Dolomitic Microfossil Preservation in a Cryogenian Microbial Reef, Arkaroola (South Australia)

Microbial dolostones that now form the Arkaroola reef, northern Adelaide Rift Complex, and were deposited during the Cryogenian interglacial period, display a wide range of meso-scale stromatolite morphotypes in outcrop. Microcharacterisation involving optical microscopy, combined scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) and nanoscale secondary ion mass spectroscopy (NanoSIMS), was undertaken to identify microbial textures and relics in stromatolite samples. Microbial textures identified are micritic streaky and micropeloidal (grumous) microstructures as well as microcolumns, micritic threads and micritised tubular outlines resembling the Girvanella calcimicrobe. Within these microbial textures, multiple biotic components are discerned, including intact and flattened tubular bacterial sheaths, smooth and nanogranular non-crystalline extracellular polymeric substances (EPS), filamentous and spherical bacteriomorphs, as well as coccobacilli-like moulds. The biotic-mineralogical interface is characterised by microcrystalline rhombic dolomite, nano-sized pyrite spheroids and nano-sized flaky silicates in direct contact with biotic components. Interpreted as a mixed community of filamentous and coccoidal (cyano)bacteria, these microbes were important contributors to reef building. Preservation of microfossils was likely facilitated by synsedimentary silicification of EPS and bacterial sheaths. Microbial metabolic processes lowered kinetic barriers for authigenic precipitation of clay minerals on bacterial surfaces serving as nucleation sites. These synsedimentary clays strengthened initially soft microfabrics and potentially helped establish chemical conditions that promoted early diagenetic precipitation of protodolomite. The findings of this study show that carbonates may intricately preserve microfossils where associated with synsedimentary authigenic clays and microcrystalline dolomite. These results contribute to a deeper understanding of the formation and growth of microbial reefs during the Cryogenian interglacial period.