It has long been our planet’s greatest and oldest murder mystery. Roughly 445 million years ago, around 85 percent of all marine species disappeared in a geologic flash known as the Late Ordovician mass extinction. But scientists have long debated this whodunit, in contrast to clearer explanations for Earth’s other mass extinctions.
“The Ordovician one has always been a little bit of an oddball,” said Stephen Grasby of the Geological Survey of Canada.
Now he and David Bond of the University of Hull in England say they have cracked the case in a study published last month in the journal Geology. Widespread volcanic eruptions unleashed enough carbon dioxide to heat up the planet and trigger two pulses of extinction separated by 1 million years, they report. If true, it places the first grand wipeout of life on Earth in good company: Many of the other major mass extinctions are also thought to be victims of global warming.
Scientists have offered a range of culprits — including toxic metals and radiation released from a distant galaxy — but the favored explanation has long been global cooling.
Toward the end of the Ordovician, Earth underwent widespread glaciation. That could have caused the shallow seas to disappear, which provided optimal conditions for a variety of organisms.
But some scientists, including Keith Dewing, who is also at the Geological Survey of Canada but was not involved in this research, have struggled with this hypothesis. Geological evidence shows that both pulses of the extinction were quite abrupt, but glaciation often waxes and wanes over millions of years.
“You had to shoehorn your data in a little bit to get it to fit,” he said of that explanation.
Dr. Bond and Dr. Grasby reached their volcanic hypothesis after collecting Ordovician rocks from a small stream in southern Scotland. They then shipped those rocks to Vancouver, British Columbia, where the specimens were heated in a lab until they released large amounts of mercury — a telltale sign that volcanoes had rocked the epoch.
The rocks also emitted molybdenum and uranium — geochemical proxies that suggest the oceans were deoxygenated at the time. Only warming so easily robs the oceans of oxygen, they say, asphyxiating the species that live there.
Think of a bottle of cola. “If it’s been in the fridge, it stays nice and fizzy because the gas in that carbon dioxide stays in the liquid,” Dr. Bond said. “But if you leave it on a sunny table outside and it gets really warm, then that gas quickly dissociates out of that liquid and you end up with a flat coke.”
These findings allowed the team to paint a new picture — one that doesn’t discount the glaciation at the time, but suggests that the cooler climate was punctuated by global warming events triggered by volcanic eruptions.
“It all just seemed to fit together quite nicely,” Dr. Bond said.
In the story they tell, Earth’s crust began to break open just before both pulses of extinction. Giant cracks released walls of lava that erupted hundreds of feet into the air and extended for hundreds of miles. So many flows could have deposited lava up to a million square miles away, plus mercury and enough carbon dioxide into the atmosphere to drive global warming. That, in turn, caused a cascade of effects, from punching holes in the ozone layer to reducing oxygen in the ocean.
“This wasn’t an oddball cooling event,” Dr. Grasby said. “It joins the club as another ‘death by warming.’”
Seth Finnegan, a paleobiologist at the University of California, Berkeley, who was not involved in the research, has questions about the study’s mercury data. It’s possible, he says, that mercury (from later volcanic eruptions or elsewhere) moved into Ordovician rock shortly after the extinction pulses.
Dr. Dewing said that if one rock showed an anomalously high mercury signature, it might be cause for concern. But the team saw it in rock after rock after rock.
“It’s a very pronounced change right at that point,” Dr. Dewing said. “So it’s not just one bad data point.”
The new hypothesis points toward a number of tests that scientists can now undertake, like studying Ordovician rocks in other locations for the same signatures. That alone is a huge step beyond the global cooling hypothesis, which Dr. Dewing said “was almost more like a belief system.” In addition, scientists can attempt to pinpoint the volcanic region that dates back to that time (as they have done with other periods of mass extinction).
“The real smoking gun would be to find a big volcanic province,” said Paul Wignall, from the University of Leeds who was not involved in the study.
If the hypothesis holds, the first mass extinction will match many of the others. For some scientists, that only provides more impetus to better understand these events — which can no doubt yield further insight into anthropogenic impacts today as we also pump carbon dioxide and other greenhouse gases into the atmosphere.
Although it might sound mind-boggling, Dr. Finnegan argues that we’re releasing greenhouse gases at a rate that equals or exceeds these major extinction events.
“These are not worlds that you want to inhabit,” Dr. Finnegan said.