Over 99% of all organisms that have ever lived on Earth are now extinct. Most of them died quietly. However, in Earth’s history there have been five major mass extinction events – known as the Big Five – in which many species have gone extinct at the same time.
Each of the five great events caused at least a 40% loss of all species on Earth. Yet humans hold a particular grudge against the most recent, which brought the 160-million-year-old dinosaur story to a sudden end. It was the Cretaceous-Paleogene extinction, and it happened 66 million years ago, wiping out about 75% of all species on Earth at the time. With the exception of sea turtles and crocodiles, no four-legged animal over 25 kg survived.
After decades of heated debate, scientists have decided on two main theories about the causes of this extinction. The first possibility is the impact of an asteroid that created the 180 km-wide Chicxulub impact crater in Mexico’s Yucatán Peninsula. Second, a series of eruptions in a volcanic area known as the 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 12 km-wide asteroid is to blame.
A closed case?
The study examined rock samples collected from the crater, which is now underwater. They found a layer of earth mud mixed with “space dust” containing the element iridium, which can be found in high concentrations in meteorites but which 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 of the Early Paleogene, the geological period that began immediately after the extinction. This thin layer of sediment exhibited iridium concentrations of one part per billion, compared to 0.04 part per billion in the earth’s crust.
The asteroid is believed to hit the planet at around 20 km per second, more than 50 times faster than the speed of sound in air. Not only did it destroy the immediate surroundings, but it also sent a cloud of vaporized rock and microscopic dust with high levels of iridium that passed through the Earth. Global cloud cover has blocked sunlight, cooled the Earth’s surface for decades to 10 ° C, and triggered a global “impact winter”. The cold and dark regime was followed by a halt in large-scale photosynthesis, disruption of food webs around the world, and collapse of ecosystems.
Iridium spikes in the dust from this era have been found in over 100 places around the world, from America, Asia, Europe, Oceania, to Antarctica. These were first identified in the results of the 1980s.
The first studies did not win a global consensus because the evidence could not link the high concentrations of iridium to the Chicxulub crater. But this new study provides that crucial link, and also imposes a significant time constraint. The dust must have settled in just a few decades – less than 20 years – after impact.
The Armageddon of the Dinosaurs
Although the sudden nature of this extinction is supported by changes in the fossil record, the records of microscopic organisms rather indicate a long-term change. This is the argument for a volcanic extinction, with a series of eruptions occurring over tens of thousands of years.
Massive and prolonged volcanic eruptions have been linked to other extinction events of volcanism – such as the Siberian trap eruptions, which marked extinction at the end of the Permian period. In the case of the dinosaur extinction, however, it could not have been caused by volcanism alone.
The Deccan traps released more than ten million cubic kilometers of materials and gas, causing long-term global warming of between 2 and 4 ° C, 150 to 300 thousand years before the extinction, just before the blow of the asteroid.
The eruption of the Deccan Traps lasted for several million years, beginning long before the asteroid impact. In fact, the main phase of Deccan Traps volcanism around 66 million years ago could have been triggered by the Chicxulub impact.
Increased nutrients in the oceans led to plankton blooms and low oxygen levels in the oceans, but the oceans were not completely oxygen free. The Deccan volcanism could have induced a biotic stress but not the total disappearance of the organisms.
Dinosaurs could have their end of the world in an unfortunate double issue of an asteroid volcano combo or a single giant impact. Either way, the large asteroid played a key role. This new study found the missing piece of evidence that links the extinction of the dinosaurs to the impact of Chicxulub, and that it happened in the blink of an eye.
This article by Queenie Hoi Shan Chan, Senior Lecturer in Earth Sciences, Royal Holloway is republished from The Conversation under a Creative Commons license. Read the original article.