The atmosphere has changed drastically over Earth's 4.6 billion year lifespan. Anomalous patterns among the isotopes of sulfur hold great promise for providing insight into the evolution of atmospheric chemistry on both Earth and Mars. The enigmatic signals known as sulfur mass-independent isotope fractionation (S-MIF) are seen in aerosol particles in the stratosphere, within ice core sulfates, in Martian meteorites, as well as among the most ancient sedimentary rocks on the planet, with ages of up to 3.8 billion years old. It is remarkable that this unique signature generated by the photolysis of SO2 (Farquhar 2001) remains preserved in the rock record, providing a direct record of an ancient reducing atmospheric chemistry. In particular, the presence of S-MIF is a defining signature of sedimentary rocks laid down prior to Earth's great oxidation, which occurred approximately 2.4 billion years ago. Subtle variations in the magnitude of S-MIF may reflect changes in the concentrations of CO2, biogenic sulfur compounds, or the presence of a Titan-like organic haze during periods of Earth history (Ueno, 2009 ; Halevy, 2010 ; ; Zerkle, 2011 ; Claire, 2012). Understanding how the signature is formed, transported, deposited, and stored, will enable us to constrain the composition and structure of the paleo atmosphere.

This project will primarily involve numerical modelling of atmospheric chemistry, specifically using a 1-D photochemical model (Zerkle, 2011 ; Claire, 2012) enhanced to study the 4 isotopes of sulfur. The model will be validated against existing geochemical data from Earth system environments, and applied to the study of sulfate aerosols in the modern atmosphere as well as constrain atmospheric chemistry before Earth's great oxidation. The project is part of an international collaborative effort investigating the influence of biological forcing on the atmosphere in the time period surrounding the great oxidation, and so provides the potential for additional Earth Science skills such as field work and/or measuring carbon and sulfur isotopes given student interest.  The numerical skills learned would be broadly applicable in a career in atmospheric chemistry or other quantitative Earth Sciences, and require a motivated student with at least a 2.1 honours degree in chemistry, mathematics, physics, computing, or a branch of environmental science with good numerical ability. The student will be actively encouraged to network through attendance at conferences, workshops and meetings at institutional, national and international levels. The project will be co-advised by Professor Andy Watson and would form a solid foundation for a career in Earth or planetary sciences.

Claire M; Kasting J; Buick R & Stueeken E "The magnitude of mass-independent sulfur fractionation in the Archean atmosphere" PNAS, in review, 2012.

Farquhar J; Bao H; Thiemens M "Atmospheric influence of Earth's earliest sulfur cycle" SCIENCE (289) 5480, p.756-758, 2001.Halevy I; Johnston D.; Schrag D. "Explaining the Structure of the Archean Mass-Independent Sulfur Isotope Record" SCIENCE (329) 5988 p. 204-207, 2010.

Ueno Y; Johnson M; Danielache S; et al. "Geological sulfur isotopes indicate elevated OCS in the Archean atmosphere, solving faint young sun paradox" PNAS (106) 35 p. 14784-14789 , 2009

Zerkle, A; Claire, M; Domagal-Goldman, S. et al. "Redox state of the Neoarchean Earth environment" NATURE GEOSCIENCES, DOI: 10.1038/NGEO1425, 2012.