Secret Life of Waves

Duke Researchers Are Finding That the Waves We Never See May Play a Big Role in Shaping the Planet

by Tinker Ready

Waves help define the ocean. They mark the boundary between land and sea, shape the beach and warn of approaching storms. But, beyond the swells familiar to beachcombers and surfers, other, less visible waves also give form to the sea. Offshore and deepwater waves sweep up sand and other forms of sediment as they churn with the tide. Now, Duke scientists are taking a closer look at this interplay between sand and sea. In two separate labs, researchers are exploring the relationship between offshore waves, the shoreline and the continental slopes. In the process, they are turning up some surprising clues to coastal sedimentation patterns that have not been well understood up to now.

Work by a team including geologist Lincoln Pratson found that "internal" or underwater waves - in addition to earthquakes - play a role in forming the inexplicably shallow gradient of continental slopes in ocean basins. Also based in the Nicholas School's Earth and Ocean Sciences Division, A. Brad Murray analyzes wave angles and sediment transport, but from the nearshore perspective. Rather than relying on traditional simulation models, he's brought a new tool to the field. Using the principles of chaos theory and nonlinear dynamics, Murray and his team discovered that they could explain shoreline changes based on the angle between offshore waves and the beach.

While Murray's work takes him just beyond breaking waves, Pratson's takes him far out to sea, to the continental margin. A Columbia-trained geologist, Pratson spent much of his career studying the evolution of the submarine seascape that marks the transition from shallow to deep water. Once the edge of the continent's waterfront, the margin's plains give way to a gradual slope. There, Pratson said, a thick accumulation of sediment contains "the most complete record of the earth's history." In addition to holding clues to the earth's past, the slope also feeds its future by storing and recycling the ocean's vital nutrients. Much of Pratson's work has revolved around the role of sedimentary processes that shape this important underwater terrain.

"One of the things I'm interested in is the stability of the continental slope and what controls it," he said. "And a big indicator of its stability is its gradient."

The quest to understand this deep-sea landscape has some practical implications. Oil companies involved in offshore drilling need to know whether the shelf and the waters above can support their structures. And instability, in particular landslides and earthquakes, can have devastating consequences. The 1998 tsunami that killed 3,000 people in Papua New Guinea was most likely generated by a submarine landslide.

But ultimately, the slope research was driven by "scientific curiosity," Pratson said. "The slope should be on the order of 10 degrees steeper than it is. The question then becomes - why is it so low?"

There was quite a bit of evidence supporting the existing theory - that underwater earthquakes and avalanches knock sediment down and keep the slopes from becoming steep, Pratson said. That is one reason the role of underwater waves and tides had not been explored in the past. Still, the avalanche theory fails to fully answer the question of why the slopes are so gradual, he said.

Submarine avalanches will lower the angle of the continental slope, Pratson said. "However, they generally require that the continental slope achieve a steep gradient before it becomes unstable and avalanches. On average, the slope of the continental slope is much less than this critical or threshold angle."

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