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

Minor sulfur isotope anomalies indicate the absence of O-2 from the Archean atmosphere. A rich dataset showing large variations in magnitude and sign of Delta S-33 and Delta S-36, preserved in both sulfates and sulfides, suggests that further constraints on Archean atmospheric chemistry are possible. We review previous quantitative constraints on atmospheric Delta S-33 production, and suggest that a new approach is needed. We added sulfur species containing S-33 and S-34 to a 1-D photochemical model and describe the numerical methodology needed to ensure accurate prediction of the magnitude and sign of Delta S-33 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 Delta S-33 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 Delta S-33 formation. We find that differential absorption through the Danielache et al. (2008) cross sections is capable of altering predicted Delta S-33 as a function of multiple atmospheric variables, including trace O-2 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 Delta S-33 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 O-2, as soon as additional experimental measurements of MIF-S producing processes become available. (C) 2014 The Authors. Published by Elsevier Ltd.