The Worst Day in Earth’s History Contains an Ominous Warning
Robinson Meyer
5 days ago
©
NASA / Reuters
The
worst day in the history of life on Earth, so far, happened almost
exactly 66 million years ago, when an asteroid roughly the size of
Manhattan slammed into the Yucatán Peninsula.
You
may know the story. The asteroid—which arrived, probably,
in June or July—immediately drilled a 20-mile hole into the
planet’s surface, vaporizing bedrock and ejecting it halfway to
the moon. The planet shuddered with magnitude-12 earthquakes,
loosing tsunamis across the Gulf of Mexico. Some of the ejected
debris condensed in orbit and plunged back to Earth as searing
spheres of molten glass, which torched the land and turned forests
into firestorms. Other debris remained high in space, where it
blocked the sun’s rays and began to chill the surface of the
planet.
By
the time it was all over, about 75 percent of all species on Earth
had died, including all non-avian dinosaurs. The event, which ended
the Cretaceous Period and began the Tertiary Period, is named the
K-T extinction.
Since
1980, when the K-T impact hypothesis was first
proposed,
the Day the Dinosaurs Died has attained almost mythic significance.
But questions remain about the theory. None of the Earth’s other
big mass extinctions were caused by asteroid impacts. Why did this
one end the 180-million-year reign of the dinosaurs?
A
new paper,
published this week in the Proceedings
of the National Academy of Sciences,
offers a possible answer: The impact changed the chemical content
of the ocean, rendering seawater more acidic and inhospitable to
the tiny plankton that form the base of the marine food chain.
Combined with the other impacts of the asteroid—darkened skies
and a snap of global cooling—this ecologic disruption doomed much
of life on Earth.
The
finding may be satisfying for asteroid fans, but it is an ominous
one. Ocean acidification, a hallmark of the planet’s previous
mass extinctions, is happening again today.
How
does an asteroid prompt an extinction? It chooses the right
location. The Yucatán Peninsula was an excellent one, says
Pincelli
Hull,
an author of the paper and a geology professor at Yale. The
peninsula is essentially an “old buried reef,” she told me, an
accumulation of dead coral and other sea life that is now more than
a mile thick. When the asteroid hit, untold megatons of that old
organic material—rich in nitrogen and sulfur—instantly became
dust and shot up into the atmosphere.
Soon
it began to rain down, falling now as nitric oxide and sulfuric
acid. “It was raining brimstone and acid from the sky,” Hull
said. The air would have reeked of acrid smog and burnt matches. As
the acid fell, it accumulated in the oceans, wearing away the
shells of the small, delicate plankton that serve as the basis of
the marine food chain. Within a few centuries of the impact, ocean
acidity had spiked by at least 0.3 pH units.
This
spike in ocean acidification may have lasted for less than 1,000
years. But even that pulse “was long enough to kill off entire
ecosystems for sure,” Hull said. Ocean acidification also likely
worsened other sweeping environmental changes wrought by the
impact, such as the years-long darkness caused by orbiting debris
and ash from the global wildfires.
With
this new finding, it now appears that all three of the worst mass
extinctions in Earth’s history featured huge spasms of ocean
acidification. They include the K-T extinction; the
End-Triassic Extinction,
when volcanoes in New Jersey killed 75 percent of all species; and
the
dread End-Permian Event,
the worst extinction in the history of the planet, which killed
roughly 85 percent of all species and nearly sterilized the oceans.
Scientists call that event “the Great Dying.”
And
that pattern is worrying, because the oceans are acidifying again
today. Carbon dioxide—the same air pollutant that causes global
warming—also dissolves in the oceans and increases the acidity of
seawater. Since the late 1980s, the planet’s oceans have become
about 0.02 pH units more acidic every decade, according
to a report last month
from the Intergovernmental Panel on Climate Change. More than a
fifth of all modern carbon pollution has already dissolved into the
oceans, the report also found.
©
Provided by Atlantic Media, Inc.
The researchers collected more than 7,000 of these tiny
fossils—each half the size of a grain of sand—from
Geulhemmerberg Cave in the Netherlands, the only place on Earth
that contains fossils from the oceans in the decades and centuries
after the K-T impact. (Michael J. Henehan / PNAS)
Modern
acidification is not yet at the same magnitude as the K-T pulse.
It’s “moving toward that scale, but it’s not quite there
yet,” Hull said. What unites our world and the K-T period, she
said, was that a number of environmental catastrophes can overlap
with ocean acidification to produce a major upheaval.
“You
should think of [ocean acidification] as the straw that broke the
camel’s back” during the K-T extinction, she said. “It’s
dark, it’s really cold after the impact—and the ocean has
acidified.”
Chris
Lowery, who studies the oceans of the past at the University of
Texas at Austin, told me that the paper represents “a big leap”
in our understanding of the extinction. “We’ve known for a
while that there was some amount of ocean acidification due to the
Chicxulub impact, but this is the first time that the acidification
has actually been quantified,” he said in an email, referring to
the town in the Yucatán Peninsula for which the impact crater is
named.
While
paleontologists have long hypothesized about how an asteroid impact
could
produce
the K-T extinction, this is some of the first evidence that
supports those mechanisms, he added. And while the impact struck
Mexico, the crucial evidence for this study came from a cave in the
Netherlands that preserves fossils from the oceans in the decades
or centuries immediately after the impact. The scientist Michael
Henehan,
then a postdoc in Hull’s lab, collected more than 7,000 tiny
plankton fossils from the cave—each half the size of a grain of
sand—and crushed them to analyze their chemical signatures.
“It
was a herculean effort to get these measurements,” Hull said.
“There’s just one place in the world where we think these
fossil preserved.” (Henehan is now a professor at the German
Research Center for Geosciences.)
Two
years ago, another
study
found the first geological evidence of global cooling following the
impact, another proposed mechanism.
Notably,
the study’s findings do not support the idea that enormous
eruptions from volcanoes in modern-day India, called the Deccan
Traps, prompted the surge in ocean acidification and resulting mass
extinction. That hypothesis has a small number of ardent advocates,
among them Gerta Keller, a geologist at Princeton who was the
subject of a profile in this magazine last year.
But
“this study pretty definitively shows that those eruptions had no
effect on ocean chemistry,” Lowery said. In an email, Keller
disputed the paper’s dating of the impact, arguing the asteroid
actually struck Earth “over 100,000 years” prior to the
extinction’s start.
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