Vanadium and other elements in Greenland ice cores. M. M. Herron, C. C. Langway Jr, H. V. Weiss, P. Hurley, R. Kerr and J. H.

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1 Vanadium and other elements in Greenland ice cores M. M. Herron, C. C. Langway Jr, H. V. Weiss, P. Hurley, R. Kerr and J. H. Cragin Abstract. Chemical analysis for Na, Cl, AI, Mn and V of surface snows and deeper ice core samples from station Milcent, Greenland, indicates a terrestrial or marine origin for these constituents. Pre-100 enrichment factors, based on average crustal composition, are high for Zn and Hg and appear to be related to the volatility of these elements. A comparison of pre-100 and concentrations of V and Hg shows no decided increase due to industrial production, yet the relative abundance of Zn increased from 12 to 32 over this time period. The chemical composition of ancient ice is extremely useful in interpreting modern aerosols. Vanadium et des autres éléments dans des carottes de glace du Groenland Résumé. L'analyse chimique pour Na, Cl, Al, Mn et V des neiges de surface et des échantillons plus profonds de carottes de glace provenants de la station Milcent, au Groenland, indique que ces constituants ont une origine et terrestre et marine. Des facteurs d'enrichissement d'avant 100, fondés sur une composition de croûte moyenne, sont élevés pour Zn et Hg et semblent en rapport avec la volatilité de ces éléments. Une comparaison des concentrations de V et Hg d'avant 100 et de ne manifeste aucune augmentation marquée attribuable à la production industrielle, et pourtant l'abondance relative de Zn a augmenté pendant cette période de 12 à 32. La composition chimique de la glace ancienne est extrêmement utile à l'interprétation des aérosols modernes. The atmospheric concentrations of trace elements, particularly those which may include an anthropogenic fraction, have been the subject of much recent study (Zoller et al, 13; Chester and Stoner, 14; Zoller et al, 1'4; Duce et al, 15). The crustal enrichment factor, EF, used to tentatively identify possible pollutants, is defined as EFJ5T = WA^SAMPLE (1A1) CRUST where WA1) SAMPLE and WAi) CRUST are the concentration ratios of the element X to aluminium in the sample and in average crustal material, respectively. An element whose aerosol source is primarily crustal weathering processes should have an EF value near 1. A significantly greater EF indicates input from local or additional sources, selective volatilization at the source (Zoller et al, 11 A), selective transport to, or selective removal at the deposition area. One method of distinguishing naturally high enrichment factors from man's contribution to the atmospheric burden is the analysis of chemical impurities within ice cores from the Greenland ice sheet. Pollutant Pb and S have been identified in modern Greenland snows by comparison with concentrations in pre-100 ice strata (Murozumi et al, 16; Koide and Goldberg, 11; Weiss et al, 15; Cragin et al, 15). Mercury was identified as a possible pollutant in north Greenland ice (Weiss et al, 11a), though work in southern Greenland disputed this (Weiss et al, 15). Extreme variations in Hg concentrations in four north Greenland samples were interpreted as suggesting that the natural Hg burden was highly variable (Carr and Wilkniss, 12). This paper presents the results of a study designed to determine the elemental abundances of Na, Cl, AI, Mn, V, Zn, and Hg in Greenland ice and snow over the past several hundred years, and to help shed light on the significance of modern aerosol analyses. 8

2 Vanadium and other elements in Greenland ice cores During the 13 field season of the Greenland Ice Sheet Program a field camp was established at station Milcent (0 18'N, 44 35'W). A continuous 400-m long 12-cm diameter ice core was recovered and surface pit samples were collected 2-3 km from the camp (pit 1 was always upwind). The sampler wore a clean room coat, surgical face mask, hair covering and powder-free polyethylene gloves throughout the collection, and the pit walls were trimmed back 10 cm with a precleaned stainless steel shovel. One-year channel samples, as determined by stratigraphie features, were obtained with a polyethylene scoop and placed in polyethylene containers and bagged for shipment frozen to the US. All bottles and field implements had been precleaned with a leach in 1 % HN0 3, five rinses with distilled deionized water (DW) and two rinses in double distilled deionized water (DDW). In the laboratory, the surface samples were melted in a microwave oven in their original containers and immediately transferred to hot nitric acid treated (HNAT) one-litre linear polyethylene bottles. These bottles had been filled with reagent grade HNO s, and heated to 80 C-0 C for periods greater than 1 h, rinsed five times with DW, twice with DDW and twice with sample meltwater. This treatment was proven effective in preventing adsorption of 48 V and 203 Hg for over 32 d in separate 400-ml aliquots of the pit 4 sample. The concentrations used were 6.8 ng 48 V/kg (5400 cpm) and < 1 ng 203 Hg/kg (2400 cpm). The ice core samples were cleaned by the 'dry' cleaning procedure (Langway et al., 11 A), which involves extensive rinsing of the core with DDW in a class 100 clean area. The three deep samples not used for the Hg study were stored in conventional polyethylene one-litre bottles prepared by leaching with % Ultrex HN0 3 for several days at room temperature followed by the usual rinsing procedure. A concentration of 10 ng 48 V/kg was added to a 250-ml aliquot of the pit 4 sample in one of these bottles and quantitatively recovered after 21-d storage. Zinc concentrations were measured on the unpreconcentrated samples using flameless atomic absorption with stopped nitrogen flow during atomization. Eppendorf pipette tips were found to give erroneously high concentrations until they were given the hot nitric acid treatment. Mercury concentrations were measured on one-litre aliquots of the meltwater by neutron activation following the procedure of Williams et al. (14). Before radiochemical Hg yields could be determined, the surface samples were destroyed and a yield of 0. was assumed. Five-millilitre aliquots of the same meltwater were analysed for Na and CI by instrumental neutron activation analysis (INAA). The remainder of the samples, 600 ral-21. in volume, were evaporated down to 20 ml in a precleaned quartz flask in a clean air station. They were transferred to 50-ml teflon beakers in a glass enclosure under positive filtered N 2 pressure and evaporated to 5 ml and transferred to HNAT polyethylene irradiation vials (Weiss et al, 11b). A spike of 0.2 ng 48 V added prior to the preconcentration was quantitatively recovered. The samples were then analysed for V, Al, and Mn by INAA. All counting was done with a 5 cm 3 Ge(Li) detector coupled to a 406 channel analyser and peaks were integrated by computer. The full data for Milcent snow and deeper ice is presented in Table 1. Each core sample represents exactly two years' accumulation as determined by interpretation of the oxygen isotope profile (Dansgaard et al, personal communication). The elemental concentrations are in /xg/kg. The Cl:Na ratio of 1.8 throughout the profile matches the ratio of bulk sea water and confirms the marine origin of these two elements. Based on average sea-water composition, the contribution of sea spray to the other elements is minimal.

3 100 M. M. Herron et al. Mean crustal enrichment factors using Al as the reference element are presented in Table 2. The enrichment factors are calculated on the basis of the mean crustal abundance of Taylor (164). Also given in Table 2 are enrichment factors for aerosol collections from Chester and Stoner (14) for Atlantic Ocean samples, Duce et al. (15) for Atlantic Ocean samples, and Zoller et al. (14) for South Pole aerosol samples. TABLE 1. Elemental concentrations in Milcent snow and ice samples (all concentrations in fig/kg) Sample Depth [m] Age Na CI Al V Mn Zn Hg Pit 2 Pit 3 Pit BD 2 BD Assumes radiochemical yield of Below detection limit of 6 ng. 3 Estimated age. TABLE 2. Taylor (164) Enrichment factors at Milcent based on average crustal abundances of Enrichment Factors Sample Al V Mn Zn Hg Average Average South Pole 2 Atlantic Ocean 30 N 3 Atlantic Westerlies ± ± ± ± Assumes average Al concentration of 8.5 /xg/kg in the Milcent core (J. Cragin, personal communication). 2 From Zoller et al. (14). 3 From Duce et al. (15). 4 From Chester and Stoner (14). The Hg concentrations are the highest yet reported for Greenland snow and ice, and there is no indication of significant Hg input in modern times. One interpretation is that the hot nitric acid treatment of the linear polyethylene bottles prevented adsorption on the container walls and resulted in more representative concentrations. Sample contamination is not considered very likely as samples from Point Barrow, processed and analysed identically to the Milcent samples, showed low and constant concentrations. The storage containers for the study of Weiss et al. (11a) were prepared using a cold nitric acid leaching

4 Vanadium and other elements in Greenland ice cores 101 (Murozumi et al., 16) and the samples were stored unacidified for three years in liquid state prior to analysis for Hg. In the study of Weiss et al. (15) a cold acid leach was used and samples were analysed 12 h after melting (Weiss and Bertine, 13). Bottle preparation was not discussed in the paper by Carr and Wilkniss (12). The crustal enrichment factors for V and Mn in Milcent ice are near 1 and do not show any increase in modern deposits. This suggests a common origin due to crustal weathering. Duce et al. (15) reported a vanadium EF of 1 for aerosol samples collected at 30 N over the Atlantic Ocean attributable to the burning of V-enriched fossil fuels. Zoller et al. (14), on the other hand, found a vanadium EF of for aerosol samples at the South Pole. The fact that fossil fuel-derived V does not appear to reach the polar regions may be related to the relatively low volatility of V compounds. The pre-100 zinc EF in Milcent ice is 12, increasing to 32 for snows. This indicates not only that zinc is selectively volatilized, transported to and/or precipitated in Greenland, but also that the atmospheric load of Zn has increased in modern times. The fossil fuel mobilization of V and Zn has been estimated at 12 and x 10 g/year respectively (Bertine and Goldberg, 11). Yet the Milcent evidence suggests that industrial Zn and not V reaches the Greenland ice sheet. At present it appears that elemental enrichment factors in pre-100 Greenland ice may be explained adequately in terms of crustal abundances and relative volatilities. Thus Hg and Zn have high EFs while those of V and Mn are near unity. Apparently the presence or absence of a pollution fraction in modern snows is also affected by relative volatilities. Efforts to expand the list of enrichment factors of ancient and modern precipitation are presently underway. Acknowledgements. The authors gratefully thank E. D. Goldberg and M. Koide for laboratory use. This work was supported by National Science Foundation's Office of Polar Programs. REFERENCES Bertine, K. K. and Goldberg, E. D. (11) Fossil fuel combustion and the major sedimentary cycle. Science 13, Carr, R. A. and Wilkniss, P. E. (12) Mercury in the Greenland ice sheet: Further data. Science 181, Chester, R. and Stoner, J. H. (14) The distribution of Mn, Fe, Cu, Ni, Co, Ga, Cr, V, Ba, Sr, Sn, Zn and Pb in some soil-sized particulates from the lower troposphere over the world ocean. Mar. Chem. 2, Cragin, J. H., Herron, M. M. and Langway, C. C. Jr (15) The chemistry of 00 years of precipitation at Dye 3, Greenland. US Army Cold Regions Research and Engineering Laboratory Research Report Duce, R. A., Hoffman, G. L. and Zoller, W. H. (15) Atmospheric trace metals at remote northern and southern sites : Pollution or natural? Science 18, Koide, M. and Goldberg, E. D. (11) Atmospheric sulphur and fossil fuel combustion. /. Geophys. Res. 6, Langway, C. C. Jr, Herron, M. and Cragin, J. H. (14) Chemical profile of the Ross Ice Shelf at Little America V, Antarctica. /. Glaciol. 13, Murozumi, M., Chow, T. J. and Patterson, C. (16) Chemical concentrations of pollutant lead aerosols, terrestrial dusts and sea salts in Greenland and Antarctica snow strata. Geochim. et Cosmoch. Acta 33, Taylor, S. R. (164) Abundance of chemical elements in the continental crust: A new table. Geochim. et Cosmoch. Acta 28, Weiss, H. V., Koide, H. and Goldberg, E. D. (11a) Mercury in a Greenland Ice Sheet: Evidence of recent input by man. Science, Weiss, H. V., Koide, M. and Goldberg, E. D. (11b) Selenium and sulphur in a Greenland Ice Sheet. Science 12,

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