Various indicators, such as the presence of detrital uranium oxide and iron sulphide grains in sediments older than about 2.3 Ga and the appearance of terrestrial sediments stained red by the presence of ferric (Fe-3) oxides thereafter, have long been used to suggest that atmospheric oxygen was a mere trace before that time. Generation of oxygen through photosynthesis by simple organisms, principally blue-green bacteria, could have led to an oxygenated atmosphere when their productivity exceeded the tendency for oxygen to be consumed by reaction with reducing agents, such as abundant ferrous (Fe-2) iron in sea water, and by burial of carbon-rich dead organic matter. That method is a central plank in the Gaia hypothesis. However, geochemical considerations suggest another scenario for oxygenation (Catling, D.C., Zahnla, K.J. and McKay, C.P. 2001. Biogenic methane, hydrogen escape, and the irreversible oxidation of early Earth. Science, v. 293, p, 839-843). Unless carbon burial exceeded the rate at which reductants supplied to the outer Earth (including exposure of buried carbonaceous sediments) by geological processes consumed oxygen, the atmosphere would remain low in oxygen.
Lacking in oxygen, the early atmosphere would have been able to support build-up of methane from biogenic processes – today methane is soon oxidized to carbon dioxide and water. Carbon isotope evidence suggests that early life was dominated by methanogens, and such organisms alive today are genetically very primitive. Consequently, methane is a good candidate for keeping average surface temperature above the freezing point of water at a time when the Sun’s output of energy was considerably lower than it is now. All hydrogen-bearing compounds become dissociated high in the atmosphere, to release hydrogen atoms, and they readily escape the Earth’s gravitational pull. Fortunately, this does not happen now because the only significant H-compound, water, cannot rise above the tropopause. The decline in temperature upwards acts as a cold trap for water. Were this boundary not in place, and it is largely due to the presence of ozone in the stratosphere which absorbs radiation to give higher-level warming, Earth would long ago have lost most of its water, as did Mars and Venus. In the early atmosphere, methane would not have been “cold trapped”, and nor is it today. So, during that period, hydrogen would steadily have leaked from the Earth.
The chemical outcome of such a simple process would have been a steady decline in the reducing capacity of the Earth as a whole, for hydrogen is a powerful reductant. Because most of our planet’s hydrogen was locked in water from the time of its accretion, its escape must have resulted in a net gain of oxygen somewhere in the Earth system. Increased methane productivity by methanogen bacteria during the Archaean and early Proterozoic would have enhanced this tendency for the whole Earth to become more oxidizing. Catling et al. argue that the continental crust became more oxidized, so that any gases released from it by metamorphism would become less reducing. That would have reduced the tendency for immediate consumption of oxygen produced by photosynthetic organisms, culminating in its eventual ability to exist in the atmosphere in balance with biological processes at around 2.3 Ga.
Zircons’ window on the Hadean
The oldest tangible rocks that are not completely changed by deep-crustal metamorphism are those of Isua in West Greenland. Interleaved with gneisses that originated probably from calc-alkaline intrusions are rocks formed at the Earth’s surface around 3.8 Ga ago. The general scene represented by this Akilia Association is in many respects familiar – the operation of plate tectonics, rapid generation of what was to become continental crust, abundant evidence for the action of liquid water and even the isotopic traces of living organisms. That 750 Ma after the Earth’s accretion the last two were present is no surprise. The oddity is that, despite decades of effort, there is still no sign of continents older than 4 Ga. That crustal rocks which had undergone considerable evolution from their mantle source did exist in the missing half-billion years emerged from the discovery of detrital zircons as old as 4.4 Ga in much younger Australian sedimentary rocks. Some of the rare, tiny grains show isotopic evidence that the magmas in which they formed had contact with liquid water at the surface.
As well as containing sufficient uranium to allow the dating of single grains by the U-Pb method, zircons also contain hafnium, which is chemically very similar to zirconium. Measurable quantities of 176Hf add to common 177Hf by the decay of 176Lu, giving a potential dating technique. However, zircon contains only minute traces of lutetium, so that its 176Hf/177Hf ratio remains that of the ultimate source of its host rock. Relative to hafnium, lutetium is more likely to remain in the residue left by partial melting of the mantle, or so theory suggests (geochemists can only deduce this from various lines of indirect evidence). Consequently, mantle that has sourced continental crust builds up 176Hf from the time such crust formed., whereas continental crust has significantly lower levels. Studying hafnium isotopes in very old zircons is therefore a means of seeking periods when significant amounts of continental crust separated from the mantle. Because such tiny amounts of the radiogenic hafnium are involved, an accurate decay constant for 176Lu is vital (Scherer, E., Münker, C. and Mezger, K. 2001. Calibration of the lutetium-hafnium clock. Science, v. 293, p. 683-687). Zircons from the oldest rocks in Greenland, Canada, Australia and South Africa fall into two, complementary groups; those with slight enrichment in 176Hf and those with slight depletion. Simple geochemical theory seems to indicate that indeed magmas similar to those that contributed to formation of the bulk of continental crust did form as early as 4.4 Ga ago. However, zircons with younger Archaean ages show little sign of deviant hafnium, which suggests that a large proportion of the mantle was not involved in early sial formation. Hadean continental material no doubt formed, but not much. That is no surprise, for involvement of surface-derived water in mantle melting above zones where earlier lithosphere returns to the mantle, whatever their form, seems inevitable in a planet noted for its high water content. That is the basic “recipe” for the formation of silica-rich magmas.
Two things stem from this work: the probable futility of seeking Hadean continents; the unlikelihood that the chemical heterogeneity of the mantle stemmed from Hadean continet formation on a massive scale.
See also: Kramers, J. 2001. The smile of the Cheshire Cat. Science, v. 293, p. 619-620