Self-assembled silica-carbonate structures and detection of ancient microfossils

We have synthesized inorganic micron-sized filaments, whose microstucture consists of silica-coated nanometer-sized carbonate crystals, arranged with strong orientational order. They exhibit noncrystallographic, curved, helical morphologies, reminiscent of biological forms. The filaments are similar to supposed cyanobacterial microfossils from the Precambrian Warrawoona chert formation inWestern Australia, reputed to be the oldest terrestrial microfossils. Simple organic hydrocarbons, whose sources may also be abiotic and indeed inorganic, readily condense onto these filaments and subsequently polymerize under gentle heating to yield kerogenous products. Our results demonstrate that abiotic and morphologically complex microstructures that are identical to currently accepted biogenic materials can be synthesized inorganically. Evidence for the oldest known fossils has rested with reported microfossil filaments from the 3.49-billion-year-old Dresser For-mation (1) and the slightly younger (3.465 billion years) chert in the Apex basalt (2–5), in the Warrawoona Group of the Pilbara cra-ton in Western Australia. Optical and elec-tron microscopic studies confirmed the pres-ence of complex segmented filaments in these rocks that are similar in morphology to modern cyanobacteria (1–5). Raman spectra of the Dresser and Apex filaments reveal the presence of carbonaceous residue (6, 7). We demonstrated the formation and growth of filamentous materials under alkaline, mildly hydrothermal conditions, whose morpholo-gies and Raman spectra closely mimic those of the Warrawoona microfossils. We obtained crystal aggregates by mixing a water-soluble barium salt with alkaline sodium silicate solutions at ambient temperature and pressure, at a pH ranging from 8.5 to 11. The Na and Si concentrations in silica solution re-action mixtures were 7.5 and 9 mM, respec-tively. Most of the silicate in our solutions was likely to be in the form of dimeric Si2O(OH)6 ions (8). Reactions were typically carried out in diffusive contact with the ambient atmosphere, so that CO2 dissolution was significant, becaus