Sulfur Systematics and Carbonate Diagenesis in the Mitchell Plateau, Indiana

Abstract

Two endmember processes, carbonic acid speleogenesis and sulfuric acid speleogenesis, describe a dichotomy in karst science between epigenetic and hypogenetic caves. This binary may be a useful model for the differences between caves and karst regions but does not acknowledge a spectrum of cave formation. This ambitious longitudinal study of groundwater geochemistry in the classic karst landscape of the Mitchell Plateau, Indiana provides a new understanding of speleogenesis in the Midcontinent. The Bluespring karst basin is drained by sinkholes connected to the Bluespring Caverns cave system, previously described as a type example of an epigenic floodwater diversion maze cave. The adjacent, larger Lost River karst basin is drained by a network of surface streams that sink into the Lost River cave system, a subject of intense study since the late 1800s. To assess the role of sulfur in the Mitchell Plateau, water was sampled from 4 sites across the karst basins from February 2019 to November 2020 and analyzed for carbon, sulfur, and other ionic content as well as isotopes of dissolved inorganic carbon, sulfate, oxygen, and hydrogen. Our analysis reveals elevated sulfur at or near the terminus of karst basins but differently sourced: hydrogen sulfide evolved from organosulfurcompounds in petroleum seeps in Bluespring Caverns versus the gypsum and anhydrite beds of the St. Louis Limestone at Orangeville Rise, a terminal spring of the Lost River karst basin. Diverse sulfur sources have opposed implications for carbon‐sulfur cycle linkages, potentially accelerating carbon flux in the Bluespring karst basin from sulfuric acid dissolution while decelerating it in the Lost River karst basin because of common ion effect related calcite precipitation. We synthesize new and old data, geochemical modeling, geologic structure, carbonate chemistry, rock core, cave gypsum, and petroleum seeps, and conclude with a shifting paradigm for speleogenesis in the Midcontinent. We argue that speleogenesis in the Mitchell Plateau is not epigenetic or hypogenetic, but instead polygenetic with competing processes varying across space and time.