The Early History of Atmospheric Oxygen: Geological Evidence - Oxygen: A Four Billion Year History - page 98

metallic iron is rarely found at the Earth surface, so we won't consider
it any further. Let's go into the backyard and check on the pyrite cube
(from chapter 5 ) that we purposefully left out in the rain. I don't want
to give too much away, but if it has been there a while, we should see
reddish-brownish iron minerals forming on the surface. This is basically
rust. Most iron at the Earth surface either has become, or wants to be,
rust in the presence of oxygen (think about a car after 5 to 10 years of
salted winter roads). Iron in this form is said to be oxidized, which in
chemical terms means that, compared to the metallic form, each atom
has given up three electrons. This is ferric iron and it is written as Fe 3+ .
From our perspective here, we can think of it as: oxygen + iron = rust,
and rust is immobile.
My grandparents used to have a cottage on the shore of Lake Michi-
gan, which was supplied with water from a deep well. We were told not
to drink the water, which is strong encouragement for a 7-year-old boy
to drink up. Anyway, the water had a particular metallic flavor that
many of you probably know. I'd place a glass of freshly collected water
on the table, and within minutes, brownish rust began to form on the
sides of the glass. The metallic flavor was from iron in its reduced form,
which from a chemical perspective, means that for every atom, it has
one more electron than iron in the oxidized form. This is ferrous iron
(we met ferrous iron first in chapter 2) and it is written as Fe 2+ . Ferrous
iron is soluble and quite mobile in water, but as my childhood experi-
ment also demonstrated, ferrous iron will react with oxygen to form in-
soluble rust.
This is the key to understanding BIFs. Their massive scale and fine
laminations mean that the iron was transported in soluble form through
the ocean depths, and hence in the absence of oxygen. As this ferrous
iron was mixed into the upper layers of the ocean it was oxidized, per-
haps by the same phototrophic iron-oxidizing bacteria we met in chap-
ter 2, or perhaps by small amounts of oxygen in the surface waters pro-
duced by cyanobacteria (if they were present). To be honest, we don't
really know for sure how it was oxidized, but the net result was the rain
of insoluble iron oxides to the ocean floor, and the subsequent for-
mation of BIFs. We will explore this in more detail in chapter 9 , but
the anoxic conditions required for the deep-ocean transport of ferrous