The Preservation Potential of Biosignatures in Neoarchean Stromatolites of the Lower Transvaal Supergroup, Botswana

Astrobiologists have used stromatolites as an ecosystem model for exploring the interactions between microbes and minerals leading to formation and preservation of biosignature in the rock record. Stromatolites can provide information on early life on Earth and can serve as analogues for early life elsewhere in the universe. Here, a multi-tool combination of optical petrography, Scanning Electron Microscope coupled with Energy Dispersive Spectroscopy (SEM-EDS), and Raman spectroscopy techniques is adopted for studying biosignatures preserved in the Neoarchean stromatolites of the Ramonnedi Formation of southeastern Botswana. The goal is twofold: to identify and characterize any potential biosignatures preserved in ca. 2.4 Ga old carbonates, and to assess the potential of this approach for the detection and characterization of biosignatures from other planets (e.g., future Mars sample return missions). The petrographic investigation of stromatolitic dolostone revealed the presence of putative microbial fabrics such as laminations and microbial peloids, but no filaments were observed. Further SEM-EDS analysis of these fabrics revealed microbial-mediated clay minerals. Raman spectroscopy identified preserved carbonaceous material with peak temperature of 326 ± 30°C to 392 ± 50°C equivalent to the host rocks’ diagenetic history. Although microbial signatures are rarely preserved within dolomitized carbonates, the exceptional biosignature preservation within the Ramonnedi Formation stromatolites, such as carbonaceous materials, microbial peloids and fenestrae, makes them a potential analogue for the exploration of similarly ancient carbonate rocks on Mars.