What finished off the non-avian dinosaurs, along with three-quarters of the planet’s species, 66 million years ago? A trio of geochemists in the US claim the famous meteorite impact usually attributed to their extinction was preceded by a massive bout of volcanism, a few hundred thousand years earlier. This weakened ecosystems and set up species for the asteroid to deal the knock-out blow.
Their work was published in Nature Communications.
The cause of the mass extinction at the boundary of the Cretaceous and Palaeogene periods is still intensely debated. While the meteorite that struck what is now Mexico has traditionally been the frontrunner, megavolcanism in India – which spewed monstrous floods of lava and gases for tens of thousands of years to form the Deccan Traps – has gained traction.
Most scientists think a combination of the two dealt a “one-two punch”. But some say the impact shook up the Earth to trigger the eruptions, while others say the volcanoes were already in full flow when the asteroid hit. The problem is untangling the timing between the two. Which came first – the blast from outer space or rumblings deep within the Earth?
Dating events millions of years old is tricky – the most reliable dating methods have error margins of a few hundred thousand years – and an incomplete fossil record doesn’t help either.
So Sierra Petersen and Kyger Lohmann from the University of Michigan and Andrea Dutton from the University of Florida turned to mollusc shell fossils from Seymour Island in Antarctica, which grew during the last few million years of the Cretaceous and across the extinction boundary (around 69 to 65.5 million years ago).
They used a new geochemical technique called carbonate clumped isotope paleothermometry to ascertain sea temperature changes during the period.
The temperature of the ocean’s surface dictates what isotopes of elements – atoms with different numbers of neutrons in their nucleus – are present. Warm water, for instance, tends to see more of the lighter oxygen isotope 16O evaporate, leaving a higher ratio of 18O in the ocean. And molluscs keep a record of this 18O/16O ratio, and other isotopic ratios, in their calcium carbonate shells.
Over time, the molluscs died and were buried in layers, and their shells dug up and analysed.
Petersen, Dutton and Lohmann, analysing many different isotope ratios in 29 fossil shells, found a temperature spike that coincided with the onset of Deccan Traps volcanism and another, smaller peak around the time of the asteroid impact.
Before either of the pulses, though, was a period of sustained warmth. Around 68.7 and 67.8 million years ago, temperatures rose from around 5 °C to 14 °C.
The water stayed warm (around 9 – 12 °C) for a million years or so, then gradually cooled to 4 °C by 66.25 million years ago.
Then only around 150,000 years before the meteorite impact and around the time the Deccan Traps started to stir, spewing greenhouses gases into the atmosphere, came the first temperature spike – around 7.8 °C (plus or minus 3.3 °C).
This was followed by cooling and a second, smaller peak of 1.1 °C (plus or minus 2.7 °C) at the time of the impact. And over the next 200,000 to 400,000 years, the temperature dropped to pre-impact levels.
The mollusc-measured spikes in temperature also coincided with marine species extinctions.
The first pulse, dated 66.23 million years ago, saw 10 species disappear from that layer. Nine were benthic, or bottom-dwelling. The second pulse saw 14 species wiped out, but only six were benthic.
This, the researchers write, suggests different kill mechanisms for each – such as volcanism versus impact.
The reason those species didn’t die in the initial period of sustained warmth is, they add, because the temperature rose gradually. When faced with sudden volcanism and impact heat spikes, though, they were unable to adapt in time.
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