A gigantic asteroid is usually fingered as the sole culprit in the extinction of the dinosaurs, but that isn’t the whole story. Fossilized shells from that period have now revealed that Earth’s systems were already stressed before the impact, as volcanic eruptions may have heated the oceans and raised mercury levels to dangerous heights.
Technically, dinosaurs were a victim of climate change – the question is what triggered it. An asteroid impact is only directly deadly to those animals unlucky enough to be in the direct line of fire. Instead, the majority of life on Earth would have succumbed to the effects of climate change that followed.
The impact would have kicked up huge amounts of rock and dust, which hung around in the atmosphere for years to come. Sulfuric acid would have rained down, making the oceans more acidic. Meanwhile, the dust cloud would have blocked sunlight for months, killing off many plants. Those two things together would have devastated the bottom rungs of the food chain, rippling upwards and upsetting most life on Earth.
But while the asteroid impact story is regarded as the leading hypothesis, it’s not the only one. Another major contender is huge volcanic eruptions, most likely those that created the Deccan Traps rock formations in India. Covering an area of roughly 500,000 km2 (200,000 mi2), this is one of the biggest such formations in the world.
The eruptions that created the Deccan Traps began about 66 million years ago – roughly around the time the dinosaurs went extinct – and lasted off and on for about a million years. During that time, they would have outgassed immense amounts of carbon dioxide, which would have caused major climate change.
And now, researchers from the University of Michigan have found evidence supporting the role of the Deccan Traps. By studying fossilized mollusk shells from around 66 million years ago, the team found evidence of warming oceans and higher mercury levels at the time – both of which are symptoms of volcanic eruptions.
The fossil shells were collected from Antarctica, the US, Argentina, India, Egypt, Libya and Sweden. By analyzing the isotopic composition of the carbonate in them, the team is able to determine how warm the waters were when the shells formed, and they can also figure out how much mercury was in the water. Importantly, these markers appeared in the fossil record before the time of the impact.
“For the first time, we can provide insights into the distinct climatic and environmental impacts of Deccan Traps volcanism by analyzing a single material,” says Kyle Meyer, lead author of the study. “It was incredibly surprising to see that the exact same samples where marine temperatures showed an abrupt warming signal also exhibited the highest mercury concentrations, and that these concentrations were of similar magnitude to a site of significant modern industrial mercury contamination.”
Mercury is, obviously, something to be avoided in the environment, but it’s often found in heavily-contaminated areas around power plants and mines. For comparison’s sake, the team examined mercury levels in modern clam shells taken from an industrial site in Virginia – and found similar readings.
“The modern site has a fishing ban for humans because of high mercury levels,” says Sierra Petersen, co-author of the study. “So, imagine the environmental impact of having this level of mercury contamination globally for tens to hundreds of thousands of years.”
The researchers aren’t claiming that the asteroid impact didn’t happen – instead, they’re saying that both could have contributed. The asteroid impact may have accelerated the climate change that the Deccan Traps had already set into motion.
The team says that studying shells for these kinds of markers, which are both preserved side by side, could help improve our understanding of ancient – and possibly upcoming – climate change events.
The study was published in the journal Nature Communications.
Source: University of Michigan
