SUITMA 2005 Cairo - Ancient soil pollution with heavy metals and isotopic evidence of the cultural layers in urban Nanjing, China
SUITMA 2005 Cairo
| This abstract is about one of the papers of the Historical sites theme of the SUITMA 2005 symposium. |
| Ganlin Zhang,i | Feng-Gen Yang,i |
| Wen-Jun Zhao,i | Yu-Guo Zhao,i |
| Jin-Ling Yang,i | Zi-Tong Gong.i |
- i - Institute of Soil Science, Chinese Academy of Sciences, Nanjing, China.
Soils in urban area are often artificially formed and preserved as cultural layers in which soil materials are mixed with artifacts. The vertical change of soil properties denotes the historical change of human activities, and therefore urban soil can be considered as a record of history of urban development. We studied a deep profile of urban soil in Nanjing, China by intensively sampling at every 5 cm. Soil samples were determined for heavy metal content. Charcoal from several layers was found and was dated using 14C to recognize archaeological cultural layers that considered as formed in different Chinese dynasties. Lead isotope ratios were determined by thermal ionization mass spectrometry (TIMS) technology.
The study found that artificial deposition, i.e., the formation of cultural layer started about 1700 years ago and lasted until recent age in Nanjing. All cultural layers above the original loess were heavily contaminated by Cu, Pb and Zn, but not significantly by Co, Cr and Ni. Lead content varied from 100 mg kg-1 to more than 2000 mg kg-1, Cu varied from about 100 mg kg-1 to more than 5000 mg kg-1, Zn from 70 mg kg-1 to more than 2000 mg kg-1. There were several historical periods in which heavy metals accumulated, indicating significant contamination by human activities, such as ancient ore smelting for various purposes and use of heavy metal-containing materials for handicraft manufacture. In general, soil magnetic susceptibility increased with the succession of ancient dynasties until recently, and there was a significant correlation between heavy metal content and soil magnetic susceptibility.
Various Pb isotope ratios of the cultural layers differed substantially from that of the original undisturbed loess in the deepest position. 206Pb/207Pb value decreased gradually from the bottom layer to top layer, although with occasional exceptions, indicating a long lasting mixing of extraneous source Pb during the entire history since c.a. 300 AD. In comparison with the isotope ratios of lead ores of different sources and that of aerosols, it was illustrated that the source of Pb in cultural layers might come from lead ores of southern China in the earlier dynasties, while Pb from north China might have contributed to the Pb source in the more recent years, suggested by the lowering of 206Pb/207Pb ratio in the near surface cultural layers. Cu and Zn were believed brought in simultaneously with Pb. However, the impact of petrol burning on cultural layers was basically excluded, considering the relatively short history of petroleum use in this area.
