Ancient seawater found in Chesapeake Bay impact crater

by Sara E. Pratt
Friday, July 18, 2014

When the bolide struck in the Late Eocene, it shattered the bedrock, truncated aquifers and left a crater roughly twice the size of Rhode Island and nearly as deep as the Grand Canyon. The resulting structural low is likely responsible for the formation of the Chesapeake Bay.

Deep saline groundwater has previously been found at several locations under the Atlantic coastal plain, including in Georgia, North Carolina, Maryland and New Jersey. Explanations for those brines have included evaporite dissolution, osmosis and evaporation from heating associated with the bolide impact.

Now, new evidence gleaned from deep cores drilled at the tip of the Delmarva Peninsula indicates at least some of that salty water might actually be ancient seawater that has been trapped belowground for tens of millions of years.

Water samples retrieved from boreholes more than a kilometer deep revealed salinity levels of 70 parts per thousand — twice as salty as today’s ocean. Such salinity levels are consistent, the authors suggested, with water originating in the Early Cretaceous North Atlantic (ECNA), a closed basin.

Previous studies have indirectly analyzed the chemistry of ancient seawater by examining geochemical, isotopic and paleontological evidence from rocks and minerals recovered from deep sediment cores. For example, in one recent study published in Geology, researchers analyzed fluid inclusions in Neoproterozoic marine halites from Western Australia that revealed the sulfate chemistry of 830-million-year-old seawater, pushing back the previous earliest measurements of seawater chemistry by about 300 million years. The new study, however, is the first to analyze ancient seawater itself.

“Our study identifies ancient seawater in situ and provides a direct estimate of its age and salinity,” the authors wrote. “Moreover, we suggest that it is likely that remnants of ECNA seawater persist in deep sediments at many locations along the Atlantic margin.”

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