When water seeps into the ground it picks up minerals, usually calcium carbonate. This mineral-rich water can drip into caves, leaving mineral deposits behind. These deposits can be dated through radioactive decay.
Variation in the thickness of layers can tell researchers how much precipitation occurred over time. With one more twist, using the ratios of heavy to light isotopes of oxygen present in the layers, researchers can find the temperature at which the water originally condensed.
Researchers have used the technique to examine the Mawmluh Cave in the Indian state of Meghalaya, an area thought to be the rainiest place on Earth. The technique helped to establish a relationship between monsoon rainfall and sea surface temperatures in the Pacific Ocean, known as the El Niño Modoki.
‘Now that we have shown that the Mawmluh cave record agrees with the instrumental record for the last 50 years, we hope to use it to investigate relationships between the Indian monsoon and El Niño during prehistoric times such as the Holocene,’ says Jessica Oster, a researcher who worked on the project.
The Holocene saw a period of global climate warming around 6,000 to 9,000 years ago. Temperatures were four to six degrees higher than today, around the increase predicted if human-caused global warming continues unchecked.
Understanding the historical monsoon patterns could help researchers predict how it will change in the future. The monsoon provides the area with 75 per cent of its annual rainfall.
The researchers used stalagmites with high concentrations of uranium for the analysis, including columns that had probably broken off in an 8.6 magnitude earthquake that hit Assam in 1950. The new stalagmite growth was perfect for analysis.