Cave stalagmites show that flooding destroyed Liangzhu City

The Liangzhu excavation site, on the Yangtze River Delta, southwest of Shanghai in China, is home to the remains of a 5,300-year-old civilisation. Liangzhu City was once an advanced culture, with hydraulic infrastructure including dams, reservoirs and canals. Despite not having metals, the Neolithic people of Liangzhu operated a complex water system that allowed agriculture to flourish.

There is still much to learn about Liangzhu, which was only declared a UNESCO World Heritage Site in 2019. It was inhabited for 1,000 years, but it’s still not entirely clear what caused the city’s downfall, which was thought to be abrupt.

“A thin layer of clay was found on the preserved ruins, which points to a possible connection between the demise of the advanced civilisation and floods of the Yangtze River or floods from the East China Sea. No evidence could be found for human causes such as warlike conflicts,” says geologist Christoph Spötl, from the University of Innsbruck, Austria.

“However, no clear conclusions on the cause were possible from the mud layer itself.”

But Spötl, along with Chinese, Austrian and US colleagues, have found more evidence to support this conclusion. They’ve spotted geological indicators in nearby caves that suggest that the city’s decline coincided with unusually wet weather conditions.

A paper describing their research is published in Science Advances.

“These caves have been well explored for years. They are located in the same area affected by the Southeast Asian monsoon as the Yangtze Delta, and their stalagmites provide a precise insight into the time of the collapse of the Liangzhu culture, which, according to archaeological findings, happened about 4,300 years ago,” Spötl explains.

Moisture affects stalagmite growth, so determining the age of specific parts of the stalagmites can help researchers to figure out wetter and drier periods in the caves’ history.

The researchers used carbon isotope analyses on the stalagmites to determine humidity levels when they were formed, and uranium-thorium dating to find their age. Uranium-thorium dating can determine a rock’s age to around a 30-year window.

“This is amazingly precise in light of the temporal dimension,” says Spötl.

The researchers found that between 4,345 and 4,324 years ago, there was a period of extremely high precipitation. This coincides with the decline of Liangzhu.

“The massive monsoon rains probably led to such severe flooding of the Yangtze and its branches that even the sophisticated dams and canals could no longer withstand these masses of water, destroying Liangzhu City and forcing people to flee,” says Spötl.

The researchers determined that the humid conditions persisted for another 300 years after these proposed floods.