Ancient seawater entombed in ancient impact crater yields secrets

The world's oceans 100 million to 145 million years ago were more than twice as salty as they are today, U.S. scientists studying ancient sediments say. Sediments retrieved from a crater formed when a 2-mile-wide asteroid or comet struck off the Atlantic coast of North America 35 million years ago containing remnants of Early Cretaceous North Atlantic seawater allowed them to make a direct estimate of the water's age and salinity, the researchers reported in the journal Nature. "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 U.S. Geological Survey hydrologist and lead author of the study. "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 56-mile wide Chesapeake Bay oceanic impact crater, one of just a few discovered around the world and the largest crater found in the United States, is now buried beneath between 400 to 1,200 feet of silt, sand and clay, which has entombed the ancient seawater within it. Starting around 100 million years ago, the researchers estimate, North Atlantic salinity began to change to that of modern seawater as the Chesapeake Bay crater was transitioning from being a closed rift basin to an open ocean.