The Salt of the Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 85

the early oceans altered them fundamentally. Over a billion years and
more, the early Earth possessed bizarre iron seas and sulphide seas.
When those were swept away, our planet—as almost every animal and
plant would agree—became a better place.
Iron Seas, Sulphide Seas
There are different kinds of saltiness. The world's oceans, today, have
a chemistry dominated by sodium, potassium, chloride, and sulphate.
However, if you travel in desert regions and stumble across dried-up
lakes there you may likely find their saltiness to be quite different, as
in the bitter lakes where sodium sulphate is dominant because the
rivers that feed them bring in specific, local chemistries.
You don't usually have to travel so far to see a different kind of salti-
ness. Take a walk in coal-mining districts, or in an area with thick
peat bogs, and you will likely see springs here and there bringing
water that, when it reaches the surface, becomes orange, opaque, and
slimy: rather unpleasant, in short. This water is iron-rich, and you are
looking at a faint echo of the Earth's earliest oceans.
Water that contains no oxygen—water that was typical of the first
2 billion years of the Earth's oceans—can contain large amounts of
dissolved iron, many times more than can be found in both typical
freshwater and seawater in today's oxygen-charged world. When
oxygen comes into contact with such water, the iron immediately
forms an insoluble hydroxide—a particle of rust—and becomes a
part of that orange slimy stuff you can see in the iron-polluted
streams.
Early oceans contained iron in abundance, and on the sea floors
from early Archaean times, more than 3 billion years ago, thick layers
of iron oxides accumulated that today form giant iron ore deposits
which provide nearly all of the iron and steel that we use today. It used
to be thought that this was a reflection of early oxygen-producing