A huge wave crashed into the New York City region 2,300 years ago, dumping sediment and shells across Long Island and New Jersey and casting wood debris far up the Hudson River.
(BBC)-The scenario, proposed by scientists, is undergoing further examination to verify radiocarbon dates and to rule out other causes of the upheaval.
Sedimentary deposits from more than 20 cores in New York and New Jersey indicate that some sort of violent force swept the Northeast coastal region in 300BC.
If we’re wrong, it was one heck of a storm
It may have been a large storm, but evidence is increasingly pointing to a rare Atlantic Ocean tsunami.
Steven Goodbred, an Earth scientist at Vanderbilt University, said large gravel, marine fossils and other unusual deposits found in sediment cores across the area date to 2,300 years ago.
The size and distribution of material would require a high velocity wave and strong currents to move it, he said, and it is unlikely that short bursts produced in a storm would suffice.
“If we’re wrong, it was one heck of a storm,” said Dr Goodbred.
Landslide or asteroid?
The origin of such a tsunami is also under debate. An undersea landslide is the most likely source, but one research group has proposed that an asteroid impact provided the trigger.
In 300BC, barrier beaches and marsh grass embroidered the coast, and Native Americans walked the shore.
Today, a wave of the proposed size would leave Wall Street and the Long Island Expressway awash with salt water.
Atlantic tsunamis are rare, but could be triggered by submarine landslides
An Atlantic tsunami was rare but not inconceivable, said Neal Driscoll, a geologist from Scripps Institution of Oceanography, who is not associated with the research. But verifying one that is 2,000 years old is tricky.
Earthquakes, underwater landslides, or a combination of the two were the most frequent Atlantic tsunami triggers, said Professor Driscoll.
The 1929 Grand Banks tsunami, in Newfoundland, which killed more than two-dozen people and snapped many transatlantic cables, was set in motion by a submarine landslide set off by an earthquake.
Dr Goodbred imagines that the New York wave was on the Grand Banks scale – three to four metres high and big enough to leap over the barrier islands; but that it did not reach the magnitude of the 2004 Sumatran tsunami.
The evidence is buried under metres of sediment in New York and New Jersey.
Dr Goodbred first proposed the link between the layers of unusual debris found in sediment cores and a tsunami while studying shellfish populations in Great South Bay, Long Island.
He extracted many mud cores with incongruous 20cm layers of sand and gravel.
Their age matched that of wood deposits buried in the Hudson riverbed and marine fossils in a New Jersey debris flow in cores gathered by other researchers.
The fist-sized gravel he found in Long Island would require a high velocity of water – well over a metre per second – to land where it did, said Dr Goodbred.
Among the fossils and shells sandwiched in the organic black mud of Sandy Hook Bay, New Jersey, Marine Geologist Cecilia McHugh of Queens College, City University of New York, discovered mud balls made from red clay that matched iron-rich sediments found onshore.
The balls form their spherical shape only through vigorous reworking, said Dr McHugh, and they do not form in small storms.
“I didn’t think much about it until we dated the deposit and came up with the same date that Steve did on Long Island,” she said.
It prompted her to check cores extracted from the upper continental slope 200km offshore.
She discovered a 2,200-year-old layer of sand and mud, on top of sedimentary layers 8,000 to 14,000 years old.
Dr McHugh says such relatively young debris is not found that far out on the slope, and the date is close to that of the New York and New Jersey samples.
Age of a storm
The age and nature of the material make tsunami verification a challenge.
The radiocarbon dates of the debris are accurate to within a century, said Dr Goodbred. But the only evidence that a dramatic event took place thousands of years ago is common coastal debris – wood, sand, shells and rock.
Researchers must discern whether it was strewn by a tsunami or a hurricane, or another large storm, such as a “nor’easter”, said Professor Driscoll.
Unusual layers in sediment cores may be a sign of an ancient tsunami
“Understanding the origins of these deposits can be difficult,” he added.
While tsunamis can occur in any ocean, they are most common in the Pacific and Indian Oceans where continental plates collide.
There, large undersea earthquakes are relatively common.
In the Atlantic, where the plates spread, tsunamis are rare, which means Atlantic tsunamis are not well studied, said Bruce Jaffe, of the United States Geological Survey.
There is little research on tsunami debris in the variety of northeast coastal environments – riverbeds, marine bays – where the New York debris layers were found. There are few modern analogues to compare them with for identification, he said.
“Grand Banks is the only unequivocal tsunami in the Atlantic on the Northeast coast because there were eye-witness accounts and the deposits matched that of other modern tsunamis,” said Dr Jaffe.
To rule out the possibility of a severe storm, said Professor Driscoll, tsunami groups should collect more core samples to see whether the distribution of the debris is consistent.
Dr Goodbred said teams were planning to do just that. And this would confirm that the deposits are not quirks of local geology.
The researchers would also repeat carbon dating on cores to verify ages, said Dr Goodbred, but he has a hunch the tsunami theory will win out.
“We’re building a case of circumstantial evidence that is getting harder and harder to ignore,” he said.
While many geologists say a submarine landslide is the likely trigger of a tsunami, a group led by geologist Dallas Abbot thinks a space impactor may have set off the massive wave.
Some researchers suspect a space impactor may have been involved
Her team discovered material in the New Jersey and Hudson River cores dated to 2,300 ago, and believe it to be meteoritic in nature. This includes 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 Dr Abbott, from the Lamont Doherty Earth Observatory of Columbia University.
She theorised 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. No crater has yet been found.
Many geologists and other scientists remain sceptical of the asteroid evidence so far; but proof of an asteroid impact is not necessary to build the case for a massive wave.
As Dr Goodbred pointed out: “The tsunami story stands on its own without the impact.”
for National Geographic News
|May 4, 2009|
|A prolonged lull in solar activity has astrophysicists glued to their telescopes waiting to see what the sun will do next—and how Earth’s climate might respond.The sun is the least active it’s been in decades and the dimmest in a hundred years. The lull is causing some scientists to recall the Little Ice Age, an unusual cold spell in Europe and North America, which lasted from about 1300 to 1850.The coldest period of the Little Ice Age, between 1645 and 1715, has been linked to a deep dip in solar storms known as the Maunder Minimum.
During that time, access to Greenland was largely cut off by ice, and canals in Holland routinely froze solid. Glaciers in the Alps engulfed whole villages, and sea ice increased so much that no open water flowed around Iceland in the year 1695.
But researchers are on guard against their concerns about a new cold snap being misinterpreted.
“[Global warming] skeptics tend to leap forward,” said Mike Lockwood, a solar terrestrial physicist at the University of Southampton in the U.K. (Get the facts about global warming.)
He and other researchers are therefore engaged in what they call “preemptive denial” of a solar minimum leading to global cooling.
Even if the current solar lull is the beginning of a prolonged quiet, the scientists say, the star’s effects on climate will pale in contrast with the influence of human-made greenhouse gases such as carbon dioxide (CO2).
“I think you have to bear in mind that the CO2 is a good 50 to 60 percent higher than normal, whereas the decline in solar output is a few hundredths of one percent down,” Lockwood said. “I think that helps keep it in perspective.”
For hundreds of years scientists have used the number of observable sunspots to trace the sun’s roughly 11-year cycles of activity.
Sunspots, which can be visible without a telescope, are dark regions that indicate intense magnetic activity on the sun’s surface. Such solar storms send bursts of charged particles hurtling toward Earth that can spark auroras, disrupt satellites, and even knock out electrical grids.
In the current cycle, 2008 was supposed to have been the low point, and this year the sunspot numbers should have begun to climb.
But of the first 90 days of 2009, 78 have been sunspot free. Researchers also say the sun is the dimmest it’s been in a hundred years.
The Maunder Minimum corresponded to a profound lull in sunspots—astronomers at the time recorded just 50 in a 30-year period.
If the sun again sinks into a similar depression, at least one preliminary model has suggested that cool spots could crop up in regions of Europe, the United States, and Siberia.
During the previous event, though, many parts of the world were not affected at all, said Jeffrey Hall, an astronomer and associate director at Lowell Observatory in Flagstaff, Arizona.
“Even a grand minimum like that was not having a global effect,” he said.
Wild Cards and Uncertainties
Changes in the sun’s activity can affect Earth in other ways, too.
For example, ultraviolet (UV) light from the sun is not bottoming out the same way it did during the past few visual minima.
“The visible light doesn’t vary that much, but UV varies 20 percent, [and] x-rays can vary by a factor of ten,” Hall said. “What we don’t understand so well is the impact of that differing spectral irradiance.”
Solar UV light, for example, affects mostly the upper layers of Earth’s atmosphere, where the effects are not as noticeable to humans. But some researchers suspect those effects could trickle down into the lower layers, where weather happens.
In general, recent research has been building a case that the sun has a slightly bigger influence on Earth’s climate than most theories have predicted.
Atmospheric wild cards, such as UV radiation, could be part of the explanation, said the University of Southampton’s Lockwood.
In the meantime, he and other experts caution against relying on future solar lulls to help mitigate global warming.
“There are many uncertainties,” said Jose Abreu, a doctoral candidate at the Swiss government’s research institute Eawag.
“We don’t know the sensitivity of the climate to changes in solar intensity. In my opinion, I wouldn’t play with things I don’t know.”
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