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The Chicxulub crater is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is offshore, but the crater is named after the onshore community of Chicxulub Pueblo (not to be confused with the larger coastal town of Chicxulub Puerto). It was formed slightly over 66 million years ago when an asteroid, about in diameter, struck Earth. The crater is estimated to be in diameter and is buried to a depth of about beneath younger . It is one of the largest impact structures on Earth, alongside the much older Sudbury Basin and Vredefort impact structures, and the only one whose peak ring is intact and directly accessible for scientific research.
The crater was discovered by Antonio Camargo and Glen Penfield, geophysicists who had been looking for petroleum in the Yucatán Peninsula during the late 1970s. Penfield was initially unable to obtain evidence that the geological feature was a crater and gave up his search. Later, through contact with Alan R. Hildebrand in 1990, Penfield obtained samples that suggested it was an impact feature. Evidence for the crater's impact origin includes shocked quartz, a gravity anomaly, and in surrounding areas.
The date of the impact coincides with the Cretaceous–Paleogene boundary (commonly known as the K–Pg or K–T boundary). It is now widely accepted that the devastation and impact winter resulting from the impact was the primary cause of the Cretaceous–Paleogene extinction event, a mass extinction of 75% of plant and animal species on Earth, including all Dinosaur.
The Alvarezes, joined by Frank Asaro and Helen Michel from University of California, Berkeley, published their paper on the iridium anomaly in Science in June 1980. Almost simultaneously Jan Smit and Jan Hertogen published their iridium findings from Caravaca, Spain, in Nature in May 1980. These papers were followed by other reports of similar iridium spikes at the K–Pg boundary across the globe, and sparked wide interest in the cause of the K–Pg extinction; over 2,000 papers were published in the 1980s on the topic. There were no known impact craters that were the right age and size, spurring a search for a suitable candidate. Recognizing the scope of the work, Lee Hunt and Lee Silver organized a cross-discipline meeting in Snowbird, Utah, in 1981. Unknown to them, evidence of the crater they were looking for was being presented the same week, and would be largely missed by the scientific community.
In 1978, geophysicists Glen Penfield and Antonio Camargo were working for the Mexican state-owned oil company Petróleos Mexicanos (Pemex) as part of an airborne magnetic survey of the Gulf of Mexico north of the Yucatán Peninsula. Penfield's job was to use geophysical data to scout possible locations for oil drilling. In the offshore magnetic data, Penfield noted anomalies whose depth he estimated and mapped. He then obtained onshore gravity anomaly from the 1940s. When the gravity maps and Magnetic anomaly were compared, Penfield described a shallow "bullseye", in diameter, appearing on the otherwise non-magnetic and uniform surroundings—clear evidence to him of an impact feature. A decade earlier, the same map had suggested a crater to contractor Robert Baltosser, but Pemex corporate policy prevented him from publicizing his conclusion.
Penfield presented his findings to Pemex, who rejected the crater theory, instead deferring to findings that ascribed the feature to volcanic activity. Pemex disallowed release of specific data, but let Penfield and Camargo present the results at the 1981 Society of Exploration Geophysicists conference. That year's conference was under-attended and their report attracted little attention, as many experts on impact craters and the K–Pg boundary were attending the Snowbird conference instead. Carlos Byars, a Houston Chronicle journalist who was familiar with Penfield and had seen the gravitational and magnetic data himself, wrote a front-page story on Penfield and Camargo's claim, but the news did not propagate widely.
Although Penfield had plenty of geophysical data sets, he had no rock cores or other physical evidence of an impact. He knew Pemex had drilled exploratory wells in the region. In 1951, one well bored into what was described as a thick layer of andesite about down. This layer could have resulted from the intense heat and pressure of an Earth impact, but at the time of the borings it was dismissed as a lava dome—a feature uncharacteristic of the region's geology. Penfield was encouraged by William C. Phinney, curator of at the Johnson Space Center, to find these samples to support his hypothesis. Penfield tried to secure site samples, but was told they had been lost or destroyed. When attempts to return to the drill sites to look for corroborating rocks proved fruitless, Penfield abandoned his search, published his findings and returned to his Pemex work. Seeing the 1980 Science paper, Penfield wrote to Walter Alvarez about the Yucatán structure, but received no response.
Alvarez and other scientists continued their search for the crater, although they were searching in oceans based on incorrect analysis of glassy from the K–Pg boundary that suggested the impactor had landed in open water. Unaware of Penfield's discovery, University of Arizona graduate student Alan R. Hildebrand and faculty adviser William V. Boynton looked for a crater near the Brazos River in Texas. Their evidence included greenish-brown clay with surplus iridium, containing shocked quartz grains and small weathered glass beads that looked to be . Thick, jumbled deposits of coarse rock fragments were also present, thought to have been scoured from one place and deposited elsewhere by an impact event. Such deposits occur in many locations but seemed concentrated in the Caribbean Basin at the K–Pg boundary. When Haitian professor Florentine Morás discovered what he thought to be evidence of an ancient volcano on Haiti, Hildebrand suggested it could be a telltale feature of a nearby impact. Tests on samples retrieved from the K–Pg boundary revealed more tektite glass, formed only in the heat of asteroid impacts and high-yield nuclear weapon.
In 1990, Carlos Byars told Hildebrand of Penfield's earlier discovery of a possible impact crater. Hildebrand contacted Penfield and the pair soon secured two drill samples from the Pemex wells, which had been stored in New Orleans for decades. Hildebrand's team tested the samples, which clearly showed shock-metamorphic materials. A team of California researchers surveying satellite images found a cenote (sinkhole) ring centered on the town of Chicxulub Pueblo that matched the one Penfield saw earlier; the cenotes were thought to be caused by subsidence of bolide-weakened lithostratigraphy around the impact crater wall. 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. The crater was named for the nearby town of Chicxulub Pueblo. Penfield also recalled that part of the motivation for the name was "to give the academics and NASA naysayers a challenging time pronouncing it" after years of dismissing its existence.
In March 2010, forty-one experts from many countries reviewed the available evidence: twenty years' worth of data spanning a variety of fields. They concluded that the impact at Chicxulub triggered the mass extinctions at the K–Pg boundary.; 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 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.
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 (anhydrite 25–30%) and minor amounts of shale and sandstone (3–4%) underlain by approximately of continental crust, composed of Igneous rock 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.
A cloud of hot dust, ash and steam would have spread from the crater, with as much as 25 trillion metric tons of excavated material being ejected into the atmosphere by the blast. Some of this material escaped orbit, dispersing throughout the Solar System, while some of it fell back to Earth, vaporizing upon re-entry. The rock heated Earth's surface and ignited wildfires, estimated to have enveloped nearly 70% of the planet's forests. The effect on living creatures even hundreds of kilometers away was immense, and much of present-day Mexico and the United States would have been devastated. Fossil evidence for an instantaneous extinction of diverse animals was found in a soil layer only thick in New Jersey, away from the impact site, indicating that death and burial under debris occurred suddenly and quickly over wide distances on nearby land. Field research from the Hell Creek Formation in North Dakota published in 2019 shows the simultaneous mass extinction of myriad species, combined with geological and atmospheric features that are consistent with the impact event.
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. This global dispersal of dust and would have led to a sudden and catastrophic effect on the climate worldwide, instigating large temperature drops and devastating the food chain. Researchers stated that the impact not only generated an environmental calamity that extinguished life, but it also induced a vast subsurface hydrothermal system that became an oasis for the recovery of life. 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 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. For a decade or longer, sunlight would have been blocked from reaching the surface of Earth by the dust particles in the atmosphere, cooling the surface dramatically. Photosynthesis by plants would also have been interrupted, affecting the entire food chain. See also a similar 1998 report by the same group. 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".
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).
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.;
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 .
Red beds of variable thickness, up to , overlay the granitic basement, particularly in the southern part of the area. These continental 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 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 , bands of cataclasite and ultra-cataclasite and some shear zone. This deformation sequence is interpreted to result from initial crater formation involving acoustic fluidization followed by shear faulting with the development of cataclasites with 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.
On the Yucatán peninsula, the inner rim of the crater is marked by clusters of , 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 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. (1998). 9781862390171, Geological Society. ISBN 9781862390171
A 2007 Nature report proposed a specific astronomical origin for the Chicxulub asteroid. The authors, William F. Bottke, David Vokrouhlický, and David Nesvorný, argued that a collision in the asteroid belt 160 million years ago between a diameter parent body and another diameter body resulted in the Baptistina family of asteroids, the largest surviving member of which is 298 Baptistina. They proposed that the Chicxulub asteroid was also a member of this group.; 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 Disrupted planet of comets could potentially increase impact rates. In 2021, Avi Loeb and a colleague suggested in Scientific Reports that the impactor was a fragment from a disrupted comet.; 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.
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