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SCIENCE

Seawater from early Cretaceous period found under Chesapeake Bay

Scientists discover that saltwater trapped in a meteor crater below Chesapeake Bay in US northeast dates from early Cretaceous period

PUBLISHED : Monday, 18 November, 2013, 5:30am
UPDATED : Monday, 18 November, 2013, 12:43pm

Not only is the Chesapeake Bay in the United States so enormous it can be seen from space, it essentially came from outer space.

An asteroid or huge chunk of ice slammed into earth about 35 million years ago, splashing into the North Atlantic sea and sending tsunami as far as the Blue Ridge Mountains. It left a 90-kilometre-wide hole at the mouth of what is now the bay.

But a newly published research paper written by US Geological Survey scientists shows that wasn't the end of it. While drilling holes in southern Virginia to study the impact crater, the scientists discovered "the oldest large body of ancient seawater in the world", a survivor of that long gone sea, about 800 metres underground near the bay.

"What we essentially discovered was trapped water that's twice the salinity of [modern] seawater," said Ward Sanford, a USGS hydrologist. "In our attempt to find out the origin, we found it was early Cretaceous seawater. It's really water that's from the North Atlantic."

The findings showing that the water is probably between 100 million and 150 million years old were published on Thursday in the journal Nature.

The Chesapeake Bay Impact Crater was discovered in 1999 by a tandem of USGS and Virginia Department of Environmental Quality scientists.

They theorised that a huge rock or chunk of ice slammed into an ancient ocean, sending enormous pieces of debris skyward and forcing monster tsunami hundreds of kilometres inland. Over centuries, the crater became hidden under 120 to 36- metres of sand, silt and clay, hampering its discovery.

The bay crater is shallower and smaller than another off the coast of Mexico, which most scientists believe caused the extinction of dinosaurs.

Five years after the Chesapeake crater's discovery, Sanford's USGS team started drilling at Cape Charles, Virginia to study how the earth's crust absorbed the blow.

As the team drilled about 800 metres from the surface, it encountered standing water. They first thought it was salty water that shows up at coastal drill sites. Saltwater is found underground all over the world all the time, often because of huge salt deposits in the ground.

In this case, "we didn't hit any salt while drilling" at the Cape Charles site, 1.5 kilometres from the bay, Sanford said.

So researchers considered the possibility of boiling, when a meteor impact is so forceful that it heats water and increases its salinity. But after further tests, the boiling theory didn't make sense.

Results from more testing showed the water was twice as salty as today's ocean water. When they analysed its chemistry, they found high levels of chloride and bromide, the fingerprint of seawater from another time.

More tests and digging through research established that the chemistry was consistent with the "vast halite deposits created during the Jurassic and Cretaceous periods in the Gulf of Mexico and South Atlantic Basins", the research paper said.

In other words, the groundwater at Cape Charles had the same salinity as the long-gone North Atlantic sea.

The USGS said the discovery would help scientists to better understand the hydrology of the area, at the very least.

The discovery would also help explain a higher rate of sea-level rise around Norfolk, Virginia.

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