Chalk reveals greatest underwater landslide

Column of sediment from 637 to 639 meters below the ocean floor shows layers of chalk (white) deposited during underwater landslides 65 million years ago. Norris, et al./Geology

The cataclysmic event that wiped out the last dinosaurs also triggered the greatest underwater landslides ever, scientists report.

A chunk of a giant comet or asteroid slammed into Mexico’s Yucatan peninsula about 65 million years ago, setting off earthquakes with magnitudes estimated at 10 to 13. The seismic shock waves generated meter-high vibrations in Earth’s crust all along the east coast of North America.

New research indicates that this shaking sent sediment from shallow waters sliding off the continental shelf. Once the ooze settled, it may have blanketed a region as large as 3.9 million square kilometers on the deep ocean floor—an area more than twice the size of Alaska.

The researchers examined sediment drilled from the seafloor near Bermuda at depths of about 4,000 meters. Within those cores, 65-million-year-old layers of chalk contain fossil and geologic markers, including fragments of ancient land plants and pollen.

“You don’t [normally] find that in open ocean sites,” says John Firth, a paleontologist at Texas A&M University in College Station. He and his colleagues also found large amounts of fossilized algae that typically grew in much shallower water, he says.

The chalk layers that the scientists examined lie nearly 640 meters beneath the seafloor. Previous acoustic studies had suggested a chalk layer at this depth underneath an enormous area of the western North Atlantic. In the December Geology, the scientists argue that this layer, known as Horizon A*, is in fact debris from massive landslides triggered by the impact in the Yucatan.

“This was never recognized before,” Firth says. The researchers say that the layering pattern they see within the chalk could have been produced by moving sediment.

Furthermore, Firth says that the deeply buried chalk layer contains remnants of only the small, juvenile forms of a protozoan that normally appears in a range of sizes. Chalk in previously drilled cores off the coast of Florida revealed only large varieties of the organism. The scientists suspect that as the underwater landslide began, large protozoa came to rest close to the continent while small ones were carried out to sea.

Firth and his colleagues at Woods Hole (Mass.) Oceanographic Institution point out that small particles in clay lying above the chalk resemble pulverized stone that could have been thrown into the atmosphere from the extraterrestrial rock’s impact. These particles, seen previously in ocean-floor cores, would have settled out of the atmosphere and water after the landslides, the researchers say.

Other scientists agree that the case for underwater landslides in the western North Atlantic appears sound. Geologist Tim Bralower of the University of North Carolina in Chapel Hill says he found similar evidence in sediment around the Gulf of Mexico and the Caribbean.

Richard Olsson, a geologist at Rutgers University in Piscataway, N.J., also recently reported dramatic repercussions of the Yucatan impact. His team found evidence that seismic shock waves stimulated a giant landslide off the coast of New Jersey and generated a tsunami that roared ashore there.

Still, Olsson takes issue with the claim by Firth and his colleagues that all of Horizon A* was deposited during the landslides. Too little is known about the layer to draw that conclusion, he says, adding, “The impact event…generated a great deal of devastation, and we are just beginning to sort [the record] out.”

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