Trace element perspective into the ca. 2.1-billion-year-old shallow-marine microbial mats from the Francevillian Group, Gabon

Abstract

The sedimentary fabrics of Precambrian mat-related structures (MRS) represent some of the oldest convincing evidence for early life on Earth. The ca. 2.1 billion-year (Ga) old MRS in the FB2 Member of the Francevillian basin in Gabon has received considerable attention not only because they contain remnants of microbial mats that colonized large areas in oxygenated, shallow-marine settings, but they also contain evidence for ancient multicellular organisms that thrived on these microbial mats using them as a food source. Despite these insights, what remains lacking is a full characterization of the geochemical composition of the MRS to test whether the bulk composition of fossilized MRS is distinct from the host sediments (sandstones and shales). Here, we show that the trace element (TE) content of microbial textures belonging to pyritized MRS, poorly pyritized MRS, and "elephant-skin" textures (EST) is highly variable and differs from that of the host sediments. The poorly pyritized MRS contain a unique matrix with embedded Ti-and Zr-rich minerals and syngenetically enriched in TE. The EST, some of which are developed along the same stratigraphic horizon as the poorly pyritized MRS, display a distinct distribution of TE-bearing heavy minerals, suggesting a local difference in physical conditions during sedimentation. Similarly, high chalcophile-element (CE) content in pyritized MRS relative to the host sediments of the FB2 Member further points to local bacterially influenced enrichments with high rates of microbial sulfate reduction during early diagenesis.The geochemical relationship between the MRS and the Francevillian sediments (e.g., FB, FC, and FD formations) indicates that specific biological pathways for CE enrichments (i.e., microbially controlled accumulation) are not apparent. Our findings highlight bulk-rock TE distinction between the 2.1-billion-year-old MRS and their host sediments, but also indicate that environmental conditions, such as hydrodynamic regime and water-column redox chemistry, may simply overwhelm any potential biological signal. Our data suggest that the microbial impact may have only passively influenced TE enrichment in the studied sediments, implying that TE concentrations in MRS are a poor biosignature. Importantly, this work indicates that bulk TE geochemistry does not unveil specific microbiological processes in the rock record, which is consistent with the observed patterns in modern analogues.

Authigenic kaolinite and sudoite in sandstones from the Paleoproterozoic Franceville sub-basin (Gabon)

“…The basin fill consists of 1.0 to 2.5-km-thick Paleoproterozoic siliciclastic sedimentary succession commonly referred to as the Francevillian Group [Figure 1a; Weber, 1968]. Importantly, the Francevillian deposits were not affected by metamorphic processes and have only been subjected to diagenetic and low-temperature hydrothermal processes [Aubineau et al, 2020, Bankole et al, 2016, Gauthier-Lafaye and Weber, 1989. Specifically, the Franceville sub-basin records a maximum burial depth and temperature of about 2500 m and 80°C, respectively [Ngombi-Pemba et al, 2014, Weber, 1968, which corresponds to burial conditions of a common geothermal gradient.…”

“…(b) Lithostratigraphic column of the FB 2 Member in the Moulendé Quarry displaying sampling positions of both microbial mat structures and host sediments (red arrows). Images in (a) and (b) are modified from Aubineau et al [2020] and Aubineau et al [2019], respectively. from the oldest to the youngest.…”

Benthic redox conditions and nutrient dynamics in the ca. 2.1 Ga Franceville sub-basin

“…The former, including "elephant-skin" textures (EST), domal buildups, and discoidal microbial colonies, consist of the mat layer itself generated through mat propagation, while the latter, such as wrinkle and "Kinneyia" structures, arose from structures that relied on the mat communities for their preservation/protection. Individual MRS vary between 0.2 and 3 mm in thickness; this heterogeneity has been attributed to changes in physical and chemical environmental factors (Aubineau et al, 2020). According to their sulfur content (0.15 to 29.37 wt.…”

“…%) (cf. Aubineau et al, 2020). Detailed descriptions of textural diversity and elemental geochemistry of MRS are provided in Aubineau et al (2020Aubineau et al ( , 2018.…”

“…In order to provide further insight into N cycling and microbial biogeochemistry at the end of the LE, this study focused on the MRS and their associated host sediments of the Francevillian Group FB2 Member. Both have been previously characterized for their mineralogy and elemental composition (Aubineau et al, 2020(Aubineau et al, , 2019. Here, we use iron-based redox analyses and redox-sensitive element concentrations to constrain local redox conditions in the depositional setting.…”