Tikalon Blog by Dev Gualtieri

The Burgess Shale

April 16, 2018

There are missed opportunities when scientists are focused on their own field of science to the exclusion of others. Quantum mechanics is usually presented as a triumph of physics, but its development had significant impetus from unanswered questions in chemistry, especially the organization of the periodic table. Francis Crick, co-discoverer of the secret of life (more prosaically, the molecular structure of DNA), started as a physicist, but his fame arose from his further excursion into molecular biology.

Luis Alvarez (1911-1988), who was awarded the 1968 Nobel Prize in Physics for his work in elementary particle physics, is known also for the hypothesis, co-authored with his son, Walter Alvarez (b. October 3, 1940), that the demise of the dinosaurs was caused by a meteor impact event.[1-2] He brought physics to bear on the problem through the use of neutron activation analysis as a sensitive method for trace element detection.

Luis Alvarez (left) and his son, Walter.

I wrote about this father-son team in a previous article (Near Earth Asteroids, October 5, 2011).

(Lawrence Berkeley Laboratory photograph, via Wikipedia Commons))

Iridium is a rare element in Earth's crust, existing as just 0.001 ppm by weight, which is forty times less of an abundance than gold. That's why its presence in the clay layer that marks the division between the Cretaceous and Paleogene Periods is unusual. While Earth does obtain about ten million kilograms of iridium per year from the infall of micrometeorites, it's distributed uniformly on Earth's surface, so the concentration in any one place should be very small.

The iridium concentration in this layer was at least two orders of magnitude greater than expected. It's now believed that all this iridium came from the impact of an asteroid, 10-15 kilometers (6-9 miles) in diameter. The vestige of this impact is thought to be the Chicxulub crater, centered on Chicxulub, Yucatán, Mexico. This crater is 150 kilometers (93 miles) in diameter and 20 km (12 miles) deep. Such an impact would have been a natural disaster that caused most species living at the time to become extinct.

There goes the neighborhood!

While it may seem that the pteranodons in this image are flying quite high, there are some large modern birds that fly at tens of thousands of feet.

NASA image, photo P-45062, Chicxulub impact site by Donald E. Davis

As did most students, I hated reading the books assigned by my teachers. These were selected as being classics in their respective fields of literature and history. It was only later that I discovered other books in these subject areas that were actually interesting to me. One of these was economist Thorstein Veblen's, The Theory of the Leisure Class.[3] In this book, Veblen writes about conspicuous consumption, the purchase of lavishly expensive, or ephemeral and impractical, goods and services by wealthy individuals as a display of their wealth. I wrote about this in an earlier article (The Physics of Inequality, May 11, 2017).

Taking the example of many illustrious scientists, I endeavored to expand my reading beyond the physical sciences. The origin and evolution of life on Earth was especially interesting to me, since I had read as a child a magazine account of Stanley Miller's 1953 experiment. In this experiment, Miller produced amino acids, the organic chemical precursors to life, from synthetic lightning in a flask containing gases representative of Earth's early reducing atmosphere (see figure). I wrote about this experiment in a previous article (The Miller Experiment, July 30, 2014)

Schematic diagram of Stanley Miller's 1953 abiogenesis experiment.

Our present understanding of Earth's early atmosphere is that it was different from Miller's mixture. Instead of ammonia, methane, hydrogen and water, it was more like carbon dioxide, carbon monoxide, nitrogen, and water. Some hydrogen and methane would have come from volcanic sources, as would lightning.

(Modified illustration by Ned Shaw, Indiana University.)

A few million years after the first simple organic chemicals appeared on Earth, chemical complexity increased. There's fossil evidence of this, as described in Melvin Calvin's 1969 book, Chemical Evolution.[4] As Calvin writes in the preface to this book, "...there must have been a period of time in the earth's history that encompassed the transition between a non-living molecular population on its surface and a population of molecular aggregates that we would call living." I wrote about Calvin's book in an earlier article (Chemical Evolution, June 30, 2014)

The fossil record in later years gives us a look at the simple aquatic organisms flourished on the early Earth. One problem with fossil preservation of these early organisms is that they were composed mostly of soft body parts, such as legs, gills, and antennae, things that are quite impermanent. The Burgess Shale Formation in British Columbia is important to paleontology, since its shale preserved the soft body parts of these animals.

While Burgess Shale has provided a wealth of Middle Cambrian period fossils, such as those of the trilobyte that I discussed in a previous article (Trilobite Sex, March 13, 2017), there are many more fossil-bearing Burgess Shale-type deposits known worldwide.[5] Geologists from Yale University (New Haven, Connecticut), the University of Oxford (Oxford, United Kingdom), and Pomona College (Claremont, California) have identified a mineral profile of which shale compositions will yield fossils.[6-7]

A fossil of Waptia from the Cambrian Burgess Shale Formation in British Columbia. The Waptia is shrimp-like creature about 5.5 centimeters long that lived about 508 million years ago.

(Yale University photo by Susan Butts.)

It's not surprising that the particular chemical composition of shale, especially that of its precursor clay minerals, will affect fossil preservation of soft body parts. The research team did powder X-ray diffraction crystallography of 213 Cambrian shales from 19 sedimentary successions on four continents.[6-7] Their aim was the comparison of the shales that preserved soft tissues with those that only contained fossilized shells or skeletons.[7]

Logistic regression and classification tree methods showed that soft body fossils are more likely to be found in those sediments that are rich in berthierine/chamosite and poor in celadonite and illite. Such compositions are likely the result of a high kaolinite/smectite ratio in the original sediment, and the enhanced iron concentrations affected the early diagenesis (fossilization).[6] The mineral, berthierine, is toxic to decay bacteria.[7] Says study coauthor, Oxford's Ross P. Anderson,

"Berthierine is an interesting mineral because it forms in tropical settings when the sediments contain elevated concentrations of iron. This means that Burgess Shale-type fossils are likely confined to rocks that were formed at tropical latitudes and that come from locations or time periods that have enhanced iron."[7]

fossil of Marrella from the Cambrian Burgess Shale Formation in British Columbia.

The Marrella, which is a small arthropod less than 2 centimeters long that lived about 508 million years ago, is the most common fossil of that period.

(Yale University photo by Susan Butts)

Models derived from this analysis of clay mineralogy are able to predict fossil-bearing Cambrian shales with about 80% accuracy.[6-7] Says Derek Briggs, a professor in the Yale University Department of Geology & Geophysics, "This discovery is important because it will help us to narrow the search for exceptionally preserved fossils in thick sequences of Cambrian and Precambrian rocks, which harbor critical clues to the early evolution of animal life on Earth."[7] The study may even aid in a search for possible fossil life on Mars.[6-7] Funding for this research was provided by the NASA Astrobiology Institute, the National Science Foundation, and other organizations.[7]

The Burgess Shale

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