Earth_Magazine_October_2017

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T

he end-Triassic mass extinction

exterminated up to three-quar-

ters of all species on land and

in the oceans 201 million

years ago. This die-off opened up ecolog-

ical niches and allowed for, among other

changes, dinosaurs to diversify and spread

across terrestrial ecosystems during the rest

of the Mesozoic. Volcanism has long been

implicated in the extinction, but whether

it had a major impact on the planet at the

time has remained unclear. In new research,

scientists observed elevated mercury con-

centrations in extinction-aged rocks from

around the world. Because volcanism is the

main nonanthropogenic source of mercury

in the environment, the findings suggest

that volcanic activity was likely the main

extinction trigger at the end of the Triassic.

“There was a big increase in volca-

nism happening precisely at the time of

this extinction,” says Lawrence Percival,

a recent University of Oxford doctoral

graduate and lead author of a new study

in Proceedings of the National Acad-

emy of Sciences. Volcanic rocks dating to

around the time of the extinction occur

in Africa, Europe and North and South

America. These rocks form the Central

Atlantic Magmatic Province (CAMP),

which covers roughly 7 million square

kilometers and which originally erupted

as Pangea was splitting and the Atlantic

Ocean began forming. The similar timing

of the extinction and the CAMP erup-

tions is well established, “but that on its

own doesn’t prove that the volcanism was

actually having any impact on the Earth

system,” Percival says.

To investigate the extinction trigger,

Percival and his co-authors examined

six different sedimentary sections — one

each in Argentina, Austria, Canada,

Greenland, Morocco and the U.K. — that

span the extinction boundary. Of those

sections, five showed spikes in mercury

levels, Percival says, with concentrations

increasing by a factor of five.

Within 100,000 to 200,000 years of

the first spike seen in the sediments,

which coincides with the extinction’s

onset, there were at least three addi-

tional “pulses” of increased mercury

concentrations. “This suggests there was

repeated, large-scale volcanism going

on,” Percival says. “The big spikes in

mercury also match evidence for a big

increase in [atmospheric] carbon diox-

ide,” he says, which “strongly supports

that the carbon dioxide increase at this

time was volcanically sourced.” This

pulsed volcanism likely led to prolonged

greenhouse gas-induced climate change,

Percival explains, which would have

made it harder for life to survive during

the extinction.

The new study offers “quite a com-

prehensive investigation of mercury

excursions associated with the end-Triassic

mass extinction,” says Alyson Thibodeau, a

geochemist at Dickinson College in Car-

lisle, Pa., who was not involved in the

work. “It was interesting that they looked

not only at marine sections, but also at

terrestrial aquatic environments.” Thibo-

deau says that finding the mercury spikes

in both types of environments implies that

the source of the mercury was “not just

some sort of environmental perturbation

that’s related to the ocean” alone, but that

it was likely widely deposited from the

atmosphere. That strengthens the case that

it is volcanic in origin because it means the

mercury occurred as a gas.

Also, because the mercury spike occurs

in marine sections, the CAMP eruptions

must have been large enough to impact

areas far from where the lava actually

flowed, says Paul Olsen, a paleontologist

at Columbia University who was not

involved in the study. The fact that the

eruption signal reached the marine realm

“is especially important,” Olsen says,

because it also enables researchers to tie

the marine extinctions to the eruptions.

The end-Triassic is not the only

extinction event associated with sedi-

mentary mercury spikes. For instance,

mercury spikes potentially related to vol-

canism also correspond with the timing of

the end-Ordovician extinction, Percival

notes. He says the next task is to see if

such mercury anomalies align with other

extinctions as well.

Lucas Joel

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