Stalagmites preserve a record of climate change that goes back hundreds of thousands of years. But unlike other climatic archives, such as marine sediments and ice cores, stalagmites can be precisely dated. That’s because Earth’s uranium-234, a daughter of 238U, steadily decays into thorium-230 with a half-life of 245 000 years. As rainwater passes through soil and bedrock to reach a cave, it carries trace amounts of dissolved U with it. Being water-insoluble, Th is left behind. But as radiogenic 230Th accumulates anew in a stalagmite, it also radioactively decays (to radium-226) with a half-life of 76 000 years. From the 230Th/234U and 234U/238U concentration ratios in a stalagmite layer, the layer’s age can be determined.
The rainwater’s oxygen isotopes are also incorporated into the carbonate mineral layers that form the stalagmite. The 18O/16O ratio is thought to be...
