7 The Death of the Dinosaurs

Many thanks to Dr. Bill Ward for his recounting to me the history of UNO’s involvement in studying the Yucatan Impact Crater. Bill was my mentor when I started at UNO and my best-man when I got married. He guided me through tenure and extreme ignorance about much of academia and life. I owe him a lot. 

The major sections of geologic time are separated by big event—the evolution of species with hard parts that could be preserved in rocks, the Great Dying (a mass extinction) and the extinction of the dinosaurs. The University of New Orleans, where I used to teach, contributed to solving the mystery of how the latter happened.

A widely accepted hypothesis about the cause of the extinction of the dinosaurs at the end of the Cretaceous Period is an impact of a meteorite or comet (bolide) in the Yucatan Peninsula, Mexico. The bolide is believed to have been about 6 miles in diameter, impacting the Yucatan at a low angle, vaporizing rock, setting off catastrophic fires and tidal waves, and blanketing the Earth with dust, which led to acid rain and a plunge in global temperatures. The angle was such that central and western North America were particularly ravaged. As the dust settled out of the atmosphere, large amounts of water vapor remained, causing a rapid rise in temperature to follow the cooling. Thus the dinosaurs and other Cretaceous life were hit with a one-two punch that caused massive annihilation. The acceptance of this hypothesis is relatively new—the previous edition of our historical geology book doesn’t even mention it. Evidence stored at the UNO Geology Department helped provide the support that led to acceptance.

Former professor Al Weidie has had a Mexican connection dating back to his graduate school days working on Gulf Coast geology. (As part of his student accomplishments, he brought home a lovely wife, Ana, from Saltillo, Mexico.) After coming to UNO and helping start the Department of Earth Sciences, Dr. Weidie convinced PEMEX, the Mexican petroleum company, to ship him some cores from wells that PEMEX drilled in the Yucatan Peninsula. (Cores are samples of the rock being drilled through.) When Dr. Bill Ward joined the faculty in 1970, the cores were available for a thesis project by his first graduate student, Bob Marshall.

Cores from near the Cretaceous-Paleogene boundary proved to be difficult to understand. A breccia there was particularly prob-lematic, causing some to accuse Dr. Ward of looking at concrete that had been drilled out in the wells. However, in the breccia were fragments of anhydrite, limestone, and dolomite, and the matrix was dolomitized. Dr. Ward reasoned that all the anhydrite chunks and splinters in the breccia, which spread across the peninsula, would amount to a lot of anhydrite. Its likely source was the Middle Cretaceous, deeper underground. How could it have gotten to its present location? Anhydrite is very soluble, so it had to have been moved quickly and without a lot of water. The mechanism Dr. Ward hypothesized at the time was block faulting. Years later, electric logs and seismic would show that this explanation couldn’t be supported.

In the years after Marshall’s thesis, a high concentration of iridium at the Cretaceous-Paleogene boundary was found in many places around the globe. The likely source of iridium is extrater-restrial, in comets or meteorites. Its wide distribution at a single time indicated the possibility of a massive impact. The timing also corresponded with the end of the Age of Reptiles. The hunt was on for a large crater caused by the impact. Geophysical evidence indicated the presence of such a crater in the subsurface of the Yucatan Peninsula.

Geologists seldom believe geophysicists unless there is supporting evidence from rock data. The PEMEX wells were known to exist, including one near the center of the proposed impact crater, but the cores from them were supposedly lost. However, someone came across a reference to Marshall’s thesis and contacted UNO. In the cores, they found shocked crystals of quartz that have been impacted violently like hitting them with a hammer. Other evidence soon followed, including finding glass microspherules throughout the Caribbean and as far away as Wyoming. In 1995, Dr. Ward and others authored an article that appeared in Geology, presenting much of the petrographic evidence from the subsurface sedimentary rocks for the bolide impact, including a thin section of anhydrite fragments wrapped in altered glass.

As part of a long series of programs on dinosaurs narrated by Walter Cronkite, there was a segment on dinosaur extinction and its possible relation to the Yucatan impact. A camera crew came to UNO and filmed a geologist going into the lab where the cores were stored, across the hall from Dr. Ward’s office. The geologist had a flashlight in his hand, looking like an archaeologist entering King Tut’s tomb. Suddenly he discovered the long-missing cores. Of course, the geologist passed up a light switch at the door. It looked as if UNO didn’t pay the electric bill. Yet there is no doubt that the missing cores helped shed light on the cause of the extinction of the dinosaurs.

Notes on Geologic Time:

The concepts of absolute and relative time are crucial to understanding geology, especially the geology of Iowa. Geology embraces time in ways unlike most other sciences, except perhaps cosmology.  Small changes, such as erosion, add up over long periods of time to create big changes, like the Grand Canyon. Slow movements, perhaps an inch a year, add up to distances that move entire continents from equator to pole or shove rocks from deep in the earth to heights tens of thousands of feet above sea level.

The very idea of time is really strange if you think about it.  What is a day? A concept measured by the rotation of the Earth.  So what’s a year? 365 days, my students often reply. Afraid not.  Some years. Not leap years. and even that fails on the century mark. Because a year isn’t based on the rotation of the Earth. It’s based on the orbit of the Earth around the Sun, which takes something like 364 and a quarter days, but not quite. (Actually, we can’t even agree on what a year is. See [here].) Regardless, the main idea is that we tell time by something that occurs pretty regularly—the swing of a pendulum, the vibration of a quartz crystal, of the orbit of the Earth. These things are our best guess at absolute time.

The regular event that geologists use to date the age of rocks is radioisotopic decay. The most famous is carbon-14. Carbon-14 breaks down into nitrogen at a regular rate—half of it changes top nitrogen in about 5700 years (its half-life). This is particularly useful for dating organic material, such as the wood in that Neanderthal’s campfire or the Hopi’s hut. But other radioisotope’s half-life is much longer, like [potassium or uranium, which can be used for much longer time spans. It’s like the difference between a stopwatch and a calendar.

But things also change over time. New kids are born, and we suddenly have a younger sister or brother. Similarly, mud settles out of water, accumulates in layers, so the newest layers are atop the older. How old? We don’t know. But the youngest are on top. This is relative time. The early geologists did not have access to lab equipment, but they could see the layering of sedimentary rock, find fossils within the rock, and determine which rocks were older and younger. This principle was used by geologists to create the Geologic Time Scale

Dating

relative dating: younger or older. Each day when I bring the mail from the box and dump it into a pile on the counter, after a week, the older mail is on the bottom of the pile. For more examples, see [here.]

absolute dating: a specific age, often in years. A year is a trip around the sun, not a specific number of days. (There are actually about 365.2422 days in a year, which we deal with by designating leap years.) The point is, we tell time based on something that occurs regularly, such as the swing of a pendulum or vibration of a crystal. For the geologist, the clock is the radioactive decay of a variety of isotopes elements.

atomic number: An element is identified by the number of protons (positively charged) in its nucleus.

isotope: Also present in an atom’s nucleus are neutrally charged neutrons. The number of neutrons plus the number of protons determines the isotope. Carbon has six protons—that’s what makes it carbon. But forms of carbon with 6, 7, or 8 neutrons exist. 6 + 6 = 12, making carbon-12, a stable form of carbon. But carbon-14, with 8 neutrons, is unstable, breaking down regularly, about half converting to nitrogen over 5,730 years.

Another use for isotopes: Because of the different weights of various isotopes, they respond differently to gravity and mo-mentum. This allows us to purify uranium for building nuclear weapons and to estimate past temperatures based on oxygen isotopes in water preserved as ice in ancient glaciers.

Geologic Time Scale

Precambrian: the oldest and largest of Earth’s times, beginning about 4.5 billion years ago and ending 541 million years ago when the first fossils with hard parts were found preserved in rocks. Life previous to that left traces that early geologists did not have the equipment to observe.

Paleozoic Era: extending from the end of the Precambrian to 252 million years ago, the Paleozoic was the time of tremendous diversification of life, but it ended with the greatest mass extinction of Earth’s history. For us here in Dubuque, two periods withing the Paleozoic are particularly important: the Ordovician (under campus) and the Silurian (younger and under the Dubuque Airport.)

Mesozoic Era: known as the Age of Reptiles, especially dinosaurs, rocks of this age are mostly missing from Iowa, especially Eastern Iowa.

Sedimentary rocks are mostly deposited in water, and this part of Iowa was above sea level during the Mesozoic. Thus, we had erosion, not deposition. The Mesozoic came to an end with a bang, an impact from space that annhilated the dinosaurs 66 million years ago.

Cenozoic Era: known as the Age of Mammals, the death of the dinosaurs opened up opportunities for mammals to spread, grow, and dominate. The Cenozoic continues through today. Starting about 2.58 million years ago, the Pleistocene Epoch was time of widespread continental glaciation.