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Quantum chemical and chemical kinetics calculations of sulfur and sulfur exchange reactions related to mass-independent fractionation of sulfur isotopes
PI: Lyons, James
The discovery (Farquhar et al. 2000) of mass-independent fractionation (MIF) in sulfur isotopes preserved in Archean and Paleoproterozoic rocks has solidified our understanding of the rise of O 2 in the ancient Earth atmosphere. Photochemical modeling has shown that a maximum concentration of O 2 of 10-5 times the present atmospheric level is necessary to maintain distinct reservoirs of sulfur MIF in the Archean atmosphere. All of these conclusions have been reached without an understanding of exactly what the chemical mechanisms of sulfur MIF were in the ancient atmosphere. Elucidating these mechanisms will allow for more refined constraints on O 2 , and, more importantly, will yield quantitative constraints on the sulfur concentration in the ancient atmosphere. In this study we will perform high-level quantum chemical (also called ab initio) calculations of the rate coefficients for a suite of sulfur reactions and sulfur exchange reactions that are likely to be of importance in a low O 2 atmosphere. In addition, we will incorporate the ab initio results into chemical kinetics models of the Archean atmosphere with the goal of determining which of several possible sulfur MIF mechanisms are most important. The proposed work is complementary to experimental investigations of MIF signatures produced during photolysis of sulfur-containing gases ongoing in the laboratory of J. Farquhar at the University of Maryland. The expected results from this study are as follows: i) an accurate determination of potential energy surfaces and rate coefficients for several important sulfur reactions and sulfur exchange reactions; ii) incorporation of these rate coefficients into a chemical kinetics model of a low O 2 atmosphere, and evaluation of several possible sulfur MIF mechanisms. Ultimately, we expect to be able to constrain the atmospheric concentration of sulfur (most likely as SO 2 ) based on the sulfur MIF mechanism. The proposed study fits within the Exobiology and Evolutionary Biology program under the research area “Early Evolution of Life and the Biosphere”. By bringing some of the tools utilized in modern atmospheric chemistry (i.e., high-level quantum chemistry methods), we believe we can contribute significantly to understanding the full implications of sulfur MIF to the environment of the Archean and Paleoproterozoic Earth.
