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EvidenceforhighsalinityofEarlyCretaceousseawaterfromtheChesapeakeBaycrater

ARTICLEinNATURE·NOVEMBER2013

ImpactFactor:41.46·DOI:10.1038/nature12714·Source:PubMed

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Evidence for high salinity of Early Cretaceous sea water from the Chesapeake Bay Crater Ward E. Sanford1, Michael W. Doughten1, Tyler B. Coplen1, Andrew G. Hunt2 & Thomas D. Bullen3

Published in the journal Nature, vol. 503, no. 7475, p. 252-256. 14 Nov 2013, doi:10.1038/nature12714

1USGS, Mail Stop 431, Reston, VA 20192, USA; 2USGS, Federal Center, Box 25046, Denver, CO 80225, USA; 3USGS, McKelvey Bldg 15, 345 Middlefield Rd, MS 420, Menlo Park, CA 94025, USA

High-salinity groundwater more than 1000 metres deep in the Atlantic Coastal Plain of the United States has been documented in several locations1,2, most recently within the 35 million-year-old Chesapeake Bay impact crater3-5. 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) seawater. We find that the seawater is likely 100-145 million years old and that it has an average salinity of about 70 per mil, which is twice that of modern seawater 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 paleontological analyses of solid materials in deep sediment cores. In contrast, our study identifies ancient seawater in situ and provides a direct estimate of its age and salinity. Moreover, we suggest that it is likely that remnants of ECNA seawater persist in deep sediments at many locations along the Atlantic margin.

Hypersaline groundwater in the deep Atlantic Coastal Plain of North America was discovered in several locations decades ago. Off the coast of Georgia, hypersaline brines (>100 g/kg) associated with anhydrite deposits exist at depths > 1.2 km in Lower Cretaceous strata1. On Cape Hatteras, North Carolina, drilling in 1946 recovered brine from 2.0 km with a salinity of 117 g/kg9. In southeastern Maryland, actual and estimated chloride concentrations of 40 and 50 g/kg were reported2,10 at 1.0 and 1.2 km. In southern New Jersey, at 1.0 km, a chloride concentration was observed at 25 g/kg.2 Observations have also indicated that the position of the freshwater-saltwater transition zone extends tens of kilometers farther inland near the southernmost Chesapeake Bay11. This “inland wedge” was eventually recognized as collocated with the Chesapeake Bay impact structure4,6, and hypersaline water was detected there at Kiptopeake, Virginia at 0.6 km depth (Fig. 1)4. Two original theories for the origin of the hypersalinity at the crater, evaporite dissolution and reverse osmosis6, have now been dismissed for the lack of proximal evaporites and the necessary hydrogeologic conditions, respectively12.

The Chesapeake Bay impact structure was created by a bolide impact 35 Myr ago in the Late Eocene3,6. Deep drilling by the U. S. Geological Survey (USGS) at Cape Charles, Virginia13,14 and the USGS and International Continental Drilling Program (ICDP) near Eyreville Neck, Virginia15,16 recovered core and revealed a stratigraphic section documenting the sediment and bedrock disruption associated with the “wet-target” impact. The test hole at Cape Charles14, drilled in 2004, revealed post-impact sediments underlain by synimpact sediment-clast resurge, crystalline-clast breccias, and suevites down to the bottom of the hole at 823 m. Wells at this site were installed at depths of 418 and 692 m. The deep hole near Eyreville, drilled in 2005, revealed post-impact sediments down to 444 m, sediment-clast resurge breccias down to 1,096 m, a granite megablock(s) down to 1,371 m, suevites, cataclasites, and crystalline-clast breccias down to 1, 551 m, and schist

and pegmatite allochthonous blocks down to 1,766 m (Fig. 2a). Drilling did not reach the bottom of the excavated crater. Drill-stem casing was left in the hole down to 1,330-m, which allowed for later sampling and chemical analysis of formation water.

Before data were retrieved from deep coring, impact heat and associated evaporation was suggested as a source of the elevated salinity. Simulations of the hydrothermal response to the impact suggested that hydrothermal and vapor phase conditions could exist during and following impact17. To investigate the thermal history of the crater and overlying sediments, vitrinite reflectance was measured on sediment cores from the Eyreville Neck and Cape Charles sites18. Results showed a thermal signature of upward movement of groundwater in the 10,000 yrs or so following impact, but a heat balance based on the suevite temperature and the quantity of hypersaline groundwater in the crater demonstrated there was inadequate heat to account for the excess salinity solely by steam production and loss19.  

 

  Figure 1 | Map showing locations of Chesapeake Bay Crater and coreholes. Map showing location of the Chesapeake Bay impact structure and sites of various deep coreholes and wells19. The black and red circular lines mark the outer edges of the inner and outer craters. The black-filled circle marks the location of the ICDP-USGS deep corehole and well15,16. The red-filled circle marks the location of the USGS corehole and wells at Cape Charles13,14. The blue-filled circle marks the location of the USGS corehole and wells at Kiptopeake4,5.

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Figure 2 | Chloride and isotope results from the deep ICDP-USGS corehole. Results from the ICDP-USGS corehole in the Chesapeake Bay impact crater near Eyreville, Virginia, including (a) drilling depths and sediment types recovered, (b) dissolved-chloride concentrations, (c) chloride-bromide mass ratios, (d) oxygen-isotope measurements of the pore water, and (e) dissolved-strontium and dissolved-boron isotope measurements. Open circles are 87Sr/86Sr mole ratio results, open squares are boron-isotope results. The solid red curved lines are simulation results assuming values of molecular diffusion (Dm) and upward fluid flux (q) from compaction of 2×10-11 m2 and 3.5 m/Myr, respectively. Additional details of the simulation methods can be found in the On-line Methods Section and Supplementary Information. Pore water was extracted from cores at the Eyreville site over the entire 1,766 meters. Ten-cm-long core samples were taken approxi-mately every 10–20 m, and water was extracted from the core by high-speed centrifuge. Analyses were performed to determine concentrations of major cations and anions (Extended Data Fig. 1), pH, alkalinity, and isotopic compositions of hydrogen (δ2H) and oxygen (δ18O) of water, dissolved boron (δ11B) and strontium (87Sr/86Sr), and sulfur (δ 34S) and oxygen (δ18O) of dissolved sulfate19. The dissolved chloride concentration at the site increases with depth continuously through the post-impact section, surpassing seawater concentration well before the top of the crater-fill deposits (Fig. 2b). Concentrations continue to increase with depth in the sediment-clast breccia erratically until reaching approximately 38 g/L (twice modern seawater) by 900 m. The Cl-Br mass ratio in pore waters at the site is relatively constant through the entire section at a mean value of 262 (modern seawater=292) (Fig. 2c). This rules out halite dissolution as a cause of elevated salinity20 and is indicative rather of a paleoseawater that is slightly depleted in chloride relative to bromide, consistent with the vast halite deposits created during the Jurassic and Cretaceous in the Gulf of Mexico and South Atlantic Basins. The δ18O values of the pore water at the site increase with depth (Fig. 2d) in a pattern similar to that of chloride. Strontium isotope ratios are known to have varied substantially in seawater since the Mesozoic (Early Cretaceous 87Sr/86Sr mole ratios varied between 0.7073 and 0.7075)21, and thus might be a good indicator of ancient seawater, but samples analyzed here reflect leaching of strontium22 from continental-sourced Potomac Formation sediment clasts hosting the water (Fig. 2e). Boron isotopic compositions also suggest a strong water-rock interaction, as δ11B values differ substantially from seawater (11 ‰) (Fig. 2e) and are consistent with movement through and interaction with igneous rocks in the crater and/or pre-impact crystalline basement (Extended Data Fig. 2).

Figure 3 | Helium-based age estimates for the deep Chesapeake crater groundwater. Estimated groundwater ages in the Mid-Atlantic Coastal Plain of North America based on dissolved helium concentra-tions. Blue diamonds represent estimates of groundwater ages from wells sampled in the coastal plain sediments of Maryland25. These ages are based on 4He-accumulation rates calibrated against 14C-based and 36Cl-based ages calculated for a set of wells in the same geologic formation as the crater wells25. Red triangles represent estimates of age for the groundwater in the Chesapeake Bay impact crater sampled at the well near Eyreville, Virginia. These latter age estimates are a linear extrapolation using the 4He-accumulation rates calculated from the Maryland samples.

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Figure 4 | Plate-tectonic reconstruction of the ECNA Ocean. Plate-tectonic reconstruction of the North Atlantic Ocean through the Early Cretaceous Epoch8 at (a) 145, (b) 130, (c) 115, and (d) 100 Myr ago. Red lines represent modern coastlines. Grey shapes represent continental plates and fragments. Open circle on the Atlantic coast of North America is the site of the late Eocene (35 Myr-old) Chesapeake Bay impact crater (CBIC). The maps are an attempt to delineate the edge of the continents, and do not show the continental regions that were covered by ocean water during the global Cretaceous transgression. They were constructed from a software program located at: http://www.odsn.de/odsn/services/paleomap/paleomap.html.

The curvature of the chloride and δ18O profiles suggests diffusion and slow upward advection as controlling processes23. One-dimensional transport simulations were carried out to test this hypothesis. The advection-diffusion equation was solved with an upper boundary condition that allowed for post-impact sequences of sediment to be deposited over time and for the upper boundary salinity to alternate between freshwater and saltwater, depending on the inferred position of the coastline from the known depositional environments and hiatuses (Extended Data Fig. 3). The simulated chloride values could be made to match the observed values very closely in the upper 500 m of the section (Fig. 2) when a diffusion coefficient (Dm) of 2×10-11 m2 was used along with an upward fluid flux (q) of 3.5 m/Myr and original chloride concentrations of 38 g/L in the sediment-clast-breccia section. Observed variable concentrations in the sediment-clast breccias lower than those simulated are attributed to clay clasts that are retaining brackish water19. The δ18O profile was also simulated with the same diffusion coefficient and upward fluid flux, and initial δ18O values (relative to the VSMOV-SLAP) of –1.0 and –0.8 ‰ in the coastal plain and sediment-clast breccias. The simulated δ18O values also make a close match with the observed data down to 500 m. Observed and

simulated δ37Cl values suggest possible isotopic effects related to the impact, but are also consistent with an ECNA origin for the pore water (Extended Data Fig. 4).

The minor inflections in the Cl and δ18O data can only be replicated by including sedimentation and temporal salinity changes at the upper boundary. The overall goodness-of-fit to these simulations (Extended Data Fig. 5) is evidence that the crater fill has been undisturbed by lateral groundwater movement since the time of impact, with the solutes being influenced only by diffusion and advection consistent with burial and compaction of fine-grained sediment23. The 38-g/L initial condition for the chloride simulation represents water that was trapped in and below the coastal plain sediments before the bolide impact occurred. Simulation of chloride concentrations in the pre-impact sediments confirms that most of the pore water would have remained undisturbed between the end of the Early Cretaceous period and the time of impact (Extended Data Fig. 6). The impact disrupted the sediment and basement crystalline rocks, along with their pore water, redepositing them in the crater. Because much of the crater fill is composed of clasts many meters in diameter, most of the initial pore water would have been retained through the collapse and resurge phases

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after impact16. Vertical pore-fluid flux due to loose-sediment compaction immediately following the resurge was estimated to be ~30 m/kyr18. But the much slower 3.5 m/Myr flux over the intervening 35 Myr indicated by the chloride and δ18O profile is consistent with loading rates during post-impact sedimentation.

Dissolved gas samples were collected from the deep wells at Cape Charles and Eyreville. At Eyreville, well casing in corehole B was left in situ at the very top of the granite megablock(s). Helium concentrations and isotope ratios (R/Ra) for deep Cape Charles and Eyreville groundwater were found to be 5.0×10-3 and 4.0×10-3 ccSTP/g, and 0.44 and 0.04, respectively. The low R/Ra number at Eyreville suggests this sample’s helium concentration represents predominantly in-situ helium accumulation over time24, and so we used that concentration to calculate a He-based groundwater age. Based on recent estimates of 4He accumulation rates in the coastal plain sediments25, the helium-age of the saline groundwater would be 180 ± 80 Myr (Fig. 3) given the variability in accumulation rates and the extrapolation to the much older age. The age can be further constrained to the Early Cretaceous (145–100 Myr ago), as the Lower-Cretaceous Potomac Formation (LCPF) is the oldest local sediment, resting directly on the crystalline basement under the coastal plain26. Given that the depositional environment of the LCPF is coastal fluvial and deltaic, the water in the LCPF and the upper crystalline basement would have been replaced with Early Cretaceous North Atlantic (ECNA) seawater during the global Cretaceous transgression.

For the North Atlantic, the Cretaceous Era was a time of transition from a closed rift basin to an open ocean (Fig. 4). During the Jurassic Era, the early North Atlantic must have had a very high salinity, evidenced by the fact it was depositing salt as evaporite deposits around its border27. Rifting that would lead to the South Atlantic was only beginning during the Early Cretaceous. As small openings to the ECNA waxed and waned and eventually became wider during the Early Cretaceous, the salinity would have fluctuated but slowly declined overall from highly saline to eventually a near-modern value. Anoxic events recorded in the ECNA suggest a lack of basin-wide circulation28, and some δ18O evidence from foraminifera in cores suggest a salinity higher than modern seawater29,30. By the end of the Early Cretaceous, the openings, especially to the South Atlantic, were becoming substantially wider (Fig. 4d). Even a small opening could lead to substantial dilution of salinity. As a modern example, the only opening to the Mediterranean Sea is the Strait of Gibraltar, yet its salinity (38 ‰) is only about 10 percent above that of modern ocean salinity. By the beginning of the Late Cretaceous (100 Myr ago) North Atlantic salinity was probably near that of modern seawater.

We conclude that the saline water currently present in the Chesapeake Bay impact crater’s resurge breccia and crystalline blocks is a remnant of ECNA seawater, and that the ECNA had an average salinity of about 70 ‰, or twice that of modern seawater. In addition, given that several other locations along the North American Atlantic Coastal Plain have hyperseawater salinities at depth, we believe remnants of ECNA seawater likely persist at many other locations along the Atlantic margin as well.

 References 1. Manheim, F., Paull C. Patterns of groundwater salinity changes in a deep

continental-oceanic transect off the southeastern Atlantic coast of the USA. J Hydrol. 54, 95-105 (1981).

2. Meisler H. The occurrence and geochemistry of salty groundwater in the Northern Atlantic Coastal Plain. U. S. Geol. Surv. Prof. Paper 1404-D. (1989).

3. Poag, C., Koeberl C., & Reimold, W. The Chesapeake Crater—Geology and Geophysics of a Late Eocene Submarine Impact Structure. Springer, Berlin (2003).

4. McFarland, E., Bruce, T. Distribution, origin, and resource-management implications of ground-water salinity along the western margin of the Chesapeake Bay impact structure in eastern Virginia. U. S. Geol. Surv. Prof. Paper 1688, K1-K32 (2005).

5. McFarland, E., Groundwater quality data and regional trends in the Virginia Coastal Plain, 1906-2007. U. S. Geol. Surv. Prof. Paper 1772. (2010).

6. Poag, C. Chesapeake Invader: Discovering America’s Giant Meteorite Crater. Princeton Univ. Press, Princeton (1999).

7. Sanford, W. Heat flow and brine evolution following the Chesapeake Bay bolide impact. J. Geochem. Explor. 78-79, 243-247 (2003).

8. Hay, W. et al. Alternative Global Cretaceous Paleogeography, in Barrera, E. and Johnson, E. (eds), The Evolution of Cretaceous Ocean/Climate Systems. Geol. Soc. Am. Spec. Paper 332, 1-47 (1999).

9. Spangler, W. Subsurface geology of Atlantic Coastal Plain of North Carolina. Amer. Assoc. Petrol. Geol. Bull. 34, 100-132 (1950).

10. Radford, L., Cobb, L., & McCoy R. Atlantic Coastal Plain Geothermal Drilling Program, DOE/Crisfield Airport No. 1 Well, Somerset County, Maryland. Dept. of Energy Tech. Rep. ET/28373-1 (1980).

11. Cederstrom, D. Chloride in groundwater in the coastal plain of Virginia. Virginia Geol. Surv. Bull. 58, 1-384 (1943).

12. Neuzil, C., Provost, A. Recent experimental data may point to a greater role for osmotic pressures in the subsurface. Water Resour. Res. 45, 1-14 (2009).

13. Sanford, W. et al. Drilling the central crater of the Chesapeake Bay impact structure: A first look. Eos Trans. AGU 85, 369,377 (2004).

14. Gohn, G. et al. Site report for USGS test holes drilled at Cape Charles, Northampton County, Virginia, in 2004. U. S. Geol. Surv. Open-File Rep. 2007-1094 (2007).

15. Gohn, G. et al. Chesapeake Bay impact structure drilled. Eos Trans. AGU 87, 349, 355 (2006).

16. Gohn, G. et al. Deep drilling into the Chesapeake Bay impact structure. Science 320, 1740-1745 (2008).

17. Sanford, W. A simulation of the hydrothermal response to the Chesapeake Bay bolide impact. Geofluids 5, 185-201 (2005).

18. Malinconico, M., Sanford, W., Horton, J. Postimpact heat conduction and compaction-driven fluid flow in the Chesapeake Bay impact structure based on downhole vitrinite reflectance data, ICDP-USGS Eyreville deep core holes and Cape Charles test holes. Geol. Soc. Am. Spec. Paper 458, 905-930 (2009).

19. Sanford, W. et al. Pore-water chemistry from the ICDP-USGS core hole in the Chesapeake Bay impact structure—Implications for paleohydrology, microbial habitat, and water resources. Geol. Soc. Am. Spec. Paper 458, 867-890 (2009).

20. Davis, S., Whittemore, D., & Fabryka-Martin, J. Uses of chloride/bromide ratios in studies of potable water. Ground Water 36, 338-350 (1998).

21. Jones, C., Jenkyns, H., Coe, A., & Stephen, H. Strontium isotope variations in Jurassic and Cretaceous seawater. Geochim. Cosmochim. Acta 58, 3061-3074 (1994).

22. Bullen T., Krabbenhoft, D. & Kendall, C. Kinetic and mineralogic controls on the evolution of groundwater chemistry and 87Sr/86Sr in a sandy silicate aquifer, northern Wisconsin, USA. Geochim. Cosmochim. Acta 60, 1807-1821 (1996).

23. Ingebritsen, S., Sanford, W., & Neuzil, C. Groundwater in Geologic Process. Cambridge Univ. Press, New York (2006).

24. Oxburgh, E., O’Nions, R., & Hill, R. Helium isotopes in sedimentary basins. Nature 324, 632-635 (1986).

25. Plummer, L. et al. Old groundwater in parts of the upper Patapsco aquifer, Atlantic Coastal Plain, Maryland, USA: evidence from radiocarbon, chlorine-36 and helium-4. Hydrogeol. J. 20, 1269-1294 (2012).

26. McFarland. E., & Bruce, T. The Virginia coastal plain hydrogeologic framework. U. S. Geol. Surv. Prof. Paper 1731 (2006).

27. Evans, R. Origin and significance of evaporites in basins around Atlantic margin. Am. Assoc. Petrol. Geol. Bull. 62, 223-234 (1978).

28. Arthur, M., & Natland, J. Carbonaceous sediments in the North and South Atlantic: The role of salinity in stable stratification of early Cretaceous Basins. Deep Drilling in the Atlantic Ocean: Continental Margins and Paleoenvironment 3, 375-401 (1979).

29. Friederich, O., Erbacher, J., Moriya, K., & Wilson, P. Warm saline intermediate waters in the Cretaceous tropical Atlantic Ocean. Nat. Geosc. 1, 453-457 (2008).

30. Wagner, T. et al. Rapid warming and salinity changes of Cretaceous surface waters in the subtropical North Atlantic. Geology 36, 203-206 (2008).

   

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METHODS Core Sample Collection. At the Eyreville ICPD-USGS site, 10-cm-long sections of core were removed every 10-20 m for pore-water analysis. Cores from core hole A were 8.5 cm in diameter from 126 to 591 m and 6.35 cm from 592 to 941 m, whereas those from core hole B were 6.35 cm from 738 m to 1101 m and 4.76 cm from 1101 m to 1706 m. Most of the unconsolidated cores removed for pore-water analysis were bisected, and one half was returned for archival purposes. All core split samples were collected before the cores were washed. A few millimeters of the outer rind of each sample were removed with a knife to remove visible drilling mud from the outside of the core and any that may have invaded the outer edge of the core. Core sections were then placed in glass jars that had been flushed with nitrogen, sealed, and refrigerated at 10 °C. The jarred samples were then transported and stored at a USGS laboratory in Reston at 5 °C until pore-water extraction. Core Sample extraction. The core samples were placed in one of two USGS high-speed centrifuges in Reston, Virginia, and Denver, Colorado, to extract pore water. Samples were removed from the jars and pulverized, and subsamples were spun at room temperature at 40,000 rpm for ~30 min. Once collected, the pore-water samples were analyzed for pH, alkalinity, and specific conductance19 by using Hach small-volume test kits. The remaining water was then separated into vials for various analyses and refrigerated. Water and solute analyses included major anions and cations, δ2H and δ18O of water, 87Sr/86Sr of dissolved strontium, dissolved organic carbon (DOC), and δ34S and δ18O of sulfate19. To ensure the samples were not contaminated by drilling fluids, selected samples were also analyzed for polar-organic compounds that were characteristic of the drilling fluids31. Drilling fluids were not detected in any of the samples. Dissolved ion and isotope methods. Analyses for cations and anions and the stable isotopes of water were performed in USGS laboratories in Reston, Virginia. The anions and cations were analyzed by ion-chromatography and inductively coupled plasma-optical spectroscopy, respectively. Hydrogen and oxygen isotopes of water were measured, respectively, by gaseous hydrogen32 and carbon dioxide equilibration33 and dual-inlet isotope-ratio mass spectrometry. Sulfur and oxygen isotopes of dissolved sulfate were analyzed by conversion of sulfate to barium sulfate34 and analysis by continuous flow isotope-ratio mass spectrometry35. Strontium and Boron isotopes were measured in a USGS laboratory in Menlo Park, California by inductively thermal ionization mass spectrometry22, and negative thermal ionization mass spectrometry36, respectively. Chloride stable isotopes were analyzed at the University of Chicago by continuous-flow isotope ratio mass spectrometry37. Helium sample collection. In the deep wells at both Eyreville and Cape Charles, water samples were retrieved by using a 6-L, stainless steel Kemmerer sampling flask attached to a portable cable winch fitted with a digital depth counter. The deep wells were first purged to ensure that pristine formation water was in the well casing. After being lowered to the near the bottom of the well casing, the Kemmerer flask was closed by a sender weight and retrieved. The dissolved gas samples were then transferred to a 0.30 m length of 1-cm-diameter copper tubing, first allowing the water sample to flow from the Kemmerer sampler through the tubing. Once gas bubbles were purged from the tubing, it was crimped on both ends and sealed with refrigerator clamps. Helium measurement methods. Dissolved helium and helium isotope ratios were measured in the USGS Noble Gas Laboratory in Denver, Colorado. The dissolved gases were separated from the formation water in the laboratory and analyzed in an ultra-high vacuum extraction system. Total pressure of the extracted gases was measured on a capacitance manometer. Helium was separated from the sample gas using a cryogenic cold trap and analyzed for isotopic composition statistically on a MAP-215-50 mass spectrometer38. Helium data interpretation. Helium accumulation rates were not estimated or calibrated in this study, but were calibrated against 14C and 36Cl abundances in another recent study near the same vicinity25. The calibration results from that recent study allowed us to make an estimate of the age using helium. We acknowledge that the estimate is a large extrapolation from the measured ages and thus represents an estimate that is far from precise (our error bars are 80 Myr). The validity of the extrapolation is supported, however, by the fact that the helium from the calibration study was extracted and measured from a very similar section of the Potomac Formation that the helium from this study was extracted from. In this respect, “deep crustal” contributions of helium that might be present would have contributed to the calibrated and extrapolated He values in a similar fashion. As additional evidence, deep sources of helium often have a mantle, 3He, signature. We measured 3He/4He ratios (R/Ra) (reported in the main text of this article) and found the value at the deep drill site to be close to zero, suggesting the helium was solely from crustal in-situ decay, with a negligible mantle source. In contrast, the helium at the center of the crater had a value of R/Ra substantially above zero, perhaps influenced by a deeper source originating through deep crustal fractures that were generated during the impact. In general

we consider the helium-based age to be one piece of circumstantial evidence in this study. It is not proof of the age, but it is consistent with the other pieces of evidence. The age is further constrained to not be older than Early Cretaceous (by the age of the sediment) and not younger (because the North Atlantic then became hydrologically open). Strontium and boron isotopes. Rock-water interaction is indicated by observing a combination of the dissolved strontium and boron isotopes (Extended Data Fig. 2). The values do not represent modern seawater—especially boron (11B), which has a modern seawater value approximately +40 per mil relative to NIST SRM 951 boric acid. The data do trend, however, in the direction of water-rock interaction with igneous or igneous-derived sediments, the latter two of which have typical values that could easily fall in the range indicated by “Pre-Impact Basement Igneous Rock” in extended data figure 2. Mixing with modern seawater is also suggested by the isotopes, which is consistent with the 35 million years of diffusion that has occurred between the ECNA and modern seawater in the crater. Concentration profile simulation. The process of molecular diffusion and upward advection of groundwater was simulated by solving the advection-diffusion equation for the flow of groundwater in a porous medium23:

t

C

z

Cq

z

CDm

2

2

(1)

Where Dm is the coefficient of molecular diffusion, [L2/T]; C is the concentration, [M/L3]; z is the vertical distance [L], q is the specific discharge [L/T], φ is porosity, and L, T, and M, are the dimensions of length, mass, and time, respectively. For the simulation of δ37Cl profiles (supplementary information), equation (1) must be solved for both 35Cl and 37Cl and the δ37Cl values calculated from the two results39.

Equation 1 was solved for the vertical concentration profiles of chloride and δ18O in the crater from the land surface down to 1,400 meters depth (Extended Data Fig. 3). A finite difference approximation method was used with a spatial discretization of 15 m, and time steps of 25 thousand years. The finite difference equation was solved in space and time explicitly using an Excel spreadsheet. The upper boundary condition represented the top of the sediment column as it accumulated over 35 million years since impact40. The chloride and δ18O boundary conditions at the top of the profile were allowed to switch back and forth between fresh and sea water conditions, depending on the environment at the time. The upward advective fluid flux, q, represents the upward movement of pore water being slowing expelled by compaction of the entire sediment column over time. In reality the fluid flux would have varied with depth and over time, but here a single value was used as a simplifying assumption. This assumption was adequate to fit the data, and the additional complexity of variations in q was not justified here based on scatter in the data that was being fit. The evolving concentration profiles are shown in extended data figure 3. Porosity values were assigned that approximated those from the cores19—0.45 from 0 to 230 m, 0.60 from 225 to 345 m, 0.45 from 345 to 445 m, and 0.3 from 445 to 1,400 m. As with the fluid flux, the porosity varied more with depth and over time, but this approximation was adequate to fit the data.

The molecular diffusion coefficient and fluid flux value, q, were adjusted until a best fit was made with both profiles. The best fit values and profiles for chloride and δ18O are shown in extended data figures 5a and 5e, respectively. The uncertainty in these values was investigated by varying the boundary conditions and parameter values (Extended Data Fig. 5). Simulations without the change in the concentration or sedimentation history at the upper boundary, or either, show poorer fits. The plots shown include adjusting the values of Dm and q to obtain the best fit possible. The values of Dm and q were also adjusted upward and downward by factors of two, to show the sensitivity of the fit to the parameter. The sum of squared errors (SSE) is listed for all simulations, and increases substantially for all of the alternate simulation conditions other than the global best-fit values. References 31. Rostad, C., & Sanford, W. Polar organic compounds in pore waters of the

Chesapeake Bay crater deep core hole: Character of the dissolved organic carbon and drilling fluid detection. Geol. Soc. Am. Spec. Paper 458, 891-903 (2009).

32. Revesz, K. & Coplen T. Determination of the δ(2H/1H) of water, U. S. Geol. Surv. Techn. & Meth., book 10, chap. C1 (2008).

33. Revesz, K. & Coplen T. Determination of the δ(18O/16O) of water, U. S. Geol. Surv. Techn. & Meth., book 10, chap. C2 (2008).

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34. Carmody R. et al. Methods for collection of dissolved sulfate and sulfide and analysis of their sulfur isotopic composition. U. S. Geol. Surv. Open-File Rep.. 97-234 (1997).

35. Brenna, J. et al. High-precision continuous-flow isotope ratio mass spectrometry. Mass Spectr. Rev. 16, 227-258 (1997).

36. Barth, S. Boron isotopic analysis of natural fresh and saline water by negative thermal ionization mass spectrometry. Chem. Geol. 143, 155-261 (1997).

37. Sturchio, N. et al. Stable isotopic composition of chlorine and oxygen in synthetic and natural perchlorate. Perchlorate, Environmental Occurrence, Interactions and Treatment. Springer, New York, 93-107 (2006).

38. Bayerle U. et al. A mass spectrometric system for the analysis of noble gases and tritium from water samples. Environ. Sci. Technol. 34, 2042-2050 (2000).

39. Mazurek, M. et al. Natural tracer profiles across argillaceous formations: The CLAYTRAC project. University of Bern, Villigand, Switzerland (2009).

40. Browning, J. et al. Integrated sequence stratigraphy of the postimpact sediments from the Eyreville core holes, Chesapeake Bay impact structure inner basin. Geol. Soc. of Amer. Spec. Paper 458, 755-810 (2009).

Acknowledgements. Haiping Qi of the USGS in Reston assisted with δ2H, δ18O, and δ34S measurements of water and dissolved sulfate. Neil Sturchio of the University of Chicago anlayzed δ37Cl. Daniel Webster and Nicole Bach of the USGS extracted pore waters from the cores in Reston, Virginia. Justin Little, Christine Johnson-Griscavage, and Joshua Koch of the USGS extracted pore waters from the cores in Denver, Colorado. Jerry Casile of the USGS in Reston sampled the deep wells for dissolved gas analysis at Eyreville and Cape Charles. Wilma Aleman-Gonzalez, Nicole Bach, Colleen Durand, Lucy Edwards, Jennifer Glidewell, Julie Kirshtein, Tom Kraemer, Dan Larsen, Michael Lowit, Holly Michael, Jim Murray, Amanda Palmer-Julson, Ellen Seefelt, Jean Self-Trail, Mary Voytek, Daniel Webster, and Brendan Zinn all helped collect core samples during the 3 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 U. S. Government.

Author Contributions Ward Sanford compiled and interpreted the data, performed

the transport simulations, and wrote this article. Mike Doughten analyzed pore waters for the major anions and cations. Tyler Coplen analyzed the pore waters and dissolved sulfate for their δ2H, δ18O, and δ34S values. Andrew Hunt analyzed the well-water samples for the total dissolved helium and helium isotope ratios. Thomas Bullen analyzed the pore waters for the boron and strontium isotopic compositions and interpreted their values. SUPPLEMENTARY INFORMATION Overall chemical composition of the pore waters. In addition to measurements of chloride and bromide, concentrations of other ions, including calcium, strontium, magnesium, sodium, potassium, sulphate, and bicarbonate were measured in the pore waters (Extended Data Fig. 1)19. There has been scientific speculation on how much the major-ion composition of seawater has changed through geologic time. Although the preservation of ECNA seawater at this site has the potential to provide such information, the concentration profiles of the other major ions suggest that diagenesis has altered their concentrations substantially. Clay diagenesis and albitization can account for much of the increase in calcium concentration at the expense of sodium and magnesium. Sulphate reduction has likely consumed much of the dissolved sulphate in the crater breccias, although elevated sulfate values are present in some post-impact sediments. The calcium-magnesium and sodium-potassium ratios suggest there has been a substantial trend in these processes with depth. Preimpact Salinity in the Sediments. The conclusion that the chloride concentrations in the deep crater-fill sediments can be traced back to the ECNA assumes that concentration levels did not change substantially

between the Early Cretaceous and the time of impact in the Late Eocene. That time frame spans the time between 100 and 35 Myr ago. This 65 Myr span is long enough for molecular diffusion to have diluted the sediment pore waters to some degree, and so calculations are warranted to demonstrate that the dilution from the diffusion of lower chlorinity waters overlying the sediments would not have been substantial. Such a calculation was performed (Extended Data Fig. 6) using estimates of a pre-impact sediment thickness of approximately 650 m for the Early Cretaceous age sediments of the Potomac Formation, and 150 m for the overlying sediments that were deposited in the Late Cretaceous, Paleocene, and Eocene time periods40. The advection-diffusion equation (see online-only methods section) was solved for a 65 Myr time period beginning with an initial concentration of 38 g/L chlorinity. The post-Early-Cretaceous-age sediment was added linearly over the 65 Ma with an upper boundary condition of the modern chlorinity of seawater (19 g/L). The same values for the diffusion coefficient and upward fluid flux were used that were calibrated for the post-impact simulation. The simulated chloride concentration profiles for the end of the Cretaceous (65 Myr ago) and the time of impact (35 Myr ago) indicate that the chloride concentrations of three-quarters of the thickness of Early Cretaceous sediments remained virtually undisturbed between the time they were deposited and the time of impact. Much of these same sediments were rearranged into the crater by landslides and avalanches following the impact.

An important process of note that allowed for the chloride concentrations to remain undisturbed is the continual slow compaction of the sediments. The upward expulsion of pore waters counteracts the downward displacement of chloride by diffusion from above, which otherwise would have had a larger (but far from complete) diluting effect on the pore water in the Early Cretaceous sediment. In addition, a substantial amount of the uppermost crystalline basement in the form of granite, schist, and pegmatite blocks (which also would have contained ECNA seawater) was also disrupted during the impact and transported into the crater. None of the simulations in this study included a numerical grid that compacted with the sediments. As in the post-impact simulation, the upward compaction fluid-flux value used is an average, and in reality the value would vary both in space and time. We believe, as in the post-impact simulation, that the compaction parameters are not known with enough certainty to warrant this additional detail to be included in such simulations at this time, although such a more rigorous simulation in the future might reveal additional insights into the details of how this system may have evolved. Chlorine Isotopes. We did not measure any 36Cl isotopic abundances because 36Cl abundances could not be measured on the very small amounts of water obtained from the cores due to analytical sample-size requirements. 36Cl abundances were not measured in the well sample in which helium was measured in for two reasons—(1) 36Cl has a half-life of about 300 thousand years, and (2) there was likely still a few percent modern drilling fluid in the sampled well water. The combination of these two meant that a very old (tens of millions of years) water could easily be contaminated by a few percent modern drilling water and appear to give an age of 1-to-2 million years, whereas that interpreted age would actually be erroneous. The same problem does not exist for helium because the formation water was extremely high in helium and the few percent of contaminant drilling water has virtually zero helium.

Chlorine stable isotopic compositions are affected by evaporation in salt deposits, but the differences are relatively small and they do not trend substantially with age41, and thus would not be a good indicator of age. However, we measured δ37Cl on numerous samples down to depths of 1,000 m. Dissolved chloride becomes more depleted in 37Cl downward to the top of the impact sediment at 444 m, and then becomes more enriched in 37Cl downward into the resurge breccias (Extended Data Fig. 4). Two conceptual models were tested to explain the δ37Cl observations: (1) isotopic fractionation by molecular

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diffusion, and (2) the decay of a low (negative) δ37Cl anomaly established at the time of impact. Numerical simulation of isotopic fractionation associated with the best-fit advection and diffusion parameters would give the magnitude of the observed δ37ClSMOC data, but yield a much different pattern (Extended Data Fig. 4b). Simulation of the decay of an anomaly from the time of impact can be made to match the δ37Cl data if the initial anomaly is around -14 per mil (Extended Data Fig. 4c). An initial anomaly such as this would have resided in, and might have been associated with, hydrothermal activity at or following the time of impact. No known geologic fluid has been observed with a δ37Cl value as low as -14 per mil42. Gases in high temperature volcanic fumaroles have been observed with values as high as +12 per mil43, suggesting residual hydrothermal water could be left with values on the order of -12 per mil. Thus it is not implausible that heating and boiling of seawater at the time of impact left the resurge breccia pore waters initially with values of δ37Cl in the range of -14 per mil. The total chloride observations and simulations (Fig. 2) make it clear that any such hydrothermal event did not make a substantial impact on the total chloride concentrations and produce the elevated salinity in the deep pore water. Whatever the mechanism responsible for the observed δ37Cl anomaly, the δ37Cl simulations demonstrate that

the observed isotopic values deeper than 800 m, which are close to zero and similar to that of Jurassic44 and modern45 seawater, are consistent with a ECNA seawater origin of the high-salinity pore water.

References 41. Eastoe, C. et al., Stable chlorine isotopes in Phanerozoic evaporites.

Appl. Geochem. 22, 575-583 (2007). 42. Barnes, J. et al., Chlorine isotope variations along the Central

American volcanic front and back arc. Geochem. Geophys. Geosy. 10, 1-17 (2009).

43. Sharp Z. D. et al., An experimental determination of chlorine isotope fractionation in acid systems and applications to volcanic fumaroles. Geochim. Cosmochim. Ac. 74, 264-273 (2009).

44. Eastoe, C. et al., Stable chlorine isotopes in halite and brine from the Gulf Coast Basin: brine genesis and evolution. Chem. Geol. 22, 343-360 (2001).

45. Shirodaker, P. et al., Influence of air-sea fluxes on chlorine isotopic composition of ocean water: Implications for constancy in delta-Cl-37—A statistical inference. Environ. Int. 32, 235-239 (2006).

 

 

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 waters

19.   Chloride and bromide results are given in figure 2 (main paper).  The vertical dashed black lines represent concentrations or values for modern seawater.  For additional discussion see Supplementary Information available online. 

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Extended Data Figure 2 | Plot of Strontium and Boron Isotope measurements. Plot showing water-rock interaction of pre-impact igneous basement and resurge blocks according to mixing lines of dissolved strontium and boron isotopes. Blue diamonds are red squares represent analyses of pore waters from cores of pre-impact and post-impact sediments from the Eyreville drill hole. For additional discussion see extended Methods section available online.

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Extended Data Figure 3 | Detailed results from chloride and 18O transport simulations. Results from the numerical simulation of chloride and oxygen-18 diffusion and vertical advection. Shown are the (a) transient change in the top model boundary condition over time, including the upward migration of the sediment-water interface and the changes in chlorinity, (b) simulated change in chloride concentration in depth and time, and (c) simulated change in oxygen-18 abundances relative to VSMOW (Vienna Standard Mean Ocean Water) with depth and time. Red lines are simulated values, and black diamonds shown in the 1 and 0 million year panels are observation data from cores for comparison. Simulations are using the best fit parameter values of Dm = 2×10-12 m2/sec and q = 3.5 m/Myr. For additional discussion see the expanded Methods section available online.

 

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Extended Data Figure 4 | Stable chloride isotope results. Major lithologies (a) 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 (b) isotopic fractionation using a ratio of diffusion coefficients of 37Cl/35Cl of 1.002 as observed in other argillaceous materials39, and (c) a δ37Cl anomaly in the resurge breccias created at the time of impact 35 million years ago (Ma). The value of -14 per mil for the anomaly was the optimal value to fit the observed data. For additional discussion see Supplementary Information available online.

 

 

 

 

 

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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). Alternate 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 error (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 the expanded Methods section available online.

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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, and at the time of the bolide impact, respectively. For additional discussion see Supplementary Information available online.