Journal of Volcanology and Geothermal Research, 42 (1990) 363-372 Elsevier Science Publishers B.V., Amerstam--Printed in the Netherlands

363

Radiometric studies of Lake Nyos (Cameroon) sediments: evidence of strong mixing and excess 2l°pb

M. PIBOULE a, M. POURCHET b,d, R. BOUCHEZ c, J. AMOSSE a, P. BRENAC d, J. MALEY d, J.F. PINGLOT b and C. VINCENT b

aInstitut Dolomieu, CNRS, N ° 69, Univ. Joseph Fourier, Grenoble, France bLaboratoire de Glaciologie et de G~ophysique de l'Environnement, CNRS, Grenoble, France

Clnstitut des Sciences Nucl~aires, Univ. Joseph Fourier, Grenoble, France dLaboratoire de Palynologie, OSTROM, UR-A3, USTL, MontpeUier, France

(Received and accepted April 23, 1990)

Abstract

Piboule, M., Pourchet, M., Bouchez, R., Amosse, J., Brenac, P., Maley, J., Pinglot, J.F. and Vin- cent, C., 1990. Radiometric studies of Lake Nyos (Cameroon) sediments: evidence of strong mix- ing and excess 216pB. In: F. Le Guern and G.E. Sigvaldason (Editors) The Lake Nyos Event and Natural CO 2 Degassing, II. J. Volcanol. Geotherm. Res., 42: 363-372.

A one-meter-long sediment core was extracted from the middle of Lake Nyos six months after the August 1986 gas burst disaster. Major and trace elements have been determined in twenty samples, including alpha and gamma spectrometry for the radioisotopes.

Preliminary results show that: -- 137Cs is present in uniform concentrations throughout the core. The measured activity (7.3 Bq kg -1) corresponds to a total deposit of approximately 1800 Bq m -2 in the one-meter-thick sediment layer, similar in magnitude to the average amounts deposited at these latitudes by atmospheric thermonuclear tests. - 21°pb, with a half-life of 22 years, is also uniformly distributed, showing a measured activity of 233 Bq kg -1, three to four times higher than the values observed in other Cameroon lakes. The activity ratio of 21°pb to 226Ra is about 4. These data are interpreted as fol]ows: - The uniformly distributed 137Cs and 21°pb concentrations suggest complete mixing of the top of sediments at the sample location.; - From the amount of deposited 137Cs found in the core, this sediment mixing process does not appear to have involved significantly more than that top layer. - The 21°pb excess, not supported by 226Ra, can be explained by an upwards release of radon gas (222Rn) prior to the CO 2 burst.

I n t r o d u c t i o n

As p a r t of a mul t i -d i sc ip l ina ry pa laeoc l imat ic a n d p a l a e o e n v i r o n m e n t a l inves t iga t ion con- ducted in C a m e r o o n (Maley et al., 1990- th is volume), we ex t r ac t ed a s ed imen t core f rom the

middle of L a k e Nyos, six m o n t h s a f t e r the

Augus t 1986 gas b u r s t disaster . The object ive was to search for evidence of the or igin of the gas burs t . This p a p e r p r e sen t s the r e su l t s of the chemica l and X-ray dif f ract ion ana ly se s con- ducted on these sed imen t s t oge the r wi th a

0377-0273/90/$03.50 © 1990 Elsevier Science Publishers B.V.

364 M PIBOULE ET AL.

radioactivity study based mainly on 137Cs and 21°pb measurements. The results throw new light on the process of the gas burst event.

Geological setting

Lake Nyos is located in a recent maar, lying on a crystalline substra tum locally composed of monzonitic granites. This base is covered by a relatively thin, recent basaltic flow from a lava fountain, indicating an early effusive phase. The basaltic layer is cut by the crater and out- crops at its northeastern border under the maar ejecta. The relationship between the formations confirms that the effusive phase has preceded the crater explosion.

The Nyos cone is low and crescent-shaped; it is made up of breccia and of pyroclastic ejecta rich in granitic blocks torn away from the subs t ra tum as well as in peridotite nodules car- ried up from the source region by the basalts.

The crater was formed by violent explosions, mainly phreatic origin. It has an irregular, elongated shape with subvertical walls and a centre depth of 220 meters (Fig. 1). The cone and the inner rim of the crater are marked by five small valleys. Two small streams flow into the lake; to the South, they contribute to the mixed terrigenous and volcanic alluvial depos- its. The sediments are deposited upstream of the crater and on its rim. The bathymetric survey shows that they are cut by a narrow channel extending downwards. The pyroclastic deposits of the cone have been heavily eroded by gullying, producing a predominantly volcanogenic lake sediment input. The uniform grass cover on the slopes surrounding the crater implies weak erosion and a relatively low non-volcanogenic fines input.

The date of origin of the maar is not accurate- ly known, however the fresh morphology of the cone suggests an age of about 400 years (Lockwood and Rubin., 1989)

Core extraction apparatus

A Mackereth piston-type coring device was used, it was fitted with a removable sleeve made of semirigid polyethylene designed to hold the extracted core. The piston was triggered by a sensing mechanism when the coring device was within 2 cm of the lake bottom.

Sampling

Degassing of the core during its rise to the surface resulted in the expansion of the top 50 cm of sediment. The rest of the core was not significantly affected by this phenomenon and the density variations in the sampled material are similar to those measured for other Cameroon lakes (Ossa, Barombi-Mbo, Oku and Bambuluwe).

The recovered core was subsequently cut into thin samples: 2 cm thick at the top (0 to 54 cm), 1 cm thick between 54 and 68 cm, and 2 cm thick at the bottom of the core (68 to 130 cm). These samples were oven-dried at 60°C in the laboratory.

Sediment description

The sample used for X-ray diffraction ana- lysis came from the lower part of the core 1 0 6 - 1 0 8 c m depth). It consisted mainly of black silt, rich in organic matter. The X-ray dif- fraction studies indicate the presence of clays (kaolinite, vermiculite, illite), together with gibbsite and a high proportion of amorphous phases. The presence of kaolinite (do01 = 7.24 A, do92 = 3.6 A, . . . ) and of gibbsite (d0o 2 = 4.87 A) is confirmed by the high thermal sen- sitivity of the main diffraction peaks observed and by their total disappearance after the sam- ple was heated. Only very small quantities of il- lite (do01 - 10 A) were detected. Its presence was nevertheless confirmed by the persistance

RADIOMETRIC STUDIES OF LAKE NYOS (CAMEROON) SEDIMENTS: EVIDENCE OF STRONG MIXING AND EXCESS 2i°pb 365

L AKE NYOS N

i Wet -zon e 1

~2oo- Contour Line

t cL.-CU_

, , u - ~'" ' r )

- - ) • . . j , '

c--'S T i i

J ) \ ,--..~

k L t <oo:. S A M P L E

Fig. 1. Lake Nyos area: the map elevations are relative to the estimated 1100 m elevation of the lake and the elevation accuracy is about 30 m.

of the doo 1 peak after heating. The same is true for the observed traces of vermiculite (and/or in- terstratified illite-vermiculite) (do01 = 14 A, with an intensity drop when heated and an in- crease in the 10-fi, peak after heating). Note the absence of carbonate. For the entire set of dif- fraction patterns, only very low to low peak in- tensities are observed, indicating a minor pro-

portion of crystalline material in the sample. This also suggests a marked predominance of amorphous phases (aluminium hydroxides) con- firmed by the high background noise.

Chemical analysis (Table 1) of a second sam- ple (124-126 cm depth) confirms the alum- inous nature of the sediment. Reported in the chemico-mineralogical Moine diagram [K/(A1-

366

TABLE 1

Chemical analysis of Lake Nyos sediments (124-126 cm)

1 2 3

SiO 2 39.52 38.27 A1203 20.35 20.28 Fe203 tot. 6.51 6.32 MgO 0.48 1.13 CaO 0.75 0.80 Na20 0.45 0.46 K20 1.79 1.84 TiO 2 1.70 1.48 MnO 0.05 0.05 P205 0.78 0.52 Pftot.300 26.68 Pf(100") 0.99 Pf(300") 25.30

Total 99.06 97.44

Zn = 75 ppm Cu = 29 ppm Co = 18 ppm Ni = 108 ppm Cr = 128 ppm

1 XEDS-F. Keller-Institut Dolomieu. UJF. Grenoble 2 XRF P. Capiez-Lab. Petrologie-Univ. Lyon I. Lyon 3 Atom. Abs. Amosse-INST. Dolomieu. UJF. Greno- ble

Na) and (Fe+Mg)/(A1-Na), Fig. 2) (Piboule, 1979)], such a totally argillaceous sediment should correspond to a mixture of kaolinite, il- lite and montmorillonite, supported by the X- ray diffraction study for the first two com- ponents. These two phases could originate from the weathering of the granitic subst ra tum in the region.

K/(At-Na) Fetdspath K

0.5- 0 0 O:g ~(Fe+Mg)/(k[-Na)

Kaolinife-Anocfhife Earl:~afes Fe-i'~J

Fig. 2. Chemical and mineralogical Moine's diagram for the 124-126-cm sample. The two values repre- sent two distinct measurements.

M. PIBOULE ET AL.

Finally, the high Ni, Cr, TiO 2 and A120 3 con- tents of the analyzed sediment indicate that the material is largely derived from leaching by acidic water; this view is supported by the absence of carbonates and the expansion of the core. Accordingly no smectites, characteristic weathering products of basaltic lava, were detected by X-ray diffraction, since they are definitely incompatible with carbon dioxide- rich water (low pH).

Radiometr ic methods

Alpha spectrometry

21°pb (22.3 yr) was analyzed by alpha spec- t rometry of 21°po (138.4 days) using Hasanen 's method (1977) on samples taken at 10-cm inter- vals along the length of the core.

High-sensitivity gamma spectrometry

Two samples (Nyos 1 and Nyos 2), one from each end of the core, were analyzed using a gamma spectrometer with a low-level, high- purity germanium detector, specially designed for analysis of geologic materials with ac- tivities in the milli-Becquerel range (Ma, 1984). In particular, this device can measure uranium and thorium contents down to the 0.1 ppm level, and evaluate the degree of radioactive desequilibrium in natural materials (Con- domines et al., 1987). Table 2 and Figure 3 show the main natural radioisotopes of the uranium and thorium families which can be measured by gamma spectrometry. This equip- ment was also used to measure 134, 137Cs (605; 662; 795keV), l°3Ru (498keV) and other radioactive fallout from the atmospheric nuclear tests of the sixties and the Chernobyl accident (Ma, 1986; Bouchez et al., 1986; M~li~res et al., 1988).

Results and d i scuss ion

Evidence of sediment mixing is provided by

RADIOMETRIC STUDIES OF LAKE NYOS (CAMEROON) SEDIMENTS: EVIDENCE OF STRONG MIXING AND EXCESS 21°pb 367

kBi

-op b

="'Th

44.5 63.3 92.6

:3,Th =26Ac

E 0

u

=3s U ="Ra

-- 11wi

21:~pb

2"pb =14pb J

==a~ IL 2 1 4 ~ ) ~ H

CL '2'At,o,-" It F[ 11 Ji " ~ - ~ " I I [ j " - I " O ~l IL

241 295 352 ~

v Annihilation

=OSTi

2'4Bi

I",;'Cs

kr~,l,,~l~&.~,rm.~ ~ r r g Y (key)

609 662

Fig. 3. Pa r t i a l gamma- ray spectra of the 128-130-cm sample. Plot ted along with the energy of nuclides are the energy values of gamma- rays from 137Cs and from the nuclides of the 238U serie.

368

TABLE 2

Gamma-ray energies of U and Th series

Nuclides Gamma Series energy (keV) 238U 235U 232Th

21°pb 46.5 * 234Th 63.3 * 226Ra 186.0 * 235U 185.7 212pb 238.6 214pb 295.2 * 228Ac 338.4 214pb 351.9 * 2°ST1 583.1 214B1 609.3 * 228Ac 911.1

two independent radiometric measurements in- volving 137Cs and 21°pb radioisotopes.

137Cs measurements

137Cs is an artificial radioisotope injected in- to the Earth 's atmosphere over the past four decades by atmospheric tests.

137Cs is generally used for dating subactual sediments because its diffusion coefficient is small, and also because it is quickly and ir- reversibly adsorbed by clay.

In the top sample of the core (in fact a set of samples from 0 to 18 cm), the measured 137Cs activity is 8.1 + 0.8 Bq kg -1 (Fig. 3). The value for the bottom sample (128-130 cm) is 6.5 _+ 0.7 Bq kg -1 (Table 3). From the average

M. PIBOULE ET AL.

sedimentation rates observed in other Came- roon lakes (24 mg cm -2 per year for Lake Ossa on the Sanaga River and 21 mg cm -2 per year for Lake Barombi-Mbo, geologically similar to Lake Nyos), the Nyos bottom layer was deposited several thousand years ago, long before nuclear testing. As a result, the high 137Cs activity measured in the bottom layer can only be explained by the mixing of the sedi- ment with tha t of the top layers.

More precisely, we have calculated that the 137Cs activity per square meter for the core ex- tracted from the middle of the lake corresponds to the mean value of the 137Cs-fallout following the nuclear tests. First, a calculated value of 400 Bq m -2 was obtained, taking in account a 137 Cs activity of the core sediment of 7.3 Bq kg -1 (the average of two measurements for the top and the bottom of the 130 cm-long core), a dry weight per unit area of 222 kg m -2 for the sediment, and a reduction factor of 4 due to a ratio of around 8 between the catchment area and the lake area to multiply by a catchment ef- ficiency of around 50% (Stiller, 1979; Drisham and Joshi, 1980; Dominik et al., 1984). Thus, the 137Cs activity of the core seems to ade- quately correspond to a 137Cs catchment after a fallout of around 400 Bq m -2 in the Nyos maar area. Then, that value has to be compared with that corresponding to the total 137Cs fallout over the entire nuclear test period for the same latitudes. At Singapore (lat. 1 ° N), for example, the integrated 137Cs fallout (AERE, 1981), after correction of radio-active decay, gives an amount of 415 Bq m -2. Therefore, if the

TABLE 3

Activity of 137Cs and 21°pb excess in lake Nyos sediments

Sample 137Cs activity 21°pb activity 21°pb/226Ra (bq kg- 1) (bq kg- 1

Nyos 2 (surf.) 8.1 ± 0.8 180 ± 22 4.2 _+0.4

Nyos 1 (130 cm) 6.5 ± 0.7 255 ± 30 3.5 _+ 0.4

RADIOMETRIC STUDIES OF LAKE NYOS (CAMEROON) SEDIMENTS: EVIDENCE OF STRONG MIXING AND EXCESS 2]°pb 369

137Cs activity of the 130-cm-long core-sediment is representative of the mean activity of the lake sediment, then, nearly all the 137Cs activi- ty that has been caught in the lake sediment does exist in no more than a thickness of 130 cm, the length of the core. Therefore, taking in- to account the uncertainties of various mea- surements and estimates, it would appear that the mixing of the Lake Nyos sediments does not affect a thickness exceeding 2 meters.

2 1 ° p b m e a s u r e m e n t s

The 21°pb (RaD) observed in lake sediments is usually of two origins. The first 21°pb part is slowly decaying, because it is supported by long-life parents (from226Ra to 238U) of the 238U series present in sediments. Over a hundred-year period (approximately five half- lives of 21°pb) the supported 2i°pb activity re- mains constant as long as the geochemistry of the catchment region does not vary. The second 2i°pb part is not supported 226Ra, and is com- ing from the steady emission of radon gas (222Rn) at the surface of emerged continents (Lambert et al., 1982). As a result of the short half-lives of the daughter 222Rn (Table 4), the decay rate is governed by the half-life of 2i°pb (T1/2 ~ 22.3y). The unsupported 21°pb is cur- rently used for sediment dating over the last hundred years (Turekian et al., 1977).

At Lake Nyos, alpha and gamma- spectrometry analysis give a 2i°pb activity of 233 _+ 30 Bq kg - i (Table 5). That value is

TABLE 4

Schematic 238U series

> 2.5 x 105y 23S U . . . . . . . . . . . . . . . . . . . . . . . . . . , . 2 3 0 T h

1620y 3.8d

7.5 x 104y _,226Ra

22.3y 2 2 2 R n ................. 21opb ...........................

138d 2i°po 2°6pb (stable)

roughly constant over the entire length of the sediment core. As the alpha spectrometry measures only the 21°po activity, a comparison with the 2i°pb activity has been performed us- ing our low-level gamma spectrometer. The results (Table 3) show that the 2i°pb activity is nearly equal to that of 2i°po activity: (21°po/21°pb) = 1.06 _+ 0.10. Therefore, radioac- tive equilibrium between 2i°pb and 2i°po seems virtually reached for the Lake Nyos sediments. Further support for this hypothesis was obtained when a second alpha measure- ment, carried out six months later on the (98-100cm) sample, showed no significant variation of 2i°pb-activity (209 and 221 Bq kg-i) .

In conclusion: (a) the uniformly distributed 137Cs and 21°pb activity over the one-meter- long core suggests complete mixing of the top layers of Lake Nyos sediments, at least at the sample location; (b) this sediment mixing does not seem to have significantly involved more than the top layer, because the amount of 137Cs-activity measured 'in the core is mat- ching the i37Cs-fallout in the Nyos maar area from the entire nuclear tests.

TABLE 5

Activity of 2i°pb by alpha spectrometry

Depth 2i°pb Activity (cm) (Bq kg- 1)

0 -4 215 8-12 245

14-16 226 52-54 303 59-60 240 68-70 221 78-80 253 88-90 220 98-100 209

120-122 207-196

Mean value 233(2 30)

370 M. PIBOULE ET AL.

The 21°pb budget

From the experimental results (Tables 3, 5), and taking into account the dry weight of sediments (222 kg m-2), a total 21°pb activity of 52 kBq m -2 (_+ 10%) is obtained. Such an amount of (21°pb) in Lake Nyos sediments is around (3.8) times higher than that of Barombi- Mbo lake: (21°pb) = 13.6 kBq m -2 (___ 10%). Those values hold for the same mass-thickness of sediments.

Because the geological environment is similar for those two lakes, we suggest that the (21°pb)s, , supported by (226Ra), should be ap- proximately the same. As our alpha measure- ments for Barombi-Mbo give: (210pb)s = 10 kBq m -2 (_ 10%), then, at Nyos, the "free" part, unsupported, should be: (21°pb) c - 40 kBq m -2, around 10 times high- er than at Barombi-Mbo lake (13.6-10 kBq m-2). Yet more, if the geological hypothesis holds, the high free 21°pb activity at Nyos should appear as a strong radioactive dise- quilibrium between (21°pb) and (226Ra):

(210pb/226Ra) = [(210pb)u + (21°Pb)s)]/(21°pb) S

(40 + 10) /10 = 5

The radioactive disequilibrium has been measured on the two samples (Nyos 1 and Nyos 2) from gamma-ray spectra (Fig. 3) using our low-level gamma spectrometer. The measure- ments give (Table 3) values of: (21°pb/226Ra) = 4.2 for Nyos 2 (top sample), and of (3.5) for Nyos 1 (bottom sample), both with (+ 10%) uncertain- ty. These results give directly (21°pb)u:

(21opb)u = 210pb _ (21opb/2261~)

= 38.5 kBq m -2

They show that the geological hypothesis is good in a first approach but may be refined by radiometric measurements. Nevertheless, as compared with other Cameroon lakes, there is

a strong excess of free 210pb into Nyos lake sediments: (21°pb) u - 3 8 k B q m -2 and (21°Pb)u/(226Ra) - 2.8, instead of 3.4 and 0.33 respectively at Barombi-Mbo. Let us note the apparent positive difference (+ 20%) between the top and bottom samples, due possibly to a recent catchment because sampling was done six months after the burst.

The source of the strong 21°pb excess is evidently the radon decay. As the common at- mospheric radon contributes a small part ( - 10%), because our alpha-measurements on various Cameroon lake sediments, at Barombi- Nbo, Ossa and Oku, give a value of around 4 k Bq m -2. Therefore, the main part of the ex- cess ( - 30 kBq m -2 is due to uncommon radon which may have originated in 3 different ways:

(1) From the deep volcanic substra tum of the maar underlying the lake sediments.

(2) From water seepage directly into the lake. (3) From the atmosphere, by deposition on the

surface of lake, of gases emanating from the regional volcanic substratum.

The deep origin under the maar appears more likely for the following reasons:

The local volcanic substra tum must contain may more discontinuities than the regional substratum. This would facilitate either radon bursts of regular radon outgasing. The rocks ly- ing column under the maar were much faulted by the violent phreatic explosions which gave birth to the Nyos crater.

Thus the 21°pb catchment efficiency from un- common atmospheric radon is probably small because the maar is largely open in the NNW part of its outlet. Furthermore, the lake is not a good collector to trap gas and atmospheric dust. To estimate the 21°pb and 21°po deposit it would be useful to measure the evolution of 21°pb and 21°po activity in the lake waters. Recently, about few weeks after the burst, little 21°pb (if any) had been found (Tazieff et al., 1986) in the lake.

To test possible 222Rn catchment from the seeping waters, we propose to measure the 21°pb activity gradient throughout sediment

RADIOMETRIC STUDIES OF LAKE NYOS (CAMEROON) SEDIMENTS: EVIDENCE OF STRONG MIXING AND EXCESS 21°pb 371

cores in two years, when the sediment mixing is reduced and the 21°pb-210po radioactive equilibrium is reached. Let us note that the 21°pD catchment is likely a long process, as previous gas bursts occurred in the past cen- turies. They are recorded in the collective memory of the highlands population (Shanklin, 1989).

Conclusions

(1) Sediment mixing at the time of the gas burst was strong enough evenly to distribute throughout a two-meter-thick layer the 137Cs activity that was accumulated in a thin top layer.

(2) At Nyos, the measured 21°pb excess is about 10 times higher than that for most Cameroon lakes. That 21°pb is coming from an uncommon (or "abnormal") flux of radon gas emitted over a long period prior to the disaster. Par t of it may originate from the atmosphere or/and from the natural seeping waters for geological reasons. We suggest that most of it has a deep-seated origin. Radon is coming out through the underlying much faulted rocks. Thus the local radon flux should be much higher than the regional radon flux. It should be positively correlated with the local CO 2 flux and the evolution of the deep magmatic events.

Acknowledgements

This expedition received full agreement from A. Babale, Minister of High Education and Scientific Research of Cameroon, and also from the Prefect of Wum. We enjoyed the technical support of the Yaound~ ORSTOM Center (Ph. Mathieu, Director) and from the Yaound~ Hydrologic Research Center (E. Naah, Direc- tor). We received support from the Cameroon Army, from A. Zogning (CGN, Yaound~), and from ELF-Douala geologists. This scientific contribution is part of an ORSTOM Program about the Cameroon palaeoenvironments studies. (J. Maley, UR-A3) and Programm

GEOCIT). The field operations were funded by ORSTOM, France.

References

A.E.R.E. R-10088, 1981. ISBN-0-70-580983.8 Bouchez, R., Bruandet, J.F., Ma, J.L., Amosse, J. and

Piboule, M., 1986. Une ~tude sur la contamina- tion radioactive du thym du Haut Var. Rapport 86-3 du Groupe de G4ochronologie de Grenoble.

Condomines, M., Bouchez, R., Ma, J.L., Tanguy,J.C., Amosse, J. and Piboule, M., 1987. Short-lived radioactive disequilibria and magma dynamics in Etna volcano. Nature, 325: 607-609.

Dominik, J., Burrus, D. and Vernet, J.P., 1984. Removal of the fallout 137Cs from the watershed and its deposition in Lake Geneva sediments. In- teractions between sediments and water. Geneva, ISBN, 0.905941.10.1.

Durham, R.W. and Joshi, S.R., 1980. The 21°pb and 137Cs profiles in sediment cores from Lakes Matagami and Quevillon, Northwest Quebec, Canada. Can. J. Earth Sci., 17: 1746-1750.

Hasanen, E., 1977. Dating of sediments, based on 21°po measurements. Radiochem. Radioanal. Lett., 31(4-5): 207-214.

Lambert, G., Polian, G., Sanak, J., Ardouin, B., Buis- son, A., Jegou, A. and Le Roulley, J.C., 1982. Cycle du radon et de ses descendants. Application

l'~tude des ~changes troposphere-stratosphere. Ann. Geophys., 38 (4): 497-531.

Lockwood, J.P. and Rubin, M., 1989. Origin and age of The Lake Nyos maar Cameroon. In: F. Le Guern and G.E. Sigvaldason (Editors), The Lake Nyos Event and Natural CO 2 degassing, I. J. Volcanol Geotherm. Res., 39: 117-124.

Ma, J.l., 1984. Etude de deux nouvelles m~thodes de g~ochronom~trie: datation de formations carbona- t~es par la spectrom~trie ~ du d~s~quilibre de la famille de l'uranium. Datation RPE de l'~mail dentaire fossile uranif'ere, Thesis, Grenoble, 148 pP.

Ma, J.L., 1986. Etude par spectrom~trie ~ des retom- b~es radioactives provenant de Tchernobyl. Rap- port 86.1 du Groupe de G~ochronologie de Greno- ble.

Maley, J., Livingstone, D.A., Giresse, P., Thouveny, N., Brenac, P., Kelts, K., Kling, G., Stager, C., Haag, M., Fournier, M., Bandet, Y. and Zogning, A., 1990. Lithostratigraphy, volcanism, paleomagnetism and palynology of Quaternary lacustrine deposits from Barombi-Mbo (West Cameroon): preliminary results. In: F. Le Guern and G.E. Sigvaldason (Editors), The Lake Nyos

372 M. PIBOULE ET AL.

Event and CO 2 Degassing, II. J. Volcanol. Geotherm. Res., 42: 319-335.

M61i~res, M.A., Pourchet, M., Pinglot, J.F., Bouchez, R. and Piboule, M., 1988. Chernobyl 134Cs, 137Cs and 21°pb in high mountain lake sediment: measurements and modelling of mixing process. J. Geophys. Res., 93(D6): 7055-7061.

Piboule, M., 1979. L'origine des amphibolites. Appro- che g~ochimique et math~matique. Application aux amphibolites du Massif Central franqais. Doct. Etat., Lyon, 1979, 1003 p.

Shanklin, E., 1989. Exploding Lakes and maleficent water in Grassfields legends and myth. In: F. Le Guern and G.E. Sigvaldason (Editors), The Lake

Nyos Event and Natural CO 2 Degassing, I. J. Volcanol. Geotherm. Res., 39: 233-246.

Stiller, M. 1979. Sedimentation patterns in Lake Kinneret (Tiberias). Isotopes in lake studies, I.A.E.A., Vienna, ISBN 92.0.141179.0.

Tazieff, H., Faivre-Pierret, R.X., Le Guern F. and Chevrier, R.M., 1986. La catastrophe de Nyos, R~publique du Cameroun. Contribution du C.F.R. N ° 811, CNRS. CEA, Gif sur Yvette, 76 pp.

Turekian, K.K., Nozaki, Y. and Benninger, L.K., 1977. Geochemistry of atmospheric radon and radon products. Annu. Rev. Earth Planet. Sci., 5: 227-255.