Althoughoxygen isthe most abundant element inEarth's crust, due to its highreactivity it mostly exists incompound (oxide) forms such aswater,carbon dioxide,iron oxides andsilicates. Beforephotosynthesis evolved,Earth's atmosphere had little freediatomicelementaloxygen (O2).[2] Small quantities of oxygen were released by geological[3] and biological processes, but did not build up in thereducing atmosphere due to reactions with then-abundant reducing gases such asatmospheric methane andhydrogen sulfide and surfacereductants such asferrous iron.
Oxygen began building up in theprebiotic atmosphere at approximately 2.45 Ga during theNeoarchean-Paleoproterozoic boundary, apaleogeological event known as theGreat Oxygenation Event (GOE). The concentrations of O2 attained were less than 10% of today's and probably fluctuated greatly. Around 500Mya a second event known as theNeoproterozoic Oxygenation Event lead to oxygen levels similar or even higher than the present. The increase in oxygen concentrations had wide-ranging and significant impacts on Earth'sgeochemistry andbiosphere. Detailed connections between oxygen and evolution remain elusive.
Oxygen is both a result of biological activity and a key enabler.Photosynthesis produces oxygen while plants and animals usingaerobic respiration consume it. Consequently theevolution of life is closely related to the concentration of available oxygen. Understanding the relationship between oxygen and evolution would aid in seeking evidence ofextraterrestrial life inexoplanet data.[4]: 252 Oxygen concentration plays a key role in the geochemical composition of sedimentary rocks, making oxygen concentration important for geology and sedimentary rocks important for understanding oxygen concentration over geologic time.[5]
Due to limitations in measurements, oxygen concentration in the atmosphere or oceans over much of the early parts of Earth's history remains controversial.[6] The consensus view includes these phases:
The timing and character of the oxygenation events have been the subject of many discussions.[6]
Techniques for estimating oxygen at different times in the past are called "paleo-oxybarometers". An ideal technique would rely on trapped gas or fluid in a well-dated rock layer, but such examples are scarce. Most measurements are indirect analysis of oxidation in sedimentary rocks to infer oxygen in ancient atmosphere or oceans. Ocean analysis is especially challenging becausetectonicsubduction replaces the ocean floor every 200 million years. Many new techniques have been developed but their analysis and comparison have lead to additional debates rather than consensus on the oxygen history of Earth.[8]
Earth's early atmosphere had a very low concentration of oxygen, probably less than 0.001% of present day levels. While details are not well known, measurements ofmass-independent fractionation of sulfur in sedimentary sulphides and sulfates rule out significant oxygen before around 2.45Gya.[1] Continuing sources of oxygen during this period would have beenphotodissociation of water followed by escape the hydrogen product[9] or ofsulfur dioxide.[10]
Between 2.45 and 2 Gya, oxygen began to build up in the atmosphere. This time coincided with major changes in geochemistry and biology on the Earth but which changes are causes and which are results are debated.[1]: 905 Widespread production of oxygen bycyanobacteria which evolved around this time is suggestive, but substantial evidence suggests that cyanobacteria appeared at least 2.7Ga and perhaps well before that. Geological effects like volcanism, weathering, and burial of chemically altered rocks may be important.[11]: 6 Oxygen began to persist in the atmosphere in small quantities about 50 million years before the start of theGreat Oxygenation Event.[12] By 800Mya oxygen in the atmosphere was between 1% and 18% of the present atmospheric level.[11]
After the Great Oxygenation Event, the Earth entered a long period ofeuxinia known as theBoring Billion.[citation needed]} Although the atmosphere had become oxidative with the presence of free oxygen, the oxygen level was still very low (about 0.1%) in both the atmosphere and the ocean. By around600 Mya during theNeoproterozoic, however, oxygen levels began to rise significantly.[13] During theCambrian, atmospheric oxygen 5-10% concentration, around half its current value. It rose in pulses above 15% of the atmosphere during theDevonian "Age of the Fishes", reaching 25% (above modern 20%) in thePermo-Carboniferous.[4] This second great oxygenation event has been reported to be associated with the evolution ofnitrogen fixation in cyanobacteria,[14] or the rise of more robusteukaryoticphotoautotrophs (i.e.algae) viaendosymbiosis and increasedphosphorus removal from the ocean.[15]
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Atmospheric oxygen is the most conspicuous sign oflife on Earth. The evolution ofphotosynthesis and the rise of oxygen-producingcyanobacteria are major events inevolution. Photosynthetic oxygen eventually accumulated in the atmosphere, transforming both the surface of the planet and the nature of life[11]
Despite these connections, the details are not simple. Theevolution of life and the geological history of oxygen share many similar patterns, but the relationship between these two histories remains uncertain. For example, the rise in oxygen concencentration and the rise the maximum size of organism have similar histories, but evidence the oxygen concentration limits size is inconclusive. As more evidence for lower and variable levels of oxygen before the Neoproterozoic has emerged, simple relationships between life and oxygen have been harder to justify.[16]