Climatic Data, Cave Records

CAVES HAVE PROVIDED climate researchers with several useful ways to learn about past climates. In caves that are connected to the surface, plant (particularly pollen) and animal remains allow scientists to study floral and faunal successions over time, which often reflect concurrent climate shifts. Cave deposits, collectively called speleothems (for example, stalagmites and stalagtites) provide interesting information that, though geochemical techniques are still being refined, promises a detailed and well-constrained record of past climate variability.

Caves are commonly found in limestone (calcium carbonate, CaCO3) bedrock settings. When precipitation moves through a soil horizon, it picks up dissolved carbon dioxide (CO2) produced in organic decomposition, thereby becoming mildly acidic. The acidic groundwater then slowly dissolves CaCO3 bed-rock, forming caves over time. Speleothems form when dissolved CaCO3 precipitates from groundwater upon interaction with cave air. As groundwater carrying dissolved CO2 and CaCO3 reaches the cave interior, CO2 degasses as a result of the large difference in partial pressure of CO2 between the soil and the cave air. The water is then supersaturated with respect to CaCO3, causing mineral crystal precipitation on cave surfaces.

Speleothems are ideal terrestrial climate records because they can be accurately dated using uranium series dating, which circumvents problems in carbon dating that complicate other terrestrial records, and because they exist in relatively stable cave environments that experience little or no erosion and may therefore grow continuously, often for hundreds of thousands of years. They can also be found all over the world, at high and low latitudes and elevations, and, therefore, have the potential to provide a wide range of terrestrial climate records.

Climate records are interpreted from speleothems by examining growth rate, stable isotopie composition, organic matter inclusions, trace and minor element composition, and variations in banding or crystal structure.

A cross-section of a stalagmite, for instance, reveals a very thin banding pattern. Uranium series dating indicates that these bands are often annual, especially in areas with a strong rainy season signal (as in monsoon areas). The relative thickness of these bands represents changing growth rates and can provide information about relative amounts of rainfall from year to year, or even variations in biological activity at the surface. [Page 231]

Speleothems, such as these formations in Carlsbad Caverns National Park, may hold valuable climatic data.

Increased biological activity adds more CO2 to the soil, making the groundwater more acidic and better able to dissolve CaCO3, leading to increased mineral precipitation inside caves. Bands of sediment or sandy material indicate complete cessation of speleothem deposition, which is usually interpreted as severe drought, or severe cold, as in glacial times when there is less meltwater available and groundwater may be completely frozen. Sea-level changes can also be inferred from cave shapes, and mineral or organic growth layers on speleothems in seaside caves.

Stable isotopes of oxygen and carbon, and trace element concentrations are analyzed within the carbonate or in pockets of trapped water called “fluid inclusions.” Minor or trace elements can be included in speleothem structure, either via groundwater or airborne dust particles, and can represent changes in temperature or air circulation, or geologic occurrences such as nearby volcanic eruptions. In-cave processes such as mineral recrystallization, evaporative processes (especially in caves near the surface), and whether or not the CaCO3 was deposited in isotopie equilibrium with groundwater, may influence isotopie composition.

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