MULTIPLE SULFUR AND OXYGEN ISOTOPE FRACTIONATION FACTORS DURING UV PHOTOLYSIS OF SO 2

The mass-independent fractionation (MIF) of sulfur isotopes during UV photolysis of SO₂ is a key result that has been used to relate the MIF-S found in many Archean rocks to the concentration of O₂ in the Archean atmosphere. MIF-S during SO₂ photolysis only occurs at UV wavelengths that would be adsorbed by significant concentrations of O₂ and O₃, so the presence of MIF-S implies that O₂ concentrations were very low in the Archean atmosphere. Since the study of Farquhar et al. (2001), relatively little further direct experimental research has been devoted to studying MIF during UV photolysis of SO₂.

This study has performed UV photolysis experiments using a Nd-YAG laser (213 nm wavelength) and a low pressure Hg lamp (wavelengths of 185 & 254 nm), for a 5% SO₂–95% He gas mixture within synthetic quartz reactor vessels equipped with a gas syringe sampling valve. Stable isotope analyses were performed using the SO⁺ fragment of SO₂ gas, and the concentration of SO₂ gas was also measured. Analyses were performed both with and without passing the SO₂ gas through a tube packed with quartz at 890°C, which buffers δ¹⁸O of the SO₂ to a constant composition.

MIF-S results obtained with the low pressure Hg lamp show excellent agreement with the results of Farquhar et al. (2001), yield very large values for ³³α_S and ³⁴α_S, and demonstrate a relatively efficient mechanism for generating MIF-S, as large negative values of Δ³³S are produced at relatively modest values of ln F and δ³⁴S. Values of δ⁴⁹ and δ⁵⁰ differ between the O-isotope buffered vs. non-buffered experiments, indicating that O isotope fractionation also occurs during SO₂ photolysis. Furthermore, using ³³α_S and ³⁴α_S values determined for the O-isotope buffered experiment, the results for the non-buffered experiment allow for calculation of the values of ¹⁷α_O and ¹⁸α_O, and suggest that extensive MIF of O isotopes is also taking place. In comparison, results using the Nd-YAG laser show that both S and O isotope fractionation occurs, but indicate that little or no MIF-S or MIF-O takes place.

Results illustrate the utility of the experimental design for studying multiple S and O isotope effects during UV photolysis of SO₂, and suggest the possibility that MIF of oxygen may have been produced during the Archean, in addition to MIF of sulfur.

Farquhar et al. (2001) JGR 106: 32829-39.