How to Make a Seashell – Just Add Water!

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.

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