he Great Oxidation Event (GOE), also called the Great Oxygenation Event, was a time period when the Earth's atmosphere and the shallow ocean first experienced a rise in oxygen, approximately 2.4–2.0 Ga (billion years ago) during the Paleoproterozoic era.[2] Geological, isotopic, and chemical evidence suggests that biologically-produced molecular oxygen (dioxygen, O2) started to accumulate in Earth's atmosphere and changed it from a weakly reducing atmosphere practically free of oxygen into an oxidizing atmosphere containing abundant oxygen, causing many existing species on Earth to die out.The event was caused by cyanobacteria producing the oxygen, which stored enough chemical energy to enable the subsequent development of multicellular life forms.
The composition of the Earth's earliest atmosphere is not known with certainty. However, the bulk was likely dinitrogen, N
2, and carbon dioxide, CO
2, which are also the predominant carbon- and nitrogen-bearing gases produced by volcanism today. These are relatively inert gases. The Sun shone at about 70% of its current brightness 4 billion years ago, but there is strong evidence that liquid water existed on Earth at the time. A warm Earth, in spite of a faint Sun, is known as the faint young Sun paradox.[7] Either carbon dioxide levels were much higher at the time, providing enough of a greenhouse effect to warm the Earth, or other greenhouse gases were present. The most likely such gas is methane, CH
4, which is a powerful greenhouse gas and was produced by early forms of life known as methanogens. Scientists continue to research how the Earth was warmed before life arose.
An atmosphere of N
2 and CO
2 with trace amounts of H
2O, CH
4, carbon monoxide (CO), and hydrogen (H
2), is described as a weakly reducing atmosphere. Such an atmosphere contains practically no oxygen. The modern atmosphere contains abundant oxygen, making it an oxidizing atmosphere.[9] The rise in oxygen is attributed to photosynthesis by cyanobacteria, which are thought to have evolved as early as 3.5 billion years ago.
The current scientific understanding of when and how the Earth's atmosphere changed from a weakly reducing to a strongly oxidizing atmosphere largely began with the work of the American geologist, Preston Cloud, in the 1970s. Cloud observed that detrital sediments older than about 2 billion years ago contained grains of pyrite, uraninite,and siderite, all minerals containing reduced forms of iron or uranium that are not found in younger sediments because they are rapidly oxidized in an oxidizing atmosphere. He further observed that continental redbeds, which get their color from the oxidized (ferric) mineral hematite, began to appear in the geological record at about this time. Banded iron formation largely disappears from the geological record at 1.85 billion years ago, after peaking at about 2.5 billion years ago. Banded iron formation can form only when abundant dissolved ferrous iron is transported into depositional basins, and an oxygenated ocean blocks such transport by oxidizing the iron to form insoluble ferric iron compounds. The end of the deposition of banded iron formation at 1.85 billion years ago is therefore interpreted as marking the oxygenation of the deep ocean. Heinrich Holland further elaborated these ideas through the 1980s, placing the main time interval of oxygenation between 2.2 and 1.9 billion years ago, and they continue to shape the current scientific understanding.
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