Chicxulub crater
Chicxulub crater | |
---|---|
Chicxulub impact structure | |
CR type carbonaceous chondrite | |
Location | |
Coordinates | 21°24′0″N 89°31′0″W / 21.40000°N 89.51667°W |
Country | Mexico |
State | Yucatán |
The Chicxulub crater (
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 tektites in surrounding areas.[3]
The date of the impact coincides with the
Discovery
In the late 1970s, geologist
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.[8] Almost simultaneously Jan Smit and Jan Hertogen published their iridium findings from Caravaca, Spain in Nature in May 1980.[11] 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.[10]: 82 [12] There were no known impact craters that were the right age and size, spurring a search for a suitable candidate.[6] 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.[10]: 83–84 [12]
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.[14]: 20–1 Penfield's job was to use geophysical data to scout possible locations for oil drilling.[5] 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 "bullseye", 180 km (110 mi) in diameter, appearing on the otherwise non-magnetic and uniform surroundings—clear evidence to him of an impact feature.[5][3] A decade earlier, the same map had suggested a crater to contractor Robert Baltosser, but Pemex corporate policy prevented him from publicizing his conclusion.[14]: 20
Penfield presented his findings to Pemex, who rejected the crater theory, instead deferring to findings that ascribed the feature to volcanic activity.[3] Pemex disallowed release of specific data, but let Penfield and Camargo present the results at the 1981 Society of Exploration Geophysicists conference.[12] 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.[14]: 23
Although Penfield had plenty of geophysical data sets, he had no rock cores or other physical evidence of an impact.
Alvarez and other scientists continued their search for the crater, although they were searching in oceans based on incorrect analysis of glassy
In 1990, Carlos Byars told Hildebrand of Penfield's earlier discovery of a possible impact crater.[16]: 50 Hildebrand contacted Penfield and the pair soon secured two drill samples from the Pemex wells, which had been stored in New Orleans for decades.[3] Hildebrand's team tested the samples, which clearly showed shock-metamorphic materials.[5] 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.[17] 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.[18] Hildebrand, Penfield, Boynton, Camargo, and others published their paper identifying the crater in 1991.[10][15] The crater was named for the nearby town of Chicxulub. 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.[3]
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.
Impact specifics
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, 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 spherules from Gorgonilla Island, Colombia, obtained a slightly different result of 66,051,000 ± 31,000 years ago.[22] The impact has been interpreted to have occurred in Northern Hemisphere spring 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.[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, with an estimated depth at the centre of the impact of approximately 650 meters (2,130 ft).[26] The seafloor rocks consisted of a sequence of Jurassic–Cretaceous marine sediments, 3 kilometers (1.9 mi) 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 35 kilometers (22 mi) of continental crust, composed of igneous crystalline basement including granite.[27]
There is broad consensus that the Chicxulub impactor was a
Effects
The impactor's velocity was estimated at 20 kilometers per second (12 mi/s).
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,[6] while some of it fell back to Earth, heated to incandescence upon re-entry. The rock heated 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 devastated.[5][10]: 10–13 [6] Fossil evidence for an instantaneous extinction 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.[6]
Due to the relatively shallow water, the rock that was vaporized included sulfur-rich gypsum from the lower part of the Cretaceous sequence, and this 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, instigating large temperature drops and devastating 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.[41][42] 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, due to more water vapor being available to react with the vaporized anhydrite. This could have made the impact even deadlier by cooling the climate and generating acid rain.[43]
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 living things. Production of carbon dioxide caused by the destruction of carbonate rocks would have led to a sudden greenhouse effect.[15]: 5 For over 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.[44][45] 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.[46]
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".[47]
Post-discovery investigations
Geophysical data
Two
In 2005, another set of profiles was acquired, bringing the total length of 2D deep-penetration seismic data up to 2,470 kilometers (1,530 mi). This survey also used
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 deeply enough to reach the ejecta deposits outside the main crater rim, UNAM-5, 6 and 7. In 2001–2002, a scientific borehole was drilled near the Hacienda Yaxcopoil, known as Yaxcopoil-1 (or more commonly Yax-1), to a depth of 1,511 meters (4,957 ft) below the surface, as part of the International Continental Scientific Drilling Program. The borehole was cored continuously, passing through 100 meters (330 ft) 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.[51]
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 1,335 meters (4,380 ft) below the seafloor.[52]
Morphology
The form and structure (morphology) 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 130 kilometers (81 mi) from the crater center, and is a ring of
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 100 meters (330 ft).[26]
Geology
Pre-impact geology
Before the impact, the geology of the
Red beds of variable thickness, up to 115 meters (377 ft), overlay the granitic basement, particularly in the southern part of the area. These continental
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.[58]
Impact melt rocks are thought to fill the central part of the crater, with a maximum thickness of 3 kilometers (1.9 mi). 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.[59]
The "pink granite", a granitoid rich in
The peak ring drilling below the sea floor also discovered evidence of a massive hydrothermal system, which modified approximately 1.4 × 105 km3 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,[62] when the entire surface of Earth was affected by impactors much larger than the Chicxulub impactor.[63]
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 1,000 m (3,300 ft).[15]: 3 The K–Pg boundary inside the crater is significantly deeper than in the surrounding area.[15]: 4
On the Yucatán peninsula, the inner rim of the crater is marked by clusters of cenotes,[64] 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.[15]: 4 [65] From the cenote locations, the karstic aquifer is clearly related to the underlying crater rim,[66] possibly through higher levels of fracturing, caused by differential compaction.[67]
Astronomical origin of impactor
In September 2007, a report published in
The Baptistina family was subsequently considered an unlikely source of the Chicxulub asteroid because a spectrographic analysis published in 2009 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.[69] 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 made an asteroid from the family highly unlikely 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.[70] In 2010, another hypothesis 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.[71] In July 2021, a study reported that the impactor likely originated in the outer main part of the asteroid belt, based on numerical simulations.[72]
The original 1980 paper describing the crater suggested that it was created by an asteroid around 6.6 kilometers (4.1 mi) in diameter. Two papers published in 1984 proposed the impactor 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.[28] In February 2021, four independent laboratories reported elevated concentrations of iridium in the crater's peak ring, further corroborating the asteroid impact hypothesis.[73] 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—the long-standing leading candidate among scientists.[74] This was followed by a rebuttal published in Astronomy & Geophysics that June, which charged that the paper ignored the fact that the mass of iridium deposited across the globe by the impact (estimated to be approximately 2.0×108–2.8×108 kg (4.4×108–6.2×108 lb)) was too large to be created by a comet impactor the size required to create the crater, and that Loeb et al. had overestimated likely comet impact rates. They found that an asteroid impactor was strongly favored by all available evidence, and that a comet impactor could be effectively ruled out.[28]
See also
- Barberton Greenstone Belt
- List of impact craters on Earth
- List of possible impact structures on Earth
- Nadir crater
- Permian–Triassic extinction event
- Timeline of Cretaceous–Paleogene extinction event research
- Tenejapa-Lacandón Formation
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