Earth Institute News Archive
posted 05/30/00
Seafloor off Mid-Atlantic Coast Highly Charged with Gas. Vigorous Gas Expulsion Could Weaken Shelf Edge.
Columbia University, NYC
A team of scientists investigating whether possible cracks along the outer continental shelf off the mid-Atlantic coast might lead to a tsunami-causing landslide has discovered that the entire area is charged with gas.
Based on preliminary results from a just-completed two-week cruise to the area, the scientists say the suspected cracks are a system of large depressions along the shelf edge that appear to have been excavated by gas erupting through the seafloor.
The scientific team:
(standing l. to r.) Jeff Weissel (Senior
Scientist, LDEO), Mark Capone (Vassar undergraduate), John Goff
(Collaborating Scientist, UT), Wayne Spencer (Senior Technician), Neal
Driscoll (Chief Scientist, WHOI);
(seated l. to r.) Bill Lyons (graduate
student, MIT), Luigi Tosi (visiting scientist, Venice, Italy), Kurt
Sternlof (writer/photographer, LDEO).
"We don't know the source of the gas," team leader Neal Driscoll of the Woods Hole Oceanographic Institution (WHOI) said. "But it is clear that gas has played an important role in the formation of these features. The gas is trapped under layers of sediment on the shelf edge until some circumstance causes it to escape, blowing holes in the seafloor to form these large pockmark features we thought were cracks."
In a paper published in the journal GEOLOGY days before their May 7, 2000 departure for the Mid-Atlantic coast, Driscoll and colleagues Jeffrey Weissel of the Lamont-Doherty Earth Observatory of Columbia University and John Goff of the University of Texas Institute for Geophysics had speculated that rising gas might play a part in triggering shelf-edge collapse. Even so, they were surprised at the quantity of gas and the apparent vigor of the "blowout" process.
"Our seismic data show that gas is pervasive in and around the blow-outs," Goff said. "Gas has a characteristic signal, which commonly shows up as a bright, high-amplitude reflection that obscures any deeper signals."
"We were a bit taken aback, to be sure," Weissel said. "A great deal more work is needed before any definitive statements can be made. Regardless of the source, it is apparent that gas charging plays a critical role in the area, and that the ongoing process of blowouts could conceivably weaken the shelf edge and contribute to a submarine landslide/tsunami scenario."
The scientists say the features, some as large as 2,000 meters (6,500 feet) across, 50 meters (165 feet deep) and up to 5,000 meters (16,400 feet) long, occur in a line along the shelf edge. That suggests there is some geological mechanism at work underneath the features, or their trend and shapes might be due to their proximity to the shelf edge. A submarine landslide and resulting tsunami along the shelf, although a very low risk when compared to the chance of hurricanes and severe storms striking the area, remains possible.
Jeffrey Weissel with 3-D color chart of shelf.
The May 7-20 research cruise was funded under a grant from the Marine Geology & Geophysics program of the National Science Foundation (NSF), and was conducted aboard the Research Vessel Cape Hatteras, operated out of Beaufort, North Carolina, by Duke University. The NSF grant will also cover the next year of data synthesis and analysis, and the eventual publication of the detailed results.
Gas Trapped by Ancient Delta Layers
The section of continental shelf in question is known to be landslide prone -- an enormous slide occurred just to the south some 16,000 to 18,000 years ago at the end of the last ice age. But whether that long-ago collapse resulted from a similar process of gas charging and shelf-edge blowouts, and if so when the next one might occur, remains to be determined.
3-D perspective view of the seafloor
"What we can say with some confidence is that these blowout features occur where they do because layers of relatively impermeable sediment draped over the edge of the shelf form a trap that catches and accumulates the gas," Goff said. "This
allows pressure to build until the gas breaks through the seafloor, presumably
throwing sediment up into the water column where it is carried away by
the bottom currents."
3-D diagram of blow-out features and gas.
These confining layers appear to be remnants of an ancient delta that reached far out from the current coastline during the last ice age, when sea levels were much lower than today, Driscoll said. "We recovered samples of silty clay, sand and gravel from the bottom that are consistent with a deltaic setting. Where these deposits are absent, the gas appears to percolate harmlessly to the surface," Driscoll said.
"The apparent size and violence of the gas-release episodes does concern us as they could pose hazards in and of themselves," Weissel said. "Shallow gas blow-outs have damaged or destroyed oil drilling rigs in the Gulf of Mexico and the North Sea -- something we should keep in mind when future work is conducted here."
Onboard Research
The primary tool used during the recent cruise was SUBSCAN, a state-of-the-art seafloor and sub-bottom imaging system, designed by EdgeTech of Boca Raton, Florida in collaboration with researchers at Florida Atlantic University and Woods Hole Oceanographic Institution. The device combines side-scan sonar with a chirp seismic reflection unit to
provide the highest resolution imaging available of both the seafloor
and the subsurface geology to a depth of 50 meters or more -- all at a
single pass.
Photos of research aboard R.V. Cape Hatteras
Towed behind the ship over the seafloor, SUBSCAN was in the water 24 hours a day for eleven straight days, creating a dense grid of overlapping data sections at just a few miles an hour.
"We call it mowing the lawn," Weissel said. "It
isn't exactly glamorous, but it's effective."
Maps of R.V. Hatteras ship track.
The scientists rotated on 12-hour watches to monitor the equipment, record the data and make preliminary interpretations, returning with more than 2,000 kilometers (1,250 miles) of geophysical data. They also took sediment core samples in several locations using a gravity core.
A seismic section across a blow-out feature.
Future Research
Although they now know that gas expulsion is the primary force at work on the shelf edge, the teams says further research will be necessary to completely unravel the story and determine the real risk to adjacent coastal communities of a submarine landslide and associated tsunami.
Additional work will take a closer look into the subsurface to determine what is happening at depth. Collecting more samples of the sediments involved to determine age and timing is important, as is determining the origin of the gas and whether the venting process supports any specialized biological activity.
"When it comes to science, surprise is generally a good thing," Driscoll said. "You have to approach any research effort with an open mind, willing to rethink your working hypothesis as new data comes in. This is a fascinating area of research, and we've only just begun to understand the processes at work."
Still images and video available upon request.
CONTACT:
Kurt Sternlof
Senior Science Writer, Columbia Earth Institute
Columbia University
914-365-8747 kurt@ldeo.columbia.edu
05/02/00 news story:
GEOLOGY May 2000 paper
The Geological Society of America
Potential for large-scale submarine slope failure
and tsunami generation along the U.S. mid-Atlantic
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