International audienc

Large impacts provide a mechanism for resurfacin g 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 peak s transition to peak ri ngs. 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. Ex pedition 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

The Cretaceous/Palaeogene mass extinction eradicated 76% of species on Earth. It was caused by the impact of an asteroid on the Yucatán carbonate platform in the southern Gulf of Mexico 66 million years ago , forming the Chicxulub impact crater. After the mass extinction, the recovery of the global marine ecosystem-measured as primary productivity-was geographically heterogeneous ; export production in the Gulf of Mexico and North Atlantic-western Tethys was slower than in most other regions, taking 300 thousand years (kyr) to return to levels similar to those of the Late Cretaceous period. Delayed recovery of marine productivity closer to the crater implies an impact-related environmental control, such as toxic metal poisoning , on recovery times. If no such geographic pattern exists, the best explanation for the observed heterogeneity is a combination of ecological factors-trophic interactions , species incumbency and competitive exclusion by opportunists -and 'chance'. The question of whether the post-impact recovery of marine productivity was delayed closer to the crater has a bearing on the predictability of future patterns of recovery in anthropogenically perturbed ecosystems. If there is a relationship between the distance from the impact and the recovery of marine productivity, we would expect recovery rates to be slowest in the crater itself. Here we present a record of foraminifera, calcareous nannoplankton, trace fossils and elemental abundance data from within the Chicxulub crater, dated to approximately the first 200 kyr of the Palaeocene. We show that life reappeared in the basin just years after the impact and a high-productivity ecosystem was established within 30 kyr, which indicates that proximity to the impact did not delay recovery and that there was therefore no impact-related environmental control on recovery. Ecological processes probably controlled the recovery of productivity after the Cretaceous/Palaeogene mass extinction and are therefore likely to be important for the response of the ocean ecosystem to other rapid extinction events.

The Phanerozoic paleontological record is marked by several biological extinction events. One of them, at the Cretaceous/Tertiary (K/T) boundary, was responsible for the demise of about 50% of genera and 75% of species, including the dinosaurs. These drastic and abrupt changes in the development of life on Earth puzzled paleontologists in the past. Many a cause was put forward to account for them, amongst them climate changes, disease, or overspecialization. About 20 years ago, Alvarez et al. [1980] discovered a high iridium concentration in an Italian K/T boundary clay layer.They proposed that the iridium was derived from an extra‐terrestrial impact 65 Ma ago, and that the impact was the cause for the K/T boundary extinctions. The iridium layer was subsequently found at K/T boundary locations worldwide. Further evidence for a K/T impact came from the discovery of shocked quartz, nano‐diamonds, glass spherules, and nickel‐rich spinels in microkrystites in the iridium‐rich layer. There was evidence for an impact event, but no crater.

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The ~180-km-diameter Chicxulub peak-ring crater and ~240-km multiring basin, produced by the impact that terminated the Cretaceous, is the largest remaining intact impact basin on Earth. International Ocean Discovery Program (IODP) and International Continental Scientific Drilling Program (ICDP) Expedition 364 drilled to a depth of 1335 m below the sea floor into the peak ring, providing a unique opportunity to study the thermal and chemical modification of Earth’s crust caused by the impact. The recovered core shows the crater hosted a spatially extensive hydrothermal system that chemically and mineralogically modified ~1.4 × 105 km3 of Earth’s crust, a volume more than nine times that of the Yellowstone Caldera system. Initially, high temperatures of 300° to 400°C and an independent geomagnetic polarity clock indicate the hydrothermal system was long lived, in excess of 106 years.

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35In this paper we investigate the formation of the Cretaceous-Paleogene (K-Pg) boundary 36 layer through numerical modeling. The K-Pg layer is widely agreed to be composed of meteoritic 37 material and target rock from the Chicxulub impact site, that has been ejected around the globe 38 and mixed with local material during final deposition. The observed composition and thickness of 39 the K-Pg boundary layer changes with azimuth and distance from the impact site. We have run a 40 suite of numerical simulations to investigate whether we can replicate the observational data, with 41 a focus on the distal K-Pg layer and the impact glasses at proximal sites such as Beloc, Haiti. 42Previous models of the K-Pg ejecta have assumed an initial velocity distribution and tracked the 43 ejecta to its final destination. Here, we attempt to model the entire process, from impact to the 44 arrival of the ejecta around the globe. Our models replicate the observed ejecta thickness at 45 proximal sites, and the modeled ejecta is composed of sediments and silicate basement rocks, in 46 agreement with observational data. Models that use a 45° impact angle are able to replicate the 47 total ejecta and iridium volume at distal sites, and the majority of the ejecta is composed of 48 meteorite and target sediments. Sub-vertical impacts generate too little iridium, and oblique 49 impacts of ≤ 30 degrees generate too much. However, in contrast to observations, models that 50 involve ballistic transport of ejecta lead to ejecta thickness decreasing with increasing distance, 51and are unable to transport shocked minerals (quartz and zircon) from the Chicxulub basement 52 rocks around the globe. We suggest that much of the K-Pg ejecta is transported non-ballistically, 53and that the most plausible mechanism is through re-distribution from a hot, expanding 54 atmosphere. The results are important for future investigations of the environmental effects of the 55 Chicxulub impact. 56