By Will Sager
Everybody knows the “Crater of Doom” story because it combines all the elements of a good story: dinosaurs, explosions, fires, and stuff from outer space. If you only recently became sentient, here is a quick recap. One day, about 66 million years ago, the dinosaurs were happily doing whatever dinosaurs do and – ka-blam! – an asteroid of about 10 km in diameter smashed into Earth near Chixulub on the Yucatan peninsula, causing environmental mayhem that wiped out all the non-avian dinosaurs and about 75% of species on the planet. Now there is a new wrinkle to the story. A team of marine geologists report finding another crater that may be the little brother to the Chixulub crater (Nicholson et al., 2022).
The Chixulub story itself is relatively recent. Geologists have known since the first rudimentary fossil timescales that there was a mass extinction event at the boundary between the sedimentary rocks of the Cretaceous and Paleogene periods (the boundary is often abbreviated K-Pg)*. The Cretaceous period was dominated by dinosaurs and their relatives whereas the Paleogene period saw the mammals take over. Until 1980, the preferred explanation for this event was an outburst of volcanic activity. But that changed when Nobel Prize-winning physicist Luis Alvarez and his team reported finding high concentrations of the element iridium at the K-Pg stratigraphic boundary (Alvarez et al., 1980). They reasoned that because iridium is rare on Earth, but contained in some asteroids, that the extinction event was caused by an asteroid impact. The theory was gradually accepted as iridium was found to be present at the K-Pg stratigraphic boundary in many locations across the globe. Moreover, other scientists worked out the connections between the impact and environmental catastrophe. The impact caused tsunamis, global wildfires, rapid heating of the atmosphere, and the collapse of photosynthesis because of a “nuclear winter” debris shroud. The icing on the cake was the discovery of the Chixulub impact crater just offshore of the Yucatan peninsula in 1991 (see first figure below; Hildebrand et al., 1991). This crater has the size to indicate a large impactor and the right age to match the K-Pg extinction. In criminology terms, scientists found the gun to match the gunpowder residue. Although there are still paleontologists who argue that the Chixulub impactor was not the cause (or sole cause) of the mass extinction, the Crater of Doom story is widely accepted.
Because the Earth is covered ~71% by ocean, it is likely that most impacts have been in the oceans. Craters in the ocean are hard to find because they found a good place to hide (ask a submarine commander). Exploration of the oceans is incomplete and luck is needed to find something under the sea. Chixulub was found because the area is one in which widespread oil exploration has occurred, so there was ample geophysical data to show what lay beneath the seafloor. Nevertheless, it was 11 years between identification of the iridium layer and the Chixulub impact crater.
Luck and oil exploration are at it again. Nicholson and his team studied seismic profiles collected for oil exploration to find another crater, which they dubbed the Nadir crater (after a nearby seamount of the same name). Seismic profiles are created by sending sound waves into the subsurface. The waves interact with the layering and bounce back to receivers. The recorded waves are used to create an image of the subsurface, which looks like a two-dimensional x-ray of the layering (see second figure). Nadir crater was found on the continental margin of Guinea at a depth of about 900 meters (see first figure). The crater itself is about 9 km in diameter and 200 meters in depth, but it is buried beneath several hundred meters of sediment (see second figure). The seismic images show that the crater is surrounded by faults that penetrate deeper layers, has a central uplift, and is encompassed by a halo of damage. The authors argue that these features are characteristic of impact craters. Moreover, they did impact model calculations to make an approximation of the crater, which fits an impactor about 400 m in diameter crashing into sediments beneath 800 meters of water. The age of the crater matches the Chixulub impact because the crater is located at the interface between Creteacous (Maastrichtian) and Cenozoic (Danian) sediment layers, with ages defined by fossils recovered from exploration wells in the region.
What impacts did this impact have? Modeling by the research team suggests that the impact released energy equivalent to 5,000 megatons of TNT (330,000 times the Hiroshima atomic bomb) that created an air blast with winds of about 470 km/hr out to 50 km radius. It created seismic waves equivalent to a magnitude 7 earthquake and likely caused widespread underwater avalanches of sediment. Near the crater, the impact probably caused a 2 km high “ejecta curtain” that settled into a 500-meter tsunami wave (see third figure). Moreover, the impact intruded into carbon rich sediment layers, so it probably released huge quantities of methane, a powerful greenhouse gas.
Was the Nadir impactor related to the Chixulub impactor? It is impossible to tell at this point. Double and multiple impacts are relatively common on other planets, suggesting that asteroids often travel in groups. The exact timing of the Nadir crater is still not determined precisely, so it may be a somewhat different age, but the coincidence is suggestive. The separation of the two craters (today about 8,000 km, but about 5,000 km at 66 million years ago when Africa and North America were closer together) is rather far for a closely paired body. The authors speculate that the impacts may be similar to that of comet Shoemaker-Levy which broke up by a close pass to Jupiter several years before plowing into the great planet in a subsequent orbit in 1994.
The Nadir impact hypothesis makes the K-Pg boundary even more interesting by suggesting that it was caused by more than one impactor. Given the expanse of unexplored oceans, there may be other K-Pg craters. Moreover, this research suggests that asteroid deflection schemes may have to account for more than one body.
*The terminology is confusing to the non-geologist. K comes from the German “kreide”, which means chalk, a characteristic rock of the Cretaceous. In geologic timescales, the 66 Ma extinction event is boundary is the dividing line between several timescale divisions (which are based on fossil turnover). In descending order: it is the boundary between the Mesozoic and Cenozoic eras, the Cretaceous and Paleogene periods, the Late Cretaceous and Paleocene epochs, and the Maastrichtian and Danian ages (whew!). To top it off, “Tertiary” was commonly used in the past as a synonym for Cenozoic, but it is considered an archaic term. “Cretaceous-Tertiary” was commonly used as a name for the K-Pg boundary, but Tertiary is passé. The preferred usage is Cretaceous-Paleogene, because both are geologic periods (no mixing of different types). But K-Pg is common because geologists.
Map view of the crater outline (upper left) and seismic sections crossing the crater. Black lines denote faults. (From Nicholson et al., 2022)
Impact modeling of the Nadir crater. (From Nicholson et al., 2022)
References
Alvarez, L. W., W. Alvarez, F. Asaro, and H. V. Michel, 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science, v. 208, p. 1095-1108.
Hildebrand, A. R., G. T. Penfield, D. A. Kring, M. Pilkington, A. Camargo Z., S. B. Jacobsen, and W. V. Boynton, 1991. Chixulub crater: A possible Cretaceous/Tertiary boundary impact crater on the Yucatán peninsula, Mexico. Geology, v. 19, p. 867-871.
Nicholson, U., V. J. Bray, S. P. S. Gulick, and B. Aduomahor, 2022.The Nadir crater offshore west Africa: A candidate Cretaceous-Paleogene impact structure. Science Advances, v. 8, 14 pp., doi: 10.1126/sciadv. abn3096.
Further Reading
Alvarez, W., 2008. T. Rex and the Crater of Doom. Princeton University Press.
Wall, M., 2021. Asteroid that killed the dinosaurs: Likely origin and what we know about the famous space rock.
Greshko, M., 2022. Dinosaur-killing asteroid most likely struck in spring. National Geographic online.
https://www.nationalgeographic.com/science/article/dinosaur-killing-aste...
Article Access
Alvarez et al. (1980)
https://www.science.org/doi/abs/10.1126/science.208.4448.1095
Hildebrand et al. (1991)
Nicholson et al. (2022)
https://www.science.org/doi/10.1126/sciadv.abn3096
