There's nothing lucky about having an asteroid wipe out three quarters of all life on Earth, but a new study from Tohoku University in Japan has put a number on just how unlucky the dinosaurs were. According to the new research, there was only a 13 percent chance of the mass extinction event that occurred 66 million years ago, and if the asteroid had struck almost anywhere else on the planet, the dinosaurs could have survived.

The Cretaceous period came to an abrupt end about 66 million years ago, when an asteroid estimated to be about 10 km (6.2 mi) wide smashed into what is now the Yucatan Peninsula, Mexico. Although death would have been pretty instant for anything unlucky enough to have been in the direct line of fire, the impact alone didn't spell global destruction. Vaporized rock and soot was ejected into the atmosphere in huge amounts, blocking out sunlight, causing a nuclear winter effect, disrupting photosynthesis and triggering the collapse of the food chain.

The researchers on the new study hypothesized that the severity of the climate changes would vary depending on where the asteroid hit. Areas with higher levels of sedimentary organic material would throw more soot into the stratosphere, cooling the Earth more dramatically than if it hit in places with lower hydrocarbon concentrations. To test the idea, the researchers used a global climate model to estimate the temperature anomalies that would be caused by different levels of soot in the stratosphere.

Based on the hydrocarbon-rich rock at the impact site, the model indicated that the soot thrown into the air would cool the Earth by a devastating 8 to 11° C (14 to 20° F) on average, with a drop as drastic as 17° C (31° F) over land and 5 to 7° C (9 to 13 F) in the seawater, to a depth of 50 m (164 ft). Rainfall over land would have dropped by 70 to 85 percent. Altogether those figures paint a grim picture for life on Earth.

This chart shows just how unlucky the dinosaurs were: a mass extinction that devastating could only occur if an asteroid struck a hydrocarbon-rich area (those marked in orange), making up just 13 percent of the Earth's surface at the time(Credit: Kunio Kaiho)

The Tohoku team then looked at how widespread these hydrocarbon-rich areas would have been at the time. The researchers found that they were mostly marine coastal margins, concentrated along shorelines where algae could deposit more organic matter into the sediment. These areas, the team found, covered just 13 percent of the Earth's surface.

Had the asteroid struck somewhere in the other 87 percent of the planet, life on Earth could have taken a very different path. While an impact from a space rock that big would still have caused widespread destruction no matter where it hit, the Tohoku researchers say that the mass extinction event that followed wouldn't have been quite so severe, even going as far as to say that some species of dinosaurs may have persisted beyond that clear boundary.

The research was published in the journal Scientific Reports.

Source: Tohoku University

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