IN-SITU CORRELATED ELECTRON AND X-RAY MICROSCOPY OF PUTATIVE ARCHEAN BIOSIGNATURES: A NOVEL APPROACH FOR ASSESSING THE BIOGENICITY OF ANCIENT ORGANIC MATTER

Due to controversy in recent years regarding the biogenicity of ancient microfossils and stromatolites in Archean cherts from Western Australia, new methods and procedures for robustly assessing biosignatures are being developed. Laser Raman spectroscopy is one technique that has proved quite useful for probing the structure and bonding of organic matter. An alternative approach is to use a combination of transmission electron microscopy (TEM), scanning transmission x-ray microscopy (STXM), and related spectroscopic techniques, electron energy loss near-edge structure spectroscopy (ELNES) and x-ray absorption near-edge structure spectroscopy (XANES). While TEM allows for near-atomic spatial/imaging resolution, the synchrotron radiation source for STXM allows for very fine energy/spectroscopic resolution. These techniques can be performed on the same exact sample, providing for correlated analyses of the structure and bonding environment of in situ organic matter with both high spatial and high energy resolution.

Correlated TEM and STXM allow for sensitive visual and spectral probing of the onset of graphitization, the relative proportions of aromatic and aliphatic carbon bonding, and the presence and relative abundance of organic functional groups, such as carbonyl or phenol groups. This information can be used to infer a general structural model of the organic matter. Correlated TEM and STXM have been used to compare the organic matter preserved in the controversial 3.5 Ga Apex and Strelley Pool Cherts with 2.0 Ga Gunflint kerogen. The structural and compositional similarities between all three samples, despite different formation ages, locations, and geologic histories, imply that the organic matter preserved in the Western Australian cherts are indeed biogenic.