Five major mass extinctions punctuate the history of life on Earth. The first is the Late Ordovician mass extinction, which began about 445 million years ago, triggered by a severe ice age and subsequent global warming that exterminated more than 85% of all marine species. Why the ice age that sparked the event was so drastic, however, is not clear. Recent research has suggested that large-scale volcanism before and during the extinction may be to blame, but evidence for volcanism has remained scant—until now. In a new study, researchers report the discovery of rock layers formed about the time of the extinction that are rich in the chemical element mercury, which they say is a telltale sign of volcanic activity.
Geochemist David Jones of Amherst College in Amherst, Mass., lead author of the new study, collected Late Ordovician rocks from the Monitor Range mountains in Nevada, as well as from Wangjiawan in south China. When he analyzed the rocks in his laboratory, he found mercury concentrations that shot through the roof around the time of the great die out. Because volcanoes are the largest natural source of mercury to Earth’s surface, discovering such a surge suggested that a connection may exist between the volcanism and the extinction.
“Our baseline values for the Nevada rocks are around 6 parts per billion,” Jones said. However, in rocks just preceding the extinction, mercury levels surge to “3000 parts per billion.” The same kind of surge occurs in the south China rocks, in which mercury levels reach far above an average concentration of about 50 to 100 parts per billion to almost 400 parts per billion, he added.
Studying spikes in mercury concentrations is one of the best ways to try and correlate volcanic activity with an extinction, particularly when there is a dearth of actual volcanic rock evidence in the rock record, explained Alyson Thibodeau, a geochemist at Dickinson College in Pennsylvania who was not involved with the study. Thibodeau said that Jones and his colleagues “are the first to identify mercury anomalies associated with this particular mass extinction.”
“It’s quite convincing,” agreed David Bond, a paleontologist at the University of Hull in the United Kingdom who was not involved in the work. For the first time, Bond said, scientists now have strong evidence for volcanism occurring just prior to and during the extinction. Before, the Late Ordovician mass extinction (LOME) was the only one of the “Big Five” extinctions not associated with volcanic activity. This is partly because the quality of the rock record worsens the farther back in time one goes. “There are no real remains of the volcanic rocks, so you have to look wider and farther afield for signs of volcanism,” like the mercury record, he said. The mercury concentrations detected by Jones and his team “are pretty big,” according to Bond. He added that the concentrations’ large magnitude “fits with a large igneous province,” or an LIP, which is an area of widespread volcanism.
Jones and his colleagues published their findings in the May issue of Geology.
Volcanism as an Extinction Trigger
It’s well known to geoscientists that LOME began during the Hirnantian age as large ice sheets grew over the single supercontinent that existed at that time, Gondwana, and reached their maximum extent between 445.2 and 443.8 million years ago. What’s more, those ice sheets dwarfed those of Earth’s most recent ice age, which reached its maximum extent about 20,000 years ago, Jones explained. “Geochemical evidence suggests that total ice volume during the Hirnantian glacial maximum was significantly more—perhaps fifty percent more—than the [most recent glaciation] when three to four-kilometer thick ice sheets existed in North America and northern Eurasia,” he wrote in an email. As sea surface temperatures in places like the tropics plunged by about 5°C, more than half of the animals in groups like brachiopods and trilobites died off.
Jones thinks that the volcanism for which he and his team have found evidence likely helped drive the glaciation. “The scenario that we’re suggesting is a one-two punch,” he said. Although volcanoes emit carbon dioxide, which, at sufficient concentrations, can warm the planet, Jones explained that previous modeling work suggests that weathering of volcanic rock can “lead to a drawdown of carbon dioxide in the atmosphere through the chemical weathering of that province.” This is thought to be the case not just for the LOME glaciers but for other glaciations, like the so-called Snowball Earth event wherein ice sheets covered much of the planet, Jones explained. In addition, volcanoes also emit sulfur dioxide gas into the atmosphere, which can form sulfate ions that scatter incoming solar radiation, cooling the planet in the process.
Unclear Link Between Volcanism and Cooling
Although other researchers warmly welcomed the mercury evidence of volcanism, the cooling part of the scenario has raised eyebrows.
“To be honest, I find the discussion here not hugely convincing,” said Seth Finnegan, a paleontologist at the University of California, Berkeley, who was not involved in the study. “There are a lot of effects that are being thrown around but not really quantified well, so I think it would be very useful to do some modeling.”
Bond said that he is also skeptical of that part of the paper. “Part of the issue is that sulfur dioxide, while it can drive global cooling, only does so over very short timescales: 5, 10 years—not the long geological timescales that carbon dioxide works on,” he said. Also, volcanism is often thought of as an agent of warming, so teasing out how it may also cause cooling is tricky, he explained.
Still, according to Jones, volcanism does not necessarily mean warming will ensue because how much carbon dioxide ancient volcanoes emitted into the atmosphere depended on how carbon-rich the sedimentary rocks surrounding the extruding lavas were. “The amount of carbon dioxide released would depend critically on the geology of the region in which the eruption occurred,” he wrote, because the lava would help release the gas into the air. Further study of the geology will help reveal whether or not this was the case.
However the volcanic cooling hypothesis pans out, the biggest revelation in the new work is that there is now evidence that each of the five major mass extinctions coincided with widespread volcanism, said Thibodeau. The new findings “may imply that a large igneous province is almost a precondition to have a mass extinction,” she said.
—Lucas Joel (email: [email protected]), Freelance Writer