Sky & Telescope - October 2021 - 21

JOANNA MORGAN (2)
" In every single section you look at
all around the world, life is happy
before the impact and then it's
clearly devastated. " -Joanna Morgan
K-Pg boundary. That theory was put forth again in 2014 when
physicists suggested that dark matter could have dislodged a
comet and again this year when a different team argued the
impactor was a fragment from an icy comet that flew too
close to the Sun.
But these nuances aside, the impact theory is taught in
astronomy and geology courses alike, and it appears in science
museums across the globe and even in children's books.
It is so well known, in fact, that if you were to take an informal
poll at a party, everyone would likely say that it was an
asteroid that spelled doom for the dinosaurs.
" The asteroid hypothesis is pretty much guaranteed at this
point, " Gulick says.
Fire and Ash
And yet there are scientists who deny the accepted dogma.
When Gerta Keller (Princeton University) peers into the geologic
record, she doesn't see a knife-sharp die-off coincident
with the asteroid impact. Rather, she finds a gradual decline
of fossils that began when volcanic eruptions across modernday
India belched greenhouse gases into the atmosphere.
The steady change appears across the globe. In the late
1980s, Keller analyzed rock sections in Tunisia and Texas to
find that some populations of single-celled marine organisms
called foraminifera started to decline 300,000 to 400,000
years before the impact. Her team released a similar story
in 2012 when they discovered that plankton suffered severe
losses early on. And in 2014, a separate team of scientists saw
that some populations of amphibians at a site in Montana
also dwindled before the asteroid struck.
" It may be presented as a closed book, but it's definitely
not, " says Courtney Sprain (University of Florida), who later
worked on the Montana samples.
The culprit is obvious to Keller. Travel to India and you will
see soaring plateaus and river canyons carved out of basalt -
volcanic deposits known as the Deccan Traps that erupted at
the end of the Cretaceous. In 2008, Keller's team determined
that the lava flows preceded the extinction, but their exact
timing has long been a crucial question. So from 2013 to
2016, Keller and her colleagues made several trips to Pune,
India, where roads have been carved into the Traps - revealing
layer atop layer of lava flows and providing a peek into
their history. Her team cut samples from the lithified windborne
ash that was occasionally sandwiched between the lava
layers and sent the rocks home.
Back in the lab, they dated crystals within the ash to find
that there were four distinct pulses of eruptions, with the
biggest occurring merely tens of thousands of years before
the extinction. Those results, published in 2019, help to paint
a different picture of the extinction event. Add in both the
fact that species were clearly stressed during that time and a
discovery in 2018 that the global temperature increased once
the Deccan Traps kicked into action, and it looks like the
dinosaurs' fate was sealed long before the asteroid bore down
on Earth.
It sounds wild, but it isn't unprecedented. On four other
occasions, animal life has been nearly destroyed in planetwide
extinctions - all of which potentially came after volcanic
eruptions.
IMPACT DEBRIS
Taken from about
650 meters below
the seafloor in the
Gulf of Mexico, this
core contains suevite,
a rock composed
of fragments of
limestone and granite
as well as impactmade
melt, all mixed
together when the
crater formed.
DRILLING IN CHICXULUB Researchers used the liftboat Myrtle
to drill into Chicxulub Crater in 2016. In this location, the ocean
is about 18 meters deep; the boat was raised about 15 meters
above water level, to shield it from buffets by currents and tides
during drilling.
sk yand tele scope .o r g * OCTOBER 2021 21
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Sky & Telescope - October 2021

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