The Salt of the Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 73

Then there is the skeleton-former of choice for molluscs, corals,
stony algae, sea butterflies, and those giant armoured protozoans the
foraminifera that live both at the sea surface and on the sea floor. This
is calcium carbonate, and its wholesale extraction from the sea floor
by these organisms forms, and buries, enormous masses of limestone,
thus taking its ingredients—calcium and carbon—out of the water
and converting them into rock. The chemistry may be simple enough,
but the architecture of these calcium carbonate shells and skeletons is
dazzlingly sophisticated. The elegant shape of a scallop, cowrie shell,
or coral that one can appreciate with the naked eye is beautifully engi-
neered in itself. But take a scanning electron microscope to a tiny por-
tion of such a shell, or to the whole skeleton of a foraminifer no bigger
than a sand grain, and magnify it a few thousand times. Then there
appear marvellously shaped and interlaced crystals of the shell min-
eral encased (in life) in an organic matrix. More finely sculpted and
assembled than a Fabergé egg, each is a reminder of the almost story-
book complexity encapsulated within, seemingly, the most common
and humble of biological constructions.
This particular import-export pathway is modulated by life—but
is not dependent on it. So much calcium and carbonate is carried by
rivers into the sea that the bulk of the ocean waters are saturated in
calcium carbonate, and it can precipitate out of the seawater of its
own accord. On a hot sunny day off the Bahamas, for instance, espe-
cially if a drying wind is blowing, the surface of the sea can suddenly
turn milky white. 'Whitings', as these are called locally, are made of
millions of tiny needle-like crystals of calcium carbonate that have
spontaneously precipitated out of the seawater—they then drift
slowly to the sea floor to form a layer of white carbonate mud. If our
planet was suddenly turned lifeless, whitings would be forming all the
time as part of the natural regulation of the calcium carbonate com-
ponent of the saltiness of the oceans.