HR: 0800h
AN: V51A-0521    [Abstracts]
TI: Fe and S isotope variations in cyanobacterial mats: modern analogues of ancient stromatolites
AU: * Severmann, S
EM: SilkeS@geology.wisc.edu
AF: Dept. of Geology & Geophysics, University of Wisconsin, Madison, WI 53706 United States
AU: Johnson, C M
EM: clarkj@geology.wisc.edu
AF: Dept. of Geology & Geophysics, University of Wisconsin, Madison, WI 53706 United States
AU: Beard, B L
EM: beardb@geology.wisc.edu
AF: Dept. of Geology & Geophysics, University of Wisconsin, Madison, WI 53706 United States
AU: Yevenes, M
EM: myevenes2002@yahoo.es
AF: Instituto de Investigaciones OceanolĒgicas /Posgrado en OceanografĄa Costera, Universidad AutĒnoma de Baja California, Ensenada, 22800 Mexico
AU: Huerta-Diaz, M A
EM: mhuerta@uabc.mx
AF: Instituto de Investigaciones OceanolĒgicas /Posgrado en OceanografĄa Costera, Universidad AutĒnoma de Baja California, Ensenada, 22800 Mexico
AU: Thamdrup, B
EM: bot@biology.sdu.dk
AF: Danish Center for Earth System Science, University of Southern Denmark, Odense M, 5230 Denmark
AU: Hoehler, T
EM: tori.m.hoehler@nasa.gov
AF: NASA Ames Research Center, Mail Stop 239-4, Moffett Field, CA 94035-1000 United States
AU: Welch, S
EM: swelch@ems.anu.edu.au
AF: Department of Earth and Marine Sciences, The Australian National University, Canberra, ACT 0200 Australia
AB: Iron and sulfur isotope variations in modern microbial mats from the hypersaline ponds of the Guerrero Negro salt works, Baja California Sur, Mexico have been investigated. Cyanobacteria are the primary producers of this mat ecosystem. The oxygen concentrations in the surface 2 mm of the mats alternate between oxygen-rich and oxygen-free over a diel cycle. Previous work indicates that heterotrophic metabolism is dominated by sulfate reducing bacteria whereas direct metabolic processing of Fe, such as dissimilatory Fe(III) reduction or photoautotrophic Fe(II) oxidation, is negligible. Extremely high rates of sulfate reduction are observed near the mat surface, which coincides with the highest $\delta$$^{34}$S values of sedimentary sulfides (total inorganic sulfide, TRIS). The overall $\delta$$^{34}$S values of TRIS are -19 to -46 $\permil$ decreased relative to seawater sulfate. The absence of significant S isotope variations in sedimentary sulfide below the surface 1 cm indicates that sulfate does not become limiting within the microbial mats. Iron isotope compositions of pyrite, expressed as $\delta$$^{56}$Fe and normalized to average igneous rocks, varies between -1.9 and -0.2 $\permil$. These values are in the range of previously reported Fe isotope compositions of sedimentary pyrites from Archean shales, Banded Iron Formations and modern continental margin sediments (-2.5 to -0.5 $\permil$). Similar to S isotopes, the highest values in $\delta$$^{56}$Fe are observed in the surface 1 cm of the mat. The dominant processes that control S isotope compositions are microbial processing of S, including bacterial sulfate reduction and S disproportionation; inorganic fractionations, e.g. during conversion of H$_{2}$S, are negligible. In contrast, Fe isotope variations are the cumulative expression of multiple microbial and inorganic reactions, including reductive dissolution, inter-mineral fractionations, ligand-promoted dissolution and redox reactions. In this study we examine how these fundamentally different controls on S and Fe isotope fractionations can be related to the isotope variations in a modern microbial mat. The aim is to provide a framework for the interpretation of S and Fe isotope compositions in ancient sedimentary environments.
DE: 4870 Stable isotopes
DE: 4803 Bacteria
DE: 1045 Low-temperature geochemistry
DE: 1615 Biogeochemical processes (4805)
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting