HR: 09:15h
AN: V21F-06    [Abstracts]
TI: What Does the Absence of Mass-independent Fractionation of Sulfur Isotopes at 2.8-3.2 Ga say About the Early Atmosphere?
AU: * Goldman, S D
EM: sgoldman@geosc.psu.edu
AF: The Pennsylvania State University Department of Geosciences, 437 Deike Building, University Park, PA 16803 United States
AU: Kasting, J F
EM: kasting@geosc.psu.edu
AF: The Pennsylvania State University Department of Geosciences, 437 Deike Building, University Park, PA 16803 United States
AB: The presence of mass-independent fractionation (MIF) of sulfur isotopes in Archean sedimentary provides evidence for a low-O₂ atmosphere prior to 2.4 Ga (1). Recent data hints at the possibility that S-MIF vanished transiently some time between 3.4 and 2.7 Ga (2). The absence of S-MIF after 2.4 Ga is commonly attributed to the rise of O₂ in the atmosphere, since the presence of free O₂ would have oxidized all sulfur before removal from the atmosphere, thereby erasing any MIF that had existed between reservoirs (3). However, if free O₂ did not appear in the atmosphere until 2.4 Ga, then why did S-MIF disappear for hundreds of millions of year prior to 2.7 Ga? Could S-MIF have been eliminated from the rock record without the presence of free O₂ in the atmosphere? Two different mechanisms will be discussed. The first possibility is that H₂ levels decreased sufficiently to oxidize all MIF, but were still high enough to prevent free O₂ from building up to appreciable levels in the atmosphere. Stabilization of H₂ at these intermediate levels could have been triggered by a number of mechanisms controlling the H₂ budget, the most promising of which is changes in the biogeochemical processing of sulfur itself (4). Before the advent of bacterial sulfate reduction (BSR), seawater sulfate would have reacted with Fe in basalts, removing sulfur from the surface in oxidized form. As removing sulfur from the surface as sulfate requires oxidation of SO₂, this implies that H₂ must have been generated by the geochemical S cycle. After the advent of BSR sulfur would be buried in reduced form as pyrite. Burial of sulfur as pyrite would require reduction of SO₂, thus the biogeochemical S cycle would have consumed H₂. This change in S cycling likely would have impacted the H₂ budget more than any other change in element cycling, other than direct changes in the H₂ fluxes into and out of the system. The second possibility is that the atmosphere was reduced with respect to the sulfur system and all sulfur was removed from the atmosphere in reduced form, which would have prevented the sulfur from being deposited in separate reservoirs that could have preserved the S-MIF signal. This can be explained by recent calculations that suggest that H escape from the top of the atmosphere was diffusion limited, and therefore much slower than previously believed (5). As long as methanogens were not present to draw H₂ levels down, H₂ could have grown been a major component of Earth's early atmosphere. We are currently testing these hypotheses by running 1-D photochemical models of the Archean atmosphere over a wide range of H₂ concentrations to see which H₂ boundary conditions can cause the elimination of S-MIF of sulfur isotopes. If these boundary conditions can be explained by phenomena that could have been contemporaneous to the cessation of S-MIF, then the absence of S-MIF does not necessarily imply the presence of free O₂ in the atmosphere. 1. Farquhar, J., Bao, H., and Thiemans, M. Atmospheric influence of Earth's earliest sulfur cycle. Science 289, 756-758 (2000). 2 Y. Watanabe and H. Ohmoto, Early Earth Symposium. Tokyo, May, 2005. 3. Holland, H.D. Volcanic gases, black smokers, and the Great Oxidation Event. Geochim. Cosmochim. Acta 66, 3811-3826 (2002). 4. Pavlov, A.A. and Kasting, J.F. Mass-independent fractionation of sulfur isotopes in Archean sediments: strong evidence for an anoxic Archean atmosphere. Astrobiology 2, 27-41 (2002). 5. Tian, F., Toon, O.B., Pavlov, A.A., and De Sterck, H. A hydrogen rich early Earth atmosphere. Science 308, 1014-1017 (2005).
DE: 0325 Evolution of the atmosphere (1610, 8125)
DE: 0330 Geochemical cycles (1030)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005