MICROBIALITE GROWTH AND DISTRIBUTION WITHIN FISH LAKE, MICHIGAN

The study of microbialites can help modern researchers understand Earth’s beginnings as a life-supporting planet. Microbialites are organo-sedimentary structures formed from the precipitation of calcium carbonate by the photosynthetic activities of cyanobacteria. This research analyzes the formation environment and distribution of freshwater microbialite nodules in Fish Lake, Michigan. The carbonate nodules are only present in certain locations within the lake and the lake bottom sediment is predominantly marl. The distribution and density of the nodules was mapped by field survey and analyzed using ArcGIS. Measurements were taken of the size and weight of nodules at two lake locations (shallow and deeper depths) within a one-meter square area. There was an average of 40 nodules per square meter. The average diameter of sampled microbialites was 12 cm and the average weight was 540 grams. Given that the area of the lake with microbialites is mapped as 3,225 m2, we estimate there could be 129,000 nodules in the lake (despite microbialites only occupying about 0.5% of the entire lake area). Based on rates of sedimentation from aquarium experiments, the average amount of sediment produced per nodule per day is 5.5g. Scaling this up, the calculated sediment production for a 3-month period is approximately 1824 kg, helping to explain the abundance of carbonate sediment in the lake.

We analyzed the oxygen isotope signatures of the growth layers of six nodules to explore the timing of when the microbialites form. Carbonate samples were taken from the interior and exterior layers of five nodules, with ten closely spaced carbonate samples taken from one additional nodule for high definition analysis. The 𝛿18O (VPDB) values for the twenty samples ranged between -9.05 and -8.23, with a mean value of -8.66. Given seasonal temperature fluctuations in the lake, the small variation in 𝛿18O suggests that nodule growth took place within a short time interval, perhaps during the summer. Microbialites are useful as modern analogues to help understand ancient stromatolites, which were responsible for the early production of oxygen and subsequently the origins of life on earth.