A transmission electron microscopy study of silica and kerogen biosignatures in ∼1.9 Ga Gunflint microfossils

Microfossils preserved in chert from the ∼1.9 Ga Gunflint Formation (Schreiber Beach, Ontario, Canada) were studied with transmission electron microscopy (TEM) and analytical TEM (ATEM). Our goals were to uncover the style of silicification relative to the distribution of organic matter, and to evaluate the distribution and evolution of organic matter, at submicroscopic spatial scales. Petrographically the microfossils typically display filamentous or coccoidal morphologies, and consist of quartz crystals surrounded by kerogen along grain boundaries. ATEM analysis revealed that quartz associated with kerogen consists of 200-500nm-sized, round crystallites, whereas the chert matrix is comprised of randomly oriented, polygonal microquartz (5-10 μm). Silica spheroids found within some fossils consist of quartz subgrains in an amorphous to poorly crystalline matrix, suggesting that precipitation of opaline silica on organic matter occurred with subsequent but incomplete transformation to α-quartz. Some coccoidal microfossils surround large euhedral quartz crystals (up to 5 μm in diameter) that appeared to have influenced the distribution of kerogen during crystal growth. These euhedral quartz crystals commonly contain elongated (50-100 nm) iron-rich crystallites. Energy-loss, near-edge structure analysis of kerogen associated with a coccoidal microfossil showed that it is composed of amorphous carbon with no evidence of graphitization. TEM results revealed significant differences in the style of silicification between microbe-shaped microfossils and their surrounding chert matrix, as well as the presence of amorphous kerogen.