Isotopic hydrology and paleohydrology of the Madagi (Kenya)-Natron (Tanzania) basin during the late quaternary

Palaeogeography, Palaeoclimatology, Palaeoecology, 58 (1987): 155 181 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

155

ISOTOPIC HYDROLOGY AND PALEOHYDROLOGY OF THE MAGADI (KENYA)-NATRON (TANZANIA) BASIN DURING THE LATE QUATERNARY C. H I L L A I R E - M A R C E L 1'2 a n d J. C A S A N O V A 2 1GEOTOP, Universitd du Qudbec ~ Montrdal, P.O. Box 8888, Station A, Montrdal, Qud, H3C 3P8 (Canada) 2Laboratoire de Gdologie du Quaternaire, CNRS Case 907 Campus de Luminy, 13288 Marseille, Cddex 9 (France) (Revised manuscript received and accepted July 10, 1986)

Abstract Hillaire-Marcel, C. and Casanova, J., 1987. Isotopic hydrology and paleohydrology of the Magadi (Kenya)-Natron (Tanzania) basin during the Late Quaternary. Palaeogeogr., Palaeoclimatol., Palaeoecol., 58: 155-181. Three generations of stromatolites mark the recent extension of paleolakes in the Magadi-Natron basin on the border between Tanzania and Kenya. The last two generations are observed some 50 m above the modern lakes' level; they are respectively attributed to the beginning of the last interglacial and to the Pleistocene-Holocene transition. Analysis of the morphology and texture of the stromatolites reveals an ecological zonation between 0 and 10-14 m below the shoreline. Their relatively high 51so and 513C values reflect a long residence time for the paleolake water, which would have been favorable to the establishment of an isotopic equilibrium between atmospheric CO2 and the dissolved inorganic carbon, despite deep carbon inputs in relation to carbonititic volcanism. A study of the 1sO, 2H and J3C contents in the hydrothermal springs, perennial rivers and lacustrine brines indicates that, during the last period of significant renewal of the deep saline groundwater, precipitation was depleted in heavy isotopes when compared to modern precipitation. This difference corresponds to a drop of about 600 1000m in altitudinal "hydroclimatic" zones. Finally, a comparison between the modern and paleohydrological data leads us to believe that the stabilization of the paleolake shorelines at the same altitude can be attributed to control by the water table level in the aquifer of the basaltic plateaus east of the basin.

Introduction The last decade brought up new and more precise chronological data about Late Quaternary climatic changes in intertropical Africa (see for i n s t a n c e S t r e e t - P e r r o t a n d H a r r i s o n , 1984). T h e t i m i n g of t h e L a t e P l e i s t o c e n e E a r l y H o l o c e n e h u m i d e p i s o d e is n o w p a r t i c u l a r l y w e l l d o c u m e n t e d a n d s o m e e v i d e n c e s of analog conditions during the Last Interglacial h a v e a l s o b e e n f o u n d ( G a v e n et al., 1981; H i l l a i r e - M a r c e l et al., 1986). D u r i n g e a c h of these humid phases, most lake levels rose in 0031-0182/87/$03.50

t h e e a s t A f r i c a n Rift. H o w e v e r , t h e h y d r o l o g i cal c o n d i t i o n s a l l o w i n g p o s i t i v e p r e c i p i t a t i o n / evaporation budgets are still debatable. Season° ality changes, real p r e c i p i t a t i o n i n c r e a s e or lower temperatures and evaporation rates are e q u a l l y i n v o k e d (cf. N i c h o l s o n a n d F l o h n , 1980). I n t h i s a s p e c t , t h e i s o t o p i c c o m p o s i t i o n of p a l e o l a k e c a r b o n a t e s m a y be of s o m e h e l p to r e c o n s t r u c t p a l e o h y d r o l o g i c a l c o n d i t i o n s . U n f o r t u n a t e l y , t h e 1so c o n t e n t of p a l e o l a k e w a t e r s a n d t h e r e f o r e t h a t of c a r b o n a t e a r e n o t d i r e c t l y r e l a t e d t o t h e i s o t o p i c c o m p o s i t i o n of paleoprecipitation. They are mainly governed

~2 1987 Elsevier Science Publishers B.V.

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157

by the hydrological setting which includes the type of lake (groundwater fed, stagnant water sheets in valleys, terminal lakes, "pluviometer" lakes) and the water balance (Fontes and Hillaire-Marcel, 1982). Similarly, 513C values in carbonates depend on several variables, notably the turnover times and the mixing conditions. Very seldom, equilibrium conditions with atmospheric CO 2 are achieved. This is of basic importance for the discussion of 14C age validities in lacustrine carbonates. Consequently, the methodology for paleoclimatic reconstructions based on isotopic studies is site specific and knowledge of the modern hydrology of the area is a prerequisite. In this article, we will compare the modern and Late Quaternary hydrology of one of the

largest drainage basins of the Gregory Rift Valley in East Africa, using isotopic data on modern surficial and ground waters and on algal limestones deposited at the margin of paleoshorelines. The basin of lakes Natron and Magadi, on the Tanzanian-Kenyan border, is one of the lowest points on the east branch of the Rift (Fig.l). Today the two lakes, nearly dried up and covered with a fairly thick crust of trona, receive a small amount of water during the rainy season (cf. Surdam and Eugster, 1976; Jones et al., 1977; Eugster, 1980). A few pools of water near the hydrothermal springs and at the outlet of the perennial rivers persist during the dry season, especially in Lake Natron. However, the depression was occupied by much deeper, if not larger, lakes

Fig.2. S t r o m a t o l i t e f o r m a t i o n s . (a, b): 130,000 140,000 yr. (a) p e t a l - t y p e b i o h e r m s ( b a t h y m e t r i c z o n e - 1 . 5 t o - 2 . 5 m); (b) o n c o l i t e p l a t e ( b a t h y m e t r i c z o n e - 7 t o - 10 m). (c, d): 12,000 10,000 y r B.P.; (c) e n c r u s t e d s u r f a c e ( b a t h y m e t r i c z o n e - 3 t o - 5 m); (d) b l o c k of e n c r u s t e d b a s a l t ( b a t h y m e t r i c z o n e - 8 t o - 10 m).

158

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on several occasions during the Pleistocene (Baker, 1958; Thouveny and Ta~eb, 1984; Hillaire-Marcel et al., 1986). The most recent lacustrine episodes resulted in a nearly continuous belt of algal stromatolites some 50 m above the modern water level (Fig.2). Three distinct generations of stromatolites, occasionally overlapping in stratigraphic position (Fig.3), are observed (Casanova, 1987). The oldest exceeds the limits of the Th/U dating method (> 200,000 yr); the intermediate one is dated at about 130,000 yr and the most recent between 12,000 and 10,000 yr B.P. (cf. HillaireMarcel et al., 1986). Stromatolitic formations

of the same type, with ages ranging from Pliocene to the Holocene, have already been documented in most of the Rift lakes (cf. Casanova, 1986, for an exhaustive review of the relevant papers). The most detailed studies were conducted on Lake Turkana stromatolites (see Vondra et al., 1971; Johnson, 1974; and Johnson and Reynolds 1976); their isotopic composition was examined in detail by Abell et al. (1982). In relation to the preceding, the M a g a d i - N a t r o n stromatolites reveal several particularities; notably, the perfectly preserved morphostratigraphic position of several distinct generations. Firstly, we will discuss

159 their morphology and structure in relation to their paleobathymetric distribution. Then, based on a study of their 180 and 13C contents, we will attempt to constrain the paleohydrology through comparison with the present-day isotopic hydrology of the basin.

these were stable during the entire construction phase.

E c o l o g i c a l z o n a t i o n and m o r p h o l o g y o f the M a g a d i - N a t r o n s t r o m a t o l i t e s

The level of this paleolake was 655 m, that is, 47 m higher than the present level of Lake Natron. The stromatolites formed during this period were found in some ten sites equally distributed around the M a g a d i - N a t r o n basin, on relatively gentle slopes. On the steeper slopes, especially on the fault planes, erosion eventually wiped out all trace of this generation. A detailed morphological zonation has been established for this generation of stromatolites from the paleoshore to a depth of - 1 0 m (type section of the Kipalagfi Plateau, cf. Casanova, 1987; Fig.4): bioherms are largely dominant. The carbonate fraction in this generation consists of a low magnesian calcite precipitated in situ by Cyanophyceae. The structure of the stromatolitic crust depends on the type of algal development. The most common structures are flat or cylindrical crusts formed by the simple superposition of laminae, and a columnar pattern (Casanova, 1987).

Stromatolites from the high lake stand at the end of the Middle Pleistocene (ca. 130,000-140,000yr)

The stromatolites of the three generations are not equally preserved. The oldest (>200,000 yr) was found in situ in only one spot: about 80m above the modern Lake Natron, that is, 20 m above the most recent stromatolitic shoreline. The other samples of this generation were found as detritic pebbles covered by later generations. We will place special emphasis on the latter, which are better preserved and which form an observable double-belt of stromatolites almost all the way around the Magadi Natron lakes. The limits of the paleolake during the last two episodes were determined through geomorphological criteria (beaches and shorelines marking off islands and peninsulas) and biological criteria (ecological zonation of the stromatolites). The consistency and continuity of the ecological zonation indicates clearly that the stromatolites developed only during high stands and that

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Stromatolites from the high lake stand at the end of the Late Pleistocene (ca. 12,000-10,000yr B.P.) The limit of this paleolake is 656 m, that is, 48 m higher than the present Lake Natron and 56 m higher than Lake Magadi. The stromatolites formed during this period were examined at about 50 sites around the lakes. Occasionnally, they encrust those of the preceding generation. To the NW of the basin, along the Nguruman fault escarpment, they are observed at two different altitudes (676 and 656 m), probably due to vertical movement during the corresponding high lake stand (Hillaire-Marcel et al., 1986). A morphological zonation is also observed from the paleoshoreline to a depth of - 1 0 m (type section of the Mo~nik River plateau; cf. Casanova, 1987; Fig.5). Bioherms are not as frequent as they were in the previous generation, notably below a depth of 3m, where laminar type incrustations are characteristic. Regardless of its bathymetric and geographic position, the structure of a stromatolitic crust of this generation is always composed of three distinct parts: a basal sole with flat interwoven laminae, a dark middle part made up of fan-shaped colonies, and a light colored upper part comprised of columnar laminae.

The two lower zones are generated by two different bacterial populations; the precipitated carbonate being a high magnesian calcite. The columnar zone is generated by Cyanophyceae; the precipitated carbonate being again a low magnesian calcite (Casanova, 1987). One of the thickest bioherms of this generation yielded ages of 9860___150 yr B.P. (UQ-951) and 9710_+100yr B.P. (UQ-953) respectively for the basal and top layers, thus indicating the scale (102 years) of the growth period.

Stromatolites - - paleobathymetric indicators? It is generally acknowledged in the literature that lacustrine stromatolites are excellent indicators of the bathymetric zone from 0 to - 1 m (Von der Borch et al., 1977; Abell et al., 1978; Talbot and Delibrias, 1980; Osborne et al., 1982; Krylov, 1982, etc.). The M a g a d i - N a t r o n stromatolites, characterized by an ecological zonation from 0 to - 1 4 m, clearly show the possibility of development of crust-forming Cyanophyceae in the zones below the s.s. shoreline. These facts concur with the observations made at Green Lake, where stromatolites develop down to a depth of - 8 m (Eggleston and Dean, 1976) and at Lake Wondergat, where dragging has revealed the

161 presence of active stromatolites as deep as - 6 0 m (Gow, 1981). Therefore, the use of stromatolites as paleobathymetric indicators must be subject to great caution. Around lakes N a t r o n and Magadi, some isolated samples gave a margin of altimetric error of as much as 10 m, which is very significant in terms of the amplitude of Q u a r t e r n a r y lacustrine variations.

Microstructure of the M a g a d i - N a t r o n stromatolites An analysis of thin sections in stromatolitic microfacies shows a textural relationship between generations (Fig.6), despite the morphological differences observed. In particular, there seems to be a general absence of detrital

particles trapped and retained by algal filaments. The construction of the stromatolite results from the biological precipitation of calcite by the algae and/or the bacteria. The calcium carbonate is extracellular; it forms around isolated filaments or between filaments grouped in colonies. The micritic, microsparitic and sparitic structures are generated by algae as well as by bacteria. Locally, large angular detritic particles (quartz, feldspar), originating from the crystalline basement outcropping east of the basin, are interspersed between the laminae or constitute the nuclei of oncolites, but they never intervene in the construction of the stromatolitic laminae. The lamination is therefore a direct reflection of the growth of the algal mat. Furthermore, the most common laminar organization in the

Fig.6. Principal microfacies of stromatolites. 130,000 140,000yr generation: (a) repetitive algal lamination interrupted by organic film of bacterial origin; (b) oncolites with algal laminations. 12,000-10,000yr B.P. generation; (c) repetitive laminations of algal origin; (d) fan-shaped bacterial colonies. Scale: 1 mm.

162

samples studied results from the superposition of a doublet composed of a wide, light-colored lamina and a thin dark one. The light laminae, ranging from 30 to 300/~m in size, contain algal filaments and represent a favorable growth phase for Cyanophyceae. The dark laminae, ranging from 2 to 40 pm, are composed of one or more films rich in organic matter but without algal filaments; this can be interpreted as a bacterial development phase unfavorable to Cyanophyceae. Consequently, the MagadiNatron stromatolites reflect basically long periods favorable to algal growth, alternating with short episodes of interrupted growth. The doublet, composed of a light lamina and a dark lamina of equivalent thickness and characteristic of a climate with widely contrasting seasons (Casanova, 1987), is practically absent here. It would therefore seem that during the high lake stands when the stromatolites were built, there were very slight seasonal contrasts.

Isotopic composition (blsO and j~3C) of stromatolites Approximately one hundred samples were subject to isotopic analyses, some of them seriated in the specimens and some of them conducted on aliquots of carbonate representative of each specimen. The results, in b-units (vs. PDB-standard; cf. Craig, 1961a), are shown in Figs.7 and 8. The three generations are characterized by relatively high b180 and b13C values (Fig.7). In this respect, the M a g a d i - N a t r o n stromatolites differ from their analogues in other areas of the Rift. In the Haddar Valley (HillaireMarcel et al., 1982), around Lake Turkana (Abell et al., 1982) and in the Suguta Valley (Hillaire-Marcel et al., unpublished), such high values are more rare; only the Lake Bogoria stromatolites have nearly as high values (Casanova, 1986). Later we will discuss the paleoclimatic significance of this difference. The b180 and b13C ranges defined by the three generations cannot really be distinguished from each other. This seems to indicate that the geochemical conditions that

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prevailed during the calcite precipitation were very similar from one lacustrine episode to the next. And since the two most recent paleolakes reached nearly identical levels, the overall paleohydrological conditions probably differed very little. Before going into in depth interpretation of the 180 and 13C contents of the stromatolites, we will examine the present-day hydrology of the basin.

Isotopic hydrology of lakes Natron and Magadi During the dry season, as we have already mentioned, the two lakes are fed by numerous hydrothermal springs and, in the case of Lake Natron, several perennial rivers (Enwase Ngiro, Engare Sero, Peninj, Moinik, etc.) that drain the relatively shallow and dilute groundwater of the basaltic and Precambrian plateaus to the west and the same hydrothermal systems. Highly detailed studies of these springs have

163 been published (cf. for example Jones et al., 1977 and Eugster, 1980 for Lake Magadi; Ghedari, 1984 for Lake Natron). As a whole, the hydrothermal waters are highly alkaline and of the sodium carbonate and chloride types. The carbonatitic volcanism characterizing the Oldoinyo Lengai volcanic complex (Fig.l) may of course be related to the carbonated property of the water. In the present article, we will examine particularly the water isotopic composition (180, 2H) and that of the TIDC (Total Inorganic Dissolved Carbon; lsC, 1"C). Samples were taken from some 40 springs around two lakes. Their 51sO values (vs SMOW) ranged from - 4.4 to - 1.3°o and their 52H values from - 34 to - 15%o (Figs.9 and 10). However, most (29/39) have less variable isotopic compositions, e.g. about - 3 to -4%0 for oxygen; in all probability, the springs are basically fed by a homogenous groundwater body. The deviation towards higher heavy isotope contents, observable in some springs, can be explained in part by a high rate of evaporation near their outlets; this was the case for springs with low flows and/or very high temperatures (e.g. Little Magadi, Fig.9: 5180 = - 1.3%o; 52H = - 15%o; t°>80°C). These same hydrothermal inflows are of course characterized by a high concentration of dissolved salts. In the case of the Little Magadi hot springs, which we just cited, the dissolved solids constitute 24.6 g1-1 (cf. Jones et al., 1977). The evaporation effect can also be observed along the perennial watercourses and is particularly noticeable in the Enwase Ngiro river, which is the longest and drains the largest part of the basin (Fig.ll). As might have been supposed, the pools of free water in the two lakes show heavyisotope contents that are variable but always higher than those of neighboring springs ( - 3 . 5 < 51 sO _< + 0.9%0 in October 1982 in Lake Natron). Finally, the interstitial brine in the sediment (alternating clay and layers of trona; cf. Baker, 1958) is by far the richest in heavy is(~topes (~2H=+24%o; 5180=+8%o at 7 m depth in the central part of Lake Magadi).

The 52H-5180 values of the 11 springs sampled around Lake Magadi (Fig.10) show a linear relationship (r = 0.89) with a slope of 4.7, which is typical for waters having undergone fairly heavy evaporation (cf. for example Fontes, 1976). It is interesting to note that the interstitial brine in the Lake Magadi sediment fits very satisfactorily onto the regression line defined only by the hydrothermal springs. Contrary to expectations, the water's geothermal circuit left no apparent isotopic trace such as a selective increase in 180 contents which may have been produced by exchanges with surrounding altered rocks (cf. Craig, 1963; Gonfiantini et al., 1973; Fontes, 1976). In this case, however, consideration must be given to the large, deep reservoir of CO2, linked to the carbonatitic volcanism of the Oldoinyo Lengai (cf. Hay, 1983). Isotopic exchanges with this reservoir would tend to make the water selectively poorer in xso (cf. Ferrara et al., 1965), thereby offsetting the effect of the exchanges with silicates or carbonates. The aforementioned absence of isotopic signals therefore does not mean that there were no exchanges between the aqueous phase and the deep aquifer rocks during the water's geothermal circuit. The high salt concentration (cf. Eugster, 1980) would tend to indicate the contrary. Finally, no significant contribution of magmatic (juvenile) water could be seen (Fig.12). Discussion

From the above data, we will try to decipher the main paleohydrological characteristics of the high lake level stages. During the corresponding humid episodes, a significant recharge of the deep Rift aquifer certainly occurred. Therefore, groundwaters may have preserved an isotopic "memory" of the paleoprecipitation at their origin. That will be our first point of discussion.

The hydrothermal water origin The regression line representing partial evaporation of the Lake Magadi springs inter-

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sects with the one representing meteoric water (Craig, 1961b) at a point whose coordinates (52H = - 37%o; 5180 = -6%0) are relatively close to those of the spring water that is the poorest in heavy isotopes (Fig.10). The meteoric mark appears even more distinct when one examines the 1sO and 2H contents of the dilute water

springs along the Nguruman fault escarpment. The two sites sampled gave 51sO and 52H values that fit exactly onto the meteoric water line. However, it is likely that the deep groundwater at the very origin of the hydrothermal springs was significantly renewed during one or several climatic episodes differ-

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ing slightly from the present conditions. This hypothesis is suggested by the hydrogeological model proposed by Eugster (1970, 1980). Based on concentrations and ion ratios, Eugster considered the springs to be a mixture of three components (Fig.13): (1) the deep groundwater

of the Rift, warm and relatively brackish (ca. 3% by weight of solids); (2) the Lake Magadi brines, which plunge towards the south end of the lake and are recirculated (10-30°//o salt); and (3) surficial dilute groundwaters with subpresent recharge (~0.1°//o salt). In terms of

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%o

Fig.10. ~2H and ~lsO of waters sampled in October 1983 in the Lake Magadi area. The hydrothermal springs accept a regression line (r = 0.89) with a slope of ca. 4.7, close to t h a t observed for precipitation having undergone heavy evaporation ( ~ 5).

6180 (%0) d o w n s t r e a m Enwase ngiro river

SPRINGS

Fig.9. 1sO contents of some hydrothermal springs around lakes Magadi and Natron.

isotopic composition, the latter can be represented by inflows of freshwater from the Nguruman fault escarpment, which in fact is collected for consumption by the Magadi community (5180 = - 3.9%0; 52H = - 20%o). The interstitial brines, pumped into the middle of the lake, could represent the " h e a v y " component (~lsO= +8%0; 32H= +24%0); the deep one would then have 1sO and 2H contents equal to those of the intersection of the meteoric water line and the regression line (~180=-6%o; ~2H = - 37%0; cf. supra) or even lower (Fig.12) if it is admitted th at the points corresponding to the hydrothermal springs must be within the triangle demarcated by the three components. According to this hypothesis, the last component would have the following coordinates: &1sO = - 7%0; &2H = - 45%o). Before examining

-3"9e~ ~

t/~k

-3.1 e- 1.6

e

Magadi Lake Natron

.)o - O . 1

Fig.ll. Increase in 180 content of the Enwase Ngiro river, downstream (October 1983 field trip).

the paleohydrological significance of this hypothetical isotopic composition of the deep groundwater, we will give a look to t hat of modern precipitation. During the three field trips (1982, 1983 and 1984) made in the late autumn, i.e. at the beginning of the minor rainy season (about 30% of annual precipita-

169 8SMOW 2H

~ q,~),-" £/

Shallow

oroo.Owa, ry DEEP EXCHANGES -50-

water assumed to come from the s h a l l o w dilute g r o u n d w a t e r bodies. These, then seem to represent the present-day meteoric contribution. The M a g a d i - N a t r o n drainage basin has an area of 23,000 km 2 (Fig.14). Altitudes vary from 600 to 3000 m; current average temperatures decrease from about 30°C (Magadi) to about 10°C at 3000 m of altitude. Precipitation (50year average for Magadi) varies from less t h a n 5 0 0 m m yr -z to more than 1750mm yr -1. Conversely potential evapotranspiration decreases from 2400 mm yr 1 (Magadi) to less t h a n 1400 mm yr t at the highest altitudes. The mean renewal altitude for the s h a l l o w g r o u n d w a t e r can thus be estimated at 1900 m. By attributing the mean 6zsO value of about -4%0 to the precipitation at this altitude, we can extrapolate an isotopic composition range for the entire basin between 0 and -7.3%0, assuming that the gradient (-0.3%o/100 m of altitude change) measured by Tongiorgi (in Gonfiantini, 1971) on the slopes of Kilimanjaro, 1 5 0 k m SE of lakes N a t r o n and Magadi, is applicable here (Fig.15). Moreover, it is clear that the heavy-isotope c o n t e n t of the present-day precipitation is relatively low, considering the a n n u a l average temperatures in the basin. This is explained by a c o n t i n e n t a l effect (cf. Craig, 1961b) despite

\ ~

(%0) +50-

~'&~

/ / ~ _V~-,'f~S~I ~

..=::~J~" m ~ ~'~

~ brines

~DEEP ~WITH

JT

EXCHANGES ROCKS

@

Hypothetic deep groundwater

Juvenile water

6SMOW180 (%o)

Fig.12. 3180 and 52H of the components of the Magadi hydrothermal springs. tion vs. 70% from February to April), we had the opportunity to collect about ten rainwater samples. These are probably not representative of the average a n n u a l precipitation, especially since the air was rarely saturated during these minor rainfalls, as can be seen by the evaporation lines (52H vs. 51so, Fig.10) they define. At their peak, however, the rains had 51so and 52H values of about - 3 . 8 0 o and -25%0 respectively, close to those measured in the fresh-

Interstitial brines ] 6 1 8 0 > > o (+8%o) I

:0'

/

Hydrothermal springs S ~-6-38%o ~

~ + +/~+ /~<-~-

....~ +

+

+

Shallow groundwater

6180=-4%o

Deep groundwater 6180 = - 6 / - 7 % o 613TIDC =-4.3%o S = 30%0

Fig.13. Isotope composition of the components of Eugster's hydrological model (1980). The springs originate from a mixture of deep ground water, surficial ground water and brines.

170

Precipitation

ET MAU NAROK

NAROK 1890m ~m 1112)

NAROK 1890m

VIAGADI SODA Co 613m mm 20

lS0~ ~

5025-

2 0 0 ~

0

I

~100 0,~,,~ ......

LOLLOND( 2134m

ocl(~) " 2

I\

I\ o ,o~f~,, ~ . . 3 ~ s

U~

25

50km

o

Fig.14. Average precipitation and potential evapotranspiration for the Magadi-Natron basin.

2000-- PRECIPITATION 1000-- (mm'yr-1)

i

MAT ~t ~10°C

r

3 0 0 0 - ELEVATION (m)

6SMOW180 ~ -7.3%o

Tgradient -0.3%o/lOOm

2000-

N16°C

~-4.0%o

~29°C

~

1000-600 , 0

10.~103

20.~103

CUMULATIVE SURFACE ( k m 2 )

Fig.15. Altimetric distribution of basin surface, precipitation and their average 180 content.

0%0?

171

the relative proximity of the Indian Ocean (ca. 700 km at the closest point). From the preceding we can conclude that the isotopic composition of paleoprecipitation recorded in the deep groundwater reservoir of the M a g a d i - N a t r o n basin cannot be simply transcribed in terms of paleotemperatures. Assuming ~180 values of - 6 to -7%0 for paleoprecipitation (cf. supra), these would translate into a 600 1000 m drop of the altitudinal climatic zones in relation to the present situation, i.e. a temperature drop of about 3-5°C in reference to the present-day thermal gradients. Later, we will discuss the possible age of such paleoclimatic episodes.

LITTLE

-0,2(9.5 - 5 . 4 (8.9 +0.8

-

-

]A value of -5.4%0 was observed for the TIDC in a spring on the west shore of Lake Magadi. This spring trickles into highly organic mud whose origin can be understood in light of the fact that during our field trips, we baptised this bay Flamingo nursery. It is probable that the decomposition of organic matter produces CO 2 very poor in heavy carbon, as we observed in other pools in Lake Natron for example (&]3C=-9.9%o), where flamingos and marabou storks also abound. In the case of the abovementioned spring, the ~3C therefore does not seem representative of the deep influx.

- 0.9 (9.80) +1.0(>11)

~+0.7(10.1)

L. MAGADI

+0.9

---0.9

(9.80)

""-'--0.2(9.55) ~-~-- - 1.2 ( 9 . 6 2 )

(7.80)

1.8 (8.4) 2.1 (9.75) 0.2 (9.37) 0.5 (9.77)

The isotopic composition of dissolved carbon For the Magadi Natron waters in general, TIDC-13C contents are extremely variable. The hydrothermal springs, for example, have 613C values ranging from -4.3%0o to + 0.7%o, with pH values between 9 and 10. The springs close to Oldoinyo Lengai probably best represent the contribution of deep carbon originating from carbonatitic magma. In fact, Hay (1983) concluded that the alkaline carbonates of the Kerimasi (adjacent to the Lengai) were of purely magmatic origin; furthermore, he considered that a carbonatitic magma usually, if not always, has a natrocarbonatitic composition. This would largely explain the general chemical characteristics of the hydrothermal springs. As for the springs SE of Lake Natron, on the flanks of the Lengai, they would have the lowest 13C contents (minimum (~13C= 4.3%o)1; this isotopic composition seems to correspond with that of a magmatic

~_

:: L A K E NATRON

~/

i

j -

O. 1 (9.60)

I

lOkm

I

1.1

-1.7 -2.9~ -8.6---3.9 _..ffa _4.3/`.

8PDB13TIDC(pH) -2.03

Fig.16. 13C contents of TIDC in springs (dots), lake brines and pools (triangles) and rivers (squares).

carbon at the end of its initial differentiation (613C-- -50/oo; Cfo Taylor et al., 1967; Pineau and Javoy, 1969). At the other extreme, the interstitial brines in the Lake Magadi sediments have the highest 13C content observed (613C= +1%oo). The values for the hydrothermal springs are interspersed between these two poles, thus reflecting the mixture of deep contributions with brines. Several factors besides the mixing can account for the variations of TIDC isotope composition: (1) release of CO: near the spring outlets; (2) carbonate precipitation; (3) local CO/ contributions from organic mud interstratified with layers of salt. Furthermore, the lake brines are rapidly undersaturated in relation to the atmospheric

172 CO 2, inasmuch as the trona precipitation occurs from equimolar proportions of CO3 Eand HCO3-, thus reducing the bicarbonate content in the residual brine (Eugster, 1980). The lake brines therefore move fairly rapidly towards an at least partial isotopic equilibrium with atmospheric CO:. Along some of the rivers draining the hydrothermal springs, we also observed a tendency towards 13C enrichment through isotope exchange with atmospheric CO z. Such is the case for the Engare Sero river (Fig.17). In the Enwase Ngiro river, this effect is largely masked by the biogenic carbon added as the river crosses the swampy plain NE of the Shompole volcano. The preceding allows us to understand the wide range of 14C activities in the hydrothermal springs. The deep inflows from the Lengai vicinity are almost completely free of radioactive carbon: subrecent travertines revealed 14C activity levels of less than 1.4% (in comparison with the reference value of 13.56 dpm g-~). Similarly, the springs in the northern part of Lake Magadi have very low ~4C contents (<10%; Fig.18). The interstitial brines, which we have seen tend to balance their dissolved carbon with atmospheric CO 2, have the highest 14C levels (40-50%). The

613C (TIDC) Engare

downstream sero river

~. " .--"'-'"... "L-_~ Natron~..: ~-----'-'_--' " - ~ - 9.9 ~ - - - - - Z - - ~ - - - - - - ] (above Lake

m-'°r u ;C

L I T T L E MAGADI 9.8

-+ 2.1-----..

~.~

4.6 -+0.2~

~7.7 + 0 . 8 ~/~3 4 . 3 - + 0 . 4 ,,,~42.3

-+ 0 . 5

TRONA CRUST BRINES

,....--9.6-+2,1 L. MAGADI "'"--50.9 -+ ^~'*" - STREAM SMALL -+1.5 -+3.1

t

~

LAKE ..

NATRON

: !:it

::i:;

I

1Okra I

14C CONTENT (%NBS) OF SPRINGS

Fig.18. ~4C activity of TIDC in springs and brines of Lake Magadi.

recirculation of brines towards the south (cf. Eugster, 1970, 1980) can probably explain the intermediate 14C levels of the springs in the south part of Lake Magadi and the north part of Lake Natron around the Shompole volcano (ca. 17%; cf. Fig.18). Obviously, in such a context, the 14C is of no use in assessing the age of the last significant recharge period of the deep groundwater. The contributions of surface COE (atmospheric and biogenic CO2) are largely overshadowed by those of magmatic CO2.

Conclusions about the present-day isotopic hydrology Fig.17. Variations in 13C content of the Engare Sero river TIDC, downstream (progressive equilibration with atmospheric CO2).

From what precedes, we will retain a few factors as a basis for determining the paleohy-

173

drological conditions that let to the deepening of the lakes during the Late Quaternary: (1) During the last (?) significant recharge episode of the deep groundwater at the source of hydrothermal springs, the precipitation was poorer in heavy isotopes ( - 7 _ < 3 1 s o < - 6 % 0 ) than it is today (-4%o_<51so). Expressed in terms of altitude, this difference corresponds to a downward shift of 600 to 1000 m in climatic zones. The hydrogeological history of this ground water may of course be much more complex. Several significant recharge periods may have produced a composite isotopic print. Since the Rift trachyte aquifer is characterized by a fracture porosity, we are however justified in assuming significant recharge during the last humid episode, which would therefore largely account for the 180 and ZH contents observed today. (2) The carbon of the hydrothermal springs is mostly of deep origin, linked with the carbonatitic volcanism of the region. The isotopic nature of these inflows appears in the 13C content (6~3C = -4%0) and especially in the nearly total absence of lac. Later on, this second point will lead us into a more detailed discussion of the significance of the ~4C activity measured in the stromatolites of the last paleolake. (3) The ponds remaining at the surface of Lake Natron during the dry season show highly variable ~sO contents ( - 3.5 _<6 ~80 _< + 0.9 before the 1982 rainy season). Although these values are higher than those of the springs and rivers feeding the lake pools they reflect a fairly high renewal rate of the water. Longer residence times for the water would result in a large increase of heavy oxygen content, due to continuous surface evaporation (see the example of groundwater controlled lakes in Sahara; Hillaire-Marcel, 1982). This is another point to which we will return in our discussion of paleolake isotopic composition.

Isotopic composition of paleolake waters In a previous chapter, we saw that the three generations of stromatolites did not really

differ from each other in terms of 15C and 180 contents. We will now give special attention to the last generation, which is the best represented of the three. Our conclusions on the paleohydrology of the corresponding lacustrine episode will probably be applicable, in some measure, to the preceding episodes. 6180 values of the stromatolites range from 0 to + 4%0 (Fig.8). Assuming a plausible temperature of about 25_+ 5°C for the paleolake water, and supposing that the calcite was precipitated in isotopic equilibrium with the water, we can calculate the domain of its oxygen isotope composition (61sO vs. SMOW): +2_+1%o (cf. Craig, 1965). However, the stromatolite 1sO contents, especially in the last generation, can be seen to be much more homogenous in the part of the belt surrounding Lake Natron (51sO vs. PDB = + 2.6 + 0.4%0; average for 20 samples), compared to that surrounding Lake Magadi. The paleobathymetry of the basin (Fig.l) explains this difference. The many shallow bays to the north created conditions favorable to large variations in isotope composition, due to evaporation periods alternating with episodes of heavy stream inflows, notably on the axis of the Enwase Ngiro paleoriver. The Lake Natron stromatolites therefore define a more precise 1sO average content for the paleolake water: 51so vs. SMOW= +4.4+1.4%o. These figures are given only as an indication. When we analyze the 13C contents, we will see that the calcite precipitation can in fact occur off isotopic equilibrium. The high 61sO values in particular must be regarded with caution. But the fact remains that the ~sO contents were distinctly higher in the paleolake than in Lake Natron today, and higher still than the values attributed previously to the precipitation ( - 7 _ < 6 1 s o < - 6 % 0 ) that recharged the deep groundwater of the basin. The paleolake, with a level of + 50 m above the present one and a surface area of 1959 km 2 (vs. 1203 km 2 today), could not have been filled unless there was a favorable precipitation/evaporation balance, i.e. paleohydrological conditions characterized by precipitation lower in ~so than it is today. We are therefore led to conclude that the

174

paleolake water was considerably enriched in ~80 in relation to its direct meteoric supply (runoff) and possibly its indirect supply (shallow groundwaters). This enrichment cannot be explained without assuming a long enough residence time to result in a high, relatively stable 180 content under steady conditions (5180 = +4.4_1.4%o; cf. Fig.19). It would be purely academic to try to push this reasoning any further, since we have no way of finding out the exact rate of renewal for

the paleolake water. However, it is interesting to dwell for a moment on the fact that the paleolake level stabilized at 656 m long enough to allow stromatolite construction for nearly 2000 years. There seem to have been three major phases of growth at approximately 11,500, 10,300 and 9000yr B.P. (cf. HillaireMarcel et al., 1986), all of them at about the same altitude. During each phase, the paleolake level remained stable for several hundred years (cf. Casanova, 1987). Even more surpris-

ELEVATION (m) 700-

/e

/ / /tf 656m LIMIT

650-

600

0

t

"" / ~ " ~ ~"83) ~ ' 1 9 5 9 )

1500km 2

2000km 2

lOkm 3

60km 3

1 lOkm 3

I

t

14" • /

lO00km 2

I

/

I

6SMOW180

(%o)

~ O"

' ~ l - p ) ( 6 n - 1 - F - . L - v L n ( 1 - p ) ) - ( p ' - p ) 6 n - 1 +p'6o

~

6°"-7%° EL-v ~-13%o

./LChange in w a t e r budget ~ - - - ( m a x i m u m lake volume)

6n - ( 1 - p ) ( S n - l - E L - v L n ( 1 - p ) ) + p ' 6 o

p V EVAP. =

,_ (p-E)easin P - -VL-'-~

-5" p-E=170mrn x Ssasin

0

5'0

1()0

150 Seasonal cycles

Fig.19. (a) Paleolake surface and volume in relation to shoreline altitude. (b) Model of the evolution of its 180 contents in the case of a positive precipitation-evaporation balance (+ 170 mm).

175 ing is the fact that during the previous lacustrine episode (around 140,000-130,000 yr), the paleolake came within 2 m of the same level (ibid.). The first causes that come to mind to explain such a fact are geomorphological. But in this case, there is no sill beyond which the paleolake could have drained into another depression. As we mentioned earlier, the M a g a d i - N a t r o n basin is the lowest point in the eastern branch of the K e n y a n Rift. Moreover, the basin shows no sign of morphological discontinuities that would account for a sudden increase in the evaporatory surface of the paleolake above 656 m. The sharply rising walls of the Rift explain the nearly linear relationship (Fig.19) between volume and depth. Here again, there is no particular discontinuity at 656 m. Consequently, we cannot explain the stabilization of the paleolake by citing a sudden change in its precipitation/evaporation balance in relation to the morphology of the basin. We are therefore led to consider a control of the paleolake through the piezometric level of the shallow groundwater of the basin. On the axis of the Rift, from Lake Naivasha in the north to Lake M a n y a r a in the south (Fig.l), the hydraulic gradients indicate groundwater flows toward the Magadi Natron basin. Given the altitude of the paleolakes that occupied each of the depressions at the end of the Pleistocene (cf. Holdship, 1976), there is no reason to postulate an inversion of the hydraulic gradients during the episo2e. The only explanation we can see for the stabilization is the paleolake's direct connection with the water table in the basaltic plateau aquifer to the NW along the Ngurumann fault escarpment. Over more than 40 km, the 660-m contour travels along the base of the fault plane, which rises up almost vertically, over 400-800 m of deleveling. At the altitude of ca. 660 m, the paleolake would have been in equilibrium with the water table of the basaltic plateaus. For the water level to exceed this limit, the precipitation/evaporation balance would have had to increase by a considerable amount, given the relative dimensions of the basaltic plateau aquifer and those of the M a g a d i - N a t r o n drainage basin.

Biogenic and other isotopic effects during CaCO 3 precipitation On the whole, the isotopic composition of the stromatolite carbon covers a wide range (+2.6
= 5(HCO~)- 5(CO~-)-- + 1.6%~ (cf. Mook et al., 1974; McCrea, 1950)

176 Thus, 5(CO32-) = -5.8%0. If the pH were to increase, it can be seen that the 5(CO32-) would tend toward that of the TIDC, that is, -4.3%o. In this case, we can conclude that the maximum 313C value of the dissolved carbon is that of the original TIDC. A variant can be considered, if there is carbonate precipitation (calcite and/or trona, for example). Calcite is enriched in '3C in relation to CO32 by about 3-4%0 in the thermal range under consideration (Mook et al., 1974); trona also, but of a smaller amount (+ 1 to ÷0.7%0; Matsuo et al., 1972; +0.9%o measured at Lake Magadi). In both cases, the substraction of a compound enriched in 13C in relation to TIDC, considered in a closed system, can only result in a gradual depletion in heavy carbon, following a Rayleigh equation. Of course, the system is not closed, and exchanges with atmospheric CO 2 tend to establish the fundamental chain of equilibria of the carbonates, i.e. CO2 gas+-~CO2 dissolved+-*HCO~+-~CO~-+-* CaCO3 Assuming a value of -7.5%0 for the gaseous CO2 (cf. Craig and Keeling, 1963), at 25°C and with the successive equilibria achieved, we obtain calcite and dissolved carbon g-values of about +2.5%o and -1%o respectively. At the same temperature, the bicarbonate will show a 513C of around +0.5%0 (cf. supra). It is therefore clear that, if there is equilibrium with atmospheric CO2, since an increase in pH will tend to increase the proportion of CO32- in relation to the bicarbonate, a decrease in 13C will be observed in the TIDC. However, the calcite must maintain its isotopic composition, because the extreme isotopic equilibrium between CO2-gas and CaCO3 remains. From these observations we can conclude that the alkalinity and pH cannot explain the stromatolites' high 13C contents, nor the fluctuation of these contents in parallel with 'sO. If we compare the 513C values in the dissolved carbon of the present-day water in the basin with those of the ancient carbonates, we can reasonably hypothesize an isotopic equilib-

rium between the paleolake TIDC and atmospheric CO 2. Indeed, we have seen that the minimum 513C observed in the stromatolites (÷ 2.6%0) corresponds exactly with the isotopic composition of a calcite in equilibrium with atmospheric CO 2 at 25°C. Of course, the atmospheric CO 2 isotopic composition (-7.5%0) was perhaps not the same as today during the high lake stand; the preceding hypothesis is therefore subject to some reservations. In any case, we must explain the observed high 513C values (513C=+3.76±0.45%o, southern area of the paleolake; + 4.43 ± 0.45%0, northern area of the paleolake) and, also, find the cause of the parallel fluctuations in 1so and 13C (Fig.9). The first variable to take into account is the temperature of the isotopic equilibria (CO2~ CaCO 3 and H20+-*CaCO3). The thermodependence of the two fractionations (D gives a d513C/d51SO ratio of about 0.6 at 20-30°C (see for example the equations of Craig, 1965 and of Fontes and Pouchan, 1975). What of the isotopic shifts observed in the stromatolites? The slopes of the regression lines, based on 51sO and 513C values, vary from 0.4 to 0.85 - - at least, for those that accept significant correlation coefficients (0.70-0.88). This is not very conclusive. Moreover, in our hypothesis, variations as high as 2%0 in 513C values within a single stromatolite would have to be explained by extreme thermal divergences; 15-20°C, which seems rather excessive. We are also unable, within this hypothesis, to explain the highest 513C values: + 5.4%0 for this generation in Lake Magadi. We would have to assume a temperature of 0°C at the time of the calcite precipitation! Such high 513C values are occasionally observed along hydrothermal streams in relation to degassing processes. They are exceptional in lakes. Given the hydrological conditions characterizing the intertropical regions, the paleolake must have experienced seasonal and pluriannual variations in its water lso/160 ratio. The fluctuations may have been smaller in those days than they are now; nevertheless, their existence is proven by the rhythmic growth

177

(Fig.8) and do not seem to present any linear tendency. The simultaneous variations in heavy isotopes of carbon and oxygen are therefore applicable only to the growth of individual stromatolites. Calcite precipitation is no doubt linked to the assimilation of CO 2 by Cyanophyceae. The latter are known to use preferably CO 2 with light isotopes, notably 12CO2, rather than 13CO2 (cf. Abelson and Hoering, 1961). As a result, during periods of intense photosynthetic activity, the surrounding environment - - in this case, TIDC - - tends to become enriched in heavy isotopes. Similar effects can be observed with higher forms of plant life (cf. Lowden and Dyck, 1974) and in foraminifera symbiotic with algae (Williams et al., 1981). The precipitated calcite therefore

pattern of the stromatolites. The simplest explanation for the fluctuations in 180 would rather be the alternation between humid periods (influx of meteoric water depleted in 180) and dry periods (evaporation and enrichment in 180 of residual water). This explanation has the particular advantage of accounting for the fact that the northern part of the paleolake (Magadi), shallower and fed directly by the Enwase Ngiro paleoriver, shows wider variations in stromatolite 3180 values (+ 0.1 to + 4.4) than those observed in the deeper part of the paleolake more to the south (Natron: + 1.6 to +3.3%0). It now remains to explain the parallel fluctuations in 13C. First of all we will restate that the stromatolite isotopic compositions, on the whole, cover a large surface

6 1 3 C %o

6180%o +2

+3

A14C

+2

+3

+4

I

I

I

%

15

20

25

I

I

I

Fig.20. 180, ~3C a n d ~4C c o n t e n t s of a t r a v e r t i n e pipe N W of L a k e M a g a d i , f o r m e d on t h e N W s h o r e of t h e p a l e o l a k e a r o u n d 12,000- 10,000 yr B.P.

178 CaCO s in equil. with atm. CO 2 at 25°C I STROMATOLITES %.,r (CaCO3)

=Atm. CO2 O~

100 HCO 5 in equil, with aim. CO 2 at 25°C

k÷" "/ x +%~__. "t~'_~ II . . . . .

I ll//

..-?. n" ~e 50 v 0

TIDC (=HCO 5) Oldono Lengai springs -=deep TIDC / carbon LAKE MAGADI . / f E n a a r e Sero SODA S P R I N G S / " / travertine / ~ o ~ o . . . . . ~". (CaCO3) /" "x

[

....

0 -5

=ooo,

.

~

/TRAVERTINE .~..1:_~," PIPE • (CaCO3)

/ " ~ " "

.

0

.

.

::E~:::~ Preferential uptake of 12CO2 by algae or plants and relative enrichment in SC of surrounding TIDC

.

-5

6PDB13C (%o)

Fig.21. Comparative laC and 14C contents of deep TIDC and lacustrine carbonates (the 14C activity of the stromatolites at time 0 was considered to be that of carbonates in equilibrium with atmospheric CO2).

becomes richer in 13C at that moment. Since algal photosynthetic activity is a function of temperature and light, it can be seen that during periods of heavy precipitation, the 1so content in the water decreases, the lake becomes deeper and more turbid, and the light and temperature decrease, so of course the algal mat tends to reduce its photosynthetic activity. The calcite then records a decrease in its 13C content, which approaches that of equilibrium with atmospheric CO2. Conversely, during dry periods, evaporation increases the proportion of heavy oxygen in the water, the water level falls, there is more light and the temperature increases. Photosynthesis then also increases (up to emersion, of course), the ~3C content in the ambient TIDC (immediately surrounding the algal mat) rises, and the calcite becomes enriched in heavy carbon. Strictly speaking, these are not kinetic effects; the calcite is able to maintain its isotopic balance with its immediate environment. However, the possibility of kinetic effects is not excluded. Biogenic limestones are often depleted in heavy isotopes when compared to limestones that have been precipitated more slowly (cf. for example calcite precipitation by foraminifera: Vinot-Berthouille and Duplessy, 1973). In our case, such effects are not to be

excluded, notably when the algal mat tends to precipitate the calcite very actively. But on the whole, we can disregard them since the &~3C values exceed those that would be expected under equilibrium with atmospheric CO2. 14C activity of paleolake TIDC Except for the biogenic isotope fractionations, the 14C activities of the stromatolites should reflect that of the atmospheric COz at the time, in the light of the preceding discussion. In fact, it is interesting to compare the stromatolites' average 14C activity with that of the carbonate in the hydrothermal mounds formed during the last high lake stand (cf. Hillaire-Marcel et al., 1986 and Figs.20 and 21). These travertine pipes show a relative 14C depletion of about 25% when compared to the stromatolites formed during the same period. In comparison with the present day (Fig.21), the hydrothermal influxes of the time therefore present a very different hydrological budget: the contribution of surface water (ca. 75%) was greater than that of deep groundwater. The high lake stand, then, was indeed a phase of significant recharge for the deep groundwater. Isotopic exchanges between the paleolake TIDC and atmospheric CO 2 was no doubt

179

favored by the long residence time of the water, as shown by the high 5~sO values. Later, during the Holocene, lakes Magadi and Natron experienced several short episodes of partial filling. No stromatolite construction took place, but these episodes are evidenced here and there by accumulations of calcitic oncolites (cf. Casanova, 1986). As for the 14C activities and 13C contents measured in these carbonates, we are in a position to believe that the lake TIDC was able to approach equilibrium with atmospheric CO 2 on several occasions: the most recent oncolites show a 14C activity of 72.5%. This value stands as further proof of the tendency towards atmospheric CO2-TIDC equilibrium. Conclusion

Compared to other lake indicators, stromatolites are treasure-houses of information. Like most biogenic carbonates, they keep a record of the time gone by (~4C, Th/U; cf. HillaireMarcel et al., 1986) but also an isotopic print of their paleoenvironment of deposition (~3C, 1sO). In addition, they are valuable because they precisely mark ancient shorelines. In our study, stromatolites enabled us to Carbon cycle

Soda

,,,~pring ~'~ ~

~ = \

Water cycle

Atm CO2 14C ~ 100% Travertine pipe Evaporation '~ I {spring-mounds) |1 (14Cca 75%) J ~, St.... tolites I I~-_.____~)~ [ Lake water I TIDC [ Or anic ~ ~ I 8180 _- +4.5%0

~..~DDe e

establish that the high lake stands at the end of the Pleistocene stopped at the same altitude (ca. 650 m) and that the paleolake levels were probably controlled by the water table of the basaltic plateaus. Their stable-isotope contents, when compared with present-day hydrological conditions, pointed to a long residence time of the paleolake water, which favored the achievement of an isotopic equilibrium between atmospheric CO/ and the inorganic dissolved carbon. Therefore, the 14C activity levels measured in the M a g a d i - N a t r o n stromatolites may be converted to real 14C ages. The parallel variations in 13C and 1sO during the stromatolite growth was linked to the variable development of algal mats in relation to hydrological conditions in the paleolake. Low contents in heavy isotopes indicate periods of heavy precipitation during which increased turbidity, higher lake levels and lower temperatures combined to reduce algal photosynthetic activity and thereby prevent the relative enrichment in 13C of the ambient dissolved carbon. The result is the precipitation of a lighter calcite. During periods of intense evaporation, the opposite effects prevailed. The filling of the M a g a d i - N a t r o n basin

~ p~ 114c~100% \l e e,pwater \ "~.~//g

TT -

/

reservoir 14C-TIDC - 0% 613C : -4 5%o?

\~~ ~ ] ~

Deep water reservoir 6180 = -7%0 ?

Paleolake Natron and Magadi

Fig.22. Paleohydro-isotopicoverview of the paleolake.

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180

during the humid periods of the Quaternary probably corresponds to the phases of significant recharge of the deep saline groundwaters of the Rift. Since this water has kept an isotopic record of the last humid episode (12,000-10,000 yr B.P.), from a "hydroclimatic" point of view it reflects a drop of about 600-1000 m in altitudinal climatic zones. This would be adequate to establish a favorable precipitation/evaporation balance. However, because we have no data on the amplitude of the seasonal variations in the paleoclimate, our interpretation can go no further.

Acknowledgements Our geological trips (1982-1983), directed by Dr. M. Taieb (CNRS), were financed by the CNRS of France and NSERC-Canada. They were made possible by the cooperation of the Kenyan and Tanzanian authorities. We are also grateful for the assistance of the Magadi Soda Company and the MasaY people. The isotopic analyses were financed by NSERC-Canada and conducted in the GEOTOP laboratory of the University of Quebec at Montreal. The collaboration of Ms. N. Page (teledetection and cartography) and Ms. O. Carro (isotopic geochemistry) was of great value. The comments of Messrs. J.-C. Fontes (Univ. ORSAY) and P. Pag~ (UQAM) were largely taken into account in the preparation of the final manuscript. M. Laither was responsible for the illustrations.

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