Unique rock formations in caves display a climate record of their own and serve as proxy indicators of past moisture conditions. In the southwestern United States, more than 100 caves exist whose formations tell a story of what the climate was like long ago. In addition, because the formations in caves are preserved underground, they are protected from the harsh weathering and erosional processes to which features on the Earth's surface are subjected.
The wonderland of geological formations found in caves is formed when water soaks through the ground and picks up minerals, the most common being calcium carbonate. When the water drips into caves, it is laden with calcium carbonate. As the water drips, it leaves behind the mineral deposits, which are the same types of hard white deposits that collect on sink faucets and showerheads in homes. As the minerals accumulate, they form the iciclelike rocks that hang from cave ceilings called stalactites. Minerals drip off the stalactites and are deposited on the ground directly below them, creating another deposit from the ground up called a stalagmite. The calcium carbonate can collect in many interesting shapes, such as flowstone and popcorn.
Caves are another proxy of past climate change because they are indicators of past water conditions, which provide important information about past rainfall and temperature. They are formed by the subsurface action of water, like a type of huge subterranean plumbing system. Many massive subterranean cave systems, such as Carlsbad Caverns in New Mexico, exist throughout the world, through which water does not currently flow but did in the past.
Cave formations give scientists clues as to what past climatic processes were like in an area. This is the Chinese Theater formation located in Carlsbad Caverns National Park, New Mexico. (National Park Service)
Caves initially form from the dissolving of carbonate rocks and the formation of cavities and passageways. This occurs in an area just below the water table in the zone of saturation, where the continuous mass movement of water occurs.
The second stage in cave development occurs after the water table lowers. During this stage, the solution cavities (the hollow openings left after the removal of the calcium carbonate in limestone by carbonic acid) are abandoned in the unsaturated zone, where air can enter. This leads to the deposition of calcite, which is what forms the dripstone features.
The mineral formations in caves are collectively referred to as spe-leothems. As long as water is actively flowing into caves, speleothems continue to grow in thin, shiny layers because mineral deposits are continually being added. The amount of growth they experience is a direct indicator of how much groundwater percolates into the cave. If there is only a small amount of growth, this may indicate that the existing climate was dry. Conversely, rapid growth may indicate that the climate at the time of deposition was extremely wet.
Geologists can also tell the difference between when speleothems are actively growing and when they stopped growing. When they are active, they have a smooth, wet appearance. When they have stopped growing and are inactive, the outside becomes dirty and eroded, making it look dull.
As speleothems form and grow layer upon layer, scientists have been able to date the individual layers in some caves by measuring how much uranium (a radioactive element used for dating; see chapter 3) has decayed. This is possible because uranium from surrounding bedrock seeps into the water and forms a carbonate that is incorporated into each layer as it forms. As the uranium (parent element) decays into thorium (daughter element), it adheres to the clay in the bedrock instead of escaping into the groundwater and into the speleothems. Because the thorium does not escape, the newest layer added to the speleothem contains no thorium. The uranium in the deposit then decays into thorium, enabling scientists to radiometrically date the deposit by measuring the ratio of uranium to thorium.
By dating each individual layer of a speleothem, paleoclimatologists can compile a record of how groundwater levels have changed over the lifetime of the formation. According to paleoclimatology experts at the National Oceanic and Atmospheric Administration (NOAA), however, using information from just one cave is not sufficient to infer climate conditions over broad areas. Scientists must look at caves in several geographic locations and search for similar patterns of growth in spe-leothems to accurately infer regional climate change.
One area scientists are looking into with speleothem research is analyzing oxygen isotope records (oxygen-18 and oxygen-16). Through the analysis of tiny bubbles in the speleothems that contain trapped air and water, scientists can look at the oxygen isotopic ratio to calculate past air temperatures. As discussed earlier, the ratio of these two isotopes in water varies based on air temperature, the amount of precipitation, and the amount of water locked in ice caps and glaciers worldwide. Scientists believe that the 18O-16O ratio in cave deposits may be able someday to be directly correlated to understanding global climate conditions.
To support this, scientists have been able to use this oxygen isotope ratio to determine the origin of rainfall in certain geographic areas, that is, whether it was from coastal or inland sources. According to NOAA, by analyzing the oxygen isotope ratio, it is possible to track seasonal changes and rainfall patterns, and since caves exist worldwide, speleo-thems may have the potential to play a key role in understanding the land-based global climate record.
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