ORIGIN, CHEMISTRY, AND GEOMICROBIOLOGY OF TERRACED, CRYSTALLINE MIRABILITE MOUNDS ON THE SOUTHEASTERN SHORE OF GREAT SALT LAKE, UTAH

During the fall of 2019, four crystalline mounds formed on the southeastern shore of Great Salt Lake (GSL) in northern Utah, near Great Salt Lake State Park. Soon after, several more mound complexes were discovered on nearby Antelope Island. Recent historic low lake levels permitted sulfate-saturated spring waters to emerge on the exposed shoreline. When the spring water encountered cold winter air, sodium sulfate precipitated in the form of the mineral mirabilite (Na₂SO₄·10H₂O). Mirabilite precipitates at temperatures below ~10°C in dry climates; when the temperature warms, mirabilite dehydrates to a white powdery mineral called thenardite (Na₂SO₄). Mirabilite-saturated spring waters either pool in shallow sediment-collapse depressions or, with sufficient hydraulic head, precipitate subaerial crystalline spring mounds. The mounds are comprised of clear, bladed and tabular mirabilite crystals (1 to 10 cm in length; 1 to 5 cm in width) that form terracette pools and mini-rimstone dams due to cascading spring water flow. The waters have a potent sulfur odor and an oxidation-reduction potential (ORP) of -250 mV, pH of 7, densities of 1.103–1.1169 g/cm³, and temperatures of 10–15°C, as compared to nearby lake waters that had an ORP of 12 mV, pH of 7, density of 1.0872 g/cm³, and temperatures of 0–3°C. Green cyanobacteria and other microbial communities thrive in the spring waters and some are occluded within mirabilite crystals. The spring waters have a δ³⁴S of 20‰ and sulfide concentrations up to 6200 µM, implying sulfate reducing bacteria are present.

Currently, the south arm (SA) (south of the earthen railroad causeway) of GSL is undersaturated in sulfate. However, pre-causeway historic reports indicate mirabilite precipitated in the SA during the winter and through wave action, accumulated along the leeward southeastern ooid shore (similar to processes occurring in the north arm today). In the spring, the shoreline mirabilite dissolved and reprecipitated in the shallow subsurface (<100 cm) as an ooid cement. It is hypothesized that this subsurface layer of mirabilite is the source of the sodium sulfate-saturated spring water. Ongoing research will help to understand the chemistry and flow of groundwater, the mechanisms for crystalline mound formation, and the origin and role of the associated microbial community.