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That Time It Rained for Two Million Years

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May 22, 2018

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That Time It Rained for Two Million Years
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  • Imagine a world so warm that the ocean feels like a hot tub.
  • Huge volcanic eruptions have pumped the air full of globe-warming carbon dioxide.
  • And with the continents locked together from pole-to-pole in the supercontinent of Pangea,
  • the world is hot, flat, and very, very dry.
  • Early reptiles and mammal ancestors thrive in this sweltering land, dominating a landscape
  • that’s still struggling to recover from the Permian extinction.
  • This is Earth, 250 million years ago, at the beginning of the Triassic Period.
  • But then, starting around 234 million years ago, the climate suddenly changed, for the
  • wetter.
  • The rains finally came to this hot, dry world.
  • And then they stayed … for two million years.
  • This period of intense rain killed off many of the early reptiles and … confused the
  • heck out of the geologists who found the flood deposits millions of years later.
  • This time is known as the Carnian Pluvial Episode, and it set the stage for a new group
  • of animals to take over the world:
  • the dinosaurs.
  • Evidence of just how hot and dry the world was at the start of the Triassic is trapped
  • in the land beneath our feet.
  • Rocks from that period are mostly swaths of red sandstones and soil deposits from dry
  • woodlands, with no sign of the coal swamps that had covered the world during much of
  • the Permian.
  • And one major reason that the world was so dry was the shape of Pangea.
  • With all the continents locked together, rain clouds couldn’t move much past the coastlines,
  • and there were no big mountain ranges to break up the low, arid land.
  • Now, dinosaurs did exist in this dry, post-Permian world.
  • But they were still vying for their place among early reptiles and reptile-like mammal
  • ancestors.
  • The dominant carnivores back then were the early crurotarsans, a broad group of croc-like
  • animals that included reptiles like pseudosuchians and phytosaurs.
  • For example, there was Ornithosuchus, which had long hind legs and could actually stand
  • up when it wanted to run, which I imagine would’ve been both awesome and terrifying to
  • actually see.
  • But even stranger than these were the rhynchosaurs, herbivores with parrot-like beaks and, sometimes,
  • cheekbones to die for, all on a chubby lizard body.
  • And although there were no true mammals, there were dicynodonts -- the closest things to
  • them at the time.
  • They’re actually more closely related to us than, say, Dimetrodon, despite being scaly,
  • four-legged creatures with bills and tusks.
  • All of these fascinating creatures were widespread for most of the Triassic, with one species
  • of dicynodont – called Lystrosaurus – being so common all over the world that its fossils
  • were actually used to help construct the idea of Plate Tectonics
  • But these animals, adapted as they were to life in a dry climate, were in for a big shakeup.
  • Most of what we know about the history of the climate comes from plant fossils and rock
  • types.
  • And in the early 1990’s, two British geologists found rocks that didn’t match the dry climate
  • of the Triassic that they knew.
  • Instead of finding red, slowly-deposited sand, they found thick layers of river rocks, sediments
  • from giant lakes, and evidence of coal swamps.
  • All of these were signs of massive rainfall, over the course of some two million years.
  • But stranger still, these traces of a suddenly wet climate turned out to be everywhere, from
  • England to the Americas to Israel, in regions that were far apart at the time.
  • That meant the rise in rainfall must have been world-wide.
  • Initially, other geologists were skeptical.
  • Couldn’t these rocks just be explained by a lot of big local floods?
  • Well, over the next two decades, reports of more and more weird rocks kept trickling in,
  • and they kept pointing to a world that was getting wetter and wetter.
  • The rocks revealed coal deposits in Austria, traces of ancient lakes in Italy, wet soils
  • in Utah, and giant rivers in China.
  • And they all dated to the same window of time -- between 232 and 234 million years ago.
  • In time, this phenomenon came to be known as the Carnian Pluvial Episode, or CPE.
  • The Carnian is the name of the geologic age within the Triassic when this all happened.
  • And for what it's worth I seriously thought about naming this episode the Chronicles of Carnia
  • but I didn't because that would have been dumb.
  • And “pluvial” means rain, and it rained a whole awful lot.
  • For example, one estimate suggests that the average annual rainfall in what’s now Utah
  • almost quadrupled, reaching a peak of 1400 millimeters, or about 55 inches of rain a year.
  • For context, that’s how much that a temperate rainforest gets today, like say, in the Pacific
  • Northwest.
  • And this would have happened over, and over, and over again, all around the world.
  • It was not one big flood; it was more like floods every year, all over the place, for
  • two million years.
  • And with all this rain, things were bound to change - and one of the biggest changes
  • was the sudden abundance of dinosaurs.
  • In rock dated to the start of the Carnian Pluvial Episode, dinosaurs account for about
  • 5% of the fossils of terrestrial vertebrates.
  • But by the end, they make up more than 90% of those fossils.
  • So what made the dinosaurs so suddenly successful?
  • Were they better off than their competitors in this newly wet world?
  • Or did other animals simply die, leaving them to rule the world by default?
  • The key might not have been the rain itself, but what the rain brought with it: a proliferation
  • of giant plants!
  • During this time, we begin to see lots of large conifers, and big coal-forming plants,
  • like the primordial-looking Bennettitales.
  • For herbivores, this change in food supply could have been a game changer.
  • For example, rhynchosaurs were abundant, but they were also … short.
  • And they couldn’t stand on their hind legs to reach higher leaves.
  • This would’ve been fine in a dry environment, where plants tend to stay close to the ground.
  • But in a wet forest, rhynchosaurs would’ve only been able to eat smaller plants, or whatever
  • leaves and fruit fell to the ground.
  • Meanwhile, dicynodonts were herbivores, too, but they didn’t have teeth.
  • And, they also didn’t use gastroliths, the rocks that some animals - like birds - swallow
  • to help digest plant material.
  • Without teeth or gastroliths, dicynodonts would have had a hard time eating anything
  • fibrous, like wood.
  • And sure enough, fossils of dicynodont poop from this time have been found to contain
  • the digested remains of mostly soft ferns, with only very small amounts of wood.
  • By comparison, some plant-eating dinosaurs - which had both teeth and gastroliths - left
  • us poop fossils that are up to 85% wood!
  • Which is a lot of fiber.
  • So as the climate became wetter, soft small ferns were quickly replaced by tall woody
  • conifers, which the dicynodonts and rhynchosaurs didn’t eat.
  • And without the rhynchosaurs and dicynodonts, then the carnivores -- those crurotarsans
  • -- would’ve lost a lot of their food supply.
  • So maybe, instead of being better-adapted to this new environment, dinosaurs were just
  • the only major group of reptiles left standing.
  • Or, y’know, squatting.
  • Even though we don’t know exactly why the rain helped the dinosaurs, we do know that
  • dinosaurs became a lot more abundant during the Carnian Pluvial Episode.
  • And the dicynodonts, the rhynchosaurs and many of the early crurotarsans soon went extinct.
  • Now, there’s still the question of: What made it rain for two million years in the
  • first place?
  • Well, right before the rains came, some 235 million years ago, a huge burst of volcanic
  • activity took place in Alaska and British Columbia.
  • Today it’s known as the Wrangellian eruptions.
  • These eruptions lasted for more than 5 million years, churning out a layer of lava that got
  • to be 6 kilometers thick, and releasing enough CO2 to raise temperatures by about 3 to 10
  • degrees Celsius worldwide.
  • And, over a very long time, warmer temperatures can create a wetter climate, because they
  • can speed up the water cycle, driving more evaporation of surface water into the atmosphere,
  • among other things.
  • And in fact, because of this increase in atmospheric moisture, the CPE is also sometimes called
  • the Carnian Humid Episode.
  • But for what it’s worth, my favorite name for this episode by far is The Wet Intermezzo.
  • Which I think is delightful so let’s try to bring that term back, OK?
  • Anyway, after about a million years of volcanic activity, the atmosphere became so warm and
  • wet that rain could finally reach even Pangea’s vast interior.
  • OK but then, how did it stop?
  • Well, when carbon levels are really high for a really long time, our planet can be pretty
  • good at getting some of the extra carbon back into the ground where it belongs.
  • Plants take it in and store it in their tissues; weathering and eroding rocks absorb it; and
  • the oceans soak it up to form carbonate rocks like limestone.
  • So as the Wrangellian eruptions slowed down, the carbon cycle was eventually able to stabilize,
  • excess CO2 was reabsorbed from the atmosphere, and the CPE gradually came to an end.
  • By the time the eruptions had completely ended 230 million years ago, the world had returned
  • to a classically hot, dry Triassic climate that only ended when Pangea began to break
  • up.
  • But even though the Carnian Pluvial Episode was short - only 2 million years – its impacts
  • on life were permanent.
  • All that rainfall helped conifers spread and diversify, leading to the pine trees we know
  • today.
  • And while the start of the Triassic may have been the land of weird, croc-like-things running
  • on their hind legs by the time the rains had ended, the world
  • had fully entered the age of dinosaurs.
  • Thanks for joining me for this wet intermezzo!
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  • And if you haven’t already and I don't know why you wouldn't have
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At the beginning of the Triassic Period, with the continents locked together from pole-to-pole in the supercontinent of Pangea, the world is hot, flat, and very, very dry. But then 234 million years ago, the climate suddenly changed for the wetter.

Thanks as always to Nobumichi Tamura for allowing us to use his wonderful paleoart: http://spinops.blogspot.com/

Thanks to Franz Anthony, Julio Lacerda and Studio 252mya for their illustrations. You can find more of their work here:
Julio Lacerda: https://252mya.com/gallery/julio-lacerda
Franz Anthony: https://252mya.com/gallery/franz-anthony

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