Sulfur Isotope Fractionation Due to SO2 Photolysis in the Atmosphere
Abstract
The discovery of unusual (i.e. mass-independent) sulfur isotope fractionation (or MIF) in Archean and Paleoproterozoic sedimentary rocks has promised to yield insights into the rise of O2 and the nature of the sulfur cycle on ancient Earth [1], but interpretation has been hampered by the lack of a clear mechanism for the sulfur isotope signature. Proposed MIF mechanisms include SO2 photolysis [1-4], atmospheric S3 (thiozone) formation, and thermal sulfate reduction in sediments [5]. Studies focusing only on SO2 photolysis, including measurements of isotopic cross sections [6], have yielded results differing greatly from theory [4], and have resulted in improbable interpretations [7]. In addition to ancient rocks, there are sulfur isotope MIF signatures in polar ice core sulfates associated with massive Plinian eruptions over the past ~1000 years (e.g., [8]). The ice core MIF signatures differ significantly from the ancient Earth MIF signatures, suggesting a different source mechanism. SO2 photolysis can generate sulfur isotope MIF signatures in two ways: 1) self-shielding by an optically-thick column of SO2, and 2) isotope-dependent differences in absorption line intensities and widths, which are espcially important for optically-thin conditions. The MIF signatures in ice core sulfates appear to be consistent with self-shielding in an optically-thick plume, but the Archean MIF clearly is not. To address the optically-thin case, we've made high-resolution ultraviolet cross section measurements of the sulfur isotopologues of SO2 made with the UV FTS at Imperial College. We measured cross sections at 1 cm-1 spectral resolution for 32SO2, 33SO2, 34SO2 and for a 36SO2/34SO2 mixture. Incorporating these cross sections into a simple atmospheric photochemical model with a solar UV flux, we find sulfur MIF signatures for SO and S that.are consistent with the Archean pyrites. We also find that additional mass-dependent fractionation during self-shielding by 32SO2 places an upper limit on the SO2 mixing ratio of ~1 ppb. Our results imply that SO2 photolysis alone is responsible for most of the Archean sulfur MIF record, and that sulfur MIF is a good proxy for the rise of O2 in the earliest Paleoproterozoic. [1] Farquhar (2000) Science 289, 756-758. [2] Farquhar (2001) JGR 106, 32829-32840. [3] Pavlov & Kasting (2002) Astrobiology 2, 27-41. [4] Lyons (2007) GRL 34, L22811. [5] Watanabe et al. Science 324, 370-372. [6] Danielache et al. (2008) JGR 113, D17314. [7] Ueno et al. (2009) PNAS 106, 14784-17789. [8] Baroni et al. (2007) Science 315, 84-87.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.B23M..04L
- Keywords:
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- 0370 ATMOSPHERIC COMPOSITION AND STRUCTURE / Volcanic effects;
- 0454 BIOGEOSCIENCES / Isotopic composition and chemistry;
- 0488 BIOGEOSCIENCES / Sulfur cycling;
- 1041 GEOCHEMISTRY / Stable isotope geochemistry