EVIDENCE FOR HYPERSALINE NORTH-ATLANTIC SEAWATER TRAPPED IN THE US ATLANTIC COASTAL PLAIN SINCE EARLY CRETACEOUS TIME

Sediment and rock cores from the deep ICDP-USGS Chesapeake Crater drill hole revealed deep pore water chemically similar to seawater, yet at nearly twice the salinity. Ten-cm-long cores were taken about every 10-20 m along the 1.766 km deep borehole and pore water was extracted using high-speed centrifuging. The water samples were analyzed for major and minor elements and ²H and ¹⁸O. The vertical chemistry profiles revealed a monotonically increasing salinity with depth up to about 1 km, where chloride concentrations peaked at about 38 g/L. Chloride-bromide ratios throughout the section are close to that of modern seawater. A 1D advection-diffusion model of the profile with sedimentation history can accurately reproduce the chloride and isotope profiles, yielding results that suggest this is a system that has been dominated by molecular diffusion, compaction-driven vertical upward pore-water seepage, and a transport initiation time that dates to the time of the bolide impact and crater refilling at 35 Ma. The required initial condition of 38 g/L chloride suggested the coastal sediments contained pre-impact water virtually twice the salinity of modern seawater—but the origin of the hypersalinity was an enigma.

High helium concentrations in water extracted from deep in the borehole suggested an age of greater than 100M years. The earliest Cretaceous sediment at the site is 145 Ma, constraining the possible age of the potentially connate water to 100-145 Ma. Additional 1D numerical simulations demonstrate that seawater emplaced 125 Ma would not have diffused out of the sediment column in the 90 M years between emplacement and the bolide impact. Paleogeographic reconstruction of the continents confirms that the North Atlantic Ocean at the time was a nearly closed basin, pointing to the hypersaline groundwater being remnants of ancient ocean water entrapped there during the Early Cretaceous Period circa 125 Ma. This is the first direct evidence of hypersalinity in the North Atlantic during the Early Cretaceous. It also underscores the extremely long potential residence times of deep saline pore water in the Atlantic Coastal Plain, and the potential for ancient ocean water to be trapped in many other deep locations along the Atlantic Margin.