In December, I listened to the Radiolab “Apocolyptical” show which was all about the Cretaceous-Paleogene boundary event. Famously, in 1980, Walter Alvarez and colleagues described an iridium anomaly at the K/Pg (also known at K/T) boundary which was subsequently specifically tied to an extraterrestrial impact event. The crater from this impact is around 200-km wide off the coast of the Yucatán in Mexico, known at the Chicxulub Crater.
But what happens when a 10 km (6 mile) wide rock smashes into Earth’s surface? When this object made impact, hot, melted rock was ejected and these little rock balls (spherules) rained down and can be found in deposits of K/Pg age rocks worldwide. There is little debate that this enormous disruption to the entire Earth was responsible for the large-scale extinction we see at the K/Pg boundary. But what, more specifically, did the molten rock and rock vapor have to do with it?
Back to the Radiolab. I was surprised as I listened to it that the hosts were taking an angle of “everything you have learned about dinosaur extinctions is wrong!” From my own experience, it is taught that the final nail in the coffin for dinosaurs, plants, and other animals was a long-lasting “impact winter.” The amount of debris kicked up into the atmosphere would have been so extreme, it would have blocked out the sun, reduced photosynthesis, and caused a cooling period. This cooling period, although it probably only lasted about 2,000 years, would be devastating to ecosystems worldwide. I don’t think there is much doubt an impact winter actually happened, but the stance this radio show took was that this was absolutely not the cause of the massive vertebrate extinction.
Their source on this was work done last year, mainly through computer modeling, that calculated the infrared (IR) radiation heat pulse and subsequent probability of global wildfires caused by molten ejecta re-entering the atmosphere. Douglas Robertson published a comprehensive review of the heat-fire hypothesis, noting that by calculating the kinetic energy converted to IR radiation by ejecta re-entering the atmosphere, a temperature could be reached on the surface of the Earth that is sufficient to ignite plant matter and tinder, causing global wildfires.
Much of this review is dedicated to addressing the problems with this hypothesis raised by other researchers. This often comes down to the morphology and fine structure of the soot found in K/Pg age rocks. While proponents of the heat-fire hypothesis say that soot found is clearly from widespread forest fires, the research of scientists like Claire Belcher suggests otherwise. Her research indicates that these soot deposits are not soot from the burning of plant matter, but actually hydrocarbon combustion from the impact site. Additionally, she puts forth compelling evidence that while there would have been IR radiation coming from the spherules, it mainly would have been shielded from the Earth by the spherules actually settling and forming a cloud in the atmosphere. The spherules’ interaction with the atmosphere potentially prevented the surface of the Earth from being completely incinerated. Of course, the other camp argues the charcoal deposits have signs of coming from ignition of plants and not of hydrocarbons. The debate rages on.
So was every living thing not underground or in the water burned up almost immediately by an IR radiation pulse? Or was there not truly enough heat reaching the Earth’s surface to cause such widespread fire, and extinction was driven mainly by other factors such as acid rain or impact winter? It is hard to know if we will ever be 100% certain, but I find the debate fascinating. Do I think it is right to tell the public everything the know about the dinosaur extinction is wrong? Honestly, I don’t think it is the best way to go about things, because there is still so much debate on this topic and scientists have not reached a consensus, and may never have a unified theory. What do you think about this debate? Any strong opinions either way? If you are an educator, what do you teach your students?
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