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The deep sea is full of sand, but exactly how it got there is surprisingly hard to study.
“It’s a potentially paradigm-changing thing.”
Just as surprising is how important deep-sea sand can be. Dense, yet porous layers called turbidites can be packed with oil, and when they are not, their absorbency might make them prime sites for carbon capture and sequestration.
In a new study, scientists highlighted factors that predict where and when turbidites might form. That information could help them pinpoint ancient deposits lying kilometers below Earth’s surface. “It’s a potentially paradigm-changing thing,” said Jacob Covault, a senior research scientist at the University of Texas at Austin who was not involved in the study.
How Sand Descends
Underwater avalanches, called turbidity currents, carry sediment from the continental shelf to the deep sea. Once it gets there, it can form porous, sometimes oil-filled turbidites. Starting in the mid-20th century, oil companies began prospecting ancient turbidites, predicting where they are on the basis of the factors that might have spawned them.
The oil companies thought of sea level as a main control on turbidity currents, said lead author Zachary Burton of Stanford University. When sea levels are low, the thinking went, the continental shelf can accommodate less sediment, so it is more likely to slough off into the deep sea.
Burton and his colleagues tested this idea by searching decades’ worth of stratigraphy data. They looked for evidence of turbidites formed during the Cenozoic era (about 65 million to 35 million years ago). In total, they found 59 deposits spanning every continent except Antarctica. “This was the first global compilation that looked to the ancient rock record [for turbidites],” said Burton.
Their findings called the dominant understanding into question: Turbidites formed during the warmest times of the Cenozoic—times when the sea levels would have been high. Though not the absolute first of their kind, these findings were by far the oldest and most widespread, and they were situated during some of Earth’s highest seas. Sea levels alone, then, couldn’t explain turbidite formation.
Burton and colleagues theorized a new conceptual model that linked turbidity currents to other climate-driven factors like precipitation, extreme storms, and river drainages that could have pushed sand to the sea. These factors can load the offshore environment with sediment and “overwhelm” the otherwise spacious continental shelf, Burton said.
“It might not be as simple as sea level,” Covault said. “You can have climate forcings that promote a whole bunch of sediment supply…and that’s how you get sediment into deep water, even when sea level is globally high.”
The Surprising Value of Sand
Sand—even when it doesn’t form turbidites—is a valuable resource used in products ranging from smartphones to toilets.
Sand—even when it doesn’t form turbidites—is a valuable resource used in products ranging from smartphones to toilets. Concrete, made of sand and water, is the second most used material in the world. The old understanding of turbidity currents suggested that during high seas, sand should remain close to shore, said Covault. But the new model suggests that despite high seas, valuable sand might be spirited away to inaccessible depths.
Turbidites aren’t just candidates for resource extraction; they might also be used for carbon sequestration. Carbon capture and storage reservoirs are thought to have a lot of the same kinds of requirements as oil reservoirs, Burton said.
When choosing a possible reservoir for carbon storage, “you want to try to identify where the pockets of sand are of significant size and then where the muddy parts are that seal them up,” Covault said.
Some scientists say that deep-sea carbon storage comes with major caveats. It captures only a portion of existing carbon dioxide emissions, for starters, which does little to outweigh overall emissions. And if sequestered carbon dioxide leaked out of deep-sea reservoirs, it could acidify the environment, spelling disaster for many marine creatures.
In addition to being a resource and reservoir, sand can also be a hazard. A better understanding of turbidity currents will help protect infrastructure such as transoceanic cables, oil and gas platforms, and wind turbines that plunging sediment could destroy, Burton said. “If these events are becoming more frequent, that has implications for any infrastructure we have on the seafloor.”
—Emily Shepherd (@emilyshep1011), Science Writer