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55 million years ago, just 10 million years after the
extinction of nearly all the dinosaurs, Earth’s average
temperature shot up, and fast. This warm period is known as the Paleocene-Eocene
Thermal Maximum, or PETM. We know it was caused
by a massive amount of carbon entering Earth’s atmosphere, and that this warming
happened over a comparatively short time. So, the PETM is the closest
thing we have in Earth’s history to the modern climate crisis.
Except, instead of all that carbon coming from human
activity, a 2022 study suggests it could have come from an
entire continent splitting apart. The PETM has been a big mystery
for paleoclimatologists who are scientists who
study the climate in Earth’s past. During this event, temperatures
at the oceans’ surface suddenly shot up by about
5 degrees Celsius on average, and the ocean acidified to the
point that many deep-sea species were wiped out.
It’s one of the most extreme heating events in Earth’s
history, and while scientists have been studying it for
decades, they still don’t know definitively what caused it.
They do know that the warming was driven by extra carbon in the atmosphere, and especially extra
carbon dioxide.
That’s because the oceans both warmed and acidified.
And while many gases can warm the planet, only CO2 dissolves in the ocean to make it more acidic.
But it would’ve taken more than just like a quick burst of
gas to cause this kind of warming. In fact, researchers
calculated that it would have taken 10 to 15 trillion tonnes
of carbon. That’s four to six times
more than what humans have released since 1850.
And what we’ve done already, here, is enough to start wreaking havoc on the planet.
Now, the cause of the modern climate crisis is clear and
straightforward: Our world revolves around fossil fuels, and
burning them releases gasses that warm the Earth. But 55 million years ago,
it’s not like ancient birds were sitting around burning tons of oil and coal.
Instead, for that much carbon to enter the atmosphere
naturally, scientists know it had to come from multiple
sources. They’re just not sure what all of them are.
They do know that volcanoes were part of it, but the total
amount of volcanism happening at this time only accounts
for about a fifth of the carbon. So, the bulk of it had
to come from somewhere else. And a study published in 2022 might have found where.
In their paper, researchers looked at volcanic rocks from
sediment cores: cylindrical samples of material drilled
from the seafloor. The material in these
cores forms as sediment from the water’s surface drifts down to the
seafloor and builds up over time. So,
they tell us what sorts of material was floating around the oceans as far back as
millions of years ago.
These cores in particular were taken from the North
Atlantic Ocean in the 1980s. But they had basically been
sitting around in a warehouse since then, waiting to be
analyzed. And when the COVID-19 pandemic shut down most
normal research, it was finally their time to shine. Scientists
took the opportunity to dive into the archives to dust off
these samples. They found that
the core sections that came from around the time of the PETM were full of ash
deposits, indicating a lot of volcanism. Now
that sounds a lot like support for the theory that the PETM was caused by volcanoes.
But when they looked at the chemistry of lava deposits that
occurred around the same time as the ash deposits, they
found something strange about the chemistry of the lava.
Lava deposits from around the same time as the PETM
were oddly high in elements like titanium and magnesium.
Eventually, these elements disappeared from the cores
and the rest of the lava deposits from this time looked
more like normal lava deposits. But that initial spike of high
magnesium, high titanium lava led the researchers to think that
some of the volcanism might have come from
a different source than normal. And that made them look at the subcontinental
lithospheric mantle, which is a deep layer of the earth
between the crust and mantle. This layer is high in carbonates,
minerals with a high amount of carbon and oxygen in them.
But here’s the weird thing: This layer of the crust normally
exists way deeper in the planet than we can get to with
conventional tools. So, for these metals
to have made it to the sediment cores, something must have been happening to take those
carbonates from deep in the Earth, and throw them all the
way up to the surface, where they could eventually drift
down to the seafloor. And scientists think
this happened during one particular, dramatic geological event: the break-up of a
supercontinent. Before the continents
got to their modern positions, all the land on Earth was bunched up into one enormous
supercontinent, called Pangea. Eventually, it split apart in three
stages. The last stage formed what’s now
the North Atlantic Ocean, and that occurred at roughly the same time as the PETM.
This was a violent time, and material from deep in the
planet could’ve been brought up to the surface, including
those carbonates. And if that happened, a lot of carbon
would have been released into the air. Because these minerals
formed at super high pressures deep in the Earth. So, when they reached lower
pressures at the surface, they would’ve broken apart,
releasing that CO2. The release of carbon
dioxide would have been short-lived, like a really big burp. But in
that time, a lot of carbon would have gotten into the atmosphere.
If this is true, it could explain where a lot of the carbon
released during the PETM came from! That said, other researchers not
involved with the study were quick to point out
that the PETM was still likely caused by more than one event.
So, while the breakup of Pangea could have been a significant factor, other effects
might have put even more carbon in the atmosphere.
For example, as the planet warmed, frozen soils called
permafrosts could’ve started melting, releasing their
stored carbon into the air. But this research adds another
piece to the story. And with Earth’s climate changing rapidly,
and scientists doing their best to learn what this
means for our future, understanding extreme warming events like the PETM
is more important than ever.
This kind of climate research can involve some really big
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