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  • So you just clicked on this video, you saw the title - you probably think you already

  • know the answer to this question.

  • But as it often turns out, the science is more complicated than what we have been told.

  • First, let's take a look at the story we've heard, and why we're so sure that an asteroid

  • impact was a big part of the dinosaurs not existing anymore.

  • [Intro]

  • I'm not gonna lie to you.

  • It was a really bad day.

  • Some time around 66 million years ago, a giant asteroid about the size of Manhattan careened

  • into Earth.

  • It smashed into the ocean on the shores of what we now think of as the Gulf of Mexico

  • and caused lasting damage to the planet.

  • The energy of the impact itself vaporized every living thing within a day's walk.

  • Then huge chunks of the meteorite that had been thrown up into the atmosphere started

  • raining back down as molten glass.

  • The ensuing cloud of ash and dust that was vaulted into the atmosphere obscured the sun

  • for decades, creating a dim, cold climate plagued by acid rain and firestorms.

  • Over the millennia that followed, at least 50% of the species on Earth disappeared - 20%

  • of the world's species of sharks, rays, and skates; nearly 60% of the plants; 98% of the

  • warm-water corals; and of course, 100% of the dinosaurs.

  • For most of us, that collision has become a kind of shorthand explanation for why the

  • dinosaurs disappeared, and because we associate asteroids with extinction, it's a scenario

  • that still scares us.

  • But as our knowledge expands, we've come to see that what killed the dinosaurs, like all

  • of the other great extinctions that happened before, was probably a lot more complex than

  • just a big rock falling into the ocean.

  • Because it turns out, there was more than one disaster movie playing at the cineplex

  • that was Earth 66 million years ago.

  • It was such a calamity that it literally, in geological terms, marked the end of an

  • era.

  • The dinosaur dominated period of life that we knew as the Cretaceous Era was suddenly

  • over.

  • The new normal, marked by the rise of mammals, would be called the Paleogene Era.

  • The giant die-out that created this transition is now known as the Cretaceous-Palogene Extinction

  • Event, or the K-Pg for short.

  • And like many things in science, there's not a whole lot of consensus about what caused

  • the K-Pg, but there are a few facts that everyone agrees on.

  • First, about 66 million years ago, the climate changed, a lot, all over the world.

  • We can tell by changes in the types of marine fossils from both before and after the event,

  • along with differences in soil chemistry and other evidence that the climate became much

  • cooler really fast - by 7 degrees Celsius in some places.

  • The transition was so abrupt that the layer of rock that separates the before picture

  • from the after picture is easy to spot with a well-trained eye.

  • It's known as the Cretaceous-Palogene Boundary, and the chemistry of this boundary of rock

  • is itself another weird clue, because it shows up all over the world in the same way - a

  • sudden appearance of a layer of clay that's rich in iridium.

  • Iridium is a metal, and if you've never heard of it, that's because it's very rare on earth.

  • We tend to find it most often in meteorites and other celestial deposits.

  • The last clue that everyone agrees on is simply extinction itself - tons of living things

  • just disappear from the fossil record.

  • Tyrannosaurus Rex, the biggest land predator at the time, Bennettitales, a group of tall

  • plants with thick armored trunks, and whole passel of reptiles that lived alongside the

  • dinosaurs and are still sometimes mistaken for them.

  • Aquatic reptiles like mosasaurs and plesiosaurs, and pterosaurs - those freaky flying reptiles

  • - all of them, gone.

  • So the reason that the Asteroid Impact Theory is so compelling is that it explains pretty

  • much all of those things.

  • The theory first came about because of that iridium.

  • In 1980 geologist Walter Alvarez was studying clay deposits in Italy that dated back about

  • 65 million years and found that they contained hundreds of times more iridium than normal,

  • and the more of that stratum of the earth that he sampled in different parts of the

  • world, the more iridium he found.

  • Since iridium is rare on earth but plentiful in space-rocks, Alvarez and his father - Nobel

  • prize winning physicist Luis Alvarez - hypothesized that it was a sign of an enormous asteroid

  • impact.

  • Based on the amount of iridium they'd found, they figured that the collision was caused

  • by a giant metallic rock, about 10 kilometers across - big enough create an impact with

  • the energy of 180,000,000 megatons of TNT.

  • Clearly, it was a game changer.

  • Alvarez and son theorized that this collision must have been what wiped out the dinosaurs.

  • Soon, scientists stared finding other signs of a colossal asteroid impact - like shocked

  • quartz.

  • At K-Pg sites around the world, the quartz looked like it had been transformed - its

  • crystals rearranged by tremendous pressure that you'd never suspect to find on earth's

  • surface.

  • Then they found tektites - big tear-dropped shaped pieces of glass that form when rocks

  • are thrown into the atmosphere by an impact and then melt on re-entry.

  • So all the signs pointed towards a huge, ugly collision.

  • But there was one thing missing - a crater!

  • Scientists new about a lot of different impact craters around the world, but none of them

  • matched the magnitude of the one that the Alvarezes envisioned.

  • Then researchers started noticing that a lot of the evidence they were studying - the iridium,

  • the quartz, the glass - was all concentrated in North America, so they focused their search,

  • using new technology, like satellite imagery and scans that measure the density of rock

  • beneath the surface, and in 1991 they found what they were looking for - a giant crater

  • just off the coast of Mexico's Yucatan Peninsula, about 180 kilometers across - about the size

  • you'd expect a 10 kilometer asteroid to make, and marked by huge disruptions in the layers

  • of rock at about 66 million years ago.

  • It was named after the nearby town of Chicxulub, and that is how the biggest piece of the puzzle

  • came together - the Asteroid Impact Theory, also known as the Alvarez Hypothesis.

  • So, like I said, this theory checks off a lot of the boxes when it comes to the evidence

  • we have.

  • Explains the abundance of iridium in the rocks?

  • Check!

  • Accounts for the drastic and sudden change in climate?

  • Check!

  • Was capable of killing 50% of life on earth?

  • Double-Check!

  • So the rest is history - by which I mean it became accepted as historical fact, and it's

  • the quick and easy answer to the question of what killed the dinosaurs.

  • Now, you're not going to find many scientists who say that there was never any impact or

  • that the theory is totally off-base, but there are many who think that there were other factors

  • at work and that the extinction might well have started before, and the asteroid came

  • to finish the job.

  • It turns out, for instance, that the impact occurred in the middle of the most severe

  • volcanic activity Earth has ever seen.

  • India is home to one of the largest lava flows in the world, with volcanic rock covering

  • an area the size of France and, at some places, nearly two and half kilometers thick.

  • These formations are known as the Deccan Traps, and they were formed by a series of mind-blowingly

  • huge eruptions that scientists think began about 500,000 to a million years before the

  • Yucatan Impact, and kept going for hundreds of thousands of years afterward.

  • Such violent, prolonged eruptions would have had an unmistakable effect on the climate,

  • sending ash and dust into the atmosphere, obscuring sunlight, killing vegetation, and

  • altering weather patterns around the world.

  • And, sure enough, fossils show changes in things like microscopic marine life that suggest

  • a steep dive in temperature before the impact by as much as five to eight degrees Celsius.

  • Now the big-picture impacts of this kind of climate change, whether it was caused by volcanoes

  • or an asteroid, are probably pretty obvious, but the thing is we're not sure how exactly

  • this led to the dinosaur wipe-out.

  • For instance, despite the fact that we think of dinosaurs as reptiles, they probably weren't

  • cold blooded, but from what we can tell about their physiology, they weren't warm blooded

  • like modern birds either.

  • So they might have been somewhere in between, or different types of dinosaurs could have

  • had different types of metabolisms.

  • In either case, they weren't able to adapt as well or as quickly to changing temperatures

  • as mammals or other kinds of animals.

  • Another problem dinosaurs likely suffered was simply their own success.

  • They were by far the largest animals on land, and lots of fossil beds suggest that sometimes

  • they lived in dense populations.

  • So dinosaurs were probably competing with each other for food, water, and territory

  • by the time the curtain started to fall.

  • Throw in a tiny little variable, like a 7 degree change in temperature, and you can

  • see how things would fall apart pretty quickly.

  • While some dinosaurs might have given in pretty quickly to the cold, the plant eaters may

  • have dwindled more slowly as huge swathes of plant life started to disappear, and then

  • without any herbivores to prey on, the meat eaters began to vanish.

  • But that's just one scenario; in addition to volcanoes exploding and space rocks hurtling

  • toward us, it also seems that the oceans were disappearing, not because of changes in climate

  • but because of changes in the plates of the Earth's crust.

  • Again, about 65 million years ago, the usually dynamic rifts where plates meet under the

  • ocean floor began to stagnate.

  • It's like the plates just stopped moving, and once they stopped, those rifts began to

  • settle, sinking down into the earth's mantle.

  • As the sea floor dropped, ocean levels dropped with them.

  • This phenomenon, known as marine regression, completely changed the map of the Earth.

  • Inland seas, like the one that used to run through the middle of North America disappeared,

  • and more than 28 million square kilometers of land suddenly became exposed in less than

  • 100,000 years.

  • Marine regressions don't happen often, but when they do, they are usually linked to extinction

  • events because not only do they remodel life on land, turning shorelines into planes and

  • lakes into deserts, they also wreck everything for aquatic life.

  • Marine regressions cause big changes in ocean depth and temperature and can create huge

  • oxygen-poor dead zones.

  • This is probably why fossils show enormous die-offs in plankton, for instance, tens of

  • thousands of years before the Chicxulub impact.

  • With these centers of the food web gone, it makes sense that other saltwater creatures

  • like sharks and rays and corals, and reptiles like mosasaurs and plesiosaurs, would not

  • be far behind.

  • Again, it could have played out in a number of ways, and in a way, it's kind of fun to

  • consider all the different scenarios.

  • Like if the Chicxulub impact never happened, the world would be a very different place1

  • We might still have a world with pterosaurs and armored trees and me riding a triceratops

  • at the rodeo.

  • I definitely wouldn't do that, let's be honest.

  • I don't even want to ride a horse, a tame, docile, old horse, I'm like "Oww, no."

  • But if there were no asteroid, a lot of dinosaurs and marine life would probably be gone anyway.

  • In that case, the Chicxulub impact may have just been the last nail in the coffin - something

  • that turned an already really hard day into one of the worst days in the history of the

  • planet.

  • The point is, we still don't have one single answer to what caused the K-Pg extinction

  • event, though we generally talk about it like we do.

  • But the more clues we tease out of the ground, the better we'll be able to understand what

  • can cause such a diverse group of animals that once ruled the earth to all but disappear.

  • Thanks for watching this SciShow Infusion, especially to our Subbable subscribers.

  • To learn how you can support us in exploring the world, just go to subbable.com/scishow.

  • And as always, if you want to keep getting smarter with us, just go to youtube.com/scishow

  • and subscribe!

So you just clicked on this video, you saw the title - you probably think you already

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是什么真正杀死了恐龙?(What Really Killed the Dinosaurs?)

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    joey joey 發佈於 2021 年 04 月 27 日
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