Every breath you take is predominantly composed of nitrogen (78%) and oxygen (21%), with minor amounts of argon, carbon dioxide, water vapour, and other trace gases – a recipe that is unique to Earth in the Solar System. The composition of the atmosphere, however, has been far from constant during the history of the Earth and for decades scientists have been fascinated by reconstructing its changing composition.

Image: Modern stromatolites at Shark Bay, Western Australia. Stromatolites are rock structures built up by layers of cyanobacteria, microorganisms that contribute to the addition of O2 to the atmosphere. Their role in oxygen production dates back billions of years and similar structures to the stromatolites at Shark Bay have been found in rocks as old as 3.5 billion years before present.

Attention is often given to oxygen since it is not only vital to the respiration of most life on the planet, but it also provides protection, via the ozone layer, from the harsh UV radiation generated by the Sun. Most scientists now recognise that the atmosphere of the early Earth was nearly devoid of oxygen and that during two main events the amount of oxygen rose in substantial amounts. The first of these events occurred approximately 2.3 billion years ago (Ga) and is now referred to as the Great Oxidation Event (GOE). The second occurred roughly 600-500 million years ago (Ma) and paved the way for the evolution of life. The presence of atmospheric oxygen requires oxygenic photosynthesis, a process performed predominantly by cyanobacteria, and a steady-state of oxygen consumption that doesn’t exceed its production. Detecting the presence of an oxygenated atmosphere, therefore, provides clues regarding the evolution of life and the transition to an Earth habitable by complex multicellular life. But, how do scientists detect the presence or absence of oxygen in the atmosphere millions and even billions of years ago?

The difficulty in studying the composition of the atmosphere at different points in the Earth’s history rests in the lack of physical samples of air. In the recent record of the Quaternary (2.588 Ma to present) record, air trapped in deep ice cores in Antarctica and Greenland has allowed scientists to reconstruct the greenhouse gas composition of the atmosphere as far back as roughly 800,000 years. Deeper in the geological record, however, scientists must become increasingly more creative and rely on inference and contraints from materials that form at the surface and interact with the atmosphere. Often times, the presence or absence of oxygen can be detected, but reconstructing the amount of O 2 in the atmosphere is far more difficult.

Throughout the Phanerozoic eon (the last 542 Ma), the levels of atmospheric oxygen are likely to have fluctuated within the range of 10-25% in order to support the life present at the surface and in seas. During this time period, models that integrate the biogeochemical cycles of carbon and sulphur, the main elements that consume and remove oxygen from the atmosphere, are used to reconstruct possible fluctuations in atmospheric O 2 levels. The models are then constrained and tested against independent lines of biological and geological evidence, both from the fossil record and controlled experiments. For example, following the evolution and proliferation of vascular plants, charcoal (indicative of forest fires) has been detected in the rock record as far back as nearly 350 million Ma. Charcoal provides a rough minimum estimate of 12-13% O 2 in the atmosphere, since fire is difficult to sustain below these levels. By extension, the occurrence of abundant charcoal at roughly 300 Ma (in the Carboniferous) has been used to suggest that atmospheric oxygen levels may have been significantly elevated (up to 30-35%) at the time. Leaps in animal evolution, as well as their size, survival, and reproductive success have also been correlated with changing atmospheric oxygen levels (see Berner, 1999, Berner et al., 2007 and references therein for more information).

Stepping back into the early history of the Earth, one of the most fascinating changes that occurred was the transition from an anoxic to oxic atmosphere – the Great Oxidation Event. What then are the lines of evidence for an atmosphere barren of oxygen? As early as the 1960s (e.g., Holland 1962, Cloud 1968), scientists examining the Paleoproterozoic (2.5 to 1.6 Ga) rock record noticed a transition in the mineralogy of sediments that were deposited in shallow waters, such as rivers and streams. In old sediments (>2.4 Ga), it became apparent that minerals such as uraninite and pyrite (with the reduced forms of U and Fe) were transported in the shallow water, which is not possible in an oxygenated atmosphere. Both minerals are unstable and will rapidly decay in the presence of oxygen. By contrast, younger sediments are barren of these minerals, indicating that oxygen had accumulated to levels high enough to overcome their stability at the surface. This transition also marks the first appearance of ‘red beds’ in the geological record, which are sedimentary rocks (e.g., sandstones and shales) that are rich in Fe oxides – their formation requires the presence of oxygen at the surface or in shallow groundwater. Decades of supporting research followed these observations (see Farquhar et al., 2011), mainly focussed on better understanding the elements that are redox-sensitive (gain or lose electrons during chemical reaction) and changed in their behaviour on Earth after the influx of oxygen (e.g., sulphur, phosphorus, and the transition metals). It is now almost unanimously agreed that the GOE drastically changed the type of sediments and minerals that could form on Earth, and which elements could be released into the oceans from the lands – all of which had a profound influence on biological evolution.

Currently, several research teams across the world continue to investigate the accumulation of oxygen in Earth’s atmosphere and its interconnection with the evolution of life. What exactly triggered the transition and allowed the accumulation of oxygen is still debated. Recent work has focussed on attempting to unravel how early oxygenic photosynthesis developed and the first ‘whiffs’ of oxygen were produced and if a more precise understanding of how the oxygen levels rose and fluctuated before and after the GOE can be established (was it actually one big, unidirectional event or did O 2 build-up in a step-wise pattern?). New insights are continually being generated through detailed sampling and the application of new techniques with advancing state-of-the-art mass spectrometers. Postgraduate students Meabh Gallagher and Michael Babechuk at Trinity College Dublin, led by Prof. Balz Kamber, are contributing to this area of study.

One unique way to study the composition of the atmosphere is through the investigation of ancient paleosols (buried fossil “soils” of the past), since they formed in direct contact with the atmosphere. This type of research was pioneered by the late Heinrich D. Holland, who noticed that paleosols older than 2.2 Ga show evidence that Fe was removed during chemical weathering. This is only possible if the fluids percolating through the soil have insufficient oxygen to form Fe oxides from the conversion of Fe2+ to Fe3+. Most soils forming at the surface today are rusty red and orange in colour due to the presence of Fe and Mn oxides. Soils younger than 2.2 Ga exhibit Fe behaviour akin to modern soils, indicating sufficient oxygen levels to form similar Fe oxides. Within reasonable assumptions related to other factors of soil formation, the full retention of Fe places a semi-quantitative lower limit of atmospheric O 2 of roughly 1% of the present atmospheric levels. Researchers at TCD are revisiting these very old paleosols with more in-depth sampling and new geochemical techniques to search for further clues of how ‘soils’ developed differently in anoxic and oxic environments. An example of a well-preserved paleosol is highlighted in a recent publication in the Canadian Journal of Earth Sciences. The paleosol in question formed 1.85 billion years ago and is one of the most famous examples from Canada (Holland et al., 1989). The publication is available with open-access temporarily here. These studies are also relevant to understanding how the surface of other rocky planets evolved with different atmospheric compositions, such as on Mars, which is another research area at TCD led by Dr. Mary Bourke.

Michael Babechuk

Ph.D. student, Department of Geology, Trinity College Dublin

Other blog posts by this author:

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Further information for readers interested in changes in atmospheric oxygen levels and the GOE are listed below.

Some useful terms (definitions adapted from Oxford Dictionary of Science, 6th edition):

Cyanobacteria – a phylum consisting of two groups of photosynthetic eubacteria (blue-green bacteria and chloroxybacteria). The blue-green bacteria (formerly known as blue-green algae, or Cyanophyta), comprise the vast majority of members, are unicellular and occur in all aquatic habitats.

Photosynthesis – the chemical process by which green plants, algae, and certain bacteria synthesize organic compounds from carbon dioxide and water in the presence of sunlight. It can be summarized by the equation:

CO 2 + H 2 O –> CH 2 O + H 2 O + O 2

References and further reading:

Babechuk, M.G., and Kamber, B.S., 2013. The Flin Flon paleosol revisited. Canadian Journal of Earth Sciences, 50: 1223-1243 (open-access link)

Berner, R.A., 1999. Atmospheric oxygen levels over the Phanerozoic. Proceedings of the National Academy of Sciences of the United States of America, 96: 10955-10957

Berner, R.A., 2006. GEOCARBSULF: A combined model for Phanerozoic O 2 and CO 2 . Geochimica et Cosmochimica Acta, 70: 5653-5664.

Berner, R.A., VandenBrook, J.M., Ward, P.D., 2007. Oxygen and evolution. Science, 316: 557-558

Cloud, P. E., 1968. Atmospheric and hydrospheric evolution on the primitive Earth. Science, 160: 729-736.

Farquhar, J., Zerkle, A.L., and Bekker, A., 2011. Geological constraints on the origin of oxygenic photosynthesis. Photosynthesis Research, 107: 11-36.

Frimmel, H.E., 2005. Archaean atmospheric evolution: evidence from the Witwatersrand goldfields, South Africa. Earth-Science Reviews, 70: 37-41.

Holland, H.D., 1962. Model for the evolution of the Earth’s atmosphere. In: Petrologic Studies: A volume in Honor of A.F. Buddington (eds. Engel, A.E.J. et al.), pp. 447-477. Geological Society of America. Colorado, USA.

Holland, H.D., Feakes, C.R., and Zbinden, E.A. 1989. The Flin Flon paleosol and the composition of the atmosphere 1.8 BYBP. American Journal of Science, 289: 362-389.

Kump, L.R. 2008. The rise of atmospheric oxygen. Nature, 451: 277-278.

Lüthi, D., et al., 2008. High-resolution carbon dioxide concentration record 650,000-800,000 years before present. Nature, 453: 379-382.

Rye, R., and Holland, H.D. 1998. Paleosols and the evolution of atmospheric oxygen: a critical review. American Journal of Science, 298: 621-672.

Scott, A.C., 2000. The Pre-Quaternary history of fire. Palaeogeography, Palaeoclimatology, Palaeoecology, 164: 281-329.

Sessions, A.L., Doughty, D.M., Welander, P.V., Summons, R.E., Newman, D.K., 2009. The continuing puzzle of the Great Oxidation Event. Current Biology, 19: R567-R574.

Sverjensky, D.A., and Lee, N., 2010. The Great Oxidation Event and mineral diversification. Elements, 6: 31-36.

Ulrich, T., Long, D.G.F., Kamber, B.S., and Whitehouse, M.J., 2011. In situ trace element and sulphur isotope analysis of pyrite in a Paleoproterozoic gold placer deposit, Pardo and Clement Townships, Ontario, Canada. Economic Geology, 106: 667-686.

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