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.