HOW WELL DO SULFUR ISOTOPES CONSTRAIN OXYGEN ABUNDANCE IN THE ANCIENT ATMOSPHERES?

Oxygen abundance in the ancient terrestrial atmos-pheres is a fundamental and poorly resolved problem. Fortunately, discovery of mass-independent fractionation (MIF) in sulfur isotopes in the Archean/Early Proterozoic sediments and lack of MIF in sulfur in the younger rocks [1] placed a strict upper constraint on the amount of oxidants and oxygen in the ancient Earth’s atmosphere prior to 2.3 Gyr ago [2]. Preservation of MIF in the Archean/Early Proterozoic sediments requires sulfur deposition from the Archean at-mosphere in the variety of oxidation states (SO2, H2S, S8, H2SO4 etc.) so that sulfur-bearing species would not completely rehomogenize sulfur isotopes before incor-poration into the ancient rocks. Pavlov & Kasting [2] showed that it would be only possible if the ancient pO2 level were less than 10-5 PAL (present atmospheric level).

However recent measurements by [3] in the Antarctic ice core showed a small but non-zero MIF in sulfate correlated with the large volcanic eruptions, raising questions about the sulfur MIF sensitivity to oxygen levels.

We put sulfur isotopes into a 2-D photochemical/microphysical model and looked at how strong the initial atmospheric fractionation of sulfur had to be in order to be preserved in some measurable quantity at the surface upon deposition. We will discuss two possibilities to preserve small sulfur MIF in the oxidizing atmosphere: (1) preservation through polar sulfate mesopause aerosols (2) preservation through the alter-native pathway of SO2 oxidation [3]. We will specifically focus on how both of these mechanisms can be affected by the large volcanic eruptions and overall oxygen levels.

References: [1] Farquhar J. et. al. (2000) Science, 289, 756-758. [2] Pavlov A. A. and Kasting J. F. (2002) Astrobiol-ogy, 2(1), 27-41. [3] Savarino J. et. al. (2003) GRL, 30(21), Art. No. 2131.