagues wondered whether any aerobic, or oxygen-liking, microbes found there might be revivable. After “feeding” microbes from the collected sediments with nutrients including carbon and nitrogen, the team tracked the organisms’ activity based on what was consumed.
The aerobic microbes in the sediments turned out to be a highly diverse group, consisting mostly of different types of bacteria belonging to large groups such as Alphaproteobacteria and Gammaproteobacteria (SN: 9/14/17). Nearly all the microbes responded quickly to the food. By 68 days after the experiment’s start, the total number of microbial cells had increased by four orders of magnitude, from as little as about 100 cells per cubic centimeter to 1 million cells per cubic centimeter.
Those increases weren’t just among the youngest microbes. Even in the sediment sample containing the most elderly — about 101.5 million years old — up to 99.1 percent of the microbes were revived. More

Microbe communities living in the seafloor off Peru haven’t bounced back from a deep-sea mining experiment 26 years ago. The populations are still reduced by 30 percent in this part of the South Pacific Ocean, researchers report April 29 in Science Advances. From 1989 to 1996, the DISturbance and reCOLonization, or DISCOL, experiment plowed grooves […] More

Nearly a decade after the Deepwater Horizon oil spill in the Gulf of Mexico, computer simulations suggest that the toxic pollution extended much farther than satellite images first indicated.   Those images, taken after the spill dumped nearly 800 million liters of oil into Gulf waters, helped to determine which areas would be temporarily closed […] More

The noise of shipping vessels traveling through northern Canadian waters is causing Arctic cod to sacrifice much of their foraging and feeding in order to flee the area until ships move away, researchers report. The findings — the first to gauge how shipping noise could affect Arctic fish — are cause for concern as climate […] More

Moving slowly and stealthily through the Pacific Ocean, a robotic glider with a microphone captured a cacophony of sounds from ships, whales and underwater explosions. The glider’s journey, across 458 kilometers off the Washington and Oregon coast and down to 650 meters, demonstrates that gliders could be effective tools to help map ocean noise levels, […] More