Speleothems are stalactites on the roof or stalagmites on the floors of limestone caves. Surface rainfall percolates through the soil and eventually may make it to the roof of a cave. The rainfall is rather acidic and becomes enriched with carbon dioxide in the soil, so that then it can dissolve limestone (calcium carbonate), often enough that the concentration becomes saturated, before dripping from the cave roof. The drops then lose water by evaporation, and the limestone within the lost water is precipitated out of solution, forming speleothems. Other minerals may be deposited as well.
Speleothems contain very small amounts of Uranium 238, previously dissolved in the initial rainwater. However, the uranium decays radioactively into U234, and then into Thorium 230, at a known rate. So the thorium/uranium ratio accurately indicates the age of the deposit. Also, the oxygen in the deposited limestone is either the normal isotope O16, or ‘heavy oxygen’ O18, according to the surface temperature of the ocean from which the oxygen came in the previous evaporation of water. Likewise, the ratio of two carbon isotopes in the deposit depends on soil conditions and vegetation types around the time of deposition, and hence, indirectly, on ambient temperatures at that time. In short, measuring the three isotope ratios of layers within a speleothem shows climatic conditions at known times past.
Measurements in New Zealand by J. Hellstrom of the Australian National University show the last glacial maximum 20,000 years ago, and a smaller cold period around 12,000, approximately coincident with the Younger Dryas in the northern hemisphere (1).
(1) Black, M. 1997. How stalagmites keep climate records. ANU Reporter (Australian National University).