Asian monsoons and ice-age cycles

Stalagmites, such as the ones shown here from Sanbao cave in central China, preserve a record of climate change that goes back hundreds of thousands of years. And unlike other climatic archives, such as marine sediments and ice cores, the stalagmites can be dated precisely. That’s because Earth’s naturally occurring uranium-234 steadily decays into thorium-230 with a half-life of 245 000 years. When water passes through soil and bedrock to reach the cave, it carries U with it. But as the ²³⁰Th grows, it also radioactively decays, with a half-life of 76 000 years; the concentration of ²³⁰Th/²³⁴U in a stalagmite layer determines the layer’s age. Oxygen isotopes in rainwater are also preserved in the stalagmites, and the variation in ¹⁸O/¹⁶O over time is a proxy of both local and regional climate conditions, such as the amount of rainwater falling on the cave. That rainfall, in turn, reflects the strength of the Asian Monsoon, a vast wind system that carries air and moisture from the Indian Ocean and the tropical western Pacific Ocean into the Indian subcontinent and southeastern Asia. Aided by recent advances in ion mass spectrometry of U and Th, Hai Cheng (Xi’an Jiaotong University) and his colleagues have now constructed a record that spans 640 000 years of rainfall over Sanbao cave—an extension of nearly 300 000 years over the record compiled in earlier studies.

Monsoon intensity waxes and wanes with the amount of solar heat received over the Northern Hemisphere. The heat varies with Earth’s tilt, precession, and orbital eccentricity. Armed with the new record, Cheng and company compared the monsoon cycles with those orbital cycles and the roughly 100 000-year cycle associated with the comings and goings of the ice sheets. The comparison confirmed a strong synchrony between exceptionally weak monsoons that marked dry periods in East Asia, warming periods in the precession cycles, and episodes of catastrophic ice melting. (H. Chen et al., Nature 534, 640, 2016 .)