Early Archaean Stromatolites: Microbe-Sediment Interactions on the Early Earth

Microbial-sedimentary biosignatures are one of the most significant offerings from the early fossil record, potentially enabling us to trace life back to Early Archaean beginnings. However, the biogenicity of Earth's oldest ‘biosignatures' has been a matter of debate.

Stromatolite units of the 3.45 billion year old Strelley Pool Formation (SPF), Western Australia, provide an opportunity to integrate possible signatures of microbe-sediment interactions across the full spectrum of resolutions — from basin analysis to microfabrics — in order to seek definitive evidence of biology on the early Earth. Large scale observations show that the SPF stromatolites formed a carbonate reef-like buildup during transgression of an isolated shallow marine platform, developing a diverse array of morphotypes in seemingly ecological distribution in peritidal environments, between periods of evaporation-induced crystal formation and local exposure. Small and medium scale evidence (microfacies, stromatolite architecture and fabric analyses) show that, although mineral precipitation was an important process on the platform, formation of the stromatolites cannot be attributed solely to abiotic chemical precipitation. Despite extensive recrystallization, the sedimentary fabrics and microfacies provide a highly detailed record of co-variations in local sedimentary regime, lamina geometry and stromatolite accretionary architecture. There are also changes in stromatolite shape or distribution that coincide with changes in environment; and variations in stromatolite shape or laminar fabric across unchanging environments. These relationships between fabric, sedimentary facies and stromatolite morphologies provide insights to: 1) local microbial mediation of sediment deposition (and consequent stromatolite formation); 2) environmental forces upon stromatolites/microbial communities; 3) the likely response strategies that microbes adopted; and 4) the resultant effect on stromatolite morphology. This integrated, multiple-resolution approach has proven useful not only in revealing new types of evidence for the origin of the SPF stromatolites, but also for identifying general principles that can be applied to other similar studies.