| Carbonaceous microbe-like features preserved within 3.46 Ga Apex chert and stromatolite-like structures preserved in the 3.45 Ga Strelley Pool Chert, both in Western Australia, potentially represent some of the oldest evidence of early life on Earth. However, it has been suggested that the Apex microbe-like features are composed of abiotic carbonaceous material generated by Fischer-Tropsch-type (FTT) synthesis within an ancient hydrothermal vent, while the Strelley Pool stromatolite-like structures may be built by purely abiotic trapping of FTT organic matter. In this study, the general structure, carbon bonding, and light element composition of carbonaceous material preserved within these two ancient cherts was analyzed in situ using a combination of transmission electron microscopy (TEM), scanning transmission x-ray microscopy (STXM), and secondary ion mass spectrometry (SIMS). Carbonaceous material within the cherts appears amorphous and is located between quartz grains, at triple junctions, and occasionally within hexagonal fluid inclusions. Both Apex and Strelley Pool carbonaceous material are largely ungraphitized but include abundant polyaromatic carbon, along with carbonyl and phenol functional groups. Apex and Strelley Pool carbonaceous material also contains minor amounts of nitrogen, sulfur, and phosphorus. These structural and chemical features are similar to those observed from bona fide kerogen within the 2.0 Ga Gunflint Formation but are quite distinct from those of amorphous carbon, disordered graphite, and experimentally-produced FTT carbonaceous material. These observations indicate that ancient carbonaceous material preserved in Apex and Strelley Pool chert is likely the biogenic remains of early microorganisms.; A potential mechanism is presented for the natural abiotic synthesis of organic matter in an ancient hydrothermal vent, followed by maturation into an abiotic kerogen-like carbonaceous material. Although various types of organic molecules have by synthesized in FTT experiments, these reaction products are not refractory enough to survive diagenesis. Furthermore, it is unlikely that similar light element compositions could be generated in abiotic FTT material from different hydrothermal systems. Because microbial processes regularly generate refractory material able to mature into kerogen, a biogenic origin for these ancient carbonaceous materials is much more likely. |
