Iridium and the Terminal Cretaceous Event

There have been a number of natural catastrophes in Earth’s past that wiped out huge numbers of plant and animal species- -in effect,”resetting the clock” and requiring a new start for much of life on Earth. One such catastrophe took place at the end of the Permian era, some 251 million years ago. But perhaps the most famous one occurred about 65 million years ago at the end of the Cretaceous, causing the dinosaurs to go extinct (except for a small number that evolved into modern birds). Evidence that the terminal Cretaceous event was triggered by the impact of an asteroid with Earth began to emerge in the early 1980s, when Luis and Walter Alvarez discovered a thin layer of iridium in rock strata (layers) 65 million years old at widely separated sites around the world. Iridium is very rare on Earth’s surface but is much more common in some kinds of meteorite. The two scientists calculated that the impact of such an object, about 10 kilometers across, could have spread the right amount of iridium around the globe in the cloud of debris (fragments) blasted out by the impact. The idea was greeted with skepticism at first, but more traces of debris from an impact occurring at the time of the death of the dinosaurs began to turn up as geologists searched for evidence for or against the hypothesis. To most people, the most decisive evidence came in the early 1990s, when a geological feature at Chicxulub in the Yucatan Peninsula of Mexico was identified as the remains of an impact crater in exactly the right place and with exactly the right age to connect it to the terminal Cretaceous event.

There remained one doubt. Iridium does exist within Earth, and it is brought to the surface by volcanic eruptions. It still seemed just possible that volcanic eruptions on a truly massive scale could have put the iridium layer in place, and such eruptions would, of course, be bad news for life on Earth. There was even a known volcanic event that could have been responsible. Huge eruptions at roughly the right time spread vast amounts of lava across what is now west-central India, forming a structure known as the Deccan Traps. This is one of the largest volcanic features on Earth, more than two kilometers thick and covering an area of more than a million square kilometers. But it is not a unique feature. The Siberian Traps were produced in one of the largest known volcanic events since the start of the Cambrian, which lasted for a million years and spanned the Permian-Triassic boundary, about 250 million years ago. This coincided with the terminal Permian extinction event, which killed some 90 percent of species existing at the time; in fact the eruption of the Siberian Traps almost certainly caused this extinction.

So it is reasonable to implicate the formation of the Deccan Traps with the extinction at the end of the Cretaceous, and it is certain that such events have played a part in the history of life on Earth. And it also seems possible that life at the end of the Cretaceous was already under stress because of the environmental changes associated with the formation of the Deccan Traps when the meteorite struck. All of these possibilities encouraged a great deal of work over the best part of the next two decades, culminating in a meeting of 41 geologists, paleontologists, and other researchers who reviewed all the data and published their conclusions in the journal Science in 2010. They found that the volcanic activity lasted for about 1 .5 million years, much longer than the time span over which the famous iridium layer was deposited, and that the eruptions began about half a million years before the terminal Cretaceous event. During that time, ecosystems did not change dramatically, but they suddenly collapsed at the time of the Chicxulub event. In light of all the data, the team concluded that a large asteroid impact 65 million years ago in modern-day Mexico was the major cause of the mass extinctions.

1.There have been a number of natural catastrophes in Earth’s past that wiped out huge numbers of plant and animal species- -in effect,”resetting the clock” and requiring a new start for much of life on Earth. One such catastrophe took place at the end of the Permian era, some 251 million years ago. But perhaps the most famous one occurred about 65 million years ago at the end of the Cretaceous, causing the dinosaurs to go extinct (except for a small number that evolved into modern birds). Evidence that the terminal Cretaceous event was triggered by the impact of an asteroid with Earth began to emerge in the early 1980s, when Luis and Walter Alvarez discovered a thin layer of iridium in rock strata (layers) 65 million years old at widely separated sites around the world.?Iridium is very rare on Earth’s surface but is much more common in some kinds of meteorite. The two scientists calculated that the impact of such an object, about 10 kilometers across, could have spread the right amount of iridium around the globe in the cloud of debris (fragments) blasted out by the impact. The idea was greeted with skepticism at first, but more traces of debris from an impact occurring at the time of the death of the dinosaurs began to turn up as geologists searched for evidence for or against the hypothesis. To most people, the most decisive evidence came in the early 1990s, when a geological feature at Chicxulub in the Yucatan Peninsula of Mexico was identified as the remains of an impact crater in exactly the right place and with exactly the right age to connect it to the terminal Cretaceous event.