Research reveals 30 million years of East Asian summer monsoon evolution

East Asian Summer Monsoon (EASM) plays an important role in the formation of the region’s climate and ecosystem. This is an important driver of seasonal precipitation patterns that preserve agricultural productivity and water resources in East Asia. Additionally, EASM promotes heat and moisture transport, adjusts the energy balance in the region, and affects large-scale atmospheric circulation.

However, understanding the evolution of Cenozoic EASM is complicated by the lack of extensive, continuous, high-resolution monsoon records.

To address this challenge, a research team led by Professor Wang Shimming of the Institute of Oceanography (IOCAS) at the Chinese Academy of Sciences, together with collaborators, quantitatively reconstructed the history of EASM precipitation over the past 30 million years. This was achieved using sediment samples from IODP site U1501 in the northern South China Sea. Their findings were published in Innovation Geoscience.

The researchers have developed a quantitative model based on the behavior of silicate minerals during the weathering process. They examined the relationship between strong (or weak) warm, wet monsoons and intensified (or weakened) chemical weathering driven by sedimentary clay minerals.

This relationship shows that while a higher (or lower) proportion of kaolinite correlates with enhanced (or weakened) weathering, illite exhibits the opposite trend. Using linear regression, the team correlated with clay mineral assemblies in modern East Asian rivers with precipitation and temperature.

The model was then applied to sedimentary cores in the northern South China Sea, allowing researchers to isolate the temperature effects from the weathering index and quantitatively reconstruct the evolution of EASM precipitation over the past 30 million years.

“We have seen that global temperature fluctuations and the uplift of the Qinghai Tibet Plateau act as a ‘dual engine’ due to temperature changes that shape the long-term trend in blue rainfall.”

The plateau’s uplift acted as a huge “air conditioner,” rebuilding the atmospheric circulation, disrupting the original hydrothermal cycle more than 10 million years ago, allowing East Asia to maintain relatively moisture during the subsequent global cooling period.

This study also found a relationship between EASM precipitation and temperature changes regulated by atmospheric CO2 levels. In light of current global warming caused by increased CO2 concentrations due to human activity, EASM precipitation could rise significantly.

However, paleoclimate studies should be approached with caution in practices of interpreting the past using current conditions, as they can disrupt the rhythmic evolution of climate between the present and the past. “Future research needs to carefully examine the complex interactions between geological processes and climate change,” emphasized Professor Wan.