Cite: Catling, D.C. (2008) in "Seventy Mysteries of the Natural World" (ed. M.J. Benton) Thames and Hudson Ltd., London, pp.69-71 D AVID CATLING Where did the oxygen in the atmosphere come from? 14 A breath that fleets beyond this iron world ALFRED TENNYSON oday the air consists of 21% molecular carbon dioxide and water. oxygen (O ), but this was not always so. Cyanobacteria are the most numerous oxy- T 2 Indeed, for almost the first half of Earth genic photosynthesisers – the oceans teem with history, the air contained less than one part per them, and their ancestors were probably just as million of oxygen. With so little oxygen, animals plentiful. DNA analysis shows that chloroplasts – and multicellular plants (p. 27) could not evolve, bacteria-sized sites of photosynthesis within and only single-celled organisms existed. plants and algae – are descendants of cyanobac- Prochlorococcus The atmosphere first became oxygenated at teria. Evidently, long ago, a cyanobacterium took is the most numerous 2.4–2.3 billion years ago. But at that time, oxygen up symbiotic residence inside another cell that cyanobacterium in concentrations probably only reached a few per was the ancestor of modern plants and algae. the oceans. cent of present levels. It was not until a second At first sight, then, we might suppose that Ancestral marine increase in oxygen, around 580 million years ago, when cyanobacteria evolved, oxygen would have cyanobacteria on the early Earth that widespread animal respiration became pos- accumulated as a result of photosynthesis. But were the architects sible. While oxygen appears to be essential for geochemical evidence suggests that cyanobacte- of oxygen in the complex life, planets are constructed with chemi- ria produced oxygen for several hundred million atmosphere. cals that consume oxygen, so oxygen should not years or more before oxygen levels rose at 2.4 accumulate. Earth’s oxygen-rich atmosphere is billion years ago. To understand what happened therefore rather mysterious. Fortunately, the requires us to consider carefully the loss of oxygen comings and goings of oxygen on the modern as well as its source. Earth provide hints for solving this great puzzle. Both respiration and the decay of organic matter consume oxygen, reversing the gains The oxygen balance sheet: gains and losses through photosynthesis, thus producing no net Oxygen is The principal source of oxygen is biological: it is a oxygen. But a tiny fraction (0.1–0.2%) of organic produced when organic carbon byproduct of a particular type of photosynthesis carbon escapes oxidation by being buried in sedi- and pyrite are called oxygenic photosynthesis. This is a process ments (mostly marine), leaving oxygen behind. buried, but is whereby certain bacteria and green plants consumed by ‘reductants’, convert carbon dioxide and hydrogen extracted volcanic gases substances that from water into organic matter such as carbohy- oxygen react chemically drates, using the energy from sunlight. Oxygen is with oxygen, which are released released from the process as a waste product: metamorphic weathering gases from hot rocks that Carbon dioxide + water + sunlight = organic matter + oxygen melt (volcanism) or metamorphic CO2 + H2O + sunlight = CH2O + O2 The reverse process is aerobic respiration, rocks that do not melt. Other whereby the organic matter is combined with reductants include oxygen to provide metabolic energy, releasing organic carbon or pyrite burial minerals on land. 69 70MystNatWor_069_071_Part_2_14 14/1/08 11:46 Page 70 14 THE EARTH WHERE DID OXYGEN IN THE ATMOSPHERE COME FROM? 14 Anaerobic air and the advent of oxygenic cyanobacteria The red line shows At 2.4 billion years photosynthesis visible fossils of algae estimated ago, oxygen levels animals amounts of rose from less than Before photosynthesis developed, the break- 100% atmospheric 1 ppm (parts per down of water vapour (H O) in ultraviolet sunlight oxygen over time, million), up to 2 10% would have produced oxygen. This process is only based on 0.3–0.6%. Shortly geological after 0.58 billion a net source of oxygen when hydrogen subse- 1% n evidence and years ago, oxygen e g quently escapes into space, so that the products y 0.1% models. Before life rose to levels that x o c existed, at 4.4 supported of water decomposition cannot recombine. i r e 0.01% billion years ago, animals. The h Notably, water vapour condenses in the lower p s the oxygen history of fossils is o 0.001% atmosphere, preventing its hydrogen from escap- m t concentration was shown above the A ing. Consequently, before life, volcanic gases 1 ppm less than 1 ppt graph. would have scavenged oxygen down to less than (parts per trillion). 0.1 ppt one part per trillion. Surprisingly, atmospheric oxygen did not accumulate as soon as photosynthesizers 4.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.1 0.8 0.6 0.4 0.2 0.0 evolved. Biomarkers (diagnostic organic mole- Time before present (billion years ago) cules) in 2.7 billion-year-old sedimentary rocks In the Grand Thus, on geological timescales, the net gain of indicate the presence of cyanobacteria as well as of reductants. Perhaps methane was the key. In an encouraging more oxygen production. Biological Canyon of Arizona, oxygen is actually equivalent to the burial rate of eukaryotes (single-celled organisms with cell anoxic atmosphere, methane reaches concentra- proposals are that weathering was accelerated by thick red beds of organic carbon. nuclei) that used local O – some 300 million years tions hundreds of times greater than today’s 1.8 lichen, or that fecal pellets from newly evolved In the pre- Permian-age 2 oxygenated Esplanade Some organic matter is used by microbes to before oxygen rose at 2.4 billion years ago. parts per million. Ultraviolet light decomposes zooplankton hastened organic burial. Alterna- atmosphere, Sandstone form make pyrite (FeS2) from seawater sulphate. Conse- A plausible explanation for why little atmos- methane in the upper atmosphere, causing tively, moderately high levels of biogenic methane would the top of this quently, the burial of pyrite accounts for roughly pheric oxygen accumulated is that a glut of hydrogen to escape and therefore Earth to methane throughout the ‘boring billion’ pro- decompose at high tower of rock, altitude allowing O’Neill Butte. Red 40% of net oxygen production today. The ancient reductants – chemicals that consume oxygen – oxidize. So high quantities of methane may have moted hydrogen escape and oxidized the Earth. hydrogen to beds are found oceans before 2.4 billion years ago were largely depleted it. Then, a time came when the oxygen subtly encouraged oxygen to rise. In the Phanerozoic eon (542 million years ago escape from the only in rocks devoid of sulphate, so at that time organic carbon produced from organic carbon burial exceeded to present), oxygen levels have probably varied Earth, leaving younger than burial was the main cause of oxygen production. the geological sources of reductants. At this The ‘Great Oxidation Event’ between extremes of 10% and 30%. Thanks to corresponding 2.4–2.3 billion amounts of years ago when Oxygen is easily lost. Geothermal activity and tipping point, oxygen flooded the atmosphere The rise of oxygen 2.4–2.2 billion years ago is high oxygen, the world has remained fit for oxygen behind. the atmosphere volcanoes release gases, such as hydrogen, which until oxygen levels reached a plateau where called the ‘Great Oxidation Event’. Despite this animals, but exactly how oxygen rose from much This has been was oxygenated. consume oxygen. In the ocean, oxygen reacts oxygen production was balanced by losses to name, oxygen levels remained limited and subse- lower Precambrian concentrations still remains proposed to explain how the with dissolved minerals and gases from hot continental weathering. quent biological evolution progressed slowly. somewhat enigmatic. Archaean Earth seafloor vents. Finally, oxygen dissolved in rain- But what caused this rise of oxygen? Either These years of stasis have been dubbed ‘the oxidized. water reacts with minerals in the process of oxygen production increased or consumption boring billion’. During this time, much deep sea- weathering. For example, ‘red beds’ are river- decreased. Some favour the former idea, arguing water perhaps remained anoxic, limiting banks, deserts and floodplains with a reddish that the growth of early continental shelves pro- biological evolution. Escape into Space pigmentation arising from an iron oxide coating moted organic carbon burial and thus oxygen Oxygen finally rose a second time around 580 on mineral grains produced from the reaction of production. Others question this hypothesis: million years ago from a few per cent to greater Carbon dioxide (CO ) Hydrogen (4 H atoms) iron minerals with atmospheric oxygen. organic matter extracts the light carbon isotope, than 15% of present levels. Afterwards, Ediacaran 2 When losses balance production, the amount carbon-12, from seawater, but seawater carbon, animal fossils (see p. 30) appear at 575 million Methane of atmospheric oxygen remains steady. Today, recorded in ancient limestones, does not show a years ago, and then Cambrian animals at 542 about 80–90% of the oxygen produced from steady decrease in carbon-12 content. million years ago. Carbon dioxide organic and pyrite burial is lost to oxidative Instead, oxygen consumption by reducing The cause of the second rise of oxygen remains Microbial Biosphere weathering, while 10–20% reacts with reduced gases could have abated. Proponents of this unsolved, but various ideas have been suggested. Oxygen (O ) gases in the atmosphere and is also lost. ‘Reduced’ theory note that excess hydrogen-bearing reduc- Geological proposals include the enhanced pro- 2 or ‘reducing’ gases are those that tend to react ing gases in the pre-oxygenated atmosphere duction of clays that adsorbed and buried organic with oxygen and become oxidized, e.g., hydro- would cause hydrogen to escape to space, which matter, or the construction of a supercontinent Water (2H O) gen, which oxidizes to form water vapour.