QUANTITATIVE DESCRIPTIONS OF THE SPATIAL ORGANIZATION OF ~1.9 BILLION-YEAR-OLD MICROBIAL BUILDUPS

Stromatolites, laminated constructions built (at least in part) by microbes, serve as some of the earliest evidence of life in Earth's sedimentary rock record. These fossilized structures encode critical information about paleobiological and paleoenvironmental conditions. By better describing stromatolite morphogenesis—for example, constraining how biology (i.e., microbial activity) and physical parameters (i.e., seawater chemistry) contribute to form—scientists can generate new insights into the evolution of life and environment. Like for other fossils, morphological analyses hold the key to an improved understanding of these ancient constructions. At the centimeter- to meter-scale, such analyses involve: i) the shape of singular stromatolites and ii) the spatial relationships between individuals within a larger population.

Here, we describe exceptionally preserved bedding plane exposures of stromatolites in rocks of the Paleoproterozoic Pethei Group, Great Slave Lake, Northwest Territories, Canada. We use handheld photography and Structure from Motion (SfM) to build three-dimensional (3D) reconstructions of the bedding planes. Next, we trace individual stromatolites in our reconstructions and then quantify their organization (e.g., orientation, connectivity, and distribution); the resulting organizational metrics provide insights into growth dynamics. To deconvolve the relative contributions of biotic and environmental forcings to stromatolite morphology and organization, we merge our quantitative descriptions with micron- to decameter-scale sedimentological and geochemical observations. Finally, to improve inferences about morphogenesis, we compare our findings to other studies of microbially mediated constructions, both extant and fossilized in the sedimentary rock record.