The origin of sulfur isotope mass-independent fractionation in Archean rocks
Abstract
The discovery of unusual sulfur isotope fractionation in Archean and Paleoproterozoic (>2.4 Ga) rocks has promised to yield insights into the rise of O2 and the nature of the sulfur cycle on ancient Earth, but interpretation has been hampered by the lack of a clear mechanism for the sulfur isotope signature. Proposed mechanisms for the unusual sulfur isotope signatures (termed mass-independent fractionation or MIF) include SO2 photolysis, atmospheric S3 (thiozone) formation, and thermal sulfate reduction in sediments. Here we report high-resolution ultraviolet cross section measurements of the sulfur isotopologues of SO2 that demonstrate that photolysis yields a sulfur MIF signature due to small offsets in the cross sections (~1-2 %) for low abundances of SO2 (~0.1 ppb), with additional MIF due to self-shielding by 32SO2 at higher abundances (up to ~3 ppb). The results imply that SO2 photolysis alone is responsible for most of the Archean sulfur MIF record. The required atmospheric abundances are consistent with variable SO2 injection rates from volcanic plumes, with rates comparable to modern day to ~ 10 times modern. A recent suggestion that OCS was an abundant greenhouse gas in the Archean, based on low-resolution SO2 cross section data, is not supported by our results.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.U13A0037L
- Keywords:
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- 0325 ATMOSPHERIC COMPOSITION AND STRUCTURE / Evolution of the atmosphere;
- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 5225 PLANETARY SCIENCES: ASTROBIOLOGY / Early environment of Earth;
- 8409 VOLCANOLOGY / Atmospheric effects