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Humans have been polluting the environment for at least 4000 years. So say scientists who have analysed sediment from the South China Sea – but not everyone is convinced.

Several early civilisations hit a crisis point about 4000 years ago. The global climate cooled, and this has been linked to the in ancient Mesopotamia, the of South Asia and the fall of the Liangzhu culture in what is now eastern China.

Cooling was also felt on Hainan island off China’s southern coast, according to at the China University of Petroleum, Qingdao, at the Qingdao Institute of Marine Geology, and their colleagues. The cooling led to an increase in heavy metal pollution in the South China Sea.

Xu and his colleagues looked at two sediment cores from just south-east of Hainan. They calculated “enrichment factors” for several metals. A value of 1 or below indicates no enrichment, while .

Farms not fish

The enrichment factors of cadmium and lead hovered around 1 before the 4000-year mark, but afterwards increased to about 1.5. In other words, human activity on Hainan 4000 years ago led to a subtle but detectable uptick in heavy metal pollution in the South China Sea, says the team.

The group suggests that the change was linked to the global cooling at the time, when Hainan would have cooled and dried. Lower monsoon activity would have triggered a reduction in coastal upwelling, lowering marine productivity and encouraging Hainan’s inhabitants to focus on farming rather than fishing.

Run-off into the sea from farms would have included heavy metals, which were accumulating in soil because of the use of metal tools.

at the French Research Institute for Exploitation of the Sea in Plouzané was involved in , derived from the Red River that runs through China and Vietnam. There, the evidence of human-related pollution from lead, copper and arsenic began only 1800 years ago.

Questionable link

Toucanne says the uptick in pollution 4000 years ago is gradual rather than clear-cut. “But they use this precise age to discuss global climate and the relationship with human activities,” he says. It is questionable whether there is a direct link between the climate cooling 4000 years ago and the increase in pollution off Hainan.

There is also evidence of , says at Durham University, UK. “So increased aridity at Hainan around this time isn’t for certain.”

Still, Innes thinks the proposed scenario is plausible. “Declining productivity offshore could have led to increased agriculture on Hainan island,” he says.

However, at Louisiana State University is unconvinced. He says other evidence suggests pollution from metalworking on the mainland . “I find it difficult to believe that little Hainan island was much more developed 4000 years ago.”

However, there is precedent for significant pollution that long ago. In 2013, at the Japan Agency for Marine-Earth Science and Technology in Yokosuka found . He has since found evidence that . “But the amount we are talking is so low that it is not considered harmful or dangerous for human [health],” says Jiménez-Espejo.