【気候科学】アジアモンスーンの64万年の歴史

A new analysis of stalagmites from a cave in China provides the most detailed and accurate record to date of variations in the Asian monsoon, extending the record to 640,000 years ago. The findings are published in this week’s Nature. Approaching the limits of what is achievable with uranium/thorium dating, the study suggests that cyclical changes in the level of solar radiation reaching the Earth may have contributed to the end of the seven most recent ice ages.

Oxygen isotope levels from cave formations can be used to determine past climate conditions (such as temperature and monsoon strength), and combining the oxygen isotope record with uranium/thorium dating can give a very precise date for those climate conditions. Hai Cheng and colleagues use a recently improved uranium/thorium dating technique on four stalagmites collected from deep - 1.5 kilometres from the cave mouth - within the Sanbao Cave in central China, along with previous records, to construct a composite oxygen isotope record that details the strength of the Asian Monsoon from 640,000 years ago to ad 1950.

The authors then combine their record with a marine oxygen isotope record to determine the exact timing of the end of the last seven ice ages. They confirm that changes in Northern Hemisphere solar radiation (insolation) caused by the Earth’s precession cycle - the gradual shift in the orientation of Earth's axis of rotation, similar to a wobbling top, over approximately 20,000-year cycles - drove the end of the last seven ice ages. They also show that the changes in solar radiation influence the observed millennial variations in monsoon strength.