Speleothems, from the Greek words for cave and deposit, include the familiar descending stalactites, upward-growing stalagmites and more continuous sheets called flowstones. They grow slowly, at rates of between 1 mm a year to 0.001 mm a year. This growth can persist for many thousands of years before being interrupted. As with trees, speleothems commonly display annual layers which may be visible when a sample is polished, or recognised through chemical analyses. Ideally, a sample will accumulate regular, parallel layers, allowing the investigator to study its growth through time, and see changes in environmental conditions.
These speleothems grow in caves within limestone rock. Typically the limestone has cracks which help to guide the flow of water, but there may also be porous parts where water is stored for a few years. Water dripping through the cave may come from cracks in bedrock, or more slowly through the pores. It therefore represents a mixture of rain and melting snow from different seasons that fall on the land above.
Inside a cave, conditions are highly protected in comparison to the outside. There is little temperature variation, but it is approximately equal to the average annual outside temperature. Changes in the drip rate of water in the cave mark wetter and drier seasons. The air circulation is often stronger in one season, usually the winter, resulting in a lowering of cave air carbon dioxide (CO2). This CO2 is released by dripping water, having originally been dissolved along with limestone in the soil zone above. The larger the loss of carbon dioxide from the water, the larger the amount of calcium carbonate deposited, and the faster speleothems grow. Fast growth is typical of warm, wet conditions, whereas cold or dry result in the cessation of growth.
Just as the rate of growth of speleothems is sensitive to the external conditions, so are many aspects of their chemical composition. The oxygen atoms in rainwater molecules carry a signal of the history of the air mass which carried the rain. The heavier atoms are found in less abundance in rain or snow falling from air that has cooled greatly (even to the extent that water can be typical of a season and the season typical of a year), another characteristic found in the dripping water in the cave. In this way the changing composition of the stalagmite through time tells stories of shifting climates. For example, the summer patterns of East Asian monsoon activity gradually change over thousands of years as the Earth’s orbit wobbles. Likewise changes in the abundance of carbon atoms, or the amounts of particular elements can be analysed and interpreted, for example in terms of changing aridity.
A great feature of speleothems is that their layered structure can usually be dated very precisely and accurately by the decay of atoms that were originally incorporated inside them. Because of this, and their growth over thousands of years, speleothems have emerged as one of the most important sources of information about climate and environmental change over the last few hundred thousand years.