Show notes
Join Jesse and Chris as we explore the hypothesis that an asteroid ended the Cretaceous AND the age of the reptiles. It's a very compelling scientific story of discovery.
This has been hotly debated back and forth for a long long time, with consensus being reached only recently (since I was in graduate school)
- Other options include a massive volcanic outpouring - Deccan Traps in India erupted >1 million cubic km of lava in a million year period, with 50% of that coming before the mass extinction event.
- The question is really about what killed so much stuff? How does the Earth respond to massive impacts? There have been other large meteorite impacts (and big volcanic eruptions) with no mass extinction - why this one?
- Was the Earth already in a sorry state (lots of CO2 and mercury in the atmosphere) before the extinction?
- Flood basalt volcanism is tied to other major mass extinction events (end-Permian and end-Triassic)
As we look at the evidence, we bounce around the globe. We begin in Italy.
1- Gubbio, Italy. Layers of ancient limestone that are now folded and uplifted. Working from bottom to top- site became a magnet. Why? It has a thin layer of dark clay. Walter Alverez became v. Interested in this. He studied the foraminifera below and above. He found that at the top of the white limestone, the vast diversity of the forams abruptly went missing.
2- Spain - 1500km away. On the Atlantic coast outside of Zumaia. 2 beaches here which are of interest to geologists because they are situated among the longest set of continuous rock strata in the world. Jan Smit was studying the forams here. In these layers, Smit found a strange clay layer - just like Alvarez. In these rocks, they were steady, consistent, for over a million years, then all of the sudden, the clay layer. There is no evidence of anything happening and then all of the sudden, the clay layer and bang, they’re gone.
How long did this take to change?
3- Walter brought in his dad, Louie Alverez. He brought in his knowledge of Astrophysics. He found the layer to be rich in IR (also iron, nickel and cobalt). It contained over 30 X the amount in surrounding rock (measured from the clay at Gubbio). Could it come from an Asteroid. He hypothesized that the IR would be spread all over the world as a blanket. So he could calc. The amount that was laid down all over Earth. Knowing the amount that is in Asteroids and Comets, they can calc. The size of the Asteroid.
5- Texas on the Brazos River. 65 my this was the bottom of a sea. We found, Allen Hildebrand, strange sed deposits across the basin. 7 million years of boring mud there. This was under water. Then suddenly, we see the mud got eroded. Suddenly, there were huge boulders in the mud. Only a giant Tsunami could do this.
6- Haiti: Report of volcanic rocks. They were actually ejecta full of shocked qtr and spherules. They were also full of melted rocks called tektites.
7- From Jan Smit: The boundary clay has two parts: lower layer - just 2-3 mm. It’s the most enriched in IR, nickel, FE, and cobalt. Also has spherules (crystalline and .5 mm). Remains of micro Tektites. Also has the shocked quartz. Highly diagnostic from shock pressures.
9- Shocked minerals are diagnostic of impacts
10- In 2019 in North Dakota, a new discovery - The Tanis (fossil) site. Evidence of a tsunami deposit. Has rocks with 3 distinct fish fossils - paddlefish, sturgeon, and ammonites. They pulled gills out of the fish and found a lot of tektites inside the gills - they were swallowing them while still alive.
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