Sulfur mass-independent fractionation patterns in the broadband UV photolysis of sulfur dioxide: Pressure and third body effects
Abstract
The production of mass-independent fractionation (MIF) in the products of the middle/far UV photolysis of SO 2 isotopologues has been linked to large S-MIF signatures observed in the pre-2.45 Ga rock record, and provides a valuable proxy for the evolution of atmospheric O 2. The origin of such large MIF signatures in photolytic products has recently been ascribed to the optical self-shielding of SO 2 isotopologues in photon-limited regions of the lower atmosphere. To better characterize the origin of such a signal we have performed pressure-variant broadband UV photolysis experiments of SO 2 and SO 2-He mixtures. Low pressure (~ 20 Torr) photolysis of SO 2 produces extractable sulfur with large δ 34S ≈ 120-180‰, Δ 33S ~ 20‰, and Δ 36S/Δ 33S ≈ - 2.5, while near atmospheric pressures produce sulfur with Δ 33S ~ 2‰, and Δ 36S/Δ 33S ≈ - 12. Similar experiments conducted with variable pressures of helium bath gas produce a similar diminution in Δ 33S, and in both cases a Δ 36S/Δ 33S ratio that is a linear function of pressure over several orders of magnitude. We postulate a photochemical and kinetic origin for the large S-MIF observed in these static cell experiments, produced from the excited state predissociative dynamics of SO 2, but cannot expressly rule out self-shielding as a contributor to fractionations observed here.
- Publication:
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Earth and Planetary Science Letters
- Pub Date:
- June 2011
- DOI:
- 10.1016/j.epsl.2011.04.004
- Bibcode:
- 2011E&PSL.306..253M