DETERMINING THE BIOGENICITY OF RESIDUAL CARBON WITHIN THE APEX CHERT

Preserved carbon with microfossil morphologies observed in the Apex Chert has been accepted as the oldest evidence for life on this planet until recently. One key issue under debate is the nature of the carbonaceous material--whether it is kerogenous or graphitic. Using transmission electron microscopy (TEM), we can visualize the order or disorder of the carbonaceous material at the atomic scale. In addition, energy-dispersive x-ray spectroscopy (EDS) gives information about the chemical composition of the specimen, while electron energy loss spectroscopy (EELS) gives information about both the composition and electronic structure of the material. Carbon atoms in graphite have only sp³ hybridized electron orbitals, whereas carbon atoms in kerogen and amorphous carbon contain various hybridized and unhybridized orbitals. Therefore, EELS can be used to distinguish even disordered graphite from kerogen and other disordered carbon phases. Previous studies of kerogen associated with microfossils in the Gunflint Formation show that the EELS spectrum of kerogen is very similar to that of amorphous carbon and quite dissimilar to that of graphite.

We have used these TEM techniques to study residual carbonaceous material in the Apex Chert. No features similar to the proposed microfossils were studied due to their absence in our small sample. However, it may be assumed that the carbon in these features should be similar to the residual carbon. This residual carbon appears amorphous in high-resolution TEM images, is very pure, and is mostly distributed along grain boundaries between quartz crystals. This material is also sometimes found in ~200 nm, euhedral, hexagonal fluid inclusions within quartz crystals. EELS spectra of the residual carbon are very similar to those of Gunflint kerogen and amorphous carbon spectra. These results indicate that the residual carbon in the Apex Chert is either kerogen or amorphous carbon. This does not prove the biogenicity of the proposed microfossils, but it does limit the possible theories for their formation. In addition, our observations suggest that the original distribution of carbon has been altered during the history of the chert, mostly likely through transport by hydrothermal fluids.