NEW PEAT BOG RECORD OF ATMOSPHERIC LEAD DEPOSITION SINCE THE ROMAN PERIOD AT ETANG DE LA GRUÈRE, JURA MOUNTAINS, SWITZERLAND: TOTAL Pb CONCENTRATIONS, ENRICHMENT FACTORS, ISOTOPIC COMPOSITION, AND ORGANOLEAD SPECIES

W. Shotyk¹*(shotyk@geo.unibe.ch), D. Weiss ², M. Heisterkamp³, A.K. Cheburkin⁴, and F.C. Adams³: 1, Geological Institute, University of Berne, Baltzerstrasse 1, CH-3012 Berne, Switzerland; 2, Earth, Atmospheric and Planetary Sciences, MIT, 77 Massachusetts Avenue, Bldg E34‑246, Cambridge, MA 02139, USA; 3, Department of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610 Antwerp, Belgium; 4, EMMA Analytical Inc., General Delivery, Elmvale, Ontario L0L 1P0 Canada

ABSTRACT

As an independent evaluation of the record of post-Roman atmospheric Pb deposition at Etang de la Gruère, a second, 100 cm long peat core (2K) was collected two years after core 2F. Both cores were analyzed for total Pb and other trace elements using the EMMA XRF, and Pb isotopes (²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb) using TIMS. Each core was age dated using both ²¹⁰Pb (gamma spectrometry) and ¹⁴C (decay counting). The 2K and 2F cores show similar temporal variations in Pb concentrations, enrichment factors, and isotopic composition. The 2K core was also analyzed for organolead species (DEL, TEL, DML and TML) using GC-MIP AES. The spatial and temporal variations in organolead species concentrations and Pb isotopic composition are consistent with the history of leaded gasoline consumption since its introduction (to Switzerland) in 1947.

INTRODUCTION

The ombrogenic peat bog "Etang de la Gruère" (EGR) in the Jura Mountains of Switzerland has provided a continuous record of atmospheric Pb deposition since 12,370 ¹⁴C yr BP (Shotyk et al., 1998). The isotopic composition of Pb (summarized as the ratio ²⁰⁶Pb/²⁰⁷Pb), combined with total Pb concentrations and ²¹⁰Pb age dating confirmed using radionuclide (²⁴¹Am) and pollen chronostratigraphic markers (primarily Cannabis), was used to construct a geochemical mass balance which argued against significant post-depositional migration of Pb within the peat profile (Shotyk et al., 1996, 1997). Very similar temporal trends in Pb enrichment factor (Pb EF) and Pb isotope ratios reported for EGR were observed in three other Swiss peat bogs (Weiss et al., 1999a), adding further strength to the view of peat bogs as reliable archives of atmospheric Pb deposition. As an independent check on this interpretation, the isotopic composition of Pb was measured in samples of Sphagnum moss which had been collected from peat bogs since 1867, and stored at the Herbarium of the University of Geneva; the results were remarkably similar to the variations preserved by the peat cores taken from four Swiss peat bogs (Weiss et al., 1999b).

As a further evaluation of the reproducibility of the peat bog archives of atmospheric Pb deposition, we have undertaken a detailed investigation of a second peat core (2K) from EGR; this second core was prepared and analyzed in the same way as 2F for total Pb and Pb isotopes, and dated using both ²¹⁰Pb and ¹⁴C. Moreover, as part of a Ph.D. thesis about organolead species in the environment (Heisterkamp, 2000), selected samples from the 2K core were also used to measure the following organolead compounds: dimethyllead (DML), trimethyllead (TML), diethyllead (DEL) and triethyllead (TEL). By including these analyses, we are able to evaluate independently the chronology of the introduction, growth, and subsequent decline in leaded gasoline consumption which has dominated the accumulation of atmospheric Pb since leaded gasoline was first introduced in Switzerland in 1947.

MATERIALS AND METHODS

Sample collection and preparation

A monolith of peat ca. 10 x 10 x 100 cm was collected at EGR on 22.6.93 (core 2K) using a Wardenaar peat coring device; the 2K core was taken approximately 4.5 m E of the 2F core which was collected on 26.8.91. Samples from the 2K core were prepared as described previously (Shotyk, 1996).

Lead and selected trace element analyses of solid peat samples

Lead and Ti were measured at EMMA Analytical Inc. (Elmvale, Ontario, Canada) using the Energy-dispersive Miniprobe Multielement Analyzer (EMMA) with Mo Kβ (energy of 19.6 KeV) as the exciting wavelength. The instrument was calibrated as described in detail elsewhere (Shotyk et al., 2000). Lead was previously measured in triplicate, in the 2K core, using Mo Kα as the exciting wavelength (17.44 keV), and the new values (obtained using Mo Kβ) are in excellent agreement with previous measurements (α = 0.955β + 1.57; r²=0.997, n=32). Lead was also measured in acid digests of the 2K samples using ICP-MS, and the values are very similar (ICP-MS = 0.927 (β)XRF - 0.66; r²=0.996, n=33).

Lead isotopes

Following microwave-assisted acid dissolution of peat samples (Weiss et al., 1998), Pb isotope ratios were measured in slices 2K1-13 and slice 2K15 using solid-source thermal ionisation mass spectrometry (TIMS) with a VG Sector mass spectrometer (Weiss et al., 1999a,b).

Alkyllead species

A simplified derivatization method for the speciation analysis of organolead compounds followed by gas chromatography ‑ microwave induced plasma atomic emission detection (GC-MIP AES) was developed (Heisterkamp, 2000). An in-situ butylation using tetrabutylammonium tetrabutylborate in an acetate buffer medium of pH 4.0 with simultaneous extraction of the derivatized organolead species into hexane was performed; for the analysis of peat samples, the desorption of the different species from the matrix by acid leaching is also included in that step. This speciation analysis is capable of quantifying all relevant organolead species, namely trimethyllead (TML: (CH₃)₃Pb⁺), dimethyllead (DML: (CH₃)₂Pb²⁺), triethyllead (TEL: (C₂H₅)₃Pb⁺) and diethyllead (DEL: (C₂H₅)₂Pb²⁺). Detection limits are at the sub- ng/l range, and the accuracy of the method was confirmed by analysis of a standard reference material (BCR-CRM 605, road dust).

Age dating

Age dating of the youngest peat samples (ca. past 150 years) was obtained using ²¹⁰Pb (Appleby et al., 1997). Selected peat samples from deeper layers were dated using ¹⁴C decay counting (Radiocarbon Lab, Physics Institute, University of Berne) as described elsewhere (Shotyk et al., 1998).

RESULTS AND DISCUSSION

Pb and Ti concentration profiles, and Pb EF

There are pronounced peaks in Pb concentration at AD 1979 (± 2) and AD 1881 (± 9), repectively, with the older peak considerably greater than the younger one (Fig. 1a). However, there are elevated concentrations of Ti (Fig. 1b) at similar depths to the older, deeper Pb peak. To take into account the variations in amount of mineral material (mainly soil dust) on the Pb concentration profiles, Pb was normalized to Ti. This procedure yields the metal Enrichment Factor which was calculated as

EF = ([Pb]/[Ti])_peat / ([Pb]/[Ti])_{Earth`s crust}

where [Pb] refers to the total concentration of Pb measured in the peat sample (µg/g) and [Ti] to the total concentration of Ti. The calculated EFs (Fig. 1c) indicate the extent of Pb enrichment in the peats, relative to the abundance of the Pb in the Earth`s crust where Pb = 14.8 µg/g and Ti = 4010 µg/g (Wedepohl, 1995). The Pb EF profile also shows two pronounced peaks, but the older peak (at 1932 ± 3) is smaller than the younger peak (at 1975 ± 2). The difference in age and intensity of Pb accumulation viewed as Pb concentrations in the peat profile (Fig. 1a) versus Pb EF (Fig. 1c) illustrates the danger in viewing metal concentrations alone, without also considering the abundance of mineral material (Shotyk, 1996).

Figure 1 Pb and Ti concentrations, Pb EF, and 206Pb/207Pb ratio in the 2K core from EGR. Age dates over 40 cm were obtained using 210 Pb, older dates are conventional radiocarbon yrs BP.

Chronology of Pb enrichments

The chronology and intensity of Pb enrichment during the Roman Period and Late Medieval Period recorded by the 2K profile is remarkably similar to that reported earlier for 2F (Shotyk et al., 1998). With respect to Pb enrichments during the early part of the present century, caused mainly by coal-burning and industrial production, these are evident later in the 2K core (1932 ±3) compared with 2F (1905 ±6 to 1920 ±4). The maximum Pb EF in the two cores are dated at 1975 (±2) and 1979 (±2) in core 2K and 2F, respectively.

Isotopic composition of Pb and its variation with time

The variation in ²⁰⁶Pb/²⁰⁷Pb in the 2K core is very similar to that reported earlier for 2F. In sample 2K13 dated 1837 (±31), the value is 1.18015 ± 0.0002 which, compared with the background value of ca. 1.2 (Shotyk et al., 1998), reflects the importance of anthropogenic Pb from both industrial lead production and coal burning (Weiss et al., 1999a). The isotopic composition of Pb becomes progressively less radiogenic from the beginning of the Industrial Revolution onward, but the greatest change is seen in sample 2K9, dated 1946; this corresponds with the introduction of leaded gasoline in Switzerland in 1947 (gasoline Pb is characterized by especially low ²⁰⁶Pb/²⁰⁷Pb ratios). The lowest ²⁰⁶Pb/²⁰⁷Pb ratio is found in peat dating from 1988, and the most recent sample measured (corresponding to 1993) shows an increase, back toward more radiogenic values; this latest value, however, is far removed from natural, background values.

The maximum Pb EF pre-dates the minimum ²⁰⁶Pb/²⁰⁷Pb in both cores: in the 2K core, the maximum Pb EF is at 1975 (±2) and minimum ²⁰⁶Pb/²⁰⁷Pb (1.1254 ± 0.0002) is dated at 1988 (±2); in the 2F core, the max. Pb EF is 1979 (±2) and the lowest ²⁰⁶Pb/²⁰⁷Pb ratios are 1.1231 ± 0.0002 dated at 1985 (±2) and 1.1232 ± 0.0002 dated at 1989 (±2). Thus, Pb EF began its decline prior to the beginning of the gradual phasing out of leaded gasoline; this probably follows the introduction of various industrial emission control technologies for reducing pollutive emissions generally, but especially from fossil fuel consumption, industrial production of metals and manufactured products, and municipal waste incineration.

Figure 2 Total alkyl Pb, dimethyllead, and diethyllead concentrations in the 2K core from EGR. Age dates are from 210 Pb age dating of this (2K) core. These results supercede a chronology given previously (Heisterkamp and Adams, 1999) which was plotted using ESTIMATED age dates obtained by correlating Ti concentrations in the 2K profile with that of 2F.

Abundance of organolead compounds and variation with time

The first quantifiable occurrence of organolead compounds is in peat dated at 1946 ± 3, and this is consistent with the introduction of leaded gasoline in Switzerland in 1947. The lack of measurable concentrations of alkyllead compounds in peat samples pre-dating this indicates a lack of significant vertical downward migration of these compounds, implying efficient retention of these species by peat. The maximum concentration is found in 1988; the lowest ²⁰⁶Pb/²⁰⁷Pb ratio occurs at the same time, revealing the point of greatest gasoline lead impact. While total alkyllead has certainly gone into decline, probably as a result of the gradual phasing out of leaded gasoline, there remains significant concentrations of alkyllead compounds in the most recent (1993) peat samples. The enhanced lipid solubility and toxicity of organolead compounds compared with inorganic Pb gives rise to justifiable concern for their dispersion and accumulation in the environment.

The concentrations of alkyllead species are very low, with the sum of DML, TML, DEL, and TEL never exceeding 0.02 % of total Pb in any given sample. However, in alpine snow and ice, the relative importance of alkyllead to total Pb is similar (Heisterkamp et al., 2000). Thus, decomposition of organolead species within the bog is an unlikely explanation of the low abundance of these species compared with total Pb. While virtually all of the Pb in gasoline is present as alkyllead compounds, very little of these remained preserved in environmental archives such as snow, ice, and peat: either very little survives the combustion process, the atmospheric residence time is very low, or both.

In alpine snow and ice, DEL and TEL were below the lower limit of detection in most samples, with DML and TML by far the dominant forms of total alkyllead (Heisterkamp et al., 2000). In contrast, DEL and TEL are more abundant in the peat samples than DML and TML. The difference in relative abundance of methylated compared with ethylated species may result from a number of factors including the greater thermal stability and much higher vapour pressure of tetramethyllead compared with tetraethyllead, the longer atmospheric lifetime of methylated lead species (Wang et al., 1997), and the much higher elevation of the alpine site (4250 m) compared with the peat bog at EGR (1005 m).

SUMMARY AND CONCLUSIONS

The chronology of Pb EF and temporal evolution of the isotopic composition of Pb in the 2K core is similar to the results published previously for the 2F core which was collected two years earlier. This similarity strengthens the view that peat cores from ombrogenic bogs faithfully preserve the record of atmospheric Pb deposition, yielding chronological records which are reproducible. The Pb EF profile shows clearly that anthropogenic Pb has dominated the supply of atmospheric Pb to this region continuously since the Roman Period. The chronology of changes in ²⁰⁶Pb/²⁰⁷Pb ratio and the abundance of alklylead species provide a consistent reconstruction of the changes in atmospheric Pb accumulation rates supplied by leaded gasoline use.

ACKNOWLEDGEMENTS

Financial support from the Swiss National Science Foundation (Grant 21-55669-98 to W.S.) is sincerely appreciated. Thanks to Peter Appleby, Radiometric Research Centre, University of Liverpool, for the ²¹⁰Pb age dating, and to Steve Reese of the Radiocarbon Lab, University of Berne, for the ¹⁴C age dates. Special thanks also to Marlies Gloor, WSL, Birmensdorf (now at EMS Dottikon) for ICP-MS measurement of Pb in the 2K core.

REFERENCES

Appleby, PG, Shotyk, W, and Fankhauser, A (1997) Water Air Soil Pollut. 100:223-231.

Heisterkamp, M (2000). Ph.D. Dissertation, Dept. Chem., Univ. Antwerp.

Heisterkamp, M and Adams, FC (1998) Fresenius J. Anal. Chem. 362:489-493.

Heisterkamp, M and Adams, FC (1999) ERCA Vol. 3, From Urban Air Pollution to Extra-Solar Planets, pp. 227-253.

Heisterkamp, M, Van de Velde, K, Ferrari, C, Boutron, CF and Adams, FC (2000) Env. Sci. Technol. 33:4416-4421.

Shotyk, W (1996) Environ. Revs 4(2):149-183.

Shotyk, W , Cheburkin, AK, Appleby, PG, Fankhauser, A, and Kramers, JD (1996) Earth Planet. Sci. Lett. 145:E1-E7.

Shotyk ,W, Cheburkin AK, Appleby PG, Fankhauser A, Kramers JD (1997) Water Air Soil Poll. 100:297-310.

Shotyk W, Weiss D, Appleby PG, Cheburkin AK, Frei R, Gloor M, Kramers JD, Reese S, van der Knaap WO. (1998) Science 281:1635-1640.

Shotyk, W., Blaser, P, Grünig, A, and Cheburkin, AK (2000). Sci. Tot. Environ. 249:257-280.

Wang, Y, Turnbull, AB, and Harrison, RM (1997) Appl. Organometal. Chem. 11:889-901.

Wedepohl KH. (1995) Geochim. Cosmochim. Acta 59:1217-1232.

Weiss, D., Shotyk, W., Schäfer, H., Loyall, U., Grollimund, E., and Gloor, M. (1999). Fresenius J. Anal. Chem. 363:300-305.

Weiss D, Shotyk W, Appleby PG, Cheburkin AK, Kramers JD. (1999a) Environ. Sci. Technol. 33:1340-1352.

Weiss D, Shotyk W, Gloor M, Kramers JD. (1999b) Atmos. Env. 33:3751-3763.