Wednesday, February 25, 2009

New York Tsunami 300 BCE

This is a neat story that likely got little coverage outside New York, if there. It does tell us that meteorite strikes should not be discounted as sources of tsunami events.

That alone is useful. Tsunami events are mapable in the sedimentary record. Event markers can be checked for. Local impact tsunami events can be sorted out. This report tells geologists to question unusual sediments exposed to tsunami events for an impact signature.

Any crater here, if it could be pinpointed, is likely to be quite close by.

I hope that this means that this particular unique event was never replicated over the past ten thousand years. We presume the 1159 BC Hekla event came from the wrong direction and strength to leave any sign not long since eroded away.

It actually is a good measure of how often a particular stretch of coast line might be impacted by a significant meteor generated tsunami. The risk is real though very small and in the modern era we have some semblance of warning and possible response.

Meteorite Strikes, Setting Off a Tsunami: Did It Happen Here?


Published: December 29, 2008

The tsunami washed over Fire Island and, to the west, waves perhaps as high as 20 feet spilled into Lower Manhattan. The furious onrush of water left sediment a foot and a half deep on the Jersey Shore, and debris cascaded far up the Hudson River.

A Columbia scientist has reported finding carbon spheres, above, in sediment, indicating a meteorite.

No, there’s no need to rush to higher ground, commandeer a rowboat in Central Park or empty the closet to grab the rubber boots. This disaster occurred about 2,300 years ago, though how bad it was, or even if it was a tsunami, remains in dispute.

But several geologists have collected evidence indicating that something very big and unusual occurred in waters near the New York area around 300 B.C., give or take a century. And
Dallas Abbott, a research scientist at Columbia University’s Lamont-Doherty Earth Observatory, is asserting that a meteorite, landing somewhere in the Atlantic, generated the tsunami.

Someone at the tip of Lower Manhattan then would probably have seen “something coming in,” Dr. Abbott said. “Then you would hear a big bang, maybe a series of bangs, something that sounded like gunfire or cannons. It would be a really, really loud noise. And then you would be knocked to the ground by the air blast. And then you would be inundated by the tsunami.”

While not nearly as severe as the tsunami that killed more than 180,000 people in South and Southeast Asia in 2004, “it would have been a bad day to end all bad days,” she said, “in all senses.”

Although American Indians had long been living in and around the area that became New York, Dr. Abbott said there was no archeological evidence of a tsunami or known legends of, say, a terrible flood. She has built her case with diamonds, very tiny ones.

At a meeting of the
American Geophysical Union in San Francisco earlier this month, Dr. Abbott reported finding minute carbon spheres and smaller-than-dust diamonds in sediment layers, which she said were the distinctive calling cards of a meteorite’s impact.

“I think it’s pretty convincing,” Dr. Abbott said. “We always find the impact ejecta in the tsunami layer, never outside.”

A few years ago, the geologist
Steven Goodbred, then at the State University of New York at Stony Brook, was not looking for tsunamis or meteorites when he first examined sediment cores taken along the South Shore of Long Island. Dr. Goodbred was interested in the history of oysters in that area. But in the very first core, he saw a strange layer several inches thick containing fist-size gravel.

“We started joking immediately, ‘It’s a tsunami,’ ” recalled Dr. Goodbred, now a professor at
Vanderbilt University in Nashville.

Subsequent cores, taken in Great South Bay, also contained that layer, deposited about 2,300 years ago. When Dr. Goodbred presented his findings at a conference a couple of years ago, he failed to convince other scientists. They said the layer was more likely caused by a big storm, not a tsunami.

“Even if it was a storm, it was the mother of all storms,” Dr. Goodbred said, pointing out that the devastating hurricane that passed directly over Long Island in 1938 generated less than an inch of sediment.

Then Dr. Goodbred met other scientists who had found similar sediment layers nearby.
Cecilia McHugh, a professor at Queens College, had seen a sediment layer a foot and a half thick at Sandy Hook in New Jersey. That, too, was laid down about 2,300 years ago. And Frank Nitsche, another research scientist at Lamont-Doherty, had discovered a layer of wood debris in sediment cores from the upstate reaches of the Hudson River.

Then Dr. Abbott joined the project and found possible evidence of a meteorite.

But the arguments of a meteor causing a New York tsunami are still regarded skeptically by many, if not most, geologists. For one, no one has found any craters.

The evidence hinges most strongly on the tiny diamonds, presumably formed by the ultra-high pressures of impact.

The carbon atoms inside some of the diamonds are lined up in a hexagonal crystal structure instead of the usual cubic crystals. The hexagonal diamonds have been found only within meteorites and at impact craters, said Allen West, a geologist who performed the diamond analysis for Dr. Abbott’s New York sediments.

But unless researchers find a crater in the ocean floor, an Indian legend telling of a day of fire and water or many more thick sediment deposits, convincing other scientists of what they believe happened 2,300 years ago will continue to be an uphill battle.

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