MICROBIAL AND MINERAL IMPACT ON TEXTURE IN INCIPIENT DENDROLITIC MICROBIALITES, LITTLE HOT CREEK, CA

Microbial mats represent one of Earth’s oldest ecosystems and may be preserved in the rock record as carbonate microbialites, though mechanisms of mat lithification, including microbial influence on texture, remain unclear. Modern systems where mechanisms of texture formation can be tested, especially in specimens resembling ancient microbialites are rare. Here, we investigate incipient CaCO₃ microbialites from Little Hot Creek (LHC), a hot spring system in eastern California, where actively-lithifying microbial mats have a dendrolitic (shrub-like) texture strikingly similar to certain Neoproterozoic and Cambrian specimens. The similarity in texture and rapid lithification timescales (weeks-months) provide a unique opportunity to examine microbialite texture formation in situ with relevance to ancient microbialites.

We used fluorescently labelled embedded coring to preserve both the microbial and mineralogical phases within the LHC dendrolites in life position with micron scale resolution. Texture was characterized using reflected light microscopy, petrographic thin section analysis, X-ray micro computed tomography, and epifluorescence microscopy. Petrographic thin section analysis in transmitted light and with epifluorescence revealed an arborescent microfabric of rounded spar calcite crystals varying in size and interconnectedness, all suspended in a microbial meshwork. Organized growth of the microbial community provides the template for calcite grain nucleation, evidenced by an abundance of small, non connected spar grains in newer, branching regions of the dendrolite, suggesting a mechanism of branching driven by microbial growth, followed rapidly by calcite grain nucleation. Extensive boreholes (~5um diameter) were found in multiple planes within spar grains, likely made by endolithic cyanobacteria. We characterize several consistent relationships between microbial and mineral fabrics that contribute to overall texture—including their relationship to endolithic microboring—and quantitatively document their appearance in several incipient microbialites in Cone Pool. By conducting this process on samples which maintain both minerals and microbes in life position, we gain insight into the formation of sub-mm scale textures found in ancient microbialite specimens and their dependence on microbial position and activity.