How Reliable Is the <Superscript>210 </Superscript>Pb Dating Method

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 environmental 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. The water con- ~-spectroscopy. Proper corrections were made tent and the densities of the dry sediment samples for decay after sampling, and during sample were used to assess sediment compaction. An preparation and measurements. The 'supported' accurate pycnometric determination is important 2mpb activity (in equilibrium with 226Ra of the because the dry density varies considerably sediments) was obtained from the constant ac- (2 g cm- 3 at the top, 2.6 g cm- 3 at the bottom of tivity of the deepest samples in the cores and was the cores). These changes in density are due to subtracted from the measured total activities. In decreasing concentrations of organic matter with several samples the 226Ra activity was also mea- depth; the changes are also manifested by coIour sured on a proportional counter or by the y-rays changes. The samples from Lobsigensee were re- of 2~4pb using a Ge(Li) well detector. For these covered in October 1987 by a freeze-drilling tech- measurements the samples were sealed in the 164

ZLirich

Tiefenbrunnen

r!km~

Distance (kin) 0 0.5 .5 2.5 20 I- cross-section I / Herrliberg .--E 60

OberriedeJ ~3 100 \~l lasm/! 140

Fig, 2. Locations of the 21°pb studies in Lake Zurich, Insert: Cross section throttgh the lake at Oberrieden - Herrliberg with sampling positions~ Cores from 34 m depth and from ttle cross section (Moser e~ aL, i99I and this work), from 61 m depth (Erten et al., 1985). measuring tubes (10 cm length, 0.9 cm dia). These air. These data are followed by an account of activities agreed with the 'supported' activity in earlier and of recent applications of the 21°pb the deep samples. The determination limits dating method in our laboratory, Please refer to (Currie, 1968)were 1 mBq, 30 mBq and 170 mBq, Fig. 1 for the localities of sampling. for ;l°Po, al°Bi and 21°Pb, respectively (7 days counting). 2z°Pb concentratio~zs in air samples

Results and discussion A very basic assumption of the 21°pb dating method is a constant flux of 21°Pb to the envi- In the following we present long-term measure- ronmental archives. This flux depends i)on the ments of the specific 2mpb activity (Bq m- 3) in dry and wet fallout of 2mPb from the atmosphere 165

Table 1. Comparison of activity measurements in the uppe "- results of these measurements are presented in most sediments of Lake Zurich. Pb-210 by 47 keV ?-ray, Table 2. There are considerable and irregular Bi-210 by fi-counting and Po-210 by c~-spectroscopy (activities in dpm g- i, dpm = decay per minute). changes in the activities of the monthly samples. These variations are due to the prevailing local Mass Activity conditions, and to weather and wind patterns on depth Pb-210 Bi-210 Po-210 a larger scale. No significant correlations were (g cm 2) found either, with the amount of precipitation or Tie~nbrunnen 0.16 9.1±1.7 9.6±0.3 9.5±0.3 with the ambient temperature. Despite the large July89, 0.36 9.7±1.6 9.3±0.3 10.8±0.3 variability of the monthly samples (130-1250 #Bq oxic 0.85 10.6±1.9 9.3±0.3 9.2±0.3 m- 3) one observes a remarkable constancy of the 1.47 7.9±I.5 7.0±0.3 8.5±0.3 values (460 ± 20 #Bq m- 3) if the annual means are considered. A similar constancy is obtained in Center of Lake 0.09 24.6_+ 2.2 23.0 ± 0.6 23.7 + 0.7 July 89, 0.20 i8.3 + 2.4 22.5 ± 0.6 22.4_+ 0.5 the 11-year means of each month (e.g. for the suboxic 0.50 8.5k2.1 15.6+0.4 14.2±0.4 mean of all values of January, or February, etc.). 0.77 16.5 ± 2.4 I6.5 + 0.5 13.6 ± 0.4 Therefore, one can conclude that in the near- surface air no seasonal variations of the specific 21°pb activity occur. This contrasts strongly with and ii)on the transport of 21°pb in dissolved 21°Pb in air at higher altitudes which shows a very and/or particulate form from the catchment area expressed seasonal variability (Gfiggeler etal., to the investigated system. Contribution (i) is 1991, see also below). Unfortunately, our 2I°pb easier to quantify than contribution (ii). measurements of the air filters do not allow cal- We have measured the dust-bound atmo- culation of the natural flux of 21°pb close to the spheric concentrations of al°pb during 11 years earth surface, nor do they allow estimation of the by pumping large volumes of air through filters respective contributions by wet or dry precipita- which were changed at monthly intervals. The tion.

Tab~ 2. Activity ofPb-210 (#Bqm 3)in the air of Fribourg, Switzerland. Monthly samples colected on filters between 1973 and 1983.

Year Monthly mean 1973-1983 1973 t974 1975 1976 1977 1978 1979 1980 I981 1982 1983

January 760 400 360 300 430 340 450 570 370 530 380 440 ± 40 February 380 420 670 770 220 530 370 490 720 770 530 530 + 50 March 490 660 330 540 400 190 190 370 130 470 410 380 ± 50 April 210 730 280 460 260 340 260 360 630 380 190 370 _+ 50 May 330 360 460 410 280 350 320 350 270 360 230 340 ± 20 June 410 180 320 520 280 420 380 130 340 380 340 340 ± 30 July 420 420 480 300 350 440 390 260 330 550 770 430 ± 40 August 800 620 380 580 380 470 390 460 610 1250 680 600 _+ 80 September 610 600 670 290 530 450 650 240 530 940 560 550 ± 60 October 560 200 550 700 590 640 680 970 290 290 770 570 ± 70 November 440 370 420 380 380 1000 370 510 510 500 870 520 ± 60 December 420 210 630 540 460 650 370 240 240 450 580 440 ± 50

460 ± 30

Annual 490 430 460 480 380 490 400 410 410 570 530 460 mean ± 50 _+ 50 ± 40 ± 50 _+ 30 + 60 _+ 40 ± 60 ± 50 ± 80 ± 60 _+ 20 210pb activity [dpm/kg]

..,,k % 0 0 C:) O I f t I I I I , , , ,, I\\t , , l i I l I I ~ I I

0 0 n ¢'1)

::r r--1 3

O-

F-

O I I i I,If i , I , I ,,I\\l l i I i I , tl I I , I I I,I I t 167

Dating with e~°Pb concentrations) should lead to relatively low deposition rates compared to periods with a pre- In the following we present examples of 2~°pb dominantly continental climate. A mean accumu- applications which were performed in collabora- lation rate offirn of 32 cm y- 1 resulted from these tion with others at the Laboratorium ftir Radio- measurements. This accumulation rate is in good chemic of the University of Bern, Switzerland. In agreement with values obtained from tritium the first part we reproduce two published appli- (1963) and from Sahara dust layers (G~tggeler cations of 21°Pb, namely the dating of ice of a cold et aI., 1983). The results from this ice core dem- (T < 0 ° C) alpine glacier and of sediments from onstrate that dating of cold alpine glacier ice is Lake Constance. In these examples the 2~°pb possible with the 21°pb method. method works very satisfactorily. The third in- The measurements of 2mpb in the Colle Gnifetti vestigation fi'om Lobsigensee, a small eutrophic ice samples show, furthermore, that the inventory lake, is new. Here the 21°pb method is not appli- and the mean flux of 2~°Pb is about 3.6 dpm cm - 2 cable. The final examples are from Lake Zurich and 0.11 dpm cm-2 y-1, respectively, which is and show with published and new results that much lower than for the lakes in the Swiss planes. dating with 21°pb has to be interpreted with great Several processes contribute to this low value. care. i) Colle Gnifetti is exposed to strong winds which remove about one third of the annual snow de- Ice of a cold alpine glacier posits, ii)The concentrations of 21°pb in air are Figure 3 shows the results of the 2~°Pb dating lower at higher elevations and iii) G~ggeler et al. method applied to a 65-m long ice core from Colle (199t) have demonstrated on Jungfraujoch Gnifetti (Monte Rosa, Switzerland, 4450 m alti- (3400 m), Switzerland, a very expressed seasonal tude, G~iggeler et al., 1983). The temperature of variation of the 21°Pb concentrations in air the tim/ice was about -15 °C, thus excluding (200 #Bq m - 3 in summer, 30 #Bq m- 3 in winter). melting processes which could lead to a migration of 21°pb with percolating water. Based on the low Dating sediments of Lake Constance temperature the assumption of a closed system is In Fig. 4 we present dating results of an oxic sedi- fulfilled and the decay of 2a°Pb follows approxi- ment core of Lake Constance (von Gunten et al., mately an exponential curve. The flatter part 1987). The decay of 21°Pb is exponential in this above a depth of 12 m water equivalent (w.e.) and core. A mean sedimentation rate of 0.11 + 0.02 g the steeper decay of the activity of 21°pb below cm- 2 y- 1 results from a least square fit through 20 m w.e. can be reproduced reasonably well the data. This value agrees with sedimentation by a model which accounts for an effect of de- rates deduced from the 1963 bomb peak of 137Cs. creasing annual ice layer thickness with depth The sedimentation rate obtained with the ~37Cs (G~iggeler et at., 1983). This eft?ct has been dem- marker depends on the assumption of a complete onstrated in a glaciological model (Haefeli, 1961). core recovery (von Gunten et al., 1987). This was The scattering of the individual data can tenta- assured by measuring 7Be (tl/2 53d) in the top- tively be explained with variations in the supply most sediments and by a 21°pb inventory of of 21°pb. Colle Gnifetti is situated on top of the > 9070 of the atmospheric input. The excellent alpine range and is reached by air masses of mari- agreement between the values obtained from time and continental origin. Therefore, periods the CRS (constant rate of supply, Appleby & with a mainly maritime climate (with lower 21°Pb Oldfield, 1978) and CIC (constant initial concen-

Fig. 3. a. 21°Pb activity in a 65-m long firn/ice core from Colle Gnifetti (Monte Rosa), Switzerland. Core depth given in water equivalents (i.e. corrected for the density of the ice). b. Data of Fig. 3a averaged for core sections of 2 m water equivalent. Solid lines represent least square fits to the data points. Reproduced by courtesy of the International Glaciological Society from Jour- nal of Glaciology, Vol. 29, No. 101, 1983, p, 165, Fig. 5. 168

=- 21°pb Activity (dpm g-l)

01 0.2 0.5 1 2 5 10 [ I "[ ' ' ' I '"'''1 '

1970

2- ' 1960 3 9 Ipkm "~~R~hein

1950 4

1940 A 5 E O 1930 6 (-

e0 1920 7 I

C~ 8 1910

9- 1900

10- 1890

L L ~ I , , t , 1, ~ ~ ~ I ,

Fig. 4. Activities of 'unsupported' 21°pb in a sediment core from Lake Constance. Solid line: least square fit to the data points (von Gunten et al., 1987). Courtesy Birkh~tuser Verlag, Basel, Switzerland. tration) models for several cores (Table 3) con- Constance prove that dating with 21°pb furnishes firms the constancy of the accumulation rate for correct results for oxic and undisturbed lake sedi- the period assessed by these measurements. Fur- ments. At the coring location 21°pb results almost thermore, comparable results for the sedimenta- completely from atmospheric input, allochtho- tion rate were deduced fiom a significant turbidite nous contributions are negligible. layer of the year 1900 and from settling sediment particles collected in pans (sediment traps) at e~°Pb measurements in Lobsigensee various depths in the water column (von Gunten Lobsigensee is a small shallow lake with a surface et aL, 1987). The results of the samples of Lake area of about 0.04 kin2; it is situated about 20 km 169

Table 3. Comparison of sedimentation rates in Lake Con- stance by the CRS and CIC models (P.G. Appleby, personal 1986 communication). / lo Sample Sedimentation Sedimentation rate rate CRS model CIC model (gcm 2y-,) (gcm-2y-1)

SM1 (Dominik et al., 1981) 0.125 0.112 SM2 (Dominik et al., 1981) 0.086 0.097 B (von Gunten et aL, 1987) 0.107 0.123 i 133 I / 1963 & E

NW of Berne, Switzerland. The lake is sur- # I ._> I rounded by fertilized farm land and is highly I eutrophic. A core of 35 cm length has been ex- 4 I tracted for 21°pb and 137Cs measurements. The 4x results of these determinations are shown in I I Fig. 5. It is evident that 2mpb activities remained I I rather constant within + 20 per cent. We believe I that an intensive bioturbation is responsible for I I the observed 21°pb and 137Cs activity pattern in tl these sediments with a high content of organic matter and nutrients for (micro)-fauna. The vari- 2 i I I ability in the activities of 21°pb may reflect varia- o lO 20 30 4C tions in the growth and decay of aquatic biota Depth (cm) which depends in part on external parameters. It Fig. 5. Activities of 'unsupported' 21°pb (m) and 137Cs (A) is obvious from Fig. 5 that the 2mpb method is in sediments from Lobsigensee, a small eutrophic lake. The not applicable to date these sediments. Here, two 137Cs peaks correspond to 1963 (bomb fallout) and 1986 the ~37Cs markers may be more appropriate for (Chernobyl). Both, 21°pb and 137Cs are affected by bioturbation. Depth not corrected for density variations. dating, even considering the very broad and dis- turbed peak shapes. The recovery of sediment cores from shallow lakes is not a complicated resulting from this and a few very small rivers is task and sampling losses at the top of the cores negligible. The take has very steep shores and a are expected to be negligible. flat region of about one kilometer between the shores (see Fig. 2). The lake sediments are oxic in Dating sediments of Lake Zurich the more shallow parts and suboxic in the deeper A considerable effort has been undertaken to date locations. The dimensions of Lake Zurich and sediments of Lake Zurich which is an important estimated residence times for several radionu- environmental archive due to its location in a clides are shown in Tabte 4. highly populated and industrialized area. Lake Erten et al. (1985) have measured 21°pb pro- Zurich is well suited for these investigations be- files of three sediment cores which were extracted cause it receives the major input of 2mpb directly in 1979 and 1981. They observed (see Fig. 6) an from atmospheric fallout. The main inflowing extended plateau region in the 21°pb activity at River Linth passes through another lake (Walen- the top of the cores corresponding to about 25 see) and enters (as Linthkanal) a part of Lake years of sediment accumulation. In the deeper Zurich which is separated by a dam before arriv- part of the core the 2mpb activity decayed expo- ing at the main lake. The transport of particles nentially with depth. An inventory of only about 170

Table 4. Dimensions of Lake Zurich and residence qmes for some radionuclides (from Wieland et al, (1991) and Santschi et al. (1990)).

Lake Zurich Surface Drainage Maximum Average Water Pb-210 Po-210 Cs-137 basin depth depth residence time residence time residence time residence time

67.2 km 2 1892 km 2 137 m 49 m 1.0 y 1.0 month 10-26 months 7 months

50 per cent of the atmospheric 21°pb fallout was the interstitial water of sediments (P. Santschi, found. These authors explained the very large personal communication). This result supports 21°Pb deficit and the plateau-like region at the top the hypothesis of a remobilization of 21°pb. An- of the 21°pb activity profile by a remobilization other explanation for the observed plateau region process which occurs under suboxic or anoxic of the 21°Pb activity profile could be an accelera- conditions of the sediments. Mixing of the top- tion of the sedimentation rate in more recent most layers of the sediments (Robbins etal., years. This assumption is in agreement with the 1977), hereby producing a plateau in the 21°Pb somewhat higher sedimentation rates deduced profile, was excluded as an explanation due to from counting of the annual layers within the several facts given by Erten etal. (1985). In ad- sediments (distinct light and black laminations dition, traces of dissolved 21°pb were identified in which correspond to summer and winter depos-

= Unsupported Pb-21o Activity(dpmlg)

10-1 10 0 101 1979 1970 ~ "-" 19631_~ 1960 1950 ! 1940 137Cs 1930 U) 1920 0,. "0 1910 :T

¢JO 1900

3 5 ! 1890 b~ v I Core I 1880 6 - 1870 1860 7

Fig. 6. Activities of 'unsupported' 21°pb and 137C$ in a core from Lake Zurich (Erten et al., 1985). Solid lines: least square fits to the data points. Note the constant 21°pb activity in the uppermost 6 cm (1.5 gcm- 2). The undisturbed peak of 137Cs (units in dpm g- ~) rules out mixing at the top of the sediments. Courtesy of Birkh~iuser Verlag, Basel, Switzerland. 171 its, respectively; Erten et al., I985). Calculations time markers in the sediments representing a span with the CRS model using the data of Erten et al. of 23 years. These markers allow measurement (1985) support the assumption of a higher sedi- of sedimentation rates for this period with high mentation rate over the past 60 years, i.e. since accuracy. Furthermore, different sediment cores ca. 1920 (P. G. Appleby, personal communica- can be compared unambiguously by these mark- tion). However, the very large deficit in the in- ers. Data of the 137Cs measurements in the inventory of 21°pb at this location (see Table 5), vestigated core samples of Lake Zurich are pre- contradicts a faster sedimentation in more recent sented in Table 6. Problems in the 21°Pb method times. Despite the problems with the plateau re- are easily recognized if the sedimentation rates gion which cannot be explained unambiguously, from this method are compared with those result- dating of these cores by the 2~°Pb method was ing from the 137Cs markers (Table 5). Possible possible and a reasonable agreement with the explanations for deviations :from the real sedi- 137Cs time marker of 1963 was established mentation rate can then be sought. (Table 5). Figure 7 shows 21°pb (21°Po) profiles of Lake In order to verify the remobilization postulate Zurich sediments from four different locations of Erten etal. (1985), we measured additional (see Fig. 2) and from different sampling dates. 21°Pb profiles in several sediment cores of Lake The water depth at the sampling locations varies Zurich (Moser et at., 1991 and new data). These from 34m to 135 m. The samples from Ober- cores are from different locations in Lake Zurich rieden, Herrliberg and the Center of the lake be- (see Fig. 2) and were taken at various times of the long to a cross section through the lake. The sam- year. pling locations and the seasons influence the The Chernobyl reactor accident in 1986 has chemical conditions (e.g. the redox potential) at produced an additional pronounced activity peak the bottom of the water column and probably of 137Cs. This peak can easily be measured by within the sediments. Based on the results of gamma-ray spectroscopy in lake sediments of Erten et al. (1985) and Benoit and Hemond (1990, Switzerland. Together with the t37Cs peak from 1991), we expected to find reflections of the vary- 1963 (bomb fallout), there are now two distinct ing conditions in the different sediment cores.

Table 5. Sedimentation rates, Pb-210 fluxes and inventories in Lake Zurich (errors _4- 10~, t sigma).

Sample Sedimentation Sedimentation Pb-210 Flux I Flux 2 rate Cs-137 rate Pb-210 inventories (dpm cm- ~- y 1) (dpm cm 2 y- 1) (gcm-2y 1) (gcm 2y-l) (dpmcm-2)

Tiefenbrunnen July 89 0.19 0.i5 40.6 1.36 1.28 March 90 0.20 0.13 36.8 1.24 1.16 Center of Lake July 89 0.12 0.09 44.1 1.50 1.39 March 90 0.14 0.t7 60.9 1.91 1.92 Sept. 90 0.15 0.t6 71.3 1.93 2.25 Herrliberg (111 m) Sept. 90 0.26 0,40 76.1 2.36 2.40 Oberrieden (105 m) Sept. 90 0.15 0.24 58.0 1.78 1.83 Erten et al. (I985) Core I 0.07 0.06 18.1 0.57 Core II 0.07 0.09 16.1 0.51 Wieland et aL (t989) 0.973 Schuler et aL (1991) 0.833 mean 1984-1987 i Calculated from sedimentation rate and surface activity obtained by a fit through the data points (see text). 2 Calculated from inventories (see text). 3 Sediment traps, center of iake, 172

Table 6. Cs-137 in Lake Zurich sediments (errors +_ 5~o, i sigma) I.

Tiefenbrunnen March 90 Center of Lake March 90 Oberrieden Sept. 90 Herrliberg Sept. 90

Mass depth Cs-137 Mass depth Cs-137 Mass depth Cs-137 Mass depth Cs-137 (gcm -2) (dpmg 'I) (gcm -2) (dpmg -1) (gcm -2) (dpm g- l) (g cm- a) (dpm g- i)

0.32 12.08 0.10 25.19 0.09 7.76 0.34 12.20 0.82 16.16 0.31 20.94 0.42 20.54 0.77 15.56 1.45 3.72 0.51 13.35 0.81 21.06 1.25 19.38 2.14 7.47 0.74 64.95 1.24 4.56 1.76 20.2t 3.03 4.57 1.29 3.27 1.68 2.87 2.72 2.77 3.95 6.70 2,60 2.48 2.26 4.44 4.79 6.25 4.90 9.41 3.54 7.45 2.94 5.58 6.31 8.06 5.86 9.93 4.02 22.04 3.55 9.28 7.01 14.19 6.86 4,41 4.58 7. t7 4.15 10.10 7.65 16.53 7,79 2.05 5.13 5.52 4.76 4.40 8.38 6.80 8.74 0.75 5.74 0.67 5.41 1.67 9.91 1.07

i The distribution of Cs-137 in the additional cores (see Table 5) is similar.

The cores from Tiefenbrunnen (34 m depth) (e.g. Fig. 6, Erten et al., 1985). At Herrliberg the are from the oxic part of the lake. The two 21°pb activity of 21°Pb decreases only slowly, corre- profiles of the cores of July 1989 and March 1990 sponding to a high apparent sedimentation rate of are practically identical. The exponential decay of 0.4 g cm -2 y-t. This rate disagrees with 0.26 g 21°Pb corresponds to a mean sedimentation rate cm- 2 y - 1 as estimated from the two 137Cs mark- of (0.14 + 0.01) g cm- 2 y- i, in slight disagree- ers (see Table 6). The apparent sedimentation rate ment with (0.19 +_ 0.01) g cm - 2 y- 1resulting from on the left shore (Oberrieden) is also relatively the ~37Cs markers (see also Tables 5 and 6). In high (0.24 g cm - 2 y - 1), whereas the rate deduced these cores the 2~°Pb inventory is somewhat from the 137Cs markers (0.15 g cm -2 y-l) is higher than 100~o if an atmospheric flux of2~°Pb rather similar to that of other locations in this to the lake of 0.013 to 0.016 Bq cm- 2 y- 1 (about lake. The large scatter of the data and the appar- 1 dpm cm -2 y--J) is assumed (Turekian etal., ently too high 2t°Pb sedimentation rates in these 1983; Erten et al., 1985; Schuler et al., 1987; cores could tentatively be explained by small Dominik et al., 1987; Wieland et al., 1989; Schuler slides and slide-induced mixing of the sediments et al., 1991). The too high 21°Pb inventory as in this steep part of the shores. The observed compared to the atmospheric flux suggests an un- effects are indeed larger at the steeper slopes close known additional source of 21°pb besides the at- to Herrliberg than at Oberrieden (see Fig. 2). This mospheric input or more probably a higher sedi- simple explanation for the unexpected 21°Pb re- mentation rate in the last 30 years. The scatter in sults is supported by extended tails in the 137Cs the data does not allow to fully support this con- peaks of both profiles (Moser etal., 1991, see clusion. However, the 137Cs measurements Table 6). The inventories of 21°Pb in these pro- (Table 5) agree with the assumption of a higher files are about twice the atmospheric input (see sedimentation rate since 1963. Table 5). This can be explained by the postulated Figure 7 presents also 21°pb activities in cores additional contributions of sliding matter from from the steep shores of the orographically left the slopes above. (Oberrieden) and right (Herrliberg) hand side of Figure 7, finally, shows the analyses of 21°pb of the lake of September 1990. In both cores one cores taken at 135 m depth in the flat part of Lake observes a larger scatter in the data than in the Zurich. The profiles of March and September cores from Tiefenbrunnen and other locations 1990 are in close agreement, but as in the cores 173

20 2O July 89 March 90 Sept. 90] Tiefenbrunnen -41-- | LIDQ 10 10 ,~"a_-_A Center of Lake o 135m E P (3. b 5 5 ._> < < 0 O ''m z~ ~- 2 ~- 2 Cq C,I ± .6 O. [L "

0.2 0.2 I i ,, 1 0 5 10 15 20 0 5 10 15 20 Mass Depth (g cm-2) Mass Depth (g cm-2)

201 Oberrieden 20 I Herrliberg !sept.90 i 1o. ~" 10 L__--~

E 5 • O. 5- 5 >-

> >

D 0,5 D 0.5

0,2 I ...... 1 0.2 I t 0 5 10 15 20 O 5 10 15 20 Mass Depth (g cm-2 ) Mass Depth (g cm -2 ) Fig, 7. Profiles of 'unsupported ' 21°pb in sediment cores from different locations in Lake Zurich (see Fig. 2). Tiefenbrunnen, 34 m depth, oxic sediments, Center of lake, 135 m depth, suboxic. Herrliberg, right shore, 111 m depth, suboxic. Oberrieden, left shore, 105 m depth, suboxic. Solid and dotted lines: least square fits to data points. Errors in activity +_ 5%, in mass depth _+ 0.t g cm- 2. 174 from the lake shores there is much more sc~,tter Lake Zurich 2t°Pb is expected to dissolve together of the data than at Tiefenbrunnen or other loca- with manganese oxides and to reprecipitate under tions (e.g. Erten etaL, 1985). The data of July more oxic conditions. Dominik et aL (1983) found 1989 deviate from the cores of 1990 and suggest in sediments of Lake Geneva that about 40 ~o of a smaller sedimentation rate. This smaller value 21°pb was residing in a Fe/Mn-phase. Therefore, results mainly from the deepest samples in this if Mn dissolves during periods with a relatively core which exhibit similar activities at various low redox potential, 2~°Pb would also dissolve depths. We cannot give a reasonable explanation and could re-enter into the water column of the for this observation, but it shows that significant lake. Due to particle transport and circulation deviations can occur even in cores which were processes, manganese and other substances will taken at precisely the same depth (measured by accumulate in the deepest part of the lake (Im- Sonar) and within an estimated small area of boden & Schwarzenbach, 1985). Indeed, we about 100 m 2. Again, the simplest explanation for found that manganese concentrations in the sedi- this result would be the accumulation of material ments at this location were ten times higher than resulting from small slides at the steep slopes of at Tiefenbrunnen. Figure 8 shows manganese and the lake. These slides are expected to occur from decay corrected 21°pb depth profiles in sediments time to time in a stochastic manner, thus produc- from Tiefenbrunnen (34 m) and from the center ing variations in the 21°pb activities. The high of the lake (135 m). At Tiefenbrunnen the con- inventories of 2~°pb at this site, corresponding to centrations of manganese are rather constant at twice the atmospheric input, seem to support this hypothesis. However, the sedimentation rates at the center are similar (mean value 0.14g cm -2 y-~) to other locations in the lake and the dis- 20 tance from the steep shores is rather large for a significant transport of solid material. Further- 0 more, one observes undisturbed narrow peaks of 137Cs (see Table 6 and Moser etal., 1991) which E ...... a.,,,.a ...... A ...... a ...... ,A question the assumption of a transport of solids. o Therefore, the given explanation for the high 2~°pb inventories and the scattering of the data by slides 3 from the shores is questionable. In the following paragraphs, we propose another more likely mechanism. In the center of the lake (135 m depth) the redox conditions change from slightly oxic (4 mg O2 L-1) to anoxic (beiow detection) with a rather ., Mn large variability from year to year (analyses of the Water Management Laboratory of Zurich, personal communication). These changes lead to the \ d .,~ .... ~---,~ well known iron-manganese cycles (e.g. Davison, 0.2 2~ - -t~, - ~z~- - - .~,. - - - .,~. -...~ .... i 1985) and are also manifested in the annual lay- 0 2 4 6 8 ers within the sediments (see above). Benoit & Mass depth (g cm -2) Hemond (1990, 1991) have related the chemical Fig. 8. Manganese (O, A) and 2a°pb (0, A) depth profiles behaviour of 2~°pb to these cycles. We have ob- in sediment cores from Lake Zurich (March, 1990). a and c (circles) Center of Lake; b and d (triangles) Tiefenbrmmen. served a similar cyclic behaviour of manganese Errors: _+ 5%, uppermost vaIue of 21°Pb + 25% (y-ray mea- and heavy metals in river sediments (von Gunten surement). Each data point of the 21°Pb activity decay- etal., 1991). At the suboxic to anoxic sites in corrected to sampling date. 175 about 0.2 to 0.3 mg g- 1. In contrast, at the cen- tory of 21°Pb. A comparison of the expected in- ter of the lake, one observes concentrations of ventory, resulting from an atmospheric input of manganese in the 2-mg g- 1 range with an irregu- 1 dpm cm-2 y 1, with the effective 21°pb inven- lar variability with depth within a factor of two or tories at the different locations demonstrates a three. The decay corrected 21°Pb activities in the rather large disagreement at the cross section be- profiles from these locations demonstrate quite tween Herrliberg and Oberrieden and at the site similar patterns to Mn: a uniform decay corrected of Erten et aI. (1985). We have discussed possible activity distribution at Tiefenbrunnen and a large reasons for the excess of 21°pb in the profiles from scatter at the center of the lake. Furthermore, the the cross section through the lake and at the site average specific 21°pb activity (dpm g- 1) is about ofErten et al., t985 (see above). The explanations a factor of two higher in the center of the lake than are tentative and need further explorations. Lake at Tiefenbrunnen. Figure 8 supports the results of Zurich is very suitable for additional investiga- Benoit & Hemond (1990, 1991) and indicates that tions of these processes because the input of 21°Pb the concentrations of Mn and 21°pb are indeed is dominated by atmospheric fallout (see above). significantly influenced by the redox behaviour of In Table 7 we compare sedimentation rates and manganese. Our preliminary results of 21°Po and fluxes of 21°pb in several Swiss lakes. Calcula- 21°pb measurements in the interstitial water of tions are, whenever possible, also applied to the sediments from Lake Zurich support the hypo- data from the literature. Both, sedimentation rates thesis of a mobility of these radionuclides. and fluxes vary considerably, depending on at- In Table 5 we compile 21°pb inventories and mospheric and allochthonous inputs of 21°Pb to fluxes at different locations of Lake Zurich. The these lakes. In Lake Constance the flux of 21°pb fluxes were calculated in two ways. i) by the prod- corresponds to the atmospheric input of about uct P = rCo, where r is the sedimentation rate and 1 dpm cm - 2 y - 1. In Lake Zurich we found lower Co the decay corrected unsupported activity of as well as higher fluxes than the atmospheric con- 21°pb. ii) from the formula P = 2A(0), where 2 is tribution. This may be the result of a redistribu- the decay constant of 21°pb and A(0) the invert- tion of 21°Pb at different locations of the lake due

Table 7. Pb-210 sedimentation rates and fluxes in Swiss lakes.

Sedimentation rate Flux 1 Flux 2 References (gcm - 2 y - 1) (dpm cm 2 y - 1) (dpm cm- 2 y .. t)

Lake Constance 0,11 ± 0.01 1.05 + 0.133 von Gunten et al. (1987) 0.06 to 0.13 0.44 to 1.23 Dominik et al. (1981) Lake Zurich 0.09 _+ 0.03 0.81 _+ 0.273 0.6 _+ 0.13 Erten et al. (1985) Tiefenbrunnen 0.14 +_ 0.01 1.3 + 0.1 1.2 _+ 0.1 Moser et al. (1991) Center of iake 0.14 _+ 0.01 1.9 + 0.2 2.1 _+ 0.1 Moser et al. (1991) Greifensee 0.14 _+ 0.02 0.50 ± 0.073 0.33 + 0.03 Wan et al. (1987) Lake Lucerne 0,042 +_ 0.014 Bloesch et aL (1982) Weggis 0,71 +_ 0.243 Horw 0.25 ± 0.083 Lake Bienne 0.13 to 1.67 0.58 + 7.33 M~ieller (1982) Lake Geneva 0.12 to 0.15 1.2to 1,43 Dominik et al. (1989)

(cm y- I) Lake Morat 1,1 + 0,1 Bergerioux et al. (1980) Lobsigensee 1.2 + 0.1 (Cs-137) This work

Caiculated from sedimentation rate and surface activity obtained by a fit through the data points (see text), 2 Calculated from inventories (see text). 3 Calculated from the data in literature. 176 to sediment slides and/or seasonally varying chemical remobilization. More investigations redox conditions. The fluxes of 21°Pb into the are needed for a better understanding of these sediments of Greifensee and Lake Lucerne are processes. much smaller than the atmospheric influx. This Some of the high inventories of 21°pb in Lake indicates an unknown process of removal of 2~°pb Zurich can tentatively be explained by sliding from these lakes. We expect that this could be due matter from the steepest sections of the shores to a redissolution from the sediments and a trans- and by accumulations of 21°Pb which are report of 2~°pb out of these lakes. In Lake Bienne, lated to the manganese redox cycles. finally, the fluxes vary by more than one order of Too low inventories of 21°Pb in Lake Zurich magnitude. This is due to a locally variable al- are attributed to a redissotution of 21°Pb to- lochthonous contribution of the major inflowing gether with Mn during reducing conditions. River Aare. Whereas the high fluxes can be ex- Dating with 21°pb should be supported by the plained by allochthonous material, it is more dif- measurement of the 1986 and 1963 peaks of ficult to find reasons for fluxes which are much ~37Cs. In Europe these peaks are easy to mea- smaller than the input from the atmosphere. Ten- sure by gamma-ray spectroscopy and may be tative explanations for these deficiencies in 2~°Pb used to connect the 21°Pb results with an ab- have been proposed by Benoit & Hemond solute time scale and to recognize problems of (1990, 1991). the ~l°Pb method. Both, Lake Morat and Lobsigensee are very eutrophic. Their high sedimentation rates are probably due to the intensive growth and decay Acknowledgements of aquatic biota, mainly algae. It is quite clear that dating of sediments of a lake with very high bio- The authors are indebted to H. V01kle for collect- logical productivity- is questionable since the ing and supplying air samples and to F. Weg- activity of biota is significantly influenced by ex- maller for measurements. B. Amman has fur- ternal parameters, e.g. nutrients and climatic con- nished samples from Lobsigensee. S. Bollhalder, ditions. A. Lueck, M. Sturm, E. Wieland and A. Zwyssig The data of Table 7 and the discussion of the helped during collection and treatment of the present paper show clearly that several questions samples from Lake Zurich. M. Sturm and remain open concerning sources and sinks of E. Wieland have contributed by discussions. 21°pb and its behaviour in lacustrine sediments. The very careful and constructive reviews of These facts should always be remembered when P. G. Appleby and two anonymous reviewers applying the 2~°Pb method for dating purposes. have considerably improved the manuscript. We thank EAWAG for the use of the boats, the Water Management Laboratory of Z/irich and R. Keil Conclusions for chemical analyses, E. R0ssler for Figs 2 and 3, and R. Lorenzen for typing. Work was sup- - The average specific activity of 2~°pb (#Bq ported by the Swiss National Science Founda- m-3) in the near-surface atmosphere in Swit- tion. zerland is very constant. - Swiss lakes show very different sedimentation rates and variable ratios of atmospheric to al- References lochthonous inputs of 21°pb. - Dating with 21°Pb is not always possible. Rea- Appleby, P. G. & F. Oldfield, 1978. The calculation of 21°Pb dates assuming a constant rate of supply of unsupported sons for problems are rapidly growing biota 2~°pb to the sediments. Catena 5: 1-8. due to eutrophication, bioturbation, unstable Begemann, F., H. yon Buttlar, F. G. Houtermans, N. Isaac & sediments leading to mechanical mixing and E. Picciotto, 1953. Application de la m6thode du RaD/t la 177

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