Correlative microspectroscopy of biogenic fabrics in Proterozoic silicified stromatolites

Questions surrounding the biogenicity of ancient stromatolites have perplexed geobiologists for decades. Abiotic processes can produce superficially stromatolite-like structures; moreover, stromatolites frequently fail to preserve organic materials and cellular traces of their microbial architects. Using spatially correlated optical and electron microscopy coupled with Raman and FTIR microspectroscopy, we show that silicified stromatolites from the Tonian Skillogalee Dolomite (Flinders Ranges, South Australia) contain exceptionally well preserved microbial mat fragments and microbially induced sedimentary structures. These organic-rich layers exhibit mat-like laminations with low degrees of inheritance and reflect interactions between microbial communities and their environments, i.e. growth, sediment trapping and binding, and reactions to early diagenesis, and are inconsistent with abiotic formation. Although accounting for a minor proportion of the volume of the stromatolites, these kerogenous relics are demonstrably syngenetic and comprise aromatic and aliphatic organic materials, likely preserved due to early and rapid silicification. Constraining the origins of such lamination-scale features can elucidate relationships between morphogenesis and diagenesis and may assist in the resolution of controversies surrounding stromatolite biogenicity in deep time.