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  • Living creatures are amazing at building their homes from just about anything.

  • But sea-dwelling creatures are particular wizards: microscopic coccolithophores, coral-building

  • algae, and giant snails engineer their own building material, like magic, by pulling

  • two dissolved chemicals -- calcium and carbonate -- out of the water to form solid shells of,

  • surprise, calcium carbonate.

  • The reason those shells don't dissolve back into calcium and carbonate as soon as they're

  • built is that ocean water is already holding as much calcium and carbonate as it can, so

  • the mineral forms much more easily than it dissolves.

  • At least, that's the way it works near the surface, where the shell-builders live.

  • But at greater depths, water isn't quite as saturated with calcium and carbonate, and

  • thus calcium carbonate is easier to dissolve.

  • So, unlike shallow coastal waters where shells of dead creatures build up on the seafloor,

  • out in the deep ocean, there's a depth at which calcium carbonate starts to break apart

  • and empty shells dissolve before reaching the bottom.

  • This "dissolving depth" depends on the concentration of calcium and carbonate already in seawater

  • - if the concentration is high, shells sink deeper before their calcium carbonate dissolves.

  • And if the concentration is low, the "dissolving depth" moves closer to the surface, meaning

  • the deepest intact shells begin to dissolve.

  • But this is a feedback loop - shells that dissolve add more calcium carbonate to the

  • water, making it harder for other shells to dissolve and lowering the "dissolving depth".

  • Basically, chemistry in the deep ocean stabilizes the concentrations of calcium and carbonate

  • in seawater, which is why the upper part of the ocean is saturated with calcium and carbonate

  • (and perfect for shell-building) to begin with.

  • Except, we forgot to take into account the chemistry of another key part of the ocean

  • - the atmosphere!

  • At the ocean's surface, a small proportion of gases like oxygen and carbon dioxide dissolve

  • into the water (dissolved oxygen, for example, allows sea creatures to breathe).

  • And when the concentration of the gases in the atmosphere rises or falls, so does the

  • amount of gas dissolved in the oceans.

  • If it weren't for the ocean's own balancing act, any incoming carbon dioxide would be

  • bad news for shell builders, because more CO2 means less CO3.

  • That might sound weird, but it's just the way the chemistry plays out: dissolved CO2

  • molecules combine with water to form what's called carbonic acid, which in turn combines

  • with carbonate to form hydrogen carbonate.

  • Simply put, when carbon dioxide in the atmosphere increases, carbonate in the ocean decreases

  • and shell-building gets harder to do - at least for a moment.

  • Given enough time, the physics and chemistry of the ocean will cause the "dissolving depth"

  • to rise, and more shells on the sea floor will return their calcium and carbonate back

  • to the water, restoring normal levels.

  • There are situations where the oceans can't keep up this balancing act, though.

  • For example, if so much carbon dioxide were added to the ocean that the dissolving depth

  • rose high enough, all shells everywhere in the ocean might start dissolving.

  • While possible, this is a lot less pressing than the risk that, for a time, CO2 levels

  • change faster than the ocean can compensate, so that even if it would eventually stabilize

  • and allow shell formation at the surface, it would take centuries to do so.

  • During that time, the upper reaches of the ocean, where most of the amazing shell-builders

  • live, might become a barren wasteland.

  • And speaking shellfishly, THAT would be

  • a clamity.

Living creatures are amazing at building their homes from just about anything.

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如何制作贝壳-只需加水!(How to Make a Seashell - Just Add Water!)

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    joey joey 發佈於 2021 年 04 月 30 日
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