COURTESY OF XIANGDONG LI AND SHIHUA QI
Shihua Qi (left) and a student of the China University of Geosciences collected a 330 centimeter sediment core from a lake in central China during the June monsoon season in 2002.Xiangdong Li of Hong Kong Polytechnic University and colleagues used established lead-isotope-ratio methods to identify human-made lead versus naturally deposited lead. They found the anthropogenic signature in a sediment core from a lake in a part of China that has been only lightly impacted by modern industrial operations. The low background signal from atmospheric deposition allowed the researchers to tease out the anthropogenic signature left by earlier mining and metalworking.Li and his team used a technique that measures the lead isotope ratios 206Pb/207Pb and 208Pb/207Pb, which are lower for anthropogenic lead than for natural deposits. Greek and Spanish lead mines from ~3000 B.C. have been fingerprinted using these lead isotope ratios (Environ. Sci. Technol. 1997, 31, 3413–3416). Past research also used lead isotope signatures to reveal European mining and metallurgy history in peat and lake deposits in Switzerland and Sweden, respectively, back to the Bronze Age and to trace lead levels as far back as 12,000 years ago.In China, Li and his colleagues took a 330-centimeter-long core from the center of Liangzhi Lake in central China’s Hubei Province. Although sediments sometimes reflect yearly and seasonal cycles of deposition, they also get disturbed by storms or by animals that live at the sediment–water interface. Such mixing means that a lake sediment core rarely contains detail going back 5000 years.To solve that problem, the researchers used 14C to date the layers. They then searched for lead in the sediments by using ICPMS (inductively coupled plasma mass spectrometry). The researchers also assessed other trace elements, including copper, nickel, and zinc. Putting dates to concentrations, they found that before 3000 B.C., lead hovered at low levels—which probably represent natural background levels for the region.During the Bronze Age, the sediment core’s lead levels peaked, and many of the other measured elements increased gradually. After rapid increases 2000 years ago, concentrations of copper, nickel, lead, and zinc leveled out. Still, the scientists observed subsequent peaks that matched China’s Warring States Period (475–221 B.C.) and the early Han Dynasty (206 B.C.–A.D. 220) as well as the onset of modern industry in the 1800s and 1900s.“It’s an eloquent story,” says Joe Graney, a geochemist at Binghamton University. But he says that Li and colleagues seem to be emphasizing atmospheric deposition without taking into account land-use changes associated with agriculture. Those sedimentary signals could be responsible for some of the lead peaks, in addition to atmospheric deposition of human-worked lead, he suggests.A new generation of instruments could further solidify the dating and isotope correlations, Graney and others advise. Some instruments are so sensitive now that a researcher can collect a sample almost by “scratching with a fingernail,” says Stanley Church of the U.S. Geological Survey. This analytical capability could allow museum curators and geochemists to fingerprint artifacts. They eventually could trace the objects’ geologic and geographic provenance while establishing mining—and environmental pollution—history, he says.Mining records based on lead are more common in Europe and the U.S., says Graney, who has used the technique on Great Lakes sediments for a history of the past two centuries. Church surmises that the new work, which he calls “a nice summary of the correlation of geochemical change and the impact of man on the environment,” could be quite popular with archaeologists working in China. “It does demonstrate nicely the capabilities of the method,” he comments.“What I find fascinating,” says Li, “is the similar signal on the European side. . . . How did these people communicate the technology? Maybe we can use lead isotopes [to look at] mixing ores in the smelting process or [at] other global signatures at other locations” to see whether ancient metallurgy traveled from Rome to the East, or vice versa. —NAOMI LUBICK