Distinguishing cellular from abiotic spheroidal microstructures in the ca. 3.4 Ga Strelley Pool Formation

Abstract

The morphogenesis of most carbonaceous microstructures that resemble microfossils in Archean (4-2.5 Ga old) rocks remains debated. The associated carbonaceous matter may even-in some cases-derive from abiotic organic molecules. Mineral growths associated with organic matter migration may mimic microbial cells, some anatomical features, and known microfossils-in particular those with simple spheroid shapes.Here, spheroid microstructures from a chert of the ca. 3.4 Ga Strelley Pool Formation (SPF) of the Pilbara Craton (Western Australia) were imaged and analyzed with a com-

Distinctive microfossil supports early Paleoproterozoic rise in complex cellular organisation

“…Having said this, there is some coarse similarity between the cell‐like spheres within the SA microfossils and spheroidal structures recently described from the c. 3.4 Ga Strelley Pool Formation (SPF; Type 4 structures of Coutant et al, 2022), in that both consist of a sub‐spherical morphology with an inner vesicle (cell) that is surrounded by an outer wall (sheath). However, there are prominent differences in overall colony/cluster appearance and configuration between these two examples.…”

“…However, there are prominent differences in overall colony/cluster appearance and configuration between these two examples. Specifically, the cell‐like spheres of the SA microfossils are consistently aggregated into an overall spherical, colonial morphology of regular size and shape, with tens of aggregates observed across multiple thin sections (Figures 1–3), whereas the spheroidal structures of the SPF were observed within only one cluster, and this had a contorted, non‐uniform overall shape (Figure S17c, left side; Coutant et al, 2022). Moreover, the SPF spheroidal structures are loosely arranged and sit among a thick concentration of nanoparticulate OM that is distributed both in and around the structures (Figure S17c, middle and right side), which may suggest cluster formation via adhesion of individual cells rather than cell division (e.g.…”

“…McCollom et al, 1999), and are also inconsistent with pseudo‐fossil formation through localised mobilisation of OM, as this has been described from rocks that have experienced extensive, syn‐sedimentary hydrothermal input and subsequent high‐temperature (~350°C) metamorphism (e.g. Coutant et al, 2022); occur in a stratigraphic sequence with other fossils of unambiguous origin, that is, multiple varieties of microbialites (Barlow et al, 2016; Nomchong & Van Kranendonk, 2020) and other bona fide microfossils with cellular preservation (Barlow & Van Kranendonk, 2018; Fadel et al, 2017; Schopf et al, 2015); occur within multiple petrographic thin sections produced from different samples that were collected from kilometres apart along strike; occur in a distinct and repeated microfossil assemblage (termed NBC texture 3 ; Barlow & Van Kranendonk, 2018; Barlow, 2019) alongside other kerogenous microfossils of markedly different, filamentous morphotype (Figure S5c,d); each have a three‐dimensional morphology of consistent size and shape (Figures 1–3; Figure S6); exhibit both a morphology and size range that is consistent with that known to be produced by life (see Section 5.2.3); are of kerogenous composition and are not composed of any minerals other than the microquartz in which they are preserved (Figure 4; see also Barlow & Van Kranendonk, 2018); and have an in situ carbon isotopic composition that is consistent with biological fractionation (δ 13 C of ~ −30‰; Hayes et al, 1999). …”

“…McCollom et al, 1999), and are also inconsistent with pseudo-fossil formation through localised mobilisation of OM, as this has been described from rocks that have experienced extensive, syn-sedimentary hydrothermal input and subsequent high-temperature (~350°C) metamorphism (e.g. Coutant et al, 2022);…”

“…However, the grainsize of the microquartz within the microfossils and their surrounding rinds is much finer than the size of either the compartments or the individual cell-like spheres(cf. Figures 5b, 6b and 5d,e,g and 6c,d), and botryoidal quartz (known to provide a template for OM resulting in the formation of vesicular shapes, seeCoutant et al, 2022) has not been observed. This makes a diagenetic origin via OM mobilisation along quartz boundaries or templating on quartz crystals highly unlikely.An alternative abiotic mode of formation of the honeycomb compartments and cell-like spheres could be OM mobilisation via diffusion in a silica gel prior to quartz mineralisation (e.g Coutant et al, 2022)…”