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Geotimes
 Published by the American Geological Institute
November 2000
Newsmagazine of the Earth Sciences

News Notes


Hydrothermal circulation
Undersea earthquake a blessing in disguise
 

Like their counterparts on land, earthquakes undersea can rattle homes several kilometers away from the epicenter. But when a “big one” hits, undersea creatures living along hydrothermal vents might not know about it for days. The effect one such undersea earthquake had on vent systems off the coast of Washington has shocked and excited oceanographers.

“We thought, ‘something’s not right. Something broke,’” says Paul Johnson of the University of Washington. Equipment sitting on the bottom of the ocean floor last year at a site on the Endeavour segment of the Juan de Fuca Ridge acted as expected for 10 months. It dutifully recorded the ebb and flow of warmer fluid slowly leaking from diffusive vents as a result of the tide overhead.

Unlike black smokers, the chimneys of Earth that gush out fluid with temperatures of 350 to 400 degrees Celsius, diffusive vents are large swaths of oceanic crust. They are areas where fluids from Earth’s interior percolate up through cracks and mix with sea water, cooling anywhere between two and-a-half to 40 degrees Celsius, before peacefully oozing out. They are not areas prone to wild temperature changes.

That’s why Johnson and colleagues were confused when they pulled up the equipment last summer and saw records of sudden, wild fluctuations in temperature from June through mid-July. Then he learned about the earthquake on June 8. He called up NOAA scientists Robert Dziak and Christopher Fox in Newport, Ore., to confirm. It was a big one for that area, they said — 4.5 — and happened right on the Endeavour. A swarm of smaller aftershocks followed for about a week.

Most earthquakes that impact hydrothermal vent systems are associated with underground volcanic eruptions and occur along the ridge axis where vents tend to cluster, says Maurise Tivey of Woods Hole Oceanographic Institution, who did not contribute to the study. This one was different. It hit 7.5 kilometers off axis, away from the vents, and was of tectonic origin, the authors say. And it was the first nonmagmatic, tectonic event recorded to send a shock wave through a hydrothermal system. The tectonic disturbance stirred things up enough to give the researchers a peak into how long it takes hydrothermal fluids to circulate from one spot to another.

About four or five days after the quake, depending on the vent’s location, “bang!” Johnson says, “the output of the vents can go up by a factor of 10.” Of course, as with any flushing of a system, the output can be a little messy. “Once you increase the flow it flushes all this biological material out into the water column. It looks like floating tissue paper.” But, he says, it is actually bacterial mucus and sulfur. “It’s what the bacteria excrete — probably to hang on to the rocks so they’re not swept out by the hydrothermal circulation.” The bacterial mats may play a role in the vents’ mysterious temperature fluctuations.

After jumping in output and temperature increase, the vents showed dramatic oscillations in temperatures that lasted for about a month. “We don’t know what’s causing these,” Johnson says. In their report, the scientists proposed that the abundant flocculent material might plug up the system in different areas while the flow cleans out sub-surface plumbing in other spots. This may be one way biology is influencing the circulation of the vents.

Other hypotheses in the report proposed that newly cracked zones opened below the vents or that fluid convection after the earthquake became unstable. “If it was just a step increase, we would have said ‘All right something happened below the crust.’ But these oscillations were strange,” Johnson says.

He would like to explore the similarities to another oscillation, which is unrelated to temperature. In oil reservoirs, explosions, pumping or natural disturbances can lead to oscillations in the pressure of the flow.

“Now a hydrothermal fluid system is probably a reasonable analog of an oil reservoir,” Johnson says. “Certainly you have porous rock and you have fluid flowing through it.” He plans to calculate the resonant frequencies of the hydrothermal reservoirs while he waits for the next set of data from newly placed equipment on the sea floor.
 
 
At left: The gray bar indicates the June 8, 1999, earthquake that caused temperatures of vent fluids (in red) and ocean bottom waters (blue) to rise. 
Reprinted by permission from Nature 407:176, copyright 2000, Macmillan Magazines Ltd.

Christina Reed



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