Insights into modes and timing of microbialite mineralization from mobile microbialite grains in Great Salt Lake, Utah, U.S.A.

ABSTRACT Microbialites are among the earliest indications of life on Earth and interpreting the formation mechanisms—both abiotic and microbially mediated—of mineral assemblages and microfabrics in these structures is a crucial part of understanding Earth’s geobiological record. To decode ancient microbialites in this way, we must understand how they are formed through the study of modern analogs. Previous modern analog studies of microbialites from Great Salt Lake and other closed basin lakes have proposed a microbialite mineralization model in which extracellular organic matrix in the biofilm is first permineralized by a poorly crystalline Mg-Si phase before being replaced by aragonite, which is itself later partially replaced by dolomite. To explore how this model may apply to other microbialite morphotypes and better constrain the timing of these stages of mineralization, we characterized two sediment cores from the southern shoreline of Great Salt Lake, in which the sediments are composed dominantly of mobile microbialite grains, using light microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffractometry, analyses of stable-isotope ratios of C and O, and radiocarbon dating. Unlike previous studies of in situ microbialite mounds, our core samples provide a series of snapshots in time as mineralization of microbialite grains proceeds spanning some 4200 years. We document evidence of the first two stages of mineralization (permineralization of cyanobacterial cells by Mg-Si and replacement of Mg-Si by aragonite), but did not observe authigenic dolomite formation. We interpret that the conditions necessary for authigenic dolomite formation are not present throughout Great Salt Lake and that dolomite formation may instead be concentrated in hot spots such as the area adjacent to Antelope Island Marina. The microbialite grains were significantly permineralized by Mg-Si in under ∼ 340 years, and nearly completely mineralized by aragonite after ∼ 2000 years. Finally, we find evidence that the δ13C and δ18O values of aragonite in the microbialite grains preserve a record of lake-water δ13C and δ18O values driven by changes in lake level, even though much of the aragonite forms postdepositionally. This observation suggests that stable-isotope data from lacustrine microbialites that form through this type of mineralization process can still be reliable paleoenvironmental records.