Carbon isotopic signatures of individual archean microfossils(?) from Western Australia

New types of carbonaceous filamentous microstructures have been identified in silica veins at two new localities in the similar to3.5 Ga North Pole ar ea of Western Australia. Their carbon isotopic compositions were measured i n situ by secondary-ion mass spectrometry. The carbonaceous filaments are s imilar to1 mum wide, 10 to 100 mum long. and are permineralized in a fine-g rained (similar to1 mum) silica matrix. They are morphologically divided in to three types (i.e.,spiral, thread-like, and branched filaments). Their si zes and morphologies resemble modern and previously reported fossil bacteri a. These similarities and their complex three-dimensional geometry suggest that they may represent morphologically preserved fossil bacteria. delta C- 13 values of the carbonaceous filaments range from -42 to -92 parts per tho usand, which strongly suggest that they are composed of biologically fixed organic compounds, possibly via the reductive acetyl-CoA pathway or the Cal vin cycle. This is consistent with the hypothesis that autotrophs already e xisted on the Archean Earth.