A BIOGEOLOGICAL ANALYSIS OF CARBONATE MICROBIALITE NODULES THAT FORM IN FISH LAKE, SOUTHWESTERN MICHIGAN

The intersection between the biologic and geologic realms can be found in the study of microbialites. We explore what conditions influence microbialite growth in a freshwater, alkaline, lacustrine environment and provide insight into how cyanobacteria influence carbonate precipitation.

Carbonate nodules are patchily distributed along the lake bottom of the eastern shoreline of Fish Lake in Orangeville Township, Michigan. Where present the nodules are abundant, elsewhere much of the lake bottom consists of loose marl mud. The nodules average around six inches in diameter, are most commonly tabular in shape, and have been found forming around freshwater bivalves and sticks. The nodules have a cauliflower appearance, and the outer layer is rich with cyanobacteria. The surfaces and much of the interior of the nodules are porous and friable, in contrast to the more laminar layering common in most stromatolites. Inspection of nodule thin sections reveal layering that appears to correlate with differences in porosity and density of carbonate material. Preliminary microscopy of the outer layer of microbialites reveals the dominance of cyanobacteria with numerous thin filaments and a few thick filaments. Crystalline tubes visible around some thin filaments suggest biomineralization, as cyanobacterial photosynthetic activity locally promotes precipitation of carbonate. Other authors have interpreted similar nodule textures at Pavilion Lake in Canada to reflect high photosynthetic biomass growth rates.

We also examined water chemistry and hydrologic conditions to help explain the microbialite environment of formation. Mini-piezometers were used to collect samples of groundwater at several locations and to confirm groundwater discharge into the lake. Lake and groundwater samples were sent for analysis of major cations (Ca, Mg, Na, K) and anions (HCO₃, SO₄, Cl). When compared to the lake water, groundwater samples had greater values for all measurements, including conductivity, calcium concentration, and alkalinity (~ 5x). Further investigation is needed to constrain the environmental conditions that control the distribution of these nodules. These modern day microbialites may give clues and glimpses into the distant past, as their ancestors provide the earliest evidence of life.