Sulfate-rich spring seeps and seasonal formation of terraced, crystalline mirabilite mounds along the shores of Great Salt Lake, Utah: Hydrologic and chemical expression during declining lake elevation

Abstract

As Great Salt Lake (GSL) elevation declined below ∼1278.6 m (4195 ft) beginning in the fall of 2019, crystalline mounds formed on the southeastern shore, near Great Salt Lake State Park, Utah. Soon after, several more mound complexes were discovered on nearby Antelope Island. Recent historic low lake levels permitted sulfate-saturated spring brines to emerge on the exposed shoreline. When the spring waters encountered cold winter air, sodium sulfate precipitated in the form of the mineral mirabilite (Na2SO4·10H2O). Mirabilite-saturated spring waters pool in shallow sediment-collapse depressions or, with sufficient hydraulic head, precipitate subaerial crystalline spring terraced mounds. The mounds are composed of clear, bladed, and tabular mirabilite crystals (1 to 10 cm in length; 1 to 5 cm in width) that form mini pools and rimstone dams due to cascading spring water flow. The waters have elavated μM concentrations of sulfide, 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 undetectable sulfide, an ORP of 12 mV, pH of 8.2, density of 1.0872 g/cm3, and temperatures of 0°–3 °C. Green algae and probable sulfide oxidizing bacteria thrive in the spring waters and some are occluded within the mirabilite matrix and fluid inclusions. Sulfur δ34S and oxygen δ18O isotopes of brine and mirabilite sulfate are enriched +18.8–20.41‰ and + 12.2–14.5‰, respectively, indicating groundwater and mirabilite are in equilibrium for oxygen. Brine sulfide is heavily fractionated (δ 34S of ∼ − 30 ‰) relative to sulfate, indicating active sulfate reduction in subsurface aquifers that feed the spring seeps and which is consistent with higher HS− concentrations relative to adjacent lake waters. The recognition of such biosignatures retained in mirabilite provides further insights for the preservation potential of biological material in extraterrestrial sulfate minerals.Today, Gilbert Bay (south arm of GSL) is undersaturated with respect to mirabilite. However, pre-earthen causeway historical reports (prior to 1959) indicate mirabilite precipitated in the south arm during the winter and through wave action accumulated along the leeward southeastern oolitic shore (similar to current processes occurring in the hypersaline Gunnison Bay, the north arm of GSL). Upon solar irradiance in the spring, the shoreline mirabilite dissolved and reprecipitated in the shallow subsurface (<1 m) as an ooid cement. We hypothesize that this subsurface layer of mirabilite is the source for active lake margin sodium sulfate-saturated spring brines. The emergence and surface exposure of the brines and associated chemical sediments are an additional contributing solute source that may increase GSL salinity during lake level decline more than evaporation alone.