Chapter 8 - Speleothems

Abstract

Speleothems are mineral formations occurring in limestone caves, most commonly as stalagmites and stalactites or slablike deposits known as flowstones. Stalactites (which hang from the ceilings of caves) often have a hollow core, with growth occurring around this central orifice, whereas stalagmites are solid and grow incrementally at the drip site. Thus, stalagmites are generally selected for paleoclimatic analysis. The extensive distribution of karst landscapes means that studies can be undertaken on a worldwide basis. Speleothems are primarily composed of calcium carbonate, precipitated from groundwater that has percolated through the adjacent carbonate host rock. Certain trace elements may also be present (often giving the deposit a characteristic color), and one of these, uranium, can be used to determine the age of a speleothem, as discussed in the succeeding text. Seasonal variations in the trace element composition of dripwaters may also be used to identify annual layers. Deposition of a speleothems results from evaporation of water or degassing of carbon dioxide from water droplets. Evaporation is normally only an important process near cave entrances; most speleothems from deep within caves therefore result from the degassing process. Water that has percolated through soil and been in contact with decaying organic matter usually accrues a partial pressure of carbon dioxide exceeding that of the cave atmosphere. Thus, when water enters the cave, degassing of carbon dioxide occurs, causing the water to become supersaturated with calcite, which is thus precipitated.