Geological Sulfur Isotopes Indicate Elevated OCS in the Archean Atmosphere, Solving Faint Young Sun Paradox

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 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 Edited by Mark H. Thiemens, University of California at San Diego, La Jolla, CA, and approved July 14, 2009 (received for review March 31, 2009) Distributions of sulfur isotopes in geological samples would pro- photolysis would not affect the isotopic composition of aerosol vide a record of atmospheric composition if the mechanism pro- (8, 9). (ii) In extreme volcanic events resulting in highly elevated ducing the isotope effects could be described quantitatively. We atmospheric SO2 concentrations, SO2 photoexcitation below the 32 33 34 determined the UV absorption spectra of SO2, SO2, and SO2 dissociation threshold at 220 nm generates electronically excited and use them to interpret the geological record. The calculated SO2* that reacts with SO2 to form SO and SO3, which is assumed isotopic fractionation factors for SO2 photolysis give mass inde- to produce MIF (9). The resulting SO3 combines with water to pendent distributions that are highly sensitive to the atmospheric form sulfuric acid and may carry the MIF signal. Under low- concentrations of O2,O3,CO2,H2O, CS2,NH3,N2O, H2S, OCS, and oxygen conditions, SO2 photolysis is the prime candidate for SO2 itself. Various UV-shielding scenarios are considered and we producing Archean MIF (8). conclude that the negative ⌬33S observed in the Archean sulfate Despite mechanisms that can explain production and preser- deposits can only be explained by OCS shielding. Of relevant vation of MIF in aerosols, no previous model can reproduce the multiple isotope composition of photochemically produced sul- Archean gases, OCS has the unique ability to prevent SO2 photolysis by sunlight at ␭ >202 nm. Scenarios run using a photochemical fate aerosol. Archean sulfate deposits show uniformly negative ⌬33 box model show that ppm levels of OCS will accumulate in a S values (1, 7, 10), which have been claimed to match CO-rich, reducing Archean atmosphere. The radiative forcing, due laboratory studies of SO2 photolysis at 193 nm (2). However, the to this level of OCS, is able to resolve the faint young sun paradox. solar spectrum is a broad continuum, not a sharp laser wavelength. Photolysis using a sun-like continuum (high pressure Xe Further, the decline of atmospheric OCS may have caused the late lamp) yields positive ⌬33S values of product sulfate (2), which is Archean glaciation. incompatible with the geological record. To resolve the apparent ͉ ͉ discrepancy, it is necessary to understand the wavelength de- mass independent fractionation carbonyl sulfide (OCS) pendence of the mass independent isotope effect. Recent the- sulfur dioxide (SO2) ͉ photochemistry ͉ greenhouse gases oretical studies have examined the UV absorption spectra of SO2 isotopologues (11) and have suggested that SO2 self-shielding ass independent fractionation (MIF) of sulfur isotopes has may have caused the Archean MIF record (12). However, the Mbeen found in geological samples older than 2.3 billion theoretical spectra necessarily contain several approximations years (Ga) and is believed to be produced by the photochemical and do not agree with recently obtained isotopologue-specific reactions of gaseous sulfur compounds in a low-oxygen reducing experimental spectra (13). The most straightforward way to atmosphere (1–4). The rise of atmospheric oxygen (O2 and O3) determine the MIF effect is via direct measurement of UV would have oxidized atmospheric sulfur compounds to sulfate, absorption by SO2 isotopologues. preventing the accumulation of isotopic fractionation in two In a recent article, we presented the absorption spectra of reservoirs. Moreover, it would have blocked UV sunlight inhib- three SO2 isotopologues from 190 to 330 nm (13). Based on these iting the supposed photochemical process(es) producing the spectra, we present here the multiple isotope fractionation isotope anomaly. In contrast, given reducing conditions, two factors for SO2 photolysis as a function of wavelength. Further, 33 isotopically distinct sulfur species such as elemental sulfur we predict the ⌬ S value of sulfate aerosols under several (polysulfur) and sulfate could be formed photochemically and atmospheric UV-shielding scenarios and present a new inter- precipitate at the surface, where the MIF signal could be pretation of Archean atmospheric chemistry, which can explain preserved in sediment (3–5). Sulfur MIF in geological samples is the geological record of MIF. This approach is similar to our regarded as indicating an anoxic atmosphere. earlier experimental work with N2O, which was used in the This assertion relies on assumptions as to the origin of the isotopic analysis of the Anthropocene atmosphere (14). MIF, although the mechanisms are still poorly known. Broad Results agreement in the ⌬36S/⌬33S ratio (ϷϪ1) in pre-2.3 Ga sedimen- Wavelength-Dependent Isotope Effect of SO Photolysis. Fig. 1 shows tary rocks (1, 6, 7) and those produced in the laboratory by SO 2 2 the spectra of the fractionation and mass independent anomaly photolysis suggest that photodissociation of gas-phase SO2 most likely accounts for MIF in the Archean (2). Indeed, SO2 photolysis is a key rate limiting step, particularly in a low-O2 reducing Author contributions: Y.U., M.S.J., and N.Y. designed research; S.O.D. and C.E. performed atmosphere (4, 8). In addition, two mechanisms have been research; M.S.J. and N.Y. contributed new reagents/analytic tools; Y.U., M.S.J., S.O.D., and proposed whereby sulfur MIF could be formed under oxidizing A.P. analyzed data; and Y.U., M.S.J., and S.O.D. wrote the paper. conditions in the modern atmosphere: (i) Photolysis of SO3 at The authors declare no conflict of interest. altitudes greater than Ϸ50 km, where reaction between SO3 and This article is a PNAS Direct Submission. H2O is slow, is claimed to be responsible for the MIF found in 1To whom correspondence should be addressed. E-mail: [email protected]. modern stratospheric aerosol (8). In contrast, at lower altitudes, This article contains supporting information online at www.pnas.org/cgi/content/full/ SO from SO2 photolysis reforms SO2, so fractionation in SO2 0903518106/DCSupplemental. 14784–14789 ͉ PNAS ͉ September 1, 2009 ͉ vol. 106 ͉ no. 35 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0903518106 Downloaded by guest on September 28, 2021 shielding gas 60 20 800 10 A CO2 SO2 [‰] Ε 40 H2O OCS 33 [‰] 400 λ NH CS 3 2 20 Ε O2 O3 33 0 0 -100 -50 01019 50 100 150 and λ 19 34 ε ε -400 -20 10 [‰] 34 -40 -800 1024 1018 1016 -60 21 10 1023 1017 CS2 -80 -16 B 24 17 10 1018 10 10 ] SO2 2 O3 -100 error 1018 1019 10-18 Fig. 2. Process-dependent two- and three-isotope fractionations from SO2 OCS photolysis calculated for different columns of each shielding gas. The number 10-20 H2O adjacent to each point shows the overhead column of shielding gas (mole- 2 O2 NH3 cules/cm ). Points are spaced at one log unit intervals. Calculations were 10-22 terminated when opacity became Ͻ0.01. The star shows result for solar UV Cross section [cm spectrum without atmospheric attenuation. CO2 10-24 1013 photolysis in a given atmospheric scenario (15). The values of 34␧ [ϭ 1,000(34J/32J Ϫ 1)] and 33E[ϭ 1,000(33J/32J Ϫ (34J/32J)0.515)] are /nm] C 2 based on the isotopologues’ photolysis rates J, which are sensitive 1012 to the concentrations of UV-shielding molecules in the atmosphere. We consider CO2,H2O, O2,O3,NH3,CS2, OCS, and 1011 SO2, which possess significant absorption cross-sections in the wavelength region of the SO2 photodissociation band (190–220 nm). These few molecules are part of a greater number of 490 10 10 atmospherically relevant molecules that were investigated (SI Text). Actinic flux [photon/s/cm 109 Fig. 2 shows the resulting isotope effects of SO2 photolysis. 190 200 210 220 The isotope effects for different shieldings intersect at Ϸ34␧ ϭ 33 Wavelength [nm] Ϫ39‰ and E ϭϪ51‰. This region represents photolysis in an atmosphere with negligibly small opacity. When the column Fig. 1. UV spectra of isotopic fractionation factors, absorption by atmo- density increases, three types of behavior are seen. First, an spheric molecules and actinic flux. (A) Wavelength-dependent photolysis increase in CO2,H2O, NH3,CS2,orO2 concentration produces fractionation and three-isotope constants recalculated from high-precision a similar negative shift of 34␧ and 33E (Fig. 2), because these gases absorption spectra of SO2 isotopologues (13). The blue and red lines represent 34 32 34␧ generally attenuate wavelengths shorter than 202 nm (Fig. 1). In S/ S fractionation ( ␭) and the deviation from a mass-dependent relation- 34␧ 33 33 contrast, O3 and OCS shielding shift and E toward more ship ( E␭), respectively. Note that SO2 photolysis above and below 202 nm gives generally negative and positive 33E␭, values, respectively. The definition positive values because they attenuate wavelengths longer than of 33E␭ is different from that found in ref. 13. (B) The absorption spectra of the 202 nm. In contrast, an increase of O3 or OCS has the opposite 34 UV-shielding molecules. (C) Present-day solar spectrum (black; see ref.

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