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How to Date 100 Million-Year-Old Seawater

Geologists are piecing together why Virginia's groundwater is so damn salty.
November 20, 2013, 8:45pm

via Wikimedia Commons

There’s mysterious and extra-salty groundwater 1,000 meters below Chesapeake Bay that geologists now think is 100 million-year-old seawater, preserved “like a prehistoric fly in amber” by a comet or meteorite smashing into the Earth. We take it for granted that scientists can deduce the age of rocks and whatever they find in rocks. But how do you know when you’ve pulled out a sample of seawater from the age of the dinosaurs?

I emailed Ward Sanford, a hydrologist for the US Geological Survey and one of the authors of the study that was published in Nature. According to Sanford the key is reading the chemical composition of the water, and comparing it to water found in wells whose ages are known. “We dated the water using the helium concentration,” Sanford said.

This required observing the composition of the ground around the water and projecting how it interacts with saltwater backwards for millions of years. “We were able to establish the rate at which helium is produced in the sediment and used that rate along with the helium concentration in the crater to date the water,” Sanford explained.

The water under the bay is twice as salty as contemporary ocean water, 70 parts-per-million versus the 35 ppm currently lapping at the beach. It also bears the signature of the prehistoric seas—high levels of chloride and bromide.

via USGS

The Chesapeake Bay was blasted into existence 35 million years ago. When the comet or meteor hit, it left a 56-mile-wide hole in the ocean floor and created gigantic tsunamis that may have reached as far as the Blue Ridge Mountains, some 110 miles away. The impact cracked the Earth’s crust as deep as seven miles and scattered the bedrock and continental shelf, sending shocked quartz as far away as the coast of New Jersey.

Effects of the impact are still evident today—the York and James Rivers make sharp turns to the northeast near the outer rim of the crater the impact left, as opposed to flowing southeastwardly into the Atlantic like most rivers. All of the subterranean aquifers in the region were disrupted as well, smashed by the impact. In their place is now what the researchers believe is the “"the oldest large body of ancient seawater in the world."

People have known about this deposit of salt water for decades, digging up briny water from hundreds of meters below the ground, far from the coast. This is known as Virginia’s “inland saltwater wedge,” and researchers now think it is linked to the prehistoric ocean.

“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," said Jerad Bales, acting USGS Associate Director for Water, in a press release. “This study gives us confidence that we are working directly with seawater that dates far back in Earth’s history.”