Big splash from an ancient fall.

Big Splash From an Ancient Fall

It has been 50 million years since anextraterrestrial body landed in the ocean off the Nova Scotia coast, but the crater left over from this impact is currently making a big splash among scientists. The crater is the first such structure ever identified in the ocean, reports a pair of Canadian geologists. Moreover, its identification may help scientists who are trying to determine whether an earlier impact led to the extinctions of the dinosaurs.

Hundreds of known craters on landrecord the impact of meteorites, asteroids or even comet nuclei. Scientists have posited that many more bodies must have fallen into the ocean, given that water covers more than 70 percent of the earth's surface. Indeed, several sites have been proposed as possible marine impact craters, but "this is the first time that an impact crater at sea has been identified," says Georgia Pe-Piper of St. Mary's University in Halifax, Nova Scotia.

Interest in this structure, knwon as theMontagnais crater, dates back to 1974, but the site was initially misidentified and its true identity had eluded scientists. Now Pe-Piper and colleage Lubomir F. Jansa of the Bedford Institute of Oceanography in Dartmouth, Nova Scotia, report in the June 18 NATURE that they have accumulated enough geochemical and seismic evidence to confirm that it is an impact crater.

The crater, at least 45 kilometers indiameter, is located on the relatively shallow continental shelf, 200 km southeast of Nova Scotia. While it sits in only 113 meters of water, the crater itself is 2,800 meters deep. It closely resembles impact craters on land: A central mountain, left over as an artifact of the impact, rises from the inside of the crater. The researchers believe the object that created the crater must have been 2 to 3 km in diameter.

In recent years, a controversy overprehistoric impacts has been making waves in the scientific community as geologists debate whether a large impact 65 million years ago could possibly have initiated a round of mass extinctions concurrent with the boundary between the Cretaceous and the Tertiary periods (SN: 5/16/87, p.309). Scientists gauge extinctions at this K-T boundary by a line of demarcation in sedimentary rocks: Below the line, the fossil evidence of flora and fauna is rich; above the line, in rocks slightly younger than 65 million years, the evidence is poor, indicating that many species perished at the time of the boundary.

The evidence for such a devastatingimpact rests in the unusual concentrations of iridium that geologists have been finding at the K-T boundary around the world. Iridium is rare on the earth's crust, but impact sites often have relatively high concentrations of it, leading some geologists to conclude that an extraterrestrial source provided the anomalous iridium at the K-T boundary. As yet, however, scientists have been unable to positively identify the crater from this proposed impact, although some possible sites have been suggested (SN: 6/7/86, p.356).

Finding the Montagnais crater will enablegeologists to test one aspect of the K-T impact theory, says Blyth Robertson of the Geological Survey of Canada in Ottawa. Since previous impact sites were located on land, and the surrounding areas are often subject to high erosion rates, geologists have yet to find conclusive evidence that an impact could eject extraterrestrial iridium, which would then settle far from the original crater, says Robertson. However, a well-preserved sedimentary record surrounds the submerged Montagnais site, and geologists can trace this record away from the crater and search for an iridium layer. "This gives us the opportunity to prove that the iridium anomaly in an ejecta [debris] layer can be traced back to the cratee," says Robertson.

This same sedimentary record, saysPe-Pipe, will allow scientists to test "the influence of such impacts on the marine ecosystems." Specifically, paleontologists will try to determine whether Montagnais caused any localized extinctions in the areas around the crater .

Other scientists are equally stirred bythe identification of a submerged crater. When informed of the Montagnais structure, geophysicist H. Jay Melosh told SCIENCE NEWS, "It sounds wonderful. It's about time--there clearly must be craters down there, but they're hard to see." Melosh, from the University of Arizona in Tucson, says analysis of the crater will also "Tell us something about the mechanisms of formation of craters with a water overburden." This knowledge, he says, may help in locating other submerged craters.