ENERGY ABSORPTION SPECTROSCOPY OF CARBONACEOUS MATERIAL IN PRECAMBRIAN CHERTS

Ancient carbonaceous material (CM) is preserved in many Precambrian black cherts, including the 3.5 Ga Apex Cherts and 3.4 Ga Strelley Pool Chert in Western Australia, purported to contain the earliest fossil evidence of life on Earth. Although the CM in some of these ancient cherts definitely has a biogenic origin (such as the Gunflint Formation), the biogenicity of other CM is unknown or suspect (such as the Apex and Strelley Pool cherts). In much of this latter category, both biogenic and abiotic origins have been proposed for the CM. While the biogenic origin requires some form of microbial organisms as a source of carbon, the abiotic model involves an organic synthesis process, most likely Fischer-Tropsch-type (FTT) reactions. Not much is known about FTT processes in natural environments, and there is no single test for whether a particular CM has a biogenic or abiotic origin. Microscopic spectroscopy techniques are a good way to investigate the biogenicity of ancient CM by observing the hydrocarbon structure and functional groups at atomic or near-atomic scales. Micro-laser Raman spectroscopy has proven useful in this regard, but electron energy-loss spectroscopy (EELS), x-ray absorption near-edge structure spectroscopy (XANES), and transmission electron microscopy (TEM) imaging provide a complementary suite of techniques for high spatial and high spectral resolution data.

This study compares EELS, XANES, and high-resolution TEM images of CM preserved in the Gunflint Formation, Apex Chert, and Strelley Pool Chert with carbonaceous products created by FTT synthesis using a montmorillonite catalyst. Spectra of the Precambrian CM are all similar and show the presence of aliphatic carbon and abundant aromatic carbon. In addition, the high energy resolution of XANES reveals peaks due to carbonyl (C=O) functional groups in all chert samples. Spectra of FTT carbons also show the presence of aliphatic and aromatic carbon but contain much larger carbonyl peaks, which are even visible in some EELS spectra. These FTT carbons are immature materials compared to ancient CM, but it is possible that burial and diagenesis could transform FTT carbons into mature, kerogen-like material with an energy absorption spectrum that matches those of the ancient CM.