Dating of Greenland ice cores by microparticle concentration analyses. C. U. Hammer

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1 Dating of Greenland ice cores by microparticle concentration analyses C. U. Hammer Abstract. Seasonal variations of microparticle concentration in 6000 samples were compared with S( 18 0) and gross ^-activity analyses and may be used for dating beyond the ranges of the latter techniques. Datation des carottes de glace en Groenland par analyses de la teneur en microparticules Résumé. Les variations saisonnières de la teneur en particules mesurées dans 6000 échantillons sont comparées avec les analyses en S( 18 0) et activité j8 totale; elles peuvent être utilisées pour dater au-delà des possibilités offertes par ces deux dernières techniques. INTRODUCTION A reliable time scale is the fundamental basis for all kinds of physical and chemical studies on ice cores. Radioactive dating is often too inaccurate, and counting seasonal stable isotope cycles from the surface down to great depths is possible only on polar glaciers with a high accumulation rate (250 mm water equivalent per year). In low accumulation areas diffusion in the firn results in gradual isotopic homogenization of the snow pack (Johnsen, 1977). Since dust particles do not diffuse, the seasonal dust concentration variations in the deposits, first demonstrated by Hamilton and Langway (1967), may be used for accurate dating where the isotopes fail. TECHNIQUE A check on the applied preparation technique was carried out as follows : Water filtered by a 0.45 fim millipore filter was frozen. The artificial ice core was stripped by a razor blade leaving only the central part for the sampling. This was done at - 10 C in a laminar air filter bench by a special quartz heating device. The meltwater was transferred into plastic beakers by a glass pipette and kept at 10 C until analysis within 5 h. No contamination was observed. The samples were analysed by the standard Coulter counter (model Z B I) and/or a light scattering technique that offered the following advantages : (1) Very small samples are needed (0.2 g, i.e. 25 to 50 times less than the Coulter technique). (2) No addition of chemicals is needed. (3) The analysis is faster and can be performed under normal laboratory air conditions. (4) The signal includes light from particles with radii less than 0.35 /xm, which is the effective Coulter counter limit. On the other hand, it is a disadvantage that no size distribution is obtained, at least when only one scattering angle (90 C) is used. Yet, some information is gained when combining with Coulter measurement in just one size interval (Hammer, in preparation). All Coulter counter measurements are given in terms of the number of particles with radii larger than 0.5 pm per gram (N 0.Jg). The light scattering results are 297

2 298 C. U. Hammer given in terms of millivolt amplifier output, 1 mv corresponding to N 0. s /g approx , somewhat dependent on the size distribution. OBJECT Most of the ice cores studied in this work were recovered under the USA- Denmark-Switzerland joint Greenland Ice Sheet Programme. Table 1 lists the sites of recovery, the time intervals studied, and sampling frequencies (sampling frequency applied; usually number of samples per annual layer as calculated from mean accumulation rate, density, vertical strain etc.). TABLE 1 North Century Dye 2 Dye 3 Milcent Crête Summit Site Camp Position 66 N46 W 65 N 44 W 70 N 45 W 71 N 37 W 72 N 38 W 75 N 42 W 77 N 61 W Period various Sampling frequency variable RESULTS In Fig. 1 Coulter counter and light scattering data from a pitwall at Crête are shown along with corresponding values of total j3-activity and S( 18 0). Evidently the two series of dust data exhibit the same seasonal pattern. The deposition of dust exhibits some common features at all stations such as a maximum in late winter/early spring and a minimum in autumn/early winter, to judge from corresponding S( 18 0) variations (Figs. 1-5). On the average, the dust peak is found approximately one third of the annual layer thickness above the S( 18 0) minimum. It appears from Fig. 4 and, particularly, Fig. 1 that the dust peak does not occur simultaneously with maximum fallout of total js-activity, which suggests that, unlike the fission products, the airborne dust has not been transported to the ice sheet via the stratosphere. (The measurements were performed on the same core.) The reason for the dust peak early in the year is probably because the westerlies are regularly blocked by a quasistationary anticyclone in spring, causing strong meridional wind components either at the same time as, or a little after, the dry season sets in over North America. The 398-m deep ice core from Dye 3, south Greenland, was initially dated by measuring seasonal S( ls O) variations (743 cycles). The interpretations of the isotope as well as the dust profile are ambiguous for some 15 per cent of the cycles. However, the ambiguities are randomly scattered in both of the profiles, which makes a cross dating beneficial, leaving doubt about only 2 per cent of the cycles. Consequently, the accuracy of the resulting time scale is estimated at ± 2-3 years at AD 1600 and ± 4 years in the thirteenth century. Figure 2 shows a few seasonal 8( 18 0) and dust-cycles close to the surface. The dating is a result of a cross check on ^-activity, S( 18 0) and dust profiles. At North Site, where the accumulation rate is only 140 mm water equivalent per year the 8( 18 0)-profile is smoothed by diffusion, and to a higher degree than it appears in the figure, because the two annual layers indicated by arrows in the dust profile are more or less obliterated in the S( ls O)-profile. It is tempting to reconcile the highest dust peak with the Hekla eruption in spring At

3 Dating of Greenland ice cores 299 Dye 2 the S( 18 0)-profile is shortly disturbed by percolation, according to js-measurements (not shown in the figure), but it is interesting that the dust profile seems less disturbed, possibly because the summer layers that melt are low in dust. Finally, preliminary analyses have been performed on the Camp Century deep ice core. Figure 5 shows the dust profile along a 30-cm increment from 1159 m NORTH SITE %«6(o 18 ) Crête MILCENT %o 6(o 18 ) mV DYE 2 %o &(o 18 : dph/kg "102 Total 8 activity 90 light scattering Bxsptti FIGURE 1. Total j8-activity, particle counts, 90 light scattering and 8( ls O) at station Crête in the period AD. FIGURE 2. Seasonal S( l s O) and dust concentration variations at North Site, AD, Milcent, AD, and Dye 2, AD. depth, corresponding to an estimated age of years according to flow model calculations by Dansgaard and Johnsen (1969). Counting peaks leads to an estimated annual layer thickness (A) of mm, in essential agreement with the theoretical value 21 mm. Results on other increments are shown in Table 2. So far, they are much too sparse to justify any conclusion as to the Camp Century time scale.

4 300 C.U. Hammer SUMMIT TOTAL-B DUST Sto 18 ) ACTIVITY " J dph/kg FIGURE 3. Seasonal 3( 18 0) and dust concentration variations at Dye 3 in the period AD. FIGURE 4. Total ^-activity, seasonal S( w O) and dust concentration variations (90 light scattering) at Summit, AD.

5 Dating of Greenland ice cores 301 FIGURE 5. Seasonal dust concentration variations at Camp Century, BP. TABLE 2 Depth [m] Age [years] mm ice measured calculated Number of years measured CONCLUSION Seasonal variation of the deposition of microparticles makes it feasible to accurately date firn and ice cores from most of the Greenland ice sheet, > particularly when the analyses are combined with p and S( 18 0)-measurements. The range of dating exceeds that of the stable isotope method, particularly in low accumulation areas. Acknowledgements. This study was funded by the US Advanced Research Projects Agency (contract DA-ENG G42) and by The Danish Natural Science Research Council. REFERENCES Dansgaard, W. and Johnsen, S. J. (1969) A flow model and a time scale for the ice core from Camp Century, Greenland. J. Glacial. 8, Hamilton, W. L. and Langway, C. C. (1967) A correlation of microparticle concentrations with oxygen isotope ratios in 700 year old Greenland ice. Earth Planet. Sci. Lett. 3, Johnsen, S. J. (1977) Stable isotope homogenization of polar firn and ice. In Isotopes and Impurities in Snow and Ice (Proceedings of the Grenoble Symposium, August-September 1975), pp : IAHS Publ. no. 118.