A 4,500-year chemical archive
The mud at the bottom of Erhai Lake is a a meticulously layered archive of environmental history. Each layer contains chemical traces from the atmosphere and surrounding land, capturing moments in time. Scientists from the University of Pittsburgh extracted sediment cores from the lake bed to read this history, analyzing them for heavy metals. Their findings reveal a 4,500-year timeline of human industry and its atmospheric consequences in China's Yunnan province, a region long known for its mineral deposits.
The first faint, but clear, chemical signs of metallurgy appear in sediment layers dated to 1500 BCE. An increase in copper concentrations marks the beginning of the Bronze Age in the region. For thousands of years after this initial signal, the levels of metallic pollutants remained relatively low. The lake's chemical signature changed dramatically around 1100 CE. Concentrations of lead, silver, zinc, and cadmium began to rise sharply, showing an industrial boom.
The Yuan Dynasty silver rush
The pollution reached its zenith during the Mongol-led Yuan Dynasty, which ruled from 1271 to 1368 CE. Kublai Khan, the grandson of Genghis Khan, founded the dynasty and established government-operated silver mines in Yunnan around 1290 CE. The demand for silver was immense, as it was an important part of the empire's tax revenue. The sediment cores from Erhai Lake show this imperial focus.
Around 1300 CE, lead pollution in the lake peaked at 119 micrograms per gram of sediment. This concentration is three to four times higher than pollution levels generated by modern industrial mining in the same region. The smelting process used to extract silver from ore was responsible for releasing this plume of heavy metals into the atmosphere. Elements like lead, cadmium, and mercury were vaporized during heating and drifted across the landscape, eventually settling in the lake and becoming a permanent part of the geological record. The lead concentrations were high enough to potentially cause harmful effects in the lake's aquatic organisms. After peaking, the pollution levels began to decline around 1420 CE, following the collapse of the Yuan Dynasty and a subsequent decrease in large-scale mining operations.