HR: 0800h
AN: B21B-0883    [Abstracts]
TI: Searching for Biosignatures in Pyrite Framboids
AU: * Popa, R
EM: rpopa@usc.edu
AF: University of Southern California, 3651 Trousdale Pkwy. ZHS 117, Los Angeles, CA 90089 United States
AU: Schelble, R T
EM: rachels@usc.edu
AF: University of Southern California, 3651 Trousdale Pkwy. ZHS 117, Los Angeles, CA 90089 United States
AU: Douglas, S
EM: susanne.douglas@jpl.nasa.gov
AF: Jet Propulsion Laboratory, 4800 Oak Grove Dr. Mail stop 183 301, Pasadena, CA 91109 United States
AU: Nealson, K H
EM: knealson@usc.edu
AF: University of Southern California, 3651 Trousdale Pkwy. ZHS 117, Los Angeles, CA 90089 United States
AB: Framboids are submillimeter raspberry-like structures with 10-10$^{9}$ microcrystallites, often composed of pyrite (FeS$_{2}$). The origin and mechanisms of formation of framboids are poorly understood. Framboid morphology can be obtained in abiotic simulations, but are also very common in the presence of microbial communities. Although some microbial communities are rich in framboids, the role that microbes potentially play in the formation of framboids, and the impact of microorganisms on framboid characteristics remains unclear. The purpose of this study was to use morphological and geochemical differences between abiotic pyrite framboids, and pyrite framboids formed in microbial mats to determine whether framboids can be used as a biosignature in the study of ancient sediments. By manipulating the experimental conditions we tried to understand more about the conditions conducive to pyrite framboid formation, and whether there was a microbial role in the genesis of sedimentary deposits containing framboids. We studied synthetic produced (abiotic) framboids with framboids formed within microbial mats and with framboids found in geological deposits. Size distribution, microtexture, composition, isotopic composition, and mineralogy of pyrite framboids was compared. Images were obtained using environmental scanning electron microscopy and the microtextures were analyzed using shape-analysis tools providing information about the overall size distribution of framboids, number of microcrystallites per unit volume, and size distribution of microcrystallites. Synthetic framboids were made in the laboratory, and framboids from microbial mats were collected from the Mangalia (Romania) subsurface sulfidic system. Neoproterozoic (750-600 Ma) manganese carbonate deposits containing fossilized pyrite framboids were collected from a mine (Hunan Province, China). Our results show that the framboids from microbial mats have a distinct size distribution, different number of crystallites, and different sizes of crystallites than abiotic framboids, and that the framboids from the Mangalia system show variability of S isotope composition within the system. We discuss criteria to address the biogenicity of (or life intervention upon) pyrite framboids from the geological record.
DE: 3625 Descriptive mineralogy
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting