Modelling the End Cretaceous Chicxulub Impact Event suggests a bolide with a low impact trajectory.

The 66 million year ago asteroid impact event that formed the Chicxulub Crater, Mexico, marks the end of the Mesozoic Era of Earth history and has been attributed as the cause of the contemporaneous mass extinction. Numerical impact simulations combined with geophysical investigation of subsurface structure have constrained the kinetic energy of the impact under the simplifying assumption of a vertical trajectory. The trajectory angle and direction of the Chicxulub impact are not known, but a near-vertical impact is unlikely. Only one quarter of impacts occur at angles between 60
° and the vertical and only 1 in 15 impacts is steeper than 75
°.

For constant impactor size and speed, a shallower impact angle produces a smaller crater, a more asymmetric dispersal of ejected material and partitions more impact energy at shallower depths. As a result, impact direction and trajectory angle to the target plane are important impact parameters that determine, among other things, the direction of most severe environmental consequences and the volume and depth of origin of vaporised target, as well as ejecta and crater asymmetries.

Since the discovery of the Chicxulub impact structure based on diagnostic evidence of shock metamorphism and geophysical anomalies, several asymmetries in the geophysical character of the crater have been noted, which may result from oblique impact and/or impact in a heterogeneous target. Among the most obvious of these are radially oriented gravity lows to the south and northeast, and a radial gravity high to the northwest. However, these large-scale, peripheral features are all likely to be pre-existing features, unrelated to the impact. As models of the subsurface based on potential field data have inherently poor resolution and suffer from non-uniqueness, the most robust evidence of asymmetry comes from seismic reflection and refraction data. High-resolution seismic reflection images along a concentric arc outside the crater rim clearly show that the northeastern gravity low in the offshore half of the crater occurs in an area where Cretaceous and Cenozoic sedimentary rocks are particularly thick, and the northwestern gravity high occurs where this sedimentary sequence is thinnest and basement rocks are closer to surface. Given the observed correlation between the gravity signature and depth to basement outside the crater, the thickness of the sedimentary sequence is the most likely control on the offshore gravity anomaly. There is therefore no evidence that the azimuthal asymmetry in the outer gravity signature is impact related.

A nominal geographical centre of the crater is defined by the geometric centre of the crater rim demarcated by both a circular high in horizontal gravity gradient and the prominent cenote ring. Relative to this point, the centre of both the central gravity high, attributed to the uplift of dense lower-crustal rock, and the surrounding annular gravity low, which underlies the inner edge of the peak ring, are shifted several km to the west-southwest. In contrast, three-dimensional seismic velocity data indicate that the maximum uplift of the mantle beneath the crater occurs about 10 km to the north-northeast of the crater centre.

The south-westerly offset of the central gravity high relative to the crater centre was previously interpreted as indicating impact from the southwest, on the premise that central uplift motion would be directed uprange. An alternative interpretation, of a trajectory from the southeast, was proposed on the basis of a northwest–southeast elongation of the central gravity high and magnetic anomaly, and the northwest truncation of the annular gravity low. However, seismic reflection and refraction data reveal that the zone of structural uplift is not elongated towards the northwest and that the truncation of the gravity low in the northwest is a pre-impact feature of the regional anomaly caused by the shallow depth to basement in this direction. The short-wavelength component of the magnetic anomaly shows a slight (10%) elongation in the northwest–southeast direction, but is also offset to the southwest of the crater centre. The short wavelength and steep gradients of this anomaly both suggest a shallow source, probably related to the melt sheet and impact breccia and not to structural crater asymmetry. On the other hand, the long-wavelength component of the magnetic anomaly is a magnetic high elongated and offset along a direction southwest of the crater centre, consistent with a zone of uplifted basement rocks southwest of the crater centre.

In a paper published in the journal Nature Communications on 26 May 2020, Gareth Collins, Narissa Patel, and Thomas Davidson of the Department Earth Science and Engineering at Imperial College London, Auriol Rae, also of the Davidson of the Department Earth Science and Engineering at Imperial College London, and the Institute of Geology at the University of Freiburg, Joanna Morgan again of the Department Earth Science and Engineering at Imperial College London, and Sean Gulick, of the Institute for Geophysics and Department of Geological Sciences at the University of Texas at Austin, use 3D numerical modelling to examine the relationship between impact angle and structural crater asymmetries in a Chicxulub-scale peak-ring crater in a flat-layered target without lateral pre-impact asymmetry.

 

Asymmetries of the geophysical signature of the Chicxulub crater. Background colourmap shows Bouguer gravity anomaly map in the vicinity of the crater. The red circle marks the nominal position of the crater centre; the green circle marks the centre of maximum mantle uplift; the blue circle marks the centre of the peak ring (as defined by the annular gravity low surrounding the central high); the white triangle marks the location of the Expedition 364 drill site through the peak ring (Hole M0077A). The coastline is displayed with a thin white line; cenotes and sinkholes with white dots, and the city of Mérida with a white square. The dotted lines offshore mark the approximate location of the inner crater rim and the extent of faulting as imaged by seismic data. Inset depicts the regional setting, with red rectangle outlining the region shown in the gravity map. Collins et al. (2020).

Collins et al. show that the observed asymmetry in the positions of the central uplift, peak-ring centre and maximum mantle uplift, relative to the crater centre, can be attributed to the angle and azimuth of the impact trajectory. Comparison of our simulation results with geophysically constrained models of the Chicxulub crater structure is used to infer the likely trajectory and angle of the impact. The recent joint International Ocean Discovery Program and International Continental Scientific Drilling Program Expedition 364 recovered  about 600m of peak-ring rocks from the Chicxulub crater that provide additional constraints to discriminate between impact scenarios. Our simulations also reveal azimuthal variation in peakring material properties, which provide context for International Ocean Discovery Program-International Continental Scientific Drilling Program Expedition 364 core analysis.

 

Development of the Chicxulub crater for a 30
impact. The impact scenario depicted is for a 21-km diameter impactor with a density of 2630 kg/m³ and a speed of 12 km/s. Evolution of the crater up to 5 min after impact is depicted. Shown are cross-sections through the numerical simulation along the plane of trajectory, with x = 0 defined at the crater centre (measured at the pre-impact level); the direction of impact is from right to left. The upper 3 km of the pre-impact target, corresponding to the average thickness of sedimentary rocks at Chicxulub, is tracked by tracer particles (sandy brown). Deformation in the crust (mid-grey) and upper mantle (dark grey) is depicted by a grid of tracer particles (black). Tracer particles within the peak-ring material are highlighted based on the peak shock pressure recorded (white–blue colour scale); melted target material (over 60 GPa) is highlighted in red. Collins et al. (2020).

Numerical simulation results. Collins et al. performed a series of 3D simulations of impacts that produce a Chicxulub-scale crater, using the iSALE3D shock physics code. The simulations assumed a flat, two-layer target comprising crust and mantle and considered four different impact angles (90
°
(vertical), 60
°,
45
°
and 30
°
) and two impact speeds (12 and 20 km per second). Their simulations provide insight into crater asymmetries diagnostic of impact angle and trajectory in the absence of any target asymmetry. 

 

Compilation of geophysical anomalies of the Chicxulub impact structure on a Bouguer Gravity map of the crater vicinity. The coordinate system has been transformed to a cartesian grid, with the origin at the crater centre. Red circle, red dot and 1-km errorbars indicate the crater margin, centre and uncertainty, respectively, based on the outer margin of the gravity low also associated with a maximum in horizontal gravity gradient and the ring of cenotes. Green dot with 2-km errorbars indicates the location of the mantle uplift centre, which is offset 9.3 km north-northeast from the crater centre. Blue circle and dot indicate the position and approximate diameter of the peak ring, respectively, based on the annular gravity low inside the crater, which is offset approximately 7.6 km southwest of the crater centre. Orange ellipse and dot indicate the outline and centre of the short wavelength magnetic anomaly, interpreted to represent the extent of the melt sheet. Note the slight elongation in the north-northwest-south-shoutheast direction and the offset of the centre to the southwest of the crater centre. The yellow ellipse and dot indicate the outline and centre of the long wavelength magnetic anomaly, interpreted to represent uplift of mid-crustal rocks. Note the elongation and offset of this anomaly to the southwest. Collins et al. (2020).

In Collins et al.'s vertical impact simulation, crater formation is axially symmetric and consistent with previous two-dimensional numerical simulations that employed an axially symmetric geometry. Collision of the asteroid with the target surface generates a detached shockwave that propagates symmetrically from the impact site. In the first minute after impact, an excavation flow initiated by the shockwave produces a deep, bowl-shaped cavity, often termed the transient crater. The material flow depresses the crust-mantle boundary beneath the transient crater, uplifts the crust in the transient crater wall and expels the unvaporized portion of the more than 3-km-thick sedimentary rock sequence from the transient crater as part of the ejecta curtain.

 

Development of the Chicxulub crater for a 60
°
impact. The impact scenario depicted is for a 17-km diameter impactor with a density of 2630 kg m³ and a speed of 12 km/s. Evolution of the crater up to 5 min after impact is depicted. Shown are cross-sections through the numerical simulation along the plane of trajectory, with x = 0 defined at the crater centre (measured at the pre-impact level; z = 0); the direction of impact is from right to left. The upper 3 km of the pre-impact target, corresponding to the average thickness of sedimentary rocks at Chicxulub, is tracked by tracer particles (sandy brown). Deformation in the crust (mid-grey) and upper mantle (dark grey) is depicted by a grid of tracer particles (black). Tracer particles within the peak-ring material are highlighted based on the peak shock pressure recorded (white–blue colour scale); melted target material (more than 60 GPa) is highlighted in red. Collins et al. (2020).

The transient crater is unstable and collapses dramatically to produce a much flatter, broader final crater. In the vertical impact simulation, collapse manifests as uplift of the crater floor and downward and inward collapse of the transient crater rim and a surrounding collar of sedimentary rocks. Floor uplift begins directly beneath the transient crater centre and proceeds vertically upward, overshooting the pre-impact surface to form a large central uplift. At the same time, rim collapse occurs symmetrically at all azimuths, converging towards, and helping to drive up, the central uplift. Finally, the overheightened central uplift of crustal rocks collapses downward and outward, overthrusting the collapsed transient crater rim to form an uplifted ring of crystalline basement, overlying inwardly slumped sedimentary rocks from outside the transient crater. Although the spatial resolution of the numerical simulations is insufficient to resolve. This model of peak-ring crater formation is supported by geophysical data the characteristic sharp-peaked topography of the inner ring observed in extraterrestrial peak-ring craters, Collins et al. were able to identify the position and structure of the material that forms the peak ring in the numerical simulations as a 10-km-wide collar around the central uplift and recent geological drilling at Chicxulub, as well as remote-sensing data from the Schrödinger peak-ring crater on the Moon.

 

Example of how Lagrangian tracer particles were used to identify peak ring material. Peak ring material was identified as those tracers within a 10-km wide collar of the central uplift (blue tracers), and above the plane defining the base of the central uplift (green and blue tracers), at the time of maximum uplift (200 seconds in the example shown). Collins et al. (2020).

Impacts at progressively shallower angles to the horizontal result in an increasingly asymmetric development of the crater, internally, while the planform of the final impact basin remains approximately circular. As impact angle decreases, the downrange offset of the crater centre from the impact point increases; less uplift of the transient crater rim occurs in the uprange direction; and more uplift occurs in the downrange direction. Relative to the final crater centre, the deepest part of the transient crater (and depressed mantle) also shifts with decreasing impact angle, first to the uprange direction, then back towards the centre. The collapse phase of crater formation is also modified by impact angle. Uplift of the crater floor during crater collapse begins uprange of the crater centre, but has a downrange component such that the central uplift axis is tilted downrange and the centre of the uplift prior to its collapse is downrange of the crater centre. Conversely, downward and outward collapse of the central uplift occurs preferentially in the uprange direction, resulting in enhanced overthrusting of the central uplift on top of transient crater rim in the uprange direction. The net result of the downrange-directed rise and uprange-directed fall of the central uplift is a simulated peak ring with a geometric centre only modestly offset in the downrange direction.

 

Development of the Chicxulub crater for a 90° impact during the first 5 minutes. The scenario depicted is for a 16-km diameter impactor with a density of 2650 kg/m³ and a speed of 12 km/s. Shown is a cross-section through the numerical simulation along the plane of trajectory, with x = 0 defined at the crater centre (measured at the preimpact level). The upper 3-km of the preimpact target, corresponding to the average thickness of sedimentary rocks at Chicxulub, is tracked by tracer particles (sandy brown). Deformation in the crust (grey) and upper mantle (dark grey) is depicted by a grid of tracer particles (black). Tracer particles within the peak-ring material are highlighted based on the peak shock pressure recorded (white-blue colour scale); melted target material is highlighted in red. Collins et al. (2020).

The impact simulations employ an impact speed of 12 km per second, only slightly larger than the minimum possible speed, Earth’s escape velocity of 11.2 km per second. While these results are likely to be representative of the about 25% of all impacts that occur at speeds below 15 km per second, Collins et al. also conducted another suite of simulations with a more probable impact speed of 20 km per second (close to Earth’s mean and median asteroid impact speed) to examine the sensitivity of our results to impactor speed. The higher-speed impacts produced similar offsets in mantle-uplift centre and simulated peak-ring centre, and the same trends in offsets with impact angle. 

 

Development of the Chicxulub crater for a 45° impact during the first 5 minutes. The scenario depicted is for a 18-km diameter impactor with a density of 2650 kg/m³ and a speed of 12 km per second. Shown is a cross-section through the numerical simulation along the plane of trajectory, with x = 0 defined at the crater centre (measured at the preimpact level) and the direction of impact is from right to left. The upper 3-km of the preimpact target, corresponding to the average thickness of sedimentary rocks at Chicxulub, is tracked by tracer particles (sandy brown). Deformation in the crust (grey) and upper mantle (dark grey) is depicted by a grid of tracer particles (black). Tracer particles within the peak-ring material are highlighted based on the peak shock pressure recorded (white-blue colour scale); melted target material is highlighted in red. Collins et al. (2020).

An important consequence of higher impact speed is enhanced melt production caused by higher shock pressures close to the impact site. The larger melt volume complicates the interpretation of peak-ring structure in the 20 km per second simulations as the dynamics of the melt are not expected to be well captured, given the 500-m spatial resolution of the 3D simulations, and would likely continue long after the simulation end time. Nevertheless, the lateral distribution of the melt material relative to the peak-ring material at the end of the simulations. suggests that below an impact angle of 45

° there is a high concentration (thick sheet) of surficial melt in the downrange quadrant of the crater, which is likely to hinder or prevent formation of a topographic peak ring at these azimuths. Our results therefore support the idea that horse-shoe shaped peak-ring planforms are indicative of shallow-angle impacts, with the gap in the peak ring diagnostic of the downrange direction.

 

Cross-sections of the final simulated Chicxulub crater, in the plane of the impact trajectory, for a 45° (a) and 90° (b) impact angle (to the target plane) for an impact speed of 12 km per second. Impact direction is right to left in (a). Sandy-brown tracers indicate the final position of the upper 3-km of the preimpact target (sediments); red tracers indicate the position of melt; tracers with blue-white shading indicate shock pressures of peak-ring materials. The geometric centre of the crater rim defines the coordinate origin x = 0; negative x-values are downrange in (a). Collins et al. (2020).

Asymmetry in crater development produces differences in central crater structure in the uprange and downrange directions. While the centre of the simulated peak ring appears to be consistently offset downrange of the crater centre by about 5% of the crater diameter in the three oblique impacts, the centre of the mantle uplift is offset uprange of the crater centre in the 60
° impact and, to a lesser extent, the 45
° impact; and is offset downrange in the 30
° impact. This pattern of mantle-uplift offset relative to the final crater rim is a consequence of the corresponding change in the offset of the deepest part of the transient crater relative to the centre of the final crater. Geophysical observations at Chicxulub suggest the peak-ring and mantle-uplift centres are offset in different, approximately opposite directions from the crater centre. Uncertainty in the precise locations of the centres of the crater, peak ring and mantle uplift, as well as uncertainty in the crater diameter, contribute to an approximate uncertainty of 26% and 48% for the relative offset of the peak ring and mantle uplift, respectively. Comparison of these observations with our simulation results suggests that the observed configuration is most similar to the 60°
impact simulations (or possibly the 45°
impact simulation at 20 km per second).

 

Final simulated Chicxulub crater for a 30°
and 60°
impact angle. Shown are cross-sections, along the plane of trajectory, through the final simulated craters formed by a 30
° (a) and 60°
(b) impact, with x = 0 defined at the crater centre (measured at the pre-impact level); the direction of impact is from right to left. Sandy-brown tracers indicate the final position of the upper 3 km of the pre-impact target (sedimentary rock); red tracers indicate the position of melt; tracers with blue–white shading indicate shock pressures of simulated peak-ring materials. The geometric centre of the crater rim defines the coordinate origin (x = 0); negative x-values are downrange. Collins et al. (2020).

Tracer particles that track the history of material in the simulation afford analysis of the provenance of peak-ring materials and their variation with azimuth. The mean depth of origin of peak-ring materials is 10–12 km for the 45°
, 60°
and 90°
impacts, only dropping significantly, to about 8 km, in the 30°
impact. In the 30°
scenario a significant fraction of the simulated peak ring originates from the sedimentary sequence in the uprange direction; the presence of significant amounts of sedimentary material in the simulated peak ring is not consistent with geophysical interpretations or results from Expedition 364.

Collins et al. also observe a systematic change in the up/downrange difference in subsurface structure of simulated peak rings with impact angle. Similar to the situation in a vertical impact, at 60°
the simulated peak ring is formed of overthrusted granitic crustal rocks from the central uplift above down-slumped sedimentary rocks from the transient crater wall, in all directions. However, the sedimentary rocks are deeper and extend farther beneath the simulated peak ring in the uprange direction compared with the downrange direction. At 45°
and 30
° this difference is more pronounced: on the downrange side of the crater, the inwardly slumped sedimentary rocks do not extend under the simulated peak ring owing to enhanced transient crater rim uplift in this direction. This downrange configuration is inconsistent with geophysical interpretations at Chicxulub, which suggest sedimentary slump blocks lie beneath the outer portion of the peak ring at all azimuths offshore. However, pre-impact asymmetries in sediment thickness, water depth, particularly in the northeast part of the crater (and potentially in the crust), may also affect structure beneath the peak ring.

A proposed indicator of shallow-angle impact is the truncation of the peak ring in the downrange direction. Collins et al.'s numerical simulations at typical terrestrial impact speeds (20 km per second) are consistent with the production of a gap in the peak ring in the downrange direction for impact angles shallower than 45°. However, a prominent gap in the Chicxulub peak ring that might indicate a shallow-angle impact is not supported by the geophysical data. The topographic expression of the peak ring is clearly resolved in all radial seismic reflection lines through the offshore portion of the crater and is particularly prominent in the northwest seismic reflection line Chicx-B the downrange direction according to shallow-angle impact hypothesis proposed by Peter Schultz and Steven d’Hondt. While the onshore portion of the crater has not been seismically imaged, the annular negative gravity anomaly that has been shown to correlate with peak-ring position offshore is well-pronounced and continuous in this region, with no break that might indicate an abundance of melt or change in the character of the peak ring. The continuity of the geophysical signature of the peak ring therefore also supports a more steeply inclined impact trajectory.

 

Cross-sections of the final simulated Chicxulub crater, in the plane of the impact trajectory, for a 30° (a) and 60° (b) impact angle (to the target plane) for an impact speed of 20 km per second. Impact direction is right to left in (a). Sandy-brown tracers indicate the final position of the upper 3-km of the preimpact target (sediments); red tracers indicate the position of melt; tracers with blue-white shading indicate shock pressures of peak-ring materials. The geometric centre of the crater rim defines the coordinate origin x = 0; negative x-values are downrange in (a). Collins et al. (2020).

In summary, Collins et al.'s numerical simulations of oblique Chicxulubscale impacts appear to be most consistent with the internal structure of the Chicxulub crater for a steeply inclined impact angle of 45–60° to the horizontal. If the observed asymmetries in the Moho uplift, central uplift and peak ring of the Chicxulub impact structure are attributable to impact trajectory, the implied direction of impact is northeast-to-southwest. This is the opposite direction to that proposed by a team led by Alan Hildebrand in 1998, based on the offset of the central uplift relative to the crater centre. Our results indicate that uplift of the crater floor occurs in a downrange rather than uprange direction, consistent with numerical simulations of complex crater formation and geological interpretation of eroded structural uplifts at terrestrial complex craters.

 

Plan view of the crater outline (as defined by the –1 to 0 elevation contours) and the spatial distribution of peak-ring material tracers (blue) and melt tracers (red) at the end of the simulation for scenarios with an impact speed of 12 km/s and an impact angle of (a) 30°, (b) 45°, (c) 60°, (d) 90°. The geometric centre of the crater rim defines the coordinate origin x = 0; negative x-values are downrange in the oblique cases. Collins et al. (2020).

Analyses of Venusian craters have not shown a clear link between asymmetries in central crater features and direction of impact. A slight tendency for the peak-ring centre to be offset in the downrange direction was observed, but the results were inconclusive, in part owing to the relatively small number of craters used in the study. The magnitude of the offset (0.03–0.07 D) is, however, consistent with Collins et al.'s numerical simulation results. In contrast, there is no correlation between impact trajectory direction and the offset from the crater centre of central peaks in small complex craters. While Collins et al. did not simulate central peak formation in this work, their results provide a possible explanation for the absence of correlation. At steep angles, uplift of the crater floor initiates uprange of the crater centre, while at shallow angles uplift initiates downrange. If central peaks represent frozen central uplifts, offsets in either uprange or downrange direction might therefore be expected at moderately oblique angles 30–60
°.

 

Plan view of the crater outline (as defined by the –1 to 0 elevation contours) and the spatial distribution of peak-ring material tracers (blue) and melt tracers (red) at the end of the simulation for scenarios with an impact speed of 20 km per second and an impact angle of (a) 30°, (b) 45°, (c) 60°, (d) 90°. The geometric centre of the crater rim defines the coordinate origin x = 0; negative x-values are downrange in the oblique cases. At this higher impact speed, the melt volume is greater and there are  more melt tracers, which are concentrated downrange. In the 30° impact angle scenario (a), a thick layer of melt overlies the peak-ring material tracers in the downrange direction which is expected to result in no topographic expression of the peak ring at this azimuth. In such scenarios, the morphology of the final crater is expected to exhibit a horse shoe shaped peak ring, with a gap in the down range direction. Collins et al. (2020).

Impacts that occur at a steep angle of incidence are more efficient at excavating material and driving open a large cavity in the crust than shallow incidence impacts. Our preferred impact angle of about 60° is close to the most efficient, vertical scenario, which suggests that previous estimates of impactor kinetic energy based on high-resolution 2D vertical impact simulations do not need to be revised dramatically based on impact angle.

Steeply inclined impacts favour a more symmetric distribution of material ejected from the crater among both proximal and distal ejecta. Asymmetry in the distribution of ejecta was originally used by Peter Schultz and Steven d’Hondt as an argument for a shallow impact angle towards the northwest. This was based on the observation that both the particle size and layer thickness were relatively large in North American Cretaceous/Palaeogene boundry sites. Subsequent work has shown that number and size of shocked quartz grains present in the global ejecta layer decreases with distance from Chicxulub, and is independent of azimuth. In addition, the 1–3-cm-thick double layer in North America is also observed to the south and southeast of Chicxulub in Colombia and the Demerara Rise at equivalent paleodistances from Chicxulub. The global Cretaceous/Palaeogene boundary layer therefore has a more-or-less symmetric ejecta distribution, consistent with our preferred steep impact angle.

Impact angle has an important influence on the mass of sedimentary target rocks vaporised by the Chicxulub impact. Recent complementary numerical simulations of impact vapour production in oblique impacts using the SOVA shock physics code showed that a trajectory angle of 30–60° constitutes the worst-case scenario for the high-speed ejection of carbon dioxide and sulphur by the Chicxulub impact. At this range of impact angles, the ejected mass of carbon dioxide is a factor of two-to-three times greater than in a vertical impact and approximately an order of magnitude greater than a very shallow-angle (15
°) scenario. An absence of evaporites in the International Ocean Discovery Program-International Continental Scientific Drilling Program Expedition 364 drill core is consistent with highly efficient vaporisation of sedimentary rocks at Chicxulub. Our simulations therefore suggest that the Chicxulub impact produced a near-symmetric distribution of ejecta and was among the worst-case scenarios for the lethality of the impact by the production of climate-changing gases.

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Microbial life in the post-impact Chicxulub Crater.

The impact crater at Chicxulub (Yucatán Peninsula, México) is the only terrestrial crater on Earth with a well-preserved peak ring. The asteroid impact is linked to the end-Cretaceous mass extinction event, which wiped out 76% of all species worldwide, along with a near-global loss of vegetation. A collapse in Phytoplankton productivity in the world’s oceans occurred due to the sudden decline in photosynthesis as atmospheric particulates lowered light levels for years after the impact. In 2016, the peak ring of the Chicxulub crater was cored by the International Ocean Discovery Program and International Continental Scientific Drilling Program Expedition 364. A 130 m thick interval of impact melt rock and upward-fining suevite, which overlies fractured basement rock, was deposited immediately after impact. The lower suevite, rich in impact melt rock, is directly overlain by material transported via ocean resurge and then by seiches and a tsunami deposit. The overlying 0.75 m- hick, fine-grained, brown micritic limestone (the 'transitional unit'), deposited in days to years after the impact by continuing seiches and tsunami, contains microfossils of calcareous plankton and trace fossils of burrowing organisms. The transitional unit is overlain by a thin green marlstone, followed by the deposition of 'white' micritic limestone (616.55–616.24 m below seafloor) within 30–200 thousand years, representing the base of the succeeding pelagic-hemipelagic limestone deposit. 

In a paper published in the journal Geology on 17 January 2020, Bettina Schaefer, Kliti Grice, and Marco Coolen of the Western Australian Organic and Isotope Geochemistry Centre at Curtin University, Roger Summons and Xingqian Cui of the Department of Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology, Thorsten Bauersachs of the Organic Geochemistry Unit at Christian-Albrechts-University, Kiel, Lorenz Schwark, also of the Western Australian Organic and Isotope Geochemistry Centre at Curtin University, and the Organic Geochemistry Unit at Christian-Albrechts-University, Michael Böttcher of the Geochemistry & Isotope Biogeochemistry Group at the Leibniz Institute for Baltic Sea Research, Marine Geochemistry at the University of Greifswald, and the Department of Maritime Systems at the University of Rostock, Timothy Bralower, Shelby Lyons, and Katherine Freeman of the Department of Geosciences at Pennsylvania State University, Charles Cockell of the School of Physics and Astronomy at the University of Edinburgh, Sean Gulick of the Institute for Geophysics at the University of Texas at Austin, Joanna Morgan of the Department of Earth Sciences and Engineering at Imperial College, London, Michael Whalen of the Department of Geosciences at the University of Alaska Fairbanks, Christopher Lowery, also of the Institute for Geophysics at the University of Texas at Austin, and Vivi Vajda of the Department of Palaeobiology at the Swedish Museum of Natural History, present the results of a study of biomarkers within the sediments above the Chicxulub suevites, with the aim of understanding the microbial communities that developed in the post-impact crater.

Map showing International Ocean Discovery Program (IODP) Site M0077A (21.45°N, 89.95°W) at the Chicxulub crater, Mexico. Schaefer et al. (2020).

Evidence of ancient life is generally preserved in sediments as morphological fossils, trace fossils, and molecular fossils (biomarkers). Biomarkers are often well preserved in sediments even where visible mineralized fossils are absent, representing valuable signs of past life, especially microbial life. For example, in the Fiskeler Member in the end-Cretaceous boundary layer at Kulstirenden, Denmark, biomarkers showed that marine productivity recovered within a century following the Chicxulub impact. Schaefer et al. present biomarker distributions and sulfur isotopes of pyrite between 619 mbsf and 608 mbsf at International Ocean Discovery Program Site M0077A. Their aim was to use biomarkers to reconstruct the origin, recovery, and development of microbial life and to determine the paleoenvironmental conditions in the crater from the time of impact to up to about 4 million years after the impact.

Despite low organic matter content and low abundances of biomarkers, the record provided insights into the evolution of microbial communities in this exotic habitat.

The uppermost suevite, found 619.31-617.33 meters below seafloor, was deposited by a tsunami within the first day after impact. This tsunami transported reworked organic matter from outside the crater, as evidenced by the abundance and distribution of perylene and charcoal.Reworked marine inputs shown by biomarkers included long-chain alkanes, indicative of Algae or Cyanobacteria. Further, abundant long-chain steranes from Green Algae and/or land Plants, reflect a mixture of marine and terrigenous inputs. This interval also contains biomarkers derived from anoxygenic photosynthetic Sulfur Bacteria (i.e., isorenieratane, β-isorenieratane, and traces of chlorobactane and okenane). In addition, Cyanobacterial biomarkers in the form of 2α-methylhopanes and heterocyst glycolipids were observed. The latter, diagnostic for nitrogen-fixing Cyanobacteria, represent the oldest reported intact heterocyst glycolipids. From the presence of terrestrial signatures and the depositional regime, we infer that all the organic signatures are reworked materials, likely derived from carbonate platforms and coastal environments close to the site. The biomarkers listed above were also identified in overlying sediments (617.33–608.48 meters below seafloor), where they represent organisms living within the nascent crater. Here, we evaluate the oceanographic and redox conditions in the impact basin as inferred from the biological origins of these compounds.

The 200 000 years after impact is represented by the transitional unit (617.33–616.58 meters below seafloor) of fine-grained brown micrite and overlying green marlstone, and it is likely to contain the first record of microbial life after the impact. The succeeding 'white micrite' is possibly a result of calcite formed photosynthetically by Cyanobacteria that replaced the Calcareous Nanoplankton and other Algae across the Cretaceous-Palaeogene boundary. 

Schaefer et al.'s study provides the first evidence of Cyanobacteria 30 000 years. after impact at 617.33–616.58 meters below seafloor, from abundant long chain hopanes. 2α-methylhopane, considered to be a biomarker for Cyanobacterial activity, is found at levels similar to those reported for the Fiskeler Member boundary layer in the Højerup section at Stevns Klint, Denmark, are typical of marine conditions. However, this is significantly lower than those reported in Permian-Triassic and Triassic-Jurassic boundary sections.

Sterane levels were found to be low, showing low Algal inputs relative to Bacteria, particularly Cyanobacteria. In the Fiskeler Member boundary layer in Denmark, the low sterane level was assigned to a decreased Algal input, followed by an immediate increase, suggesting a rapid resurgence of Algae when solar irradiance returned to pre-impact levels. In the transitional unit, sterane level changed within multiple intervals, suggesting that the organic matter in the crater was a mixture of transported and autochthonous material, distinct from other Cretaceous-Palaeogene sites. A similar trend was observed for  2α-methylhopane and homohopane (a biomarker for Bacterial activity), consistent with anoxic-euxinic (low oxygen-high sulphide) conditions. The homohopane to hopane ratio fluctuates considerably, which is thought to have been caused by the increased preservatin of long chain hopanes under euxinic conditions through reduction and cross-linking with reduced sulphur species. The shifts in high to low sterane levels suggest that sedimentation was influenced by water movement, most likely seiches and resuspension.

Heterocyst glycolipids were observed to be low in abundance in this interval, and exclusively consisted of types of long chain heterocyst glycolipid diol and heterocyst glycolipid keto-ol, identified in coastal microbial mats, brackish-marine environments, and in axenic (monospecific) cultures of Nostocalean Cyanobacteria such as Anabaena spp. or Nodularia spp. However, long chain glycolipid diols and triols have been reported in free-living marine Cyanobacteria. It is therefore plausible that the heterocyst glycolipids found here are also derived from a marine source. The low abundance of both components, however, suggests only low productivity of nitrogen-fixing heterocystous Cyanobacteria in the first 200 000 years. after the impact. An increased influx of terrigenous nutrients would have helped to sustain Phytoplankton, as shown by the paired increase in the abundance of long-chain waxy n-alkanes and steranes.from Plants and Green Algae. 3β-methylhopanes show an increase at the top of the transitional unit (616.62–616.58 meters below seafloor) and in the white micrite, indicating the presence of Methanotrophs.

A substantial shift in the microbial community was found in the middle and upper parts of the hemipelagic limestone horizon, found between 616.62 and 616.58 meters below seafloor, and considered to have been laid down between 200 000 and 4 million years after the impact. The heterocyst glycolipid distribution patterns and abundances showed considerable changes indicating shifts in the Cyanobacterial community and an increase in Cyanobacterial productivity by two orders of magnitude compared to the transitional unit, with maximum concentrations at 613.45  meters below seafloor.

In contrast, 2α-methylhopane levels remained constant, with a slight increase at 613.45 meters below seafloor, whereas the homohopane levels increased again between 613.45 and 610.72 meters below seafloor. This increase in (Cyano)bacterial biomarkers and the concomitant rise in the abundance of nitrohen-fixing heterocystous Cyanobacteria suggest a shift toward a nitrogen-limited environment, perhaps triggered by water column stratification. Another possibility is that these organisms were allochthonous (found elsewhere) and were transported into the crater from microbial mats living in relatively shallow waters. The limestone interval between 613.45 and 610.72 meters below seafloor (roughly 64.4–63.1 million years ago) indeed indicated anoxic conditions during deposition, depicted by low pristane/phytane ratios abundant β-carotane from autotrophs, and highly characteristic photic zone euxinia biomarkers from green-green and brown-green pigmented Chlorobiaceae (e.g., chlorobactane and isorenieratane), and purple pigmented Chromatiaceae (okenane). Chlorobiaceae and Chromatiaceae are anaerobic photoautotrophs that use hydrogen sulphide (generated by sulphate-reducing Bacteria) as an electron donor and biosynthesise specific bacteriochlorophyll and accessory carotenoid pigments to capture longer wavelengths of light energy to fix carbon dioxide.Such organisms flourish in benthic mats and as plankton concentrated at the chemocline of lakes or restricted marine basins where sulphide concentrations are high within the photic zone; hence, they are indicative of photic zone euxinia conditions. In this limestone interval, total reduced inorganic sulphur was abundant, consistent with nonlimiting sulphate concentrations, water-column photic zone euxinia, and enhanced pyrite burial. Similar values have been reported for reduced sulphur in Cretaceous black shales. Diagenetic pyrite in shell fillings and sediment matrix indicates recrystallization of primary framboids. The pronounced inorganic sulphur depletion compared to the estimated value of contemporaneous seawater signifies that microbial sulphate reduction probably took place in the water column.

Associated with compelling indicators that periodic photic zone euxinia was prevalent in the Chicxulub crater from about. 64.4 to 63.1 million years ago, the molecular evidence indicates that oxygenated waters overlay the anoxic and sulphidic lower layers of the water column During this time interval, methane from anoxic sediments underlying a sulphidic water column likely migrated upward until it was oxidized by microaerophilic methanotrophic Bacteria at the chemocline, as evidenced by the presence of 3β-methylhopanes. An alternative scenario is the possibility of an oxygen minimum zone existing in the crater water.

Schaefer et al. propose a scenario where, in the initial days after the asteroid impact, debris from microbial mats containing nitrogen-fixing heterocystous Cyanobacteria and photosynthetic Sulfur Bacteria was eroded from adjacent carbonate platforms and transported by ocean resurge or tsunamis into the crater. Microbial ecosystem communities were in a constant state of dynamic flux during the early evolution of the crater. Diminution of sunlight following the impact led to a dramatic decline in cyanobacterial productivity in the crater waters. However, rapid recovery of phytoplankton occurred in the first 200 000 years, and marine primary production was fueled by an influx of terrigenous nutrients. Phytoplankton communities continued to experience rapid changes over the following 4 milllon years. The nascent Chicxulub crater basin was accompanied by major transitions in nutrient and oxygen supplies (periods of euxinia) that shaped the recovery of microbial life.

 Evolution of microbial communities in the Chicxulub crater, Yucatán Peninsula, Mexico. Schaefer et al. (2020).

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Understanding the influence of large bolide impacts on the Earth's carbon cycle and climate.

Many scientists suggest that the Earth has recently transitioned into a new period, the provisionally termed 'Anthropocene', this is defined to reflect the planet-wide effects of human activity. The Anthropocene could be described as a gigantic combustion experiment in which reduced, energy rich forms of carbon (e.g., coal, oil, gas, wood) are oxidised to CO₂, with additional significant atmospheric emissions from industrial and land-use activity. The cumulative atmospheric CO₂ release since 1750 AD is about 2000 gigatons. For comparison, the bolide strike that formed the Chicxulub structure in Mexico about 66 million years ago released between 425 and 1400 gigatons of CO₂. Thus, some large bolide impacts are comparable to the Anthropocene effect in terms of the rapid disruption of the carbon cycle and the potential for exceeding the currently unknown critical degree of perturbation. However, during the most intense bombardment period on the early Earth, the surface was poor in the carbon and sulphur rich sediments that exert the greatest control over climate perturbation. Time, therefore, provides a natural narrative for a review of environmental consequences.

In a paper published in the journal Elements on 1 October 2019, Balz Kamber of the School of Earth, Environmental and Biological Sciences at the Queensland University of Technology, and Joseph Petrus of the School of Earth Sciences at the University of Melbourne, and the Harquail School of Earth Sciences at Laurentian University, examine the relationship between bolide impacts, atmospheric carbon fluctuations, and the climate, as preserved in the Earth's rock record.

Large bolide impact events have become rare. At most, there is one strike of one roughly 10 km object every 100–200 million years. But the rich history of former bombardment is evident on the surfaces of inner solar system bodies, as well as from the few preserved impact features on Earth itself. Impact basins more than 1000 km across exist on our planetary neighbours and they are pockmarked with thousands of smaller impact features. Counting crater numbers and measuring the sizes of craters on images of the Moon’s surface qualitatively shows that most of the very large basins formed early in the history of the solar system. Dates for lunar samples constrain the bulk of the bombardment to have occurred within the first roughly 700 million years since planet formation. Importantly, however, significant events capable of producing basins hundreds of kilometres in diameter also happened in more recent history. It is instructive to study how these could have disrupted the complex interplay between biology and geology on Earth; that is, how they have affected the global biogeochemical carbon cycle. 

The Earth’s geological history is subdivided into eons, eras and periods. For the older eras, these subdivisions were defined with the appearance or disappearance of dominant rock types, whereas most boundaries of the younger eras and periods coincide with the rapid disappearance of organisms (mass extinctions).

Timeline of significant geologic events relative to geological age, subdivided into the four, colour-coded, geological eons (Hadean, Archaean, Proterozoic, Phanerozoic). The eons are subdivided into the geologic eras; the boundaries of the Proterozoic periods are shown but not named. (A) The cumulative age-distribution of bolides capable of creating basin sized impact structures on Earth (number as y-axis). (B) The ages of the six largest known terrestrial impact events (as solid lines during the Proterozoic and Phanerozoic) and the most prominent spherule layers (as broken lines, all during the Archaean). The Chicxulub (Mexico) impact is shown in red. (C) A summary of key events and preserved pieces of evidence relevant to planetary evolution. The End Cretaceous extinction event is shown as a red vertical bar. (D) Global data compilation of the carbon isotope compositions (δ13C as y-axis) of carbonate (blue symbols) and organic carbon (dark grey symbols). Kamber & Petrus (2019).

The Hadean Eon (4567 Ma to about 3850 Ma) is the oldest eon of Earth’s history, and it witnessed by far the largest number of impacts. Unfortunately, the Hadean geological record is very sparse and significant uncertainty exists about Earth’s evolution during that time. Nevertheless, three key questions are of great scientific importance because of their enduring legacy for the remainder of Earth history. They all relate to the Hadean impact history: (1) Did the Hadean bombardment deliver volatile elements, including water and carbon, to the early Earth? (2) What happened to the Earth’s vanished primordial crust? (3) Was life on Earth already established during the Hadean? 

The delivery of extraterrestrial matter to Earth happens with two dominant size classes of objects: the tiniest, and the largest. The fully formed Earth was struck by a few hundred bolides capable of causing very large (over 100 km) impact basins, and these objects contained at least one-third of the delivered extraterrestrial matter. At the other end of the spectrum, cosmic dust, having particle diameters in the micrometre range, constitutes the second significant source of extraterrestrial matter. Of the two types of impactors, comets and asteroids, comets are predominantly composed of volatile species (oxygen, carbon, hydrogen, nitrogen), whereas asteroids include undifferentiated chondritic bodies that also contain about 1.1% by weight hydrogen and about 1.8% by weight carbon, with organic molecules. Although intuitively, this suggests that the delivery of such matter to Earth during the Hadean might have contributed to the build-up of the hydrosphere and to the surficial carbon reservoir, this simple logic is complicated by impact physics.

Comparing the delivery of extraterrestrial elements of a refractory nature (i.e., with a high boiling temperature) to those of a volatile character (low boiling temperature) shows that most of the volatile cargo would be lost from cosmic dust upon its atmospheric entry, except for particles of less than 35 μm, which experience limited frictional heating. Impacts of larger bolides at velocities in excess of about 15 kilometres per second cause at least partial vapourisation of the target and intense heating to more than 10 000 K of the vapourised material, and this leads to the formation of a silicate vapour plume. The behaviour of the various chemical elements in these plumes, particularly atmospheric escape versus condensation and fallback to the Earth, is currently not fully understood. The isotopic systematics of light elements and noble gases suggest that late addition to the Earth from comets is unlikely to have been volumetrically important for water, nitrogen and carbon (the proportions of isotopes of each element in a body depend upon where it formed in the primordial disk from which the Solar System formed, with lighter isotopes mote proportionally abundant further from the Sun). Current evidence favours an origin of the terrestrial volatiles by early capture during planetary accretion rather than by late addition during very large impact events.

Regardless of the origin of volatiles, the rate at which the lunar surface was bombarded (and, by analogy, the Earth) can be reconstructed by combining crater density statistics with the known ages of rocks from the Moon’s surface. The largest uncertainty in this flux estimate arises from the paucity of samples returned from the older, more heavily cratered dark side of the Moon and the few direct dates for large lunar impact basins. There are two end-member models for the bombardment flux: one that envisages a spike in very large impacts between 3850 million years ago and 4200 million years ago (the Late Heavy Bombardment scenario) versus one that favours an exponentially decaying flux (the Accretion Tail Scenario'). With currently available data, modelling cannot unequivocally rule out either scenario. One of the strongest pieces of evidence in favour of the Late Heavy Bombardment remains the uranium/lead age line of lunar highland samples (uranium, decays into, amongst other things, lead at a known rate; it is possible to calculate the age of a mineral which would not have lead in in when it formed from the ratio between these elements) that was originally used to advance the concept of a late bombardment. This line is interpreted to date the timing of volatile element loss and homogenisation. The age conspicuously coincides with the more widespread preservation of terrestrial rocks, i.e., the Archaean–Hadean boundary. If future lunar data confirm the existence and timing of the Late Heavy Bombardment, one of the most significant environmental consequences of very large bolide impacts on Earth could have been the destruction of the protocrust. On Mars and Mercury, the ancient protocrusts persisted, despite bombardment, but the Late Heavy Bombardment on Earth may have been effective at crust destruction if the crust–mantle system had reached a vulnerable state, due, for example, to build-up of internal heat.

An artists impression of impacts on the Moon during the Late Heavy Bombardment. Australian National University.

With no supracrustal rocks of Hadean age preserved, the question of putative Hadean life and its effects on the carbon cycle cannot be studied directly. By contrast, the Archaean sedimentary record does contain samples with remains of organic (reduced) carbon, as well as carbonate, and there is clear evidence that the Archaean Earth was struck by very large bolides. No unequivocal Archaean impact basins have been found to date. Instead, the evidence for impacts comes from so-called spherule layers within sedimentary sequences. These tell-tale sediment layers are millimetre-to metre thick, laterally continuous, and contain spherules of various compositions, some with evidence for quench cooling, high pressure minerals, or shock features. The first important inference drawn from their distribution in time is that the Earth continued to be bombarded with large bolides well beyond 3850 million years ago and that the Archaean witnessed more large impacts than the later eons. Because many spherule beds are enriched in iron-loving (siderophile) elements, it has also been possible to incontrovertibly prove that some layers have had a contribution to their formation from a vapourized asteroid, for example via the isotope composition of Chromium.

A particular advantage of studying spherule beds is that they are preserved within a stratigraphic context. This provides additional sedimentological information and geochemical evidence of potential environmental disruption. Most of the well-preserved Archaean spherule beds from the Kaapvaal Craton of southern Africa and the Pilbara Craton of western Australia  show evidence for sedimentary redistribution caused by currents and/or waves. The consistent occurrence of spherules within reworked eroded local detritus rather than the pure deposits of constant thickness expected from fallout, strongly suggests that reworking was a consequence of the impact itself via tsunamis, impact-induced turbidity currents, or bottom return flows.

Previous studies have theorised that most of the spherule bed–forming bolides were 20–50 km in diameter and would have excavated transient craters of up to 100 km deep and final basins reaching several hundred kilometres in diameter. To date, no such basin has actually been discovered. One interesting area of future research is the question of shock-metamorphism of the lithospheric mantle during excavation and collapse of transient cavities well below the crust–mantle boundary. In terms of environmental and carbon isotope consequences, the impact that caused the 2629 million year old spherule layers in Western Australia and South Africa is particularly instructive because it is found within carbonate (mostly dolomite), which is conducive to chemical and isotopic analysis. The corresponding impact basin would have been about 100–150 km in diameter and, thus, represent a significant event. The carbon isotope values for reduced carbon across the spherule bed do not show an incontrovertible trend, but nonetheless indicate a significant general shift towards lighter carbon isotopes. By contrast, the shallow marine carbonate isotope values remain near constant across the spherule bed, but these data were obtained at a more limited spatial resolution that may not have captured the disruption of the global biogeochemical carbon cycle. More detailed isotopic studies across spherule beds are needed to explore to what extent the balance between the buried sedimentary reduced carbon and the dissolved oceanic carbon was disrupted by these impact events.

Carbonate is a less dominant sediment type in the Archaean supracrustal rock record than in the Proterozoic and the Phanerozoic. Therefore, it is impossible to produce a continuous global carbonate carbon isotope record that would cover all the 15 known Archaean spherule layers to test how representative the 2629 Ma event was. Notwithstanding this limitation, it is evident from the existing global compilation that the presently documented fluctuations in Archaean carbon isotope ratios were much less pronounced than in the Palaeoproterozoic and Neoproterozoic. Regardless of the potential of very large bolide impacts to temporarily disrupt the ancient carbon cycle, the apparent stability of the cycle itself, as well as the similarity of the predominant Archaean carbon isotope ratios with modern carbonate carbon, is astonishing. The oldest carbonates occur in the Isua Greenstone Belt of Southwest Greenland. They are between 3710 and 3810 million years old and, although not universally accepted as sedimentary in origin, some appear to have yielded carbon isotope ratios close to the modern-day value, and they have co-existing very light carbon preserved in putative biogenic graphite Due to the pervasive metamorphic overprint of the Isua rocks, some doubt remains as to whether the recorded carbon isotope values truly reflect the sedimentary system. Regardless, many more paired reduced carbon and carbon isotope values have been reported for younger Archaean sedimentary rocks, leaving little doubt as to the stability of the early terrestrial carbon cycle.

Photomicrographs of typical spherules collected from the 2130–1848 million year old spherule layer in northeast Midternæs, Greenland. (a) Spherule filled with finely crystalline radial-fibrous chalcedony and sericite in plane polarised light. Note marginal replacement by invasive dolomite and concentration of carbonaceous matter (black) along boundary between chalcedony and dolomite. (b) Same field of view as (a) between crossed polarisers. Arrows indicate planar interfaces between adjacent radial-fibrous aggregates. (c) Spherule filled with combination of coarser, equigranular quartz crystals (white) and fibrous sericite (grey) in plane polarised light. Sericite is unusually coarse and locally organized into radiating aggregates. (d) Part of spherule similar to (c) between crossed polarisers showing radiating sericite aggregates. Scale bars are 300 µm in (a), (b) and (c) and 100 µm in (d). Chadwick et al. (2001).

Of the six largest preserved terrestrial impact structures, three are Proterozoic in age: the 2023 million year old Vredefort impact structure in South Africa, the 1849 million year old Sudbury Basin in Canada, and the 580–590 million year old Acraman crater in South Australia. Due to deep erosion of the Vredefort structure and the lack of a confirmed corresponding impactite layer, it is impossible to reconstruct the environmental consequences of Earth’s largest preserved bolide impact. By contrast, both the Sudbury and Acraman events preserve remnant impact structures, as well as corresponding impactite layers in the sedimentary record. These two impact events are, therefore, more conducive to studying putative global environmental consequences.

The impact layer corresponding to the Sudbury Basin is found up to 700 km away in the iron-rich sedimentary strata of the Lake Superior region of North America. The layer is a breccia containing lithic fragments (some shocked), devitrified glasses of various kinds, as well as accretionary lapilli; this layer differs from the Archaean spherule beds. Of critical importance is that the breccia layer occurs within a Palaeoproterozoic sedimentary context. The bolide is believed to have hit a foreland basin covered by relatively shallow water, and the main excavated rocks were quartz-rich sandstones of the over 2200 million year old Huronian Supergroup and Archaean basement. These contained very little carbon. However, it has been argued that the bolide was likely a 15 km diameter comet (with a density of 0.6 g per cm³). If this body was similar in composition to comet Halley's Comet, which is estimated to be 18.4% carbon by mass, then the Sudbury object would have contained 195 gigatons of carbon and, if fully vapourised, would have released about 700 gigatons of CO₂, or about one-third of the CO₂ perturbation of the current Anthropocene experiment.

In the lead-up to the impact, the continental foreland basin of the Lake Superior region was ferruginous, with thick banded-iron formations being deposited. The Sudbury impact layer nearly always caps the iron formations and other ferruginous sediment, and subsequent deposition continues with different mud-sized detritus. No re-occurrence of the dominant deposition of iron formation after the impact has yet been observed. There is, thus, strong regional evidence that the Sudbury impact event caused a sharp change in basin water conditions over 700 km away. The disappearance of Palaeoproterozoic banded iron formation at about 1850 million years ago is a global phenomenon, and there is strong evidence for tsunami deposits within some of the impact layers at variable water depths, which has led to the proposal that that the Sudbury impact could have pervasively changed the regional, and probably the global, oceanic stratification, bringing to an end the long-lasting dominantly ferruginous state of the early Palaeoproterozoic deep oceans. The physical reasons for the inferred change remain to be established, however. It is currently unknown how an event such as the Sudbury impact could have disrupted the global oceanic iron supply and started the fickle oceanic states of the remaining Proterozoic. 

Stratigraphic relationship shown in six sedimentary logs between the Palaeoproterozoic Sudbury (Canada) impact layer (in blue) and the type of sediments that preceded and followed this event. Note the lack of deposition of banded iron formation after the impact event. The six logs relate to the following: Mesabi Iron Range (Minnesota, USA); Gunflint Iron Range (Minnesota, USA, and northwest Ontario, Canada); Gogebic Iron Range (Michigan and Wisconsin, USA); Iron River–Crystal Falls District (Michigan, USA); Marquette Iron Range (Michigan, USA); Baraga Basin and Dead River Basin (both in Michigan, USA). (Inset) Example of a lapilli-stone, one of the rock types that makes up the Sudbury impact layer. The rock abounds with millimetre-sized accretionary lapilli that formed in the impact plume. Photo width is 5 cm. Kamber & Petrus (2019).

The Sudbury Basin itself preserves the best-exposed and most accessible stratigraphy through a very large impact basin on Earth. It may originally have measured 170–200 km across but thanks to its remnant now being folded, there is an unparallelled opportunity to study transects from the shocked basement into the differentiated melt sheet and across the basin fill without the need for drilling. Of particular interest is the 1300 m thick unit that overlies the crystallised melt sheet. It consists of breccias and tuffs that collectively are far too thick to represent the fallback from the impact. Instead, the first 300 m of chaotic breccias most likely formed through a fuel-coolant interaction, when seawater flooded onto the superheated melt sheet The remaining stratigraphy is characterised by sustained deposition of subaqueous volcanic products (bombs, lapilli and ash) that are more mafic than the average target rocks. The observation of on-going igneous activity within a subaqueous impact basin has led to speculation that it could represent deeply sourced magmatism.

Numerical impact modelling has demonstrated that the depth of the transient cavity (created within less than a few seconds) nearly linearly increases with increasing bolide diameter, whereas the final depth of even a 500 km diameter basin is less than 3 km. The divergence in depth between transient cavity and final basin necessitates an ever-increasing material flow during the rebound and collapse of the original cavity. It has been proposed that the vertical component of this material flow could give rise to secondary decompression melting. In areas of unusually high continental heat flow and on weak plates (such as ocean basins), one environmental consequence of very large impact events could, therefore, be sustained, deeply sourced magmatism and the associated release of volatiles.

Regardless of this possibility, a final noteworthy aspect of the Sudbury crater fill is the progressive enrichment of the breccias and tuffs in reduced carbon. Studying the chemistry of the fine-grained ash-sized matrix of the crater fill, has led to the conclusion that the crater basin was likely cut off from the open ocean and so developed a distinctive water chemistry within it. The sustained magmatic activity within the basin supported base-metal deposition similar to volcanogenic massive sulfide ores, which otherwise occur at oceanic spreading sites. Apart from the destructive forces of very large impacts, one very different environmental consequence of subaqueous events could, thus, be the formation of enclosed 'ponds' (similar in shape to atolls), which contained chemical 'factories' (hydrothermal systems) producing organic molecules as potential building blocks for life. Whereas life had long been established by 1849 Ma, similar Hadean or early Archaean subaqueous impact basins should be considered as possible birth places of life and the kick-start of the terrestrial carbon cycle.

The structure of the Sudbury Basin. Natural Resources Canada/Wikimedia Commons.

The approximately 590 million year old Acraman impact occurred during a period of intense fluctuations in the carbon cycle in the late Neoproterozoic Era. The possibly 85–90 km diameter impact structure is now deeply eroded, but the corresponding impact layer can be traced for over 500 km. Palaeomagnetic data suggest a low latitude impact, which could potentially increase any resultant environmental effects. But in terms of the on-going Neoproterozoic fluctuations in carbon isotopes, the Acraman event seems to have been relatively minor, with only a small excursion towards a more negative reduced carbon isotope value; however, the detailed isotope stratigraphy is currently missing. The Acraman impactite layer coincides with a marked change in fossil plankton (Acritarch) successions and may have been more significant in terms of radiation than the preceding worldwide Marinoan Glacial Event of the Cryogenian Period. The main target lithology of the Acraman impact were acidic volcanic rocks poor in carbon. There may have been limited disruption of the global carbon cycle, although detailed carbon isotope stratigraphy is unavailable.

The remaining three largest terrestrial impact structures are Phanerozoic in age. The possible causal relationship between a large bolide impact and a Phanerozoic extinction event has been widely discussed, but there are two very clear observations. One is that there have been more significant extinction events during the Phanerozoic than there are very large impact structures to account for them; the second is that there were large impact events, such as the the 215 million year old Manicouagan Crater in Quebec, Canada, with no correlative mass extinction.

Against this backdrop, the exceptional coincidence between the End Cretaceous extinction event and the roughly180 km diameter, 66 million year old, Chicxulub impact structure in Yucatan stands out. It is still being debated whether the environmental effects of the bolide strike on their own were responsible for extinction or whether the Earth was struck at a time when its biology had already been pushed close to a tipping point by volcanic degassing and dropping sea-level. Regardless, it is widely agreed that the Chicxulub impact caused planet-wide climate disruption, as supported by the geological context of the impact site. In the late Cretaceous, the Yucatan Peninsula was a partially emerged platform composed of calcium carbonate and evaporite deposited on older sediments, themselves sitting on Precambrian basement. The bolide excavated through this 'fertile' stratigraphy at a site partly on land and partly submerged.

From a carbon cycle perspective, the presence of thick carbonate beds at the target site is of greatest relevance. The potential quantity of CO₂ devolatilised to high atmospheric altitude from bolides is dwarfed by that modelled to be ejected from a thick carbonate platform. On a 500–1000 year timescale, the effects of releasing 425–1400 gigatons of CO₂ into the atmosphere is climate warming, but in the case of Chicxulub, where limestone, sulfate and seawater were the target, the short term effect was dramatic SO₂-driven cooling with global annual mean surface air temperatures dropping by more than 20 °C, recovering only after 30 years. The bolide strike may also have caused massive wildfires and/or stratospheric emission of smoke from combustion of hydrocarbons within the target marine platform (oil and gas are produced to the north and west of the impact site). The nature of recovered molecules from incompletely combusted hydrocarbons preserved in the impact layer supports the idea that the bulk of the soot was released from reduced carbon contained within the impacted target rocks, amplifying SO₂-driven cooling along the equator and causing droughts. All this is consistent with the extinction patterns.

The End Cretaceous event, thus, emphasises a further aspect of impacts on the terrestrial surface, which is lithologically and geochemically highly diversified and evolved. Less than one-sixth of the current planetary surface has a suitable make-up to cause strong stratospheric cooling if hit by a large bolide; significant direct disruption of the global carbon cycle seems only likely from impacts onto thick carbonate targets.

The size and location of the Chicxulub Crater. Passant Rabie/Inverse.

Bolide impacts have affected the Earth’s carbon cycle in a multitude of ways. The widely held view that there are direct effects to the carbon cycle through environmental devastation and mass extinction, such as has been popularised with the End Cretaceous boundary event, is probably the exception rather than the rule. Most of the consequences of large impacts have been indirect. On the Hadean Earth, intense bombardment may have contributed to the destabilisation of the original crust, thereby possibly promoting plate motion that has become an integral part of carbon cycling through plate destruction. Subaqueous early impact basins may also have been self-contained production sites
of organic molecules that could have been potential cradles for life.

Throughout the Archaean, the Earth continued to be occasionally bombarded by large bolides, as inferred from thick beds of spherules that must have splashed down from giant melt and vapour plumes. The existing Archaean sedimentary carbon isotope record does not appear to show fluctuations of the magnitude seen in later times: however, the record is of limited temporal resolution. In general, partitioning of carbon between the reduced and oxidised pools has remained surprisingly constant. The much more pronounced carbon isotope excursions of the Palaeoproterozoic and Neoproterozoic do not coincide with known impact events. Instead, the two very large events, at 1849 and 590 million years ago, are traceable in the sedimentary record and are associated with the end of the deposition of banded iron formations and the radiation of Acritarch plankton, respectively. If future work demonstrates these to be causal relationships, they would illustrate the indirect influence of large impacts on the carbon cycle through the reorganisation of the ocean’s redox state and the disruption of biological evolution.

Most Phanerozoic mass extinctions are not coincident with very large impact events. The Chicxulub event, occurring at the End Cretaceous boundary, caused a moderate carbon isotope excursion and greatly disrupted the budget of climate-active gases in the atmosphere. This, in turn, led to a short-term abrupt cooling and a medium-term strong warming The lesson drawn for the Anthropocene is that the release of several thousand gigatons of CO₂ into the atmosphere may not leave a marked carbon isotope signal in the geological record. Instead, the Anthropocene is more likely to leave its legacy as a mass extinction from greenhouse-induced climate change on a biosphere already at a tipping point caused by habitat loss.

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