Seasonal variations of heavy metals in the 1960s Alpine ice: sources versus meteorological factors

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Abstract

We present here the first reliable data on the occurrence of Pb, Zn, Cu, Cd, Bi, Mn and Al in high altitude cold Alpine ice. They were obtained by analyzing with three different analytical techniques various sections of a 140 meter long ice core drilled near the summit of Mont Blanc at the French–Italian border. Special emphasis was given to the study of seasonal variations in ice dated from 1960/61 and 1967/68, i.e. ice deposited just before and after the opening of the Mont Blanc road tunnel. For all the metals, concentrations vary by two orders of magnitude from summer to winter, with the highest concentrations being observed in summer. For Pb, Zn, Cu, Cd and Bi, the lowest concentrations range from 0.1 pg/g (for Bi) to 50 pg/g (for Pb), i.e. values which are comparable to concentrations observed in central Greenland. Anthropogenic inputs are dominant for Pb, Zn, Cd, and Bi while inputs from natural sources are important for Cu, Mn and Al. Both source and meteorological parameters are responsible for the heavy metal content of high altitude alpine ice. These metals mainly originate from Western European sources as indicated by back air trajectories and emissions data for the different countries. The observed seasonal variations are however largely linked with the changing vertical structure of the regional troposphere, which prevented the transfer of pollutants to higher altitudes in winter but not in summer. Our data do not indicate a significant change linked with the opening of the Mont Blanc tunnel.

Introduction

During the last decade, great efforts have been made to obtain data on the occurrence of heavy metals such as lead (Pb), zinc (Zn), copper (Cu), cadmium (Cd), bismuth (Bi) and mercury (Hg), in the successive dated snow and ice layers deposited in central Greenland 1, 2. They have allowed to get unique information on past and recent changes in atmospheric pollution for these metals on hemispheric scale in the northern hemisphere. Amongst the most valuable results is the evidence of an early atmospheric pollution for Pb and Cu two millennia ago at the peak of the Greek and Roman civilizations 3, 4, 5. Highly interesting data were also obtained for the post Industrial Revolution period: they documented the increase in heavy metal pollution in the northern hemisphere from the second half of the 18th century to the 1960s–1970s and the subsequent decrease for some of them for the recent decades 6, 7. For Pb the investigations were also for Pb isotopes 8, 9and organo Pb compounds [10].

Surprisingly, little attention has been paid to the investigation of heavy metals in the successive snow and ice layers deposited in high altitude cold alpine glaciers (cold means here without any significant water percolation during the summer months from one layer to another). These glaciers are close to populated and industrialized areas, and have considerable potential to provide very interesting insight into historical changes in atmospheric heavy metals on a regional scale in Europe and other mid-latitude areas [11]. There are very few reliable data [12]which are only for surface snow [13]and do not allow time trends to be assessed. We present here new data on Pb, Zn, Cu, Cd, Bi Mn and Al in various sections of an ice core which was drilled at a high altitude location in the Alps close to the top of Mont Blanc at the French–Italian border. They were aimed at deciphering the source versus meteorological parameters which determine the heavy metals content of high altitude alpine ice. This was achieved by determining seasonal variations in the concentrations for two different time periods in ice dated from the 1960s, i.e. the decade during which the Mont Blanc tunnel was opened.

Section snippets

Field sampling and core dating

In June 1994, a 140 meter ice core [14]was drilled. We used a stainless steel electromechanical drill which was coated with polytetrafluoroethylene (PTFE) in order to minimize contamination brought to the outside of the core during drilling operations. The drilling site was situated on the east slope of Dôme du Goûter (elevation 4304 m) about 1.5 km northwest of Mont Blanc in southeastern France (see Fig. 1). Mont Blanc (altitude 4807 m) is the highest mountain in Western Europe. It is located

Pb, Zn, Cu, Cd, Bi, Mn and Al concentration profiles

Pb, Zn, Cu, Cd, Bi, Mn and Al concentrations measured in the 20 depth intervals are given in Table 1. Fig. 3Fig. 4 show the variations of the concentrations as a function of the depth for the 1960/61 and 1967/68 periods respectively. Also shown are the variations of the D/H isotopic ratio expressed in δD per mil unit which can be used to identify the different periods of the year: low δD‰ values are associated with low temperatures and correspond to winter layers while higher values are

Acknowledgements

This work was supported in France by the Institut Universitaire de France, the Ministry of the Environment, the Agence de l'Environnement et de la Maı̂trise de l'Energie, the Institut National des Sciences de l'Univers, and the University Joseph Fourier of Grenoble. In Italy, this study was performed in the framework of the Projects on Environmental Contamination and Glaciology and Paleoclimatology of the Italian Antarctic National Research Program. It was financially supported by ENEA through

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