Scientists Discover a Way of Forming Suspended Layers of Sediment

Although travel brochures might proclaim otherwise, the ocean is far from crystal clear: Bits of particulate matter—sediments, nutrients, and even tiny forms of life—pervade the water column. That material occasionally aggregates into layers, some of which can linger at intermediate water depths and extend laterally for hundreds of kilometers.

Once believed to be primarily formed by ocean bottom storms or tidal currents, such layers can also be formed by underwater eruptions of gas, researchers have now shown in the lab. Though it remains to be seen whether these findings can be translated to nature, they suggest a previously unknown mechanism behind a commonly observed marine phenomenon.

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Chaoqi Zhu, a geoenvironmental oceanographer at the Ocean University of China in Qingdao, and his colleagues never set out to study these so-called nepheloid layers. (Their name derives from the ancient Greek word nephos, meaning “cloud.”) Instead, the researchers were conducting laboratory experiments in a 400-liter tank of water to understand how earthquakes and gas outflows might cause underwater landslides and other forms of seafloor instabilities. However, when the team used an air compressor to introduce gas into the bottom of their tank, they were surprised at what they saw.

The water first became turbid. That made sense because the team had previously laid down layers of sand and clay in the shape of a wedge over half of the tank’s bottom, and that material was being dislodged by the aeration. But soon thereafter, a roughly 15-centimeter-thick layer of suspended material formed near the floor of the tank. That bottom nepheloid layer slid down the roughly 16° slope of the wedge before it detached at roughly half the depth of the tank and then moved horizontally until it reached the far end of the tank, the team noted. At that point, it thinned slightly to a width of about 10 centimeters. The researchers had inadvertently formed an intermediate nepheloid layer.

That was the first time that an intermediate nepheloid layer had been created in the laboratory from gas eruptions, said Zhu, who coled the new research with Yonggang Jia, an oceanographer also at the Ocean University of China. It was an unexpected finding given that mixing in the upper reaches of the ocean, as opposed to a process originating under the seafloor, has long been believed to be responsible for the formation of intermediate nepheloid layers.

“Nepheloid layers may influence the faunal feeding, distribution patterns, and functioning of marine ecosystems.”

“It is a new mechanism for intermediate nepheloid layer generation,” Zhu wrote in an email. The researchers reported their results in the Journal of Oceanology and Limnology.

Intermediate nepheloid layers are prevalent in the world’s oceans—they form at depths ranging from about 100 meters to thousands of meters, and they can persist in the water column for anywhere from hours to years, Zhu wrote.

These transient structures play a key role in transporting not only sediments but also organic matter and nutrients, previous research has shown. Intermediate nepheloid layers—and, in fact, nepheloid layers at any depth—have the potential to affect ocean ecosystems, Zhu said. “Nepheloid layers may influence the faunal feeding, distribution patterns, and functioning of marine ecosystems.”

On the basis of their laboratory findings, Zhu and his colleagues suggested that underwater eruptions of gas might be responsible for some of the intermediate nepheloid layers found in nature. Such eruptions could stem from gas hydrates and mud volcanoes, the researchers proposed.

Gas hydrates are repositories of frozen water permeated by trapped gases, most commonly methane. Found beneath the seafloor, they’re stable at low temperatures but are prone to melting if the local conditions warm. When that happens, they disassociate into water and gas, and the latter percolates upward through the ocean floor. Researchers have estimated that trillions of cubic meters of methane are currently trapped in gas hydrates worldwide.

Mud volcanoes are another potential source of gas in the deep ocean. These edifices are known to belch gases like methane and carbon dioxide and have been captured lofting sediments off the seafloor. Thousands of submarine mud volcanoes are believed to exist worldwide.

The idea that underwater eruptions might be responsible for some of the ocean’s nepheloid layers is intriguing, said Wilford Gardner, an oceanographer at Texas A&M University in College Station who was not involved in the research. These structures are typically linked to disturbances much higher in the water column, he said.

In 2009, Gardner and his colleagues showed that the nearby passage of Hurricane George lofted sediments off the seafloor in the Gulf of Mexico. “Energy can propagate all the way to the bottom,” Gardner said. However, it’s unknown whether gas eruptions originating from under the seafloor could have a similar effect, he said.

However, Gardner noted an inconsistency between the researchers’ experimental setup and real-world conditions: The fluid that Zhu and his colleagues used in their experiment—plain tap water—was not stratified. That’s incongruent with conditions in the real ocean given natural variations in water temperature and salinity, Gardner said. “You can’t get away from that.”

—Katherine Kornei (@KatherineKornei), Contributing Writer