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Chicxulub crater
Chicxulub impact structure
Yucatan chix crater.jpg
Imaging from NASA's Shuttle Radar Topography Mission STS-99 reveals part of the 180 km (110 mi) diameter ring of the crater. The numerous cenotes (sinkholes) clustered around the trough of the crater suggest a prehistoric oceanic basin in the depression left by the impact.[1]
Impact crater/structure
ConfidenceConfirmed
Diameter180 km (110 mi)
Depth20 km (12 mi)
Impactor diameter10 kilometers (6.2 mi)
Age66.043 ± 0.043 Ma
Cretaceous–Paleogene boundary[2]
ExposedNo
DrilledYes
Bolide typeCM or CR type carbonaceous chondrite
Location
Coordinates21°24′0″N 89°31′0″W / 21.40000°N 89.51667°W / 21.40000; -89.51667Coordinates: 21°24′0″N 89°31′0″W / 21.40000°N 89.51667°W / 21.40000; -89.51667
CountryMexico
StateYucatán
Chicxulub crater is located in North America
Chicxulub crater
Chicxulub crater
Location of Chicxulub crater
Chicxulub crater is located in Mexico
Chicxulub crater
Chicxulub crater
Chicxulub crater (Mexico)

The Chicxulub crater (IPA: [tʃikʃuˈlub]) is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is offshore near the communities of Chicxulub Puerto and Chicxulub Pueblo, after which the crater is named. It was formed when a large asteroid, about 10 kilometers (6.2 miles) in diameter, struck the Earth. The crater is estimated to be 180 kilometers (110 miles) in diameter and 20 kilometers (12 miles) in depth. It is the one of the largest confirmed impact structures on Earth, 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 impact origin of the crater includes shocked quartz, a gravity anomaly, and tektites in surrounding areas.

The date of the impact coincides with the Cretaceous–Paleogene boundary (commonly known as the K–Pg or K-T boundary), roughly 66 million years ago, and it is now widely accepted that the devastation and climate disruption from the impact was the cause of the Cretaceous–Paleogene extinction event, a mass extinction in which 75% of plant and animal species on Earth became extinct, including all non-avian dinosaurs.

Discovery[edit]

Gravity anomaly map of the Chicxulub impact area. The coastline is shown as a white line. A series of concentric features reveals the location of the crater. White dots represent cenotes (water-filled sinkholes).

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 event was caused by an impact event.[3][4] The main evidence of such an impact was contained in a thin layer of clay present in the K–Pg boundary across the world. The Alvarezes and colleagues reported that it contained an abnormally high concentration of iridium.[5][6] Iridium levels in this layer were as much as 160 times above the background level.[7] It was hypothesized that the iridium was spread into the atmosphere when the impactor was vaporized and settled across the Earth's surface among other material thrown up by the impact, producing the layer of iridium-enriched clay.[8] At the time, consensus was not settled on what caused the K-Pg boundary and the extinction, with theories including a nearby supernova, climate change, or a geomagnetic reversal.[7]: 1095  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.[9][4]

The Alvarezes, joined by nuclear chemist Frank Asaro and Berkeley paleontologist Helen Michel, published their paper on their iridium anomaly in Science in June 1980. Their paper was followed by other reports of similar iridium spikes 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.[9][10] There were no known impact craters that were the right age and size, spurring a search for a suitable candidate.[4] Recognizing the scope of the work, Lee Hunt and Lee Silver organized a cross-discipline meeting in Snowbird, Utah in 1981. Unbeknownst to those gathered, evidence of just the crater they were looking for was being presented the same week, and would be largely missed by the scientific community.[9][10]

Years earlier in 1978, geophysicists Glen Penfield and Antonio Camargo were working for the Mexican state-owned oil company Petróleos Mexicanos, or Pemex, as part of an airborne magnetic survey of the Gulf of Mexico north of the Yucatán Peninsula.[11]: 20–21  Penfield's job was to use geophysical data to scout possible locations for oil drilling.[3] In the offshore magnetic data, Penfield noted anomalies whose depth he estimated and mapped. He then obtained onshore gravity data from the 1940s. When the gravity maps and magnetic anomalies were compared, Penfield described a shallow, 180 km (110 mi)-diameter "bullseye" appearing on the otherwise non-magnetic and uniform surroundings—clear evidence to him of an impact feature.[3][12] A decade earlier, the same map had suggested a crater to contractor Robert Baltosser, but he was forbidden to publicize his conclusion by Pemex corporate policy.[11]: 20 

Penfield presented his findings to Pemex, who rejected the crater theory, instead deferring to findings that ascribed the feature to volcanic activity.[12] Pemex disallowed release of specific data, but let Penfield and Camargo present the results at the 1981 Society of Exploration Geophysicists conference.[10] That year's conference was under-attended and their report attracted scant attention, with many experts on impact craters and the K-Pg boundary 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 story on Penfield and Camargo's claim, but the news did not disseminate widely.[11]: 23 

Although Penfield had plenty of geophysical data sets, he had no rock cores or other physical evidence of an impact.[3] He knew Pemex had drilled exploratory wells in the region. In 1951, one bored into what was described as a thick layer of andesite about 1.3 kilometers (4,300 ft) 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.[3] Penfield was encouraged by William C. Phinney, curator of the lunar rocks at the Johnson Space Center, to find these samples to prove his theory.[12] Penfield tried to secure site samples, but was told they had been lost or destroyed. When attempts at returning to the drill sites and looking for corroborating rocks proved fruitless, Penfield abandoned his search, published his findings and returned to his Pemex work.[3] Seeing the 1980 Science paper, Penfield wrote to Walter Alvarez about the Chicxulub structure, but received no response.[10]

Alvarez and other scientists continued their search for the crater, although they were searching in oceans based on incorrect analysis of spherules from the K-Pg boundary that suggested the impactor had landed in open water.[9] 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.[9] Their evidence included greenish-brown clay with surplus iridium containing shocked quartz grains and small weathered glass beads that looked to be tektites.[13] 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 detonations.[3]

In 1990, Carlos Byars told Hildebrand of Penfield's earlier discovery of a possible impact crater.[14]: 50  Hildebrand contacted Penfield and the pair soon secured two drill samples from the Pemex wells, stored in New Orleans. Hildebrand's team tested the samples, which clearly showed shock-metamorphic materials.[3] A team of California researchers surveying satellite images found a cenote (sinkhole) ring centered on Chicxulub 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.[15] More recent evidence suggests the crater is 300 km (190 mi) wide, and the 180 km (110 mi) ring is an inner wall of it.[16] Hildebrand, Penfield, Boynton, Camargo, and others published their paper identifying the crater in 1991.[9][13]

In March 2010, forty-one experts from many countries reviewed the available evidence: 20 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.[17][18][4] 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;[4][19] 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.[20] A 2013 study compared isotopes in impact glass from the Chicxulub impact with isotopes in ash from the K-Pg boundary, concluding that the they were dated almost exactly the same within experimental error.[2]

Impact specifics[edit]

A 2013 study in Science obtained a mean estimate for 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, United States.[2] This date was supported by a 2015 study based on argon–argon dating of tephra found in lignite beds in the Hell Creek and overlying Fort Union formations in northeastern Montana.[21] A 2018 study based on argon–argon dating of glassy spherules from Gorgonilla Island, Colombia obtained a slightly different result of 66,051,000 ± 31,000 years ago.[22] The impact likely occurred in late Northern Hemisphere spring or summer.[23] A 2020 study concluded that the Chicxulub crater was formed by an inclined (45–60° to horizontal) impact from the northeast.[24] The site of the crater at the time of impact was a marine carbonate platform.[25] The water depth at the impact site varied from 100 meters (330 ft) on the western edge of the crater to over 1,200 meters (3,900 ft) on the northeastern edge.[26] The seafloor rocks consisted of a 3 kilometers (1.9 mi) thick sequence of Jurassic-Cretaceous aged marine sediments, 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 ~35 kilometers (22 mi) of continental crust, composed of igneous crystalline basement including granite.[27]

There is broad consensus that the Chicxulub impactor was an asteroid with a carbonaceous chondrite composition, rather than a comet.[28] In 1998 a 2.5 mm sized meteorite was described from the North Pacific from sediments spanning the Cretaceous-Paleogene boundary, that was suggested to represent a fragment of the Chicxulub impactor. Analysis suggested that it best fit the criteria of CV, CO and CR carbonaceous chondrites.[29] The impactor was around 10 kilometers (6.2 miles) in diameter[28]—large enough that, if set at sea level, it would have reached taller than Mount Everest.[9]: 9 

Effects[edit]

An animation showing the Chicxulub impact and subsequent crater formation

Hitting the earth at a speed of perhaps 30 km (19 mi) a second,[9]: 10  the kinetic energy of the impact was estimated in 1996 to be roughly 3×1023 joules,[30] more than a billion times more energy than the atomic bomb dropped on Hiroshima, Japan.[31] The impact created winds in excess of 1,000 kilometers per hour (620 mph) near the blast's center,[32] and created a transient cavity 100 kilometers (62 mi) wide and 30 kilometers (19 mi) deep that later collapsed. This formed a crater mainly under the sea and covered by 600 meters (2,000 ft) of sediment by the 21st century.[33] The impact, expansion of water after filling the crater, and related seismic activity spawned megatsunamis over 100 meters (330 ft) tall, with one simulation suggesting the immediate waves from the impact may have reached up to 1.5 kilometers (~1 mi) high.[34][35] The waves scoured the sea floor, leaving ripples underneath what is now Louisiana with average wavelengths of 600 meters and average wave heights of 16 meters, the largest ripples documented.[36][37] Material shifted by subsequent earthquakes and the waves reached all the way to what are now Texas and Florida, and may have disturbed sediments as far as 6000 kilometers from the impact site.[31][34][38][39][40] The impact triggered a seismic event equivalent to a Magnitude 12 earthquake at the impact site, with shockwaves generating the equivalent of Magnitude 9 to as high as 11 on the Richter scale.[41]

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 through the solar system,[4] while some of it fell back to earth, heated to incandescence upon re-entry. The rock broiled the Earth's surface and ignited wildfires, estimated to have enveloped nearly 70% of the planet's forests. The devastation to living creatures even hundreds of kilometers away was immense, and much of present-day Mexico and the United States would have been desolated.[3][9]: 10–13 [4] Fossil evidence for an instantaneous die-off of diverse animals was found in a soil layer only 10 centimeters (3.9 in) thick in New Jersey, 2,500 kilometers (1,600 mi) away from the impact site, indicating that death and burial under debris occurred suddenly and quickly over wide distances on land.[33] 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 consistent with the impact event.[4]

Vaporized rock, including sulphur-rich gypsum from the shallow coastal waters, was injected into the atmosphere.[33] This global dispersal of dust and sulfates would have led to a sudden and catastrophic effect on the climate worldwide, large temperature drops, and devastated the food chain. The researchers stated that the impact 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.[42][43] Researchers using seismic images of the crater in 2008 determined that the impactor landed in deeper water than previously assumed, which may have resulted in increased sulfate aerosols in the atmosphere. This could have made the impact even deadlier by cooling the climate and generating acid rain.[44]

The emission of dust and particles could have covered the entire surface of the Earth for several years, possibly a decade, creating a harsh environment for living things. Production of carbon dioxide caused by the destruction of carbonate rocks would have led to a sudden greenhouse effect.[13]: 5  Over a decade or longer, sunlight would have been blocked from reaching the surface of the 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.[45][46] A model of the event developed by Lomax et al. (2001) suggests that net primary productivity (NPP) rates may have increased to higher than pre-impact levels over the long term because of the high carbon dioxide concentrations.[47]

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."[48]

Geology and morphology[edit]

The center of the crater is near the village of Chicxulub Puerto, Yucatán.

The rocks above the impact feature are layers of marl and limestone reaching to a depth of almost 1,000 m (3,300 ft), dating back as far as the Paleocene (56 to 66 millions years ago) and therefore after the impact.[13]: 3  Below these layers lie more than 500 m (1,600 ft) of andesite glass and breccia. These andesitic igneous rocks are only found within the impact feature, as is shocked quartz.[13]: 3  The K–Pg boundary inside the feature is depressed to 600 to 1,100 m (2,000 to 3,600 ft) compared with the normal depth of about 500 m (1,600 ft) measured 5 km (3 mi) away from the impact feature.[13]: 4 

Along the edge of the crater are clusters of cenotes,[49] which suggest that there was a water basin inside the feature during the Neogene period, after the impact.[13]: 4  The groundwater of such a basin would have dissolved the limestone and created the caves and cenotes beneath the surface.[50]

Chicxulub is the only known Earth crater with a remaining impact peak ring, but it is under 600 m (2,000 ft) of sediment.[51] Analyses indicate that the impactor was large enough to create a 190-kilometer (120 mi) peak ring.[52][53] A joint United Kingdom-United States team obtained the first offshore core samples from the peak ring in 2016, surrounding the central zone of the crater. Drilling reached 1,335 m (4,380 ft) below the ocean.[54][55] Sample preparation and analysis were performed in Bremen, Germany.[51]

Pink granite, usually found deep in the Earth's crust, was found in drilling samples.[52][42] It suggests the impact was so great it shocked and melted rocks found deep in the crust, causing them to shoot up before falling back down to produce the peak rings.[52][42] The granite samples were also found to be lighter and weaker than normal granite, a result of the shock and extreme conditions of the impact.[42][56] The post-impact tsunamis were sufficient to lay down the largest-known layered bed of sand, around 100 m (330 ft) deep and separated by grain size, directly above the peak ring.[53]

The peak ring drilling below the sea floor also discovered a massive hydrothermal system filled with magma, which modified ~1.4 × 105 km3 of Earth's crust and lasted for hundreds of thousands of years; in addition, those hydrothermal systems might support the impact origin of life hypothesis for the Hadean,[57] when the entire surface of Earth was affected by impactors enormously larger than the Chicxulub impactor.[58]

Astronomical origin of impactor[edit]

In September 2007, a report published in Nature proposed an origin for the asteroid that created the Chicxulub crater.[45] The authors, William F. Bottke, David Vokrouhlický, and David Nesvorný, argued that a collision in the asteroid belt 160 million years ago 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. The connection between Chicxulub and Baptistina is supported by the large amount of carbonaceous material present in microscopic fragments of the impactor, suggesting the impactor was a member of an uncommon class of asteroids called carbonaceous chondrites, like Baptistina. According to Bottke, the Chicxulub impactor was a fragment of a much larger parent body about 170 km (106 mi) across, with the other impacting body being around 60 km (37 mi) in diameter.[59][60]

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. This makes an asteroid from this family highly improbable to be the asteroid that created the Chicxulub crater, as typically the process of resonance and collision of an asteroid takes many tens of millions of years.[61] In 2010, another hypothesis was offered which implicated the newly discovered asteroid 354P/LINEAR, a member of the Flora family of asteroids, as a possible remnant cohort of the K/Pg impactor.[62]

Four independent laboratories showed elevated concentrations of iridium in the crater's peak ring, further corroborating the asteroid impact hypothesis.[63] In the same month Avi Loeb and a colleague published a study in Scientific Reports suggesting the impactor was a fragment from a disrupted comet, rather than an asteroid which has long been the leading candidate among scientists.[64][65] This was followed by a rebuttal published in Astronomy & Geophysics in June of the same year, which charged that the paper ignored the fact that the mass of iridium deposited across the globe by the impact (estimated to be approximately 2.0 - 2.8 × 1011 grams), was too large to be created by a comet impactor, and 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 was either a CM or CR carbonaceous chondrite C-type asteroid.[28] In July 2021, a study reported that the impactor likely originated in the outer main part of the asteroid belt, based on numerical simulations.[66]

See also[edit]

References[edit]

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External links[edit]

Media related to Chicxulub crater at Wikimedia Commons

source: https://web.archive.org/web/20220216150309/https://en.wikipedia.org/wiki/Chicxulub_crater

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