The Neoproterozoic Oxygenation Event (NOE), also called the Second Great Oxidation Event, was a geologic time interval between around 850 and 540 million years ago during the Neoproterozoic era, during which the oxygen concentration in Earth's atmosphere and oceans rose significantly. Taking place after the end to the Boring Billion, a euxinic period of extremely low atmospheric oxygen spanning from the Statherian period of the Paleoproterozoic era to the Tonian period of the Neoproterozoic era, the NOE was the second major increase in atmospheric and oceanic oxygen concentration on Earth, though it was not as prominent as the Great Oxidation Event (GOE) of the Neoarchean-Paleoproterozoic boundary. Unlike the GOE, it is unclear whether the NOE was a synchronous, global event or a series of asynchronous, regional oxygenation intervals with unrelated causes.
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👉 Neoproterozoic Oxygenation Event in the context of Boring Billion
The Boring Billion, otherwise known as the Mid Proterozoic and Earth's Middle Ages, is an informal geological time period between 1.8 and 0.8 billion years ago (Ga) during the middle Proterozoic eon spanning from the Statherian to the Tonian periods, characterized by more or less tectonic stability, climatic stasis and slow biological evolution. Although it is bordered by two different oxygenation events (the Great Oxygenation Event and Neoproterozoic Oxygenation Event) and two global glacial events (the Huronian and Cryogenian glaciations), the Boring Billion period itself actually had very low oxygen levels and no geological evidence of glaciations.
The oceans during the Boring Billion may have been oxygen-poor, nutrient-poor and sulfidic (euxinia), populated by mainly anoxygenic purple bacteria, a type of bacteriochlorophyll-based photosynthetic bacteria which uses hydrogen sulfide (H2S) for carbon fixation instead of water and produces sulfur as a byproduct instead of oxygen. This is known as a Canfield ocean, and such composition may have caused the oceans to be colored black-and-milky-turquoise instead of blue or green as later. (By contrast, during the much earlier Purple Earth phase during the Archean, photosynthesis was performed mostly by archaeal colonies using retinal-based proton pumps that absorb green light, and the oceans would be magenta-purple.)
Neoproterozoic Oxygenation Event in the context of Geological history of oxygen
Although oxygen is the most abundant element in Earth's crust, due to its high reactivity it mostly exists in compound (oxide) forms such as water, carbon dioxide, iron oxides and silicates. Before photosynthesis evolved, Earth's atmosphere had little free diatomic elemental oxygen (O2). Small quantities of oxygen were released by geological and biological processes, but did not build up in the reducing atmosphere due to reactions with then-abundant reducing gases such as atmospheric methane and hydrogen sulfide and surface reductants such as ferrous iron.
Oxygen began building up in the prebiotic atmosphere at approximately 2.45 Ga during the Neoarchean-Paleoproterozoic boundary, a paleogeological event known as the Great 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 the Neoproterozoic 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's geochemistry and biosphere. Detailed connections between oxygen and evolution remain elusive.