Geoscience Reference
In-Depth Information
Chemical Lifetimes
There are those substances that, unlike iron or gold, can be dissolved
in enormous amounts within seawater. We have the chemical giants
of the ocean, chloride and sodium, and, some way behind, potassium,
magnesium, and sulphate. Water can absorb immense amounts of
these substances, much more than the modest amounts of carbonate
and the tiny amounts of iron and gold. Far more, in fact than there are
in today's seas: think of the landlocked Dead Sea, so dense with dis-
solved salts that it is impossible to sink, but difficult to swim. The
question of why the oceans are not more like the Dead Sea is some-
thing we will come to shortly.
The compounds one can make of these ions, such as sodium and
potassium chloride, are so soluble that no organism can find a means
of extracting these from the seawater to form a skeleton, despite the
abundance of such potential raw materials in water. Bones of com-
mon salt would simply dissolve away, and no sophisticated cellular
biochemistry has been evolved that might prevent this.
These ions therefore, once in the water, simply stay there—for a
very long time. There is a concept that can be used here that is as vivid
as it is useful. It is called the residence time , and it simply means the
average time a particle of anything stays within a particular system.
Thus, the residence time of an ion of calcium say, in the ocean, once
washed in by rivers, is about a million years. That is quite a long
time—by human standards certainly—but sooner or later the cal-
cium will combine with another ion, often of carbonate (perhaps
within a coral polyp or the tissues of a mollusc), to form part of a
calcium carbonate crystal, and thus be taken out of the seawater sys-
tem. As such, the average calcium ion in the oceans has been there
since before the human species evolved, and since the times when
mammoths and woolly rhinos roamed the Earth.
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