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New types of carbonaceous filamentous microstructures have been identified in silica veins at two new localities in the ∼3.5 Ga North Pole area of Western Australia. Their carbon isotopic compositions were measured in situ by secondary-ion mass spectrometry. The carbonaceous filaments are ∼1μm wide, 10 to 100 μm long, and are permineralized in a fine-grained (∼1 μm) silica matrix. They are morphologically divided into three types (i.e., spiral, threadlike, and branched filaments). Their sizes and morphologies resemble modern and previously reported fossil bacteria. These similarities and their complex three-dimensional geometry suggest that they may represent morphologically preserved fossil bacteria. δ13C values of the carbonaceous filaments range from −42 to −32‰, which strongly suggest that they are composed of biologically fixed organic compounds, possibly via the reductive acetyl-CoA pathway or the Calvin cycle. This is consistent with the hypothesis that autotrophs already existed on the Archean Earth.
Ueno et al. (Thu,) studied this question.