Abstract
Observations of Archean organic-walled microfossils suggest that their fossilization took place through both encapsulation and permineralization. In this study, we investigated microfossils from the ca. 3.0 Ga Farrel Quartzite (Pilbara, Western Australia) using transmitted light microscopy, scanning electron microscopy, Raman microspectrometry, and nanoscale secondary ion mass spectrometry (NanoSIMS) ion microprobe analyses. In contrast to previous studies, we demonstrated that permineralized microfossils were not characterized by the micrometric spatial relationships between Si and C-N as observed in thin sections. Permineralized microfossils are composed of carbonaceous globules that did not survive the acid treatment, whereas encapsulated microfossils were characterized due to their resistance to the acid maceration procedure. We also investigated the microscale relationship between the 12C14N- and 12C2- ion emission as a proxy of the N/C atomic ratio in both permineralized and encapsulated microfossils. After considering any potential matrix and microtopography effects, we demonstrate that the encapsulated microfossils exhibit the highest level of geochemical preservation. This finding shows that the chemical heterogeneity of the microfossils, observed at a spatial resolution of a few hundreds of micrometers, can be related to fossilization processes. Key Words: Carbonaceous matter—Farrel Quartzite—Fossilization—NanoSIMS—Nitrogen—Permineralization. Astrobiology 17, 1192–1202.
Highlights
Microfossil-like structures have been reported in numerous Archean rocks (e.g., Walsh, 1992; Schopf, 1993; Javaux et al, 2010; Sugitani et al, 2010)
A morphological diversity of microfossils was observed in thin section, with assemblages of lenticular, film-like (>100 mm), and spheroidal microfossils occurring either as isolated specimens or as clusters (Fig. 2)
Spheroids identified in the thin section were not found in the kerogen residue, lenticular and film-like microfossils were observed in the kerogen fraction (Fig. 3)
Summary
Microfossil-like structures have been reported in numerous Archean rocks (e.g., Walsh, 1992; Schopf, 1993; Javaux et al, 2010; Sugitani et al, 2010). The presence of organic-walled microfossils in some Archean rocks (Grey and Sugitani, 2009; Javaux et al 2010; Sugitani et al, 2015) points to the existence of other mechanisms of fossilization, such as encapsulation of microorganisms through, for example, nucleation of adjacent mineral crystals (Rainey and Jones, 2010). Through investigation of mineralization of microbial mats from Icelandic hot springs, Konhauser and Ferris (1996) proposed that encapsulation favors morphological and geochemical preservation of microorganisms. This is supported by experimental silicification of modern microorganisms, which shows that the negative effect on geochemical preservation caused by thermal alteration can be counterbalanced by encapsulation (Picard et al, 2015). Encapsulation has not yet been directly documented in Archean rocks despite its great potential for preservation of microorganisms
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