Finding Archaean atmospheric composition using micrometeorites

Modern micrometeorites (about 20 μm in diameter) from deep-sea sediments, with shiny magnetite-rich veneers (Credit: D. E. Brownlee)

The gases making up the Earth’s atmosphere and their relative proportions before 2.5 billion years (Ga) ago are known with very little certainty. Carbonate rocks are rare, indicating that the oceans were more acidic, which implies that they had dissolved more CO2 from the atmosphere, which, in turn implies that there was much more of that gas than in present air. There are few signs of widespread glaciogenic sediments of Archaean age, at a time when the Sun’s energy output is estimated to have been at 70 to 75% of its present level. Without an enhanced greenhouse effect oceans would have been frozen over; so that supports high CO2 concentrations too. The fact that water worn grains of minerals such as uraninite (UO2) and pyrite (FeS2), which are stable only in reducing conditions, occur in Archaean conglomerates is a good indicator that there were only vanishingly small amounts of oxygen in the air. That was not to change until marine photosynthesisers produced enough to overcome the general reducing conditions at the Earth’s surface, marked by the Great Oxidation Event at around 2.4 Ga (see: Massive event in the Precambrian carbon cycle; Earth-logs, January 2012. Search for more articles in sidebar at Earth-logs home page). It was then that ancient soils (palaeosols) became the now familiar red colour because of their content of ferric iron oxides and hydroxides The problem is that reliable numbers cannot be attached to these kinds of observation. A common means of estimating CO2 levels comes from the way in which the gas reacts with silicates as soils form at the land surface, estimated from carbon isotopes in soil carbonate nodules. Since the rise of land plants around 400 Ma ago the distribution of pores (stomata) in fossil leaves provides a more precise estimate: the more CO2 in air the less densely packed are leaf stomata. For the Precambrian we are stuck with estimates based on chemical reactions of minerals with the atmosphere. Until recently, one reaction that must always have been extremely common was overlooked.

When meteorite pass through the atmosphere at very high speed friction heats them to incandescence. Their surfaces not only melt but the minerals from which they are composed react very strongly with air. The reaction products should therefore provide chemical clues to the relative proportions of atmospheric gases. Both oxygen and carbon dioxide are reactive at such temperatures, although nitrogen is virtually inert, yet it tends to buffer oxidation reactions. The rest of the atmosphere comprises noble gases – mainly argon – and by definition they are completely unreactive. Pure-iron micrometeorites collected from 2.7 Ga old sediments in the Pilbara Province of Western Australia are veneered with magnetite (Fe3O4) and wüstite (FeO), thus preserving a record of their passage through the Neoarchaean atmosphere. If the oxidant had been oxygen, for these minerals to form from elemental iron suggests oxygen levels around those prevailing today: clearly defying the abundant evidence for its near-absence during the Archaean. Carbon dioxide is the only candidate. Two studies have produced similar results (Lehmer, O. R. et al. 2020. Atmospheric CO2 levels from 2.7 billion years ago inferred from micrometeorite oxidationScience Advances, v. 6, article aay4644;  DOI: 10.1126/sciadv.aay4644 and Payne, R.C. et al. 2020. Oxidized micrometeorites suggest either high pCO2 or low pN2 during the Neoarchean. Proceedings of the National Academy of Sciences, v. 117 1360 DOI:10.1073/pnas.1910698117). Both use complex modelling of the chemical effects of meteorite entry. Lehmer and colleagues estimated that the Neoarchaean atmosphere contained about 64% CO2, with a surface atmospheric pressure about half that at present. This would be sufficient for a surface temperature of about 30°C achieved by the greenhouse effect, taking into account lower solar heating. The team led by Payne concluded a lower concentration (25 to 50%) and a somewhat cooler planet at that time. Both results suggest ocean water considerably more acid than are today’s. The combined warmth and acidity would have had a fundamental bearing on both the origin, survival and evolution of early life.

See also: Carroll, M. 2020. Meteorites reveal high carbon dioxide levels on early Earth; Yirka, R. Computer model shows ancient Earth with an atmosphere 70 percent carbon dioxide. (both from Phys.org)

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