One hundred fifty–year record of lead isotopes in Antarctic snow from Coats Land
Introduction
During the last decades, various investigations have confirmed the advantages of the use of Pb isotope systematics to probe the changing anthropogenic vs. natural origins of this toxic heavy metal in well-preserved snow and ice archives of atmospheric constituents (Rosman, 2001). Most investigations, however, have been devoted to the Greenland icecap (see, e.g., Rosman et al 1993, Rosman et al 1994a, Rosman et al 1997, Rosman et al 1998a, Boyle et al 1994, Sherrell et al 2000; high-altitude alpine regions have been investigated more recently (Rosman et al., 2000).
On the other hand, there are very few reliable measurements of Pb isotopes in remote Antarctic snow and ice. The first were carried out by Rosman and coworkers, who determined Pb isotopes in four surface snow blocks, collected in 1983 to 1984 along a transect from the coastal station Dumont d’Urville to a site 433 km inland and at the geographic South Pole, and in a single sample of ancient ice dated from 7.5 kyr BP obtained at Dome C (72°39′S, 124°10′E) (Rosman et al., 1994b). The results supported earlier geochemical evidence of Pb pollution (Boutron and Patterson, 1987) and pointed to South America as a likely source of the anthropogenic Pb in Antarctic snow dated from the 1980s.
A second series of measurements by Rosman and coworkers provided preliminary evidence of Pb isotope variations in selected snow/ice cores DE08 (66°43′S, 113°12′E) and DSS (66°46′S, 112°48′E) drilled at the sites on Law Dome (Rosman et al., 1998b). Nine sections were dated 1843 to 1940, one was dated from 946 BC, and the last was ice dated from at least ∼110 kyr. These analyses proved extremely demanding because of the low Pb concentrations encountered (∼0.1 pg/g). Despite the fact that sophisticated procedures were used to decontaminate the core sections (Candelone et al., 1994) and highly developed thermal ionization mass spectrometry (TIMS) procedures (Chisholm et al., 1995) were employed, there was evidence that a number of samples were contaminated (Rosman et al., 1998b). Also at these concentrations, both the accuracy of the concentrations and the isotopic composition were highly sensitive to the value used to correct for Pb contamination introduced during the decontamination process. This problem has been subsequently addressed by Vallelonga et al. (2002), resulting in improved accuracy of the data at these low concentrations.
Rosman et al. (1999) have also reported measurements of Pb isotopes on 22.6-kyr-old ice from Dome C, i.e., ice dated from the cold terminal stage of the last ice age (Last Glacial Maximum). This Pb is less radiogenic than that found in Holocene ice but is similar to the pelagic sediments found near southern South America and volcanic rocks from Central Chile (Rosman et al., 1999).
Recently, Matsumoto and Hinkley (2001) reported Pb isotopes and heavy metal concentrations in nine ice samples dated between 1.3 kyr and 73 kyr BP from Taylor Dome (77°48′S, 158°43′E; 2374-m altitude), a near coastal Antarctic site at the western edge of the Ross Sea. They found that rock and soil dust and sea salt accounted for only a few percent of Pb present in most samples and that the Pb deposition rate to the ice was approximately matched by the output rate to the atmosphere by quiescent (nonexplosive) degassing of volcanoes worldwide. The isotopic composition of Pb in the ice was found to be similar to those of a suite of ocean island volcanoes, mostly in the Southern Hemisphere. Although pollution issues were not addressed, the depositional fluxes of metals were considered to be appropriate in modern times.
We present here comprehensive data on Pb isotopes, Pb and Ba in Antarctic snow dated 1840 to 1990. They were obtained by analyzing pit and core samples collected at a remote location in Coats Land (Planchon et al., 2001), Antarctica. Pb isotopes, Pb, and Ba were determined by TIMS, taking advantage of improvements reported by Vallelonga et al. (2002). These new data provide a clearer picture of the changing occurrence of natural and anthropogenic Pb at one of the most remote locations on our planet.
Section snippets
Sampling locations
The main sampling location (Site A) was at 77°34′S, 25°22′W in Coats Land, Antarctica. This site is of interest because it is situated in a sector of Antarctica that faces the southern Atlantic Ocean (Fig. 1a). It is adjacent to the Weddell Sea. It was then ideally situated to assess the changing natural and anthropogenic Pb inputs from South America and various subantarctic islands of the Atlantic Ocean. The site (Fig. 1b) was at 1420 m altitude, 200 km from the nearest permanent scientific
Results and discussion
Table 1 gives the results for the two snow pits (years 1920 to 1990). Table 2 shows the data for the snow core (years 1834 to 1920). These data represent the most comprehensive record of the impact of anthropogenic Pb on Antarctica that has previously been reported.
Conclusion
We have presented here the first comprehensive data on changes in Pb isotopic ratios in Antarctic snow since the mid-19th century obtained from a site in the Atlantic sector (Weddell Sea) in Antarctica. It will now be interesting to obtain similar time series from other selected sites in Antarctica, especially in the Ross Sea sector, which is another important location of deep-water formation of the World Ocean, and in the Indian Ocean sector that faces Australia. Also, it will be necessary to
Acknowledgements
This research was supported in France by the Institut Universitaire de France, the Ministère de l’Environnement et de l’Aménagement du Territoire, the Agence de l’Environnement et de la Maîtrise de l’Energie, the Institut National des Sciences de l’Univers, and the University Joseph Fourier of Grenoble. In Australia, it was supported by a research grants from the Australian Research Council (A39938047) and the Antarctic Science Advisory Committee (No. 1092) in the Glaciology Sections. In the
References (60)
- et al.
Isotopic evidence of contaminant lead in the South Atlantic troposphere and surface waters
Deep-Sea Res
(2001) Lead isotopic compositions of South Sandwich Island volcanic rocks and their bearing on magmagenesis in intra-oceanic island arcs
Geochim. Cosmochim. Acta
(1983)- et al.
Isotopic source signatures for atmospheric leadThe Southern Hemisphere
Geochim. Cosmochim. Acta
(2000) - et al.
Isotopic source signatures for atmospheric lead, IIThe Northern Hemisphere
Geochim. Cosmochim. Acta
(2001) - et al.
Heavy metal and sulphur emissions to the atmosphere from human activities in Antarctica
Atmos. Environ
(1989) - et al.
Lead variability in the western North Atlantic Ocean and central Greenland iceImplications for the search for decadal trends in anthropogenic emissions
Geochim. Cosmochim. Acta
(1994) - et al.
An improved method for decontaminating polar snow and ice cores for heavy metals analysis
Anal. Chim. Acta
(1994) - et al.
Determination of lead isotopic ratios in Greenland and Antarctic snow and ice at picogram per gram concentrations
Anal. Chim. Acta
(1995) - et al.
The occurrence and significance of Pb isotopes in pelagic sediments
Geochim. Cosmochim. Acta
(1962) - et al.
Hobbs Coast Cenozoic volcanismImplications for the west Antarctic rift system
Chem. Geol
(1997)