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The Tanis Deposit

May 6, 2019

Jokes are only funny when they don't need to be explained. While a physicist will realize that a cat will fall from a roof if his mew is too small, he will get no laughs from a listener who doesn't know that the coefficient of friction is always symbolized by mu (μ). That's why Gary Larson's Far Side cartoon about dinosaur extinction is memorable. This cartoon, which shows dinosaurs smoking cigarettes, is funny because it's common knowledge that dinosaur extinction was caused by a meteor impact, perhaps aided by concurrent factors that don't include cigarette smoking.

Financial cost of cigarette smoking (cartoon)

As this cartoon illustrates, smoking causes a financial burden as well as a health burden.

Governments have a financial interest in smoking cessation, since much of the expense of smoking-related health problems fall on them. One way governments discourage smoking, and also recoup health-related expenses, is through a cigarette tax. New York State is especially aggressive in this regard, having a cigarette tax of $4.35 per pack of twenty.

George Harrison (1943-2001) of The Beatles had many health-related issues arising from smoking. These included throat cancer and lung cancer.

One reason that cigarettes are packed as twenty items is the efficient stacking of 7, 6, and 7 cylinders. Matchbooks also contain twenty matches for the same reason, but it's unlucky to light three cigarettes from the same match.

(Portion of Wellcome Trust Photo no. L0024904, Library ref., ICV No 51428, by Reginald Mount, via Wikimedia Commons. Click for larger image.)

The Earth is constantly bombarded by meteors, but spectacular meteor impacts like the June 30, 1908, Tunguska impact and the more recent February 15, 2013, Chelyabinsk meteor[1] happen infrequently. The Tunguska event had an estimated blast-equivalent of 10 megatons of TNT, while the Chelyabinsk event had a much smaller energy of half a megaton. An average of 5-10 grain of sand sized meteors hit the Earth each hour, and this number can increase to hundreds an hour during meteor showers.

A kilogram meteor finds its way to earth about every year, a Tunguska-sized meteor hits about every century, and an extinction level impact of 50 gigatons happens a few times every million years. A real blockbuster meteor can be expected on a hundred million year time scale. Ninety percent of all species were killed off by the likely impact of a huge meteor about 250 million years ago, marked in the geological record at the Permian-Triassic boundary. A similar and more recent event at the Cretaceous-Tertiary was responsible for the extinction of the dinosaurs and the rise of mammals about 65 million years ago.

The link between meteors and extinction was found in 1980 by physicist, Luis Avlarez, his planetary scientist son, Walter Alvarez (b. 1940), and their colleagues, Frank Asaro (1927-2014) and Helen Michel (b. 1932).[2-3] Luis Alvarez is known also as the recipient of the 1968 Nobel Prize in Physics for his work in elementary particle physics.

Luis Alvarez (left) and his son, Walter.

Luis Alvarez was awarded the 1968 Nobel Prize in Physics for his development of the liquid hydrogen bubble chamber, a device for visualizing elementary particles. In this device, liquid hydrogen is held in a superheated state for a few seconds, and passage of an energetic particle produces a track of hydrogen bubbles.

This invention builds from the original bubble chamber idea of Donald Glaser (1926-2013), who was awarded the 1960 Nobel Prize in Physics.

(Lawrence Berkeley Laboratory photograph, via Wikipedia))

This discovery happened during analysis of the clay layer that marks the end of the Cretaceous Period. 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. However, their analysis of this clay layer showed a concentration that was two orders of magnitude greater than expected.

Earth's crust accrues iridium from the infall of micrometeorites, diluted over Earth's large surface area, so this larger concentration was supposed to come from a large extraterrestrial source. This excess of iridium is believed to have come from the impact of a 10-15 kilometer (6-9 mile) diameter asteroid at Chicxulub crater, which is centered on Chicxulub, Yucatán, Mexico. The remnant of this crater is 150 kilometers (93 miles) in diameter and 20 km (12 miles) deep. Such an impact was a major natural disaster, enough to have caused most species living at the time to become extinct.

The Chicxulub impact crater is located in the northwest corner of Mexico's Yucatán Peninsula. On the left is a shaded relief image of the area showing the barest outline of the remnant of the circular crater. The right image is the same area viewed by the Landsat satellite showing vegetation cover that highlights the region. The central white spot with no vegetation is the city of Merida. (NASA images (PIA03381), February, 2000, via Wikimedia Commons)

As a recent study has shown, there's more direct evidence that a meteor impact caused of the Cretaceous-Tertiary extinction than an enrichment of iridium or the outline of the Chicxulub impact crater.[4-7] Meteor impact generated a tsunami-like wave in an inland sea of what's now North America, and this tsunami killed numerous species whose remains were quickly buried by sediment and subsequently fossilized in a high state of preservation.[5]

This fossil trove is described in a recent issue of the Proceedings of the National Academy of Sciences by a team of geologists and paleontologists from the University of California (Berkeley, California), the University of Kansas (Lawrence, Kansas), Vrije Universiteit (Brussels,Belgium), University of Manchester (Manchester, UK), Florida Atlantic University (Boca Raton, Florida), the Black Hills Institute of Geological Research (Hill City, South Dakota), Florida International University (Miami, Florida), the Katholieke Universiteit Leuven (Leuven, Belgium), and the Palm Beach Museum of Natural History (Fort Lauderdale, Florida).[4] The research team included Walter Alvarez, who discovered the iridium anomaly of this extinction event in 1980.[5]

The fossil site, called Tanis, is a 1-1/2 meter thickness layer that's located in North Dakota's Hell Creek Formation.[5-6] In this layer. fossilized fish are stacked one atop another along with fossilized vegetation, insects, mammals, dinosaurs, ammonites, and other animals. It's a huge collection of different species of different stages of life that died at the same time, on the same day.[5] The fossil preservation is superb because of the quick formation of the sediment layer.[6]

A fish tail fossil from the Tanis deposit.

The excellent condition of this fossil is a consequence of the unusually rapid burial of species by sediment.

(University of California, Berkeley, image.[5])

The conjectured mechanism for such fossilization was a violent shaking that created a 30 foot wave on the inland sea in what is now North Dakota, and this wave deposited fish onto a sand bar at the mouth of a river.[5] The wave then reversed direction and left the fish stranded, at which time the fish were pelted with tektites, glass beads up to 5 millimeter in diameter, that condensed from molten material ejected by the meteor impact into the atmosphere.[5] This torrent of ejecta lasted for 10-20 minutes, and then a second large wave covered the fish with sediment, where they fossilized.[5] Says paleontologist, Robert DePalma, an author of the paper and discoverer this fossil deposit,

"A tsunami would have taken at least 17 or more hours to reach the site from the crater, but seismic waves - and a subsequent surge - would have reached it in tens of minutes."[6]

While the Chicxulub impact did create tsunamis, the wave in the inland sea was not a tsunami, but a seiche, a standing wave caused by an earthquake.[5] A tsunami would have taken 10-12 hours to travel the 3,000 kilometers from the impact site, while the exposure of the fish by a seiche would have occurred at the same time as the rain of tektites.[5] Such seiches have been observed in recent times. The 2011, 9.0 magnitude, Tohoku earthquake in Japan created a six-foot-high seiche half an hour later in a Norwegian fjord 8,000 kilometers (5,000 miles) away.[5-6]

The tektites would have impacted at speeds of 100-200 miles per hour, and it's suspected that the rain of tektites and other debris would have ignited wildfires over the Americas; and, perhaps, around the world.[5] Tektites were discovered, also, in the gills of fossil Acipenseriforma (sturgeons and paddlefish) that swam through the water with their mouths open as they were feeding.[5-6] The fossil bed also produced evidence of some undiscovered species of fish. Says DePalma "“At least several appear to be new species, and the others are the best examples known of their kind... Before now, fewer than four were known from the Hell Creek, so the site was already magnificently significant."[6]

1 millimeter spheres of glass, recovered from the Tanis fossil bed.

1 millimeter glass tektites recovered from the Tanis fossil bed.

(University of California, Berkeley, image.[5])

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