About 4.5 billion years ago, when the solar
system accreted out of a disk of gas and dust, the Earth was thoroughly
reduced. Over the course of the first 1-2 billion years our planet became
slowly, but inextricably ever more oxidized. Vast amounts of iron rich
sediments precipitated out of the oceans, known as "Banded Iron Formations" or
BIFs, indicating that reduced ferrous iron, Fe2+, converted into
ferric iron, Fe3+. This required a large, sustained supply of
In his 1984 book, "The Chemical Evolution of the Atmosphere
and Oceans," H. D. Holland estimated that, over the 2+ billion years during which the
BIFs precipitated, at least 1012 gram oxygen had to be injected into
the Earth's oceans every year. Sometime around 2.4 to 2.3 billion years ago,
the global oxidation accelerated. During this remarkable period, known as the
"Great Oxidation Event," free O2 appeared in Earth's atmosphere and
soon increased to the over 20% O2, which we now enjoy.
The Great Oxidation Event is attributed to the
invention of photosynthesis: the capacity of living organisms, using sunlight,
to split H2O and CO2 into O2 plus reduced H
and C, which in turn combine to produce organic compounds. New forms of life
appeared that could harness the newly available chemical energy: first
microbial and later multi-cellular organisms prospered in the oceans and
eventually conquered the land.
If the Great Oxidation Event can be linked to
oxygenic photosynthesis, the question remains what process might have driven
the earlier slow oxidation of Earth.
One school of thought has promoted the idea that
some form of oxygenic photosynthesis was invented very early on, soon after
the origin of life. Maybe colonies of photosynthetic bacteria, similar to
today's cyanobacteria, were building stromatolites in shallow waters along the
coasts of early continents, pumping out enough oxygen to precipitate the BIFs
and prepare the way for the stupendous rise of free O2 in Earth's
atmosphere during the Great Oxidation Event.
The "invention" of oxygenic photosynthesis so
early in Earth's history poses serious problems. Oxygen is one of the most
reactive elements in nature, and is toxic to life adapted to reducing
environments. Before pumping out oxygen as part of their metabolism, the
microorganisms must have learned how to handle this dangerous by-product of
their cellular biochemistry and how to extract the energy. They must have
learned how to detoxify those Reactive Oxygen Species, commonly referred to in
microbiologists' circles as ROS, which are the scourge of all forms of life.
One possible solution to this dilemma is that,
long before the Great Oxidation Event, the Earth might have been slowly
oxidized by some non-biological process. Such a process would have given the
microorganisms time to adapt to the changing environment or, as Dr. Lynn
Rothschild of the NASA Ames Research Center said, "It would have provided a
training ground for early life to learn how to handle oxygen."
Indeed, such a non-biological process exists.
When rocks crystallize from magmas that contain dissolved gases, mostly H2O,
or when minerals re-crystallize at high temperatures in H2O–laden
environments, water becomes an impurity in their crystal matrices, usually in
the form of hydroxyl, OH–. Even minerals that do not normally contain
hydroxyl invariably take up small amounts of water giving O3Si-OH,
more generally O3X-OH, where X can be Si4+, Al3+,
etc. Most of those will occur in the form of O3X-OH OH-XO3 pairs.
In the Earth Sciences redox reactions are
broadly discussed, usually involving transition metal cations that change their
valence states such as Fe2+ oxidizing to Fe3+. Redox
reactions involving anions are also quite popular such as reactions with sulfur
that can change from sulfide, S2-, to sulfate, SO42-,
where sulfur is in the valence state S6+. However, for some unknown
reason, oxygen anions are always considered to be frozen into their 2– valence
Years ago, while studying impurity hydroxyls in
MgO, I discovered an unusual redox reaction that involves OH– pairs:
during cooling OH– pairs in the MgO matrix change into peroxy
anions, O22–, plus H2, as indicated in Equation
1. In other words, two oxygens change their valence from 2- to 1-, meaning
that they become oxidized, while two protons become reduced to molecular H2:
OH– + OH– ó O22- + H2
subsequent years it became clear that hydroxyls in silicate minerals, also due
to some "water" being incorporated during crystallization or
re-crystallization, undergo the same type of redox reaction, oxidizing two
oxygens to the peroxy state while splitting off an H2 molecule, as depicted in this
graphic of Equation 2.
H2 is capable of diffusing away over time, even escaping to grain boundaries and
beyond. Thus, an interesting situation arises: Rocks that contain minerals with
impurity hydroxyl – essentially any rock – will acquire peroxy as a memory of
their solute H2O content. A peroxy, however, is nothing but an extra
O atom stored in the mineral structure, equivalent to half an oxygen molecule,
O3Si-OO-SiO3 ó O3Si-O-SiO3 + ½ O2
There are numerous consequences. One is rooted
in semiconductor physics. A peroxy is composed of two O–, which are
tightly bound together and inactive for all practical purposes. However, when a
peroxy bond breaks, the rock becomes a semiconductor. The reason is that an O– in a matrix of O2- is a defect electron or "hole". It is associated
with energy levels in the valence band of the otherwise insulating silicate
minerals. All mineral grains in a rock that are in physical contact with others
are also in electric contact, as far as their valence bands are concerned. In
other words, a hole in any given mineral grain in a rock is able to pass to any
neighboring grains. In fact, the holes associated with O– states
have been shown in the laboratory to travel through meters of rock as well as
through sand and soil. There is little doubt that these electronic charge
carriers are able to travel large distances through the Earth's crust, through
tens of kilometers at least.
All that is needed for these electric currents
to start flowing are: (i) a process to break the peroxy bonds, (ii) a pathway
for the charge carriers to flow.
It has been shown that stressing rocks causes peroxy
bonds to break and to release hole charge carriers that travel fast and far. This
photograph shows a 4-meter long piece of granite squeezed at one end. Upon
running a wire from the stressed end to the front end, where a copper electrode
is attached, a current of about 1 nanoampere is obtained. This current runs
along the stress gradient for hours, even days, as long as the load on the rock
is kept constant. Taking off the load causes the current to fade. Re-stressing
the rock causes to the current to come back. The process can be repeated many
times. The rock is a battery that is charged by stress and can be recharged by
When we replace the copper contact with a water
bath, into which we introduce a copper electrode, the same current flows. Using
a slab of gabbro, a rock mineralogically similar to basalt, we measure a
current on the order of 100 nanoamperes. It has been flowing for over 4 weeks
without loosing more than 30% of its initial strength. However, with water, we
see a new reaction: the holes that flow through the rock and pass through the
rock-water interface oxidize water to hydrogen peroxide, H2O to H2O2.
The reaction is quantitative, generating one H2O2 molecule
for every two hole charge carriers that cross the rock-water interface.
What does this mean for the early Earth? The
geological literature provides convincing evidence that our home planet has
been tectonically active since the earliest times. There must have been
plenty of tectonic stresses acting on the rocks that built the continents.
There must have been plenty of stress gradients, along which the same type of
hole currents were flowing that we can now demonstrate in laboratory
experiments. Wherever these currents crossed rock-water interfaces, for
instance along continental margins at subduction zones or other
mountain-building regions, water must have been oxized to hydrogen peroxide,
which in turn decomposes rapidly into water plus oxygen:
H2O2o H2O + ½ O2
This electrochemical oxidation of water must
have helped our planet to become ever more oxidized, contributing to the early
slow oxidation of Earth.
Yet, electrochemical oxidation of water was
surely not the only reaction that pushed the early Earth toward an ever higher
degree of oxidation. Global weathering has to be taken into consideration, too.
Weathering is a powerful process that dissolves rocks and wears down mountains.
Today about 3 km3 of rocks pass through the global weathering cycle
every year. When the Earth was young, the continents were bare and the rain was
more acid than today due to the higher CO2 content in the early
atmosphere. Hence, weathering rates must have been higher too, say 10 km3 per year. When weathering eats into a rock, water hydrolyzes the peroxy and
produces hydrogen peroxide, even if the H2 molecules formed
according to equation 5 still linger around:
O3Si-OO-SiO3 + 2 H2O o O3Si-OH + OH-SiO3 + H2O2
If we take a conservative estimate for the
average peroxy content in rocks, 300 parts per million, the amount of H2O2 released globally at a weathering rate of 10 km3 per year translates
into 1013 grams per year. This is 10x the amount that H. D. Holland
estimated to be necessary to precipitate the BIFs.
Thus we come to the tentative conclusion that,
through weathering and electrochemistry, peroxy in rocks provided enough
oxidation power to change the course of our planet's history. Over the course
of 1.5 to 2 billion years, peroxy forced the early Earth to slowly but
inextricably become ever more oxidized. Along the way dangerous Reactive
Oxygen Species, constantly produced at rock-water interfaces and during peroxy
hydrolysis, challenged the early microbes, archaea and bacteria. As Dr. Rothschild
so aptly put it, the ROS might have provided a "training ground" for those
early micro-organisms to learn how to deal with oxygen. They developed the
basic enzymatic defenses, which our bodies still use today to fend off the
detrimental side effects of our oxygen-based metabolism.
Thus, while the Earth was still overwhelmingly
reduced, eukaryotes joined the archaea and bacteria. Under the onslaught of
those ROS, the eukaryotes "learned" how to survive in an oxygen-spiked
environment long before free O2 gas appeared in Earth's atmosphere.
At some point the eukaryotes learned how to take advantage of the large
chemical energy that oxygen can provide. They adapted to do oxygenic
photosynthesis, to tap the energy in O2. This lead to the Great
Oxidation Event and to plenty of free O2 in Earth's atmosphere that
made our planet livable for us...and it all started with water and a little-known
solid state reaction in the rocks.