The Salt of the Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 87

Precambrian. The mass crystallization of sulphides was a very effec-
tive way of cleaning iron out of the seawater—and not only iron, but
trace elements such as molybdenum, zinc, and copper. All became
part of the ocean-wide pyrite rain. All of these elements, too, are essen-
tial nutrients to living cells, being vital components of certain complex
macromolecules of the cell machinery. And so it has been speculated
that sulphidic oceans were also nutrient-starved oceans (no matter
how much carbon might have been around). In such a view, this long-
lived chemical environment acted as a brake on biological evolution
for that billion years before oxygen managed to build up to sufficient
levels to seep into the depths of the ocean. When that happened, a little
over a billion years ago, sulphate became stable throughout most of
the water column, and the throttling of essential trace elements
stopped, allowing life to progress once more. A modern ocean, chemi-
cally speaking, had been ushered in (see Chapter 6).
Sulphidic oceans have virtually vanished from the world today,
with one significant exception: the Black Sea. This almost completely
separated arm of the Mediterranean Sea is too isolated and too deep
to be affected by the currents that stir oxygen into the bulk of the
world ocean. Below the sunlit surface, teeming with fish, most of the
2-kilometre-deep column of water is stagnant and poisonous, being
charged with hydrogen sulphide. It is also something of a magnet for
trace elements. Despite being only a tiny fraction of the area of the
global ocean, the Black Sea may be responsible for as much as half of
the total removal of molybdenum from seawater—and thus vividly
demonstrates the far-reaching effects of a change in a single chemical
parameter, that of the availability of free oxygen.
The saltiness of the ocean, then, has not been a constant of the
world ocean, nor something that has simply been building up through
geological time, nor a one-dimensional phenomenon that simply
reflects the amount of dissolved 'salt'. Rather, it has been a protean