FEATURE ARTICLE
Internal Tides and the Continental Slope
Curious waves coursing beneath the surface of the sea may shape the margins of the world's landmasses
David Cacchione, Lincoln Pratson
Proof in the (Mud) Pudding
Although Cacchione had studied internal waves extensively in the laboratory as a graduate student, his first personal experience with their effects took place years later within Hydrographer Canyon, which lies just south of Cape Cod, during a dive in NR-1, a nuclear-powered research submarine of the U.S. Navy. While Cacchione and noted marine geologist Bruce Heezen were busy mapping the seafloor and taking photographs within this canyon (one of many that cut into the continental slope in this region), strong currents buffeted the vessel. The maximum speed of these currents was about a half-meter per second near the seafloor, and they reversed direction every 12 hours or so. Unlike other nuclear submarines, NR-1 has small windows, which allowed the scientists aboard to witness the periodic pulses of muddy water the strong currents raised. These undersea mudstorms obscured the otherwise clearly visible seabed for an hour or so, just as the tidal currents reached their highest speeds. This experience convinced Cacchione that internal waves could indeed move sediment along the seafloor and motivated his continued exploration of this phenomenon.
Research dives elsewhere, along with numerous measurements from moored instruments, have since documented the universality and power of the internal tides both within submarine canyons and over other, more regular areas, where the strong bottom currents presumably have important effects. For example, investigators at the University of Washington monitored the influence of internal waves and tides using an instrumented mooring that they maintained in about 450 meters of water on the upper continental slope off northern California from 1995 to 1999. Throughout the five years of study, internal tides were consistently responsible for the strongest flows near the bottom. The fastest bottom currents reached 40 centimeters per second—pulses of colder water, which moved upslope as an advancing tidal bore, mixing the bottom waters with vigor. Oceanographers have found similar goings-on at many other places such as off Oahu, Hawaii, (at about 460 meters depth), off Virginia (at about 1,100 meters depth), and off southwest Ireland (at about 3,000 meters depth, at the base of the continental slope).
The wealth of recent evidence for the internal tide creating bottom currents strong enough to prevent deposition and sometimes even to erode accumulated sediments prompted us to reconsider the idea that this process might indeed control the geologic evolution of the continental slope. Of course, many other natural phenomena are known to affect this portion of the sea bottom—everything from the tectonic movements of the lithospheric plates to the episodic scouring action of turbidity currents, essentially underwater mudslides, which arise, for example, when a portion of the seafloor gives way, sending a dense mixture of water and sediment tumbling down submarine slopes. The rates, frequency and spatial extent of these processes vary from one continental margin to the next, which makes it rather difficult to evaluate their overall influence in shaping the continental slope. The internal tides are different. They pass over continental slopes all around the planet, every day of every year and have done so, in some parts of the globe, for more than 100 million years. Internal tides have an omnipresence that approaches the tug of gravity, making them a prime candidate in our minds for the cause of the low overall gradient of the continental slope.
Two years ago we set out to evaluate this notion. Understanding our strategy is not difficult but requires learning a little more about how internal tides work.
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