MEASURING MICROBIALITE CONTRIBUTIONS TO LAKE BOTTOM SEDIMENTATION IN FISH LAKE, MICHIGAN WITH STABLE ISOTOPES

Unique microbialites (carbonate mounds made by microorganisms) are found throughout shallow waters in Fish Lake, Michigan. Their abundance indicates important ongoing interactions between the local microbial ecosystem and water chemistry. Understanding this lake and its unique geobiology has been the subject of multiple efforts by Grand Valley State students and faculty over the last several years. These efforts have primarily focused on the geochemistry and growth of the microbialites themselves, with less effort to date on how they affect water quality and sedimentation within the lake.

Here we investigate the relationship between lake sedimentation and the activity of microbialites using a ~0.4 m core collected from the lake bottom in 2021. We used sedimentological analysis of core material, XRD, and stable isotope analysis. XRD data from the core sample and microbialites collected from Fish Lake suggest that the relative composition of these two materials are identical. Both microbialites and core material are ~100% calcite, suggesting a common origin. Core δ18O is largely constant, ranging from -8.76 to -9.24 ‰ VPDB. Bivalves collected from Fish Lake also overlap with this range, suggesting that the temperature at which all carbonate material is produced in the lake is broadly consistent. However, despite the similarities between the core sample and microbialites in XRD and δ18O, core and microbialite d13C data are significantly different. Core δ13C ranged from -6.0 to -4.9 ‰, while microbialite δ13C ranged from -4.8 to -3.0 ‰. Both of these sample sets are much more positive than lake bivalves, at approximately -9 ‰, presumably due to photosynthetic effects during microbialite formation. This spread of δ13C values suggests that the majority of lake bottom sediments have been contributed by microbialites, but also that a significant contribution comes from other sources within the lake. These results will aid our understanding of how abundant microbialites can affect sedimentation and water quality in a small inland lake.