Researchers say sedimentary deposits from more than 20 cores in New York and New Jersey indicate that some sort of violent force, such as a huge wave, swept the Northeast coastal region some 2,300 years ago, BBC News reported.
While some experts believe it could have been a large storm, other evidence is increasingly pointing to a rare Atlantic Ocean tsunami.
The size and distribution of material would require a high velocity wave and strong currents to move it, according to Steven Goodbred, an Earth scientist at Vanderbilt University.
He added it is unlikely that short bursts produced in a storm would suffice. “If we’re wrong, it was one heck of a storm,” he said.
While some experts are skeptical that it could have been a tsunami, others believe that an undersea landslide is the most likely source.
However, one research group proposed that an asteroid impact provided the trigger, since barrier beaches and marsh grass embroidered the coast, and Native Americans walked the shore in 300BC.
Neal Driscoll, a geologist from Scripps Institution of Oceanography, who is not associated with the research, said an Atlantic tsunami was rare but not inconceivable and that verifying one that is 2,000 years old is tricky.
Driscoll said the most frequent Atlantic tsunami triggers included earthquakes, underwater landslides, or a combination of the two.
He suggested that the New York wave was on the scale of the Grand Banks tsunami in Newfoundland, which killed more than two-dozen people and snapped many transatlantic cables.
He imagines it would have been big enough to leap over the barrier islands, but that it did not reach the magnitude of the 2004 Sumatran tsunami.
The link between the layers of unusual debris found in sediment cores and a tsunami were first proposed by Goodbred while studying shellfish populations in Great South Bay, Long Island.
He found that the age of extracted mud cores with incongruous layers of sand and gravel matched the age of wood deposits buried in the Hudson riverbed and marine fossils in a New Jersey debris flow in cores gathered by other scientists.
Goodbred said the fist-sized gravel he found in Long Island would require a high velocity of water to land where it did.
Tsunami verification can be challenging due to the age and nature of the material.
Goodbred said the radiocarbon dates of the debris are accurate to within a century, but the only evidence that a dramatic event took place thousands of years ago is common coastal debris like wood, sand, shells and rock.
Driscoll said researchers are left to discern whether it was strewn by a tsunami or a hurricane, or another large storm, such as a “nor’easter”.
He added that understanding the origins of these deposits could be difficult.
Tsunamis are most common in the Pacific and Indian Oceans where continental plates collide and large undersea earthquakes are relatively common, but they can occur in any ocean.
Bruce Jaffe of the United States Geological Survey said in the Atlantic, where the plates spread, tsunamis are rare, which means Atlantic tsunamis are not well studied.
Where the New York debris layers were found, there is little research on tsunami debris in the variety of northeast coastal environments like riverbeds and marine bays. He said there are few modern analogues to compare them with for identification.
Jaffe said Grand Banks is the only unequivocal tsunami in the Atlantic on the Northeast coast since there were eyewitness accounts and the deposits matched that of other modern tsunamis.
But tsunami groups should collect more core samples, according to Driscoll, to see whether the distribution of the debris is consistent enough to rule out the possibility of a severe storm.
However, Goodbred said teams are planning to do just that in order to confirm that the deposits are not quirks of local geology.
He said teams would also repeat carbon dating on cores to verify ages, and he believes the tsunami theory will win out in the end.
“We’re building a case of circumstantial evidence that is getting harder and harder to ignore,” he said.
A group led by Columbia University geologist Dallas Abbot thinks a space impactor may have set off the massive tsunami wave.
Abbot’s researchers discovered material in the New Jersey and Hudson River cores dated to 2,300 years ago, which they believe it to be meteoritic in nature.
The team found carbon spherules, shocked minerals, and nanodiamonds, which are produced under extreme pressures and temperatures.
“We didn’t find the typical shocked quartz, but that is usual for a water impact,” said Abbott.
While no crater has yet been found, she theorized that an asteroid landed in the water off the coast of New York and New Jersey, either creating the wave directly or triggering a submarine landslide.
Asteroid evidence, however, is lacking and many geologists and other scientists are skeptical of the theory; but proof of an asteroid impact is not necessary to build the case for a massive wave.
“The tsunami story stands on its own without the impact,” Goodbred said.
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