Hell Creek Apocalypse

by Nicholas Mee on September 27, 2020

In the 1970s the geologist Walter Alvarez was working with Bill Lowrie studying single-celled marine creatures known as foraminifera in the Cretaceous limestone strata of Gubbio, Italy. These organisms were plentiful in every layer until one thick layer was reached that showed no evidence of fossils at all, marking the precise location of a major extinction event.

Doom and gloom in the dying days of the Cretaceous.

It was already well established that such an event had brought the geological period known as the Cretaceous to a close about sixty-five million years ago. This was the most famous mass extinction in Earth’s history as it ended the reign of the dinosaurs. But it wasn’t just T. Rex and the other non-avian dinosaurs that were wiped out. It was also the end of the flying reptiles, the pterosaurs, and the huge marine reptiles, the mosasaurs and the plesiosaurs, as well as most birds, lizards, snakes and many other species. Modern estimates suggest at least three quarters of all animal and plant species disappeared forever. Apart from crocodiles and sea turtles, every tetrapod weighing over twenty-five kilograms was gone. And it wasn’t just the mighty beasts that were laid low as the disappearance of the humble foraminifera clearly testified. It is difficult to imagine devastation on such a scale. Throughout the world, on land and sea, every single member of most species died without offspring and the species became extinct. Those species that did pull through, were probably saved by small numbers of hardy and lucky survivors. The carnage of individual organisms must have been close to one hundred per cent.

The Iridium Spike

The interface between the fossil-rich and fossil-free layers studied by Alvarez and Lowrie is often referred to as the K-T boundary. It marks the end of the Cretaceous and the start of the Tertiary period, a now obsolete term meaning third era. Cretaceous derives from the Latin creta meaning chalk, and K is its standard abbreviation from Kreide, German for chalk. In today’s terminology, this is the K-Pg boundary, where Pg is an abbreviation for Palaeogene, the modern name for the period following the Cretaceous.

Fossilized foraminifera shells.

The cause of this apocalyptic extermination of life had been debated for many years. Alvarez felt that the disappearance of the tiny foraminifera might provide clues that were unavailable from studying much larger and rarer fossils. He consulted his father, the nuclear physicist Luis Alvarez, to see whether there might be a way to determine how quickly the fossil-free layer was deposited, as this would show whether the fossils were wiped out in a sudden catastrophic event or whether their demise was more gradual. 

His father suggested that the noble metal iridium might be the key. Iridium is rare in the Earth’s crust because it has an affinity for iron, so almost all the iridium present in the proto-Earth sank to the core along with the iron as the Earth formed. The Earth’s crust is therefore depleted of iridium compared to asteroids and meteorites. Luis saw this as an opportunity. He argued that the Earth steadily accumulates dust as micro-meteorites enter the atmosphere and that the cosmic iridium from these meteoroids must settle on the seabed at a fairly constant rate. So measuring the iridium in sedimentary rocks in Gubbio might show how quickly they were deposited.

Rock from Wyoming with a claystone layer containing iridium at a thousand times the concentration of the layers above and below. Credit: San Diego Natural History Museum, Eurico Zimbres, Wikimedia.

This idea proved to be very fruitful, but not in the way Alvarez had expected. Extremely high levels of iridium were found in the fossil-free layers at Gubbio compared to other rocks (ninety times the typical concentration), so the proposed method was useless for timing the rate of sediment deposition. But its significance was far greater. The huge iridium spike was compelling evidence for a massive asteroid impact, and the coincidence with the disappearance of the fossils could only mean one thing. The foraminifera and their fellow Earth dwellers, the dinosaurs, the pterosaurs, the ammonites and the plesiosaurs had all been eliminated in the aftermath of an asteroid strike.

If the mass extinction were due to an asteroid impact, then its effects would have been felt worldwide. Soon the K-Pg boundary was being analysed where-ever it could be found and in every such location the concentration of iridium was enhanced to the same or greater degree than the rocks at Gubbio. Furthermore, fragments of shocked quartz were found within the boundary layer along with glass beads known as tektites that are formed from tiny molten droplets produced in impact events.

The Demise of the Dinosaurs

The Alvarezes estimated the size of the asteroid that was responsible. Assuming its iridium content was similar to other meteorites and judging by the amount of iridium dispersed around the world, they estimated the asteroid’s diameter to be roughly ten kilometres. The energy released by such a rock hitting the Earth at 80,000 kilometres an hour would be something like 100 million megatons of TNT, well over ten million times the explosive power of the largest bombs in the United States arsenal. Such an impact would leave a big hole in the ground that even sixty-five million years of weathering could not erase. So where was the crater?

The Chicxulub Crater

An artist’s impression of what the Chicxulub crater might have looked like soon after an asteroid struck the Yucatán Peninsula in Mexico. Credit: Detlev van Ravenswaay.

A likely candidate was found soon after the Alvarez hypothesis was published, and its connection to the demise of the dinosaurs is now firmly established. The Chicxulub impact crater is partially submerged at the northern edge of the Yucatán peninsula in Mexico. It is 180 kilometres in diameter and the oceanic half is buried under six hundred metres of sediment.

The aftermath of the impact has been outlined by geologists and climatologists. On impact the asteroid was vaporized along with a large chunk of crust material sending a vast plume of molten rock high into the atmosphere. The re-entry of this material set the Earth’s forests ablaze releasing vast quantities of soot, smoke and carbon dioxide. It is presumed that the soot and smoke in the upper atmosphere blocked out sunlight for many years causing temperatures to plummet and making photosynthesis impossible. The consequent loss of vegetation triggered the worldwide collapse of long-established ecosystems.

Marine life would have fared no better with the removal of photosynthesizing algae at the base of the oceanic food webs. The devastation was further exacerbated by the dispersal of a sulphuric acid aerosol in the upper atmosphere formed from the vaporized impact zone bedrock, which was largely composed of gypsum (calcium sulphate). Subsequent rainfall would wash sulphuric acid out of the atmosphere, acidifying the oceans, destroying coral reefs and killing organisms with calcium carbonate shells such as ammonites and foraminifera. 

The Hell Creek Formation

Palaeontologist Robert DePalma has spent the last decade excavating an incredible assemblage of fossils, nicknamed Tanis, in the Hell Creek Formation close to Bowman, North Dakota. In the late Cretaceous the region was covered by an inland sea and Tanis was near a river estuary flowing into this sea. Tanis has preserved an amazing record of the utter devastation on the day of the impact. It is thought that the asteroid strike three thousand kilometres away in the Gulf of Mexico triggered seismic tremors that hit the region just minutes later producing a ten-metre wave that temporarily reversed the flow of the river stranding large numbers of freshwater fish such as sturgeon and paddlefish. These fish were then pelted with microtektites and other debris raining down from the sky, before being inundated with a second wave that buried them under a layer of gravel and mud.

The Tanis fossils include large stacks of fish in postures suggesting they died of suffocation. These are mixed with burned tree trunks and branches, the remains of insects, mammals, mosasaurs, hadrosaurs and even the hipbone of a triceratops. The site is pockmarked with miniature craters formed by the impact of microtektites. Even more remarkably, there are lumps of amber—fossilized tree resin—embedded with micro-tektites, and around half the fish fossils have microtektites trapped in their gills, showing that they were still alive and struggling to breathe when bombarded by debris from the asteroid strike. 

Fossil fish from Tanis. There are large quantities of microtektites matching the ejecta from the Chicxulub crater in the gill rakers of half the sturgeon and paddlefish fossils, which proves the fish were alive when the impact occurred. Credit: Robert DePalma et al.

When Did the Age of the Dinosaurs End?

Potassium-argon dating was a key component of a survey published in 2013 that refined the age of the Chicxulub crater. Potassium is found in many common minerals in the Earth’s crust. A small proportion of this potassium is the radioactive isotope potassium-40, which has a half-life of 1.248 billion years. It decays into calcium-40 and argon-40, and the second of these has proved very useful for dating ancient lavas and other once-molten rocks. Argon is an unreactive noble gas that readily escapes molten rock, so newly solidified rock does not contain any argon. As time passes, however, argon-40 atoms accumulate due to the radioactive decay of potassium-40 and they are trapped within the crystal structure of the rock. This enables geochronologists to determine the age of a rock by measuring the ratio of argon-40 to potassium-40 atoms that it contains.

The 2013 survey was the work of an international team of researchers from the United States, Scotland and the Netherlands. They carried out independent high precision dating of numerous tektite samples from the Chicxulub crater at the Berkeley Geochronology Center and the University of Glasgow. The results were unequivocal and they provide a remarkably accurate date for the impact event. We now know that the asteroid hit the Earth 66,038,000 years ago, plus or minus 11,000 years.

The survey also cemented the connection between the Chicxulub crater and the geological relics of the asteroid impact from two well-separated locations, one on the Caribbean island of Haiti and the other in the Hell Creek Formation in north-eastern Montana. High precision potassium/argon dates were obtained for tektite and volcanic ash samples found at the K-Pg boundary, as defined by the iridium spike, from these two locations. The dates at both sites exactly match the date of the Chicxulub crater to within the accuracy that is now possible.

Four decades of detective work by geologists, nuclear physicists, palaeontologists and geochronologists has given us an incredibly detailed and compelling picture of what happened one dramatic day just over sixty-six million years ago—the day that sealed the fate of the dinosaurs.


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