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photosynthesis by marine cyanobacteria—but iron oxide deposition
started in the sea over a billion years before the Great Oxidation Event
saw oxygen enter the atmosphere, to rust and redden terrestrial land
surfaces. The formation of these enormous, ancient iron deposits is
therefore now thought to relate to a primitive form of photosynthesis
that did not produce oxygen. Nevertheless, it was an effective means
of slowly beginning to take out some of the enormous amounts of
iron that had accumulated in the ocean waters (see Chapter 6).
The Great Oxidation Event, which began at about 2.4 billion years
ago, removed yet more dissolved iron from the sea, but this was not
so much by simple rust formation. A more complex mechanism
swung into action—one mediated by sulphur. The oxidation of the
land surface also oxidized sulphur compounds in the rocks, and for
the first time sulphate ions began to be washed into, and accumulate
within, the ocean waters.
At the surface of those oceans, where the water was oxygen-rich,
the sulphates remained as they were. However, in the large volumes
of oxygen-starved water below the sulphate ions were converted
into sulphide ions by bacteria as these stripped away the oxygen
atoms from the sulphur (sulphate is, after oxygen, the energy source
of choice for bacteria). The sulphide then combined with dissolved
iron to form minute crystals of iron sulphide, otherwise known as
the mineral pyrite (which also has the popular title of fool's gold). It
is certainly a beautiful mineral, with its golden sheen, and it typi-
cally crystallizes in the water column as tiny, elegant, raspberry-
shaped clusters of micro-crystals called framboids. These fall in
countless numbers on to the sea floor, taking the iron and the
sulphur with them.
This particular type of ocean—known as a sulphidic ocean—seems
to have been the predominant ocean between about 2.2 and 1.3 bil-
lion years ago—that is, for much of the Proterozoic Eon of the
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