Environ. Sci. Technol. 1997, 31, 3413-3416
Lead from Carthaginian and Roman Spanish Mines Isotopically Identified in Greenland Ice Dated from 600 B.C. to 300 A.D.† K E V I N J . R . R O S M A N , * ,‡ WARRICK CHISHOLM,‡ S U N G M I N H O N G , §,| J E A N - P I E R R E C A N D E L O N E , §,⊥ A N D C L A U D E F . B O U T R O N §,∆ Department of Applied Physics, Curtin University of Technology, GPO Box U 1987, Perth, WA 6845, Australia, and Laboratoire de Glaciologie et Ge´ophysique de l’Environment du CNRS, 54 rue Molie`re, Domaine Universitaire, BP 96, 38402 St Martin d’He`res, Cedex, France
The pollution history of the atmosphere of the Northern Hemisphere is recorded in the levels of heavy metal impurities in Greenland ice. The possibility also exists of using natural variations in the abundances of lead isotopes to trace the source of this pollution. Lead isotopes have now been measured in ancient Greenland ice with a lead concentration as low as 0.9 pg/g. The results show a depression in the 206Pb/207Pb ratio between 600 B.C. and 300 A.D., giving unequivocal evidence of early large-scale atmospheric pollution by this toxic metal. This ratio changes from ∼1.201 in ∼8-kyr-old ice to ∼1.183 about 2 kyr ago. Isotopic systematics point to the mining districts in southwest and southeast Spain as the dominant sources of this lead, giving quantitative evidence of the importance of these mining districts to the Carthaginian and Roman civilizations. Lead with a Rio Tinto-type signature represents ∼70% of the lead found in Greenland ice between ∼150 B.C. and 50 A.D. after correcting for the contribution from rock dust indexed to aluminium concentrations.
Introduction Spain was famous for its prodigious output of silver during ancient times, particularly from southern Spain at Rio Tinto in the west and at Cartagena in the east. This region was successively controlled by the Phoenicians (750-580 B.C.), the Carthaginians (535-205 B.C.) and the Romans (205 B.C.410 A.D.) (1). Spain became Rome’s greatest silver-producing region. Mines near Cartagena and Mazarron alone were estimated, from the slag heaps and workings, to have † This paper is dedicated to the memory of Professor Clair C. Patterson, geochemist at the California Institute of Technology, whose pioneering work on environmental lead has provided the scientific foundation for this study. * Corresponding author e-mail:
[email protected]; fax: +61 8 9266 2377. ‡ Curtin University of Technology. § Laboratoire de Glaciologie et Ge ´ ophysique. | Present address: Polar Research Center, Korean Ocean Research and Development Institute, Ansan, P.O. Box 29, Seoul 425-600, Korea. ⊥ Present address: Universitaire Instelling Antwerpen, Department of Chemistry, Universiteitsplein 1, 2610 Wilrijk (Antwerpen), Belgium. ∆ Also at the Unite ´ de Farmation et de Recherche de Me´canique, Universite´ Joseph Fourier de Grenoble, Domaine Universitaire, BP 68, 38041 Grenoble, Cedex, France.
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produced 2-3 times more silver than the Greek Laurion mine (2). The vast scale of mining was also in evidence further west at Rio Tinto where an estimated 6.6 million tons of Roman slag remained (3). The period 237-218 B.C., between the first and second Punic wars, was probably a period of intense mining activity by the Carthaginians in Spain. The changing fortunes of the Greek civilization were closely related to the production of silver at the silver-rich Laurion mines. Patterson (2) has estimated that 1800 t of silver (and 600 000 t of lead) was mined and smelted between ∼600 B.C. and ∼100 A.D., resulting in copious emissions to the atmosphere. About three-quarters of this production is thought to have occurred in the fifth century B.C., with practical exhaustion of the mine by ∼150 B.C. leading to the decline of the classical Greek civilization. Other deposits in the Aegean, worked between 650 and 350 B.C., were relatively small and, all together, may have approximately equaled the Laurion production (2). England was also considered to be an important mining region in the ancient world. Conquered by the Romans in 43 A.D., mining was underway in the Mendips by 49 A.D. (4). The English ores had a very low silver content and were unable to compete with the silver-rich Spanish deposits. Nriagu (4) estimates the lead production in Great Britain to be onetenth that of Spain in the period 50 B.C. to 500 A.D. Historical evidence suggests that lead production from other regions, as compared to Spain, was relatively small (Gaul ∼6%, Italy and Sardinia ∼8%, the Capathians ∼10%, and the Balkans ∼23%), with only minor changes in production between the Iron Age and the Roman Empire (4). The history of metal production by ancient civilizations is of great interest to archaeologists and historians since it was instrumental in the rise and fall of human societies and empires (4, 5). Our knowledge of this history has remained essentially qualitative until recently when the availability of contamination-free samples (6) of Greenland ice (7), dating from Greek, Roman, and Medieval periods, opened the way to more quantitative approaches. These analyses showed that the concentrations of lead (8) and copper (9) were consistent with tentative estimates of production due to early mining and smelting operations. A quantitative assessment of the relative importance of different mining districts during successive historical eras is of considerable interest. In addition to concentration measurements, a method of characterizing the sources of these emissions is needed. Lead isotopes vary widely in their abundances in nature and can be used for this purpose. This approach has been used recently by Rosman et al. (10, 11) to identify the source of lead in Greenland snow during the last 3 decades. However the technical difficulties of analyzing ancient ice are much greater because the lead concentrations are ∼100 times lower, nevertheless some isotopic analyses have been reported (12, 13). Here we report the first measurements of lead isotopes in Greenland ice dating from 7.3 ky B.C. to 1.5 ky A.D. and confirm that some contains anthropogenic lead. The isotopic signature of this lead is consistent with that expected in emissions from southern Spain (Huelva, Sevilla, Almeria, and Murcia provinces), which provides clear evidence of the importance of these Spanish mining districts to the Cathaginian and Roman civilizations.
Methods Sample Preparation. We have measured the isotopic composition and concentration of the lead in 26 sections of a 3029 m long ice core from Summit, Greenland (72°34′ N; 37°37′ W; elevation 3238 m), drilled during 1990-1992 for the
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TABLE 1. Isotopic Composition and Concentration of Lead Measured in Ancient Greeland Ice yeara
depth (m)b
206Pb/207Pbc
208Pb/207Pb
206Pb/204Pb
concnd (pg/g)
1523 A.D. 1271 A.D. 1009 A.D. 746 A.D. 473 A.D. 220 A.D. 211 A.D. 193 A.D. 128 A.D. 100 A.D. 58 A.D. 36 A.D. 21 B.C. 79 B.C. 90 B.C. 143 B.C. 266 B.C. 357 B.C. 366 B.C. 680 B.C. 962 B.C. 5295 B.C. 5764 B.C. 6257 B.C. 6779 B.C. 7313 B.C.
129.3 184.3 239.3 294.3 349.3 399.3 401.0 404.3 416.9 422.4 430.7 435.1 446.1 457.1 459.3 469.2 492.3 509.3 511.0 569.3 619.3 1230.4 1286.5 1339.3 1394.3 1449.3
1.1748 ( 0.0012 1.1836 ( 0.0025 1.1846 ( 0.0015 1.1882 ( 0.0030 1.1932 ( 0.0031 1.1941 ( 0.0028 1.1957 ( 0.0031 1.1957 ( 0.0017 1.1925 ( 0.0017 1.1921 ( 0.0018 1.1904 ( 0.0013 1.1843 ( 0.0017 1.1854 ( 0.0021 1.1907 ( 0.0018 1.1890 ( 0.0018 1.1828 ( 0.0013 1.1878 ( 0.0018 1.1932 ( 0.0038 1.1903 ( 0.0027 1.2008 ( 0.0039 1.2006 ( 0.0033 1.1973 ( 0.0037 1.2000 ( 0.0030 1.2041 ( 0.0024 1.2023 ( 0.0023 1.2023 ( 0.0029
2.4587 ( 0.0026 2.4667 ( 0.0032 2.4693 ( 0.0030 2.4704 ( 0.0030 2.4893 ( 0.0046 2.4789 ( 0.0054 2.4796 ( 0.0048 2.4831 ( 0.0060 2.4766 ( 0.0033 2.4697 ( 0.0026 2.4731 ( 0.0036 2.4665 ( 0.0029 2.4661 ( 0.0032 2.4781 ( 0.0051 2.4763 ( 0.0037 2.4685 ( 0.0026 2.4721 ( 0.0038 2.4768 ( 0.0038 2.4795 ( 0.0049 2.4843 ( 0.0044 2.4842 ( 0.0052 2.4743 ( 0.0076 2.482 ( 0.012 2.4850 ( 0.0026 2.4814 ( 0.0033 2.4858 ( 0.0051
18.27 ( 0.08 18.51 ( 0.08 18.48 ( 0.07 18.69 ( 0.11 18.62 ( 0.08 18.75 ( 0.08 18.67 ( 0.16 18.73 ( 0.20 18.63 ( 0.06 18.46 ( 0.16 18.62 ( 0.10 18.49 ( 0.05 18.43 ( 0.09 18.56 ( 0.09 18.63 ( 0.09 18.43 ( 0.04 18.61 ( 0.11 18.61 ( 0.26 18.52 ( 0.06 18.78 ( 0.17 18.87 ( 0.31 18.64 ( 0.21 18.72 ( 0.34 18.66 ( 0.13 18.70 ( 0.06 18.85 ( 0.10
5.5 2.8 4.4 2.2 1.7 1.8 1.9 2.3 2.4 2.1 3.6 3.3 3.5 6.1 2.4 3.9 3.6 1.8 2.9 0.9 1.0 1.1 1.6 1.8 1.5 1.2
a Ref 26. b Each sample integrates exactly 2 yr of ice accumulation. No data are presented for the ∼650-1200 m depth interval of the brittle zone because of the poor quality cores. c Uncertainties shown are 95% confidence intervals. d Accuracy of concentration is (20% (95% confidence intervals).
European Greenland Ice-Core Project (GRIP) (7). Five sections from this core, dating from 7313 B.C. to 5295 B.C., were chosen to represent the Holocene epoch background. Each sample represents 2 yr of ice accumulation. Each section was mechanically decontaminated by chiseling away successive concentric layers of ice from the core (6). The effectiveness of the procedure was checked by analyzing each layer for lead and demonstrating that a plateau of low concentration was achieved at the center. These samples were also analyzed by Hong et al. (8), and frozen aliquots were transported to Perth, Australia, in low-density polyethylene (LDPE) bottles for the present study. Mass Spectrometry. Lead isotopes were measured by thermal ionization mass spectrometry (TIMS), and the concentration was simultaneously determined by isotope dilution mass spectrometry (IDMS) using a 205Pb spike (halflife 1.5 × 107 yr). This approach maximizes measurement precision and optimizes sample consumption. The ultraclean procedures used to prepare the samples for isotopic analysis are described by Chisholm et al. (13) and have been employed in previous studies (10-12). Although the Pb blank was always less than 4% of the sample (18-107 pg), a correction was nevertheless made to both the concentration and isotopic composition. Measurement bias in the isotopic ratios was corrected by analyzing the NIST SRM981 lead isotopic standard. Isotopic Fingerprinting. Lead has four stable isotopes (204Pb, 206Pb, 207Pb, and 208Pb), but anthropogenic lead displays large variations in isotopic composition depending on its origin. This is because the formation of 206Pb, 207Pb, and 208Pb occurs via three different radioactive decay series: 238U (halflife 4.5 × 109 yr), 235U (half-life 7.1 × 108 yr), and 232Th (halflife 1.4 × 1010 yr). The isotopic composition of lead observed in nature will reflect the concentration of U and Th in the source material as well as its age and geological history. While the composition of lead in crustal rocks steadily changes over geological time, lead in ores remains constant because at the time of ore formation the lead was separated from the U and Th. Consequently, while modern rocks will have a 206Pb/
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FIGURE 1. 206Pb/207Pb isotopic ratio (upper curve) and concentration (lower curve) of lead in Greenland ice. Corresponding values for the Holocene background, based on measurements of five ∼8-kyr-old samples, fall between the dotted lines. 207Pb ratio greater than ∼1.20 (14, 15), lead ores may have a significantly lower value. Greenland ice with a 206Pb/207Pb ratio less than ∼1.20 must therefore contain some anthropogenic lead.
Results and Discussion Holocene Background. The isotopic composition and concentration of lead in Greenland ice is given in Table 1. Data from the five core sections dated 7313-5295 B.C. were averaged to obtain reference values for the Holocene epoch. These came to 206Pb/207Pb ) 1.201 ( 0.003, 208Pb/207Pb ) 1.482 ( 0.004, and Pb concentration ) 1.4 ( 0.3 pg/g (uncertainties are 95% confidence intervals). Isotopic Evidence for Anthropogenic Lead. Figure 1 shows the change in the 206Pb/207Pb ratio and lead concentration from 962 B.C. to 1523 A.D. The most prominent feature of the data is the dip in ratio between 680 B.C. and 193 A.D. with the two lowest values occurring at 143 B.C. and 36 A.D. The depression was followed by a steady increase just reaching the lower limit of the Holocene background by 193 A.D. From
FIGURE 2. Isotopic composition of lead in Greenland ice between 7313 B.C. and 1523 A.D. Sample dates are shown, with B.C. dates given a negative prefix. Lead ores are shown as elliptical fields: A, Aegean; B, British (Mendips, Shropshire, Derbyshire, Cornwall, and Devon) (18); E, Europe (Germany, Austria) (19, 20); L, Greek (Laurion) (17); S, Spanish, S1 (Cabo de Garta region), S2 (Cabo de Garta and Mazarron regions), S3 (Cartagena region), S4 (includes Rio Tinto region) (16); Sa, Atlantic ocean sediments near the western Sahara (21). Five samples used to represent the Holocene background are shown as filled squares. 211 A.D., the ratio decreased steadily reaching its lowest value in 1523 A.D. Between 962 B.C. and 1523 A.D., the change in isotopic ratios is mirrored, for the most part, by the change in lead concentrations. The decrease in the 206Pb/207Pb ratio to below the Holocene background after 680 B.C. is unequivocal evidence of anthropogenic pollution of the ancient atmosphere. While natural emissions may have higher ratios, only anthropogenic sources can have lower ratios. Although the broad peak of lead concentration that occurred during this period has been associated with emissions from Greek and Roman mining activities (8), the isotopic ratios demonstrate that it has an anthropogenic source. The origin of this lead remains to determined. Identification of Source Regions. The use of a 206Pb/ 207Pb/208Pb three isotope plot (Figure 2) allows the isotopic composition of the ice samples to be shown in relation to the Holocene background dust and ores known to be worked during ancient times. Elliptical fields in the figure enclose the isotopic signatures of lead from ancient mines in southern Spain, Greece, the Aegean Sea, Britain, and Europe (16-20). Ocean sediments from near the coast of northwest Africa, adjacent to the western Sahara (21), are also included to show the isotopic signature of natural terrestrial “dust” (rock dust and soil). On this plot, binary mixtures will lie on a straight line joining their isotopic compositions. We now consider the Greenland data following a chronological sequence from the prehuman Holocene times to medieval times. The five Holocene samples used to identify the composition of the background lie at one end and slightly above the trend defined by the remaining Greenland ice samples (Figure 2). Two of these samples fall within the field associated with Saharan dust. Samples dated 680 and 962 B.C. have the same low lead concentration and an isotopic composition indistinguishable from the four oldest samples analyzed. Some Saharan dust matches this composition and is a possible source of the dust. Evidence for a Saharan origin is given by Mosher et al. (22), who identified the Sahara Desert as the most likely source of reddish dust which they detected on filters at dye 3 (elevation 2479 m), in southern central Greenland. However the presence of Saharan dust at Summit which has a higher elevation has not been demonstrated. The initial decline in the 206Pb/207Pb ratio beginning after 680 B.C. (Figure 1) followed the rise of the Greek civilization
FIGURE 3. Isotopic composition of anthropogenic lead in Greenland ice. These ratios were corrected for lead in the Holocene background dust using Pb/Al ratios determined in ∼8-kyr-old ice . Uncertainties are 95% confidence intervals. and the introduction of coinage (∼650 B.C.), which promoted the use of silver. The isotopic signature of lead from the famous Laurion mine falls inside a relatively small ellipse (L) on Figure 2; however, there is no clear isotopic evidence in the data for a significant contribution to the Greenland ice from this source even in the fourth century B.C. Samples from the fifth century B.C. would offer the greatest opportunity of detecting Laurion lead, but no samples from this period were available for analysis. The dip in 206Pb/207Pb ratios shown in Figure 1 corresponds to samples dating from 680 B.C. to 193 A.D. Plotted in Figure 2, the samples form a diagonal trend that extends from the region typifying terrestrial dust toward the British (B), European (E), and Spanish (S4) fields. Because these signatures are not associated with known ores, they appear to represent mixtures of lead from two or more different sources. The historical evidence discounts mining and smelting activity in Britain (B) as a source until ∼49 A.D. and even then it was small in comparison with Spain. Contributions from Gaul, Italy, Sardinia, the Capathians, and the Balkans (E) were also considered to be relatively small (4). This suggests that the Spanish mining region, represented by S4, at Rio Tinto in southwestern Spain, is the most likely source. The dip in the 206Pb/207Pb ratios in Figure 1 is therefore consistent with significant emissions from this region. The isotopic data were corrected for lead originating from rock dust using an Pb/Al ratio of 2.15 × 10-4, based on the five Holocene background samples. Aluminium concentrations were taken from Hong et al. (23). Uncertainties of ( 20% were assigned to the Al and Pb concentrations. The corrected 206Pb/207Pb ratio of samples between 366 B.C. and 220 A.D. are shown in Figure 3. Although the 79 B.C. ice sample displayed a relatively high measured ratio in Figure 1, the corrected ratio is the same as the adjacent samples. This apparent anomaly was due to an unusually high dust concentration (23). Figure 3 also shows the 206Pb/207Pb ratio of all samples to be higher than the Rio Tinto signature (taken to be 1.164 (16)), indicating that another Pb component is present. The mines of southeast Spain are characterized by more radiogenic isotopic signatures (Figure 2, S1, Cabo de Garta region; S2, Cabo de Garta and Mazarron regions; S3, Cartagena region) and may therefore be the source of the third Pb component. This would be consistent with the historical records that report intensive mining of the Mazzaron and Cartagena regions by the Romans. These data also indicate that the ratios may be higher after ∼36 A.D., but more precise measurements are needed to confirm this trend. Evidence for any British contribution is difficult to find. The most likely source is the Mendips deposit (2) whose isotopic signature mainly occupies the upper part of field B in Figure 2. The 100 A.D. sample is the only sample, after 43
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discussion. Comments by Claude Domergue from the University of Toulouse on an earlier version of the manuscript were greatly appreciated. This research was supported by research grants from the Australian Research Council; the Australian Department of Industry, Science and Trade; and the Antarctic Science Advisory Committee. The collection and decontamination phase of the project was supported by the XII Directorate of the Commission of the European Communities and the French Ministry of the Environment, the Institut National des Sciences de l’Univers, the University Joseph Fourier of Grenoble, and the Institut Universitaire de France. This work is a contribution of GRIP organized by the European Science Foundation. FIGURE 4. Proportion of Rio Tinto-type lead in the anthropogenic component of the lead in Greenland ice. These values are based upon a two-component model including lead ores from SW (Rio Tinto) and SE Spain (Mazarron) whose isotopic ratios are shown. See discussion in text. Uncertainties are 95% confidence intervals. A.D., to show any evidence of British lead, but the available precision does not allow the signature to be uniquely identified. The absence of further isotopic evidence is consistent with a relatively small British Pb production. Figure 1 shows that the lead concentration in the ice fell to a minimum value by about 473 A.D. then increased, mirrored by a decrease in the 206Pb/207Pb ratio. From 220 A.D., there was a steady decrease in the measured 206Pb/ 207Pb; however, it was not until 1523 A.D. that the anthropogenic component could be precisely determined. Surprisingly, the 206Pb/207Pb ratio of 1.164 ( 0.004 and the 208Pb/ 207Pb ratio of 2.446 ( 0.005 are the same as the Rio Tinto signature but may represent a mixture. Relative Contributions from SW and SE Spain. In principle, isotopic systematics allow a quantitative assessment to be made of the relative contributions from southwest and southeast Spain to the anthropogenic lead found in the Greenland ice during Roman times. If the anthropogenic lead shown in Figure 3 is taken to have originated from Spain, as previously discussed, then the choice of a 206Pb/207Pb ratio for the second principal component of the mixture allows the proportion of these components to be determined. For this purpose a ratio of 1.1956 for ores from the Mazarron region was taken from Stos-Gale (16) and is shown in Figure 3. The percentage of Rio Tinto-type lead in the mixture was calculated using these parameters and is displayed in Figure 4. The plot indicates that ∼70% of the lead can be assigned to Rio Tinto between 366 B.C. and 36 A.D. There is some indication that the proportions may have decreased after this time, but more precise measurements are needed to confirm this trend. These data also allow the concentration of Rio Tinto-type lead in the ice to be calculated. In the past, lead isotopes have been used extensively to identify trade routes and to provenance and establish the authenticity of artifacts from the ancient world (24, 25). We have taken this work a step further to identify the origin of atmospheric pollution in the ancient atmosphere. The data demonstrate unequivocally that the peak of lead concentration detected by Hong et al. (8) has an anthropogenic source and when coupled with historical evidence can be associated with emissions from mining regions in southwest (Rio Tinto) and southeast Spain.
Acknowledgments We thank colleagues and students of Curtin University’s Isotope Science Research Centre, who provided helpful
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Literature Cited (1) Avery, D. Not on Queen Victoria’s Birthdaysthe story of the Rio Tinto mines; Collins: London, 1974; 464 pp. (2) Patterson, C. C. Econ. Hist. Rev. 2nd Ser. 1972, 25, 205-235. (3) Rothenberg, B.; Garcia Palomera, F.; Buchmann, H. G.; Goethe, J. W. In Mineria y metalurgia en las antiquas civilizaciones mediterraneas y europeas; Domergue, I. C., Ed.; Madrid, 1987; pp 57-70. (4) Nriagu, J. O. Lead poisoning in antiquity; Wiley: New York, 1983. (5) Nriagu, J. O. Science 1996, 272, 223-224. (6) Candelone, J.-P.; Hong, S.; Boutron, C. F. Anal. Chim. Acta 1994, 299, 9-16. (7) Dansgaard, W.; Johnsen, S. J.; Clausen, H. B.; Dahl-Jensen, D.; Gundestrup, N. S.; Hammer, C. U.; Hvidberg, C. S.; Steffensen, J. P.; Sveinbjo¨fnsdottir, A. E.; Jouzel, J.; Bond, G. Nature 1993, 364, 218-220. (8) Hong, S.; Candelone, J.-P.; Boutron, C. F. Science 1994, 265, 18411843. (9) Hong, S.; Candelone, J.-P.; Patterson, C. C.; Boutron, C. F. Science 1996, 272, 246-249. (10) Rosman, K. J. R.; Chisholm, W.; Boutron, C. F.; Candelone, J.-P.; Gorlach, U. Nature 1993, 362, 333-335. (11) Rosman, K. J. R.; Chisholm, W.; Boutron, C. F.; Candelone, J.-P.; Hong, S. Geochim. Cosmochim. Acta 1994, 58, 3265-3260. (12) Rosman, K. J. R.; Chisholm, W.; Boutron, C. F.; Candelone, J.-P.; Patterson, C. C. Geophys. Res. Lett. 1994, 21, 2669-2672. (13) Chisholm, W.; Rosman, K. J. R.; Boutron, C. F.; Candelone, J.-P.; Hong, S. Anal. Chim. Acta 1995, 311, 141-151. (14) Chow, T. J.; Patterson, C. C. Geochim. Cosmochim. Acta 1962, 26, 263-308. (15) Hamelin, B.; Grousset, F.; Sholkovitz, E. R. Geochim. Cosmochim. Acta 1990, 54, 37-47. (16) Stos-Gale, Z.; Gale, N. H.; Houghton, J.; Speakman, R. Achaeometry 1995, 37, 407-415. (17) Gale, N. H. In Thera and the Aegean World II; Doumas, C., Ed.; Aris and Phillips Ltd.: London, 1980; pp 161-195. (18) Rohl, B. M. Achaeometry 1996, 38, 165-180. (19) Russell, R. D.; Farquhar, R. M. Lead Isotopes in Geology; Interscience Publishers: New York, 1960; p 243. (20) Koeppel, V. Int. Proc. 27th Int. Geol. Congr. 1984, 12, 53-82. (21) Sun, S. S. Philos. Trans. R. Soc. London 1980, A297, 409-445. (22) Mosher, B. W.; Winkler, P.; Jaffrezo, J.-L. Atmos. Environ. 1993, 27A, 2761-2772. (23) Hong, S.; Candelone, J.-P.; Boutron, C. F. Atmos. Environ., in press. (24) Brill, R. H. Philos. Trans. R. Soc. London 1970, A269, 143-164. (25) Gale, N. H.; Stos-Gale, Z. A. Proc. Brit. Acad. 1992, 77, 63-108. (26) Johnsen, S. J.; Dahl-Jensen, D.; Dansgaard, W.; Gundestrup, N. Tellus 1995, 47B, 624.
Received for review January 17, 1997. Revised manuscript received July 7, 1997. Accepted July 14, 1997.X ES970038K X
Abstract published in Advance ACS Abstracts, September 1, 1997.
