HR: 09:30h
AN: PP41D-07    [Abstracts]
TI: Mass Independent Fractionation of Sulphur Isotopes in Precambrian Sedimentary Rocks: Indicator for Changes in Atmospheric Composition and the Operation of the Global Sulphur Cycle
AU: * Peters, M
EM: amaltheus@gmx.de
AF: Geologisch-Palaeontologisches Institut, Westfaelische Wilhelms-Universitaet, Corrensstr. 24, Muenster, 48149 Germany
AU: Farquhar, J
EM: jfarquha@essic.umd.edu
AF: Department of Geology and ESSIC, University of Maryland, College Park, Md 20742 United States
AU: Strauss, H
EM: hstrauss@uni-muenster.de
AF: Geologisch-Palaeontologisches Institut, Westfaelische Wilhelms-Universitaet, Corrensstr. 24, Muenster, 48149 Germany
AB: Large mass independent fractionation (MIF) of sulphur isotopes in sedimentary rocks older than 2.3 Ga and the absence of this isotopic anomaly in younger rocks seem to be the consequence of a change in Earth's atmospheric composition from essentially oxygen-free or to oxygen-rich conditions. MIF is produced by photochemical reactions of volcanogenic sulphur dioxide with UV radiation in the absence of an ozone shield. The products of such processes are elemental sulphur with positive and sulphate with negative Δ³³S values. Here we present isotope data (³²S, ³³S, ³⁴S) for sedimentary pyrites from Archaean and Palaeoproterozoic rocks of the Kaapvaal Craton (South Africa), the Pilbara Craton (Australia) and the Greenland Shield (Isua Supercrustal Belt). Their ages range from 3.85 to 2.47 Ga. Large positive Δ³³S values up to +9.13 ‰ in several Archaean units from the Kapvaal and Pilbara Cratons are attributed to low atmospheric oxygen at that time. Interestingly, very low Δ³³S values between -0.28 and +0.57 ‰ appear to characterize the Witwatersrand succession of South Africa (3.0 Ga). This rather small MIF signature was previously detected in rocks of the same age in Western Australia (OHMOTO et al., 2005). The signature is interpreted as a global signal, which could be the consequence of a shielding effect induced by one or more atmospheric components. The most probable chemical compounds for this process are methane and carbon dioxide. Rocks of the Kameeldoorns Fm. (2.71 Ga), Kaapvaal Craton, display also low values between -0.46 and +0.33 ‰, which are consistent with the small (absent) MIF signal in rocks of the Hardey Fm. (2.76 Ga) of Western Australia (OHMOTO et al., 2005). Very low carbon isotope values between -51 and -40 ‰ in late Archaean kerogens (2.6 - 2.8 Ga) indicate a high concentration of methane in the atmosphere (PAVLOV et al., 2001). This high methane level could produce an organic haze, which absorbed most of the UV radiation and prevented mass independent fractionation of sulphur isotopes. In Palaeoproterozoic sediments of the Brockman Iron Fm., just prior to the proposed Great Oxidation Event, we determined predominantly negative Δ³³S values between -1.07 and +0.08 ‰, which is atypical for sulphides. We interpret this negative MIF signal as a product of microbial reduction of atmospheric sulphate with an original negative MIF signature. This observation may indicate a higher concentration of sulphate in the ocean. Mass independent sulphur isotope data presented here provide a deeper insight into the major steps in atmospheric evolution and the Precambrian sulfur cycle. Ohmoto, H., Watanabe, Y., Ikemi, H. (2005) Geochim. Cosmochim. Acta 69, A 450 (abstr.). Pavlov, A.A., Kasting, J.F., Brown, L.L. (2001) JGR 106, 23267-23287.
DE: 1030 Geochemical cycles (0330)
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005