Isótopos sulfúricos geológicos indicam índice elevado de OCS na atmosfera arqueana

Geological sulfur isotopes indicate elevated OCS in the Archean atmosphere, solving faint young sun paradox

Yuichiro Uenoa,b, Matthew S. Johnsonc,1, Sebastian O. Danielacheb,c,d, Carsten Eskebjergc, Antra Pandeyb,d and Naohiro Yoshidab,d,e

+ Author Affiliations

aGlobal Edge Institute and

bResearch Center for the Evolving Earth and Planet, Tokyo Institute of Technology, Meguro Tokyo, 152-8551, Japan;

cCopenhagen Center for Atmospheric Research, Department of Chemistry, University of Copenhagen, Universitetsparken 5 DK-2100 Copenhagen Ø, Denmark; and

dDepartments of Environmental Science and Technology and

eEnvironmental Chemistry and Engineering, Tokyo Institute of Technology, G1-25, 4259 Nagatsuta, Yokohama, 226-8502, Japan

Abstract

Distributions of sulfur isotopes in geological samples would provide a record of atmospheric composition if the mechanism producing the isotope effects could be described quantitatively. We determined the UV absorption spectra of 32SO2, 33SO2, and 34SO2 and use them to interpret the geological record. The calculated isotopic fractionation factors for SO2 photolysis give mass independent distributions that are highly sensitive to the atmospheric concentrations of O2, O3, CO2, H2O, CS2, NH3, N2O, H2S, OCS, and SO2 itself. Various UV-shielding scenarios are considered and we conclude that the negative Δ33S observed in the Archean sulfate deposits can only be explained by OCS shielding. Of relevant Archean gases, OCS has the unique ability to prevent SO2 photolysis by sunlight at λ >202 nm. Scenarios run using a photochemical box model show that ppm levels of OCS will accumulate in a CO-rich, reducing Archean atmosphere. The radiative forcing, due to this level of OCS, is able to resolve the faint young sun paradox. Further, the decline of atmospheric OCS may have caused the late Archean glaciation.

mass independent fractionation carbonyl sulfide (OCS) sulfur dioxide (SO2) photochemistry greenhouse gases

Footnotes

1To whom correspondence should be addressed. E-mail: msj@kiku.dk

Author contributions: Y.U., M.S.J., and N.Y. designed research; S.O.D. and C.E. performed research; M.S.J. and N.Y. contributed new reagents/analytic tools; Y.U., M.S.J., S.O.D., and A.P. analyzed data; and Y.U., M.S.J., and S.O.D. wrote the paper.

Edited by Mark H. Thiemens, University of California at San Diego, La Jolla, CA, and approved July 14, 2009

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

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