Joanna Morgan

The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary

The Fall of the Dinosaurs According to the fossil record, the rule of dinosaurs came to an abrupt end ∼65 million years ago, when all nonavian dinosaurs and flying reptiles disappeared. Several possible mechanisms have been suggested for this mass extinction, including a large asteroid impact and major flood volcanism. Schulte et al. (p. 1214) review how the occurrence and global distribution of a global iridium-rich deposit and impact ejecta support the hypothesis that a single asteroid impact at Chicxulub, Mexico, triggered the extinction event. Such an impact would have instantly caused devastating shock waves, a large heat pulse, and tsunamis around the globe. Moreover, the release of high quantities of dust, debris, and gases would have resulted in a prolonged cooling of Earths surface, low light levels, and ocean acidification that would have decimated primary producers including phytoplankton and algae, as well as those species reliant upon them. The Cretaceous-Paleogene boundary ~65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction.

SEISMIC REFLECTIONS FROM THE MANTLE REPRESENT RELICT SUBDUCTION ZONES WITHIN THE CONTINENTAL LITHOSPHERE

In many areas of the world, the continental lithospheric upper mantle contains bright, continuous, regionally extensive, seismic reflectors. Despite the increasingly common observation of such mantle reflectors on deep seismic reflection profiles, their significance and geologic origin remain obscure. We report results from a series of seismic experiments acquired across two of the brightest of these reflectors and provide new constraints upon the composition and thickness of the reflecting region. These new seismic data reveal regionally extensive dipping and subhorizonal slabs of high-velocity (>8.4 km/s), high-density (>3500 kg/m 3 ) material, several kilometres (>2 km) thick, entrained within otherwise unremarkable upper mantle. The geometry, physical properties, and geologic setting of these mantle reflectors suggest that they represent fragments of eclogitic oceanic crust—a relict of pre-Caledonian oceanic subduction now preserved within the lower continental lithosphere. Such relict subduction zones appear to be widespread within the continental lithosphere and to exert an important influence upon the location and style of subsequent continental deformation.

The formation of peak rings in large impact craters

Drilling into Chicxulubs formation The Chicxulub impact crater, known for its link to the demise of the dinosaurs, also provides an opportunity to study rocks from a large impact structure. Large impact craters have “peak rings” that define a complex crater morphology. Morgan et al. looked at rocks from a drilling expedition through the peak rings of the Chicxulub impact crater (see the Perspective by Barton). The drill cores have features consistent with a model that postulates that a single over-heightened central peak collapsed into the multiple-peak-ring structure. The validity of this model has implications for far-ranging subjects, from how giant impacts alter the climate on Earth to the morphology of crater-dominated planetary surfaces. Science, this issue p. 878; see also p. 836 Rock samples from IODP/ICDP Expedition 364 support the dynamic collapse model for the formation of the Chicxulub crater. Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust.

Testing the resolution of a 3D velocity tomogram across the Chicxulub crater

Abstract An integrated offshore/onshore reflection and refraction experiment was shot across the Chicxulub impact crater in 1996. The refraction data were previously inverted in 3D using first-arrival travel-time tomography. A regularized inversion, in which both data misfit and model roughness are minimized simultaneously, was used to determine a smooth velocity tomogram across the inner crater region. However, the experimental geometry for the refraction data was irregular, causing concern that this velocity model might not be well resolved. In this paper, we present a suite of checkerboard tests to investigate the lateral resolution of our velocity model. The Chicxulub crater is located partly onshore and partly offshore, with its centre close to the Yucatan coastline in Mexico. The shallow water limited acquisition of marine reflection data to distances of not closer than 25 km from the crater centre, and the centre of the structure is imaged with refraction data only. Intriguing velocity anomalies were observed across the central crater region, providing constraints on the lithological and structural form of Chicxulub. A high-velocity body within the central crater is most likely to represent lower-crustal rocks that were stratigraphically uplifted during the formation of this complex crater. The concave shape of this stratigraphic uplift suggests clues to the mechanics of large-crater collapse—the rocks appear to have moved upward and outward. An inward-dipping zone of lowered velocity has been interpreted as delimiting the outer edge of a central zone of melt-rich rocks. The resolution tests presented here indicate that these observed velocity anomalies are likely to be real.

COINCIDENT NORMAL-INCIDENCE AND WIDE-ANGLE REFLECTIONS FROM THE MOHO : EVIDENCE FOR CRUSTAL SEISMIC ANISOTROPY

Abstract We explore the consequences of interpreting wide-angle crustal seismic data assuming isotropic models when the real crust may be anisotropic. We have used a simple anisotropic model to generate synthetic travel-times and have inverted these assuming an isotropic layered model for the crust. We show that neglecting the effects of anisotropy can produce estimates of crustal thickness and crustal velocity structure that are significantly in error. For realistic levels of anisotropy (∼10%) the error in crustal thickness can be several kilometres, and the error in average velocity can be ∼0.5 km/s. We show that isotropic inversion of anisotropic data can lead to an apparent mismatch between the position of the Moho inferred from normal-incidence and wide-angle data. As an example of such a mismatch we show data acquired over the continental crust offshore the north of Scotland. The dataset we have modelled includes a wide-angle expanding-spread profile, a conventional ocean-bottom seismometer profile, a high-resolution wide-angle onshore-offshore experiment, a synthetic-aperture 16-km offset CDP profile, and a conventional deep reflection profile. These data show unusually sharp, bright and continuous reflections from the Moho at all offsets. The normal-incidence reflection Moho and the Moho modelled from the wide-angle data both show the same lateral structure; however, they are offset one from the other in normal-incidence two-way travel-time. This mismatch is considerably larger than the maximum error expected in the wide-angle model, and is much greater than the accuracy with which the Moho can be picked on the reflection data. If we assume that this mismatch is caused by crustal-scale seismic anisotropy, then the synthetic results indicate that anisotropy in the crust north of Scotland is about 7%.

Full waveform tomographic images of the peak ring at the Chicxulub impact crater

Peak rings are a feature of large impact craters on the terrestrial planets and are generally believed to be formed from deeply buried rocks that are uplifted during crater formation. The precise lithology and kinematics of peak ring formation, however, remains unclear. Previous work has revealed a suite of bright inward dipping reflectors beneath the peak ring at the Chicxulub impact crater and that the peak ring was formed from rocks with a relatively low seismic velocity. New two-dimensional, full waveform tomographic velocity images show that the uppermost lithology of the peak ring is formed from a thin (∼100–200 m thick) layer of low-velocity (∼3000–3200 m/s) rocks. This low-velocity layer is most likely composed of highly porous, allogenic impact breccias. Our models also show that the change in velocity between lithologies within and outside the peak ring is more abrupt than previously realized and occurs close to the location of the dipping reflectors. Across the peak ring, velocity appears to correlate well with predicted shock pressures from a dynamic model of crater formation, where the rocks that form the peak ring originate from an uplifted basement that has been subjected to high shock pressures (10–50 GPa) and lie above downthrown sedimentary rocks that have been subjected to shock pressures of <5 GPa. These observations suggest that low velocities within the peak ring may be related to shock effects and that the dipping reflectors underneath the peak ring might represent the boundary between highly shocked basement and weakly shocked sediments.

Structural uplift beneath the Chicxulub impact structure

Models of the central structure of large impact craters are poorly constrained, partly because of the lack of well-preserved terrestrial examples, and partly because of the extreme nature of impact events. Even large impact craters take only a few minutes to form, during which time rocks from the deep crust move upward many kilometers, interacting with impact melts and breccias before settling to their final position. We construct a new model of central uplift beneath the Chicxulub crater, based upon a well-constrained 3-D velocity model, obtained by jointly inverting seismic traveltime and gravity data. The input tomographic data set has good resolution, and many rays cross the central uplift in many directions. We use laboratory measurements to convert between velocity and density. Our velocity model possesses a high-velocity zone near the crater center, and velocity gradually decreases outside this zone. We use regional refraction data to interpret these velocities in terms of a broad 80-km-wide zone of structural uplift, in which the central rocks originate from the lower crust, and the surrounding rocks from the midcrust and upper crust. This is in contrast with previous models in which the zone of central uplift is either 40-50 km or 150 km wide. Our interpretation is consistent with scaling laws, Yucatan basement lithology, other velocity data, observations at similar-sized terrestrial craters, and dynamic modeling of peak ring formation. Our model of the uplift at Chicxulub can be used to help distinguish between competing models of effective target strength in numerical models of crater formation.

An experimental assessment of the ignition of forest fuels by the thermal pulse generated by the Cretaceous-Palaeogene impact at Chicxulub

A large extraterrestrial body hit the Yucatán Peninsula at the end of the Cretaceous period. Models suggest that a substantial amount of thermal radiation was delivered to the Earth’s surface by the impact, leading to the suggestion that it was capable of igniting extensive wildfires and contributed to the end-Cretaceous extinctions. We have reproduced in the laboratory the most intense impact-induced heat fluxes estimated to have reached different points on the Earth’s surface using a fire propagation apparatus and investigated the ignition potential of forest fuels. The experiments indicate that dry litter can ignite, but live fuels typically do not, suggesting that any ignition caused by impact-induced thermal radiation would have been strongly regional dependent. The intense, but short-lived, pulse downrange and at proximal and intermediate distances from the impact is insufficient to ignite live fuel. However, the less intense but longer-lasting thermal pulse at distal locations may have ignited areas of live fuels. Because plants and ecosystems are generally resistant to single localized fire events, we conclude that any fires ignited by impact-induced thermal radiation cannot be directly responsible for plant extinctions, implying that heat stress is only part of the end-Cretaceous story.

Seismic evidence that black smoker heat flux is influenced by localized magma replenishment and associated increases in crustal permeability

Hydrothermal circulation at mid-ocean ridges is responsible for ~25% of Earths heat flux and controls the thermal and chemical evolution of young oceanic crust. The heat flux of black smoker hydrothermal systems is thought to be primarily controlled by localized magma supply and crustal permeability. Nevertheless, magma chamber characteristics and the nature of crustal permeability beneath such systems remains unclear. Here we apply three-dimensional full-waveform inversion to seismic data from the hydrothermally active Endeavour segment of the Juan de Fuca Ridge to image the upper crust in high resolution. We resolve velocity variations directly above the axial magma chamber that correlate with variations in seismicity, black smoker heat flux, and the depth of the axial magmatic system. We conclude that localized magma recharge to the axial magma lens, along with induced seismogenic cracking and increased permeability, influences black smoker heat flux.

Waveform inversion of deep seismic reflection data: the polarity of mantle reflections

Abstract The Flannan and W-reflectors are two prominent mantle features observed on seismic reflection data off the northwest coast of Scotland. They are the brightest, most laterally extensive, intra-mantle reflectors identified on a deep seismic dataset anywhere in the world. Despite extensive study, their physical origin is still the subject of speculation. We present a scheme to determine the polarity of these mantle reflectors, and constrain their upper structure using near-normal-incidence seismic reflection data. The technique exploits the convolutional model of the earth; we use a deterministic source-signature deconvolution to invert the data. We have explored the parameterization of the inversion by testing real and synthetic data. We find that it is critical to the legitimacy of the reflectivity model that many traces are stacked prior to the inversion and that the data have a good signal-to-noise ratio. Furthermore, an accurate estimate of the effective source wavelet is a fundamental requirement for obtaining a valid reflectivity model; in particular we find the deconvolution results are most sensitive to the precise value of the water depth and reflection coefficient used in estimating the sea-bed multiple train. In the case of the Flannan-reflector, the inversion shows unequivocally that it has a positive polarity. Modelling the W-reflector is less straightforward as a result of reduced signal-to-noise ratio. None-the-less, the inversion suggests a positive polarity for the W-reflector, in agreement with observations of post-critical reflections seen on wide-angle seismic data. The near-normal-incidence polarity measurements support the suggestion that both the Flannan and W-reflectors represent an eclogitic slab, presumably a relict oceanic subduction zone, preserved within the continental lithospheric mantle.