How iridium marked the end of the dinosaurs

Dinosaurs ruled the earth for 164 million years. For perspective, the human race (dating back some 315 000 years) has so far lasted merely 0.2% of this duration. However, 65 million years ago, almost all large vertebrates (on land, in the ocean or in the air) became extinct, marking the end of the Cretaceous period. Additionally, many plants and plankton disappeared at the same time. How is it that after millions of years of evolution, hundreds of species suddenly vanished from the face of the planet in a (geological) blink of an eye?

The term used when many types of species undergo global extinction in a geologically short period of time is mass extinction (Box 1). The mass extinction that wiped out the dinosaurs is known as the K-Pg (or K-T) event, occurring at a point in geological time known as the Cretaceous–Paleogene (K-Pg) boundary (formerly known as the Cretaceous–Tertiary, K-T, boundary), around 66 million years ago. Wildfires and tsunamis consumed the face of the globe. The K-Pg event led to the extinction of non-avian dinosaurs (i.e. the ones that did not evolve into modern birds). Ocean nutrient cycles also shut down, leading to many marine and terrestrial animals and plants going extinct.

Mass extinctions

Mass extinctions involve a large number and many types of species undergoing global extinction in a geologically short period of time (on the scale of thousands of years). There have been five significant mass extinctions during the Earth’s history. Four are classed as intermediate in size (including the K–Pg mass extinction). Only one is classed as major (the Permian–Triassic extinction).

The rush for an explanation

Over the decades many theories have attempted to describe what might have wiped out the dinosaurs. However, not all of them were sensible. For example, one theory from the 1980s suggested that dinosaurs spent too much time in the sun, causing them to develop cataracts, and the resulting visual impairment led them to fall off cliffs to their doom.

Many also neglected the worldwide impact of the extinction event: it took place on land, at sea and in the air. To truly understand what happened we need to consider changes in geography, oceanography and climate over that short geological period. Two key hypotheses take centre stage: a large asteroid impact or a giant volcanic eruption. However, the former hypothesis has a key piece of supporting evidence that makes it the generally accepted theory: a geological ‘fingerprint’.

An asteroid impact?

In the late 1970s, geologist Walter Alvarez (from the University of California, Berkeley) and a team of coworkers were studying the K-Pg boundary through exposed marine outcrops in Gubbio, Italy. The lower (and older) portion of exposed rock was bursting with the shells of microscopic Cretaceous marine organisms, whereas the upper (younger) layer comprised thin beds of microscopic shells of Paleogene marine organisms. Nestled between the two was a thin stratum (layer) of clay roughly 2–3 cm thick. This rock was deposited in a large quantity over a short period and marked the Cretaceous–Paleogene geological boundary.

An analysis of the clay determined anomalously high concentrations of iridium (Box 2), clocking in at 10 parts per billion (ppb), compared to the expected 0.3 ppb at the Earth’s surface. Iridium is enriched at the Earth’s core and in space, and the very low concentrations at the surface can be attributed to cosmic dust and volcanic activity. Following the iridium anomaly, the team were now tasked with finding a suitable argument for what caused the concentration of iridium to suddenly increase from the expected amount. The answer? An asteroid struck the Earth 66 million years ago, delivering the surge of iridium, and in doing so triggered the K-Pg extinction that wiped out the dinosaurs. The iridium layer is spread globally, found in both land and ocean sediments, and Alvarez and coworkers postulated the asteroid to be as much as 6 miles in diameter, passing through the atmosphere and ocean as if they were not there, forming a blast crater around 60 miles across. More evidence pointing towards an extraterrestrial impact is shocked quartz. Shocked quartz is formed under high pressures that cause its molecular structure to become distorted. Shocked quartz has been found at numerous impact sites over the globe and has become a key diagnostic tool for extraterrestrial impacts. In addition, glass spherules (minute spheres that may also accompany an extraterrestrial impact) can be found all over North American K-Pg boundary clay, accompanying the iridium rich clay and shocked quartz. The distribution of shocked quartz is focused in North America and extends into the Pacific Ocean floor, suggesting the K-Pg impact event occurred near North America. The impact crater itself has been found, buried deep under sediments off the Yucatan peninsula of Mexico (Figure 1).

Iridium

Iridium (Ir) is a transition element with atomic number 77 and relative atomic mass 192.217. It is a hard, brittle, dense silvery metal, which is extremely resistant to chemical attack. Iridium is named after Iris, goddess of the rainbow, because of the array of colours seen in its compounds (due to its nine oxidation states, from –3 to +6). As it is so resistant to corrosion, iridium is used in alloys where this property is important, for example in pen nibs, the contacts for spark plugs and bearings.

What would it have been like?

One late evening while sitting in the garden and looking up to the clear sky, you see many stars, but one is particularly bright. Oddly, the next evening it is even bigger, slowly growing into a glowing orb.

You are blinded by the flash. No noise; just a dazzling yellow flare. Moments later tremors cause the ground to vibrate with a vigour that would easily project a 12-metre-long Tyrannosaurus rex into the air. Assuming that you survived, you (and the dinosaurs) would see the sky turn a dark orange as the atmosphere (heated by the extraterrestrial compression) becomes an oven.

Rock, now molten from the impact energy, is thrown into the air producing a glassy rain shower. These silica-rich glasses, or microtektites as geologists would later observe, begin to cleave anything on the surface of the Earth in the way.

The turmoil begins to calm down, marking the end of Act 1; but only briefly. Now come the hurricanes from the sonic boom that accompanied the flash (sound travels more slowly than light), also triggering substantial volcanic activity. Soot and rock dust vented into the atmosphere block out sunlight for up to 4 months. The photosynthesis mechanism shuts down; the temperature plummets, and an impact winter begins.

Key Points

• Iridium (Ir) is a hard, brittle, dense silvery transition metal with atomic number 77, which is extremely resistant to chemical attack.
• Iridium is only present at very low concentrations at the Earth’s surface, through deposition of cosmic dust and volcanic material. However, iridium is enriched at the Earth’s core and can be found in space.
• A thin layer of iridium-rich material is found across the globe above strata rich in fossils. It is believed that this iridium was delivered to Earth in a huge asteroid that hit the planet 66 million years ago, causing a mass extinction event.
• Other evidence for the asteroid impact includes shocked quartz (distorted through high pressures) and microtektites (silica-rich glasses).

Piecing it all together

The three key pieces of evidence: the iridium layer, microtektites and shocked quartz, are the key pieces of scientific evidence pointing to an asteroid impact. This theory has gone on to spark the curiosity of people all over the globe. The story of the end of dinosaurs is a stark reminder of our place in the world, and the rich history under the surface if we dig a little deeper.

Resources

The Rise and Fall of the Dinosaurs: A New History of a Lost World by Stephen Brusatte (2019, Picador) examines the topics covered in this article and explores many fascinating areas of contemporary research in the world of dinosaurs.