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FALL 2006
Listening to the Earth from Under
the Sea
by Carole VanSickle

Using specially designed equipment positioned on the sea floor some 2,200 meters down, University of Georgia physical oceanographer Daniela Di Iorio plans to listen in on the pulse of one the planet’s mid-ocean ridges. These geologically active, uplifted areas in the ocean floor, where magma from beneath the surface of the earth is forced between the boundaries of two oceanic plates, may provide insight to everything from evolution to earthquakes.

Next year, Di Iorio will begin monitoring a hydrothermal (hot-water) vent called Hulk on the Juan de Fuca Ridge west of Vancouver Island in Canada. She’ll use an acoustical system she helped develop as a graduate student and began refining with the help of a research services consulting company to monitor the “vitals” — such as temperature fluctuations and flow speed — of the scalding metallic plumes that steadily erupt into the ocean from deep within the earth.

In 2008, she and collaborator Peter Rona of Rutgers University plan to participate in the world’s largest cable-linked sea floor observatory project, the Northeast Pacific Time-Series Undersea Networked Experiments, also known as NEPTUNE. Using extensive networks of undersea instruments connected by fiber-optic and power cables to the observatory’s shore station, they’ll continually monitor the vitals of the vent field.

“In the past, the highly acidic and corrosive, 350-degree-Celsius water that spews out of hydrothermal vents prevented extended measurements because the instruments just couldn’t take the conditions for any long period,” Di Iorio said. “With my acoustical scintillation technique — which uses a separate transmitter and receiver placed on either side of the plume to measure sound-scattering by the turbulent properties of the buoyant plume – we can invert the signals to give water movements and temperature. We don’t put the instruments into the hydrothermal fluid at all.

“And there is no telling what the data could reveal about the inner workings of the oceanic plates and the movement of sea water through the sea floor,” she added, “because a hydrothermal vent is basically our nearest communication with the magma chambers underneath the earth’s crust.”

NEPTUNE will allow Di Iorio to constantly monitor and analyze her data, but until then her instrument will work as an autonomous, battery-operated and self-contained unit that’s able to store three to four months of data on a memory card.

“We’ll head out there, recover it, upload the data and change the batteries, then redeploy it,” she said.

The first hydrothermal vent was discovered in 1977 at the Galapagos Rift, an area of great geologic activity that spawned the Galapagos Islands — a volcanic chain west of Ecuador.

“There are not a lot of times when a paradigm shift occurs and changes the way you view all aspects of your field,” Di Iorio said, “but the discovery of hydrothermal vents changed the way oceanographers look at everything from the origin of life to plate tectonics.”

Hydrothermal vents form when the earth’s crust spreads apart, allowing seawater to rush into the cracks and mix with molten lava. The water becomes “super-hot,” Di Iorio said, which results in interactions with the oceanic crust and other materials so dramatic that the water emerges from the vents with a completely altered chemical composition. The aqueous mixture of inorganic elements such as iron, sulfur and barium hurtles upward as a result of its high temperature.

Surprisingly, these plumes are the basis for unique ecosystems that exist entirely independently of sunlight [see sidebar on p. 15]. “When scientists discovered hydrothermal vents, they found an enormous biological ecosystem where microbes perform chemosynthesis — turning chemicals into nutrients — instead of photosynthesis to get their energy,” Di Iorio said. “The first research team never even thought to bring a biologist along because they figured ‘What could live at 2,200 meters, where there’s no light and no food?’ But it turns out that these microbes probably evolved to initiate the chain of events leading up to where we are today, since they are examples of the earliest life on earth.”

Di Iorio is more focused, however, on geophysically and oceanographically derived perturbations in the vent systems. She aims to determine how oceanographic changes such as flow or temperature fluctuations effect vent output.

“We’re looking at flow and temperature fluctuations of the fluid and how that information fluctuates with the occurrence of tides, inertial oscillations and seismic events. There is a very sensitive ecosystem that thrives and depends on hydrothermal fluids. Too much might cook the system and too little might starve them.”

Di Iorio will also be able to detect any shifts in the earth’s tectonic system that are likely indicative of the flexing and bending and twitching of the plate systems. “What’s really exciting are the implications for seismic investigations,” she said. “Could we eventually forecast a seismic event by looking at the characteristics of the plume? Even if not, what I’m doing should be invaluable to knowing many of the ‘causes and effects’ between geological and oceanographic systems.”

Even at 2,200 meters below the surface there is a tidal flow, and this is Di Iorio’s first target. “This flow is small,” she said, “and probably moves only 5 to 10 centimeters a second (coastal tides can move as much as 100 centimeters per second), but it’s there and interacting with hydrothermal plumes.”

Just like smoke from a factory smokestack shifts in wind currents, these plumes appear to sway back and forth (by as much as 5 to 10 meters) with the tidal flow, which creates slight periodic temperature changes in the vent area as water mixes into the flow. Di Iorio uses sound to illustrate these changes.

“Depending on water turbulence and temperature, a sound pulse will take a certain length of time to travel from the transmitter to the receiver. The sound waves will scatter forward, and these changes will be picked up by the receiver,” she said. By measuring amplitude and arrival-time fluctuations, acoustical scintillation enables scientists to create a “picture” of flow, turbulence and temperature character-istics of the water. “Sound can also be used to image a hydrothermal plume the same way that it can be used to image the sea floor or objects on the floor – by studying a reflected, scattered signal.”

By imaging a plume, scientists can determine the point at which the plumes have mixed with so much ocean water that they quit rising.

She noted that the scattering of sound can also allow scientists to measure variables such as plankton layers, fish abundance, size and the boundaries of layers of water at different levels of the ocean. Di Iorio has used acoustics to investigate the number of fish in Savannah’s Grays Reef National Marine Sanctuary and a variety of environments with active tides and currents. “Sound can tell you all sorts of things about the ocean environment,” she said.

This work is funded by a National Science Foundation CAREER grant.

For more information, contact Daniela Di Iorio at daniela@uga.edu.