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Project Progress

We are using electron microscopy to define microbial biosignatures based on structure and composition at the nanometer scale. Recent controversy over the biogenicity of microfossils from the Apex Chert has illustrated the importance of material as well as morphological biosignatures. To understand material biosignatures, we have investigated the structure and composition of kerogenous and Fe-oxide-defined microfossils of the 2.0 Ga Gunflint Formation from the Schreiber Beach and Mink Mountain localities, respectively. The Schreiber-Beach microfossils are composed of kerogen along grain boundaries between quartz grains. Electron Energy-Loss Spectroscopy (EELS) measurements indicate the kerogen is amorphous without significant graphitization. In cocoidal microfossils, the kerogen forms cell-wall like features around course quartz crystals, whereas in fibrous forms, the kerogen is disseminated throughout the filaments along grain boundaries between submicrometer-sized quartz. Mink Mountain samples contain microfossils that range from red to black, suggesting that variations in oxidation state occurred during deposition. Transmission electron microscopy (TEM) shows that the oxide grains are hematite and that the color variations come from the presence of kerogen. The kerogen in these samples also occurs outside of the oxide-defined microfossils, possibly representing extracelluar biofilms.

Hydrofluoric acid (HF) etching of Schreiber-Beach samples and subsequent secondary electron imaging with a field emission scanning electron micrograph (SEM) shows that the etched morphologies are controlled by the kerogen distribution. Kerogen sheaths are clearly visible in these samples but there is no indication of extracelluar biofilms. We are currently investigating etched samples from Mink Mountain to determine if the extracellular kerogen observed with TEM has typical biofilm morphologies.

We are now beginning work on microfossiliferous in the controversial 3.5 Ga Apex Chert from western Australia. The goal of this research is to characterize the composition, crystallinity and distribution of the kerogen to determine if this material is of biogenic or hydrothermal in origin.