SULFUR CYCLING, S ISOTOPE FRACTIONATION, AND THE RISE OF ATMOSPHERIC O2

The rise of atmospheric O₂ is linked with the geochemical cycle of sulfur in at least two important ways. S isotopes in Archean and early Proterozoic rocks are fractionated in a mass-independent manner, whereas S isotopes in younger rocks are not (1). This provides the clearest evidence to date that the atmosphere was anoxic prior to ~2.3 Ga, as photochemical processing of sulfur gases in an anoxic atmosphere is the only known mechanism that can explain the pre-2.3 Ga data (2,3). The upper limit on pO₂ prior to that time is < 10^-5 PAL (times the Present Atmospheric Level), probably < 10^-13 PAL. Either value is well below the amount required to cause oxidative weathering on the continents and can thus explain the existence of detrital uraninite and pyrite in rocks older than 2.3 Ga.

Sulfur is also important because biological reduction of sulfate and subsequent burial of pyrite in sediments is a major source of O₂. Changes in the geochemical sulfur cycle may explain why the initial rise in atmospheric O₂ postdates the first appearance of cyanobacteria (4) by at least 400 million years. The amount of sulfur entering the oceans was much lower prior to 2.3 Ga as a consequence of the lack of oxidative weathering. Consequently, the formation of pyrite in sediments and and of pyrite and anhydrite in hydrothermally altered seafloor was also much slower than today. This, in turn, reduced the SO₂/H₂O ratio in volcanic gases and may resolve the apparent problem in balancing the atmospheric redox budget during the Archean (5,6).

References: (1) J. Farquhar et al. , Science 289, 756 (2000). (2) J. F. Kasting, Science 293, 819 (2001). (3) A. A. Pavlov and J. F. Kasting, Astrobiology 2, 27 (2002). (4) J. J. Brocks et al., Science 285, 1033 (1999). (5) L. R. Kump et al., Geol. Geochem. Geophys. (on line) 2 (2001). (6) H. D. Holland, Geochim. Cosmochim. Acta, in press.