Mammoth Cave

Overview

By Matt Selensky

Caves around the world harbor myriad microbiota that thrive in these dark, energy-starved subsurface environments. Located in central Kentucky, Mammoth Cave is the longest known cave system on Earth, encompassing over 580 km of mapped passages. Although a cave-wide assessment of the its microbial ecology has never been performed, site-specific studies have elucidated intriguing characteristics of some Mammoth Cave microbes. Microbes found in the karstic sediments beneath two shallow water pools within Mammoth Cave were inferred to exhibit high diversity and total cell densities, reaching 1.4 × 10⁷ cells per g wet sediment. Densities and activity of the chemolithoautotrophic Nitrobacter sp. were determined to be significantly higher in the caves relative to the surface. These nitrifying bacteria have been suggested to play a role in the formation of widespread saltpeter (KNO₃) deposits found in the cave by oxidizing bat guano N or other surface-sourced N transported underground. A seep rich in hydrocarbons and sulfide in Marianne’s Pass brings additional energy sources into the cave for microorganisms. No published work appears to be available that describes sulfur-oxidizing bacteria in Mammoth Cave. However, a 16S rRNA gene clone survey of nearby Parker Cave demonstrates the widespread abundance of Thiothrix sp. living in the underground and euxinic Sulphur River. Caves untouched by humans inherently lack light; however, artificial lamps placed in “show cave” sections of Mammoth sustain photosynthetic algal and cyanobacterial populations. Such microorganisms are otherwise thought to be transient, washing into the cave either after heavy rains or via riverine input. Fungi commonly colonize and decompose organic matter such as rat fecal pellets or dead crickets that occasionally litter the cave passages. The most notorious fungus that is found in Mammoth Cave is Pseudogymnoascus destructans, the causative agent of white nose syndrome in bats. Other eukaryotes that are found in the cave include amoebas and other “protists” that tend to colonizing standing pools of water. It is becoming clearer that the microbial communities of Mammoth and other caves are greatly involved in the N, S, and C cycles. We are only just beginning to gain a system-level understanding of shallow subsurface microbial ecology.

Geology of Mammoth Cave

A cross section of the Mammoth Cave region, demonstrating the karst nature of the area. From Palmer (2017).

Mammoth Cave is located in central Kentucky and is the longest known cave system on Earth, encompassing over 580 km of mapped passages . Carved by dissolutional processes, the Mammoth Cave region is an example of a karst landscape. Sinkholes drain surface water into soluble limestone strata, which dissolve over geologic time. Mammoth Cave cuts through three major limestone layers, which were deposited 340-330 million years ago during the Mississippian Epoch. The St. Louis limestone is the oldest of the three; it is dolomitic, 53-60 m thick, and commonly contains shale and chert nodules. The Ste. Genevieve limestone is 35 m thick and is somewhat less dolomitic than the St. Louis stratum, though shales are still common in this layer. The Girkin limestone is the youngest, least magnesium-rich carbonate layer in the Mammoth Cave region and is 40 m thick on average.

Although the limestones of Mammoth Cave are hundreds of millions of years old, the passages themselves are much younger. An impermeable sandstone caprock exists over much of the region, which compacted and protected the underlying limestone from dissolution for millions of years. The caves themselves began forming about 30 million years ago. The formation of the caves (speleogenesis) occurred due to the flow of mildly acidic water through bedding planes. Water becomes slightly acidic when it picks up CO₂ from the atmosphere or overlying soils, where respiration produces abundant CO₂. In water, CO₂ exists in chemical equilibrium with carbonic acid (H₂CO₃); thus, as the partial pressure of CO₂ increases, more H₂CO₃ is present in solution. The flow of this weak acid through pre-existing cracks and bedding planes dissolves the surrounding limestone, producing large passages over time. Following this paradigm, meteoric water tends to enter the Mammoth Cave system through numerous sinkholes in the Pennyroyal Plateau. The water flows through the subsurface limestone until it discharges into the Green River, which has eroded through the sandstone caprock.

As a limestone cave system, Mammoth Cave is dominated by carbonate geochemistry, yielding alkaline-buffered solutions. However, some parts of the cave system exhibit distinct geochemical characteristics. Beneath the Mammoth Cave region lies a monocline, which cuts through most of the South-Central Kentucky Karst. There are several sites along this monocline where sulfide-laden brines rise to the surface, including Sulphur River in nearby Parker Cave. Marianne’s Pass within Mammoth Cave also receives sulfide-rich brines, though the origins of this seep are likely not from the same monocline structure.

References

Authored for Earth 373 Microbial Ecology, taught by Magdalena Osburn, 2020, NU Earth Page.