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Asteroid dust helps solve the mystery of dinosaur extinction

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More than 99% of all organisms that have ever lived on Earth are now extinct. Most of these just died out quietly. However, in Earth’s history there have been five major mass extinction events – known as the big five – during which many species became extinct at the same time.

Each of the big five events caused at least a 40% loss of all species on Earth. Yet humans hold a particular grudge against the most recent one, which brought a sudden end to the 160-million-year history of the dinosaurs. This was the Cretaceous-Paleogene extinction, and it happened 66 million years ago, wiping out about 75% of all species on Earth at the time. Except for sea turtles and crocodiles, no four-legged animals bigger than 25kg survived.

After decades of heated debate, scientists have settled on two leading theories about what caused this extinction. The first possibility is the impact of an asteroid that created the 180km wide Chicxulub impact crater in the modern-day Yucatán Peninsula in Mexico. Second, a series of eruptions in a volcanic area known as Deccan Traps in India.

Last week, an international team of scientists with data from four independent laboratories published a study claiming to have ended the debate. They say the 12km-wide asteroid was the one to blame.

A case closed?

The study looked at rock samples collected in the crater, which is now underwater. They found a layer of terrestrial mud mixed with “space dust” containing the element iridium, which can be found in high concentrations in meteorites but is rare in the Earth’s crust. This layer was four times thicker in the impact crater than in the surrounding area.

The team found a 5cm layer of sediment immediately below the limestone from the earliest Paleogene, the geologic period that began immediately after the extinction. This thin layer of sediment had iridium concentrations of one part per billion, compared to the 0.04 parts per billion in the Earth’s crust.