Kerogen Characterization of Microfossils in Precambrian Cherts: Evidence for Biogenicity

Currently, much of our oldest evidence of life on this planet has been called into question. It is not enough for a possible microfossil to have bacterial morphology. In addition, it must also be composed of material with an unquestionably biogenic origin. Once an organism dies, the carbon it contains is altered through diagenesis and metamorphism. Most organic material is removed or remineralized, but insoluble amorphous carbon, known as kerogen, may remain. With additional heating and pressure, this kerogen is transformed into graphite, eliminating the structural biosignature of the material. No known biological process creates graphite as a product. Oxides, which form on the external surface of bacterial cell walls, may also remain after fossilization. Many microfossils are defined not by kerogen but by arrangements of small iron or manganese oxide crystals, even though kerogen may still be associated with them. Cherts from Mink Mountain locality of the Gunflint Formation (2.0 Ga) contain black, brown, and red filaments composed of both hematite crystallites up to 1 μm and kerogen. The amount of kerogenous material determines the color of the microfossil. Those with little associated kerogen appear red, the color of hematite, while those with much associated kerogen appear black. Brown microfossils are the result of remnant carbon with little or no hematite. Kerogen is also found abundantly outside of microfossils and may possibly be the remains of ancient biofilm. The crystallinity of carbon, grading from amorphous carbon to graphite, can be measured via a variety of methods, including X-ray diffractometry (XRD), Raman spectrometry, high-resolution transmission electron microscopy (HRTEM), and electron energy loss spectrometry (EELS) in TEM. However, EELS may be the best method when dealing with small patches of carbon associated with microfossils, especially if high-resolution imaging is not possible. Information about the crystallinity is given by the morphology of the near edge fine structure of the carbon K-edge. Specifically, the relative intensities of smaller structures on the σ * peak and the width of the σ * peak both increase with increased graphitization. EELS analysis has been performed on cherts from Schreiber Beach, which contains typical, well-accepted microfossils of the Gunflint Formation (2.0 Ga). These microfossils are composed entirely of kerogen in a matrix of microcrystalline quartz. The spectrum of this kerogen is very similar to that of amorphous carbon. The biogenicity of carbon structures within the Apex Chert (3.5 Ga) is in contention. Raman spectra of these structures have been interpreted as either representing highly disordered graphite (Brasier et al. 2001) or partially graphitized kerogen (Schopf et al. 2001). The former implies an abiogenic origin, whereas the latter implies a biogenic origin. We will use EELS and HRTEM to determine the crystallinity of this carbon in the Apex Chert.