9:30 AM - 9:45 AM
[BCG06-03] Data compilation and photochemical model of sulfur mass-independent fractionation: Atmospheric evolution in the Archean
Keywords:Mass independent fractionation, Archean, Atmospheric composition in the early Earth
Elucidation of the early Earth’s atmosphere is important not only for understanding the history of the Earth but also for exoplanets. Atmospheric oxygen levels in the Archean were estimated to be several orders of magnitude lower than the present value, whereas greenhouse gases, such as carbon dioxide and methane, were estimated to be abundant1. However, the composition and its temporal change of atmosphere in the Archean remain unclear because of the lack of geochemical proxies to directly estimate atmospheric compositions. One of the direct tracers for atmospheric composition is sulfur mass-independent fractionation (S-MIF), defined by the difference from mass-dependent fractionation, that is basically found in the geologic record before 2.4 billion years ago2. The S-MIF was interpreted to have formed through photochemical reactions of SO2, offering evidence that the Archean atmosphere was anoxic based on photochemical experiments and model calculations. However, the mechanism to produce S-MIF in the atmosphere is still poorly understood because of the large discrepancies between the geologic records and experimental results or model calculations3. In particular, the Δ33S/δ34S and Δ36S/Δ33S ratios, called the Archean Reference Array (ARA), are typically known to be ca. +0.9 and ca. -0.9, respectively. However, neither photochemical experiments nor model calculations simultaneously have reproduced these values. In addition, a decrease in the Δ36S/Δ33S ratio to ca. -1.5 was observed in the Neoarchean. Previous studies have suggested that this was caused by elevated methane concentrations4,5; however, this has not been quantitatively supported by any model calculations. Thus, we compiled S-MIF data of the Nulliak supracrustal rocks (~3.9 Ga) and from the literature, as well as performed numerical calculations including multiple SO2 photochemical reactions and gas shielding effects in order to elucidate the atmospheric composition and its variations during the Archean.
We used only sulfur isotope data with Δ33S > 0 to eliminate the effect of microbial sulfate reduction. Linear regressions were obtained along the Δ33S vs. δ34S and Δ36S vs. Δ33S data, and the maximum value of δ34S was substituted into the regression lines to estimate Δ33S/δ34S and Δ36S/Δ33S ratios of the atmospheric reaction array in the Archean. The results show a positive correlation between the estimated Δ33S/δ34S and Δ36S/Δ33S ratios. This indicates that variations in atmospheric composition simultaneously caused changes in both the Δ36S/Δ33S and Δ33S/δ34S ratios. Additionally, the results indicate the Δ36S/Δ33S ratio changed from -1.7 to -0.9 in the early Archean.
The photochemical model calculations include wavelength-dependent effects in the photolysis of SO2, the self-shielding effect in the photolysis of SO2, and the intersystem crossing effect in the photoexcitation of SO2. We first performed the calculations by varying the composition of the background atmosphere, consisting of N2, CO2, and CH4, and the SO2 column density. We found that the self-shielding effect became pronounced for SO2 column densities above ~1015 molecules/cm2, and that the Δ36S/Δ33S ratio changed significantly for SO2 column densities above ~1016 molecules/cm2. Given that the Δ36S/Δ33S ratio in the geologic record is nearly constant, the S-MIF is presumed to be due to reactions under atmospheric conditions with an SO2 column density below ~1016 molecules/cm2. In addition, the two ARAs (Δ33S/δ34S ~ +0.9 and Δ36S/Δ33S ~ -0.9) were reproduced simultaneously at CH4 concentrations between 4×1022 and 9×1022 molecules/cm2, independent of the CO2 concentration. This column density corresponds to thousands to tens of thousands of ppm, suggesting that abundant methane concentrations were maintained throughout the Archean. Moreover, both the Δ36S/Δ33S and Δ33S/δ34S ratios decrease with decreasing methane concentrations. This contradicts the theory that the decrease in the slope in the Neoarchean was caused by an increase in methane concentration, and suggests that the atmosphere became temporarily oxidized.
1. Catling and Zahnle, Sci. Adv. 6, 1420, 2020 2. Farquhar et al. Science 289, 756-758, 2000 3. Ono, Annu. Rev. Earth Planet. Sci. 45, 301-329, 2017 4. Izon et al. Earth Planet. Sci. Lett. 431, 264-273, 2015 5. Izon et al. Proc. Natl. Acad. Sci. U. S. A. 114, 2571-2579, 2017
We used only sulfur isotope data with Δ33S > 0 to eliminate the effect of microbial sulfate reduction. Linear regressions were obtained along the Δ33S vs. δ34S and Δ36S vs. Δ33S data, and the maximum value of δ34S was substituted into the regression lines to estimate Δ33S/δ34S and Δ36S/Δ33S ratios of the atmospheric reaction array in the Archean. The results show a positive correlation between the estimated Δ33S/δ34S and Δ36S/Δ33S ratios. This indicates that variations in atmospheric composition simultaneously caused changes in both the Δ36S/Δ33S and Δ33S/δ34S ratios. Additionally, the results indicate the Δ36S/Δ33S ratio changed from -1.7 to -0.9 in the early Archean.
The photochemical model calculations include wavelength-dependent effects in the photolysis of SO2, the self-shielding effect in the photolysis of SO2, and the intersystem crossing effect in the photoexcitation of SO2. We first performed the calculations by varying the composition of the background atmosphere, consisting of N2, CO2, and CH4, and the SO2 column density. We found that the self-shielding effect became pronounced for SO2 column densities above ~1015 molecules/cm2, and that the Δ36S/Δ33S ratio changed significantly for SO2 column densities above ~1016 molecules/cm2. Given that the Δ36S/Δ33S ratio in the geologic record is nearly constant, the S-MIF is presumed to be due to reactions under atmospheric conditions with an SO2 column density below ~1016 molecules/cm2. In addition, the two ARAs (Δ33S/δ34S ~ +0.9 and Δ36S/Δ33S ~ -0.9) were reproduced simultaneously at CH4 concentrations between 4×1022 and 9×1022 molecules/cm2, independent of the CO2 concentration. This column density corresponds to thousands to tens of thousands of ppm, suggesting that abundant methane concentrations were maintained throughout the Archean. Moreover, both the Δ36S/Δ33S and Δ33S/δ34S ratios decrease with decreasing methane concentrations. This contradicts the theory that the decrease in the slope in the Neoarchean was caused by an increase in methane concentration, and suggests that the atmosphere became temporarily oxidized.
1. Catling and Zahnle, Sci. Adv. 6, 1420, 2020 2. Farquhar et al. Science 289, 756-758, 2000 3. Ono, Annu. Rev. Earth Planet. Sci. 45, 301-329, 2017 4. Izon et al. Earth Planet. Sci. Lett. 431, 264-273, 2015 5. Izon et al. Proc. Natl. Acad. Sci. U. S. A. 114, 2571-2579, 2017