The Time It Rained for 2 Million Years - The Carnian Pluvial Event

In the centre of the arid and ancient  supercontinent Pangaea, thousands of miles away from the sea, our time travelling  aliens have returned to witness a key moment in Earth’s history; as they arrived the rain  began to fall. Just off Pangaea’s west coast, in what is now Canada, epoch-ending volcanic  activity set off a chain of events that all but made this downpour inevitable. It would never  look the same, because this was the start of a rain that wouldn’t abate for over one million  years.

A rain that changed the course of life on Earth. A rain that allowed the dinosaurs to  take over the World in an evolutionary coup. What’s all the more surprising to me, and to  our aliens who witnessed life on Earth develop, is that the kind of event that caused this rain  is, ordinarily, the most reliable and powerful extinction event the world has ever known. 

But this one… was different, one that takes the butterfly effect to its limits, imagine:  if a butterfly flapping its wings can cause a tornado 1000 miles away, what does an eruption  100 times larger than a super volcano cause? ! I’m Alex McColgan and you are watching  Astrum.

Join me today as we discover how an extinction-level event 230 million years  ago *increased* the richness of life on Earth and accelerated the evolution of the dinosaurs;  learning as we move through Earth’s major cycles how burning fossil fuels contributed to climate  change way ahead of the industrial revolution. BODY What came before the rain? Pangaea was the largest continent  that has ever existed on earth by a long way, a record not likely to ever be beaten, given it  was the size of every current continent combined!

Its huge size meant that the centre was far  removed from coastal climates, and it therefore received very little rain, favouring the evolution  of species that required less water to survive. During this dry period around 300 million years  ago in the Carboniferous period, several species emerged that are still important today, including:  dragonflies, millipedes and spiders. Throughout this period the diapsids also exploded, a  group containing lizards and snakes as well as… archosaurs.

Now you may not know that name,  but you certainly know what this group contains… crocodiles, birds and - yes! - eventually  dinosaurs. But, we’ll come back to those later… If you've seen our previous episodes  on Ancient Earth you'll know that it was a tumultuous and unforgiving  place, with impending threats left, right and centre.

And above and below for that  matter. While asteroids smashing into the crust better grab the attention of Hollywood, it’s  under the crust where the real danger is and has always been. And it’s here that we will find  answers to what caused a million year storm.

We live on a vanishingly thin crust  that is so shallow that if the earth was represented by the entire Lord of the  Rings book trilogy, the layer harbouring all of the known life in the entire universe  would be confined to just one single page. Beneath, hot plumes rise up from the core,  mushrooming as they rise and push molten magma up against the thin crust. These huge plumes  punch through the crust wherever they meet it, completely ignoring continental fault lines,  where earth’s modern volcanic activity is concentrated (like the Pacific ring of fire). 

These plumes can release magma at the surface for over one million years in what are  known as flood basalt eruptions. It is these eruptions that are linked to the most  incredible extinction events during Earth’s history and are the probable cause of  the most destructive extinction event in history “The Great Dying”. Where ocean  temperatures rose to 40 degrees celsius!

Despite life’s ability to evolve, it is estimated  that over ninety nine percent of every species that have ever lived on earth have gone extinct.  Of course, you can’t exactly evolve out the way of a Mt. Everest sized asteroid travelling  at 20 km/s, but these volcanic processes, though slower and far less dramatic, can cause  far greater devastation over a longer period.

Evidence of flood volcanism is  scattered across the world today. The eruptions of these flood basalts result in  the creation of huge unmistakeable swathes of land like the Siberian traps in Northern Russia,  the Deccan traps in Western India and the Wrangellian Large Igneous Province (LIP) across  Canada and Alaska. They are all cooled flows of basalt rock kilometres deep making them over  one hundred times larger than super volcanoes.

When we date these flood basalts, we see that  many of these eruptions align with mass extinction events. There is one though that doesn’t. That  is our rain-maker event, that triggered the so-called Carnian Pluvial Episode or “That time  it rained for over 1 million years’.

It’s believed that volcanic activity in the Wrangellian  province is responsible for this remarkable transformation of an arid desert into an oasis  that jump started the dinosaurs’ explosion. So what separated Wrangellia’s eruption from the  rest? What made it different?

Well, I’ve got news for you - size does matter. Although this was an  extinction event – with around 30% of the ocean’s species wiped out during the CPE – (Labelled  Carnian in blue green graph) Wrangellia’s eruption was just the right size to  give life an overall boost on Earth. So the reason that the overall biodiversity was  unchanged is because the level of extinction was matched by the emergence of exciting  new species more suited to this wetter world — what the aliens witnessed was less  an extinction event and more a reinvention period.

So how can a slow eruption affect  such incredible change to Earth’s climate? To understand how this transformational  event shuffled the deck of life on Earth is to understand something that we are  living through right now: climate change. Specifically the release and production of  carbon dioxide and the release of stored methane.

Eruptions like these directly release carbon  dioxide already contained within the mantle, but they can trigger its release from other  stores too, from a source of carbon I thought only humans had used — huge reservoirs of  fossil fuels. Now, whether you remember the Fire Triangle from school or not, (Fuel,  Heat and Oxygen) I think we can all appreciate that introducing 1600ºC magma to the base of  untapped virgin coal beds is going to get spicy. Burning these crude coal beds would have  released incredible amounts of particulates and greenhouse gases, both important for  driving cloud formation and rainfall.

Just as we are seeing today, the increasing  levels of those greenhouse gases trap more of the sun’s energy, and that energy  has to go somewhere. So where does it go? Earth’s systems work to distribute  that energy and the one best placed to absorb this extra energy is… the water  cycle, which becomes supercharged.

The sun’s energy is absorbed by the  land and sea, evaporating water from the surface. Once in the atmosphere, the  water can be carried great distances, before precipitating onto land and returning  to the sea along a meandering route. The more energy that is trapped by greenhouse gases,  the faster the water cycle turns over.

In these flood basalt eruptions we  see an extreme example of the complex interplay of three major cycles (the  geological, carbon and water cycle). The formation of the Wrangellian Large Igneous  Province would have released huge volumes of CO2; the alien’s ship detected atmospheric  levels exceeding 1000 ppm — 2. 5x what they are today — increasing temperatures by  3-4 ºC.

This supercharged the water cycle, greatly increasing evaporation and cloud  formation; these clouds were increasingly able to deliver rain further and further  into the centre of Pangaea. Throughout this period the Earth became warmer and more  humid, a dramatic change in the climate. Species that had adapted to  a particular dry climate, environment or niche before the rain  were put under stress from multiple fronts.

During this turnover period in Earth’s  history, the aliens watched as old niches were seemingly destroyed as quickly as new ones  were created. The status quo was changing. Not only that, but species had to cope with a  pH shift too.

Carbon dioxide wasn’t the only gas released by the Wrangellian eruption; hydrogen  sulphide gas erupted into the atmosphere along with it. This egg smelling gas reacted with oxygen  and water to form sulphuric acid, which in this form is more well known as acid rain. As the rain  fell, the soils and oceans became inundated with acidified water, which only further contributed to  the environmental stress some species were facing.

Interestingly, a large amount of amber can be  found in the geological record from the Carnian; amber is a protective mechanism for  trees that they release when in peril, suggesting that plant life came under  significant stress during this period, too. The incredible volume of rain across the  supercontinent resulted in deluges of surface runoff. Accelerating across  and through the arid terrain these slightly acidic flows eroded the land as it went.

Some of this acidic water seeped into and eroded  small fissures in limestone and dolomite rocks. The aliens watched as rocks were literally  dissolved in front of their eyes. Over time, elaborate new cave systems were formed,  like Britain’s Triassic caves carved from carboniferous limestone (Tyherington in  Gloustershire), which provided yet more unique habitats for life to exploit.

Eventually the  flow reached the sea, carrying the eroded rocks and minerals into the ocean, further nudging  the coastal ecosystems into new territory. The ocean species were particularly hard  hit and large areas became anoxic (meaning, they lacked oxygen) and highly acidified,  which was not suitable for the existing ecosystems that inhabit those areas.  Species like conodonts, ammonoids, crinoids and green algae suffered particularly  high extinction rates during the CPE, as did reef builders.

While dinoflagellates, a  constituent of today’s ocean planktons, thrived. On their ship, the aliens moaned in disbelief  every time the ship’s AI forecasted the ongoing miserable weather. But back on land one of  the oldest known dinosaurs, Herrerasaurus was braving the elements and roaming the Earth.

, some  6 metres in length and weighing more than 300 kg, it was an outlier prior to the Carnian  Pluvial Episode when smaller reptiles and mammals were far more numerous. Its home was  the Ischigualasto Formation, a volcanic floodplain defined by its dense jungle in what is South America today - a warm and humid environment, which the Herrerasaurus was well  adapted to. So when the rains began to fall, it won the geological lottery as its habitat  spread across Pangaea.

Advancing deeper into the continent, Herrerasaurus was greeted  with literal oases. Untouched habitat that with the extinction of herbivores and other  competition meant it was to be a boom time for the Herrerasaurus and similar dinosaurs.  We can only imagine the variety and richness of habitat available for these wandering species  to find and exploit and coevolve with over time.

Further away in the Dolomites the aliens saw herds  of large dinosaur creatures roaming the plains, and their wandering footsteps have been recorded  deep in the rock. Across a 3-4 million year period spanning the Carnian Pluvial Episode, dinosaurs  went from not featuring at all in the fossilised footprints to ecological dominance, making  up over 90% of the fossilised imprints. A remarkable takeover that agrees with other records  in other parts of Pangaea during this time period.

Notably the Central European Formation  and the Ischigualasto Basin in Argentina. The 2 million year period spanning the Carnian  Pluvial Event left its mark in indelible ways, forever changing the trajectory of life on Earth  as well as the passage of water across and through its surface. The mega-monsoonal climate featured  four distinct downpours, each carving a trace through the landscape, making the biblical storm  that remade the Earth in 40 days and nights look more like a typical British summer in comparison. 

But, the 5th period of rain never came. Once the Wrangellian eruption slowed, the emission of  greenhouse and acid forming gases slowed, and it is likely that levels of carbon dioxide fell  as it was consumed by the rich flora that covered Pangaea, some of which would again become buried  and form coal fossil fuel deposits for us to use today, continuing the carbon cycle. Now that  the storm has passed we can answer our question: “If a butterfly flapping its wings can cause a  tornado 1000 miles away, what does an eruption 100 times larger than a super volcano cause?

! ” -  A complete terraforming of land, sea and air. The aliens left having witnessed the origin of  the dinosaurs dominance and a remade Earth.

Some 180 million years after the extinction  event that gave Dinosaurs their big break, another would strike, wiping them from the  face of the Earth. For the Earth giveth, and the Earth taketh away. From that point, it  has been the mammals who have come to dominate, including us humans.

A remarkable reminder both  times of how much can change on a geological whim. But that at each time of asking, as  the rules change, life adapts and goes again.