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Ancient Seawater Entombed Beneath Chesapeake Bay Crater

Nov 13, 2013 03:14 PM EST

A meteorite impact entombed ancient water beneath Chesapeake Bay about 35 million years ago, creating the oldest measurable body of seawater in the world, according to a new report by the US Geological Survey.

The high-salinity groundwater found more than 1,000 meters (0.6 miles) beneath the bay is likely remaining water from the Early Cretaceous North Atlantic Sea and could be as many as 145 million years old, the USGS reported Wednesday.

"Previous evidence for temperature and salinity levels of geologic-era oceans around the globe have been estimated indirectly from various types of evidence in deep sediment cores," said Ward Sanford, a USGS research hydrologist and lead author of a study published in the journal Nature. "In contrast, our study identifies ancient seawater that remains in place in its geologic setting, enabling us to provide a direct estimate of its age and salinity."

The oceanic impact crater beneath Chesapeake Bay is one of just a few known in the world and at 56 miles wide, is the largest crater discovered in the United States. The crater is now buried beneath between 400 to 1,200 feet of silt, sand and clay, which has entombed the ancient seawater within it.

The Chesapeake Bay impact crater is younger and in more shallow water than the impact crater off the Yucatan Peninsula in Mexico, the remnants of the impact event that is believed to have brought an end to the dinosaurs.

"The object from outer space that hit the Earth millions of years ago appears to be responsible for the salty ground water that we find at depth over a large area in the bay region," David Powars, a hydrologist with the USGS said in 1999 when the crater was confirmed.

The magnitude of the impact of the space object would have ejected an enormous amount of debris into the atmosphere and unleashed monster tsunamis that would have traveled more than 100 miles inland, the USGS said.

The impact would have "broken the plumbing," so to speak, destroying the existing arrangement of aquifers and rock layers that restrict the flow of groundwater. The markedly salty ground water around the Virginia coast is attributed to this impact crater; the outer rim of the crater appears to coincide with the boundary that separates salty and fresh groundwater, the USGS said.

"We knew from previous observations that there is deep groundwater in quite a few areas in the Atlantic Coastal Plain around the Chesapeake Bay that have salinities higher than seawater," said Jerad Bales, acting USGS Associate Director for Water. "Various theories related to the crater impact have been developed to explain the origin of this high salinity. But, up to this point, no one thought that this was North Atlantic Ocean water that had essentially been in place for about 100 million years."

"This study gives us confidence that we are working directly with seawater that dates far back in Earth's history," Bales continued. "The study also has heightened our understanding of the geologic context of the Chesapeake Bay region as it relates to improving our understanding of hydrology in the region."

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