January 6, 1997
Fluids flow fleetly under the seafloor
Hydrogeologists have taken the closest look yet at the intricate cycle of fluids that flow relentlessly beneath the seafloor. That flow, it now appears, is far more forceful than expected.
Driven by the heat of the planet's interior, water courses through pores and cracks under the ocean in earth's upper crust. The water leaches minerals as it flows, altering the crust and suffusing the ocean with important elements. This process occurs dramatically at "black smokers" and other hot vents along volcanic midocean ridges. It happens to a far greater extent along the sweeping flanks of these ridges, and possibly under most of the rest of the ocean as well. But because the seafloor is so remote, scientists have known precious little about the details.
That all changed during Leg 168 of the Ocean Drilling Program, a two-month cruise in July and August off the Pacific Northwest. An international team of researchers, led by cochief scientists Andrew Fisher of UCSC and Earl Davis of the Geological Survey of Canada, drilled as far as 1,900 feet into the seafloor on the flanks of the Juan de Fuca Ridge, where the planet churns out fresh slabs of oceanic crust.
The researchers collected the first known samples of pristine "basement" water--fluids trapped under the seafloor for many thousands of years. Chemical analysis of these and other samples points to a surprisingly energetic cycling of fluids and heat beneath the sediments that shroud the young crust.
"There's no question the water is moving much faster than we thought," Fisher says. The time it takes for the entire volume of the world's oceans to cycle through these subseafloor systems, he notes, could be "much, much shorter" than the previous estimate of one to two million years.
Fisher and his colleagues found that the upper oceanic crust beneath seafloor ridges is "overpressured," meaning that hot water surged from their drill holes. Tests also showed that the crust is most conducive to fluid flow within layers that are perhaps just 10 meters thick. "We're still hard-pressed to explain how these fluids can travel laterally within the crust for 50 to 80 kilometers, but the thermal and chemical data suggest this is exactly what happens," Fisher says.
Cochief scientist Davis adds: "We've come to realize that this kind of water circulation is much more important than we believed just a few years ago. We suspect these processes take place within a large part of the seafloor, perhaps most of it, and play important roles in forming mineral deposits and in helping to control the composition of seawater."
The Leg 168 scientists installed deep-sea observatories, called CORKs, in four of the drill holes. The CORKS will record the pressures and temperatures of fluids with exquisite sensitivity for several years; manned or robotic vessels will retrieve the data. Fisher and Davis expect the results will help paint a complete picture of the forces that impel the fluids.
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