Lake sediments reveal 7000 years of history

By Hepeng Jia/Beijing, China

Geochemists have revealed a vivid picture of mining and metal use dating back 7000 years, by tracing variations in metal concentrations in lake sediments in central China [1].

Until recently, Chinese studies on geological samples such as these had been limited to the last 100 years. However, it has often proved difficult to distinguish the historical record of metals from the more recent influence of urbanisation and industrial development, says Li Xiangdong of Hong Kong Polytechnic University.

So Li and his colleagues turned to Liangzi Lake in the central Chinese province of Hubei, which they say is an ideal place to peel off modern industrial influence.

The team analysed the concentrations of copper, lead, nickel and zinc in 300 carbon-dated samples of Liangzi Lake sediments.

Before 3000 BC, the concentrations of the metals were low, indicating their natural compositions. After that time, concentrations began to grow, a trend that probably indicates an increase in the mining and utilisation of these metals in China during the early Bronze Age, explain the authors. 

The period echoed the rise of the Chu kingdom in present-day Hubei from around 1500 BC, which became a superpower in the Warring States Period (475-221 BC) in China. 

After the beginning of the Warring States Period, a rapid increase in metal concentrations was observed, testament to the dramatic progress in the use of bronze tools - made from alloys that included mixtures of all these metals - and the mining activity associated with them.

It echoes historians’ records that the period was the peak of China’s Bronze Age.

’Meanwhile, it is interesting to note that the trend in the changes of sedimentary metals in Liangzi Lake is synchronous with those in Europe, then under the Roman Empire. This cross-continental synchronisation is a legitimate topic for historians to study,’ Li told Chemistry World.

After the significant metal concentration increases in the sedimentary record, which lasted into the Han Dynasty (206 BC - 220 AD), the growth slowed until mid-1800s, when China’s early industrialisation pushed the metal concentration dramatically higher again. 

Wang Changsui, dean of the Department of Scientific Archaeology of the Graduate University of the Chinese Academy of Sciences, welcomes the study, saying it has offered more background information for ancient human activities. ’However, we must be cautious on whether the sediment samples dated by isotope carbon have correctly represented the historical time,’ Wang told Chemistry World.