Modeling the signature of sulfur mass-independent fractionation produced in the Archean atmosphere

Minor sulfur isotope anomalies indicate the absence of O₂ from the Archean atmosphere. A rich dataset showing large variations in magnitude and sign of δ³³S and δ³⁶S, preserved in both sulfates and sulfides, suggests that further constraints on Archean atmospheric chemistry are possible. We review previous quantitative constraints on atmospheric δ³³S production, and suggest that a new approach is needed. We added sulfur species containing ³³S and ³⁴S to a 1-D photochemical model and describe the numerical methodology needed to ensure accurate prediction of the magnitude and sign of δ³³S produced by and deposited from the Archean atmosphere. This methodology can test multiple MIF-S formation mechanisms subject to a variety of proposed atmospheric compositions, yielding δ³³S predictions that can be compared to the rock record. We systematically test SO₂ isotopologue absorption effects in SO₂ photolysis (Danielache et al., 2008), one of the primary proposed mechanisms for δ³³S formation. We find that differential absorption through the Danielache et al. (2008) cross sections is capable of altering predicted δ³³S as a function of multiple atmospheric variables, including trace O₂ concentration, total sulfur flux, CO₂ content, and the presence of hydrocarbons, but find a limited role for OCS and H₂S. Under all realistic conditions, the Danielache et al. (2008) cross sections yield δ³³S predictions at odds with the geologic record, implying that additional pathways for sulfur MIF formation exist and/or the cross sections have significant errors. The methodology presented here will allow for quantitative constraints on the Archean atmosphere beyond the absence of O₂, as soon as additional experimental measurements of MIF-S producing processes become available.