The source of gypsum in Mammoth Cave, Kentucky

Mammoth Cave (Kentucky, USA) is the world9s longest human-navigablecave system. Gypsum (CaSO 4 ∙2H 2 O) crystals line manydry passages of the limestone cave and the source of sulfur for thegypsum remains uncertain. Previous workers have suggested sulfatefrom pyrite oxidation, Mississippian-aged sulfate evaporites, andPleistocene soil sulfate as possible sulfur sources. We use sulfur isotopes(δ 34 S) to constrain the gypsum sulfur source by comparing theδ 34 S values of different possible sources to that of gypsum throughoutthe cave. δ 34 S gypsum values (n = 106) from 12 different locationswithin the Mammoth Cave system are along a continuum of δ 34 S valuesfrom –12.0‰ to +12.8‰, with little variability along the crystalgrowth axis or between samples within the same cave chamber. Neithersulfur from coeval sedimentary evaporites (i.e., gypsum, anhydrite)nor from formations overlying the cave is required to explainthe δ 34 S gypsum data. Rather, the range of pyrite δ 34 S in strata immediatelysurrounding the cave is sufficient to generate the spectrumof observed δ 34 S gypsum . Modern water δ 34 S SO 4 values are similar to thehost formation δ 34 S pyrite , suggesting that oxidized pyrite from the hostformation continues to be a major sulfur source to this day. Together,these observations strongly suggest a significant local source of sulfurfor Mammoth Cave gypsum, the majority (66%–100%) of which isderived from the oxidation of pyrite in strata adjacent to the cave.