HR: 0800h
AN: B21B-0889    [Abstracts]
TI: Structural Disorder and Origin of Kerogen in the Apex Chert: A Comparison With Fischer-Tropsch-Type Carbon
AU: * De Gregorio, B T
EM: degregorio@asu.edu
AF: Arizona State University, Dept. Geological Sciences P.O. Box 871404, Tempe, AZ 85287-1404 United States
AU: Sharp, T G
EM: tom.sharp@asu.edu
AF: Arizona State University, Dept. Geological Sciences P.O. Box 871404, Tempe, AZ 85287-1404 United States
AU: Flynn, G J
EM: flynngj@plattsburgh.edu
AF: SUNY - Plattsburgh, Dept. Physics 101 Broad St., Plattsburgh, NY 12901 United States
AB: Carbonaceous filamentous features preserved in the 3.5 Ga Apex Chert in Western Australia are currently the oldest microfossils on Earth. However, the biogenicity of these microfossils has been called into question on several counts. Two points of contention concern the structural disorder and origin of the carbon in the proposed microfossils. Although the features were originally described as composed of kerogen--an amorphous, insoluble, carbon-rich material common in oil shales--recent investigations using laser-Raman spectroscopy have concluded that this carbonaceous material is partially graphitized. Unfortunately, Raman spectroscopy may not be sensitive enough to distinguish disordered carbon species. An abiotic organic Fischer-Tropsch-type (FTT) synthesis has been proposed as the source of the carbon in the Apex Chert. To address these issues, we have studied residual carbonaceous material in the Apex Chert and two FTT carbon products using transmission electron microscopy (TEM), electron energy-loss spectroscopy (EELS), and x-ray absorption near-edge structure spectroscopy (XANES). The FTT carbon was synthesized under hydrothermal conditions at 175\deg C and 250\deg C using a montmorillonite clay catalyst by Rushdi and Simoneit (Orig. Life. Evol. Biosph. 31, 103, 2001). In TEM images, the Apex Chert residual carbon is localized at grain boundaries between quartz grains, similar to kerogen distribution in microfossils from the Gunflint Formation. High-resolution TEM images do not show the presence of graphite lattice fringes. EELS and XANES spectra of this material show the large rounded peak for single C-C bonding from 290 - 300 eV and a peak for benzene ring structures at 285 eV but no graphite peak (291 eV). In addition, XANES spectra of the residual carbon show a peak due to carbonyl (C=O) at 288.5 eV. These observations indicate that the residual carbon and the carbon in the proposed microfossils is either kerogen or an abiotic kerogen-like material. EELS and XANES spectra of FTT carbon show carbon-carbon single and double bonds and carbonyl bonding, but no graphite peak. This is generally the same as spectra of Apex Chert carbon, but the carbonyl peak for FTT carbon is much more intense. However, it is possible that decarbonation of silicified FTT material during diagenesis could reduce the amount of carbonyl such that the resulting energy-loss spectrum would be similar to spectra of Apex Chert carbon. Therefore, it is possible that the proposed microfossils in the Apex Chert could be produced by preserved FTT carbon in a hydrothermal system.
DE: 9330 Australia
DE: 9619 Precambrian
DE: 5749 Origin and evolution
DE: 1055 Organic geochemistry
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting