![]() ![]() This BIF also forms conformable, bedded red jasper with orange carbonate BIF (fig. Jasper-carbonate BIF occurs as orange to red outcrops with irregular white chert bands and red decimeter-size jasper nodules (fig. The BIF is stratigraphically located between a mafic unit of basaltic composition at the base of the sequence and a basaltic to andesitic unit above ( 14). Outcrops of NSB jasper-carbonate and Fe-silicate BIF are linearly distributed along strike for more than 3 km within the locally pillowed, 3750-Ma to 4280-Ma Ujaraaluk amphibolite unit (fig. Therefore, measurements of stable isotope composition of sulfur are needed because such data are important for evaluating the presence of biological activity within the context of the oldest putative fossils on Earth. For instance, these rocks also contain millimeter-size chalcopyrite crystals ( 4) for which S-isotope compositions could inform redox conditions and have implications for biological fractionation. Questions also remain as to the biogenic origin of phosphorus and sulfur in metamorphically recrystallized minerals such as apatite and chalcopyrite, which occur in the NSB jasper-carbonate BIF. Hence, there is a need to revisit such abiotic models as a possible explanation for the recently identified microfossil-like objects in the NSB, such as those reported here. Ferruginous structures in chemical gardens may also form through diffusion-limited aggregation, which might explain occurrence of the NSB hematite filaments ( 9– 13). These “chemical gardens” are self-assembled abiotic biomorphs, or in this case prebiotic and abiotic microstructures that spontaneously precipitated from ionic solutions ( 10). However, the biological interpretation of the NSB filaments ( 4) has received criticism because some abiotic precipitates can display similar morphologies in highly alkaline solutions ( 7, 8), such as natural alkaline spring water ( 9). These putative fossils are important for understanding abiogenesis, evolutionary biology, and exobiology. The age of the NSB is still debated, but it is at least 3750 million years (Ma) and possibly as old as 4280 Ma ( 5, 6). Evidence for the oldest putative microfossils has been reported in a jasper-carbonate BIF from the Nuvvuagittuq Supracrustal Belt (NSB) in the form of hematite filaments and tubes ( 4). Eoarchean BIFs contain magnetite with heavy Fe isotopes consistent with anoxygenic photosynthetic Fe 2+ oxidation on the early Earth ( 2, 3). Collectively, the observations suggest a diverse microbial ecosystem on the primordial Earth that may be common on other planetary bodies, including Mars.įossils of primordial life or prebiotic and abiotic microstructures?įerrous iron oxidation by anoxygenic phototrophs is thought to have been an early evolving metabolism important for the deposition of banded iron formations (BIFs) before the Great Oxidation Event (GOE) ( 1). Millimeter-sized chalcopyrite grains within the jasper-carbonate rocks have 34S- and 33S-enrichments consistent with microbial S-disproportionation and an O 2-poor atmosphere. Additional clusters of irregular hematite ellipsoids could reflect abiotic processes of silicification, producing similar structures and thus yielding an uncertain origin. These microstructures are considered microfossils because of their mineral associations and resemblance to younger microfossils, modern Fe-bacteria from hydrothermal environments, and the experimental products of heated Fe-oxidizing bacteria. Here, we report images of centimeter-size, autochthonous hematite filaments that are pectinate-branching, parallel-aligned, undulated, and containing Fe 2+-oxides. If biological in origin, these filaments might have affinities with modern descendants however, if abiotic, they could indicate complex prebiotic forms on early Earth. The oldest putative fossils occur as hematite filaments and tubes in jasper-carbonate banded iron formations from the 4280- to 3750-Ma Nuvvuagittuq Supracrustal Belt, Québec. ![]()
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