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Speleothems are secondary mineral deposits in caves. The term “secondary” is used because these mineral deposits are not part of the original limestone bedrock (which may be hundreds of millions of years old) that was dissolved by groundwater to form the cavities underground in the first place. The speleothems form later from water saturated in dissolved Ca²⁺ and CO₃²⁻ ions that drips into the cave through fractures in the cave ceiling, and precipitates solid calcium carbonate (CaCO₃). These mineral deposits are like the white spots and crusts that can build up around a household faucet if you have “hard” water (i.e. water with a lot of calcium carbonate dissolved in it) and a leaky faucet. The dripping water in a cave is connected to the hydrologic cycle; it was once rainwater that percolated down through the soil and became part of the groundwater system. As the water drips in a cave, it forms layers of solid mineral deposits hanging from the ceiling like icicles (stalactites) and building up on the floor (stalagmites) of the cave. While speleothems suitable for paleoclimate research are not technically cores, their natural formation shape is already cylinder or core‐like, the pristine center of which is sampled for isotopic analyses that can provide information on changes in past temperature, the water cycle, and the carbon cycle. You will work with speleothem data from South America in Chapter 3.

1 The best speleothems for paleoclimate reconstruction form when cave conditions are just right – like Goldilocks needed her porridge just right. Think about how stalagmites form on the floor of a cave:If conditions were too wet (e.g. if the cave was flooded with groundwater), how would that affect the ability of a stalagmite to form?If conditions were too dry, where no groundwater is dripping into the cave through fractures in the ceiling, how would that affect the ability of a stalagmite to form?How would evaporation conditions differ near the entrance to a cave compared to farther back in a cave? Which of these two settings would provide a more ideal location to obtain a speleothem for paleoclimate reconstruction?

2 What would a layer of hardened mud in a stalagmite imply about the environmental history of the cave? (In other words, how could the mud have gotten there?)

3 Go to the supplemental resources to watch videos and read a short article on selecting and collecting a speleothem for paleoclimate research. Make a list of the challenges of obtaining speleothems for paleoclimate research and the strategies scientists use to overcome these challenges.ChallengesSolutions

4 Like tree‐rings, radiometric dating (in this case, U‐Th isotopic dating, which you can explore more in Chapter 3) can be used to determine the age of the speleothem layers. However, unlike tree‐rings, the layers in speleothems are not necessarily annual layers. Their accumulation rate depends on the rate of water dripping into the cave.What could you infer about regional precipitation if you see that the younger layers in a stalagmite increase in thickness?Would you expect all caves globally to show the same changes in layer thickness through time? Why or why not?