Sulfur mass-independent fractionation during SO2 photolysis in low-temperature/pressure atmospheres
Introduction
Mass-independent fractionation (MIF) is an anomalous form of isotopic fractionation that is useful as a geochemical tracer (e.g., Farquhar and Wing, 2003; Thiemens and Lin, 2019). Signatures of sulfur MIF (MIF-S) have been found in Archean sedimentary rocks (e.g., Farquhar et al., 2000a), modern stratospheric sulfate aerosol (SSA) deposited in polar ice (e.g., Savarino et al., 2003b), and Martian meteorites (e.g., Farquhar et al., 2000b). Archean MIF-S may provide insights into atmospheric composition, particularly redox states, at that time (e.g., Ono, 2017; Stüeken et al., 2020), and modern SSA MIF-S may provide insights into the occurrence of Quaternary volcanic eruptions that impacted the stratosphere (e.g., Gautier et al., 2019; Burke et al., 2019).
Sulfur isotopic compositions can be described in terms of δ33S, δ34S, δ36S, Δ33S, and Δ36S values, defined as follows (e.g., Ono, 2017):where 3x represents 33, 34, or 36, and 3xR represents the ratio 3xS/32S. The δ3xS values are usually obtained by normalizing sample values (3xRsample) against the international reference standard (3xRstandard) Vienna Cañon Diablo Troilite (V-CDT). Previous studies have indicated that Archean MIF-S is characterized by Δ36S/Δ33S ratios of −1.5 to −0.8 (e.g., Kaufman et al., 2007; Ueno et al., 2008; Zerkle et al., 2012; Thomassot et al., 2015; Izon et al., 2017; Mishima et al., 2017; Poulton et al., 2021; Chen et al., 2022) and Δ33S/δ34S ratios of ~ + 0.9 (Ono et al., 2003, Ono et al., 2009). Modern SSA MIF-S is characterized by Δ36S/Δ33S ratios of −1.56 ± 0.25 (Gautier et al., 2018) (before being updated by Gautier et al. (2018), the ratio was −4.3 (Savarino et al., 2003b), which was matched with results of SO2 photolysis experiments by Ono et al. (2013), see below) and Δ33S/δ34S ratios of +0.09 ± 0.02 (Gautier et al., 2018; Burke et al., 2019; Crick et al., 2021). Despite the abundance of MIF-S data in natural samples, the mechanism(s) responsible for its occurrence in Archean rocks and modern SSA are still debated, largely because MIF-S in natural samples has not been reproduced quantitatively in laboratory experiments (e.g., Ono, 2017).
Earlier experiments indicated that SO2 photolysis with UV irradiation at wavelengths of <220 nm causes MIF-S (e.g., Farquhar et al., 2001). In SO2 photolysis, isotopic self-shielding is known to result in strong MIF-S through changes in UV spectra caused by SO2 self-absorption (mainly by 32SO2), significantly reducing the 32SO2 photolysis rate with resulting isotopic fractionation being independent of S mass, but rather dependent on the isotopic abundance and SO2 column density (Lyons, 2007; Ono et al., 2013). The isotopic self-shielding can explain several characters MIF-S observed in SO2 photolysis experiments, such as SO2 column density dependence and total pressure dependence (Endo et al., 2022), although the mechanisms of the MIF-S in SO2 photolysis are still debated. For example, perturbations to the system induced by interstate crossings also may cause significant MIF-S (Thiemens and Lin, 2019), which is suggested by anomalous huge isotope fractionation in N2 photolysis (Chakraborty et al., 2014). However, there is no experimental evidence that the perturbation causes significant MIF-S in SO2 photolysis at the wavelength. Previous studies have shown that self-shielding in SO2 photolysis results in isotopic fractionation with Δ36S/Δ33S ratios of −4.6 ± 1.3 (Ono et al., 2013), remarkably lower than those of Archean and modern SSA MIF-S. The Δ36S/Δ33S ratio in self-shielding SO2 photolysis depends on total gas pressure with a trend of higher ratios at lower total pressures, although the ratio becomes almost constant at ~ − 2.5 below ~10 kPa (~0.1 bar; Masterson et al., 2011; Endo et al., 2019), which is inconsistent with Archean and modern SSA MIF-S. The mechanism of the total-pressure dependence of Δ36S/Δ33S ratios involves changes in absorption line widths of SO2 in self-shielding. Absorption lines of SO2 become broader with increasing pressure above ~10 kPa total pressure, whereas the SO2 Doppler width predominates at lower pressures, with the absorption line width becoming independent of total pressure (Lyons et al., 2018; Endo et al., 2019).
The temperature dependence of MIF-S in SO2 photolysis is also expected to play a role in determining Δ36S/Δ33S ratios because the Doppler width depends on temperature (e.g., Endo et al., 2022). Furthermore, the temperature dependence of the rotational population of the ground state increases absorption cross-sections at peak wavelengths at lower temperatures (e.g., Wu et al., 2000; Rufus et al., 2009), which may also contribute to the temperature dependence. It is not obvious which affects more significantly the isotope fractionations at this time.
SO2 photolysis likely occurs in the current stratosphere during large eruptions (Pavlov et al., 2005; Whitehill et al., 2015) or occurred near the tropopause on Archean Earth (Domagal-Goldman et al., 2008; Claire et al., 2014) where temperatures and pressures are (or were) low. However, most previous experiments with SO2 photolysis were performed at room temperature (Farquhar et al., 2001; Masterson et al., 2011; Whitehill and Ono, 2012; Ono et al., 2013; Franz et al., 2013; Endo et al., 2016, Endo et al., 2019; Velivetskaya et al., 2020) or high temperatures (Ignatiev et al., 2019). The only previous study of SO2 photolysis at low temperatures (225, 250, and 275 K) seems to have been that of Whitehill et al. (2015), with a total pressure of 101.3 kPa. There is thus a lack of SO2 photolysis experiments at low temperatures and low pressures, where SO2 photolysis would occur on both modern and Archean Earth.
We undertook laboratory experiments with SO2 photolysis at low temperatures and pressures, determining sulfur isotopic compositions of photolysis products. An experimental system with a reaction cell capable of cooling to low temperatures comparable to those of the stratosphere was developed (Section 2) and applied in investigating the temperature dependence of MIF-S during SO2 photolysis at low pressures, where total pressure has no effect (Section 3). Implications for the mechanisms of Archean and modern SSA MIF-S are discussed in Section 4.
Section snippets
Photochemical experiments
Photolysis experiments involved a reaction cell covered with a cooling jacket, as illustrated in Fig. 1. A D2 lamp (L1835, Hamamatsu Photonics K.K., Japan) was used as a continuous UV source, with the photolysis rate of SO2 being much higher than its photoexcitation rate. To remove the intense D2 irradiation at 161 nm, CO2 (80 kPa) was introduced into a separate cell in front of the reaction cell (Fig. 1), also aiding the prevention of condensation of atmospheric water vapor on the
Gas compositional changes
The trends in SO2, OCS, and CO2 concentrations during UV irradiation are illustrated in Fig. 2a–c, which shows concentrations normalized to initial SO2 concentrations (SO2 t =0 because the amounts of gases produced depended on initial SO2 concentrations that differed among runs). Increasing normalized OCS and CO2 concentrations with time (Fig. 2a, b) indicate that UV irradiation of CO and SO2 gas mixtures formed OCS and CO2. SO2 would have been consumed by photolysis, but reductions in
Reactions and their temperature dependence
OCS and CO2 are generated during UV irradiation of CO and SO2 gas mixtures (Fig. 2) through the following reactions (Endo et al., 2016):where M represents any third-body reaction partner. The net reaction of (R1), (R2), (R3), (R4) can be described as
Based on the stoichiometry of reaction R5, the CO2/OCS ratio should be 2. The CO2/OCS ratio became >2 at 266 K and 296 K, implying that reduced photochemical products other than
Conclusions
The temperature dependence of sulfur isotopic fractionation during SO2 photolysis at low pressures in the temperature range of 228–296 K was investigated. At a fixed SO2 column density, where isotopic self-shielding occurs, magnitudes of fractionation factors (34ε, 33E, and 36E) increase as temperatures decrease, with those at ~228 K being about four times those at room temperature. However, 33E/34ε and 36E/33E ratios are roughly independent of temperature within the range of experiments here,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We would like to thank Mayuko Nakagawa for assistance in sulfur isotope analysis. We also thank Shuhei Ono and an anonymous reviewer for constructive comments. This research is funded by JSPS KAKENHI Grant Number JP17H06456 and JP20K14593.
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