5:15 PM - 6:30 PM
[BCG04-P05] Sulfur mass-independent fractionation patterns during SO2 photolysis at low temperatures and low pressures: Implications for Archean atmospheric compositions
Keywords:Archean, sulfur isotopes, mass-independent fractionation, sulfur dioxide, photochemistry
Knowledge on redox states of planetary atmosphere is central to understanding of the climate, geochemical cycles, and habitability. However, the redox states of atmospheres, especially presence of reducing CH4, H2, and/or CO, on early Earth and early Mars remain poorly constrained because of lack of direct indicators of reducing gas molecules in the atmosphere. Mass-independent fractionation of sulfur isotopes (MIF-S) in Archean sedimentary rocks and Martian meteorites would provide unique insights into the redox states of the atmospheres. Because SO2 photolysis generates large MIF-S, it is a strong candidate of the origin(s) [1]. Since photochemical reactions of SO2 in the presence of redox-sensitive molecules, such as CO and CH4, can cause particular MIF-S trends [2], the existence of the MIF-S trends can be proxy indicators not only for the occurrence of photolysis of SO2, but also for a reducing atmosphere at the time of deposition. However, MIF-S trends (Δ36S/Δ33S, Δ33S/δ34S slopes) produced by laboratory experiments are also sensitive to reaction conditions, such as temperature and pressure [1-3]. In fact, no previous photolysis experiments, thus far, can reproduce the Archean or Martian MIF-S trends quantitatively.
Here, we focus on a temperature dependency on MIF-S trends. This is the case because MIF-S during SO2 photolysis is known to depend on total pressure, which likely results from a change in the absorption line width of SO2 [3]. Accordingly, MIF-S trends should be also highly sensitive to temperature. By extrapolating the dependency of MIF-S trends on the absorption line width, the Archean trends are expected to be reproduced by MIF-S in SO2 photolysis with narrow absorption widths at very low pressures and temperatures. In Archean’s atmosphere without the ozone layer, the stratospheric temperatures should have been lower than those of today. To examine the temperature dependency experimentally and quantitatively, we developed a new photo-cell reaction system that can be cooled down to −60ºC. We measured MIF-S during SO2 photolysis under both low temperatures and low pressures, corresponding to stratospheric conditions on Archean Earth. We present new data of temperature dependency of MIF-S, and will revisit redox states and atmospheric compositions in Archean.
References
[1] Farquhar et al. (2001). JGR, 106(E12), 32829-32839.
[2] Endo et al. (2016). EPSL, 453, 9-22.
[3] Endo et al. (2019). GRL, 46(1), 483-491.
Here, we focus on a temperature dependency on MIF-S trends. This is the case because MIF-S during SO2 photolysis is known to depend on total pressure, which likely results from a change in the absorption line width of SO2 [3]. Accordingly, MIF-S trends should be also highly sensitive to temperature. By extrapolating the dependency of MIF-S trends on the absorption line width, the Archean trends are expected to be reproduced by MIF-S in SO2 photolysis with narrow absorption widths at very low pressures and temperatures. In Archean’s atmosphere without the ozone layer, the stratospheric temperatures should have been lower than those of today. To examine the temperature dependency experimentally and quantitatively, we developed a new photo-cell reaction system that can be cooled down to −60ºC. We measured MIF-S during SO2 photolysis under both low temperatures and low pressures, corresponding to stratospheric conditions on Archean Earth. We present new data of temperature dependency of MIF-S, and will revisit redox states and atmospheric compositions in Archean.
References
[1] Farquhar et al. (2001). JGR, 106(E12), 32829-32839.
[2] Endo et al. (2016). EPSL, 453, 9-22.
[3] Endo et al. (2019). GRL, 46(1), 483-491.