Palynology of Archean microfossils (c. 3.0 Ga) from the Mount Grant area, Pilbara Craton, Western Australia: Further evidence of biogenicity

摘要

Palynological techniques have only rarely been applied to Archean rocks, mainly because of concerns about contamination. However, where microstructures are abundant and well documented, palynology can add considerably to interpretations based on thin-section examination. A modified palynological preparation method, which avoids vigorous physical or chemical methods that might cause the fragmentation of fragile specimens, was applied to black chert from the c. 3.0 Ga Farrel Quartzite of the Mount Grant area, Pilbara Craton, Western Australia, known to contain abundant microstructures of putative biogenic origin. Four main morphological types have been reported from chert thin sections: threads, films, hollow spheres and spindles. These microstructures exhibit morphological and chemical signatures consistent with a biological origin as well as showing features indicative of taphonomic degradation. Nevertheless, the possibility remains that they are artefacts. Similar structures have been attributed to physical processes, such as the accretion of fine particulate matter that has subsequently been redistributed as a result of crystal growth. By extracting structures palynologically, we have increased the probability of a biogenic origin. Firstly, examination of the morphology free of the encompassing matrix demonstrates that they are indeed three-dimensional structures with complex morphology; secondly, the extraction of entire specimens militates against them being particulate matter aggregations, which would simply disintegrate during preparation and thirdly, the chemicals used remove silicates and most other minerals, confirming the organic composition of the structures. To date, we have extracted films, hollow spheres and a few poorly preserved spindles, but preparation continues. The fact that the specimens retain their morphological integrity during palynological extraction argues strongly in favour of a biogenic origin for the microstructures.