Supersaline sediments off East Coast shed light on Atlantic Ocean’s early history
Water, water, everywhere and some of it’s been undisturbed for more than 100 million years.
Groundwater collected from sediments at the mouth of the Chesapeake Bay is thought to be 100 to 145 million years old. The ancient samples, twice as salty as modern seawater, were already trapped in sediments when an asteroid or comet landed in the area some 35 million years ago, researchers report in the Nov. 14 Nature.
Similarly salty waters in sediments off the Atlantic Coast of North Carolina, Georgia, Maryland and New Jersey could be just as old, but no one has sampled them carefully enough to find out, says Ward Sanford, a research hydrologist for the U.S. Geological Survey. Ten years ago, the USGS with the International Continental Scientific Drilling Program drilled a deep hole in the Chesapeake Bay impact crater to figure out how the 3-kilometer-wide asteroid strike had changed the Earth’s crust. What they found in early test wells surprised them: water with salinity twice that of the modern ocean.
Later Sanford and his colleagues analyzed water trapped in the cores and found evidence that it has been undisturbed since the Early Cretaceous, a geologic epoch running from about 145 million to 100 million years ago. The authors calculated the age using measurements of water’s helium concentration, which increases the longer sediments are isolated from the atmosphere. This ancient water was trapped in sediments that built up in the basin, and when the asteroid or comet impact happened 35 million years ago, those sediments broke up but maintained the high salinity in the basin.
“There haven’t been any conditions that would have driven the water out of the system,” Sanford says. “And for the diffusion that occurs naturally, there hasn’t been enough time.”
The age of the seawater could explain an anomaly for researchers: salinity that is higher than normal in pockets of water along the Eastern Seaboard. The authors suggest that during the Cretaceous, when the North Atlantic was undergoing a transition from a closed basin to an open ocean, its water was saltier. In a closed basin, salinity increases because the amount of water that comes in through precipitation is less than the water that leaves through evaporation. The salt remains.
Today, the Mediterranean Sea is nearly an enclosed basin, connected to the Atlantic Ocean only by the Strait of Gibraltar. Its salinity is around 10 percent higher than modern seawater.
Richard Norris, a paleoceanographer at the Scripps Institution of Oceanography, who is familiar with the unusually high salinity of sediments underlying the Chesapeake Bay, says the research is important for understanding groundwater resources.
“What it does tell you,” he says, “is that the water dates from around the time when the Atlantic began to open.”
W.E. Sanford et al. Evidence for high salinity of Early Cretaceous sea water from the Chesapeake Bay crater. Nature. Vol. 503, November 14, 2013, p. 252.doi:10.1038/nature12714
E. Wayman. Groundwater isolated for eons. Science News Online, May 15, 2013.