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.