Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v52b..01l&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #V52B-01
Other
0317 Chemical Kinetic And Photochemical Properties, 1028 Composition Of Meteorites (3662, 6240), 1041 Stable Isotope Geochemistry (0454, 4870), 5205 Formation Of Stars And Planets, 5225 Early Environment Of Earth
Scientific paper
Very large isotopic enrichments occur in the photolysis products of molecules that have line-type absorption spectra. Abundance-dependent line saturation, a process termed photochemical self-shielding, yields large mass-independent (MI) isotope effects, and has been proposed to have occurred in CO in the solar nebula (Clayton 2002) and SO2 in the early Earth atmosphere (Lyons 2007). The MI signatures derived from photolysis of CO and SO2 are believed to be recorded in primitive meteorites (CAIs) and in Archean/Paleoproterozoic sulfur sediments, respectively. Although self-shielding is a fairly straight-forward isotopic process, molecules with highly structured absorption spectra could, in principle, yield additional MI isotopic effects as a result of interactions of the photoexcitation state with other electronic states. Comparison of theory and experiment can be used to distinguish self-shielding from other processes. For SO2 a simplified theory (Lyons 2007) in which the sulfur isotopologue absorption spectra were approximated from measured 32SO2 spectra yields moderately good quantitative agreement with recent experiments (Pen and Clayton 2008), and confirms that self-shielding does occur during SO2 photolysis. Measurement of high resolution isotopologue absorption spectra will substantially improve the self-shielding calculations, and such measurements are in progress by several groups. Here, I will present self-shielding model results for both sulfur and oxygen isotopes. For CO most photochemical calculations (e.g., Lyons and Young 2005) have utilized 'shielding functions' (van Dishoeck and Black 1988) to quantify isotope enrichment effects. A recent experiment evaluating oxygen isotopic effects during CO photolysis (Chakraborty et al. 2008) obtained very large enrichments (~1000s ‰), as expected for self-shielding, but with δ17O/δ18O ratios for product O that range from 0.65 to 1.45 depending on wavelength, a much wider range than is possible from shielding functions. The authors interpreted these results to be evidence that CO self-shielding was not responsible for the large enrichments they measured. However, the optical depths of their gas samples were very high (~100-1000), and so self-shielding by C16O was unavoidable in their experiments. I will present line-by- line radiative transfer self-shielding simulations of their experiments. As with SO2 high-resolution CO isotopologue absorption spectra are needed to fully evaluate the isotopic effects of CO self-shielding.
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