Junqing Yu

Environmental Legacy of Copper Metallurgy and Mongol Silver Smelting Recorded in Yunnan Lake Sediments

Geochemical measurements on well-dated sediment cores from Lake Er (Erhai) are used to determine the timing of changes in metal concentrations over 4500 years in Yunnan, a borderland region in southwestern China noted for rich mineral deposits but with inadequately documented metallurgical history. Our findings add new insight into the impacts and environmental legacy of human exploitation of metal resources in Yunnan history. We observe an increase in copper at 1500 BC resulting from atmospheric emissions associated with metallurgy. These data clarify the chronological issues related to links between the onset of Yunnan metallurgy and the advent of bronze technology in adjacent Southeast Asia, subjects that have been debated for nearly half a century. We also observe an increase from 1100 to 1300 AD in a number of heavy metals including lead, silver, zinc, and cadmium from atmospheric emissions associated with silver smelting. Culminating during the rule of the Mongols, known as the Yuan Dynasty (1271-1368 AD), these metal concentrations approach levels three to four times higher than those from industrialized mining activity occurring within the catchment today. Notably, the concentrations of lead approach levels at which harmful effects may be observed in aquatic organisms. The persistence of this lead pollution over time created an environmental legacy that likely contributes to known issues in modern day sediment quality. We demonstrate that historic metallurgical production in Yunnan can cause substantial impacts on the sediment quality of lake systems, similar to other paleolimnological findings around the globe.

The isotopic response of Lake Chenghai, SW China, to hydrologic modification from human activity:

The human modification of lake hydrology is widespread in the industrial era; however, anthropogenic impacts have occurred for thousands of years in regions of the world with long histories of human occupation. Here, we use geochemical analyses of lake sediment to document the isotopic and geochemical response of Cheng Lake (Chenghai) in southwestern China to catchment modification, including the construction of a downstream dam. The Chenghai record indicates that land-use changes affected the lake/catchment by at least AD 1150 as evidenced by an increased flux of terrestrial organic matter, sediment with high concentrations of sorbed metals, and variations in stable isotopes of oxygen in precipitated aragonite. Decreases in magnetic susceptibility and metal concentrations after AD 1360 indicate a change in sediment basin dynamics. The construction of a downstream dam between AD 1573 and 1620 caused positive shifts in the isotopic composition of aragonite, indicative of increased evaporative loss and a longer water residence time. A lake-level drop by AD 1779 is accompanied by a decrease in organic carbon and a lighter nitrogen isotopic composition that suggests the oxidation of organic matter and/or reduced primary productivity. These changes closely coincide with shifts in climate variability (such as the termination of the ‘Little Ice Age’) inferred from other, regional paleoclimate records, highlighting the challenges in distinguishing between natural and anthropogenically driven environmental variations. This study demonstrates the importance of historical and cultural context in the interpretation of lake sediment records with substantial human settlement proximal to the lake system.