Chicxulub crater

In the late 1970s, geologist Walter Alvarez and his father, Nobel Prize–winning scientist Luis Walter Alvarez, put forth their theory that the Cretaceous–Paleogene extinction was caused by an impact event. Iridium levels in this layer were as much as 160 times above the background level. At the time, there was no consensus on what caused the Cretaceous–Paleogene extinction and the boundary layer, with theories including a nearby supernova, climate change, or a geomagnetic reversal. The Alvarezes' impact hypothesis was rejected by many paleontologists, who believed that the lack of fossils found close to the K–Pg boundary—the "three-meter problem"—suggested a more gradual die-off of fossil species. The aftermath of the asteroid collision, which occurred approximately 66 million years ago, is believed to have caused the mass extinction of non-avian dinosaurs and many other species on Earth. A decade earlier, the same map had suggested a crater to contractor Robert Baltosser, but Pemex corporate policy prevented him from publicizing his conclusion. on Penfield and Camargo's claim, but the news did not propagate widely. Hildebrand contacted Penfield and the pair soon secured two drill samples from the Pemex wells, which had been stored in New Orleans for decades. More recent evidence suggests the crater is wide, and the ring observed is an inner wall of the larger crater. Hildebrand, Penfield, Boynton, Camargo, and others published their paper identifying the crater in 1991. Dissenters, notably Gerta Keller of Princeton University, have proposed an alternate culprit: the eruption of the Deccan Traps in what is now the Indian subcontinent. This period of intense volcanism occurred before and after the Chicxulub impact; dissenting studies argue that the worst of the volcanic activity occurred before the impact, and the role of the Deccan Traps was instead shaping the evolution of surviving species post-impact. A 2013 study compared isotopes in impact glass from the Chicxulub impact with isotopes in ash from the K–Pg boundary, concluding that they were dated almost exactly the same, and within experimental error. ==Impact specifics==

over the Chicxulub structure (coastline and state boundaries shown as black lines) A 2013 study published in Science estimated the age of the impact as 66,043,000 ± 11,000 years ago (± 43,000 years ago considering systematic error), based on multiple lines of evidence, including argon–argon dating of tektites from Haiti and bentonite horizons overlying the impact horizon in northeastern Montana. A 2018 study based on argon–argon dating of spherules from Gorgonilla Island, Colombia, obtained a slightly different result of 66,051,000 ± 31,000 years ago. The impact has been interpreted to have occurred in the Northern Hemisphere's spring season based on annual isotope curves in sturgeon and paddlefish bones found in an ejecta-bearing sedimentary unit at the Tanis site in southwestern North Dakota. This sedimentary unit is thought to have formed within hours of impact. The site of the crater at the time of impact was a marine carbonate platform. The water depth at the impact site varied from on the western edge of the crater to over on the northeastern edge, with an estimated depth at the centre of the impact of approximately . The seafloor rocks consisted of a sequence of Jurassic–Cretaceous marine sediments thick. They were predominantly carbonate rock, including dolomite (35–40% of total sequence) and limestone (25–30%), along with evaporites (anhydrite 25–30%) and minor amounts of shale and sandstone (3–4%) underlain by approximately of continental crust, composed of igneous crystalline basement including granite. The impactor was around in diameter—large enough that, if set at sea level, it would have reached taller than Mount Everest. A 2021 study estimated the impactor had a velocity of inclined 45–60° to horizontal, impacting from the northeast. Effects The kinetic energy of the impact was estimated at . The impact generated winds in excess of near the blast's center, and produced a transient cavity wide and deep that later collapsed. This formed a crater mainly under the sea and currently covered by ~ of sediment. The impact, expansion of water after filling the crater, and related seismic activity spawned megatsunamis over tall, with one simulation suggesting the immediate waves from the impact may have reached up to high. The waves scoured the sea floor, leaving ripples underneath what is now Louisiana with average wavelengths of and average wave heights of , the largest ripples documented. Material shifted by subsequent earthquakes and the waves reached to what are now Texas and Florida, and may have disturbed sediments as far as from the impact site. The impact triggered a seismic event with an estimated moment magnitude of 9–11 . Due to the relatively shallow water at the impact site, the rock that was vaporized included sulfur-rich gypsum from the lower part of the Cretaceous sequence, and this was injected into the atmosphere. Using seismic images of the crater in 2008, scientists determined that the impactor landed in deeper water than previously assumed, which may have resulted in increased sulfate aerosols in the atmosphere as a result of more water vapor being available to react with the vaporized anhydrite. This could have made the impact even deadlier by rapidly cooling the climate and generating acid rain. The emission of dust and particles could have covered the entire surface of Earth for several years, possibly up to a decade, creating a harsh environment for biological life. Production of carbon dioxide caused by the destruction of carbonate rocks would have led to a sudden greenhouse effect. A model of the event developed by Lomax et al (2001) suggests that net primary productivity rates may have increased to higher than pre-impact levels over the long term because of the high carbon dioxide concentrations. A long-term local effect of the impact was the creation of the Yucatán sedimentary basin which "ultimately produced favorable conditions for human settlement in a region where surface water is scarce". ==Post-discovery investigations==

Geophysical data Two seismic reflection datasets have been acquired over the offshore parts of the crater since its discovery. Older 2D seismic datasets have also been used that were originally acquired for hydrocarbon exploration. A set of three long-record 2D lines was acquired in October 1996, with a total length of , by the BIRPS group. The longest of the lines, Chicx-A, was shot parallel to the coast, while Chicx-B and Chicx-C were shot NW–SE and SSW–NNE respectively. In addition to the conventional seismic reflection imaging, data was recorded onshore to allow for wide-angle refraction imaging. In 2005, another set of profiles was acquired, bringing the total length of the 2D deep-penetration seismic data up to . This survey also used ocean bottom seismometers and land stations to allow 3D travel time inversion to improve the understanding of the velocity structure of the crater. The data was concentrated around the interpreted offshore peak ring to help identify possible drilling locations. At the same time, gravity data was acquired along of profiles. The acquisition was funded by the National Science Foundation (NSF), Natural Environment Research Council (NERC) with logistical assistance from the National Autonomous University of Mexico (UNAM) and the Centro de Investigación Científica de Yucatán (CICY – Yucatán Center for Scientific Investigation). Borehole drilling Intermittent core samples from hydrocarbon exploration boreholes drilled by Pemex on the Yucatán peninsula have provided some useful data. UNAM drilled a series of eight fully-cored boreholes in 1995, three of which penetrated deep enough to reach the ejecta deposits outside the main crater rim (UNAM-5, 6, and 7). Between 2001 and 2002, a scientific borehole was drilled near the Hacienda Yaxcopoil, known as Yaxcopoil-1 (or more commonly Yax-1), to a depth of below the surface, as part of the International Continental Scientific Drilling Program. The borehole was cored continuously, passing through of impactites. Three fully-cored boreholes were also drilled by the Comisión Federal de Electricidad (Federal Electricity Commission) with UNAM. One of them, (BEV-4), was deep enough to reach the ejecta deposits. In 2016, a joint United Kingdom–United States team obtained the first offshore core samples from the peak ring in the central zone of the crater with the drilling of the borehole known as M0077A, part of Expedition 364 of the International Ocean Discovery Program. The borehole reached below the seafloor. ==Morphology==

The form and structure (geomorphology) of the Chicxulub crater is known mainly from geophysical data. It has a well-defined concentric multi-ring structure. The outermost ring was identified using seismic reflection data. It is up to from the crater center, and is a ring of normal faults, throwing down towards the crater center, marking the outer limit of significant crustal deformation. This makes it one of the three largest impact structures on Earth. This ring has a radius that varies between . The ring structures are best developed to the south, west and northwest, becoming more indistinct towards the north and northeast of the structure. This is interpreted to be a result of variable water depth at the time of impact, with less-well-defined rings resulting from the areas with water depths significantly deeper than . ==Geology==

Pre-impact geology .|right in the main square of Chicxulub Puerto commemorating the impact Before the impact, the geology of the Yucatán area, sometimes referred to as the "target rocks", consisted of a sequence of mainly Cretaceous limestones, overlying red beds of uncertain age above an unconformity with the dominantly granitic basement. The basement forms part of the Maya Block and information about its makeup and age in the Yucatán area has come only from drilling results around the Chicxulub crater and the analysis of basement material found as part of the ejecta at more distant K–Pg boundary sites. The Maya block is one of a group of crustal blocks found at the edge of the Gondwana continent. Zircon ages are consistent with the presence of an underlying Grenville age crust, with large amounts of late Ediacaran arc-related igneous rocks, interpreted to have formed in the Pan-African orogeny. Late Paleozoic granitoids (the distinctive "pink granite") were found in the peak ring borehole M0077A, with an estimated age of 326 ± 5 million years ago (Carboniferous). These have an adakitic composition and are interpreted to represent the effects of slab detachment during the Marathon-Ouachita orogeny, part of the collision between Laurentia and Gondwana that created the Pangaea supercontinent. Red beds of variable thickness, up to , overlay the granitic basement, particularly in the southern part of the area. These continental clastic rocks are thought to be of Triassic-to-Jurassic age, although they may extend into the Lower Cretaceous. The lower part of the Lower Cretaceous sequence consists of dolomite with interbedded anhydrite and gypsum, with the upper part being limestone, with dolomite and anhydrite in part. The thickness of the Lower Cretaceous varies from up to in the boreholes. The Upper Cretaceous sequence is mainly platform limestone, with marl and interbedded anhydrite. It varies in thickness from up to . There is evidence for a Cretaceous basin within the Yucatán area that has been named the Yucatán Trough, running approximately south–north, widening northwards, explaining the observed thickness variations. Impact rocks The most common observed impact rocks are suevites, found in many of the boreholes drilled around the Chicxulub crater. Most of the suevites were resedimented soon after the impact by the resurgence of oceanic water into the crater. This gave rise to a layer of suevite extending from the inner part of the crater out as far as the outer rim. Impact melt rocks are thought to fill the central part of the crater, with a maximum thickness of . The samples of melt rock that have been studied have overall compositions similar to that of the basement rocks, with some indications of mixing with carbonate source, presumed to be derived from the Cretaceous carbonates. An analysis of melt rocks sampled by the M0077A borehole indicates two types of melt rock, an upper impact melt (UIM), which has a clear carbonate component as shown by its overall chemistry and the presence of rare limestone clasts and a lower impact melt-bearing unit (LIMB) that lacks any carbonate component. The difference between the two impact melts is interpreted to be a result of the upper part of the initial impact melt, represented by the LIMB in the borehole, becoming mixed with materials from the shallow part of the crust either falling back into the crater or being brought back by the resurgence forming the UIM. The "pink granite", a granitoid rich in alkali feldspar found in the peak ring borehole shows many deformation features that record the extreme strains associated with the formation of the crater and the subsequent development of the peak ring. The granitoid has an unusually low density and P-wave velocity compared to typical granitic basement rocks. Study of the core from M0077A shows the following deformation features in apparent order of development: pervasive fracturing along and through grain boundaries, a high density of shear faults, bands of cataclasite and ultra-cataclasite and some ductile shear structures. This deformation sequence is interpreted to result from initial crater formation involving acoustic fluidization followed by shear faulting with the development of cataclasites with fault zones containing impact melts. The peak ring drilling below the sea floor also discovered evidence of a massive hydrothermal system, which modified approximately of Earth's crust and lasted for hundreds of thousands of years. These hydrothermal systems may provide support for the impact origin of life hypothesis for the Hadean eon, when the entire surface of Earth was affected by impactors much larger than the Chicxulub impactor. Post-impact geology After the immediate effects of the impact had stopped, sedimentation in the Chicxulub area returned to the shallow water platform carbonate depositional environment that characterised it before the impact. The sequence, which dates back as far as the Paleocene, consists of marl and limestone, reaching a thickness of about . which are the surface expression of a zone of preferential groundwater flow, moving water from a recharge zone in the south to the coast through a karstic aquifer system. From the cenote locations, the karstic aquifer is clearly related to the underlying crater rim, possibly through higher levels of fracturing, caused by differential compaction. ==Astronomical origin and type of impactor==

There is broad consensus that the Chicxulub impactor was a C-type asteroid with a carbonaceous chondrite-like composition, rather than a comet. In 1998, a meteorite, approximately across, was described from a deep sea sediment core from the North Pacific, from a sediment sequence spanning the Cretaceous–Paleogene boundary (when the site was located in the central Pacific), with the meteorite being found at the base of the K-Pg boundary iridium anomaly within the sediment core. It was suggested to be a fragment of the Chicxulub impactor. Analysis suggested that it best fitted the criteria of the CV, CO and CR groups of carbonaceous chondrites. A 2021 paper suggested, based on geochemical evidence including the excess of chromium isotope 54Cr and the ratios of platinum group metals found in marine impact layers, that the impactor matched the characteristics of CM or CR carbonaceous chondrites. Subsequent evidence has disproven this theory. A 2009 spectrographic analysis revealed that 298 Baptistina has a different composition more typical of an S-type asteroid than the presumed carbonaceous chondrite composition of the Chicxulub impactor. In 2011, data from the Wide-field Infrared Survey Explorer revised the date of the collision which created the Baptistina family to about 80 million years ago, allowing only 15 million years for the process of resonance and collision, which takes many tens of millions of years. In 2010, another hypothesis implicated the newly discovered asteroid 354P/LINEAR, a member of the Flora family, as a possible remnant cohort of the K–Pg impactor. In 2021, a numerical simulation study argued that the impactor likely originated in the outer main part of the asteroid belt. Some scholars have argued that the impactor was a comet, not an asteroid. Two papers in 1984 proposed it to be a comet originating from the Oort cloud, and it was proposed in 1992 that tidal disruption of comets could potentially increase impact rates. A rebuttal in Astronomy & Geophysics countered that Loeb et al. had ignored that the amount of iridium deposited around the globe, , was too large for a comet of the size implied by the crater, and that they had overestimated likely comet impact rates. They concluded that all available evidence strongly favors an asteroid impactor, effectively ruling out a comet. Ruthenium isotope ratios in impact layers also strongly support an asteroid rather than a comet nature for the impactor. == See also ==