Researchers Uncover Mystery Behind Mammoth Cave’s Gypsum

Gypsum on the ceiling of Kentucky’s Mammoth Cave. Photo by Navin75/flickr

Scientists investigating the source of sulfur in Kentucky’s Mammoth Cave have uncovered its secret.

In research published recently in the Geological Society of America Journal, Geology, a team of researchers from Washington University in St. Louis, Missouri tried to determine where the sulfer, necessary for the formation gypsum crystals that line many dry passages of the cave, came from.

Previous workers had suggested pyrite oxidation, Mississippian-aged sulfate evaporites, and Pleistocene soil sulfate as possible sulfur sources.

By comparing sulfur isotopes of different sources to 12 samples of gypsum within the cave, the researchers came to the conclusion that the major source of sulfur, between 66% and 100%, in Mammoth Cave gypsum is actually from pyrite oxidation from the caves host formation.

Furthermore, the sulfur isotopes in current water is similar to the host formation, suggesting that oxidized pyrite from the host formation continues to be a major sulfur source to this day.

The longest known cave in the world with 400 miles of suveyed passage, Mammoth Cave developed in thick Mississippian-aged limestone strata made up of the Girkin, Ste. Genevieve, and the St. Louis formations.