Massive volcanic eruptions on the Indian peninsula have long been proposed as an alternative cause for the demise of the dinosaurs. This phase of active volcanism took place in a period just before the Earth was struck by a meteorite, 66 million years ago. The effect of these volcanic eruptions on the Earth's climate has been topic of fierce scientific debates for decades. Now, climate scientists from Utrecht University and the University of Manchester show that, while the volcanism caused a temporary cold period, the effects had already worn off thousands of years before the meteorite impacted. The scientists therefore conclude that the meteorite impact was the ultimate cause of the dinosaur extinction event.
What killed off the dinosaurs—was it the Chicxulub meteorite or did the effects of massive volcanism also play a role? Many modern children's books on the history of dinosaurs include speculation on these two competing ideas.
The meteorite impact in the Gulf of Mexico roughly 66 million years ago is well researched and widely known as the defining end of the dinosaur age. But earth scientists have fiercely debated for decades whether a massive outpouring of lava on the Indian continent, which occurred both prior to and after the meteorite impact, also contributed to the demise of dinosaur populations roaming the Earth. These volcanic eruptions released vast amounts of CO₂, dust, and sulphur, thereby significantly altering the climate on earth—but in different ways and on different timescales to a meteorite impact.
Ancient peats
A new publication in the prestigious scientific journal Science Advances by climate scientists from Utrecht University and the University of Manchester now provides compelling evidence that while the volcanic eruptions in India had a clear impact on global climate, they likely had little to no effect on the mass extinction of the dinosaurs.
By analysing fossil molecules in ancient peats from the United States of America, the scientific team reconstructed air temperatures for the time period covering both the volcanic eruptions and the meteorite impact. Using this method, the researchers show that a major volcanic eruption occurred about 30.000 years before the meteor impact, coinciding with at least a 5° Celsius cooling of the climate. They also conclude that this cooling was likely the result of volcanic sulphur emissions blocking sunlight from reaching the Earth's surface.
Importantly, the scientists discovered that by around 20.000 years before the meteorite impact, temperatures on Earth had already stabilised and had climbed back to similar temperatures before the volcanic eruptions started. This period of global warming was likely aided by volcanic CO2 emissions, says Lauren O'Connor at Utrecht University: "These volcanic eruptions and associated CO2 and sulphur release would have had drastic consequences for life on earth. But these events happened millennia before the meteorite impact and probably played only a small part in the extinction of dinosaurs."
Impact winter
With the effects of volcanism practically ruled out, this would leave the Chicxulub meteorite impact as the primary cause of the dinosaur mass extinction. "By comparison, the impact from the asteroid unleashed a chain of disasters, including wildfires, earthquakes, tsunamis, and an 'impact winter' that blocked sunlight and devastated ecosystems. We believe the asteroid that ultimately delivered the fatal blow," says Rhodri Jerrett at the University of Manchester.
The fossil peats that the researchers analysed contain specific membrane-spanning molecules produced by bacteria. The structure of these molecules changes depending on the temperature of their environment. By analysing the composition of these molecules preserved in ancient sediments, scientists are able to calculate past temperatures. O'Connor adds: "This way, we were able to create a detailed 'temperature timeline' for the years leading up to the dinosaur extinction, which we can compare to the fossil record to understand the relative timing of events.".
The researchers from Utrecht University, the University of Manchester, Plymouth University, and the Denver Museum of Nature & Science, are now applying the same approach to reconstruct past climate at other critical periods in Earth's history.
