A quaint Italian town and the blast that killed the Dinosaurs

dinosaurLocated along the steep slopes of Mount Ingino in Umbria, Central Italy, the ancient town of Gubbio features a wealth of architectural jewels that testify to its millenary history. The town was founded in the 3rd century b.C. and was annexed by Rome in 85 b.C., as attested by the remanants of a Roman Theater that could host 10,000 visitors. All the churches, fountains and monuments that attract tens of thousands of tourists every year are a living proof of the ability of Italian Medieval and Renaissance architects.

Gubbio, a jewel nested in Central Italy. Photograph by C. Venturini.
Gubbio, a jewel nested in Central Italy.

However, what makes this magic place even more remarkable, is that outside the town’s ancient walls, a deep gorge (Gola del Bottaccione), excavated by a stream over the last two million years, hosts a complete stratigraphic sequence spanning more than 100 millions of years of the Earth’s history. What is most worth noting here, is a one-centimetre thick layer of clay that separates white from reddish limestone strata.

An explanatory panel marks the site where the iridium-rich clay level was first discovered by Walter Alvarez back in the 70s. Photograph by C. Venturini.
An explanatory panel marks the site where the iridium-rich clay level was first discovered by Walter Alvarez back in the 70s.

This tiny layer holds the clue to the most famous mass extinction of all time, the one that wiped out at least three quarters of all living creatures, 66 million years ago. The dinosaur-killer was an asteroid, as documented by Walter Alvarez and his father, Physics Nobel prize holder Luis Alvarez, in a 1980 Science paper that represents the outcome of the greatest scientific effort ever taken in the combined fields of planetology, astronomy and geology.

Walter and Luis Alvarez, together with Frank Asaro and Helen Michel, by analysing samples from the Bottaccione Gorge, concluded that the anomalous “spike” in the content of the element iridium found in the tiny clay layer could mean only one thing: Something came from outer space and wrought havoc to life on Earth. Today, we know that a 15-km-wide asteroid rushed through our atmosphere traveling at 25 km per second and, hitting our Planet and leaving a gigantic a scar, an impact crater as wide as 200 km, in what is now the Yucatán Peninsula, Mexico.

The asteroid scenario, although often criticized over the last four decades, has never been dismissed, especially because the layer containing abnormally high contents of iridium has been found in other locations around the world, from Spain to Denmark and New Zealand.

The asteorid blast instantly annihilated every creature, on land and offshore, within a radius of about 1000 kilometers from the impact site. What proved to be even more destructive, though, were trillions of vaporized rock particles launched high above Earth’s surface by the violence of the impact into a shallow sea area. In outer space, the particles coalesced and formed peculiar types of “spherules” that fell back to Earth and bombarded onshore areas like meteors, igniting global-scale wildfires.

In this hellish scenario, one group of creatures, which had been ruling the Earth for 160 million years, took the hardest brunt: Dinosaurs, the most fascinating among all extinct creatures, vanished almost instantly, in geological terms, in days, weeks, maybe a few months. Dinosaurs did not die alone: Thousands of other species were swept away, among which the amazing Ammonites, which had thrived in the Jurassic and Cretaceous oceans until the impact at transition from the Mesozoic to the Cenozoic wiped them out forever.

As regards the demise of Dinosaurs, a recent research published in PNAS by scientists from the University of Colorado at Boulder, has documented the catastrophic processes that led to the almost complete annihilation of those majestic creatures, along with most worldwide species. Charles G. Bardeen and his colleagues found out that a “transient climate change” took place at the Mesozoic-Cenzoic boundary, most likely triggered by mammoth quantities of soot produced as a result of the global wildfires that broke out after the blast.

The University of Colorado team conducted climate simulations that consider injection of 15,000 tons of soot into the atmosphere. According to the simulations, the soot, heated by the Sun, was lifted high up into the atmosphere, thus forming a global “shield” that reflected away most of the sunlight: As a result, continents and oceans cooled by as much as 28 °C and 11 °C, respectively; moreover, photosynthesis was made impossible for more than a year and a half. In this scenario, no major living creature could have survived, especially those, like most dinosaurs, that relied on plants for food.

In May 2017, a BBC documentary showcased a massive research effort that’s being conducted in the Gulf of Mexico, led by scientists Jo Morgan (Imperial College London) and Sean Gulick (University of Texas): The research team has suggested that the shallow sea that covered the impact site contained immense volumes of sulphur (trapped in the mineral gypsum), which were also injected into the atmosphere, worsening the effects induced by the soot and extending the “global winter” period that followed the firestorm.

The researchers argue that, had the asteroid struck a different location, such as a deep ocean like the Pacific or the nearby Atlantic, the aftermath of the blast might have been very different. An impact into deep water would, of course, have triggered mammoth-sized tsunamis, but it wouldn’t have injected sulphur in the atmosphere. An asteroid hittig a 4-km deep seawater column would have produced less vaporized rock, fewer spherules and fewer wildfires. Sunlight might have still reached Earth’s surface, and photosynthesis could have continued to function.

One could ironically argue that an asteroid hitting Earth 20 seconds earlier or later would most likely have spared the Dinosaurs, which would be still ruling the world to this day. And maybe, with such huge and fierce competitors around, we humans would never have made it.

Federico Pasquaré Mariotto (Associate Professor, University of Insubria, Italy)
Corrado Venturini (Associate Professor, University of Bologna, Italy)