Hur skapades syret?

Life started to have a major impact on the environment once photosynthetic organisms evolved. These organisms, blue-green algae (picture of stromatolite, which is the rock formed by these algae), fed off atmospheric carbon dioxide and converted much of it into marine sediments consisting of the shells of sea creatures.

While photosynthetic life reduced the carbon dioxide content of the atmosphere, it also started to produce oxygen. For a long time, the oxygen produced did not build up in the atmosphere, since it was taken up by rocks, as recorded in Banded Iron Formations (BIFs; picture) and continental red beds. To this day, the majority of oxygen produced over time is locked up in the ancient "banded rock" and "red bed" formations. It was not until probably only 1 billion years ago that the reservoirs of oxidizable rock became saturated and the free oxygen stayed in the air.

Isotopes and stellar origin
Main article: Isotopes of oxygen

Late in a massive star's life, 16O concentrates in the O-shell, 17O in the H-shell and 18O in the He-shellNaturally occurring oxygen is composed of three stable isotopes, 16O, 17O, and 18O, with 16O being the most abundant (99.762% natural abundance).[31]

Most 16O is synthesized at the end of the helium fusion process in stars but some is made in the neon burning process.[32] 17O is primarily made by the burning of hydrogen into helium during the CNO cycle, making it a common isotope in the hydrogen burning zones of stars.[32] Most 18O is produced when 14N (made abundant from CNO burning) captures a 4He nucleus, making 18O common in the helium-rich zones of stars.[32]

Fourteen radioisotopes have been characterized, the most stable being 15O with a half-life of 122.24 seconds (s) and 14O with a half-life of 70.606 s.[31] All of the remaining radioactive isotopes have half-lives that are less than 27 s and the majority of these have half-lives that are less than 83 milliseconds.[31] The most common decay mode of the isotopes lighter than 16O is β+ decay[33][34][35] to yield nitrogen, and the most common mode for the isotopes heavier than 18O is beta decay to yield fluorine.[31]