Abstract
High-salinity groundwater more than 1,000 metres deep in the Atlantic coastal plain of the USA has been documented in several locations1,2, most recently within the 35-million-year-old Chesapeake Bay impact crater3,4,5. Suggestions for the origin of increased salinity in the crater have included evaporite dissolution6, osmosis6 and evaporation from heating7 associated with the bolide impact. Here we present chemical, isotopic and physical evidence that together indicate that groundwater in the Chesapeake crater is remnant Early Cretaceous North Atlantic (ECNA) sea water. We find that the sea water is probably 100–145 million years old and that it has an average salinity of about 70 per mil, which is twice that of modern sea water and consistent with the nearly closed ECNA basin8. Previous evidence for temperature and salinity levels of ancient oceans have been estimated indirectly from geochemical, isotopic and palaeontological analyses of solid materials in deep sediment cores. In contrast, our study identifies ancient sea water in situ and provides a direct estimate of its age and salinity. Moreover, we suggest that it is likely that remnants of ECNA sea water persist in deep sediments at many locations along the Atlantic margin.
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Acknowledgements
H. Qi of the USGS in Reston assisted with δ2H, δ18O, and δ34S measurements of water and dissolved sulphate. N. Sturchio of the University of Chicago analysed δ37Cl. D. Webster and N. Bach of the USGS extracted pore waters from the cores in Reston, Virginia. J. Little, C. Johnson-Griscavage and J. Koch of the USGS extracted pore waters from the cores in Denver, Colorado. G. Casile of the USGS in Reston sampled the deep wells for dissolved gas analysis at Eyreville and Cape Charles. W. Aleman-Gonzalez, N. Bach, C. Durand, L. Edwards, J. Glidewell, J. Kirshtein, T. Kraemer, D. Larsen, M. Lowit, H. Michael, J. Murray, A. Palmer-Julson, E. Seefelt, J. Self-Trail, M. Voytek, D. Webster and B. Zinn all helped collect core samples during the three months of 24-hour drilling. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US government.
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W.E.S. compiled and interpreted the data, performed the transport simulations and wrote this article. M.W.D. analysed pore waters for the major anions and cations. T.B.C. analysed the pore waters and dissolved sulphate for their δ2H, δ18O and δ34S values. A.G.H. analysed the well-water samples for the total dissolved helium and helium isotope ratios. T.D.B. analysed the pore waters for the boron and strontium isotopic compositions and interpreted their values.
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Extended data figures and tables
Extended Data Figure 1 Results from major cation and anion analyses.
Results from the analyses of major cation and anion concentrations and their ratios in the crater corehole pore waters19. Chloride and bromide results are given in Fig. 2. The vertical dashed black lines represent concentrations or values for modern sea water. For additional discussion see Supplementary Information.
Extended Data Figure 2 Plot of strontium and boron isotope measurements.
Plot showing the water–rock interaction of pre-impact igneous basement and resurge blocks according to the mixing lines of dissolved strontium and boron isotopes. Blue diamonds and red squares represent analyses of pore waters from cores of pre-impact and post-impact sediments from the Eyreville drill hole, respectively. For additional discussion see Methods.
Extended Data Figure 3 Detailed results from chloride and 18O transport simulations.
Results from the numerical simulation of chloride and δ18O diffusion and vertical advection. Shown are the transient change in the top model boundary condition over time, including the upward migration of the sediment–water interface and the changes in chlorinity (a), the simulated change in chloride concentration with depth and time (b), and the simulated change in δ18O abundances relative to VSMOW (Vienna Standard Mean Ocean Water) with depth and time (c). Red lines are simulated values, and black diamonds shown in the 1-Myr and 0-Myr panels are observation data from cores for comparison. Simulations use the best-fit parameter values of Dm = 2 × 10−12 m2 s−1 and q = 3.5 m Myr−1. For additional discussion see Methods.
Extended Data Figure 4 Stable chloride isotope results.
a, Major lithologies drilled at the Chesapeake Bay impact crater, along with observed δ37Cl data and results of numerical simulations of δ37Cl relative to SMOC (standard mean ocean chloride) using the best-fit parameters of the chloride and δ18O advection–diffusion simulations (Fig. 2). Two conceptual models were investigated to explain the observed data, including isotopic fractionation using a ratio of diffusion coefficients of 37Cl/35Cl of 1.002, as observed in other argillaceous materials39 (b) and a δ37Cl anomaly in the resurge breccias created at the time of impact 35 Myr ago (c). The value of −14 per mil for the anomaly was the optimal value to fit the observed data. For additional discussion, see Supplementary Information.
Extended Data Figure 5 Uncertainty analysis for the chloride and 18O transport simulations.
Plots showing model uncertainty relative to the most realistic boundary conditions and global best-fit parameter values for Cl (a) and δ18O (e). Alternative boundary conditions include no sedimentation history (b, f), no transient change in the upper boundary salinity (c, g), and neither of these (d, h) for both chloride (b–d) and δ18O (f–h). Variations for the best-fit parameters including half (i, k, m, o) and twice (j, l, n, p) the best-fit values for both the coefficient of molecular diffusion (Dm) (i–l) and the magnitude of the upward fluid flux q (m–p). Filled and open circles represent those data points used in the sum-of-squared-errors (SSE) calculation, and those not used, respectively. Solid lines are simulation results. For plots i–p one of the two parameters is reset, and the other readjusted to a new best fit. For additional discussion see Methods.
Extended Data Figure 6 Simulation of pre-impact chloride profile.
Simulated chloride concentrations through the pre-impact coastal plain sediments. The black, red and blue lines indicate chloride concentrations at the end of the Early Cretaceous, at the end of the Cretaceous Period and at the time of the bolide impact, respectively. For additional discussion see Supplementary Information.
Supplementary information
Supplementary Information
This file contains the Supplementary Discussion and additional references. (PDF 189 kb)
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Sanford, W., Doughten, M., Coplen, T. et al. Evidence for high salinity of Early Cretaceous sea water from the Chesapeake Bay crater. Nature 503, 252–256 (2013). https://doi.org/10.1038/nature12714
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DOI: https://doi.org/10.1038/nature12714
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