Mammoth Cave, Kentucky, is the longest cave system in the world.
Many of the dry passages of the cave are lined with gypsum (CaSO4 * 2H2O), yet despite nearly a century of research, the source of the gypsum sulfur remains uncertain. Identifying the sulfur source is important because it reveals how fluids move through the cave, which helps geologists understand cave formation and engineers understand chemical transport in karst terrains.
To constrain the gypsum sulfur source, we measured the stable sulfur isotope ratios (δ34S) of different geological sources and compared them with that of the gypsum. The entire range in gypsum δ34S values falls within that of pyrite δ34S found in the cave hosting formations.
Sampling along the crystal growth axis suggests that the source of sulfur remained the same during the entire growth period of the crystal while δ34S of dissolved SO42 in modern seeps suggests the sulfur source is the same today.
These results suggest that the dominant source of gypsum sulfur is pyrite found locally (within ~ 20 m) and disseminated within the limestone cave walls.
Citation: The source of gypsum in Mammoth Cave, Kentucky, J. Garrecht Metzger et al., Geology; http://dx.doi.org/10.1130/G36131.1.