Resource: Cave Formation: Biogeochemical Cycles

As certain chemical elements cycle through an ecosystem, they interact with both living (bio-) and nonliving (geo-) components of the ecosystem. These elements are recycled within the system in what are known as biogeochemical cycles. Sometimes, however, they are stored for long periods of time outside the pathway in what is called a sink. Examples of carbon sinks include rainforests and caves.

Biogeochemical cycles and atmospheric changes are often linked. Global warming, for example, is a disturbance caused by deforestation and the burning of fossil fuels, which together have altered the carbon balance in the atmosphere. But biogeochemical cycles can also shape the very ground on which we walk. When plants and animals die, their organic compounds combine with oxygen, and the compounds decay. The fraction of compounds that isn't oxidized can, over millions of years, form deposits that convert to coal, petroleum, or peat. Also, shells of dead aquatic animals accumulate on the sea floor and are compressed over time to form limestone, a common surface rock.

Solution caves are typically formed by a chemical reaction between limestone and groundwater that has become slightly acidified through contact with the carbon dioxide present in most soils. The acidified groundwater — called carbonic acid — gradually dissolves away minerals as it invades cracks in the rock. If enough water is available, narrow openings can slowly widen into large cavities and passages.

Now, a radical new theory suggests an agent other than carbonic acid may be responsible for creating some of the largest cave systems in the world. Scientists have discovered that sulfur, not carbon, was responsible for carving out massive cavities from the limestone rock of Capitan Reef, the fossil reef that contains New Mexico's Carlsbad Caverns.

Certain bacteria convert organic sulfur found in oil to hydrogen sulfide gas. Several million years ago, when this gas escaped from oil deposits underlying Capitan Reef, it mixed with oxygen in the groundwater supplies and produced sulfuric acid. Sulfuric acid is capable of dissolving vast amounts of limestone minerals — far more than carbonic acid can — and enough to explain the size of Big Room, which is featured in this video segment. As Capitan Reef uplifted little by little from its origins below sea level, the level of the water table dropped in relation to the land surface. The acidic groundwater drained away to reveal a dissolved cave system with large gypsum deposits, a telltale by-product of a reaction between sulfuric acid and limestone.

To learn more about the processes that produce different cave types, check out How Caves Form.

To learn more about deposits, including gypsum, that form in caves, check out Virtual Cave.

• What are two lines of evidence that caused scientists to question the prevailing theory of cave formation?
• What evidence caused scientists to suggest that sulfuric acid was the substance that resulted in the formation of these caves?
• What does the narrator mean when he says, "life itself was helping to shape the world beneath our feet"?