Click here to see the full view of Equation 2 for this article.
Credit: Friedemann Freund, SETI Institute.
Living on a planet with an oxygen-rich atmosphere we tend to forget that our planet is an anomaly.
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 oxidizing power.
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 photo?synthesis: 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 photo?synthesis, 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 photo?synthesis 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 H?2O?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 state, O2-.
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?? ?? O22- + H2
In 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, O2:
O3Si-OO-SiO3? ?? O3Si-O-SiO3 + ? 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 re-applying stress.
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:
H2O2 ? H2O + ? 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? ?? 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.
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