Significance Chicxulub impact crater cores from the peak ring include ∼130 m of impact melt rock and breccia deposited on the first day of the Cenozoic. Within minutes of the impact, fluidized basement rocks formed a ring of hills, which were rapidly covered by ∼40 m of impact melt and breccia. Within an hour, ocean waters flooded the deep crater through a northeast embayment, depositing another 90 m of breccia. Within a day, a tsunami deposited material from distant shorelines, including charcoal. Charcoal and absence of sulfur-rich target rocks support the importance of impact-generated fires and release of sulfate aerosols for global cooling and darkness postimpact.

Abstract Highly expanded Cretaceous–Paleogene (K-Pg) boundary section from the Chicxulub peak ring, recovered by International Ocean Discovery Program (IODP)–International Continental Scientific Drilling Program (ICDP) Expedition 364, provides an unprecedented window into the immediate aftermath of the impact. Site M0077 includes ∼130 m of impact melt rock and suevite deposited the first day of the Cenozoic covered by <1 m of micrite-rich carbonate deposited over subsequent weeks to years. We present an interpreted series of events based on analyses of these drill cores. Within minutes of the impact, centrally uplifted basement rock collapsed outward to form a peak ring capped in melt rock. Within tens of minutes, the peak ring was covered in ∼40 m of brecciated impact melt rock and coarse-grained suevite, including clasts possibly generated by melt–water interactions during ocean resurge. Within an hour, resurge crested the peak ring, depositing a 10-m-thick layer of suevite with increased particle roundness and sorting. Within hours, the full resurge deposit formed through settling and seiches, resulting in an 80-m-thick fining-upward, sorted suevite in the flooded crater. Within a day, the reflected rim-wave tsunami reached the crater, depositing a cross-bedded sand-to-fine gravel layer enriched in polycyclic aromatic hydrocarbons overlain by charcoal fragments. Generation of a deep crater open to the ocean allowed rapid flooding and sediment accumulation rates among the highest known in the geologic record. The high-resolution section provides insight into the impact environmental effects, including charcoal as evidence for impact-induced wildfires and a paucity of sulfur-rich evaporites from the target supporting rapid global cooling and darkness as extinction mechanisms.

Impacts of asteroids and comets are a dominant geologic process on rocky planets (1). The largest impact structures—peak ring craters and multiring impact basins—exhibit annular rings of elevated topography surrounding their centers called peak rings. In 2016, a peak ring was drilled for the first time at the ∼200-km-diameter Chicxulub impact structure (Fig. 1) during International Ocean Discovery Program (IODP)–International Continental Scientific Drilling Program (ICDP) Expedition 364 (2, 3). Drill core showed that the bulk of the Chicxulub peak ring was formed from uplifted, fractured, and shocked granitic rocks with unusually low density and seismic velocity cross-cut by magmatic sheet intrusions and shear zones (2, 4). These results support the dynamic collapse model for peak ring formation (5), in which rocks temporarily flow like a viscous fluid, moving inward and upward to form a zone of central uplift and then, collapse outward and downward to form a peak ring (6). Within tens of seconds of the impact, a ∼40- to 50-km-radius transient cavity was formed (Fig. 2 A and B) and lined with impact melt (5). The main mass of this melt ends up inside the peak ring, forming the central impact melt sheet (Fig. 2), but some melt drapes and covers the peak ring and extends into the annular trough (7⇓–9).

Fig. 1. (A) Seismic reflection image shown in depth with full waveform velocities overlain; line runs from southeast to northwest, including the location of Site M0077, and radially outward across the annular trough. The suevite interval within M0077, the focus of this paper, is shown in red, which maps to a low-velocity zone beneath the crater floor. The map in Inset shows the location of crater rings, drill sites (in the text), the seismic image, and the direction that ocean waters reentered the crater after formation. Expansion shows (B) representative core images in stratigraphic order with depths, (C) lithologic units, and (D) lithology.

Fig. 2. Key events within the first day of the Cenozoic based on numerical modeling, geophysical data, and IODP Expedition 364 drilling. The figure includes 2 perspectives: a westerly oriented radial profile crossing inner crater rim and shallow shelf and a northeasterly oriented radial profile that crosses the opening in crater rim into the Gulf of Mexico. (A) Approaching 12-km-sized impactor over the preimpact target of the Yucatán peninsula. (B) A 100-km-wide transient crater and remnant of the impact plume consisting of vaporized/fragmented limestones, evaporites, and granitic basement rocks (timescale and geometry based on ref. 5). (C) Collapse of the transient crater with upward formation of a central uplift starting to undergo dynamic collapse (timescale and geometry based on refs. 5, 9, and 44). (D) Morphology after central uplift collapse and peak ring formation (based on refs. 2 and 9). Initial ocean resurge is depicted entering the crater with timescale based on a dam break model and undergoing MWIs. (E) Ocean resurge completes cresting the peak ring where Site M0077 was drilled. (F) Settling of debris within the now flooded crater to form the bulk of the suevite deposit that blankets the peak ring, with a zoomed-in view of processes (including seiches) and deposits capping the peak ring. (G) Tsunami entering crater from returning rim-wave tsunami and shelf collapses, with a zoomed-in view of the peak ring K-Pg deposits (timescale based on ref. 34). Rx, rocks.

Impact cratering is an extremely energetic process that results in the formation of a variety of breccia layers within and outside craters. One of the characteristic impact breccias is a polymict melt-bearing breccia, informally termed suevite, that contains shocked clasts (10⇓–12). Emplacement mechanisms for suevite vary among and within craters and with marine and nonmarine settings (13⇓⇓⇓⇓⇓⇓–20). Additionally, the sources of the material in these impactites are debated (21). For instance, occurrences of suevite have been attributed to a melt-rich flow from the overshooting central uplift during crater collapse (13) or to melt–water interaction (MWI) similar to molten fuel coolant interaction (MFCI) in volcanic processes (16, 22⇓–24).

The K-Pg impact event resulted in a globally distributed ejecta layer, which at distal sites (>6,000 km from Chicxulub), is highly condensed (2- to 3-mm thick) and contains altered microkrystites and shocked minerals (25). These impact deposits, which formally were deposited within the Cenozoic (Danian) (26), thicken and becomes more stratigraphically complex with proximity to the crater (25). K-Pg boundary sections around coastal and shelf sites in the Gulf of Mexico and Caribbean show a mixture of material delivered by airfall, shelf collapse, debris flows, and tsunami (27⇓⇓⇓⇓–32). Within the deep water Gulf of Mexico, earthquake energy from the impact triggered gravity flows on continental slopes, generating the largest known event deposit on Earth (33). Within the Chicxulub impact structure, the boundary event deposit was drilled onshore within the annular trough (sites ICDP Yaxcopoil-1 and Yucatan-6) and the central basin (sites Sacapuc-1 and Chicxulub-1) (Fig. 1) (11, 34⇓–36).

These records of the K-Pg boundary offer critical insights into the environmental effects of the Chicxulub impact and connections to the global extinction event (25, 37). The sedimentary target rocks, composed of volatile-rich marine carbonates and evaporites, have been a key focus of studies considering possible extinction mechanisms (38⇓⇓⇓⇓–43). Yaxcopoil-1 penetrated an intact Cretaceous Albian to Campanian slump block that consisted of 27% anhydrite and >70% carbonates (44, 45). Studies of the deposits outside the crater suggest that the deeper sedimentary target may have been even more evaporite rich (49 to 60% anhydrite) (46, 47). In the atmosphere, sulfate combines with water vapor to form sulfate aerosols that impede solar insolation; models of global climate response to a conservative 100 Gt of sulfur released by the Chicxulub impact into the K-Pg atmosphere indicated that global surface temperatures would have declined by >20 °C and that disruption of the Earth’s climate could have lasted ∼30 y (48). This scenario is consistent with proxy data indicating sea surface cooling in the months to years after the impact (30).

A sudden release of 425 ± 60 Gt of CO 2 and 325 ± 60 Gt of S was recently calculated by Artemieva et al. (49) using new constraints for the impact angle and direction of the Chicxulub impactor from Collins et al. (50); these larger sulfate amounts might result in prolonged cooling. Furthermore, Mössbauer analyses of boundary clay from proximal and distal sites have revealed that Fe nanoparticles, formed during the impact, served as nuclei for aerosols, causing prolonged darkness (51, 52). Ejecta descending from high altitudes can radiate heat and potentially ignite wildfires (53⇓⇓⇓–57). Soot within the K-Pg boundary layer indicates that extensive impact-induced fires occurred instantaneously or within months of the impact (58) and could have intensified global cooling (59). One of the major objectives of drilling at the peak ring was to explore evidence for the drivers of these profound environmental changes that were potentially responsible for the severity of the mass extinction at the K-Pg boundary.

IODP-ICDP Expedition 364 drilled into the offshore portion of the Chicxulub impact crater (2, 3). Site M0077 (Fig. 1) was located high on the topographic peak ring, providing a unique setting for examining the K-Pg boundary within the crater; the site was selected based on seismic images that suggested that the K-Pg boundary was located in a valley within the peak ring, which itself is elevated ∼400 m above the crater floor. We proposed that this location would preserve the full sequence of impact-related rocks and transition into the earliest Paleogene without significant erosion from postimpact slumping induced by earthquake aftershocks (Fig. 1). Site M0077 recovered core from 505.70 to 1,334.73 m below seafloor (mbsf) at nearly 100% recovery (2, 3).

Broader Implications In summary, the Expedition 364 core of the Chicxulub peak ring contains the most complete and expanded record of the immediate aftermath of the K-Pg mass extinction to date. Generation of a deep crater, with a large opening in the rim that allowed rapid flooding of the crater by seawater, produced accumulation rates among the highest known in the geologic record (130 m/d). Preservation of these extreme sediment accumulation rates within the impact basin allows us to resolve the geological processes that occurred over minutes to years after the impact event. In particular, the recovered sedimentary section lacks evaporites, supporting impact-generated sulfate aerosol production and extinction mechanisms, including global cooling and limitations on photosynthesis. The presence of melt breccia and suevite in cores suggests potential MWIs, and rounding and sorting provide evidence for ocean resurge. Finally, graded beds, a cross-bedded layer with terrestrial signatures, and charcoal provide evidence for seiches within the crater, a reflected tsunami, and some proximal fire generation within the first day of the Cenozoic.

Methods Forty-one thin sections of the suevite interval between core sections 40–2 and 87–2 were examined for the presence of gypsum (CaSO 4 · 2H 2 O), anhydrite (CaSO 4 ), and halite (NaCl) under a transmitted light microscope. Clasts identified as possible halite or sulfate phases are analyzed by small trace elemental maps on a Bruker M4 Tornado µXRF scanner following the methodology of de Winter and Claeys (75). Thin sections from Unit 1G were examined microscopically under plane and cross-polarized light using standard petrographic techniques. Bedding, lamination, ichnofabric, and other sedimentary structures were identified. Grains, matrix material, and diagenetic products were classified; their mineralogy was evaluated; and maximum grain size was measured using the microscope’s reticle. High-resolution, semiquantitative major and trace elemental maps of 53 polished thick sections between core sections 40–2 and 96–2 were produced by using the Bruker M4 Tornado µXRF scanner available at the Vrije Universiteit Brussel (75) (SI Appendix, Fig. S1). The elemental mapping measurements were executed using an Rh source and 2 XFlash 430 Silicon Drift detectors under vacuum conditions (20 mbar), with short acquisition times per spot size (1 ms per spot with a size of 25 µm) and maximized source energy settings (600 µA, 50 kV). Bulk compositions of the thick sections were quantified by identifying the X-ray peaks in a representative boundary spectrum within the high-resolution color map. Major elements were expressed as oxides (Na 2 O, MgO, Al 2 O 3 , SiO 2 , K 2 O, CaO, TiO 2 , MnO, Fe 2 O 3 ), whereas trace elements are in elemental configuration, as these are present at much lower concentration levels (P, S, Cl, V, Cr, Ni, Cu, Zn, Rb, Sr, Zr, Ba). The limit of quantification of elemental sulfur in the M4 Bruker Tornado µXRF is conservatively estimated to be 0.1 wt % (SI Appendix, Fig. S2). This limit is based on the quantification of µXRF maps of pellets prepared from carbonate reference materials NIST SRM 1d (National Institute of Standards & Technology) and BCS CRM 513 (Bureau of Analyzed Samples Ltd.). The limit of detection of S with this technique is 0.033 wt %. Back-scattered electron imaging, energy-dispersive spectrometry, and X-ray intensity mapping were used to describe the petrography of 12 suevite samples from the upper peak ring section between 620 and 708 mbsf. This work was done with a JEOL-JXA 8530F electron microprobe at Arizona State University’s Eyring Materials Center. For imaging and EDS analyses, an accelerating voltage of 15 kV, a beam current of 15 nA, and a focused beam were used. The X-ray intensity maps used an accelerating voltage of 20 kV, a beam current of 60 nA, a dwell time of 20 ms, and a beam diameter of 20 µm. These supplemental studies confirmed the absence of anhydrite or gypsum in the studied suevite. The Chicxulub cores were scanned using a Toshiba Aquilion Prime Dual Energy Helical CT scanner at Weatherford Laboratories in Houston. This produces a series of axial cross-section maps of attenuation coefficients at 2 energy levels (135 and 85 kV). Each cross-section represents 0.3 mm of core depth and has a spatial resolution of 0.25 mm. Processing of the raw CT was performed by Enthought Inc. (76). CT depth values (meters core composite depth below sea floor) are artificially lengthened relative to drillers depth (called meters below seafloor) due to overlaps in cores not being accommodated. CT images map the attenuation of X-rays at a given location in the core; this is represented using grayscale images, where darker grays are low attenuation and light gray is high attenuation (Fig. 4 E and F and SI Appendix, Fig. S5). X-ray attenuation is correlated to both the bulk density and average atomic number of a sample region. Dual-energy CT scans provide a mechanism to separate these effects (77). Line logging of drill cores is useful in the analysis of relative changes through the infill of impact craters, including slump and resurge deposits due to a marine target environment. This technique has been successfully applied to the Lockne, Tvären, and Chesapeake Bay impact structures (18−19, 78). In this method, every clast with a length axis larger than a certain cutoff size (here, 5 mm) that touches a line drawn along the core is assessed for size, roundness, and lithology (Fig. 3). Instead of using actual drill core, this technique was applied to high-resolution core photos with the use of the image analysis software jMicrovision (version 1.2.7). In the suevite (M0077 Unit 2), 2,376 clasts were analyzed between depths 672.01 and 715.93 mbsf, and an evaluation of the nature of their groundmass, whether matrix or clast supported, was done (i.e., if clasts were in contact or not with adjacent clasts). Roundness was estimated with the use of grain shape comparator (i.e., a standard diagram with drawings of grain shapes). Here, a diagram with only 4 shapes (i.e., angular, subangular, subrounded, rounded) was decided to be the most convenient (cf. ref. 79). CT images aided the lithological determinations. The lithologies were (preliminarily) classified into 17 categories that include (i) melt rocks of different colors and textures; (ii) upper target (sedimentary rock, mainly carbonates); and (iii) lower target (crystalline rock and quartzite). The granulometric data were treated statistically as variations per meter, which allowed plots of clast frequency per meter and size sorting. Owing to the large amount of data, clast vs. matrix support was plotted as a ratio per meter. Uncertainty in clast size is largest for the smallest analyzed clasts. We estimate that, at 15 pixels per 1 mm, a 5-mm grain could have an 8% error in the clast size value but that this uncertainty reduces greatly with increasing clast size. In order to efficiently examine a larger number of clasts, a deep learning model was applied to the high-resolution core photographs (Fig. 3). The pipeline has 2 stages: classification (SI Appendix, Fig. S4) and segmentation/shape analysis. In the classification step, a machine learning model is used to assign a lithology label to every pixel in the core photographs. The digital core photographs are red–green–blue images and have 3 features directly associated with each pixel location—the amount of red, green, and blue light that makes up the color of the pixel. This is local information, and classifying a pixel based only on color features neglects the spatial and textural context of a pixel. The images are passed to a pretrained convolutional neural network (VGG-16) (80), and intermediate activations are extracted, forming a hypercolumn of spatial convolutions that provide textural features useful for classification (81). Training data are created by manually labeling representative pixels that belong to each of the lithology types represented in the core images. The labeled pixels along with their associated hypercolumns are used to train a machine learning model (XGBoost) (82) to predict a lithology label for every pixel in the core images. The initial classification results are spatially regulated using a fully connected conditional random field (83). Individual clasts are observed in the classified images by identifying contiguous regions of pixels with the same lithology label that exceed a size threshold. Each clast is analyzed in terms of shape (perimeter, area), position, orientation, and aspect ratio using the image analysis package scikit- image (84). The circularity of each clast (4 × π × area/perimeter) is used as a roundness metric. The ratio of clast area to matrix area was calculated as a proxy for clast/matrix support. Sedimentologic analyses include visual observation of the split cores and point counting. Point counting to determine clast–matrix percentages was completed using JMicrovision on line-scan images. Each value represents 300 points counted on each core piece (SI Appendix, Fig. S6). During the Expedition 364 Onshore Science Party, 83 nonazimuthally oriented paleomagnetic cylinders were obtained from Units 2 and 3 (SI Appendix, Fig. S7). These samples were stepwise demagnetized using alternating field demagnetization up to at least 85 mT and measured using 2G Enterprises superconducting magnetometers at Rutgers University or CEREGE (France) or an AGICO JR-5 spinner magnetometer at CEREGE. Characteristic magnetization directions were obtained for the highest coercivity, origin-trending magnetization component within each sample using principal component analysis (85). Ferromagnetic mineralogy and Curie temperatures were determined via high-temperature magnetic susceptibility measurements using an AGICO Kappabridge susceptibility meter at CEREGE. Charcoal was first identified in smear slides using a Zeiss Axioskop microscope under bright-field light at 1,600× magnification. This illumination allowed the characteristic wood structure to be observed. The distribution and character of charcoal in samples were determined in thin sections using Zeiss Axioimager petrographic microscopes using bright-field illumination. Grains were also observed in an FEI Nova NanoSEM 630 FE scanning electron microscope. The relative abundance of charcoal was determined from the area of grains in 25 fields of view at 1,600× magnification using the NIH’s ImageJ software (SI Appendix, Fig. S8). The flooding rate of the Chicxulub crater at the approximate location of the Expedition 364 drill site on the peak ring was estimated assuming that resurge was dominated by ingress of water from the deep preimpact basin to the north–northeast (SI Appendix, Fig. S9). Analytical solutions to the 1-dimensional dam break problem (86) for 3 different dam heights ( h 0 = 0.5, 1, and 2 km) were found assuming that water entered the crater on one side through a deep channel in the rim, flowed across the crater, and reflected off the inside of the crater rim on the opposite (south–southwest) side of the crater. The different dam heights represent alternative estimates of the depth of water at the onset of the resurge. The upper estimate (2 km) represents the estimated maximum depth of the preimpact basin at the edge of the crater and is consistent with the numerical impact simulation shown in figure 5 of the work by Collins et al. (71). Flooding of the crater up to the depth of the peak ring (500 m above the crater floor) is expected to take 30 min to 1 h (SI Appendix, Fig. S9). The approximation neglects various factors that might delay the resurge, such as drag between the water and the crater floor and interactions between the flood water and the hot melt sheet, which might vaporize some of the water and generate MWI products. In order to analyze the cores for biomarkers (Fig. 4D and SI Appendix, Fig. S10), core samples (∼20 g) were surface cleaned by sonication in ultrapure water 2 times for 15 min to remove any drilling fluid. After, the samples were freeze dried and 3 times sonicated for 15 min in dichloromethane and methanol (9:1 vol/vol). After drying, the samples were ground using a pestle and mortar and Soxhlet extracted for 72 h using a mixture of dichloromethane and methanol (9:1 vol/vol). The extracts were passed through a Pasteur pipette containing activated copper powder to remove the elemental sulfur. Excess solvent was carefully removed under nitrogen. The weighed extracts were then fractionated by small-scale column liquid chromatography. The sample (up to 10 mg) was applied to the top of a small column (5 × 0.5-cm interior diameter) of activated silica gel (150 °C, 8 h). The saturated hydrocarbon fraction was eluted with n-hexane (4 mL), the aromatic hydrocarbon fraction was eluted with n-hexane and dichloromethane (4 mL, 9:1 vol/vol), and the polar fraction was eluted with a mixture of dichloromethane and methanol (4 mL, 1:1 vol/vol). The saturated and aromatic hydrocarbon fractions were analyzed by gas chromatography (GC)–mass spectrometry (MS). GC-MS analysis was performed using an Agilent 5975B MSD interfaced to and Agilent 6890 gas chromatograph, which was fitted with a DB-5MS UI capillary column (J and W Scientific; 60 m, 0.25-mm inner diameter, 0.25-μm phase thickness). The GC oven was ramped from 40 °C to 325 °C at a heating rate of 3 °C/min, with initial and final hold times of 1 and 30 min, respectively. Samples were dissolved in n-hexane and injected on column using an Agilent 7683B autosampler. Helium (constant flow 27 cm/s) was used as the carrier gas. The MS was operating with ionization energy of 70 eV, a source temperature of 230 °C, and an electron multiplier voltage of 1,706 V, scanning a mass range of 50 to 550 amu. Aromatic hydrocarbon compounds were identified by comparison of mass spectra and by matching retention times with those of reference compounds reported previously (87). The unsubstituted PAHs, including benzo(a)pyrene and perylene (the ratio of these 2 PAHs is shown in SI Appendix, Fig. S10), have a molecular ion of m/z 252. They were identified by comparing their retention times with those of reference compounds.

Acknowledgments We thank the captain and crew, drilling team, and technical staff who participated in shipboard and/or shore-based operations. We thank the editor and 2 anonymous reviewers. W. Zylberman and J. Gattacceca are thanked for assistance with paleomagnetic measurements. Data and samples can be requested from IODP. The European Consortium for Ocean Research Drilling (ECORD) implemented Expedition 364 with funding from the IODP and the ICDP. US participants were supported by the US Science Support Program and National Science Foundation Grants OCE 1737351, OCE 1736826, OCE 1737087, OCE 1737037, OCE 1736951, and OCE 1737199. J.O. was partially supported by Grants ESP2015-65712-C5-1-R and ESP2017-87676-C5-1-R from the Spanish Ministry of Economy and Competitiveness and Fondo Europeo de Desarrollo Regional. B.S. thanks Curtin University for an Australian Postgraduate Award. J.V.M. was funded by Natural Environment Research Council Grant NE/P005217/1. K. Grice thanks Australia Research Council for Grant DP180100982 and Australia New Zealand IODP Consortium for funding. The Vrije Universiteit Brussel group is supported by Research Foundation Flanders (FWO) and BELSPO; P.K. is an FWO PhD fellow. This is University of Texas Institute for Geophysics Contribution 3634.

Footnotes Author contributions: S.P.S.G., J.V.M., and G.L.C. designed research; S.P.S.G., T.J.B., J.O., B.H., K. Grice, B.S., S.L., K.H.F., J.V.M., N.A., P.K., S.J.d.G., M.T.W., G.S.C., S.M.T., C.V., G.L.C., P.C., M.J.L.C., S.G., K. Goto, N.M., G.R.O., A.S.P.R., U.R., J.S., V.V., A.W., and E.364.S. performed research; S.P.S.G., T.J.B., J.O., K. Grice, B.S., S.L., K.H.F., J.V.M., N.A., P.K., M.T.W., G.S.C., S.M.T., C.V., G.L.C., P.C., M.J.L.C., S.G., R.A.F.G., N.M., G.R.O., A.S.P.R., U.R., V.V., and A.W. analyzed data; and S.P.S.G., T.J.B., and J.V.M. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

Data deposition: All data are deposited with the International Ocean Discovery Program, iodp.pangea.de (Expedition 364).

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1909479116/-/DCSupplemental.