Journal of Paleolimnotogy 9: t61-t78, 1993. © 1993 K&wer Academic Publishers. Prinwd in Belgium. 161 How reliable is the 21°Pb dating method? Old and new results from Switzerland* H. R. von Gunten ~ & R. N. Moser Laboratorium fiir Radiochemie, UniversMit Bern, CH-3000 Bern 9, Switzerland," 1Paul Scherrer lnstitut, CIt-5232 Viltigen PSL Switzerland Received 27 January 1992; accepted 21 May I993 Key words: 2~°Pb dating, geochronology, sedimentation rates, ~37Cs, Switzerland Abstract We present a historical overview of applications of 21°Pb dating in Switzerland with a special empha- sis on the work performed at the University of Bern. It is demonstrated that the average specific activity of 21°Pb in the lower atmosphere is very constant and does not show seasonal variations. We then concentrate on new results from Lobsigensee, a very small lake, and on published and new data from Lake Zurich. Several 21°pb profiles from these lakes show obvious disturbances and a disagreement of the resulting sedimentation rate when compared to that for the 23 years defined by 137Cs peaks of 1986 (Chernobyl) and 1963 (bomb fallout). A mean sedimentation rate of about 0.14 g cm ' a y- i is found in the oxic and suboxic center part of Lake Zurich. In the oxic locations, the al°Pb flux to the sediments was close to the atmospheric input of about 1/60 Bq cm- 2 y- 1. In other parts of the lake a significant deficit in the inventory of 21°Pb was found in the sediments. This could be due to a chemical redissolution of 2~°Pb together with Mn under reducing conditions. In contrast, in the suboxic part of the lake (t35 m depth) the flux of 2~°Pb was about twice the atmospheric input. This excess is not caused by altochthonous contributions and is tentatively explained by the transport of sediment material resulting from small slides at the very steep lake shores or more probably by reprecipitation of 2~°Pb together with Mn when the conditions in the lake water become locally and seasonally more oxidizing. Dissolved 2~°Pb may migrate from locations with reducing conditions and reprecipitate under more oxic conditions. Indeed, a correlation of Mn and 2~°Pb in sediments of Lake Zurich was found. Introduction information and can be considered as environ- mental archives. Dated samples from these ar- The sediments of lakes and materials from other chives, combined with measurements of other pa- environments which accumulate with on-going rameters (chemical, physical, biological), allow time (e.g. glaciers, peats) contain valuable historic studies of environmental changes. Lead-210, a member of the 23*U decay series, provides a re- * This is the first of a series of papers to be published by this journal following the 20 ta anniversary of the first application liable method of dating sediments deposited of 21°Pb dating of lake sediments. Dr P. G. Appleby is guest during the last 100 years, a time period which editing this series. includes the most drastic changes ever produced 162 in the environment by human activities. Goldberg use of the direct gamma-spectroscopic method (1963) was the first to use the radioactive decay for the measurement of 21°Pb (G~tggeler etal., of'unsupported' 21°pb, to date glacier ice. Several 1976). years later Krishnaswami et al. (1971) applied this The first application of the 21°Pb dating tech- method to lake sediments. nique in Switzerland was performed in Greif- Interest in applications of 21°pb started at the ensee, a small lake (G~iggeler et al., 1976). Here, University of Bern, Switzerland, in the early the direct gamma-spectroscopic method was 1950's, i.e. 10 years before the mentioned contri- tested. In the following years 2~°Pb was used bution by Goldberg (1963) and has continued to to date sediments of several Swiss lakes, includ- play a significant role among the activities of our ing again Greifensee (Bergerioux et al., 1980; laboratory. Houtermans (1951) proposed the use Dominik etal., 1981; Bloesch & Evans, 1982; of the specific activity of 2~°Pb in lead in secular Erten etat., 1985; von Gunten et aL, 1987; Wan equilibrium with 23aU ('supported' 21°Pb) to as- etal., 1987; Dominik etal., 1989; Moser etal., sess mineral ages. Under the assumption of a 1991) and ice cores from an alpine glacier completely closed system and a known isotopic (Gfiggeler et al., 1983). The locations of the ap- composition of lead, it is possible to date minerals plication of the 21°Pb method in Switzerland are with one single measurement of the specific ac- shown in Fig. 1. tivity of 21°pb in a chemically separated lead Recently the unambiguous validity of the 210pb sample (Begemann et aL, 1953). In other appli- dating technique has been questioned by several cations Begemann etal. (1954) and Eberhardt authors (Erten etal., 1985; Benoit & Hemond, etal. (1955) have used 21°pb and other lead iso- 1990, 1991; Moser et al., 1991) who pointed out topes for an insight to volcanic processes. Later, that 2t°pb and also 2t°po can be remobilized in the work on 21°Pb was continued in our labora- fresh water under certain conditions. Thus, one of tory in an effort to improve the value of the half- the conditional assumptions for dating, namely life of 21°pb (yon Gunten et al., 1967). The result that lake sediments are closed systems for 21°Pb of this determination (22.2 + 1.0 years) is in per- and its daughters, is not always fulfilled. Possible fect agreement with the now recommended half- explanations for a remobilization of these ele- life of 22.26 _+ 0.22 years (HOhndorf, 1969) and ments are bioturbation, turbidites and redissolu- 22.3 y (Seelmann-Eggebert et al., 1981). Further- tion during chemical changes in the system. more, we were the first laboratory to propose the In the present paper we summarize our earlier _ ~ 1 Lake Geneva ~" 8 ~ 2 Lake Morat g~ -- 6 .,-. / 3 Lake Bienne J ~\®7 ~_ 4 Lake Lobsigen ~@5 ~ ~5 Lake Lucerne .~ @~2 .-. ..- , , ~" ]6 Lake Zurich ~ lO bwffzerlana f.~7 Greifensee (1~ .~I {~ ..,~ 8 Lake Constance E~ } d k / ~ ~"~ 9 Colle Gnifetti -- ~,~ J % ~" (Ice Core) ~9 *~ 10 Fribourg • ~ (Air Samples) Fig, I. Locations of the 2]°pb studies in Switzerland. 1) Dominik et aL (1989); 2) Bergerioux et aL (1980); 3) Mfiller (1982); 4) this work; 5) Bloesch et al. (1982); 6) Erten et al. (1985), Moser et aL (1991) and this work; 7) Gaggeler et al. (1976) and Wan et al. (1987); 8) Dominik et al. (1981) and von Gunten et al. (1987); 9) G~,ggeler et al. (1983); 10) this work. 163 work of 21°pb dating in Switzerland (including an nique (B. Ammann, personal communication). example from a glacier) and supply published One gram samples of the sediments were digested (Erten et al., 1985; Moser et aI., 1991) and new with acid, having known amounts of 2°8po or data from Lake Zurich which point to problems 2°9po tracers and stable Pb carrier present. Lead- in the applicability of the 2mpb method. We also 210, 2~°Bi and 21°po were separated on a Dowex present as yet unpublished data of air samples 1 x 2 (50-100 mesh) ion exchange column (De which confirm a constant long-time specific ac- Oliveira Godoy, 1983; Moser, 1989). The three tivity of 21°pb in the lower atmosphere of Swit- radionuclides, dissolved in 0.75 M HBr were zerland, an essential condition for the applicabil- sorbed onto the resin. Lead-210 was eluted with ity of the 210pb method. Finally we summarize the 2 M HNO 3, 21°Bi with conc. HC1 and al°Po with measurements of sedimentation rates, fluxes and conc. HNO 3. The chemical yields were 75 70 for inventories of 2~°pb in Swiss lakes. 21°po and 9070 for 21°pb. Polonium-210 was electroplated from a dilute solution (Figgins, 1961). In several samples from two locations in Experimental Lake Zurich the radioactive equilibrium between 21°pb, 21°Bi and 2~°Po was tested. The results of We report here only on those experimental pro- these measurements (see Table 1) show that cedures which were applied to produce the new radioactive equilibrium between the three nuclides sets of data. The methods used in the earlier mea- was generally achieved. surements can be found in the relevant references. Air samples Monthly dust samples were collected on the roof Sampling and sample treatment of a building in Fribourg, Switzerland, between 1973 and 1983 by pumping 6000-10 000 m 3 of air Lake sediments through cellulose fiber filters. The sampling locations of the earlier and the present work in Lake Zurich are shown in Fig. 2. All sediment cores were recovered using gravity Radioactivity measurements corers with transparent PVC-tubes of 6.3 and 12 cm inner diameter and having lengths of 30 to Lead-210 was measured via its daughter 21°Bi 60 cm. The coring operation was controlled by a (tl/2 5d) on a low background proportional Sonar system. Disturbance of the surface layers counter or by gamma-ray spectroscopy of the of the sediments during coring was negligible. We 47 keV 7-line on Ge(Li) detectors of the planar or found in the uppermost layers 7Be, 137Cs (Cher- well type. The air filters were measured on the nobyl) and often compact carpets of Beggiatoa. planar detector, VBe and 137Cs in the sediments Immediately after recovery, the cores were sec- on a well detector. Polonium-210 was assayed by tioned in 0.5- and 1-cm intervals.