MISFIT · Mass-Independent SulFate IsoTopes

Mass-Independent SulFate IsoTopes

The discovery of sulfur and oxygen isotope mass independent fractionation (S- and O- MIF) provides a direct link between atmospheric composition and signals in the rock and ice core records and a new metric for the investigation of early Earth as well as past and present day atmospheric chemistry. While the origin of O-MIF and its transfer to other oxygen-bearing compounds (e.g., H2O2, NO3-) are relatively well understood, recently controversy has emerged regarding the fundamental step lying at the origin of the S-MIF. Even the consensus that S-MIF is triggered by exposure of SO2 to UV radiation is now under debate. We should thus acknowledge that after passing the first wave of euphoria following the discovery of S-MIF in Archean and volcanic samples, the interpretation of the S-MIF does not rest on a solid mechanistic foundation. The confusion on the origin of S-MIF is particularly detrimental as the occurrence of S-MIF in both Archean rock samples and modern volcanic sulfate in polar ice offers the opportunity to examine the dependence of the MIF-signals in relation with the evolution of Earth’s atmosphere and surface environment. As such, recently NASA has placed the resolution of the origin of the S-MIF as one of the top priorities for its astrobiology program, recognizing the importance of MIF in solving the epic question of the origin of life and its interaction with the planetary environment. In this project, we aim to determine the processes responsible for S-MIF and assess the implications for the distribution of S-MIF in atmospheric sulfate, and to build a quantitative understanding of atmospheric MIF processes including its origin and transfer using O- and S-MIF model simulations, ensuring proper extraction of information embedded in sulfate MIF data. In order to achieve the objectives, we carried out a new set of chamber experiments on SO2-related production of S-MIF considering environmental conditions that are as close as possible to those of the stratosphere from where S-MIF in modern sulfate samples were observed. New protocol to process sulfate samples for S-MIF analysis was developed, and O- and S-MIF analysis were conducted on sulfate samples produced from chamber experiments. Preliminary analysis on the isotopic data suggests the role of SO2 photo-excitation in creating the observed S-MIF, with implications for the effects of SO2 isotopologues absorption cross section difference, self-shielding and intersystem crossing on the systematics of S-MIF. Further work need to be done to explore quantitatively the relative contributions of the above-mentioned processes to the observed S-MIF. Overall, the isotopic results, combined with chamber experimental parameters, will be used to constrain the origin of S-MIF, and to develop S-MIF and O-MIF isotope chemistry schemes during the formation of atmospheric sulfate. The later will be coupled and incorporated into a global chemistry-transport model (i.e., GEOS-chem) to test the proposed origin and global distribution of S-MIF following natural processes, i.e., volcanic eruptions. This work is in progress.

Data: CORDIS, © European Union