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The bulk of the free oxygen in the Archean atmosphere was derived from organic photosynthesis of carbon dioxide (CO 2) and water (H 2O) by anaerobic cyanobacteria (blue-green algae), a process that releases oxygen as a by-product. The inorganic breakdown (photodissociation) of volcanic-derived water vapour and carbon dioxide in the atmosphere would have produced only a small amount of free oxygen. Although volcanoes exhale much water vapour (H 2O) and carbon dioxide (CO 2), the amount of free oxygen (O 2) emitted is very small. It is thought that the oxygen content in today’s atmosphere must have slowly accumulated through time starting with an atmosphere that was anoxic during Archean times. These grains must have been transported by rivers from a source area, the location of which has never been found it was possibly destroyed by meteorite impacts-quite frequent on both Earth and the Moon before 4 billion years ago. In Western Australia some sedimentary conglomerates, dated to 3.3 billion years ago, contain relict detrital zircon grains that have isotopic ages between 4.2 and 4.4 billion years. The earliest terrestrial materials are not rocks but minerals. Prior to the Archean Eon, Earth was in the astronomical (Hadean) stage of planetary accretion that began about 4.6 billion years ago no rocks are preserved from this stage. The start of the Archean Eon is only defined by the isotopic age of the earliest rocks. Archean greenstone- granite belts contain many economic mineral deposits, including gold and silver. Fossil evidence of the earliest primitive life-forms-prokaryotic microbes from the domain called Archaea and bacteria-appears in rocks about 3.5–3.7 billion years old however, the presence of ancient fragments of graphite (which may have been produced by microbes) suggest that life could have emerged sometime before 3.95 billion years ago. Records of Earth’s primitive atmosphere and oceans emerge in the earliest Archean (Eoarchean Era). The Archean Eon was preceded by the Hadean Eon, an informal division of geologic time spanning from about 4.6 billion to 4 billion years ago and characterized by Earth’s initial formation. The Archean Eon began about 4 billion years ago with the formation of Earth’s crust and extended to the start of the Proterozoic Eon 2.5 billion years ago the latter is the second formal division of Precambrian time. SpaceNext50 Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!Īrchean Eon, also spelled Archaean Eon, the earlier of the two formal divisions of Precambrian time (about 4.6 billion to 541 million years ago) and the period when life first formed on Earth.Learn about the major environmental problems facing our planet and what can be done about them!
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