Geoscience Reference
In-Depth Information
past. If one were to mix river waters from different parts of the world
together into some sort of broad average, though, then one would see a
general pattern. For a start, there are almost no rivers that are anywhere
near as salty as the sea. The salt contents are typically less than 1 per
cent of that of seawater. Freshwater may not be pure, but it is nowhere
near a brine. That is understandable. That river water derives, more or
less directly, from water that has evaporated from the sea—and when
even a concentrated brine evaporates, virtually all that goes up into the
air is simply . . . water—in effect distilled water. That is the starting point,
to which the acids and dissolved minerals are added.
So—is river water simply dilute seawater? It is not close to being even
that. Sodium and chloride make up some 85 per cent of the salts in
seawater, as we have seen. In river water, together, they amount to less
than 15 per cent. Bicarbonate makes up half a per cent of seawater salt—
but nearly one-third of the dissolved solids in river water. There's 1 per
cent or so of calcium in seawater, but over 15 per cent in river water.
And so on. The chemistry of seawater and river waters are very unlike
each other—and yet the rivers are the main source of salts to the sea.
This puzzling circle can be squared—and relatively easily, too. One
just needs to consider what happens to those salts when they reach
the sea. For they can have many different kinds of journey within the
ocean waters. Each type of dissolved substance has its own trajectory,
and it is that which gives us our familiar seawater. Chemistry is one
control here, while life is another. We can turn to these first, because
they have come to work steadily and systematically, and it is nice to
see the world work in such an ordered fashion (later we will talk of
how saltiness also relies upon tectonic accidents).
The Removal
It is what goes out, quite as much as what goes in, which controls the
saltiness of the sea. For some elements simply do not dissolve easily,
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