June 09, 2000


Seafloor Off Mid-Atlantic Coast Highly Charged With Natural Gas,
Vigorous Gas Expulsion Could Weaken Shelf Edge

Amid a flurry of national media attention, a team of scientists investigating whether possible cracks along the outer continental shelf off the mid-Atlantic coast could lead to a tsunami-causing landslide has discovered that the entire area is rife with natural gas.

In fact, based on the preliminary results of their research cruise to the area in May, the scientists now believe that the suspected cracks are actually a system of large depressions along the shelf edge that were excavated by gas erupting through the seafloor. The features are up to five kilometers long, two kilometers across and 50 meters deep.

"It was an extremely interesting and exciting trip," said team senior scientist Jeffrey Weissel of the Lamont-Doherty Earth Observatory. "The amount and quality of the data we collected was phenomenal and raised a number of very interesting and unexpected questions. Analyzing it all will keep us busy for quite some time."

In a paper published in Geolog days before their May 7th departure, Weissel and his colleagues ­ principal investigator Neal Driscoll of the Woods Hole Oceanographic Institution 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.

"We don't yet know the source of the gas," said Driscoll, who did his Ph.D. work at Lamont-Doherty as Weissel's student. "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."

"We were a bit taken aback, to be sure," Weissel said, "and a great deal more work is needed before any definitive statements can be made. Regardless of the source though, 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 section of shelf in question is known to be collapse-prone ­ an enormous slide occurred just to the south some 20,000 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.

"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," Driscoll said. "Presumably this allows the pressure to build until the gas breaks through the seafloor, throwing sediment up into the water column where it can be carried away by the bottom currents."

These confining layers appear to be remnants of an ancient delta that reached far out of from the current coastline during the last ice age, when sea levels were much lower than today. Samples of silty clay, sand and gravel recovered from the bottom are consistent with a deltaic source. 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."

The potential hazards involved -- whether from a tsunami caused by a shelf-edge collapse hitting the adjacent coast, or from the actual gas expulsions themselves ­ attracted a great deal of media attention both before and after the May cruise. All the major television networks, as well as radio, newspapers and national wire services interviewed Weissel, Driscoll and Goff and reported on their findings.

"The size and vigor of the media response surprised us too, Weissel joked. "But when it comes to science, surprise is generally a good thing. You have to approach any research with an open mind, willing to rethink your working hypothesis as new data comes in. This is a fascinating area, and we've only just begun to understand the processes at work."