2006 Annual Science Report
University of California, Los Angeles Reporting | JUL 2005 – JUN 2006
Sulfur Isotopes in the Early Earth Atmosphere
This project is concerned primarily with the mechanism by which mass-independent fractionation (MIF) in sulfur was produced in the early Earth atmosphere. My recent work has focused on two possible mechanisms for producing sulfur MIF: 1) gas-phase reactions forming S3, the sulfur equivalent to O3, and 2) isotope-selective photolysis of SO2, which has been demonstrated in various laboratory experiments to yield sulfur with a MIF signature. Identifying the mechanism of sulfur MIF is essential to advancing our understanding of the implications of the discovery (Farquhar et al. 2000) of MIF in ancient sulfur-bearing rocks.
With chemistry colleagues (not NAI members), I have investigated the structure and energetics of S3. We have performed high-level ab initio calculations of the ground state and excited states of S3 using the GAUSSIAN03 and MOLPRO computational packages, respectively. The ground state work was published in August 2005 in J. Chemical Physics. The work on low-lying excited states of S3 has been accepted for publication in J. Chemical Physics later this year. These two publications show that S3, and by extension other poly-sulfur compounds, can be accurately treated by quantum chemical methods.
I have recently presented (as sole author) results on isotope-selective photodissociation of SO2 at AbSciCon, AGU, and EGS. In this work I have shown by radiative transfer calculations that the highly structured absorption spectrum of SO2 becomes slightly shifted upon sulfur isotope substitution. Photolysis of the four superimposed and slightly shifted SO2 spectra (one for each sulfur isotope) results in very large mass-dependent fractionations, and smaller but significant MIF. This is the origin of the sulfur MIF signature in Earth’s early atmosphere. With this mechanism in hand, it will now be possible to fully quantify sulfur isotope atmospheric chemistry models. I am presently writing up this work for publication.
RELATED OBJECTIVES:Objective 4.1
Earth's early biosphere