Volcanic gas bursts probably didn’t kill off the dinosaurs


Massive gas bursts emitted by
volcanoes about 66 million years ago probably couldn’t have caused a mass
extinction event that spelled doom for all nonbird dinosaurs, new research suggests.

Data on ancient temperatures,
combined with simulations of the shifting carbon cycle in the ocean, lend support to the hypothesis that a giant asteroid impact — not toxic gases emitted by Deccan Traps eruption —
was primarily responsible for the die-off,
researchers report January 17 in Science.

About three-quarters of
Earth’s plant and animal species were killed off during the extinction event at
the end of the Cretaceous Period. Sediment deposits linked to the giant asteroid impact, which struck Chicxulub in what now Mexico’s Yucatan
peninsula, form a layer known as the “KPg” boundary. This boundary marks the
transition from the Cretaceous to the Paleogene Period, and implicates the
asteroid strike in the extinction event (SN: 1/25/17).

But the Deccan Traps
eruptions, which spewed as much as 500,000 cubic kilometers of lava across much
of what’s now western India, also occurred within a million years of the
extinction. Sussing out the true killer has been challenging, because the precise
timing of the Deccan Traps eruptions has been uncertain. Scientists previously have
focused on dating the rocks — either zircon crystals embedded within ash layers between flows of lava (SN: 12/11/14), or outcrops of the lava itself
(SN: 2/21/19). Those efforts have
resulted in a range of different dates for the eruptions, some before and some
after the extinction.

Furthermore, the real dino
killer wouldn’t have been the lava — it would have been the volcanic gases: carbon
dioxide heating the planet or sulfur dioxide acidifying the oceans. “It’s the
outgassing that’s important, but it’s really hard to pin that down,” says
Pincelli Hull, a paleoceanographer at Yale University. 

Sudden, huge bursts of CO2
and sulfur dioxide have climate-altering power — and could have come
either from the asteroid impact or from the eruptions (SN: 11/2/17). So pinning down the timing
of Deccan Traps outgassing could help solve the longstanding debate.

Hull and her colleagues
turned to the temperature record preserved in sediment cores from the bottom of
the ocean, and created a timeline of global temperature shifts spanning several
hundred thousand years before, during and after the extinction event. The
researchers then examined five different scenarios for when the Deccan Traps
may have erupted, and compared them with the known temperature data.

Only two of the scenarios matched
the observed temperature data, Hull’s team found— and neither scenario could
have caused the extinction. In one scenario, the bulk of the eruptions occurred
several hundred thousand years before the KPg, causing a blip of intense
warming that was over long before the actual die-off. In the second scenario,
half of the eruptions occurred before the KPg, and half after. But the
temperature data suggest that any climate-altering impact immediately after the
KPg would largely have been muted by shifts in the ocean carbon cycle.

Those shifts are related to calcareous
plankton, tiny, floating, carbonate shell-building creatures. Plankton arose
during the Mesozoic Era, but became especially abundant when the Cretaceous began
about 145 million years ago. They were so ubiquitous, in fact, that their life
cycle — building their shells using dissolved calcium carbonate in the seawater,
then dying and sinking to the seafloor — profoundly altered the ocean’s carbon
cycle during the Cretaceous. The sinking shells were responsible for as much as
half of the global transfer of carbon from the surface ocean to the deep during
the Cretaceous, keeping the carbon cycle humming.

But the KPg extinction wiped
out almost all plankton, so the dissolved calcium carbonate stayed where it was
in the surface ocean waters. Calcium carbonate is also a powerful buffer
against acidification, which occurs when the ocean absorbs excess CO2 from
the atmosphere. So even if the second scenario does explain the die-off, and
the volcanoes emitted more CO2 after the extinction, the oceans
likely neutralized a lot of it, muting its effect on global temperatures.

In other words, Hull says,
“the mass extinction so affected global ecosystems that it hid the outgassing.”

The new study “used really
unique methods to try to answer that question” about what caused the mass
extinction event, says Courtney Sprain, a geoscientist at the University of
Florida in Gainesville. “Their conclusions [about the timing of the outgassing]
make sense.”

That doesn’t necessarily
mean that previous geochemical dates determined for the Deccan Traps lava flows
were incorrect, Sprain notes. The same technological advances that made
high-precision dating of the Deccan Traps possible were also responsible for
revealing that there might have been a lag between lava flows and outgassing,
she says.

Understanding why there
might be a timing difference between lava and gas bursts is an active area of
research in volcanology, she says. “There are still quite a few questions about
how any volcanic system erupts.”


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