Biogenicity of Morphologically Diverse Carbonaceous Microstructures from the ca. 3400 Ma Strelley Pool Formation, in the Pilbara Craton, Western Australia

Abstract

Morphologically diverse structures that may constitute organic microfossils are reported from three remote and widely separated localities assigned to the ca. 3400 Ma Strelley Pool Formation in the Pilbara Craton, Western Australia. These localities include the Panorama, Warralong, and Goldsworthy greenstone belts. From the Panorama greenstone belt, large (> 40 μm) lenticular to spindle-like structures, spheroidal structures, and mat-forming thread-like structures are found. Similar assemblages of carbonaceous structures have been identified from the Warralong and Goldsworthy greenstone belts, though these assemblages lack the thread-like structures but contain film-like structures. All structures are syngenetic with their host sedimentary black chert, which is associated with stromatolites and evaporites. The host chert is considered to have been deposited in a shallow water environment. Rigorous assessment of biogenicity (considering composition, size range, abundance, taphonomic features, and spatial distributions) suggests that cluster-forming small (<15 μm) spheroids, lenticular to spindle-like structures, and film-like structures with small spheroids are probable microfossils. Thread-like structures are more likely fossilized fibrils of biofilm, rather than microfossils. The biogenicity of solitary large (>15 μm) spheroids and simple film-like structures is less certain. Although further investigations are required to confirm the biogenicity of carbonaceous structures from the Strelley Pool Formation, this study presents evidence for the existence of morphologically complex and large microfossils at 3400 Ma in the Pilbara Craton, which can be correlated to the contemporaneous, possible microfossils reported from South Africa. Although there is still much to be learned, they should provide us with new insights into the early evolution of life and shallow water ecosystems.

Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in theca.3.48 Billion-Year-Old Dresser Formation, Pilbara, Western Australia

“…Current interpretations of the diversity of Earth's earliest life come predominantly from stromatolites (e.g., Lowe, 1980;Walter et al, 1980;Byerly et al, 1986;Hofmann et al, 1999;Allwood et al, 2006Allwood et al, , 2007Allwood et al, , 2009Allwood et al, , 2010Hickman, 2012), organic microfossils of prokaryotes and biofilms (e.g., Awramik et al, 1983;Walsh andLowe, 1985, 1999;Walsh, 1992;Hofmann, 2004;Schopf et al, 2007;Schopf and Bottjer, 2009;Sugitani et al, 2010;Wacey et al, 2011Wacey et al, , 2012Hickman 2012), and isotopic signatures of carbon and sulfur (e.g., Shen et al, 2001Shen et al, , 2009Ueno et al, 2006Ueno et al, , 2008Wacey et al, 2010). Microbially induced sedimentary structures (MISS) provide an additional way to decode life in ancient sediments (review in Noffke, 2010).…”

Multiple Generations of Carbon in the Apex Chert and Implications for Preservation of Microfossils

“…Sugitani et al (2010) showed that Raman spectra collected on carbonaceous microstructures from the 3.4 Ga Strelley Pool Formation had I D /I G ratios varying between 1.25 and 1.94. Micrometer-scale heterogeneities in CM may be explained by a variety of mechanisms, such as micrometer-scale differences in precursor chemistry of CM, and subsequently dissimilar alteration pathways of the different precursor material, allochthonous CM, or emplacement of CM during multiple hydrothermal pulses at different temperatures.…”

Astrobiology and the Possibility of Life on Earth and Elsewhere…

“…The importance of a reducing atmosphere is that the action of various energy sources on reduced C gas mixtures, including CO and CH 4 , results in the formation of a variety of more complex organic compounds, whereas using CO 2 this chemistry is less productive (Miller Sugitani et al 2010) and Schlesinger 1984;Miller 1983a, 1983b). In the presence of nitrogen, in the form of N 2 or NH 3 , important biological compounds including amino acids, purines and pyrimidines can be formed (Cleaves and Lazcano 2009).…”