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Chemical and isotopic characteristics of fluids along the Cameroon Volcanic Line, Cameroon
Jnurnol
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Eorrh Sciuncrs, Vol. 22. No. 4, pp. 433.441. 1996 Copyright 8 1996 Ekvicr
Pergamon
Scmm
Ltd
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Chemical and isotopic characteristics of fluids along the Cameroon Volcanic Line, Cameroon G. Z. TANYILEKE,’ M. KUSAKABE’ and W. C. EVANS3
‘Institute for Geological and Mining Research, Yaounde, Cameroon 21nstitute for Study of Earth’s Interior, Okayama University, Japan 3US Geological Survey, Menlo Park, California, USA (Received 10 August 1995: revised version received 10 February 1996) Abstract - Results of the chemical and isotopic analysis of the water and gases discharged from volcanic crater lakes and soda springs located along the Cameroon Volcanic Line were used to characterize and infer their genetic relationships. Variations in the solute compositions of the waters indicate the dominant influence of silicate hydrolysis. Na+ (40-95%) constitutes the major cation in the springs while Fe*’ + Mg2+ (70%) dominate in the CO,-rich lakes. The principal anion is HCO, (>90%), except in the coastal springs where Cl- predominates. Lakes Nyos and Monoun have Fe-Mg-Ca-HCO; type signatures; the soda springs are essentially Na-HCO; type, while all other lakes show similar ionic compositions to dilute surface waters. Dissolved gases show essentially CO, (>90%), with small amounts of Ar and N,, while CH, constitutes the principal component in the non-gassy lakes. Active volcanic gases are generally absent, except in the Lobe spring with detectable H3. Stable isotope ratio evidence indicates that the bicarbonate waters are essentially of meteoric origin. CO, (6°C = -2 to -8960) and He (3He/“He = 1 to 5.6RJ infer a mantle contribution to the total CO,. CH, has a biogenic source, while Ar and N, are essentially atmospheric in origin, but mixing is quite common. R&urn6 - Sur base des resultats d’analyses chimiques et isotopiques, les eaux et les gaz provenant de lacs de crathres volcaniques et de sources sodkes jalonnant l’axe volcanique camerounais ont et& caract&i&es et leurs relations g&Gtiques deduites. Des variations de composition des solutions aqueuses montrent la predominance de l’hydrolyse des silicates. Le Na* (40-95%) correspond au cation preponderant des sources tandis que le Fe2++Mg2’ (70%) domine dans les lacs riches en gaz carbonique. L’anion principal est le HCO, (=90X), sauf dans les sources &t&es oti predomine le C1-. Les lacs Nyos et Monoun ont des signatures typiques en Fe-Mg-Ca-HCO, tandis que les sources sod&s sont essentiellement caract&is&s par le Na-HCO;. Tous les autres lacs montrent des compositions ioniques similaires B celles d’eaux de surface banales. Les gaz en solution sont essentiellement le CO, (>90%), accompagnk de faibles teneurs en Ar et N,. Dans les lacs non gazeux le CH, prbdomine. Des gaz volcaniques actifs font g&?ralement defaut, sauf dans la source d,e Lobe oti du HS a et6 mesure. L’&ude des rapports d’isotopes stables indique que les eaux bicarbonatees sont essentiellement d’origine meteorique. Sur base du CO, (6’“C = -2 B -8%0) et de l’He (3He /“He=1 B 5,6 Ra) une contribution mantellique au CO, total est dbduite. Le CH, est d’origine biogenique tandis que l’Ar et le N, sont essentiellement d’origine atmospherique. Des melanges sont assez frequents. Copyright 0 1996 Elsevier Science Ltd
INTRODUCTION
Geological,
The Cameroon Volcanic Line (CVL), an approximately 1600 km long chain of Cenozoic volcanic centres, is seismically active, highly faulted and contains numerous soda springs and close to 40 crater lakes. Lethal amounts of CO, released from two of the lakes, Monoun (1984) and Nyos (1986), killed close to 1800 people (Sigurdsson et al., 1987; le Guern and Sigvaldsson, 1989). Here, the authors briefly summarize results of chemical and isotopic analyses of the water and gases from these and other crater lakes, as well as soda springs along the CVL, so as to characterize and infer their genetic relationships. The sampling and analytical procedures used are presented elsewhere (Tanyileke, 1994).
tectonic and hydrological
setting
Figure 1 shows a Precambrian basement complex of Pan-African age, Cenozoic volcanics and Mesozoic and Quaternary sediments. Typical basement rocks include schists, gneiss and migmatites, all intruded by granites and syenites. The lavas range from strongly alkaline to transitional basalts and the sediments consist of sandstones, sand and silt. The highly faulted system is underlain by the central Cameroon shear zone, a bulk N70 reactivated Precambrian wrench fault (Ngako et al., 1992) which is exposed in central Cameroon due to post-Cretaceous uplift and erosion. The central highland backbone, directly linked to this fault system, constitutes the source of the hydrographic network comprising numerous lakes, springs and rivers. 433
434
G. Z. TANYILEKE
et al.
Chemical and isotopic characteristics
Rocks + H&O3 + cations + H,SiO, + HCO, + solids.
Na+K
Mg+Fe
Ca
Figure 2. Relative cationic composition of the CVL waters showing the main groups.DSAL: dissolved springs above lake level.
RESULTS
435
of fluids along the Cameroon Volcanic Line
AND DISCUSSION
Most of the springs, excepting Would& Lobe and Loapanga, are generally cold (T <28”C), either due to waning volcanic activity or to near surface mixing with meteoric water. The pH varies within the range of 4 to 7.4, but there is a wide range in alkalinity (0 to 14,600 mg 1-l) and salinity or TDS (~10 to >20,000 mg I”), reflecting the variability in mineralization. Koutaba, the least mineralized spring (TDS =8 mg I-‘) is also the most acidic (pH=4) and CO, outgassing, a common feature of the CVL springs, is virtually absent, suggesting that there.is insufficient time or flow path (near surface reactons) for the CO,-rich fluid to react extensively with the surrounding rock following its dissolution in the groundwater. This is an indication that free CO, exists at shallow depths within the vicinity, but the observed absence of CO, outgassing is most likely a result of the low Pco, (0.31 bars). Such springs are rare, but the Nyos soda spring constitutes one further example, although it is as yet unclear whether it is derived from Lake Nyos, as suggested by Evans et al. (1994), or discharges from the same reservoir as the spring(s) feeding Lake Nyos. Hypolimnic waters of Lakes Nyos and Monoun, on the the other hand, indicate a temperature inversion (Nojiri et al., 1993; Kling et al., 1989) with alternating gradient and well-mixed layers. Bottom water temperatures have risen by less than 2°C since the explosions, but little further variation is expected of current bottom water temperatures (25.2”C at Nyos and 24.1”C at Monoun). Water chemistry In volcanic terrains (such as the CVL), the solute composition of waters is principally controlled by the silicate hydrolysis of the rock minerals and the (incongruent) reactions. can be generalized as follows:
(1)
Elemental proportions of the leached-out substances (see Table 1 for representative sites) show Na+ (40-95%) as the dominant cation in most of the springs. Ca2+ and Mg*+ dominate, but are generally less than 30%, while K+ barely constitutes 10%. In the CO,-rich lakes, Fe2+ and Mg2+ dominate, while HCO; constitutes the overwhelmingly dominant anion, except in the coastal springs where Clconstitutes the principal anion. A trilinear plot of the relative proportions of the major cations (Fig. 2) suggests segregation of the waters into three groups, with the soda springs trending towards the Na-K apex, indicating advanced water/rock interaction. The water below the chemoclines of the CO,-rich lakes indicate Fe-Mg-type waters. Observed high Mg2’contents result from preferential leaching of the host rocks, while the high Fe2+ is related to the unusual geochemical environment, which shows siderite in its sediments (Bernard and Symonds, 1989). Under carbonate equilibria and continuous CO, input, the prevailing looped reactions (Strahler and Strahler, 1973) favour siderite decomposition and thus the high Fe2+content. H,O + CO, HCO,
+ H’
FeCO, + H2C03 Fe2+ + 2HC0,
(2) (3)
Higher observed Fe2+ and HCO,- contents in the Monoun bottom waters relative to Lake Nyos are more likely the result of sustained siderite decomposition over a longer time interval, although laterite decomposition (Sigurdsson et al., 1987) and/or reduction in the presence of humic acids (Kusakabe et al., 1989) may also contribute. Gas chemistry Table 2 shows the chemical composition of the exsolved and dissolved gases (corrected for air contamination) for respective springs and lakes along the CVL. The spring gases are almost entirely composed of CO, (~90%) with small amounts of Ar, N, and barely detectable quantities of He and H2, similar to gases in Lakes Nyos and Monoun. In the non-gassy lakes, CH, dominates, while H$ and C,H, are detectable only in the Lobe spring. The presence of C,H, suggests that some of the CH, is likely to result from the thermogenic breakdown of organic matter (Chivas et al., 1987). However, the presence of residual fossil sea water in this spring, inferred from 634s (+37.2%0), indicates instead that the CH, is derived from organic sediments of marine origin. A ternary plot of the relative amounts of N,, He and Ar for assessing their source components following procedures outlined by Giggenbach (1992) is shown in (Fig. 3). All samples plot within the field delineated by
22.4 25.1 24.8 19.9 23.2 21.0 16.8 29.6 26.0 28.3 23.8 38.6 66.6
Nov-93
Nov-93
Nov-93
Nov-93 May-87
May-87
May-87
May-87
May-87
May-87
Jun-90
Jun-90
Nyos-205
Monoun-0
Monoun-15
Monoun-95 Manengouba-0
Manengouba-M-91
Barombi-Mbo
Barombi-Mbo-98
Benakuma-0
Benakuma-130
Loapanga spr. Woulde spr
24.3
Jul-87
Nov-93
Ayukaba spr.
Vgol cold spr.
24.0
Mar-93
Awing CBC spr.
23.0
23.4
Jun-90
Jul-87
19.9
Mar-93
Mar-93
24.1
Mar-93 May-87
Nyos soda spr. Sesel spr. Koutaba spr.
Kuchuantium spr.
23.5 21.0
Jul-87
Awing CBC spr. Kuchuantium spr.
28.4 47.6
Mar-93
Ndibisi spr. Lobe spr.
25.8
Nov-93
Nov-93
Nyos-50
Temp (“C)
Date
494 0
5.7
1760
6.25 1160
1337
5.97 6.21
0 1384
36.6 14600
5.08 7.37 5.62
1580
4.03
2139
6.24 6.37
973
6.92
43
9.06 194
75
6.39
1078
28
6.88
6.24
233
6.64
6.54
154
1976 62
71.8
4.83
8.12
1093
5.13
-
33.2 398.7
8.19
Alkalinity
pH
1.2 0.01
4.0
8.7
60 30
-
97
-
11
90
42
0.23
1.0
29
co.01 0.98
0.19
124
11 56
0.50
18.7
0.08
co.05
6.9
0.01
1.6
4.0
77
6.7
3.1
165
12.3 co.05
20.5
27.2
7.7 695.1 co.05
2.4
68.7 8.8
131.4
26
46.7 7.0
0.0 46.8
3.2
Fe
Ca
1.7
32
60
93
18
34
0
14 18
41
68
5.2
2.1
2.8
2.6
7.8
4.8
7.8
27.8 4.8
5.5
4.4
94.6
33.2
2.9
Mg
(mg kg-‘) of representative soda springs and lakes along the CVL
Nyos-0
Name/ depth (m)
Table 1. Chemical composition
2.0 4.0 Cl.0 1.0 1.0 3.0
6.2 8.2 2.8 2.7 4.8 9.2
72 1.0
7500 2.61
40
452
47
43
291 286
0 31
1 314
5 111
4.0
8.2
10 4200
5.6 2.0
7.2 21.7 6.2
76
2.9
6.9
87
2.7
24.88
640
8.0
3.39
1780
1.3 4.0
2.66
K
Na
0.05
-
82.0
18.3
0.5
111.5
co.1
0.4
-
co.05
6.0 111
1.2
0.07 -
35.7 44.1
61.6
1.3 -
121.3 55
3.8
81.1
-
23.1
-
-
20.5
3.1
-
14.1
46.6
-
50.1
173.2
0.11
8.2
0.12 -
0.12
35.7 10.3
0.44
16.8
49.5 41.7
0.066 0.063
6.54
F
Si
0.04
12200
53.3
74.0
48.9
59
1.4
1.8
155 2280
18
21
-
2.10
0.55
0.58
0.45
0.29
0.12
Cl
0.07
2
27.3
37.0
0.90
0.28
co.1
1.2
1.5 40
24
54.1
-
0.15
0.38
0.23
0.01
0.01
0.05
so4
Nov-93
Ngol cold spr.
-
0.188
-
0.003
-
co.002
-
0.478 -
0.002
0.0029
0.0078
0.0027
-
0.009
0.001
0.017
0.01
0.004
co.002
co.002
0.007
0.315
0.003
~0.001
HZ
-
9.88
0.14
0.24
0.03
1.55
0.04
4.11
4.24
2.100
5.132
3.235
0.030
*PS
= +37.2%0.
-
730.0
4.3
18.6
1.0
71.1
1.7
193.0
186.3
93.330
216.930
132.449
1.277
0.554
mm01 mol-r
N2
0.64
0.28
2.63
1.74
0.13
0.44
0.16
0.17
co.002
co.002
0.189
0.032
0.06
02
33
996
978
997
9.24
998
789
811
403
256
410
981
994
CO2
springs and lakes along the CVL
0.013
Ar
for representative
Hz5 <0.002 mm01 mol-’ in all springs except Lobe (0.413 mm01 mol-I).
Jul-87
Mar-93
Ayukaba spr.
spr.
Awing CBC spr.
Kuchuantium
Jun-90
Mar-93
Awing CBC spr.
Jul-87
Mar-93
Koutaba spr.
spr.
May-87
Gesel spr.
Kuchuantium
Jul-87 Mar-93
Lobe spr.* Nyos soda spr.
0.831
Jun-90
Jun-90
May287
May-87
Benakuma-130
Ndibisi spr.
0.732
May-87
Barombi-Mbo-98
Loapanga spr. Would6 spr.
May-87
Manengouba-M-91
0.0062
0.0078
Apr-92
Mar-92
He
Monoun-95
Date
Nyos-205
Name/ depth (m)
Table 2. Gas and isotopic compositions
229.3
0.01
0.488
0.552
2.111
0.034
12.53
0.738
501.4
521.8
453.6
17.84
464
CH4
-19.1
-
-16.6
-26.0
-24.5
-
-19.5
-34.8
-25.8 -11.2
-34.0
-38.0
25.0
-13.0
-6.0
-29.0
-20.0
-12.0
6D%o
-3.9
-4.1
4.9
-5.5
-
-4.0
-7.6
-4.6 -2.9
-6.1
-6.3
-4.4
-2.7
-0.8
-5.6
-4.1
-2.8
PO%0
-
-5.4
-6.5
4.9
-5.9
-5.7
0.5
-6.8 -2.6
-4.6
-6.9
-7.2
-6.7
-
-8.0
6.1
3.4
ZPC%O
5.29
1.0
1.55
0.91
4.95
2.81
3He/4He R,
438
G. 2. TANYILEKE
??
Springs
south of FSZ
et al.
O.OlN ~
* Adamaoua springs . Lake Nyos (8’7-92) Lake Monoun
??
(87-92)
1OHe Figure 3. Triangular
plot of the relative abundances
Al of N,, He and Ar in soda springs and gassy
lakes along the CVL.
*
Rain
0
LN
??
LM
0 0
SW Leakage
+
TW
??
lJ
LL ss
-60
Figure 4. Variations in 6D-PO isotopic compositions of CVL waters. Rain: rainfall; LN: Lake Nyos; LM: Lake Monoun; SW: surface water; Leakage: springs/streams issuing downslope from Lake Nyos; TW: tap water; LL: other lakes; SS: soda springs; MWL: meteoiic water line.
Chemical and isotopic characteristics
of fluids along the Cameroon Volcanic Line
439
BURIED ORGANIC Mhl-l
It--I-l
cl-
GROUNDWATER FLOW
GASFLOW
EVAI’ORATION/VEFFI‘ING
Figure 5. Conceptual the CVL.
model showing the possible sources of CO, generation, processes and flow paths along
the crustal, air and air-saturated groundwater, indicating mixing between these source components.. However, most of the Adamawa springs and gassy lakes trend towards the crustal component, suggesting a radiogenic source for the He. On the other hand, the majority of the springs south of the Foumban shear zone (FSZ) plot close to the air-air-saturated groundwater axis, suggesting a significant atmospheric contribution. SD/ ZPO isotopic ratios A standard plot of 6D versus P80 of the data set (Fig. 4) shows that most of the soda springs (SS), surface
waters (SW), rainfall, Lake Monoun (LM) and a few samples from other lakes (LL) and Lake Nyos (LN) generally plot along the global meteoric water line (GMWL) of Craig (1961), implying their meteoric origin. Although intra- and inter-group differences exist, observed 6180 shifts are predominantly due to evaporation and expected seasonal trends. While the other lakes and epilimnic water from LN and LM plot on a line with a 6D/2PO slope of 4.6, which is similar to the standard slope of 5 expected for natural bodies of water which have been subjected to excessive evaporation, the soda springs plot above the GMWL with a slope of 6.2, with the isotopically depleted
440
G. Z. TANYILEKE
samples deviating the most. Similar trends have been observed in soda and hot springs of some hydrothermal systems (Truesdell and H&ton, 1980; Bath et al., 1986) and are inherently related to the scale of circulation and recharge process or to isotopic exchange between the recharge fluids and minerals of the host rocks. Samples lying on the line suggest either short circulation times or large water:rock ratios. The offset and depleted isotopic signature may be due to prior isotopic fractionation of the source vapour (Matsuo et al., 1979; Craig and Gordon, 1965) and long circulation times coupled with topographical and geographical effects, though the presence of palaeowater along the CVL (especially in the permanently stratified lakes such as Nyos and Monoun) cannot be ruled out. 13C/12C and 3He/ *He isotopic
ratios
Measured values of 613C of dissolved or effervesced CO, relative to the PDB reference standard vary from 4.0 to -2.0%0 in Lake Nyos (excluding the high values measured shortly after the disaster in 1986), -6.6 to 3.20i00in Lake Monoun, -8.0 to -3.0%0 for the soda springs and -9.9 to 0.3%0 for the relatively non-gassy lakes. This wide range of values clearly suggests different sources and processes for the generation of the CO,. The range for magmatic carbon (-4 to -8%o) virtually spans the range for the SS, LN and LM, suggesting a magmatic source. However, 6r3C alone is not diagnostic of source since its range for biogenic CO, (0 to -30%0) overlaps the range of other carbon reservoirs. The dominant source of CO, in Lakes Nyos, Monoun and most soda springs is magmatic, considering their 3He/4He isotopic signatures: 5.6 R, for LN, 3.6 R, for LM (Sano ef al., 1990) and l-5.3 R, for the soda springs south of the FSZ. 3He/ 4He isotopic ratios are yet to be determined for the Adamawa springs, but their observed trend towards the crustal component (Fig. 3), as well as the gassy lakes, suggests the addition of radiogenic He due to the relatively prolonged residence of the fluids underground. This trend, as observed earlier by Giggenbach (1990), is in conflict with the high values reported by Sano et al. (1990) for Lake Nyos. However, both the high 3He contents and/ or high chemical He in Lake Nyos and some soda springs suggest that the CVL is a zone of enhanced mantle degassing. Unlike the CO,, most of the CH, in the gassy lakes, as well as the traces in the soda springs along the CVL, has a biogenic source. The CH, in the coastal springs, however, as well as the S of the Lobe spring, indicate fossil marine signatures. CONCLUSION Available geochemical evidence suggests that the CVL is a low temperature open system of enhanced degassing of mantle-derived CO,. Dissolution of the exsolved gas into the local meteoric groundwater
et al
produces an aggressive fluid which leaches the surrounding rocks. Under favourable conditions, the CO,-rich gases either accumulate at shallow depths, leak into the lakes or are outgassed through soda springs or quiescently through the soil surface. Figure 5 represents a conceptual model showing the possible sources of CO, generation, processes and flow paths along the CVL. Its accumulation in lakes and subsequent release could be potentially dangerous, as demonstrated at Lakes Monoun and Nyos in 1984 and 1986, respectively. REFERENCES Bath, A. H., Burgess, W. G. and Carney, G. N. 1986. The chemistry and hydrology of thermal springs on Efate, Vanuatu, S.W. Pacific. Geofhermics 15(3) 277-294. Bernard, A. and Symonds, R. B. 1989. The significance of siderite in the sediments from Lake Nyos. journal Volcunology Geothermal Research 39(2-3) 187-194. Chivas, A. R., Barnes, I., Evans, W. C., Lupton, J. E. and Stone, J. 0.1987. Liquid carbon dioxide of magmatic origin and its role in volcanic eruptions. Nature 326, 587-589. Craig, H. 1961. Isotopic variations in meteoric waters. Science 153,1702-1703. Craig, H. and Gordon, L. I. 1965. Isotopic Oceanography: Deuterium and oxygen variations in the ocean and the marine atmosphere. In: Occasional Publication 3 (Edited by Schink, D. R. and Corless, J. T.) Rhode Island University, August 1965,371~. Evans, W. C., White, L. D., Tuttle, M., Kling, G., Tanyileke, G. and Michel, R. L. 1994. Six years of change in Lake Nyos, Cameroon, yield clues to the past and cautions for the future. Geochemical journal 28,138-162. Giggenbach, W. F. 1990. Water and gas chemistry of Lake Nyos and its bearing on the eruptive process. Journal Volcanology Geothermal Research 42, 337-362. Giggenbach, W. F. 1992. Geochemical techniques in Geothermal Exploration. Application ofgeochemistry in geothermal reservoir development, UNlTAR/UNDP technical document. ~~119-144. Kling, W. G., Evans, W. C. and Tuttle, M. L. 1989. The evolution of the thermal water structure and water chemistry in Lake Nyos. Journal Volcanology Geothermal Research 39,151-165. Kusakabe, M., Oshumi, T. and Aramaki, S. 1989. The lake Nyos gas disaster: Chemical and isotopic evidence in waters and dissolved gases from three Cameroonian crater lakes, Nyos, Monoun and Wum. Journal Volcanology Geothermal Research 39,167-185. Le Guern, F. and Sigvaldsson, G. 1989. (Editors) The Lake Nyos Event and Natural CO, Degassing. [ournul Volcanology Geothermal Research 39(2-3). Le Marechal 1976. Geologie et Geochimie des sources thermominerales du Cameroon. Th&e docforuf d’Efuf. Collection memoires de l’ORSTOM. No. 59, Paris. Matsuo, S., Kusakabe, M., Niwano, M., Hirano, T. and
Chemical
and isotopic characteristics
of fluids along the Cameroon
Oki, Y. 1979. Water in the Hakone Caldera using hydrogen and oxygen isotope ratios. In: Isotopes in lake studies, IAEA, 1979. International Atomic Energy Authority, Vienna. Ngako, V., Jegouzo, P. et Nzenti, J. I’. 1992. Champ et raccourcissement et cratonisation du Nord Cameroun du Proterozo’ique superieur au Palaeozoic moyen. Academic Science Paris, Series I1 315,371-377. Nojiri, Y., Kusakabe, M., Tietze, K., Hirabayashi, J., Sato, H., Sano, Y., Shinohara, H., Njine, T. and Tanyilike, G. 1993. An estimate of CO, flux in Lake Nyos, Cameroon. Limnology Oceanography 38, 739-752. Sano, Y., Kusakabe, M., Hirabayashi, J., Sato, H., Sano, Y., Shinohara, H., Njine, T. and Tanyilike, G. 1990. Helium and carbon fluxes in Lake Nyos, Cameroon: constraint on the next gas burst. Earth Planetary Science Letters 99,303-314.
Volcanic Line
441
Sigurdsson, H., Devine, I. D., Tchoua, F. M., Presser, T. S., Pringle, M. K. W. and Evans, W. C. 1987. Origin of the lethal gas burst from Lake Monoun, Cameroon. Journal Volcanology Geothermal Research 31, l-16. Strahler, A. N. and Strahler, A. H. 1973. Environmental Geoscience; Interaction befween natural systems and man. 509~. Wiley, New York. Tanyileke, G. 1994. Fluid geochemistry of CO,-rich lakes and soda springs along the Cameroon Volcanic Line (CVL), Cameroon. Ph. D. thesis 193~. Okayama University, Japan. Truesdell, A. H. and Hulston, J. R. 1980. Isotopic evidence on environments of geothermal systems. In: Handbook of environmental isotope geochemistry, 1, The terrestrial environment (Edited by Fritz, P. and Fontes, J. Ch.) ~~179-219. Elsevier Scientific Publications B.V., Amsterdam.
$Afriwn
Eorrh Sciuncrs, Vol. 22. No. 4, pp. 433.441. 1996 Copyright 8 1996 Ekvicr
Pergamon
Scmm
Ltd
Printed m Great Britam All rights reserved 0899.53hU9h .$lS.ul + 0.w
PII: SO899-5362(%)ooo25-5
Chemical and isotopic characteristics of fluids along the Cameroon Volcanic Line, Cameroon G. Z. TANYILEKE,’ M. KUSAKABE’ and W. C. EVANS3
‘Institute for Geological and Mining Research, Yaounde, Cameroon 21nstitute for Study of Earth’s Interior, Okayama University, Japan 3US Geological Survey, Menlo Park, California, USA (Received 10 August 1995: revised version received 10 February 1996) Abstract - Results of the chemical and isotopic analysis of the water and gases discharged from volcanic crater lakes and soda springs located along the Cameroon Volcanic Line were used to characterize and infer their genetic relationships. Variations in the solute compositions of the waters indicate the dominant influence of silicate hydrolysis. Na+ (40-95%) constitutes the major cation in the springs while Fe*’ + Mg2+ (70%) dominate in the CO,-rich lakes. The principal anion is HCO, (>90%), except in the coastal springs where Cl- predominates. Lakes Nyos and Monoun have Fe-Mg-Ca-HCO; type signatures; the soda springs are essentially Na-HCO; type, while all other lakes show similar ionic compositions to dilute surface waters. Dissolved gases show essentially CO, (>90%), with small amounts of Ar and N,, while CH, constitutes the principal component in the non-gassy lakes. Active volcanic gases are generally absent, except in the Lobe spring with detectable H3. Stable isotope ratio evidence indicates that the bicarbonate waters are essentially of meteoric origin. CO, (6°C = -2 to -8960) and He (3He/“He = 1 to 5.6RJ infer a mantle contribution to the total CO,. CH, has a biogenic source, while Ar and N, are essentially atmospheric in origin, but mixing is quite common. R&urn6 - Sur base des resultats d’analyses chimiques et isotopiques, les eaux et les gaz provenant de lacs de crathres volcaniques et de sources sodkes jalonnant l’axe volcanique camerounais ont et& caract&i&es et leurs relations g&Gtiques deduites. Des variations de composition des solutions aqueuses montrent la predominance de l’hydrolyse des silicates. Le Na* (40-95%) correspond au cation preponderant des sources tandis que le Fe2++Mg2’ (70%) domine dans les lacs riches en gaz carbonique. L’anion principal est le HCO, (=90X), sauf dans les sources &t&es oti predomine le C1-. Les lacs Nyos et Monoun ont des signatures typiques en Fe-Mg-Ca-HCO, tandis que les sources sod&s sont essentiellement caract&is&s par le Na-HCO;. Tous les autres lacs montrent des compositions ioniques similaires B celles d’eaux de surface banales. Les gaz en solution sont essentiellement le CO, (>90%), accompagnk de faibles teneurs en Ar et N,. Dans les lacs non gazeux le CH, prbdomine. Des gaz volcaniques actifs font g&?ralement defaut, sauf dans la source d,e Lobe oti du HS a et6 mesure. L’&ude des rapports d’isotopes stables indique que les eaux bicarbonatees sont essentiellement d’origine meteorique. Sur base du CO, (6’“C = -2 B -8%0) et de l’He (3He /“He=1 B 5,6 Ra) une contribution mantellique au CO, total est dbduite. Le CH, est d’origine biogenique tandis que l’Ar et le N, sont essentiellement d’origine atmospherique. Des melanges sont assez frequents. Copyright 0 1996 Elsevier Science Ltd
INTRODUCTION
Geological,
The Cameroon Volcanic Line (CVL), an approximately 1600 km long chain of Cenozoic volcanic centres, is seismically active, highly faulted and contains numerous soda springs and close to 40 crater lakes. Lethal amounts of CO, released from two of the lakes, Monoun (1984) and Nyos (1986), killed close to 1800 people (Sigurdsson et al., 1987; le Guern and Sigvaldsson, 1989). Here, the authors briefly summarize results of chemical and isotopic analyses of the water and gases from these and other crater lakes, as well as soda springs along the CVL, so as to characterize and infer their genetic relationships. The sampling and analytical procedures used are presented elsewhere (Tanyileke, 1994).
tectonic and hydrological
setting
Figure 1 shows a Precambrian basement complex of Pan-African age, Cenozoic volcanics and Mesozoic and Quaternary sediments. Typical basement rocks include schists, gneiss and migmatites, all intruded by granites and syenites. The lavas range from strongly alkaline to transitional basalts and the sediments consist of sandstones, sand and silt. The highly faulted system is underlain by the central Cameroon shear zone, a bulk N70 reactivated Precambrian wrench fault (Ngako et al., 1992) which is exposed in central Cameroon due to post-Cretaceous uplift and erosion. The central highland backbone, directly linked to this fault system, constitutes the source of the hydrographic network comprising numerous lakes, springs and rivers. 433
434
G. Z. TANYILEKE
et al.
Chemical and isotopic characteristics
Rocks + H&O3 + cations + H,SiO, + HCO, + solids.
Na+K
Mg+Fe
Ca
Figure 2. Relative cationic composition of the CVL waters showing the main groups.DSAL: dissolved springs above lake level.
RESULTS
435
of fluids along the Cameroon Volcanic Line
AND DISCUSSION
Most of the springs, excepting Would& Lobe and Loapanga, are generally cold (T <28”C), either due to waning volcanic activity or to near surface mixing with meteoric water. The pH varies within the range of 4 to 7.4, but there is a wide range in alkalinity (0 to 14,600 mg 1-l) and salinity or TDS (~10 to >20,000 mg I”), reflecting the variability in mineralization. Koutaba, the least mineralized spring (TDS =8 mg I-‘) is also the most acidic (pH=4) and CO, outgassing, a common feature of the CVL springs, is virtually absent, suggesting that there.is insufficient time or flow path (near surface reactons) for the CO,-rich fluid to react extensively with the surrounding rock following its dissolution in the groundwater. This is an indication that free CO, exists at shallow depths within the vicinity, but the observed absence of CO, outgassing is most likely a result of the low Pco, (0.31 bars). Such springs are rare, but the Nyos soda spring constitutes one further example, although it is as yet unclear whether it is derived from Lake Nyos, as suggested by Evans et al. (1994), or discharges from the same reservoir as the spring(s) feeding Lake Nyos. Hypolimnic waters of Lakes Nyos and Monoun, on the the other hand, indicate a temperature inversion (Nojiri et al., 1993; Kling et al., 1989) with alternating gradient and well-mixed layers. Bottom water temperatures have risen by less than 2°C since the explosions, but little further variation is expected of current bottom water temperatures (25.2”C at Nyos and 24.1”C at Monoun). Water chemistry In volcanic terrains (such as the CVL), the solute composition of waters is principally controlled by the silicate hydrolysis of the rock minerals and the (incongruent) reactions. can be generalized as follows:
(1)
Elemental proportions of the leached-out substances (see Table 1 for representative sites) show Na+ (40-95%) as the dominant cation in most of the springs. Ca2+ and Mg*+ dominate, but are generally less than 30%, while K+ barely constitutes 10%. In the CO,-rich lakes, Fe2+ and Mg2+ dominate, while HCO; constitutes the overwhelmingly dominant anion, except in the coastal springs where Clconstitutes the principal anion. A trilinear plot of the relative proportions of the major cations (Fig. 2) suggests segregation of the waters into three groups, with the soda springs trending towards the Na-K apex, indicating advanced water/rock interaction. The water below the chemoclines of the CO,-rich lakes indicate Fe-Mg-type waters. Observed high Mg2’contents result from preferential leaching of the host rocks, while the high Fe2+ is related to the unusual geochemical environment, which shows siderite in its sediments (Bernard and Symonds, 1989). Under carbonate equilibria and continuous CO, input, the prevailing looped reactions (Strahler and Strahler, 1973) favour siderite decomposition and thus the high Fe2+content. H,O + CO, HCO,
+ H’
FeCO, + H2C03 Fe2+ + 2HC0,
(2) (3)
Higher observed Fe2+ and HCO,- contents in the Monoun bottom waters relative to Lake Nyos are more likely the result of sustained siderite decomposition over a longer time interval, although laterite decomposition (Sigurdsson et al., 1987) and/or reduction in the presence of humic acids (Kusakabe et al., 1989) may also contribute. Gas chemistry Table 2 shows the chemical composition of the exsolved and dissolved gases (corrected for air contamination) for respective springs and lakes along the CVL. The spring gases are almost entirely composed of CO, (~90%) with small amounts of Ar, N, and barely detectable quantities of He and H2, similar to gases in Lakes Nyos and Monoun. In the non-gassy lakes, CH, dominates, while H$ and C,H, are detectable only in the Lobe spring. The presence of C,H, suggests that some of the CH, is likely to result from the thermogenic breakdown of organic matter (Chivas et al., 1987). However, the presence of residual fossil sea water in this spring, inferred from 634s (+37.2%0), indicates instead that the CH, is derived from organic sediments of marine origin. A ternary plot of the relative amounts of N,, He and Ar for assessing their source components following procedures outlined by Giggenbach (1992) is shown in (Fig. 3). All samples plot within the field delineated by
22.4 25.1 24.8 19.9 23.2 21.0 16.8 29.6 26.0 28.3 23.8 38.6 66.6
Nov-93
Nov-93
Nov-93
Nov-93 May-87
May-87
May-87
May-87
May-87
May-87
Jun-90
Jun-90
Nyos-205
Monoun-0
Monoun-15
Monoun-95 Manengouba-0
Manengouba-M-91
Barombi-Mbo
Barombi-Mbo-98
Benakuma-0
Benakuma-130
Loapanga spr. Woulde spr
24.3
Jul-87
Nov-93
Ayukaba spr.
Vgol cold spr.
24.0
Mar-93
Awing CBC spr.
23.0
23.4
Jun-90
Jul-87
19.9
Mar-93
Mar-93
24.1
Mar-93 May-87
Nyos soda spr. Sesel spr. Koutaba spr.
Kuchuantium spr.
23.5 21.0
Jul-87
Awing CBC spr. Kuchuantium spr.
28.4 47.6
Mar-93
Ndibisi spr. Lobe spr.
25.8
Nov-93
Nov-93
Nyos-50
Temp (“C)
Date
494 0
5.7
1760
6.25 1160
1337
5.97 6.21
0 1384
36.6 14600
5.08 7.37 5.62
1580
4.03
2139
6.24 6.37
973
6.92
43
9.06 194
75
6.39
1078
28
6.88
6.24
233
6.64
6.54
154
1976 62
71.8
4.83
8.12
1093
5.13
-
33.2 398.7
8.19
Alkalinity
pH
1.2 0.01
4.0
8.7
60 30
-
97
-
11
90
42
0.23
1.0
29
co.01 0.98
0.19
124
11 56
0.50
18.7
0.08
co.05
6.9
0.01
1.6
4.0
77
6.7
3.1
165
12.3 co.05
20.5
27.2
7.7 695.1 co.05
2.4
68.7 8.8
131.4
26
46.7 7.0
0.0 46.8
3.2
Fe
Ca
1.7
32
60
93
18
34
0
14 18
41
68
5.2
2.1
2.8
2.6
7.8
4.8
7.8
27.8 4.8
5.5
4.4
94.6
33.2
2.9
Mg
(mg kg-‘) of representative soda springs and lakes along the CVL
Nyos-0
Name/ depth (m)
Table 1. Chemical composition
2.0 4.0 Cl.0 1.0 1.0 3.0
6.2 8.2 2.8 2.7 4.8 9.2
72 1.0
7500 2.61
40
452
47
43
291 286
0 31
1 314
5 111
4.0
8.2
10 4200
5.6 2.0
7.2 21.7 6.2
76
2.9
6.9
87
2.7
24.88
640
8.0
3.39
1780
1.3 4.0
2.66
K
Na
0.05
-
82.0
18.3
0.5
111.5
co.1
0.4
-
co.05
6.0 111
1.2
0.07 -
35.7 44.1
61.6
1.3 -
121.3 55
3.8
81.1
-
23.1
-
-
20.5
3.1
-
14.1
46.6
-
50.1
173.2
0.11
8.2
0.12 -
0.12
35.7 10.3
0.44
16.8
49.5 41.7
0.066 0.063
6.54
F
Si
0.04
12200
53.3
74.0
48.9
59
1.4
1.8
155 2280
18
21
-
2.10
0.55
0.58
0.45
0.29
0.12
Cl
0.07
2
27.3
37.0
0.90
0.28
co.1
1.2
1.5 40
24
54.1
-
0.15
0.38
0.23
0.01
0.01
0.05
so4
Nov-93
Ngol cold spr.
-
0.188
-
0.003
-
co.002
-
0.478 -
0.002
0.0029
0.0078
0.0027
-
0.009
0.001
0.017
0.01
0.004
co.002
co.002
0.007
0.315
0.003
~0.001
HZ
-
9.88
0.14
0.24
0.03
1.55
0.04
4.11
4.24
2.100
5.132
3.235
0.030
*PS
= +37.2%0.
-
730.0
4.3
18.6
1.0
71.1
1.7
193.0
186.3
93.330
216.930
132.449
1.277
0.554
mm01 mol-r
N2
0.64
0.28
2.63
1.74
0.13
0.44
0.16
0.17
co.002
co.002
0.189
0.032
0.06
02
33
996
978
997
9.24
998
789
811
403
256
410
981
994
CO2
springs and lakes along the CVL
0.013
Ar
for representative
Hz5 <0.002 mm01 mol-’ in all springs except Lobe (0.413 mm01 mol-I).
Jul-87
Mar-93
Ayukaba spr.
spr.
Awing CBC spr.
Kuchuantium
Jun-90
Mar-93
Awing CBC spr.
Jul-87
Mar-93
Koutaba spr.
spr.
May-87
Gesel spr.
Kuchuantium
Jul-87 Mar-93
Lobe spr.* Nyos soda spr.
0.831
Jun-90
Jun-90
May287
May-87
Benakuma-130
Ndibisi spr.
0.732
May-87
Barombi-Mbo-98
Loapanga spr. Would6 spr.
May-87
Manengouba-M-91
0.0062
0.0078
Apr-92
Mar-92
He
Monoun-95
Date
Nyos-205
Name/ depth (m)
Table 2. Gas and isotopic compositions
229.3
0.01
0.488
0.552
2.111
0.034
12.53
0.738
501.4
521.8
453.6
17.84
464
CH4
-19.1
-
-16.6
-26.0
-24.5
-
-19.5
-34.8
-25.8 -11.2
-34.0
-38.0
25.0
-13.0
-6.0
-29.0
-20.0
-12.0
6D%o
-3.9
-4.1
4.9
-5.5
-
-4.0
-7.6
-4.6 -2.9
-6.1
-6.3
-4.4
-2.7
-0.8
-5.6
-4.1
-2.8
PO%0
-
-5.4
-6.5
4.9
-5.9
-5.7
0.5
-6.8 -2.6
-4.6
-6.9
-7.2
-6.7
-
-8.0
6.1
3.4
ZPC%O
5.29
1.0
1.55
0.91
4.95
2.81
3He/4He R,
438
G. 2. TANYILEKE
??
Springs
south of FSZ
et al.
O.OlN ~
* Adamaoua springs . Lake Nyos (8’7-92) Lake Monoun
??
(87-92)
1OHe Figure 3. Triangular
plot of the relative abundances
Al of N,, He and Ar in soda springs and gassy
lakes along the CVL.
*
Rain
0
LN
??
LM
0 0
SW Leakage
+
TW
??
lJ
LL ss
-60
Figure 4. Variations in 6D-PO isotopic compositions of CVL waters. Rain: rainfall; LN: Lake Nyos; LM: Lake Monoun; SW: surface water; Leakage: springs/streams issuing downslope from Lake Nyos; TW: tap water; LL: other lakes; SS: soda springs; MWL: meteoiic water line.
Chemical and isotopic characteristics
of fluids along the Cameroon Volcanic Line
439
BURIED ORGANIC Mhl-l
It--I-l
cl-
GROUNDWATER FLOW
GASFLOW
EVAI’ORATION/VEFFI‘ING
Figure 5. Conceptual the CVL.
model showing the possible sources of CO, generation, processes and flow paths along
the crustal, air and air-saturated groundwater, indicating mixing between these source components.. However, most of the Adamawa springs and gassy lakes trend towards the crustal component, suggesting a radiogenic source for the He. On the other hand, the majority of the springs south of the Foumban shear zone (FSZ) plot close to the air-air-saturated groundwater axis, suggesting a significant atmospheric contribution. SD/ ZPO isotopic ratios A standard plot of 6D versus P80 of the data set (Fig. 4) shows that most of the soda springs (SS), surface
waters (SW), rainfall, Lake Monoun (LM) and a few samples from other lakes (LL) and Lake Nyos (LN) generally plot along the global meteoric water line (GMWL) of Craig (1961), implying their meteoric origin. Although intra- and inter-group differences exist, observed 6180 shifts are predominantly due to evaporation and expected seasonal trends. While the other lakes and epilimnic water from LN and LM plot on a line with a 6D/2PO slope of 4.6, which is similar to the standard slope of 5 expected for natural bodies of water which have been subjected to excessive evaporation, the soda springs plot above the GMWL with a slope of 6.2, with the isotopically depleted
440
G. Z. TANYILEKE
samples deviating the most. Similar trends have been observed in soda and hot springs of some hydrothermal systems (Truesdell and H&ton, 1980; Bath et al., 1986) and are inherently related to the scale of circulation and recharge process or to isotopic exchange between the recharge fluids and minerals of the host rocks. Samples lying on the line suggest either short circulation times or large water:rock ratios. The offset and depleted isotopic signature may be due to prior isotopic fractionation of the source vapour (Matsuo et al., 1979; Craig and Gordon, 1965) and long circulation times coupled with topographical and geographical effects, though the presence of palaeowater along the CVL (especially in the permanently stratified lakes such as Nyos and Monoun) cannot be ruled out. 13C/12C and 3He/ *He isotopic
ratios
Measured values of 613C of dissolved or effervesced CO, relative to the PDB reference standard vary from 4.0 to -2.0%0 in Lake Nyos (excluding the high values measured shortly after the disaster in 1986), -6.6 to 3.20i00in Lake Monoun, -8.0 to -3.0%0 for the soda springs and -9.9 to 0.3%0 for the relatively non-gassy lakes. This wide range of values clearly suggests different sources and processes for the generation of the CO,. The range for magmatic carbon (-4 to -8%o) virtually spans the range for the SS, LN and LM, suggesting a magmatic source. However, 6r3C alone is not diagnostic of source since its range for biogenic CO, (0 to -30%0) overlaps the range of other carbon reservoirs. The dominant source of CO, in Lakes Nyos, Monoun and most soda springs is magmatic, considering their 3He/4He isotopic signatures: 5.6 R, for LN, 3.6 R, for LM (Sano ef al., 1990) and l-5.3 R, for the soda springs south of the FSZ. 3He/ 4He isotopic ratios are yet to be determined for the Adamawa springs, but their observed trend towards the crustal component (Fig. 3), as well as the gassy lakes, suggests the addition of radiogenic He due to the relatively prolonged residence of the fluids underground. This trend, as observed earlier by Giggenbach (1990), is in conflict with the high values reported by Sano et al. (1990) for Lake Nyos. However, both the high 3He contents and/ or high chemical He in Lake Nyos and some soda springs suggest that the CVL is a zone of enhanced mantle degassing. Unlike the CO,, most of the CH, in the gassy lakes, as well as the traces in the soda springs along the CVL, has a biogenic source. The CH, in the coastal springs, however, as well as the S of the Lobe spring, indicate fossil marine signatures. CONCLUSION Available geochemical evidence suggests that the CVL is a low temperature open system of enhanced degassing of mantle-derived CO,. Dissolution of the exsolved gas into the local meteoric groundwater
et al
produces an aggressive fluid which leaches the surrounding rocks. Under favourable conditions, the CO,-rich gases either accumulate at shallow depths, leak into the lakes or are outgassed through soda springs or quiescently through the soil surface. Figure 5 represents a conceptual model showing the possible sources of CO, generation, processes and flow paths along the CVL. Its accumulation in lakes and subsequent release could be potentially dangerous, as demonstrated at Lakes Monoun and Nyos in 1984 and 1986, respectively. REFERENCES Bath, A. H., Burgess, W. G. and Carney, G. N. 1986. The chemistry and hydrology of thermal springs on Efate, Vanuatu, S.W. Pacific. Geofhermics 15(3) 277-294. Bernard, A. and Symonds, R. B. 1989. The significance of siderite in the sediments from Lake Nyos. journal Volcunology Geothermal Research 39(2-3) 187-194. Chivas, A. R., Barnes, I., Evans, W. C., Lupton, J. E. and Stone, J. 0.1987. Liquid carbon dioxide of magmatic origin and its role in volcanic eruptions. Nature 326, 587-589. Craig, H. 1961. Isotopic variations in meteoric waters. Science 153,1702-1703. Craig, H. and Gordon, L. I. 1965. Isotopic Oceanography: Deuterium and oxygen variations in the ocean and the marine atmosphere. In: Occasional Publication 3 (Edited by Schink, D. R. and Corless, J. T.) Rhode Island University, August 1965,371~. Evans, W. C., White, L. D., Tuttle, M., Kling, G., Tanyileke, G. and Michel, R. L. 1994. Six years of change in Lake Nyos, Cameroon, yield clues to the past and cautions for the future. Geochemical journal 28,138-162. Giggenbach, W. F. 1990. Water and gas chemistry of Lake Nyos and its bearing on the eruptive process. Journal Volcanology Geothermal Research 42, 337-362. Giggenbach, W. F. 1992. Geochemical techniques in Geothermal Exploration. Application ofgeochemistry in geothermal reservoir development, UNlTAR/UNDP technical document. ~~119-144. Kling, W. G., Evans, W. C. and Tuttle, M. L. 1989. The evolution of the thermal water structure and water chemistry in Lake Nyos. Journal Volcanology Geothermal Research 39,151-165. Kusakabe, M., Oshumi, T. and Aramaki, S. 1989. The lake Nyos gas disaster: Chemical and isotopic evidence in waters and dissolved gases from three Cameroonian crater lakes, Nyos, Monoun and Wum. Journal Volcanology Geothermal Research 39,167-185. Le Guern, F. and Sigvaldsson, G. 1989. (Editors) The Lake Nyos Event and Natural CO, Degassing. [ournul Volcanology Geothermal Research 39(2-3). Le Marechal 1976. Geologie et Geochimie des sources thermominerales du Cameroon. Th&e docforuf d’Efuf. Collection memoires de l’ORSTOM. No. 59, Paris. Matsuo, S., Kusakabe, M., Niwano, M., Hirano, T. and
Chemical
and isotopic characteristics
of fluids along the Cameroon
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