The carbon cycle is an example of a biogeochemical cycle in which a certain chemical element interacts with both living (bio-) and nonliving (geo-) components of an ecosystem. Whereas the influence of carbon is more frequently associated with atmospheric change — for example, an accumulation of airborne carbon may lead to global warming — another cycling element, sulfur, can shape the very ground on which we walk.
A radical new theory of cave formation suggests an agent other than carbonic acid can create some of the largest solution cave systems in the world. This agent is sulfuric acid. Sulfuric acid reacts with rock surfaces on contact, rapidly dissolving calcite, the primary mineral found in limestone. Scientists have discovered that sulfuric acid is responsible for having carved out massive cavities from the limestone rock where New Mexico's world-famous Carlsbad Caverns are located.
Kane Cave in northern Wyoming was also formed from sulfuric acid weathering. Sulfuric acid is capable of dissolving vast amounts of limestone minerals — far more than carbonic acid can — but it doesn't readily form in nature. It is a by-product of the activity of certain microbes that populate the ground beneath some caves.
In lower Kane Cave, scientists analyzing microbe populations noted that something interesting, and apparently different from what had occurred when Carlsbad Caverns formed, seemed to be happening. It was true that bacteria outside the cave system were producing hydrogen sulfide gas from sulfur stored in underground deposits. However, bacteria present within the cave — in colonies located in springs running through the cave — were also converting hydrogen sulfide gas to sulfuric acid.
To learn whether the bacteria had simply moved into an existing cave, in which case their activity was modifying what was there, or whether the entire cave-formation process was the result of microbial activity, the scientists conducted a simple test. They exposed chips of pure calcite both to acidic water alone and to acidic water with microbes present. They compared the results over time and determined that the bacteria in the cave were in fact hastening the conversion of hydrogen sulfide gas to sulfuric acid. This, in turn, was influencing how quickly the limestone rock dissolved. Thus, the efficiency with which cave formation occurs in the presence of certain bacteria indicates that the bacteria could have carved out Kane Cave.
These cave microbes prove that living organisms — as part of the sulfur cycle — can actually influence the way caves form.
To learn more about how Carlsbad Caverns were formed, check out Cave Formation: Biogeochemical Cycles.
To learn more about solution caves, decorative mineral deposits (speleothems), and karst terrain, check out Virtual Cave and Caves and Karst.
To learn about caves that form through processes other than carbonic acid reacting with limestone, check out How Caves Form and Virtual Lava Tube.