About 65 million years ago, a huge asteroid slammed into the Earth and wiped out three quarters of all life. The most famous victims were the dinosaurs, but many others, including marine life, were devastated too. And now, researchers from Yale have found evidence of just what happened in the oceans – the waters became too acidic for many animals to take.
The asteroid impact wasn’t the instant Armageddon it’s often portrayed as. Sure, anything unlucky enough to be too close to ground zero would have been killed pretty quickly, but life elsewhere on the planet would have succumbed to the onslaught of natural disasters that followed. These effects may have lingered for years afterwards.
Past studies, including drilling expeditions in the crater in Mexico, have revealed evidence of widespread wildfires, massive earthquakes and tsunamis, and acid rain in the hours and days following the impact. Huge amounts of vaporized rock would have collected in the atmosphere, blocking out the Sun for as long as 18 months.
These effects would largely have affected life on land, but the fossil record shows that vast amounts of marine life also vanished. The leading theory suggests that the vaporized rock in the air would have caused sulfuric acid to rain down and gather in the water, increasing the acidity of the oceans. Evidence of this phenomenon has been found for other mass extinction events, but not yet for this one.
That is, until now. A team of researchers led by Yale has discovered that direct evidence in the same place as the previous study – foraminifera. These microscopic creatures are a type of plankton that grow calcite shells. Conveniently, they provide one of the most detailed and long-running fossil records, spanning hundreds of millions of years.
By analyzing the chemical composition of foraminifera shells, scientists can get a good understanding of the long-term changes in marine conditions. In this case, the Yale team measured changes in boron isotopes in shells from before, during and after the time of the impact, which lines up with overall carbon dioxide levels.
Sure enough, they found that the oceans were far more acidic straight after the impact. This would have reduced the amount of calcium carbonate in the water, which many species use to build their skeletons and shells. This type of animal is already known to have died out in large numbers during this extinction event, and when they did they destabilized the rest of the ocean food chain.
“The ocean acidification we observe could easily have been the trigger for mass extinction in the marine realm,” says Pincelli Hull, senior author of the study.
According to the team, the evidence indicates that as much as half of the life in the ocean vanished fairly quickly. After that followed a long period of slow recovery.
Interestingly, the study also helps rule out alternative theories about what caused the overall extinction event. Some scientists argue that instead of an asteroid, the culprit was a long stretch of regular volcanic eruptions, lasting perhaps hundreds of thousands of years, which slowly made Earth less hospitable.
But if that were the case, the oceans should have become more acidic over a longer period of time. Instead, the researchers saw a sharp increase, consistent with a sudden event like an asteroid impact.
Not only does the study help improve our understanding of the past, it could inform our future. After all, the oceans are currently well on their way to becoming more acidic again, which is bad news for coral reefs and other marine life that depends on calcium carbonate.
The research was published in the journal Proceedings of the National Academy of Sciences.
Source: Yale University