The results of metal and Pb isotope analyses for each of the soil samples are given in Table 1. Hg content ranged from 39 to 521 ppb, with a mean of 160 ppb. The two lowest values, 39 and 45 ppb, might reflect the background Hg level in Ancient Nara because the corresponding samples (samples 203 and 38) date to the late seventh century or early eighth century (Figure 5). Samples 204, 205, 206, and 207, which also had relatively low Hg contents (59 to 92 ppb), date to the early and middle eighth century. Samples 1-A and 1-B had low Hg contents (66 and 92 ppb, respectively); these samples were obtained from heaps of soil produced during the construction of the Nara Daibutsu and, therefore, date from just before the mercury-gold gilding of the statue was undertaken. Conversely, samples 2, 3, and 210, which are younger than that of the ninth century, had Hg contents of at least 200 ppb. The maximum value of 521 ppb was observed in sample 211, which dates from between 1870 and 1960 and is, thus, probably due to modern contamination. Large amounts of Hg were released into the environment in the 19th and 20th centuries due to the operation of coal-fired power plants and the use of Hg in electric lights, batteries, electrodes for sodium hydroxide production, and wood preservation (Japan Oil, Gas and Metals National Corporation 2010).
Cu levels varied from 2.3 to 370 ppm, with a mean of 37 ppm. Samples from the early eighth century showed that the background level of Cu was around 5 ppm. The highest values (>50 ppm) were observed in the samples from Site A, which was contaminated by copper slag that splashed out of the blast furnaces when the Nara Daibutsu was cast.
Pb content varied markedly, ranging between 13 and 1,200 ppm. The highest Pb levels (>1,000 ppm) were observed in samples 208 and 204, which came from an ironsmith dump site dating from the early-middle eighth century. Samples 34 and 31 from a ditch running along the central avenue also had high Pb contents (700 to 900 ppm). Interestingly, the mean level of Pb in Ancient Nara (330 ppm) was considerably higher than the geochemical background level of 30 ppm around the modern city of Nara.
The levels of Fe, Mn, Co, Ni, and Zn in the soil were all rather low and fell within the ranges observed in modern natural soils in Japan (Imai et al. 2004). Specifically, Fe, Mn, Co, Ni, and Zn ranged between 5,800 to 61,000 ppm (mean 26,300 ppm), 92 to 2,300 ppm (mean 425 ppm), 4.8 to 32 ppm (mean 14 ppm), <29 ppm, and 20 to 140 ppm (mean 50 ppm), respectively. The contents of the latter four metals were all correlated with the content of Fe, suggesting that these five elements are present in the same clay minerals and other soil materials.
Pb isotope ratios were relatively similar among all the soil samples studied: 207Pb/206Pb ranged from 0.850 to 0.851 and 208Pb/206Pb ranged from 2.093 to 2.094. The Pb isotope compositions of the samples from the Naganobori mine (207Pb/206Pb = 0.846 to 0.847 and 208Pb/206Pb = 2.091 to 2.092) were consistent with a previous study (207Pb/206Pb = 0.848 and 208Pb/206Pb = 2.091 to 2.092; Saito et al. 2002).
Metal pollution resulting from construction of the Nara Daibutsu
The level of Hg pollution can be estimated if the size of the area over which the pollution occurred is known. Heijoyo in the Nara Basin measures about 5 km by 5 to 7 km and the soil layers in the basin are typically several centimeters to 20 cm thick (Figure 4). Consequently, if 820 kg of Hg was uniformly distributed in 5 or 10 cm thick soil layers (density 1.0 g cm−3) in a circular area with a 3 km radius in the Nara Basin, then the Hg concentration would have been approximately 600 or 300 ppb, respectively. The results of this study suggest that the Hg content in ancient soil in the middle and late eighth century (around 100 ppb) was not that high (Table 1). Even samples from the twelfth to eighteenth centuries that were collected in the district adjacent to the Nara Daibutsu had Hg levels of 200 to 300 ppb. Similar Hg contents were observed in the eighth century soil samples 100 and 106 collected in the palace grounds of Ancient Nara. The other samples had relatively low Hg contents. Assuming that the background level in this area was approximately 50 ppb before construction of the Nara Daibutsu, as estimated above, it appears that although Hg levels increased considerably when the statue was constructed, Ancient Nara was not severely polluted.
There are two possible reasons that might explain the discrepancy between the estimated and measured values: (1) Hg, which is volatile, may have evaporated from the soil (Pirrone et al. 2001), (2) Hg might have been transported to other areas over time. Regarding the second possibility, the district to the north of Nara Daibutsu is currently flat but it was a hollow depression between the eighth to the sixteenth centuries when it would have received debris and soils from fires that burned the wooden structures housing the Nara Daibutsu in 1180 A.D. and 1567 A.D. Nonetheless, samples collected from this area, i.e., samples 1 (middle eighth century), 2 (twelfth century), and 3 (sixteenth century), had relatively low Hg contents, implying that Hg transfer was not a main factor.
The soil in Ancient Nara had low Hg levels during the early eighth century; however, these levels more than doubled after the middle eighth century, possibly due to pollution from the construction of the Nara Daibutsu. In addition, there may have been other pollution sources, as a single point source of pollution would not explain the observed increase in background Hg levels after the mid-Nara era. It is well known that cinnabar has been used for a spell and/or preservative agent for more than 2,000 years (Naruse 1991; Mitochou Compilation Committee 2004), but it was relatively expensive and would likely have been used to a limited extent.
Regarding the evaporation of Hg, the vapor pressure of Hg is 0.23 Pa at 25°C. If the Hg vapor behaves as an ideal gas, then we can calculate the concentration of Hg (C, in moles per mole of atmosphere) in the atmosphere using the following equation:
where C is 2.3 × 10−6 mole, which is more than 100 times the upper limit of the modern environmental standard in Japan, 0.02 ppm. As the molar mass of Hg is 220.59 g mol−1, 4.55 × 10−4 g of Hg exists in 1 mole of atmosphere, suggesting that the inside of the Great Buddha Hall was severely polluted when gilding was undertaken and that, afterward, an appreciable amount of Hg continuously and, rather rapidly, evaporated from the soil into the atmosphere. Indeed, this evaporation would explain why less than half of the total amount of Hg used in the construction of the Nara Daibutsu was present in the soil, even as soon as the middle and late Nara period. These lines of evidence suggest that a limited area around the Great Buddha may have been severely polluted during the gilding, but that Ancient Nara was only moderately polluted with Hg since the eighth century, probably due to the construction of the Nara Daibutsu as well as the use of cinnabar in the region. The evidence does not support the hypothesis that severe Hg pollution forced the capital to be relocated from Nara to Kyoto.
High Cu contents were observed in samples 1, 2, and 3, which contained copper slag from the construction of the Nara Daibutsu. The highest level observed (355 ppm) is well above the upper limit of existing Japanese regulations (125 ppm). Conversely, samples from the other sites had relatively little Cu, and background levels were probably a few parts per million. The higher levels observed in some of the samples (i.e. >10 ppm) may have been due to the widespread use of Cu in mirrors, coins, bronze statues, and arms at the time. A more likely explanation for the presence of the grassland (instead of a forest) at Wakakusa Hill was not because of pollution, but rather because the field was burned every year as part of a New Year celebration.
In summary, our results suggest that Hg and Cu pollution accompanying the construction of the Nara Daibutsu only had a limited influence on the environment.
Pb pollution in Ancient Nara
Analysis of soils from Ancient Nara suggests that traces of ancient human activity are indeed evident in the environment. Both the extent of the contamination and the nature of the contaminants are generally consistent with the features of human activity in Ancient Nara. The contamination in samples 31 and 34 from the ancient ditch is considered to be due to urban pollution. According to historical documents, the ditch was quite dirty and contained the remains of humans, cattle, and horses, suggesting that people may have indiscriminately disposed of waste in these ditches (Archaeological Institute of Kashihara 2012a, [b]). Further, although water from the Saho River was diverted into the ditch, it was too weak to flush out the sewage efficiently (Figure 1). Hg, Pb, Fe, and Cu were also materials that were in common use in the Nara period. Hg and Pb were used for pigments, enamel, and/or lead glass; Cu and Sn were used for producing bronze coins and statues. Malachite is a copper carbonate hydroxide mineral which was used as a green pigment and Fe oxide (hematite) was used as a red pigment (Kitano 2013).
Comparing the Hg, Cu, and Pb contents of Ancient Nara to modern standards reveals that only Pb (over 330 ppm) exceeded modern levels (15 ppm for Hg, 125 ppm for Cu, and 150 ppm for Pb). The particularly high contents observed in samples 208 and 204 could be related to the high Pb content at site C. Indeed, the observed level of Pb contamination is considered harmful for human health. For example, if you inhaled 150 mg of soil per day that was contaminated with 200 ppm of Pb, and if the absorption efficiency was 40%, then 100 mg of Pb would accumulate in your body over a 20-year period and potentially cause lead poisoning (Yamada 1977). Thus, although it has been postulated that people living near blacksmiths in Ancient Nara may have suffered from lead poisoning, these smiths employed mainly iron, which means that it was unlikely that they were responsible for the lead contamination in the soil. However, further investigation is needed to determine whether Pb contamination was observed throughout the city.
The various isotopes of Pb provide an ideal tool for characterizing the original source of heavy metal pollution because the isotopic ratios are not measurably influenced by physical or chemical fractionation processes. Thus, when the 207Pb/206Pb and 208Pb/206Pb ratios of the Ancient Nara soil samples were compared to the ratios observed in various foreign artifacts (mirrors and swords from China and Korea) or in samples from various Japanese mines, the results show that the most plausible origin of the Pb in Ancient Nara was the Naganobori mine. Figure 6 shows that when the Pb isotope ratios of the soil samples were plotted against those of the Naganobori mine, the curves were similar. This mine was thus undoubtedly the source of Cu for the production of the Nara Daibutsu. Indeed, this dependence on the mine is documented in historical records, namely, on narrow strips of wood upon which official messages were written during the Nara Period (Mitochou Compilation Committee 2004): this is also evidenced by the higher relative abundance of Ag and As (Hatanaka 2003). This study reconfirmed that the Pb isotope ratios in sample 1-A, which contained drops of copper from the construction of the Nara Daibutsu, were similar to those in samples obtained from the Naganobori mine. In addition, the Pb isotopic value is the same as that in all the other solid samples in Ancient Nara. We therefore conclude that all of the Pb contamination in Ancient Nara originated from the Naganobori mine. The extraction of ore at Naganobori contributed considerably to the production of the Nara Daibutsu. However, the mining activity associated with the construction of the Nara Daibutsu may also have generated Pb pollution in the capital city, even in ancient times.
Although it is currently difficult to identify the exact source of Pb contamination, several possible sources exist. A number of studies on Pb isotopes have examined bronze coins (Mabuchi et al.
1982; Saito 2001a; Saito et al.
2002). However, it is considered unlikely that these coins would have increased the Pb content of environmental samples, such as the soils in urban areas because of distribution amount. Based on an analysis of the inorganic pigments used to decorate treasures at Shosoin, the Imperial Repository constructed to prevent damage arising from the humid Japanese climate, lead-based pigments may have been relatively popular (Naruse 2004). Although it was reported that lead carbonate (lead white) was not produced in Japan and that it was imported from China during the Nara era (Winter 1981), lead chloride produced in Japan was widely used for white paint (Naruse 1992; Winter and Emile 1988). According to a production report from a government-run workshop of Buddhist sculptors during the Nara Period in 734 A.D., red lead was made from metallic lead in order to produce lead glass (Naruse 1991). Lead isotope analysis of tricolored glaze dating from the Nara Period showed that the Pb isotopic composition was comparable to that from the Naganobori mine (Saito 2001b). Further study is required to understand the extent of environmental pollution in this ancient civilization.
Implications of pollution in the ancient city
During the seventh and eighth centuries, many religious structures, such as temples and statues, as well as tumuli, were constructed in Japan. The material requirements of large bronze statues were considerable, as exemplified by the Nara Daibutsu; and the tumuli of the previous era were associated with large numbers of terra-cotta figures, whose firing required large quantities of wood fuel. Relocations of the capital city were also frequent and entailed extensive utilization of wood resources for the construction of new buildings.
The end of the burial tradition and the beginning of the Buddhist practice of cremation occurred in the eighth century. The first person to be officially cremated in Japan was the Buddhist priest Dosho in 700 A.D., and the first emperor to be cremated was Jito in 702 A.D. According to historical records, the government recommended restraint regarding the construction of large tumuli in the seventh century (personal communication, Dr. Kinoshita). After the relocation of the capital to Ancient Kyoto in 794 A.D., no more large bronze statues were built around Nara and Kyoto. Japanese society had already started to shift away from mass consumption to a more sustainable system during the Nara period.