The Dinosaur Extinction

Walter, a competent academic in his own right, enlists the help of his father Luis Alvarez (1911-1988). Luis was a noted physicist, who developed radar systems during WWII to help planes land in poor visibility. He was later recruited by the secretive Manhattan project to develop nuclear weapons. After the war, He returned to academia, and in 1968 he went on to win a Nobel Prize in Physics. Walter, while working at Berkeley in the geology department, shows his father a small cross-section of the Gubbio K-Pg boundary clay and describes to him what he had observed throughout the various locations he had sampled. Walter asks Luis about how one might determine the time span over which the K-Pg clay boundary layer might have formed.

Think question: Imagine you are Luis Alvarez. How might you determine the time it took to deposit the K-Pg clay layer?

Luis suggests that they measure the abundance of the element, beryllium-10 in the K-Pg clay layer as this radio-active isotope is constantly forming in the atmosphere, and as a result, settles to the surface in a constant and predictable concentration. Therefore, if there is a higher abundance of beryllium-10 in sedimentary rock, that would indicate a longer span of time. With this in mind, the Alvarez’s seek the assistance of another physicist, Richard Muller, to help with the beryllium-10 measurements. They reason that determining the abundance of beryllium-10 in the clay layer and then correcting for how much would have been lost through the radioactive decay process, would give them a good idea of how long it took for the clay layer to be deposited (Berkeley 2007). The team was keen to get their measurements completed so that a time-frame could be determined. Unfortunately, they realized that documented half-life of beryllium-10 was incorrect as published. The actual decay rate was shorter than what was previously thought, so in the 65 million years since the K-Pg extinction occurred, all the beryllium-10 will have decayed into a different isotope rendering it useless as an indicator of time passed during the deposition of the clay layer.

And so, back to the drawing table for the Alvarez’s. Following the same logic as the Berylium-10 idea, Luis recalls that Platinum group elements are 10,000 times more abundant in meteorites than in the Earth’s crust. He knows that the amount entering and settling out of the atmosphere due to the dust created from meteorites burning up in the atmosphere should stay relatively constant over time. In this case, the amount of these elements in the clay layer could potentially be used as a measure for the amount of time that had passed during the deposition of the K-Pg clay layer. Luis suspects that if the clay had been deposited over thousands of years, that the scarce platinum metals would be detectable, but that if the clay layer formed quickly, the platinum metals would be undetectable. They choose to measure iridium, a platinum group metal, as it is easier to detect than platinum itself. Fortunately, Luis also knows of two physicists: Frank Asaro and Helen Michel of the Berkeley Radiation Laboratory, that have the knowledge and equipment needed to perform the analysis.

Walter, through Luis’s connection with the Berkley radiation group, gives Frank and Helen some of the samples samples from the Gubbio site containing the K-Pg clay layer. As the lab is very busy and this is really only a side project for Frank and Helen, the samples take months to process. Also, the techniques used are time consuming and equipment repairs slow the analysis even more. It is not until nine months after Walter had handed over the samples that Luis let Walter know that Frank wants to discuss his results. Based on expected levels of deposition, they suspect that iridium levels would be around 0.1 parts per billion (ppb). However, Frank and Helen determine through their analysis that there is actually 3ppb of iridium in the clay layer, an astounding 30 times more than expected based on the amounts found throughout the layers of limestone in the Cretaceous and Tertiary layers.

Think question: The results of the analysis on the clay layer showed that the concentration of iridium was 30 times the expected value. Knowing what a higher concentration in the sample should mean for the time it takes to deposit the clay (higher concentration correlates to a longer time), and knowing at least one source of iridium (meteorites from space). What can you say about these findings? How might you go about trying to find evidence to support your claims?

Think question: What are some other questions that this anomaly raises for you?

Were the high levels of iridium something that was anomalous to the clay layer in Gubbio or was it widespread? To answer this question, they need to analyze samples of the same age from other sites. Walter sets out to examine the geological structures of Stevns Klint formation near Copenhagen, Denmark. This area is known in the geological community to have a highly visible fossil rich coastal cliff spanning the Cretaceous and Paleogene periods. The chalky white cliff also bears a wide range of fossil types, but upon inspection, Walter observes that the clay layer exists at this site as well. In this location, it emitts a sulfuric smell and contains only fish bones which was in sharp contrast to the fossils above and below the layer. Walter collects more samples and sends them on to Frank and Helen for analysis. This time around, the level of iridium in the Stevns Klint clay layer is 160 times higher than the surrounding rock layers. It begins to seem to Walter that a trend could be emerging, but more testing will be required before drawing any conclusions.

Around the same time that Walter and his colleagues are exploring the Gubbio limestone, A Dutch geologist by the name of Jan Smit is working in Caravaca on the southeast coast of Spain. He, like Walter, also observes changes in the foram patterns in the rocks near the K-Pg boundary in his location. Walter contacts Jan and urges him to check the iridium levels in the samples he has collected in Caravaca. Once the tests are performed Jan lets Walter know that the levels of iridium were in fact elevated. Furthermore, samples of the K-Pg boundary from New Zealand, where he is also doing a separate investigation, also show iridium levels that were 20 times higher than the surrounding rock. At this point, Walter and the others involved become convinced that there was something going on with the K-Pg boundary and that whatever happened, it was a global in extent, rather than regional event.

Think question: The levels of iridium were much higher in the clay layer than in the surrounding rock. What could have caused this massive amount of iridium to be deposited in this layer?

The Alvarez's set out collect evidence in support of this new K-Pg extinction theory. They hypothesized that huge quantities of debris for the impact would be projected into the Earth's atmosphere, akin to what would occur from a massive volcanic eruption but at an even more prolific scale. This dust would prevent the sun's rays from reaching the primary producers thus preventing photosynthesis. This theory alone did not have enough explanatory power to set it apart for the hypothesis that photosynthesis could be inhibited from widespread, longstanding volcanic eruptions.

Think question: If an asteroid of substantial size did hit the Earth 66 million years ago, there would undoubtedly be evidence of what occurred. But what kind of evidence would be left after all these years?

One of Luis' first thoughts when considering the asteroid impact hypothesis, is that there would be a large tsunami (very large long wavelength wave in the ocean). He thought that such a tsunami would cause massive disruptions to the ocean floor and the stirred-up debris would be spread out over a large area in the water and on land. A huge crater would also support the impact hypothesis. Luis estimated, based on his calculations that the asteroid would have been roughly 10 km in diameter and would have been traveling towards Earth at a staggering 50,000 miles per hour (Carroll 2016). This asteroid would hit the Earth with such force that debris from the impact site would be ejected across the surface of the Earth creating a layer of fine material like what they have been observing in various parts of the world, what will become known as the K-Pg layer. The heat created by the impact would have also melted rock into glass and produced shockwaves that would have deformed rocks into "shocked quartz" (Figure 7) (Berkeley 2007). The impact scenario would have been a catastrophe by any definition.

Luis, Walter, Frank and Helen feel that the hypothesis fit the observations best. In June of 1980, the team's findings are published in Science. The paper was entitled: Extraterrestrial Cause for Cretaceous-Tertiary^[1] Extinction. Their take on the cause of the K-Pg extinction is met with mixed reviews. It went against the idea that mass extinctions were caused by gradual processes over millions of years, while their hypothesis suggested that the die off was relatively swift. Also, the impact hypothesis was missing a key piece of evidence: The crater left by the asteroid that supposedly caused the K-Pg extinction.

Think question: Does the absence of the crater as evidence for a cataclysmic impact of a meteor automatically make this hypothesis invalid? Support your answer.