SULFUR SYSTEMATICS AND CARBONATE DIAGENESIS IN THE MITCHELL PLATEAU, INDIANA

Two endmember processes, carbonic acid and sulfuric acid speleogenesis, are part of a dichotomy in karst science between epigenetic and hypogenetic caves. This binary is a useful model for the differences between caves and karst regions but does not acknowledge a spectrum of cave formation. Our longitudinal study of groundwater geochemistry in the classic karst landscape of the Mitchell Plateau, Indiana provides insight into this continuum in the Midcontinent. The Bluespring karst basin is drained by sinkholes connected to the Bluespring Caverns cave system. The adjacent 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 at 4 sites from 2/14/2019 to 11/06/2020 and analyzed for carbon, sulfur, and other ionic content as well as isotopes of dissolved inorganic carbon, sulfate, oxygen, and hydrogen. Our analysis reveals sulfur concentrations elevated 0.2 mmol/L and 1.8 mmol/L above atmospheric background concentrations at or near the terminus of Bluespring and Lost River karst basins, respectively. Between the basins, stable isotope values indicate that sulfur is differently sourced: hydrogen sulfide (δ³⁴S = -14.2‰) evolved from organosulfur compounds in petroleum seeps at Bluespring Caverns versus the gypsum and anhydrite (δ³⁴S = +17.91‰) beds of the St. Louis Limestone at Orangeville Rise, a terminal spring of the Lost River karst basin. The sulfur sources have opposing 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 argue that speleogenesis in the Mitchell Plateau is neither epigenetic nor hypogenetic, but instead polygenetic with competing processes varying across space and time.