Apr 01 2019

Capturing The Most Deadly Day On Earth

“When I saw that, I knew this wasn’t just any flood deposit,” DePalma said. “We weren’t just near the KT boundary—this whole site is the KT boundary!”

Unless something really unexpected happens, this is likely to be the science news story of the year, and will be on everyone’s short list for the science news of the decade. It may be the paleontological news of the century. It’s easy to get excited when news like this breaks, and maybe I’m overcalling it, but you can decide once you hear the news, if you haven’t already.

A young PhD candidate, Robert DePalma, has found a massive fossil deposit that seems to have been laid down on the actual day the asteroid hit and wiped out 99.9999% of living things and 75% of species on Earth.

The New Yorker tells the whole story in great detail, and the whole thing is worth a read, but here is the quick version. It is now clearly established that 66 million years ago a several mile wide asteroid impacted the earth at 45 thousand miles per hour, impacting near the Gulf of Mexico creating what is now called the Chicxulub crater. The impact sent tons of debris into the air, into orbit, and even around the solar system. Hot rock rained back down onto the Earth, setting fire to most of the plant life, poisoning the atmosphere, and blocking out the sun plunging the Earth into a toxic deep freeze.

It’s hard to imagine anything surviving that day or the following weeks and months, but some life squeaked through and eventually evolved into the modern assemblage of life, including humans.

How do we know this happened? Well, and event like that leaves behind some evidence. We know there was a mass extinction at what was called the K-T boundary (now the K-Pg boundary, but some still use the older nomenclature). The more fossil evidence we collect, the more sudden this mass extinction appears, although there are still some holdouts for the more gradual extinction model.

In 1980 Walter and Louis Alvarez published a paper in which they described an iridium layer at the K-T boundary, suggesting that this rare element was layered down suddenly all over the Earth from an asteroid impact. The isotope was also consistent with an asteroid origin, rather than Earthly origin, such as from massive volcanic eruptions.

Other evidence of a massive impact includes a rain of tiny glass spheres from the molten material sent up into the atmosphere. There is also shocked quartz, iridium, and nanodiamonds – geological material that can only be caused by a massive impact.

Paleontologists have previously found fossils that seemed to be from creatures that died on the actual day of the impact. However, what DePalma found was an entire ecosystem flash preserve in the aftermath of the impact. The site was referred to him by a local rancher in the Hell Creek region of North Dakota, already known as a hot bed for Cretaceous fossils. It was the site of the first T-Rex discovery, for example.

DePalma examined the site, which he is now calling the Tanis site, and eventually realized what he had found – layers of sediment with fossils, amber, shocked quartz, and other features of the impact itself. Further, he found the glass beads, called tektites, at the bottom of tiny craters. The glass spheres were still raining down when these sediments formed. Further still, he found fish so well preserved that he could see tiny tektites caught in their gills. These fish died that day.

The location is a flood valley which at the time had a large pond that would occasionally flood from a storm. So there were many sediment layers that were laid down suddenly, capturing a snap shot the ecosystem. Usually such sediment layers may represent a thousand years or more – but here they represented an actual day.

The new find is a sediment layer several feet thick. This was no storm. What DePalma and his coauthor believed happened was when the asteroid impacted Chicxulub it created a tsunami that washed over North America. The water carried with it all the life it contained, and it washed across the land picking up plants and other material. Some of that water washed into the flood valley in Hell Creek and deposited the thick layer of sediment with all the dead and dying animals.

The sediment was key, because it locked the creatures in place, and preserved them in three dimensions. The impact ejecta, including the glass spheres, then rained down into this sediment, leaving the tiny craters with the tektites at the bottom. How cool is that?

This find is consistent with the models of what happened that day, and with other evidence regarding the impact and K-T extinction. But it also provides us the best snap-shot of that day. Already there are new species discovered previously unknown to science. But we also get to see a sample of life that we know was all alive at the same time. The site will also contain evidence that scientists can use to test the computer models of the impact.

It’s estimated that this one site will likely keep paleontologists busy for half a century.

These rare findings are extraordinary. Usually we look at the geological and fossil record with very low resolution – each tiny layer represents thousands or millions of years. But occasionally big events are captured and preserved. When a volcano erupts, for example, and puts down a thick layer of ash, that can preserve a day into the geological record.

I’m not aware of anything like this, however. This was one of the biggest events in the history of life on Earth. This event is recorded all around the Earth in different ways. It makes sense that there are some locations that were just the perfect distance from the impact to have significant events occur, but not be completely destroyed, and also to happen to have the right conditions to preserve what happened. DePalma has apparently found one of these “perfect storm” sites for the K-T event.


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