Mass-independent fractionation of sulfur isotopes

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1. Story
2. The single sharpest proxy for the composition of the Archaean atmosphere, and the reason the Great Oxidation Event has a date rather than a vague interval. Ordinary chemical and biological processes sort sulfur isotopes in proportion to their mass difference. Photochemical reactions driven by short-wavelength ultraviolet light do not — they produce a mass-INDEPENDENT signature, and that signature can only be generated and then preserved if ultraviolet light reaches the lower atmosphere and if the atmosphere carries no ozone shield and essentially no free oxygen. The signal is present in sedimentary sulfides and sulfates through the Archaean and then disappears, and the disappearance is effectively irreversible: once oxygen is present, the sulfur cycle passes through a homogenising oxidised reservoir that erases it. The interpretation was published by Farquhar, Bao and Thiemens in 2000 and it converted a qualitative argument about rusting into a datable stratigraphic horizon.
3. Knowledge
4. Mass-independent fractionation of sulfur isotopes
5. The Great Oxidation Event
6. Connections
7. The Great Oxidation Event
8. The Great Oxidation Event
9. Sulfur mass-independent fractionation is proposed as an atmospheric oxygen proxy
10. LUCA — the last universal common ancestor
11. Mass-independent fractionation of sulfur isotopes
12. Banded iron formations
13. The origin of life
14. Stromatolites
15. The Cambrian explosion
16. Endosymbiosis and the eukaryotic cell
17. Mass-independent fractionation of sulfur isotopes
18. Banded iron formations
19. Evidence
20. Supports the use of mass-independent sulfur isotope fractionation as a proxy for an anoxic, ozone-free atmosphere and its disappearance as a marker of the Great Oxidation Event. V55 verification basis: this session had no network access to any source — WebFetch was egress-blocked and the WebSearch budget was exhausted — so the cited work was not retrieved and its pagination was not re-checked. Volume and page range are quoted from standing knowledge and were not re-checked.