Variations in the magnitude of non mass dependent sulfur fractionation in the Archean atmosphere

Details Variations in the magnitude of non mass dependent sulfur fractionation in the Archean atmosphere Variations in the magnitude of non mass dependent sulfur fractionation in the Archean atmosphere

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p14a..03c&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #P14A-03

Biology

[0325] Atmospheric Composition And Structure / Evolution Of The Atmosphere, [0454] Biogeosciences / Isotopic Composition And Chemistry, [5210] Planetary Sciences: Astrobiology / Planetary Atmospheres, Clouds, And Hazes, [5225] Planetary Sciences: Astrobiology / Early Environment Of Earth

Scientific paper

Recent experimental data have enabled quantitatively meaningful computations of the non-mass dependent fractionation of sulfur’s isotopes (Δ33S) that exemplify the Archean rock record. The Δ33S signal originates as a result of fine structure in the absorption cross-section of SO2 isotopologues [1], which only undergo significant photolysis in reducing atmospheres [2]. The Δ33S signal produced by SO2 photolysis varies significantly between 190 and 220 nm, and thus is strongly dependent on any other atmospheric gases which absorb photons in this range [3], as well as the height at which photolysis occurs. A model that is capable of resolving the altitude-dependent radiative transfer through a realistic self-consistent reducing atmosphere is therefore essential when making direct comparisons between atmospheric Δ33S production and the rock record. In this work, we investigate how the magnitude of Δ33S might vary as function of atmospheric composition, which in turn allows the rock record to constrain the Archean atmosphere. Other recent work on this topic using simplied atmospheric models has implicated large concentrations of SO2 [5], OCS [3], and CO2 [6] as being responsible for the variations in Archean Δ33S. We present results from an altitude-dependent photochemical model of Archean photochemistry [4] of necessary complexity to resolve the complicated redox structure of the Archean atmosphere. We show that while increased concentrations of these gases all affect Δ33S in an unconstrained model, the atmospheric conditions required for OCS or SO2 shielding are unlikely to occur in an Archean atmosphere constrained by reasonable expectations of volcanic and biogenic fluxes. Within the context of plausible Archean atmospheres, we investigate how shielding due to changing amounts of CO2, biogenic sulfur gases, and fractal organic haze [7] affect the magnitude of Δ33S produced by the Archean atmosphere, and show why simplified atmospheric modeling may lead to eroneous conclusions. [1] Danielache et al. (2008), Journal of Geophysical Research 113 D17314. [2] Pavlov and Kasting (2002) Astrobiology 2 27. [3] Ueno et al. (2009), Proceedings of the National Academy of Sciences 106 (35) 14784-14789. [4] Zahnle, Claire, and Catling (2006), Geobiology 4 271-283. [5] Lyons (2007), Geophysical Research Letters 34, L22811. [6] Halevy et al 2010 Science, 10.1126 1190298. [7] Wolf and Toon (2010), Science 238, 1266-1268.

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