How Walter Alvarez Decoded the Chicxulub Asteroid Strike
In 1978, the geophysicists Glen Penfield and Antonio Camargo were flying above the Yucatán Peninsula, just off the eastern coast of Mexico.
They were conducting an airborne magnetic survey of the Gulf of Mexico, looking for oil for Mexico's state-owned oil company, Pemex.
Analyzing their data, Penfield found an enormous south-facing underwater arc 70 km (40 mi) across. Consulting an earlier map, he found a matching arc that pointed northward. Together, the two arcs formed a circle, 180 km (110 mi) wide, and centered near the Mexican town of Chicxulub.
Penfield and Camargo presented their results to the 1981 Society of Exploration Geophysicists conference, but that conference was only sparsely attended due to members attending a separate conference on something called the K-Pg boundary.
The K-Pg Boundary
All over the world, geologists had found a white, chalky layer of sediment, and they used this boundary to mark the end of the Cretaceous Period (the last period of the Mesozoic Era), and the beginning of the Paleogene Period (the first period of the Cenozoic Era). Carbon dating showed the layer to be 66 million years old.
A team comprised of the Nobel Prize-winning physicist Luis Alvarez, his son geologist Walter Alvarez, and the chemists Frank Asaro and Helen Michel had been studying this layer, and they discovered that it contained an unusual concentration of the element iridium.
Iridium is rarely found in the earth's crust because it is a heavy element, and it sank into the earth's core while the planet was molten. Iridium is, however, very plentiful in asteroids, and the Alvarezes suggested that an asteroid must have struck the earth, and not just any asteroid.
Given the amount of iridium found in the K-Pg layer, the asteroid responsible for the K-Pg boundary must have been 10 km (6.2 mi) in diameter, or about the size of Manhattan. For comparison, the Martian moon Phobos is 11 km (7 mi) in diameter, and Mount Everest is just under 9 km (5.6 mi).
Such an impact would have released the energy of 100 trillion tons of TNT, or about 2 million times the most powerful thermonuclear bomb ever detonated. But, if such an impact happened, where was the evidence?
Chicxulub and K-Pg Converge
Besides iridium, also present in the K-Pg layer were shocked quartz granules, glass spherules, and tektites. Tektites are gravel-sized bodies composed of black, green, brown, or gray natural glass formed from terrestrial debris ejected during meteorite impacts.
These were especially plentiful in deposits around the Caribbean, and this gave scientists a clue where to look. The Chicxulub Crater in the Yucatán Peninsula perfectly fit the bill.
Based on the radioactive decay of argon, Paul Renne of the Berkeley Geochronology Center estimated the date of the asteroid strike to be 66,038,000 years ago, plus or minus 11,000 years. And, according to Renne, the dinosaurs became extinct within 33,000 years of this date.
In 2016, scientists drilled into the peak ring of the Chicxulub Crater and found that it was comprised of granite, rather than the typical seafloor rock. Granite is normally found deep within the earth.
The scientists found that the granite had been shocked, melted, and ejected to the earth's surface within minutes. They also found evidence of colossal seawater movement, or tsunami.
A study conducted in 2018 at the University of Michigan, modeled how the Chicxulub Crater tsunami would have propagated. They found waves in the Gulf of Mexico would have been 1,500 meters high, while wave heights in the Atlantic and Pacific Oceans would have been as high as 14 meters.
The asteroid impact ejected an enormous quantity of vaporized rock and sulfates into the earth's atmosphere, blocking the sun, and this had a sudden and catastrophic effect on the climate worldwide. Recent studies show that this impact winter only lasted for a brief time, then temperatures around the globe actually rose.
Researchers studying fish remains in sediment at El Kef, Tunisia, found that sea temperature had risen approximately 5°C not long after the asteroid strike, and that it stayed that way for approximately 100,000 years. They attribute the rise to the release of carbon dioxide into the atmosphere by carbonates being impacted, and from forest fires that would have been ignited by the asteroid impact.
In April 2019, a paper published in Proceedings of the National Academy of Sciences (PNAS) described the discovery of a fossil site in North Dakota that provides a "postimpact snapshot" of events after the asteroid impact. The scientists found tektites embedded in amber and in the gills of about 50 percent of the fossil fish. They also found traces of iridium.
The authors, including Walter Alvarez, theorized that the shock of the asteroid's impact caused seiches, or oscillations of water in lakes, bays, or gulfs, and these led to the rapid burial of organisms under a thick layer of sediment.
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