The mass extinction of life some 66 million years ago is marked by the extinctions of dinosaurs and shallow marine organisms. It was a supremely important event because it led to the macro-evolution of mammals and heralded the appearance of humans.
The current hypothesis for the extinction is that an asteroid impacted in present-day Mexico, forming the Chicxulub crater. The impact sent huge amounts of dust around the world, blocking out the sun, or by polluting the atmosphere with sulphates, causing harsh acid rain. This would all lead to an 'atomic winter'.
Japanese scientists now propose a new hypothesis that latitude-dependent climate changes caused by massive stratospheric soot explain the known mortality and survival on land and in oceans at the Cretaceous-Paleogene boundary. The stratospheric soot was ejected from the oil-rich area by the asteroid impact and was spread globally.
They found the soot layer would have lasted for between two and five years, depending how much soot was ejected, and conditions would have returned to normal after about ten years. They also found that the climate changes were different at different distances from the equator. Further from the equator, extreme cooling and drying would have caused the extinction of both dinosaurs and crocodilians. But the model found that closer to the equator there was drought but more mild cooling. The drought would have left land-based dinosaurs without a food source, as most plants temporarily died. But the riverine crocodiles could have continued to eat aquatic animals, most of which survived, since they don’t rely as much on land-based plants. And closer to the equator, the more mild cooling wouldn’t have been lethal to crocodiles.
The analysis is supported by evidence in the layers of the earth that were deposited shortly after the asteroid hit, where soot remnants have been found. The researchers found the soot both near the impact zone as well as on the other side of the world in Spain, suggesting a period when it covered the globe.
Furthermore, the impact might have triggered the massive volcanic eruptions at the Deccan Traps in India and these might have been partly responsible for the extinction too. They too released huge amounts of dust and gases in the atmosphere.
 Kaiho et al: Global climate change driven by soot at the K-Pg boundary as the cause of the mass extinction in Scientific Reports – 2016. See here.
 Parisio et al: 40Ar/39Ar ages of alkaline and tholeiitic rocks from the northern Deccan Traps: implications for magmatic processes and the K–Pg boundary in Journal of the Geological Society - 2016