95 million years ago, in the middle of the Cretaceous period,
a plume of magma rose up from the mantle and split open the Earth’s crust.
Massive amounts of lava spewed onto the seafloor in the area we
now know as the Caribbean…and it just didn’t stop.
With it came consequences for the entire planet.
This event, along with a smaller one in Madagascar, were the first steps of
a long chain reaction. They released enormous amounts of carbon dioxide into the atmosphere,
causing the temperature of the planet to suddenly shoot up.
And while the Earth has some powerful and pretty impressive ways to counteract
this dramatic shift in temperature, they come with tragic side effects.
So how did this eruption alter the planet, and how did scientists figure it out?
This is the story of how our planet rescued itself from extreme conditions,
at the cost of essentially suffocating the oceans for a half-a-million years.
As early as 1915, geologists searching for oil and gas learned to pay close attention
to the jet black sections of thinly layered rock known as black shales.
These layers have anywhere from 10 to 100 times more carbon than the rocks
immediately above or below them, so they’re rich sources of petroleum and natural gas.
But it wasn’t until the 1970s that geologists realized these layers all
over the world were formed by the same processes.
This means that these black shales weren’t just local deposits of carbon. They were remnants of
ancient catastrophes that had covered the entire planet…and they’d happened more than once.
And this breakthrough set off decades of investigation.
Where did all that carbon come from? How were those thin, delicate layers created?
And why do we sometimes see species of plankton going extinct right afterwards?
The most recent of these layers is the Cenomanian-Turonian boundary event, 94 million years ago.
This was just after those big volcanic eruptions in the Caribbean and Madagascar.
During this time, a type of eruption occurred called a flood basalt,
when mantle plumes literally flood parts of the surface of the Earth with lava.
And these areas are not small. Throughout the course of the eruption in the Caribbean, roughly
4 million cubic kilometers of lava erupted. That’s enough to fill the entire Mediterranean Sea.
But the impact of the lava itself was nothing compared
to the chain reaction of events it set off.
As the volcanoes released lava, they also spewed out carbon dioxide. The atmosphere
reached anywhere from 500 to over 3000 parts per million. For a somewhat worrying comparison,
all the carbon humans have emitted has got us to 417 parts per million as of 2022.
And we know this, in part, thanks to fossilized leaves.
When carbon dioxide is higher, plants develop fewer pores, called stomata, on their leaves.
So by counting these pores, scientists figured out
how high the volcanoes pushed carbon levels in the atmosphere at the time.
Now, the mid-Cretaceous was already warm,
but this rise in carbon dioxide caused temperatures to shoot up by 5 degrees Celsius,
to the highest levels the Earth had seen in the last 115 million years.
Even at the poles of the planet, the ocean reached a balmy 20 degrees Celsius.
Closer to the equator, temperatures hit 42 degrees Celsius. That’s warmer than a hot tub!
One group of fossils that tell this tale are called foraminifera.
Some species of forams have spiral shells that coil either to the left or to the right.
Scientists noticed those that coil towards the left tend to
live in colder water than those that coil towards the right.
So, researchers can use this preference to figure out how warm the water was millions
of years ago by counting how many of these tiny fossils coil each way.
We also know that, as the oceans heated up, things started to change on land, too.
Extreme heat speeds up the water cycle, causing more rain to fall on the ancient continents.
And with this, an important chemical reaction accelerated that would help
to balance out the increasing temperatures.
Now, carbon dioxide dissolves in rainwater, so when rain falls on land, it reacts with silicate
minerals in rocks like granite. This reaction, in turn, removes carbon from the atmosphere.
The process is called silicate weathering, and it speeds up when
there’s more carbon dioxide in the atmosphere and the planet is warmer.
So, by drawing down more carbon when the Earth
is warmer, this process ultimately regulates the planet’s temperature.
It’s sometimes called a ‘weathering thermostat,’ and it’s thought to be a big part of what
has kept life from completely freezing or burning up over the last few billion years.
But all that carbon has to go somewhere,
so it ends up dissolved in runoff that drains into rivers.
Along with it are other nutrients like phosphate that also erode more efficiently
when it's hotter. Eventually, this cocktail of nutrients and carbon flowed into the sea,
where it mixed with metals like iron released by the eruptions themselves.
And this abundance of nutrients meant that life in the Cretaceous oceans exploded.
This mix of chemicals fertilized massive plankton blooms and the
ancient waters turned into a warm colorful soup.
But it was too much of a good thing. The oceans
were well on their way to being thrown completely out of balance.
When all that plankton died, it sank down though the water and was eaten up by bacteria.
And all that bacteria consumed a ton of dissolved oxygen in the water.
And warmer water can’t hold as much dissolved oxygen in the first place,
so the ocean was already on the edge.
This small decrease in oxygen triggered a whole new set of processes.
Bacterial communities switched to relying on nitrogen,
and chemical reactions released phosphate from the sediments on the seafloor.
This gave even more nitrogen and phosphorus to the plankton,
causing more to grow at the surface, die, sink down, and push oxygen even lower.
This runaway cycle robbed the deep ocean of
most of its dissolved oxygen for at least half a million years.
This is known as an Ocean Anoxic Event, and it's what those layers of black shale record.
The layers preserve so much carbon because less oxygen meant that all the organic
material sinking down to the seafloor was better preserved – and there weren’t any
creatures living on the seafloor stirring up the sediment and eating up that carbon.
But half-a-million years with low oxygen had big consequences. 27% of invertebrates in the oceans
went extinct, especially those living in deeper waters that would have lost the most oxygen.
Many deep water mollusks, like some types of ammonites, went extinct,
as well as plankton, like some species of radiolarians and forams.
This was also the end for one group of bigger animals - the ichthyosaurs,
the big marine reptiles that had patrolled the depths of the seas for over 150 million years.
Overall, this was a huge tradeoff to stabilize the planet’s temperature. But it did work.
Over about 40,000 years, this process pulled a quarter of that carbon out of
the atmosphere and reduced the planet’s temperature by over 4 degrees celsius.
The Earth effectively took all the carbon that the volcanoes released
and funneled it into the ocean where it became the black shales we see today.
And we see this same pattern repeated throughout the geologic record.
For example, 252 million years ago, an eruption in Siberia caused ocean
anoxia that contributed to the end Permian extinction.
This was the biggest extinction in Earth’s history, killing 81%
of species in the ocean. It was the final nail in the coffin for
trilobites and wiped out an entire order of coral called tabulata.
Mass extinctions are almost always caused by many overlapping factors,
but time and time again throughout Earth’s history, we see flood basalt
eruptions wreaking havoc on the oceans and decimating the life within them.
And there is one other notable time when rapid rising carbon dioxide in
the atmosphere is warming the planet. Right now.
We currently are on track to reach Cretaceous levels
of carbon dioxide in the atmosphere by 2070.
As a result, we have already seen a decline of oxygen in the oceans. We may lose up to
7% of dissolved oxygen by 2100, mostly because warmer waters can’t hold as much.
But this isn’t on the same level as the anoxic events of the past, at least not yet.
Silicate weathering will pull the carbon dioxide we emit out of the atmosphere,
but it will take thousands to tens of thousands of years.
And there are far more urgent effects of the climate crisis that we will feel long before then.
So, the good news is we aren’t imminently heading for a global ocean anoxic event.
And the Earth will use its amazing ability to pull everything back into balance, eventually.
But it also means that it’s ultimately up to us to reduce our own eruption of
carbon dioxide as soon as we can. Because the past tells us that changing the carbon cycle
can have long lasting consequences. Even if the planet is able to adapt.
Hello Eons fans, it’s me, Hank.
Did you know the team that makes this show also makes another show about the
strange creatures that inhabit the earth? Except this time,
all of the creatures are currently alive and we have photos and video of them to prove it.
So join me and Sarah Suta as we introduce you to these bizarre
beasts and explore what makes these animals so weird to us.
From birds whose babies have claws on their wings, to frogs that can’t land their own jumps,
the show examines the how and why of some of the world's most amazingly strange critters.
So check out Bizarre Beasts at the link in the description!
So fossil leaves can give us clues about ancient climate, but what about the time before leaves evolved?
Check out our episode, “When it was Too Hot for Leaves”, to learn more.
And thanks to this month’s Eontologists who take our breath away!
Jake Hart, Raphael Haase, Annie & Eric Higgins, John Davison Ng,
Melanie Lam Carnevale, Addie and Tony Dai.
By becoming an Eonite at patreon.com/eons,
you can get fun perks like submitting a joke for me to read! Here’s one from Kai.
A T rex and a velociraptor are sitting at a bar. The velociraptor points to a triceratops in the
corner and says "Why is he the first to get served?" The T rex says "Because he was herbivorous."
Herbivorous? Here...
Here first?
Like, is that supposed to be "here first"?
[Kallie gasps] "Here-be-fore-us"!
Oh! Ohhhhhh...
That was a sneaky one. That was a sneaky - "here-be-fore-us", oh...
[claps hands]
Round of applause, round of applause! [laughs]
And as always thank you for joining me in the Adam Lowe studio. Subscribe
at youtube.com/eons for more Cretaceous creatures.
Closer to the equator, temperatures hit 42 degrees Celsius. That’s warmer than a hot tub!
[in disgust] Geeehhh, geeeehhhh...
That sounds awful.