How did oxygen originate on Earth?
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Great Oxygenation Event, the beginning of which is commonly known in scientific media as the Great Oxidation Event (GOE, also called the Oxygen Catastrophe, Oxygen Crisis, Oxygen Holocaust ] Oxygen Revolution, or Great Oxidation) was the biologically induced appearance of molecular oxygen (dioxygen, O2) in Earth's atmosphere ] Geological, isotopic, and chemical evidence suggests a start of around 2.45 billion years ago (2.45 Ga), during the Siderian period, at the beginning of the Proterozoic eon. The causes of the event remain unclear. As of 2016, the geochemical and biomarker evidence for the development of oxygenic photosynthesis before the Great Oxidation Event is inconclusive.b
The first microbes to produce oxygen by photosynthesis were oceanic cyanobacteria. They evolved into tuftedmicrobial mats more than 2.3 billion years ago, approximately 200 million years before the GOE.[8] The free oxygen produced during this time was chemically captured by dissolved iron, converting iron {\displaystyle {\ce {Fe}}} and {\displaystyle {\ce {Fe^2+}}}to magnetite ({\displaystyle {\ce {Fe^2+Fe2^3+O4}}}) which is insoluble in water, and sank to the bottom of the shallow seas to create massive, large scale, banded iron formations. Some of the oxygen was captured by organic matter. The GOE started after these oxygen sinks were filled to capacity.

Cyanobacteria: responsible for the build-up of oxygen in the Earth's atmosphere
The increased production of oxygen set Earth's original atmosphere off balance. Free oxygen is toxic to obligate anaerobic organisms; the rising concentrations may have destroyed most such organisms.
A spike in chromium contained in ancient rock deposits formed underwater shows accumulated chromium washed off from the continental shelves. Chromium is not easily dissolved; its release from rocks requires the presence of a powerful acid. One such acid, sulfuric acid (H2SO4), may have formed through bacterial reactions with pyrite. ]Mats of oxygen-producing cyanobacteria can produce a thin layer, one or two millimeters thick, of oxygenated water in an otherwise anoxic environment even under thick ice; thus, before oxygen started accumulating in the atmosphere, these organisms would already have adapted to oxygen. Additionally, the free oxygen would have ted with atmospheric methane, a greenhouse gas, greatly reducing its concentration and triggering the Huronian glaciation, called "snowball Earth", possibly the longest episode of glaciation in Earth's history
The first microbes to produce oxygen by photosynthesis were oceanic cyanobacteria. They evolved into tuftedmicrobial mats more than 2.3 billion years ago, approximately 200 million years before the GOE.[8] The free oxygen produced during this time was chemically captured by dissolved iron, converting iron {\displaystyle {\ce {Fe}}} and {\displaystyle {\ce {Fe^2+}}}to magnetite ({\displaystyle {\ce {Fe^2+Fe2^3+O4}}}) which is insoluble in water, and sank to the bottom of the shallow seas to create massive, large scale, banded iron formations. Some of the oxygen was captured by organic matter. The GOE started after these oxygen sinks were filled to capacity.

Cyanobacteria: responsible for the build-up of oxygen in the Earth's atmosphere
The increased production of oxygen set Earth's original atmosphere off balance. Free oxygen is toxic to obligate anaerobic organisms; the rising concentrations may have destroyed most such organisms.
A spike in chromium contained in ancient rock deposits formed underwater shows accumulated chromium washed off from the continental shelves. Chromium is not easily dissolved; its release from rocks requires the presence of a powerful acid. One such acid, sulfuric acid (H2SO4), may have formed through bacterial reactions with pyrite. ]Mats of oxygen-producing cyanobacteria can produce a thin layer, one or two millimeters thick, of oxygenated water in an otherwise anoxic environment even under thick ice; thus, before oxygen started accumulating in the atmosphere, these organisms would already have adapted to oxygen. Additionally, the free oxygen would have ted with atmospheric methane, a greenhouse gas, greatly reducing its concentration and triggering the Huronian glaciation, called "snowball Earth", possibly the longest episode of glaciation in Earth's history
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