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There have been five great extinctions in the Phanerozoic Eon. Sixty six million years ago, the End Cretaceous extinction destroyed the dinosaurs and opened the way for the little mammal insectivores of the Cretaceous to take over the world during the Age of Mammals. The catastrophe had two causes on opposite sides of the world, an asteroid impact in what is now Mexico and a flood magma event in India. This section focuses on the asteroid. The K-Pg (a.k.a. K-T boundary) or Cretaceous-Tertiary boundary marks the change around the world. No placental mammals are below the boundary, and there are no dinosaurs above it (Figure 11‑29).
Figure 11‑29. The K-Pg boundary at the end of the Cretaceous with dinosaurs below and mammals above. Credit: Anky-man. Used here per CC BY-SA 3.0. Text and pictures added.
Scientists had proposed many explanations for over a century, but Harold Urey correctly proposed in 1973 that a comet impact killed the dinosaurs. In 1974, Jan Smit was studying rocks in Spain and found that the oceans were healthy below the K-Pg boundary, but that formaminafora and even plankton went extinct in the End Cretaceous Extinction. In 1977, Smit discovered that rock samples at the K-Pg boundary had high levels of antimony, chromium, nickel, selenium, and cobalt, and he proposed that an extraterrestrial object caused the high levels of metals. In 1979, Walter Alvarez noticed that many foraminifera species existed below the K-Pg boundary in limestone layers at Gubbio, Italy, but there were no foraminafora above the boundary, which was a clay layer. The foraminafora were dead shelled animals, and the limestone would have formed in shallow seas from the decay of the calcium carbonate shells of the dead shelled animals, as in the Grand Canyon. Walter asked his father, Luiz Alvarez, who was a famous physicist to help him figure out what had caused the formation of the clay boundary at Gubbio. Luis measured concentrations of elements that rain down on earth from space at a known rate in order to determine the length of time over which the clay layer was laid down. Luis found extremely high levels of iridium in the clay layer and realized that the iridium must have come from an extraterrestrial object such as an asteroid or comet; thus, they proposed an extraterrestrial cause (Figure 11‑30) of extinction.
Figure 11‑30. Asteroid hitting Earth. Credit: Donald Davis. NASA.
Figure 11‑31. The K-Pg boundary from Wyoming. San Diego Natural History Museum. Credit: Eurico Zimbres. Used here per CC BY-SA 3.0.
When Alvarez proposed such an abrupt cause as an asteroid, paleontologists had a negative response because it violated the concept of uniformitarianism; however, evidence of an asteroid impact began to accumulate. In 1980-1981, researchers found high levels of iridium at 36 K-Pg boundary locations around the world. For example, the intermediate claystone layer in the K-Pg rock from Wyoming in Figure 11‑31 contains 1,000 times more iridium than the layers above and below. Next, Jan Smit noticed spherules, glass like beads, in K-Pg boundary layers. This was further evidence of an asteroid impact because a huge asteroid would vaporize rock. Spherules form in the atmosphere when the vaporized rock droplets cool in the atmosphere and then rain down on the earth.
Based on the amount of iridium in claystone layers, Luis Alvarez calculated that the asteroid was 10 km across. In order to finally prove that an asteroid caused the extinction, scientists needed to find the enormous crater that such a large asteroid would leave behind. Alan Hildebrand, University of Arizona graduate student, found boulders and a mixture of rocks in sediments of the same age in Texas, which was evidence of a tsunami. A 10 km asteroid would have caused enormous tsunamis, which would have traveled as fast as a jet. This was possible evidence that the asteroid might have struck in this region of the world. In his search for the asteroid, Hildebrand then discovered a forgotten study performed by oil prospecting geologist Glen Penfield, which showed that there were enormous gravitational anomalies at the northern end of the Yucatan Peninsula, which formed concentric rings, as one might expect from a crater. In Figure 11‑32, the left image shows the pattern of gravitational anomalies, and the white dots are cenotes, which are enormous holes and underground caverns that formed from the oceanic basin created by the impact. On the right side of the image, the crater outline is visible from space. Amazingly, the crater was the same size as that predicted by Luis Alvarez.
Figure 11-32a. Cenotes and gravitational anomalies around impact crater. White line is Yucatan coastline. White dots are cenotes. Credit: USGS.
Figure 11-32b. Satellite image of the Chicxulub crater on the northern part of the Yucatan Peninsula. Credit: NASA.
In 1990, Kevin Pope and Adriana Ocampo begen to examine the cenotes in the Yucatan Peninsula. In 1991, Pope and Ocampo discovered massive ejecta layers in northern Belize, one hundred feet thick, from the impact. Thick ejecta layers from the impact were also found in neighboring countries and even islands in the Caribbean Sea. Hildebrand discovered ejecta layers on the island of Haiti in the Gulf of Mexico. Scientists have measured the ejecta layer thickness as a function of distance from the impact site.[1] It was almost 60 m thick in Guayal, Mexico, 7 m thick around the northern Caribbean coast of Mexico and the US, 4 cm thick in the northern US and Canada, and 3 cm thick in Europe. The base of the ejecta has altered glass and high iron content.
As scientists studied the crater, ejecta layers, and the K-Pg boundary around the world, they determined that the asteroid was 10 km or larger diameter and traveling 50,000 mph. It caused an earthquake of magnitude 11, which shook the entire earth with 1,000 times greater intensity than a magnitude 8 earthquake. The impact energy was equivalent to 100 million megatons of TNT, which is the same as 300 million nuclear bombs. It created 1,000 mph winds and 100 m high tsunami waves, which would have been higher if not for the shallow seas in the region. Part of the vapor rich plume of ejecta reached the moon. The asteroid impact created such high temperatures that it melted rock and formed glass tektites, which scientists found in ejecta layers in Belize, Haiti, and Northern Mexico.
Figure 11‑33. Asteroid craters. Credit: NASA.
Large asteroids form complex craters because the displaced material surges back and forms a central cone. Chicxulub was a complex crater with a central peak (Figure 11‑33) that subsequently filled in with sediment. It is the third largest impact crater (180-mile diameter) in the last 2 billion years. The other two large craters formed 2 Ga and 1.9 Ga, much earlier in the history of the solar system and earth.
Pierizzo and Artemieva summarized research on asteroid impacts. [2] Scientists have found 180 terrestrial impact craters on earth. The only mass extinction known to be associated with an impact event is the End-Cretaceous extinction. Pierizzo and Artemieva found that other impacts by asteroids (Figure 11‑34) of similar size that did not result in mass extinction because they did not impact a volatile geological formation. [3] Asteroids cause long-term and global climate disasters if dust, soot, and gases are ejected into the atmosphere: 100 to 300 projectile masses can be ejected into the atmosphere if the projectile is traveling at 25 km/s.
Models indicate that global average temperature decreased by 12 0C following the End-Cretaceous impact, accompanied by a 90% decrease in precipitation. Geologic data indicates that global temperature recovered within a few decades because the overall average temperature of the ocean did not change significantly. There is geologic evidence of soot, which would have formed when organic carbon in the sedimentary layers ignited. Soot would have absorbed sunlight. The impact produced orders of magnitude more sulfur dioxide in the atmosphere than large volcanic eruptions, and this would have reduced sunlight by 50% and caused continental temperatures to be approximately at the freezing point for 10 years. Although a decade or two is a short period in geologic time, ten years of climate disruption was enough to kill off sensitive biological species such as dinosaurs.
Figure 11‑34. Impact craters in the last 2 billion years. Credit: Earth Impact Database. Carlyle S. Beals. University of New Brunswick. Public Domain
References
References
[1] Schulte, Peter, Laia Alegret, Ignacio Arenillas, José A. Arz, Penny J. Barton, Paul R. Bown, Timothy J. Bralower et al. "The Chicxulub asteroid impact and mass extinction at the Cretaceous-Paleogene boundary." Science 327, no. 5970 (2010): 1214-1218.
[2] Elisabetta Pierazzo and Natalia Artemieva, Local and global environmental effects of impacts on Earth, Elements (2012) 8:55-60.
[3] Pierazzo, Environmental effects
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Figure 11‑30. Asteroid hitting Earth. Credit: Donald Davis. NASA.
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