A window to the Precambrian
Along the western shores of Lake Winnipegosis, living structures show Earth's deep past. These are microbialites, layered structures built by communities of microorganisms, primarily cyanobacteria. They are modern, modern versions of stromatolites, which formed reefs billions of years ago and were released the oxygen that transformed Earth's atmosphere. The Lake Winnipegosis microbialites take the form of domes and brain-like mounds that can be several centimeters thick.
The process of their formation is slow and depends on a precise set of conditions. The microbial community, a film of photosynthesizing cyanobacteria and other microbes, traps fine-grained sediment from the water. In addition, their metabolic activity changes the water chemistry immediately around them, causing calcium carbonate to precipitate directly out of the water and onto the structure. Layer by layer, this combination of trapped sediment and precipitated mineral builds the microbialite over time. These are among the few known examples of such large, complex microbialite structures forming in a freshwater lake today.
A unique chemical brew
The existence of these microbialites in a freshwater lake is exceptionally rare and points to the unique water chemistry of Lake Winnipegosis. The lake is a remnant of the immense glacial Lake Agassiz, which covered much of North America at the end of the last Ice Age. Its basin sits atop Paleozoic limestone and dolomite bedrock. This geology creates unusual water conditions that enable the microbialites to form.
Saline groundwater, enriched with minerals from flowing through the Williston Basin, seeps into the lake through springs along the western shore. This influx results in elevated salinity and specific ion concentrations that are different from typical freshwater lakes. In some areas, such as Dawson Bay, conductivity can exceed 30,000 µS/cm, creating brackish and even hypersaline zones. The water is also alkaline, with a pH typically between 8.0 and 8.5, and is rich in calcium carbonate, the primary material for the microbialites. This specific combination of high mineral content and alkalinity, fed by groundwater discharge, allows the microbes to precipitate carbonate minerals, a process that usually occurs in marine environments.