Paleochemistry of plio-pleistocene lake Turkana, Kenya

Paleochemistry of plio-pleistocene lake Turkana, Kenya

Palaeogeography, Palaeoclimatology, Palaeoecology, 27(1979): 247--285 247 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Neth...

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Palaeogeography, Palaeoclimatology, Palaeoecology, 27(1979): 247--285

247

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

PALEOCHEMISTRY OF PLIO-PLEISTOCENE LAKE TURKANA, KENYA

T H U R E E. CERLING 1

Department of Geology and Geophysics, University of California at Berkeley, Berkeley, Calif. 94720 (U.S.A.) (Received July 31, 1978; revised version accepted February 28, 1979)

ABSTRACT Cerling, T. E., 1979. Paleochemistry of Plio-Pleistocene Lake Turkana, Kenya. Palaeogeogr., Palaeoclimatol., Palaeoecol., 27: 247--285. The paleochemistry of Plio-Pleistocene Lake Turkana can be estimated by using the chemistry of lakes from the Eastern Rift of Africa as an analogue. Most modern East African lakes occupy closed basins; their chemistries follow an evaporation trend defined by the precipitation of certain mineral phases with increasing alkalinity. Estimates of paleoalkalinity can be used to closely estimate the chemical composition of ancient lakes. Three methods are used to estimate paleoalkalinity. Diatoms, molluscs, and fish have certain metabolic requirements that are dependent on pH, alkalinity, or calcium levels; thus fauna and flora can be used as paleoalkalinity indicators. Exchangeable cations on clay minerals can also be used because the relative concentrations of sodium and calcium in lake waters are related to alkalinity. Absence or presence of certain minerals also can serve as a paleoalkalinity indicator. Although the latter two techniques give estimates of paleoalkalinity that are averaged over several hundred or thousand years, their estimates agree with the instantaneous estimates based on biologic considerations. This study shows that the earliest lake phase was very fresh and continued until the end of the Kubi Algi Formation. The Lower Member of the Koobi Fora Formation is shown to have been a fresh- to brackish-water lake. From the beginning of Upper Member time (about 1.8 m.y. ago) to the present, the lake occupying the Turkana Depression has varied from a brackish lake that overflowed to a closed basin lake that fell below overflow level and whose alkalinity rose to about 200 meq/1.

INTRODUCTION

Ancient lake sediments often contain exotic minerals or prolific faunal and floral remains. In most cases, only rough estimates can be made a b o u t the chemical composition of the ancient lake. They are often simply described as being fresh, saline, or alkaline although a few studies have attempted t o 1Present address: Environmental Sciences Division, Oak Ridge National Laboratory (operated by Union Carbide Corporation under contract W-7405-eng-26 with the U.S. Department of Energy), P.O. Box X, Oak Ridge, Tennessee 37830 (U.S.A.).

248

describe the paleochemistry of ancient lakes more closely (e.g., Bradley and Eugster, 1969; Surdam and Wolfbauer, 1975). Plio-Pleistocene lake sediments in East Africa provide an excellent opportunity to estimate paleochemistries because modern East African lakes can be described by a simple evaporation model which can be applied to older deposits. Sedimentary deposits surrounding Lake Turkana (Fig.l), formerly Lake Rudolf, in northern Kenya contain some of the most important paleontologic and archeologic finds in the study of hominid evolution. Over two hundred hominid specimens have been recovered from the Koobi Fora and Shungura Formations which lie to the northeast and north of the modern lake, respectively. A reconstruction of the environment in which these early hominids lived is essential to the understanding of conditions which led to the evolution of Homo. Active tectonism and an arid climate give East Africa a complex assemblage of lakes and rivers. Drowned rivers such as Lakes Victoria and Kioga, lavadammed lakes such as Lake Kivu, and many closed basin lakes are common in East Africa. Recent studies make it possible to compare a large number of lakes and to determine what controls their chemistries. This study uses

suoA~

/~L. ~ 1 LAKE ABYATA"~1 ~[~L.. LANGANO

i

..KE

O.A.

~..-:, ~- ,~.~.~

,.

E A,,A

/ /

~,.

/%

'°~'°

LAKE KIOG~3

,

i

LAKE

¢O'BISINA

/

I Io|LAKE BARINGO BOGORIA

~ttL. -o0

f KAGERA~/ C MATUN~ j

, a~© --

~ ~,

NALKURU0 t L ELMENTEITA CRATER~r~.D'LAKE NAIVASHA

s~ ~::~¢~

0''

I j ~ LAKE MAGAD,

KUSARE NANYUKt RESHITANI L. LGYARA. / o . JA' : . . I. ~ . ~. _.._------LAKES TULUSIA BIG MOMELA SMALL MOMELA LAKE E Y A\$, ~ _I U, ,L~A/ ~K|E - MANYARA -- 5"

0 t

30°

35"

KM J

~

,

,

500 I

4 O°

Fig.1. Eastern rift lakes s h o w n in relationship to the rift.

5" -

249

observations on the chemistries of these modern lakes as a basis for developing methods to estimate paleoalkalinities o f ancient East Africa lakes and applies them to the specific case of the Plio-Pleistocene sediments northeast of Lake Turkana. Late Tertiary rifting in East Africa (Baker et al., 1972) has resulted in a series of lakes (Fig.l) that stretches for 2000 km south from the R e d Sea. Erosion of fault blocks has exposed the sediments that accumulated in basins created by the rifts. Thus, all stages from modern lakes with their associated sediments to ancient lake sediments several million years old occur in near proximity and are reasonably well-exposed for study. This rifting n o t only produced numerous lake basins; great outpourings of lavas and pyroclastics have provided excellent opportunities for chronology and have resulted in a volcanic terrain which produced a series of lakes whose chemistry is predominantly alkaline. Because the sediments surrounding Lake Turkana are from the Eastern Rift, these lakes will be used as a modern analogue to the Plio-Pleistocene lake which occupied the Turkana Basin. This assumes that ancient East African lakes followed the same sodium-bicarbonate--carbonate trend as modern East African lakes. From late Pliocene to the present, the region has been a dominantly volcanic terrain with few or no outcrops of pyritic or saline sediments that would provide sulfate or chloride on weathering. Lakes from the Western Rift have significantly different chemistries (Talling and Tailing, 1965; Yuretich, 1977) and lake deposits from this region m a y need a different analogue. Likewise, it may or m a y not be applicable for some other ancient sodium-bicarbonate--carbonate lake systems. This study focuses on understanding the surface water trends of modern Eastern Rift lakes and oa determining the paleochemistry of ancient Lake Turkana. CHEMISTRY OF EAST A F R I C A N LAKE WATERS AND IMPLICATIONS F O R PALEOSALINITY

Relationship between alkalinity and composition Compositions of 25 lake waters from the Eastern Rift of East Africa were compared to investigate possible relationships between various ions and to determine the degree of saturation of each water with respect to certain mineral phases. Included in these waters were samples from several lakes from the area between the Eastern and Western Rifts (Fig.l) which represent fresh waters that have undergone a minimum of evaporation. Many of the lakes in the Eastern Rift o c c u p y elongated closed basins, flanked on either side by fault scarps, and are shallow, fluctuating water bodies that rarely overflow. Elongated, fault-controlled lakes in the Western Rift are deep because t h e y are flooded to overflow. Lakes Victoria and Kioga are shallow and were formed b y gentle tilting of the entire land surface which caused drowning of the pre-existing drainage systems (Bishop and Posnansky,

250 1960). Lake Victoria, although one of the world's most extensive bodies of fresh water (61,800 km2), has a maximum depth of 79 m (Mothersill, 1976). Crater Lake occupies a volcanic crater, as do several of the lakes studied by Kilham and H e c k y (1973) and H e c k y and Kilham (1973) in northern Tanzania. Activities of ionic species of various lake waters were calculated from Table I with the computer program SOLVEQ (Reed, 1977) which takes into account the formation of complex ions. This program is similar to SOLMNEQ (Kharaka and Barnes, 19'73) and WATEQ (Truesdale and Jones, 1974). Dissociation constants are from Silldn and Martell (1964), Truesdale and Jones (1974), and Kharaka and Barnes (1973). Results are presented in Table II. Most East African lakes axe sodium-bicarbonate or sodium-carbonate in composition (Talling and Talling, 1965). Since CO~ 2 and HCO~ are the dominant anions, the molality and activity of each o f the major cations and anions are plotted in Figs.2--5 versus alkalinity (HCO~ + CO~: in miUiequivalents per liter). Empirical formulas relating molality or activity to alkalinity follow from these data: tuNa = 0.35 x 10 -a (ALK) 1'49 Fig.2 less than 16 meq/1 Na: = 1.12 x 10 -a (ALK) l'°a Fig.2 more than 16 meq/1 rnK = 0.11 x 10 -3 (ALK) °'77 Fig.2 K: aca*2 = 0.13 x 10 -3 (ALK) °'gs Fig.3 less than 2.5 meq/1 Ca: = 0.80 x 10 -3 (ALK) -l"°s Fig.3 more than 2.5 meq/1 aMg+2 = 0.14 x 10 -3 (ALK) T M Fig.3 Mg: less than 2 meq/1 = 0.52 x 10 -3 (ALK) -°'96 Fig.3 more than 2 meq/1 me1 = 0.11 x 10 -a (ALK) l'*a Fig.4 Cl: mF = 0.039 x 10 -a (ALK) 1"°7 Fig.4 F: mso4 = 0.038 x 10 -a (ALK) °'sT Fig.5 SO: msio2 : no relationship Fig.5 SiO:

Mineral equilibria in East African waters The relative degree of saturation in the lake waters with respect to several minerals was calculated. Thus for calcite the equilibrium constant Ksp is: Ksp = (7Ca+2 inca+2)(7CO~2 m c o ~ ) = 10 -a'a7

(Robie and Waldbaum, 1968)

or:

Ksp = (aca+2) (acoi2) The degree of saturation is defined as: IAP n = log ~ = log(IAP) - log(Ksp) where IAP is the ion activity product for each sample. W h e n ~2 is greater than O, the solution is supersaturated with respect to a mineral, and when it

27.5 22.5 95 142 550

7.1 10. 20.0 15.5 24.2

4600 18000 28400 62700 132000

3800 6900 4200 4800 5900

460 767 910 1230 1900

Na

6.5 6.7 13 15.5 13

3.4 3.7 6.6 15.9 12.3

726 284 387 1080 2280

284 9.3 588 695 112

84 22 113 188 333

K

3.2

3.0 3.8 4.2 5.0

7.6 4.6 2.6 0.9 4.1

13.7 23.0 11.6 9.4 10.8

5.0 5.6 18.1 11.2 16.2

Ca

6.1

1.9 3.4 5.1 0.7

2.1 2.4 3.2 1.8 5.1

7.2 12.0 3.2 5.8 2.1

3.9 2.6 10.7 11.9 5.3

Mg 0.79 0.90 1.95 2.07 2.2

189 554 965 2096 3170

107 116.4 164 168 190

15.6 22.2 38.7 47.7 78.5

2.31 3.1 4.44 6.74 13.8

Alk

91 440 74.5 117 224

7.1 2.3 25 23 269

3.7 3.9 29 13 6.9

646 5920 6390 18300 84400

1970 6620 427 489 2786

C1

422 2730 216 4420 2190

133 830 300 745 143

130 36.7 149 285 36

4.7 4.7 19.7 23.8 37

5.1 1.8 2.8 10.5 5.2

SO4

323 79.8 361 382

28.5 8.6 56.6 110 67.5

5.41

0.47 6.65

0.95

0.32

437 583 1060 966 1550

F

5.0 54 260 155 1055

177 18 7.5 8.5

31 22.2 3.2 4.3 77

30 54.6 23.5 42.8 54

17.1 8.7 2.7 11 24.6

SiO2

a pH value from another analysis in Tailing and Tailing (1965). References: 1, Kilham and Hecky (1973); 2, Tailing and Talling (1965); 3, Baumann et al. (1975); 4, Cerling (1977); 5, Hecky and Kilham (1973); 6, Eugster (1970).

10.4 9.8 10.6 10.0 11.1

Tulusia Lgarya Hannington Natron Magadi

8.6 9.2 9.3 9.8 9.6

Nanyuki Turkana Kusare Small Momela Crater

9.4 9.5 10.1 10.4 10.0

6.60 6.95 8.8 a 9.1 9.2

Kioga Bisina Baringo Oloidien Langano

Elmenteita Eyasi Reshi~ni Big Momela Shala

6.55 7.1 8.0 7.2 8.10

Kagera Victoria Bunyoni Mutanda Naivasha

pH

Analyses of 25 East African lake waters in mg/l (except Natron and Magadi, which are in mg/kg); alkalinity in meq/1

TABLE I

1 5 5 6 6

1 5 1 5 3

1 4 5 1 5

1 1 2 1 3

1 2 2 2 1

Ref.

b~ c~

115.0 369.2 568.8 1308 3339

Tulusia Lgarya Hannington Natron A Magadi

16.97 27.39 31.22 39.11 58.06

Nanyuki Turkana Kusare Small Momela Crater

112.2 196.7 108.1 118.4 154.4

1.13 0.912 3.79 5.57 20.06

Kioga Bisina Baringo Oloidien Langano

Elmenteila Eyasi Reshitani Big Momela Shala

0.298 0.413 0.814 0.628 0.986

Kagera Victoria Bunyoni Matunda Naivasha

Na

13.51 5.05 6.91 44.63 55.33

5.45 0.17 11.04 12.85 2.08

1.85 0.47 2.38 3.85 6.66

0.160 0.159 0.304 0.364 0.28

0.0836 0.091 0.159 0.385 0.292

K

0.0036

0.0080 0.0045 0.0045 0.0066

0.075 0.0307 0.0117 0.0024 0.0087

0.260 0.415 0.178 0.116 0.081

0.101 0.120 0.335 0.210 0.297

Ca

Activities of 25 East African waters as calculated by

TABLE II

0.0075

0.0063 0.0045 0.0061 0.0011

0.032 0.021 0.019 0.0057 0.013

0.210 0.362 0.077 0.110 0.026

0.135 0.093 0.329 0.356 0.165

Mg

2.21 10.41 1.73 2.64 4.91

0.188 0.061 0.654 0.592 6.50

0.096 0.106 0.768 0.347 0.177

13.2 111.3 122.7 380.4 1753

41.2 133 8.76 9.96 56.4

C1

S O L V E Q (x 103)

0.779 2.29 0.14 1.59 0.453

0.265 1.16 0.580 1.35 0.231

0.647 0.158 0.588 0.980 0.102

0.035 0.033 0.141 0.158 0.178

0.043 0.015 0.020 0.077 0:039

SO4

1.26 0.367 2.36 4.42 2.58

0.253

0.235 0.325

0.047

0.016

14.8 15.6 26.2 19.8 13.8

11.24 2.50 12.41 12.9

F

0.010 0.340 0.360 0.901 0.843

1.76 0.163 0.027 0.017

0.471 0.263 0.035 0.027 0.616

0.499 0.908 0.340 0.544 0.642

0.285 0.144 0.044 0.123 0.397

SiO2

b~ ol to

253

/

I0

,d

= I00,000

10,000

id L I000

E

o

fl

,d2 - 100

10"3 =1o

IO-'

........

r

........

I

I IO

........

I ........ 1 ....... IO0 IO00

(meq/t)

Alkalinity

i00_

I0,000 10-17

IOOO

io-2

@

~

@/@/

o"

o

• 100

Id 3-

Q. O.

-10

IO-5

...... i

...... i

I

I0

Alkalinity

....... i

....... i I000

100

( meq/[

)

Fig.2. Concentration (filled circles) and activity (open circles) of sodium and potassium in Eastern Rift lakes.

is less than 0, the solution is undersaturated. Calcite in Lake Turkana can be used as an example: log.

(aca +2) (aco~2) 10-4.Sl 10-3.01 Kcalcite = log i0_s.37

=

0.85

This water is supersaturated with respect to calcite.

254

id3~ -I0 jd 4•

o0(d~

8 o

o

-0"1

10-6

........

;

........

l

I

........

..... r ....... iooo

[

I0

I00

Alkalinity

(meq/[)

fO-3.

.~



,,/~ ~

JO-'

r0

• ~ .o °

°° v

cl o

id 5

0

o

0.1

\ i 0 -c

........

f

........

i

I

........

fO

i

~

, , ?' .,.,

tO0

Alkalinity

........

I000

(meq/[)

Fig.3. Concentration (filled circles) and activity (open circles) of calcium and magnesium in Eastern Rift lakes.

/

I0' IO00

ioo-

- I00

• I0,000

-

I000

Cl -I0

/ io-:

......

:,FI

h

¢) o

- IOO

~

G -IO

........

II0

Alkalinity

........

I00 i

( meq/[

........

)

I000 i .......

I 0 "s , .......

i

........

i

I0

Alkalinity

........

I

I00

........

;

I000

........

( mecl/1. )

Fig.4. Concentration (filled circles) and activity (open circles) of chloride and fluoride in Eastern Rift lakes.

255

I000 i0 z.

"o" g

Jd ~-

#" oe

0 0

o

,oo

0 0



0

Fd ~

u5

.io

° I oI

°o

........

i

........

o o

i

[

io

IOO

Alkolinity

ioo0

(meg/t) -fO,O00

-I000

I0 ~

• o



d 0

o

• - I00

i0 ~ - ~,/oo~



53

° o

o

El O.

o

d 0 o •

10 -4

iO t

........

i I

o

........

o

i IO

AIKoLinity

o

o

........

o

i ........ i I00 I000

( rneq/t

qO

.......

)

Fig. 5. Concentration (filled circles) and activity (open circles) of dissolved silica and sulfate in Eastern Rift lakes.

The relative degree of saturation was determined assuming a temperature at 25°C and 1 atm pressure for several minerals commonly found in East African lacustrine sediments: calcite fluorite

log Kc

= log(aca+2) + log(aco~2 ) = - 8 . 3 7

log Kf

(Robie and Waldbaum, 1968) = log(aca+2 ) + 2 log(a F-) = --10.58 (Brown and Roberson, 1977)

gaylussite natron trona gypsum

log Kga = 2 log(aNa+ ) + log(aca+2 ) + 2 log(aco~2) + 5 log(all:O) = -9.72 (Hatch, 1972) log Kn = 2 log(aNa+) + log(aco~2 ) + 10 log(aH20) = --1.31 (Truesdell and Jones, 1974) log Kt = 3 log(aNa+ ) + 2 log(aco~2) + 2 log(aH20) -- log H + = -12.32 (Garrels and Christ, 1965) log Kgy = log(aca+2 ) + log(aso~2 ) + 2 log(aH20) = - 4 . 8 5 (Berner, 1971)

256

Solubility products for calcite, fluorite, and gypsum are considered to be quite accurate; lesser confidence is placed in the values for gaylussite, natron, and trona. The relative degree of saturation was found to be directly related to the alkalinity of the waters in each case (Fig.6). Calcite. Fig.6 shows the relationship of calcite to alkalinity. Calcite becomes supersaturated in East African lake waters at about 4 meg/1 alkalinity. Calcite is not being precipitated today in Lake Victoria (Kendall, 1969). Richardson and Richardson (1972) report that Lake Naivasha has about 4% calcite in the bottom sediments and that the bottom waters are probably at saturation. Lakes Victoria and Naivasha have alkalinities of 0.9 and 2.2 meq/1, respectively. It should be noted that calcite is generally supersaturated by about 1 log unit. Fluorite. The IAP of fluorite increases with increasing alkalinity but the data are scattered (Fig.6). This may be a result of local fluoride inputs greatly affecting the concentration of fluoride in a particular lake. Kilham and Hecky (1973) report that some nephelinites on Mount Meru have up to 0.3% F. Fluorite was observed in the sediments around Lake Elmenteita by the author and has been observed in Pleistocene lake sediments at Magadi (Eugster, 1970; Surdam and Eugster, 1976) and at Olduvai Gorge (Hay, 1970, 1976). GAYLUSSITE SATURATION / x t

-. • CALCITE SATURATION o~

-2

.t /

o

o - -

• 7" "~; - 4

-6

..... J

,

, .,,_,J

........

i

........

J

,

,,

FLUORITE SATURATION o# •

NATRON SATURATION

~i~ 0 o~

-2

o

/

.~vn"

.

/x

-4 -6

rm~ ~

/ / "

w . 4

..... I

,

.

......

n

. . . . . . . .

J

. . . . . . . .

I

.

.

.

.

.

.

I

GYPSUM SATURATION

,

,

,..,,,t

. . . . . . . .

r

. . . . . . . .

n

,

,

TRONA SATURATION

~i~ 0

/

o~-2 -4 ././

-6

.....

J

i

........

~

10

........

i'

100

,

.......

f

1000

ALKALINITY (meq/I)

. . . . . . .

~

I

,

, ,

,.n

........

10

ALKALINITY

n

100

........

I

....

1000

.(meq/I)

Fig.6. Degree of saturation of East African lake waters for calcite, fluorite, gypsum, gaylussite, natron and trona.

257

Analcime. The IAP for analcime in East African lake waters cannot be calculated because there are no analytical data for aluminum concentrations in East African lake waters. Aluminum occurs both in solution and as reactive Si-:A1 detritus which may be very important in forming analcime (Eugster and Jones, 1968; Surdam and Eugster, 1976). However, the water composition needed to form analcime either by precipitation or by reaction of Si--A1 detritus or clay can be estimated by contouring alkalinity on the activity diagram of Garrels and MacKenzie (1967) for the system Na20--SiO2--H20--A1203 (Fig.7). This diagram is used because aluminum is conserved in the equations used and does not enter the aqueous phase.

03170

"t ,Ooo I0--~

~ ~ Q,,,,~ ~500 0 190

i

~'~

8o 39

m4".B~ •a ÷

I t q l Lit

=I0~

~ •

)22

014

MONTMORILLINITE

7[] 0 6 7

2

s1 ~1 in t

KAOLINITE

1.95

022

0

-5 log

(3 S i 0 2

Fig.7. Alkalinity (meq/l) contoured on an activity diagram for the system Na20--AI203--H20~SiO 2 for East African lake waters. One point (shown by a square) was excluded in making these contours. Diagram after Garrels and MacKenzie (1967).

258 Alkalinity can be successfully contoured on this diagram because it is related to the sodium concentration and pH. This diagram suggests that an alkalinity of at least 100 meq/1 is necessary before analcime formation is favored over montmorillonite formation. Holdship (1976) calculated alkalinities using diatom assemblages in a core from Lake Manyara and found analcime~free layers to have alkalinities ranging from about 10 to 150 meq/1. Analcime-rich layers in Lake Manyara represent more alkaline conditions and are diatomfree. Although analcime is not observed to be forming in modern lakes, this provides an estimate of the compositions of waters that may react with Si--A1 detritus to produce analcime over a long period of time. Gaylussite. All waters with detectable calcium are undersaturated with respect to gaylussite (Fig.6). However, projection of the observed trend shows that gaylussite should be supersaturated at an alkalinity of about 450 meq]l. In general, this agrees with other observations of East African lakes. Although the Lake Manyara core studied by Holdship (1976) shows that the ancient lake became sufficiently alkaline to form analcime several times, it never became alkaline enough to produce gaylussite; this Indicates that the critical alkalinity needed to form gaylussite must be considerably higher than that needed to form analcime. Gaylussite crystals sometimes are found in Lake Hannington (Bogoria) which has an alkalinity of over 900 meq]l (J. A. Stevens, pers. comm., 1977). Natron and trona. Natron becomes supersaturated at about 2000 meq/l (Fig.6) and trona at about 3000 meq/1 (Fig.6). Lake Magadi is presently at trona saturation (Baker, 1958; Eugster, 1970). Gypsum. Gypsum is undersaturated in all the lakes studied (Fig.6). This indicates that the precipitation of gypsum is not relevant to the chemistry of East African lakes. Figs.3 and 5 show that because of calcite precipitation, the activity of calcium decreases faster than the activity of sulfate increases, so that supersaturation of gypsum can never be reached. Garrels and MacKenzie (1967) found this to be true in their classic study of Sierran waters. This observation has great implications for environmental interpretations of East African lake sediments because the gypsum observed in these PlioPleistocene sediments cannot be lacustrine and therefore does not indicate high salinities of lake waters. Its occurrence must be attributed to other processes, to be discussed below. Montmorillonite. Montmorillonite can form at low to moderate alkalinities in natural waters (Gac et al., 1977). Yuretich (1976) showed that montmorillonite is forming in modern Lake Turkana; Baumann et al. (1975) found high-Mg montmorillonites in Lake Shala. Gac et al. (1977) attributed Mg depletion in the Lake Chad system to montmorillonites; they found Mg depletion to

259

occur at about 2 meq/1 alkalinity. This occurs in East African waters at about 2 to 3 meq/1 (Fig.3). Sepiolite is rare in East African lacustrine deposits.

Magadiite. Magadiite is not directly related to alkalinity, but is instead indicative of high sodium, silica and pH (Hay, 1968; Eugster, 1969). East A.frican waters become sodium-rich and more alkaline as they increase in salinity through evaporation. General trends of all the major components except silica follow patterns predicted for waters in semi-arid basin volcanic terrains (Jones, 1966; Garrels and MacKenzie, 1967; Eugster, 1970; and Hardie and Eugster, 1970), although details of individual constituents vary slightly from these models. Sodium and chloride are conserved; some potassium is lost by clay-interaction; calcium is depleted because of calcite precipitation; magnesium is depleted by the conversion of S i A l detritus and kaolinite to montmorillonite (Yuretich, 1976; Cerling, 1977; Gac et al., 1977); some sulphate is lost by bacterial reduction (Berner, 1971). Silica levels may be controlled by silicate reactions as well as by diatom extraction (Hecky and Kilham, 1973). It should be noted that gypsum does not occur as an evaporite mineral in these alkaline lakes because of the strong depletion of calcium by calcite precipitation. The gypsum observed in East African Plio-Pleistocene lake deposits is most likely due to the oxidation of reduced sulfide-rich sediments after desiccation or uplift.

Implication~ for paleoalkalinity The above arguments show that the presence or absence of certain mineral phases may be direct evidence of the chemistry of ancient lakes provided that they are either primary or penecontemporaneous with deposition. Other criteria can also be used to estimate alkalinities. Diatoms (Richardson, 1968, 1969; Holdship, 1976), molluscs (page 264), fish (page 265) and exchangeable cations (page 266) have all been successfully used. Using these estimates of paleoalkalinity, all the cations and anions of Pliocene and Pleistocene East African lakes can be estimated using the equations developed earlier or by using Fig.8. Note that this diagram is very similar to the simulated evaporation trends of Garrels and MacKenzie (1967), Helgeson et al. (1969) and Gac et al. (1977). A word of caution is perhaps necessary at this point. Each of the different methods of estimating paleoalkalinity has inherent problems: for example, do the mineral assemblages reflect primary minerals formed at the time the sediments were deposited or are they a much later diagenetic assemblage? In view of these difficulties, a better estimate of paleoalkalinity would result from using several independent methods. GEOLOGIC SETTING

Modern Lake Turkana lies in a complex tectonic depression between the well-defined Gregory and Ethiopian rifts. The main north--south portion of

260 I00,000

I00,000 -.-

• I0,000

IO,O00

•I000

I000

S IO0

IO0

b

C0xx \ 0.1

O.I I

10 Alkalinity

I00

I000

(meqA)

Fig.8. Summary diagram showing the concentrations of dissolved solutes with respect to alkalinity for East African lake waters. Na, K, C1, F and SO~ are taken directly from Figs.2, 4 and 5. Concentrations of Ca and Mg are calculated from the activity (Fig.3) and average total activity coefficients derived from Tables I and II using the relationships a i = m T i ' Y T i where 7Ti is the average total activity coefficient of the ith species, and a i is the activity of the ith species. Total average activity coefficients for Eastern Rift lakes are in Cerling (1977).

the lake lies in an inactive fault trough and the extreme southern end in the modern active rift (Cerling and Powers, 1977). Miocene, Pliocene, and Pleistocene sediments and volcanic rocks around the modern lake are exposed because the lake presently stands 70 m below its overflow level (Butzer, 1971) and because late Pleistocene faulting has elevated parts of the basin floor. • Interest in the sediments northeast of Lake Turkana was aroused during Richard Leakey's 1968 expedition, which uncovered several well-preserved hominid fossils. Geologic studies were initiated by Behrensmeyer (1970), and the first detailed picture of the stratigraphic relationships was provided by Vondra et al. (1971), Bowen and Vondra (1973), and Bowen (1974), who described the relationships between interfingering alluvial fan, fluvial, deltaic, and lacustrine facies and were able to make a reasonable correlation of m a n y of the tufts t h r o u g h o u t the region (Figs.9 and 10). Findlater (1976) amplified this work and reconstructed the physical paleoenvironment of the region. Bowen (1974) divided the Plio-Pleistocene sediments into four formations. The oldest, the Kubi Algi Formation (Fig.10), consists of several finingupward cycles with volcanic conglomerates at the base and siltstones and claystones near the top. Clays of this interval are lighter in color than those higher

261 ORNL-OWG

z

78-2482

QUATERNARY, UNDIVIDED INCLUDESALLUVIUM AND BEACH SAND

GALANA B01 BEDS ,=.

oc u. 6.

o

JPPER MEMBER LOWER MEMBER KOOBI FORA FORMATION INCLUDES THE GUOMOE FORMATION IN THE [LERET AREA .C-

KUBi ALGI FORMATION

VOLCANICS, UNDIVIDED e. w

~z o ~

TURK?NA'~.~

l

to46'

/,OCAT,O,

KENYA

U /

?,

5,

,?

KILOMETERS

Fig.9. Geologic map of the East Lake Turkana area. Modified from Vondra and Bowen (1976, 1978).

in the section; they are generally light olive grey (5Y6/1) to pale yellowish brown (5Y7/2). The Kubi Algi Formation is 98 m thick in the type section. The upper half is thought by Findlater (1976) to represent a time of maximum lake expansion, and will be considered in this study. The Koobi Fora Formation lies above the Kubi Algi Formation and is subdivided into Lower and Upper Members (as redefined by Bowen and Vondra, 1978, which eliminates the Ileret Member). The Lower Member consists primarily of low-energy deposits, with a few higher-energy deposits near the basin margin. This member is the laminated siltstone facies of Bowen (1974) and of Vondra and Bowen (1976). Clays of the Lower Member are dark yellowish brown (10YR4/2), and contain few mammalian fossils. The Upper Member includes nearshore lacustrine, deltaic, fluvial, and alluvial fan deposits. It represents less stable conditions than were present in Lower Member times. The shoreline probably migrated over a low coastal plain 15--40 km wide. During high lake stands, nearshore lacustrine sediments were deposited several kilometers inland from the modern lake shore; during

262 AREAS fOI

lOS ----oz

102-

--o3

nLLJ m :E

KOOBI FORA TUFF --04

~ I °°°

hJ

AREAS - ,o

=--o2

~- -/---- o e =.,.=ms TUFF . . . . . --,4 __--15

AREAS

17

~ 2 0 Zd HASUMA TUFF

TUFF

n ~'--O7

0

I --16 ~SUREGEI

z O

ILERET TUFF COMDLEX

-~= _.._~A

tOS

I--

<~ fE

{3"

............

fn o

__Io ---,~

I~

--13 - v----ira,,,, TUFF

tY" UJ

Fig.10. Stratigraphic sections studied. The Chari Tuff is 1.3--1.4 m.y. old, the KBS Tuff is 1.8 m.y. old, the Hasuma Tuff is about 4 m.y. old (Curtis et al., 1975; Fitch and Miller, 1976). Sample numbers refer to claystones studied for exchangeable cations.

periods o f low water the shoreline was probably several kilometers west of the present shoreline, and streams cut into older lacustrine sediments. The lowest-energy deposits occur near Koobi Fora spit, and have been briefly described by Johnson and Raynolds (1976). Vigorous streams carried detritus from the volcanic highlands in the east and deposited sandstones and siltstones in the alluvial plain. These sediments contain most of the vertebrate fossils. A period of erosion and minor faulting followed deposition of the ChariKarari Tuff. The G u o m d e Formation was deposited u n c o n f o r m a b l y on this Middle Pleistocene surface, and probably represents a stand of proto Lake T u r k a n a higher than any since the end of Koobi Fora Formation times. It is made up of laminated siltstones, lenticular sandstones, bioclastic carbonates, and altered tufts. The Galana Boi sediments are the most recent deposits in the sedimentary sequence. T h e y are about 10,000 years old (Vondra et al., 1971), and were plastered on the landscape which had been eroding since G u o m d e times. They occur 2--4 km inland of the present lake shore. The Galana Boi is made up of grey diatomaceous siltstones and has numerous artifacts, algal stromatolites, and molluscs. Fig.9 shows a simplified geologic map; stratigraphic relationships are shown in Fig.10.

263 The broad picture that has emerged from the work of Bowen (1974) and Findlater (1976) is one of a large stable lake system during Kubi Algi time ancl during deposition of the Lower Member of the Koobi Fora Formation. The KBS and post-KBS lake (Upper Member) was one with a fluctuating shoreline. Wasthis change in sedimentary style tectonic or climatic? What sort of a chemistry did the ancient lake have? These questions profoundly effect the paleoecology of the region, for saline alkaline lakes support different fauna and flora from freshwater lakes. FAUNA, FLORA, AND ALKALINITIES Many factors influence the nature of plant and animal life. Salt content, level of sunshine, oxygen, and pH are all variables that help determine the community that inhabits a specific environment. East African lakes are sodium-bicarbonate--carbonate waters of varying concentrations, so alkalinity and salinity may play a major role in determining the fauna and flora living in these waters. This section will explore the known relationships of the presence and abundance of some organisms to alkalinity, and will suggest how these may help in paleoenvironmental interpretations of the sediments northeast of Lake Turkana.

Diatoms Because diatom assemblages are affected by alkalinity (Richardson, 1968; 1969; Hecky and Kilham, 1973), they have been important in reconstructing late Pleistocene and Recent paleoenvironments of East African lakes (Kendall, 1969; Richardson and Richardson, 1972; Holdship, 1976; Gasse, 1977). For example, Eunotia, Melosira ambigua or M. granulata, Coscinodiscus rudolfi, and Nitzschia frustulum indicate alkalinity ranges of 0--2 meq/1, 1--5 meq/1, 10--80 meq/l, and 10--200 meq/1, respectively (Richardson, in prep.). Few diatomaceous beds exist in the Kubi Algi and Koobi Fora Formations. Diatomites were encountered only in the upper portion of the Kubi Algi Formation, and many of these have been partly recrystallized to cristobalite. However, several good diatom separates were sent to Dr. Jonathan Richardson, who reported that the most diatom-rich sample from high in the section is almost wholly made up of Melosira granulata, indicating low alkalinities. Another diatom separate, stratigraphically lower, is made up of Cyclotella meneghiniana, Melosira agassizii, Cymbella cf. tumida, and some partly recrystallized sponge spicules, indicating slightly higher alkalinities (at least 10 meq/1). The notion of an expanded fresh-water lake during later Kubi Algi time, proposed by Findlater (1976), is supported by these two samples. Galana Boi sediments are very diatomaceous and are dominated by Rhopaladia vermiculus and Stephanodiscus astrea, with minor amounts of Cymbella, Navicula, and Surirella. This assemblage reflects an alkalinity of

264

a b o u t 10 meq/l (J. Richardson, pets. comm.). This suggests that the lake during the last period of expansion was not entirely fresh, perhaps because the outlet was short-lived so that the lake was never fresh, or as with the case of Lake Albert and the Nile River, because the outlet was so close to the major inflow that the lake had a salinity gradient and was completely fresh only in a small area b e t w e e n the O m o inflow and the o u t f l o w to the Nile. Studies of diatoms throughout the Lake Turkana Basin could shed light on this interesting possibility. Molluscs Molluscs also contribute to paleoenvironmental interpretations. Species in East African lakes can be classified as cosmopolitan (widely distributed and c o m m o n to many lakes or streams) or endemic (specific to a particular lake). Endemic species can be either phyletic or radiative. Molluscan assemblages can be divided into seven zones ( F i g . l l , after Williamson, 1978). The upper part of the Kubi Algi Formation constitutes Zone 2, made up of phyletic endemic species. WiUiamson (1978) believes that this represents a basin genetically isolated from the rest of East Africa b y either being closed or phyletic endemic

i

I

cosmopoliton exotic rodibfive endemic /

ZONES

2

5

4

5

6

7

'MARKER BEDS

member

I

FORMATION KUBI ALGI

lower KOOBI

lu~ FORA

F i g . 1 1 . M o l l u s c a n f a u n a l z o n e s , a f t e r Williamson ( 1 9 7 8 ) . Z o n e 2 e x t e n d s t o a b o u t 2 m a b o v e t h e Suregei T u f f ; z o n e 6 e x t e n d s up t o a f e w m e t e r s b e l o w t h e KBS Tuff.

because of some barrier (e.g., rapids). Zones 3 to 6 begin with the lowest black clays of the Lower Member and continue to the KBS Tuff. They include cosmopolitan, exotic, and radiative endemic species. Cosmopolitan and radiative endemic fauna could coexist for a long time in a permanent open basin that had continual genetic input from the parent stock b u t had enough stable, local environments where endemic species could develop (Williamson, 1978). The Upper Member of the Koobi Fora Formation has

265 only cosmopolitan species: Lower Member endemic species became extinct, and local evolution never persisted long enough to develop endemic species. Populations were subject to repeated genetic renewal. This could occur if salinities periodically rose so that molluscan life was extinguished, destroying any chance for endemism (Williamson, 1978). Freshwater molluscs can live in a limited range of salinities. If certain ions are too concentrated or too sparse, the ionic imbalance between the external water and intercellular fluid is too great and the organism cannot function properly. Van der Borght (1962) and Greenaway (1971) have shown that the lower limit of calcium for the African mollusc Lymnaea stagnalis is about 0.10 meq/1. Activity, however, is more important than concentration in chemical processes: this calcium concentration corresponds to an activity in fresh water of about 0.05 x 10 -3. Using an activity of Ca+2 of 5 x 10 -s as the lower critical limit for healthy molluscs, we find that they could survive in East African waters with an alkalinity range of about 0.5--15 meq/1 (Fig.3). Calcium is depleted in alkaline waters because of calcite precipitation (Fig.6). Very few, if any, lakes in East Africa have alkalinities less than 0.5 meq/1; Lake Kivu, with an alkalinity of 16 meq/1 (Talling and Talling, 1965), is the most alkaline lake in East Africa with a healthy molluscan fauna. Molluscs in Lake Turkana (alkalinity of 20--23 meq]l) are rare: only four living specimens have been found in several hundred dredge hauls by the research vessel Halcyon (T. Hopson, pets. comm.). Therefore, 16 meq/1 is considered the upper tolerance limit for normal mollusc communities in East African lakes. This has important implications on reconstructing the paleochemistry of Pliocene and Pleistocene East African lake deposits: the presence of molluscs indicates that the paleoalkalinity was less than 16 meq/1. In the PlioPleistocene sequence northeast of Lake Turkana Williamson's (1978) Zone 2 was genetically isolated from other East African molluscs but must have maintained an alkalinity less than 16 meq/1. During oldest Lower Member time the genetic barrier towards mollusc interaction was removed and free exchange with other mollusc communities occurred almost until the end of Lower Member time; at no time did the alkalinity exceed 16 meq/l in the area studied. Disappearance of the radiative endemic molluscs documented by Williamson (1978) at the end of Lower Member time marks the first basin-wide extinction due to high alkalinities. During Upper Member time, mollusc evidence suggests that the salinity oscillated several times, causing extinction and repopulation of the mollusc community, indicating a range of perhaps 10--25 meq/1. alkalinity.

Fish Fish also are affected by the composition of water. Lake Turkana is the most alkaline lake in East Africa with a normal freshwater fauna: Beadle (1974) suggests that the critical salinity is about twice that of modern Lake Turkana, and that lakes with an alkalinity of 40 meq/1 or more tend to harbor only

266

cichlid fish (e.g., Tilapia) or a dwarfed fauna such as occurs in the fresher parts of Lake Magadi (Coe, 1966). Dwarfism is c o m m o n in the cichlids, which adapt readily to new environments (Fryer and Iles, 1972). Other, less adaptable, fish are absent in this alkaline environment. Fossil fish of the Kubi Algi and K o o b i Fora Formations are as yet unstudied. Fish of the Kubi Algi Formation are rare. Remains generally consist of delicate, translucent orange bones and scales; the diameter of the vertebra is usually a b o u t 5 mm or less although rarely vertebra with a diameter of 25 mm are found. Fossils from the K o o b i Fora Formation are larger and more massive, with diameters up to 75 mm. These large fish provide evidence of alkalinities less than a b o u t 40 meq/1. Concentrations of small fish (vertebra diameter of a b o u t 4--8 mm) occur in the Upper Member at Ileret and at Koobi Fora. The most prominent of these occurrences, " t h e fish beds", is in Ueret. These concentrations of fish remains m a y well indicate that there were occasional mass deaths of fish, and their consistently small size m a y indicate high salinities and alkalinities that exceeded 40 meq/1. Neither of these localities has molluscs associated with the fish fossil concentrations.

Summary of biologic evidence Initialfaunal observations in the Kubi Algi and Koobi Fora Formations indicate that lacustrine fauna m a y be very important in reconstructing the paleochemistry of ancient Lake Turkana. The following conclusions on observations of fauna, flora, and alkalinity of East African lakes can be made: (1) Kubi Algi Formation. Diatoms indicate that the waters were fresh from mid-Kubi Algi (10 meq/l?) to late-Kubi Algi (2--5 meq/l?) time. Molluscs indicate a genetically dosed basin. Fish remains are poor and no conclusions can be drawn from them. (2) During most of Lower M e m b e r time theJake was an open basin with alkalinitiesnever rising above 16 meq/l, the upper limit for mollusc existence. (3) At the end of Lower M e m b e r time, radiative endemic species characteristicof the Lower M e m b e r became extinct. This was most likely due to the alkalinity rising above 16 meq/l. The molluscan community became extinct and was repopulated several times during Upper M e m b e r time. Several beds with numerous small fish bones m a y indicate alkalinitiesexceeding 40 meq/l. This last lake phase of fluctuating alkalinitiescontinues to the present day. EXCHANGEABLE CATIONS: INDICATORS OF PALEOALKALINITY Cation exchange is one of the most rapid chemical processes in the weathering cycle. Ports (in Keller, 1963), Carroll and Starkey (1960), and Russell (1970) have shown that exchange of the interlayer cations can occur within hours. Slower reactions also occur. In particular, cation exchange capacity

267

(CEC) decreases while the amount of fixed cations increases (Russell, 1970; Drever, 1971); potassium is fixed by illite (Weaver, 1958, 1967). Exchangeable cations were first used as a paleosalinity indicator by Spears (1973) who could distinguish marine clays from fresh- and brackish-water clays by their higher percentage of exchangeable magnesium. Exchangeable cations from the Kubi Algi and Koobi Fora Formations Twenty-four clay samples were analyzed for exchangeable cations. Eight samples were studied from the Kubi Algi Formation, the Lower Member and the Upper Member of the Koobi Fora Formation (Fig.10). None of these formations has been buried more than 400 m, so it is reasonable to assume that only low-temperature diagenetic processes have taken place. Clay minerals in this sequence are predominantly montmorillonite with occasional traces of kaolinite as determined by X-ray diffraction. Finely ground samples of clays were placed in centrifuge tubes and mixed with distilled water. After being shaken, the samples were allowed to stand for 30 min before centrifuging and decanting. This was repeated once. Then 40 ml of 1N ammonium acetate at pH 7.0 was added to replace the exchangeable cations. This was shaken and allowed to sit for 30 rain before centrifuging and decanting. This was repeated, and the second effluent was combined with the first and analyzed on a Perkin-Elmer 303 atomic absorption unit for Na, K, Ca, and Mg. Spears (1973) reports that partial dissolution of carbonate by a m m o n i u m acetate can give high Ca and Mg values. This could affect only the samples from the Upper Member of the Koobi Fora Formation because all other clays lack carbonates.

Compositions of exchangeable cations group according to their stratigraphic position (Fig.12 and Table III). Samples from the Kubi Algi Formation are calcium-rich; those from the Lower Member of the Koobi Fora Formation are approximately sodium--calcium--magnesium equivalent; and those from the Upper Member of the Koobi Fora Formation are sodium-rich. Sample 206-109 will not be included in further discussion because it is so different from other samples from adjacent horizons. Exchange of Na and Ca in montmorillonite The composition of East African lakes is dominantly sodium and bicarbonate (Table I; Talling and Talling, 1965). Very dilute lakes are equimolar in sodium and calcium, while more saline and alkaline lakes are sodium-rich and calciumpoor because of calcite precipitation. Because calcium is preferentially removed by montmorillonites (Grim, 1968; S t u m m and Morgan, 1970), clays from dilute lakes should be dominated by calcium wheras those in more alkaline lakes should be sodium rich. Exchange reactions in clays follow a regular solution model (Garrels and

268 TABLE III Exchangeable cations (in meq/100 g) in clays in the Kubi Algi and Koobi Fora formations Collection number

Sample number

Na

K

Ca

Mg

Total

Calculated alkalinity (meq/l)

3.0 2.8 1.9 1.8 1.8 2.2 1.7 1.7

13.5 28.1 10.0 7.9 15.6 23.0 5.6 14.3

3.9 1.4 1.9 3.6 2.9 2.0 5.3 5.7

54.5 73.3 49.8 76.8 55.9 61.2 57.0 55.9

11.0 7.9 11.2 22.0 10.3 8.0 21.9 10.7

32.2 28.4 30.4 23.0 30.4 26.9 17.5 32.0

2.1 1.0 1.8 1.9 2.2 2.3 1.4 2.6

11.6 12.1 15.6 16.6 20.0 7.9 12.6 8.6

10.0 12.2 14.6 14.0 9.2 18.1 17.6 14.1

55.9 53.3 61.8 55.5 61.8 55.2 49.1 57.3

11.7 10.6 9.4 7.7 8.1 13.2 7.7 13.9

4.9 13.8 9.8 17.3 42.2 37.1 16.0 20.1

1.9 1.8 1.4 2.6 2.6 2.3 2.7 3.0

45.0 39.2 39.2 85.2 11.8 41.4 51.8 20.0

9.4 7.0 5.2 7.4 8.5 10.6 6.8 4.8

61.2 61.8 55.6 112.5 65.1 91.4 77.3 47.9

1.7 3.4 2.7 2.4 13.5 5.9 3.1 6.3

32.1 26.8 26.4

0.3 1.4 1.4

11.9 12.0 12.2

12.6 12.0 11.8

56.9 52.2 51.8

11.4 10.3 10.2

Upper Member, Koobi Fora Formation C01 C02 C03 C04 C05 C06 C07 C08

007--204 103--419 103--416 101--310 102--425 102--419B 102--412 102--402

34.1 41.0 36.0 63.5 35.6 34.0 44.5 34.2

Lower Member, Koobi Fora Formation C09 C10 Cll C12 C13 C14 C15 C16

102--104 105--303Ba(avg) 105--206 105--420 105--406 102--002 102--210C 102--001

Kubi A Igi Formation C17 C18 C19 C20 C21 C22 C23 C24

200--114 200--113 200--112 200--109 206--109 206--108 208--116 208--101

a Average of three determinations: 105--303B

Christ, 1956, 1965; Truesdell and Christ, 1968): X + Y-clay = Y + X-clay or:

K = axay-chy ayax~.~ay

269 Mg

\



/ /i~-'t

- _

• •



'\

~

IDiiI

lower KOOBI

ll/ ~_ ~ _ •-



member FORA FM

\ ~

Ca



No

Fig.12. Exchangeable cations (except K) in the Kubi Algi and Koobi Fora Formations.

approximated by: _ axFXy]

n

where a x and ay are the activities of x and y in aqueous solution, ay~lay and ax-clay are the activities of x- and y-clays, Xx and Xy are the mole fraction of x and y in the exchangeable position, and n is an empirical number that relates mole fraction and activity of the exchangeable position. Montmorillonites of the Kubi Algi and K o o b i Fora Formations are calciumand sodium-rich, respectively (Table III and Fig.12). Therefore, an experiment was set up to relate calcium and sodium in aqueous solution to that in the exchangeable position in clays. Six waters were prepared to approximate the cation ratios and ionic strengths f o u n d in East African lakes b y dissolving NaCI and CaC12 in distilled water (Table IV). In the exchange experiment, 100--500 mg of montmorillonite was treated twice with 1N a m m o n i u m acetate to replace all cations with NH4+. The samples were then treated eight to ten times with 50 ml of the NaC1--CaC12 solutions to exchange the cations. This was done b y shaking the sample, allowing it to sit in the solution for at least 30 min, and then centrifuging. Cations on the clays were extracted with 1N a m m o n i u m acetate as described above. Results of the exchange reactions are shown in Table IV and Fig.13. These data fit an equation of the form: log (aNa+)2 = 5.6 + 1.8 log(XNa2"daY~ (aCa+2) \XCa~clay ] The overall composition of lake waters of the Eastern Rift of East Africa is directly related to their alkalinity (TaUing and Tailing, 1965). Activities of calcium and sodium in seventeen East African lake waters were calculated

270 TABLE IV Sodium and calcium ratios of several East African lake waters and mole fraction of sodium and calcium on clays after immersion in water of similar composition Lake

Composition (ppm) Na +

Naivasha Baringo Abaya Langano Turkana Shala

Ref.

Proxy water (ppm)

Ca 2+

24.4 95 206 550 767 5900

16.2 11.6 12.1 10.8 4.6 5.0

a a a a b c

Na+

Exchangeable cations on clay (meq./100 g) Ca2+ ' Na+ Ca2+

24.2 94.9 206 550 767 5898

16.2 17.1 12.1 10.8 4.7 5.1

2,96 12.84 19.0 29.0 38.8 46.6

40.0 30.2 28.6 19,4 8.0 0.76

aTalling and Tailing (1965); bCerling (1977); CBaumann et al. (1975).

(Table II) and are found to be related to alkalinity by the following expression (Fig.14): log (aNa+!2 = 3.2 + 3.0 log (alkalinity)

Paleoalkalinity estimates using cation exchange in montmorillonite The two above equations can be combined and an expression can be written relating alkalinity of East African lake waters to the mole fraction of exchangeable calcium and sodium on montmorillonites:

10

~

*o *o

t /

-4

log ~ a-'~-~-Q..) :

'6

o

5.6 * 1.8 log ,. Xc~_c~ay ;

J

.~

( XN.2-cI(~y"~ log ,, Xc °-clay /

Fig.13. Results of experiment relating activities of sodium and calcium in solution to the mole fraction o f exchangeable sodium and calcium on montmorillonite. Activities have been calculated from concentrations in Table IV.

271

./ *o *a

5

/ /

~

3.2 * 3.0 log (AIk) For East African lake waters

tog (Alkolinity)

Fig.14. Relationship of the activities of sodium and calcium and alkalinity in seventeen Eastern-Rift lake waters (data from Table II).

log (alkalinity) = 0.8 + 0.6 log XNa2~clay XCa~clay

where alkalinity is in meq/l. This equation was used to calculate paleoalkalinities of the samples studied assuming only sodium and calcium exchange (Table III). Paleoalkalinities of the three respective time periods are: Upper Member, Koobi Fora Formation Lower Member, Koobi Fora Formation Kubi Algi Formation

8--22 meq]l 7--14 meq/l 2--6 meq/1

This assumes that only minor diagenetic modification of the clay minerals took place and that the clays retain their original exchangeable cation ratios or at least did not exchange with waters very different from those with which they were buried. This assumption seems valid in this particular case because these paleoalkalinity estimates agree with estimates based on fauna and flora (page 266) and on mineral assemblages (see below). This study, developed specifically for cation exchange on montmorillonite in East African lakes, suggests that exchangeable cations can be used as paleoenvironmental indicators if Recent analogies are well understood and an insignificant amount of cation exchange has occurred during diagenesis. This method is of particular use, like all other methods, if there is corroborating evidence to support its conclusions. A U T H I G E N I C M I N E R A L S IN T H E KUBI ALGI A N D K O O B I F O R A F O R M A T I O N S A N D THEIR RELATIONSHIP T O T H E H I S T O R Y O F L A K E T U R K A N A

Since each species of authigenic and diagenetic minerals will form only under certain geochemical conditions, the study of a suite of such minerals

272

can be used as an indicator of past geochemical environments. The PlioPleistocene sediments northeast of Lake Turkana in northern Kenya provide an example of a section only slightly modified by diagenesis. The section is only about 300 m thick (Bowen, 1974) and so has not been subjected to the pressures and temperatures needed to drive many diagenetic reactions. These lacustrine sediments show excellent vertical zoning that can be attributed to the geochemical environment of deposition. Laboratory work included microscopic work on thin and polished sections, X-ray diffraction and chemical analyses.

Distribution of authigenic minerals Three contrasting assemblages of authigenic minerals can be recognized and can be used as a basis for defining a zone within this sedimentary sequence. The three zones are the gypsum--hematite--cristobalite zone, the calcite zone, and the calcite--dolomite--zeolite--halite zone; montmorillonite occurs in all three of them (Fig.15).

Gypsum--hematite--cristobalite zone. This zone is restricted to the upper part of the Kubi Algi Formation and corresponds with Williamson's first and second faunal zones. It is characterized by variegated clays (green, red, brown) and poorly cemented sandstones. The few well-cemented sandstones have a hematite--goethite--manganese oxide cement. Calcite is rare; molluscan aragonite shells are badly corroded and often partially replaced by iron oxides; many calcite ostracod shells are completely replaced by hematite-geothite (Fig.16). In many cases gypsum cements ostracod coquinas and is

KUBI ALGI FORMATION

I

lower member KOOBI FORA FORMATION

I

KOOBI

upper member FORA FORMATION

meters o

I

I II

I

1 JAROSITE GYPSUM

|

I

IIHII I I

I I

tf

I {

Illlflltllliillilllll

I~ II I I I

H I ]llll

H I

B

I

B

H H l i

g a • i

I IIH II lilr

AREAS 2 0 0 - 208

I I ) I

IEII

AREA

102

• I

I

D

I

II r I II I

III

OH I I

II

I

I I

I

I

HALITE lil ¢

I i



CALCtTE DOLOMITE

i

ANALCIME

II I

I I

I

CHADAZITE III

l

I

MONTMORILLONfTE CRISTOBALITE IRON OXIDES

AREA ,0, IAREA'O31

Fig.15. Distribution of authigenic minerals in a composite section of the Kubi Algi and Koobi Fora Formations (Ceding, 1977).

273 found as a surficial salt on low-lying deeply weathered clays. Jarosite occurs on some clays. This assemblage is suggestive of reduced sediments oxidized in situ. Hematite, gypsum, and jarosite are c o m m o n alteration products of pyrite (Furbish, 1963). No pyrite has been observed in the Kubi Algi Formation, even though several very good exposures of rock occur along deeply incised streams. This suggests that the pyrite was entirely oxidized before the sediments were exposed at the surface.

Fig.16. Gypsum cement in ostracod coquina that has been partially replaced by iron oxides (opaque}.

Oxidation of pyrite would produce iron sulfates and sulfuric acid (Merwin and Posnjak, 1937; Furbish, 1963). In clastic sediments attack of micas may provide potassium which precipitates jarosite. More commonly, the sulfuric acid attacks calcite and produces gypsum. Tight stratigraphic control and the replacement of ostracod shells b y iron oxides with gypsum cement suggest that the process t o o k place slowly until the pyrite was completely oxidized. Relative amounts of gypsum and hematite are related to the amount of calcium available to react with the sulfuric acid: areas with aragonite or calcite shell material result in gypsum deposition with hematite.

274 The absence of calcite in this part of the section can be attributed to either lack of calcite precipitation, which implies a very fresh lake, or to dissolution by sulfuric acid formed during oxidation of pyrite. The latter situation certainly occurred, as pointed o u t above, b u t there is no evidence for widespread calcite precipitation except as ostracod shells. Many sandstones in the Kubi Algi Formation do not have hematite, gypsum, or calcite. This suggests that calcite possibly never was precipitated, and perhaps only existed as a metastable organic precipitate. This zone also contains cristobalite which formed from the alteration of diatoms, sponge spicules, and plant phytoliths. The diatoms are predominantly Melosira, which indicate fresh waters. Montmorillonite is the only clay mineral present in the ripper part of the Kubi Algi Formation. Kaolinite, montmorillonite, and illite are all present t h r o u g h o u t the modern lake (montmorillonite is dominant near K o o b i Fora), and are being transformed to montmorillonite (Yuretich, 1976). A n y iUite or kaolinite transported into the ancient lake has since been transformed to montmorillonite. Turfs in this part of the section are unaltered; this also indicates fresh-water conditions. Calcite z o n e . This zone extends from several meters' above the base of the

Lower Member to several meters below the KBS Tuff. It corresponds to the zones 3 to 6 of Williamson (1978) and is characterized by massive black or dark-brown clays and silts with occasional calcite-cemented sandstones, ostracod coquinas, and concretionary calcite horizons. Isolated molluscs, limonite-stained calcite concretions, and hematite--goethite concretions associated with molluscs are c o m m o n in this sequence. A few limonite flatpebble conglomerates occur near the t o p of the Lower Member. Calcite, montmorillonite, and Fe--Mn oxides are the dominant authigenic minerals in the Lower Member. G y p s u m and halite occur in some places as salts on weathered clays. Many of the calcite concretions occur around mollusc shells. They are up to 10 cm in diameter and are poorly zoned with limonite and clay, and often have manganese dendrites. Well-crystallized hematite--goethite occurs around mollusc shells in hard, brittle black clays; brown to dark-grey siltstones generally harbor calcite concretions. Manganese oxides occur as dendrites and as a replacement of a molluscan coquina which forms an extensive dip slope. The ancient lake had a relatively stable shoreline with only a few minor fluctuations, and almost filled the basin (Bowen, 1974; Findlater, 1976). The few authigenic minerals can give little direct evidence of the chemistry of the original lake. Calcite and montmorillonite were b o t h stable: this indicates a range from at least 4 meq/1 to less than 100 meq/1 alkalinity. C a l c i t e - - d o l o m i t e - - z e o l i t e - - h a l i t e z o n e . This zone starts at the KBS Tuff and

includes the Upper Member of the K o o b i Fora Formation and the G u o m d e

275 Formation, which occurs above the K o o b i Fora Formation (Bowen and Vondra, 1973). Sediments of this upper zone include near-shore lacustrine, deltaic, and fluvial deposits. Near-shore lacustrine deposits can be found east of Koobi Fora spit and near Ileret (Findlater, 1976). The sediments are made up of interfingering reddish-brown claystones and siltstones, bioclastic limestones, crossb e d d e d sandstones, and algal mats. Eastward they grade into deltaic and fluvial sediments. Calcite is the most c o m m o n authigenic mineral in this assemblage. It occurs as concretions, pisolites, caliche, and as cement in sandstones and bioclastic limestones. Aragonite and calcite do not generally co-exist in the limestones: aragonite from molluscan shells has dissolved and been reprecipitated as calcite, leaving only molds as evidence of the molluscs. Near Koobi Fora spit these bioclastic limestones make excellent mapping units. Dolomite occurs as concretions and as sandstone cement. Concretions are usually 5--10 cm in diameter and are slightly pink; some nodular dolomite horizons are present. It is not clear what conditions are necessary to precipitate dolomite in East African lakes. Its c o m m o n association with zeolites in East Africa (Hay, 1973) suggests that saline conditions may be necessary to precipitate dolomite. No dolomite has been observed in East African lakes although few lakes have been studied in any detail. Modern dolomite occurrences are restricted to waters with high Mg/Ca ratios (cf. MSller and FSrstner, 1973; Folk and Land, 1975): usually waters whose calcium concentration is depleted by gypsum precipitation. Considerations of mineral saturations (Fig.6) show that gypsum is not precipitated b y evaporation in East African lake waters. This suggests several possibilities for dolomite formation that are different from previous observations. If high Mg/Ca ratios are required for dolomite formation two possibilities exist, b o t h of which require high salinities and alkalinities. Dolomite could form from waters whose Ca and Mg are below detection limits (above a b o u t 200 meq/1, Table I) if the activity of Mg is much greater than Ca in these waters; or it could form from interstitial waters whose Mg concentration is increased b y the alteration of montmorillonite to Mg-free zeolites (above 100 meq/1). Alternatively, it m a y form from highly alkaline waters with a Ca/Mg ratio near unity, since no Ca-poor, Mg-rich waters have been observed in Eastern Rift lakes. Analcime occurs in a few reddish claystones near the top of the Koobi Fora Formation. It usually occurs near beds that have numerous small fish bones and are devoid of molluscs. Analcime is stable in East African lake waters with an alkalinity greater than 100 meq/1 (Fig.7). Analcime can form penecontemporaneously with deposition in alkaline lakes (Hay, 1966; Surdam and Eugster, 1976). This suggests that analcime in East African lake sediments can be a very good alkalinity indicator because it formed during and soon after deposition of the associated sediments. Chabazite and clinoptilolite occur as alteration products of volcanic glass in the Upper Member of the Koobi Fora Formation and the G u o m d e Forma-

276 tion. They are f o u n d in deposits of saline, alkaline lakes (Hay, 1966, 1976). Mineralogic changes are not due to facies relationships. The Kubi Algi Formation and the Lower Member of the K o o b i Fora Formation b o t h have some sediments equivalent to the near-shore facies of the Upper Member of the Koobi Fora Formation and show mineral assemblages equivalent to their deep-water partners. Unfortunately, however, their outcrops are so restricted that it is n o t possible to make detailed comparisons of the minor mineral changes that m a y be taking place laterally. PALEOCHEMISTRY OF PLIO-PLEISTOCENE LAKE TURKANA: DISCUSSION AND CONCLUSIONS Three independent methods of estimating paleoalkalinity have been explored. Each has its own strengths and weaknesses, b u t the set taken together may provide a valid measure of the chemistry of the ancient lake. Kubi Algi Formation: an open freshwater lake All evidence gathered argues for the existence of a large freshwater lake. The most compelling argument lies in the molluscS. Phyletic endemic forms are f o u n d throughout the upper 40 m (WiUiamson, 1978), indicating that alkalinity levels were never high enough to cause extinction. This implies an absolute upper limit of 16 meq/1 alkalinity. Diatom assemblages also indicate fresh water: the Cyclotella--Melosira assemblage from the base indicates alkalinities greater than 10 meq]l, while an almost pure Melosira assemblage from the t o p of the formation indicates lower alkalinities (2--5 meq]l?). Fish remains are not diagnostic in the Kubi Algi Formation. Phytoliths and ostracods are c o m m o n , b u t have not yet been studied. Studies of exchangeable cations on clays corroborate the above. Using the relationship f o u n d between alkalinity and exchangeable s o d i u m and calcium on clays, one can calculate a range of a b o u t 2--6 meq/1 for the section studied. Excluding one spurious sample, the results agree with the trend towards decreasing alkalinity observed in the diatoms; the alkalinity at the base ranges from 3 to 6 meq/1 and at the top from 1 to 3 meq/1. More samples should be studied to confirm this implied change. The authigenic mineral assemblage of the Kubi Algi Formation is compatible with formation in fresh water but provides less exact estimates for paleoenvironmental reconstructions. The almost complete absence of calcite and the corrosion of aragonite shell material suggests that the waters were undersaturated with respect to calcite. If the waters were undersaturated because of extremely low calcium and alkalinity levels, it would mean that the alkalinity was less than a b o u t 4 meq/1. The abundance of iron-oxides and gypsum replacing shell material suggests an attack b y sulfuric acid that resulted from oxidation of sulfides. However, most of the section is .lacking in calcite, hematite and gypsum which suggests that calcite precipitation probably was

277 uncommon. Although not conclusive, the authigenic mineral assemblage, hematite and gypsum with very little carbonate, suggests that the lake was very fresh. These three independent lines of evidence suggest that the late Kubi Algi lake was a fresh lake, probably with an alkalinity range from about 2 to 8 meq/l. The lake was very deep, with its eastern shoreline against the basin-margin escarpments.

Lower Member, Koobi Fora Formation: fresh to slightly brackish lake Molluscs again provide an excellent starting point for paleoalkalinity estimates. The fauna of this period is characterized by cosmopolitan, exotic, and radiative endemic species (zones 3 to 6, WiUiamson, 1978). This implies that the basin was permanently fresh and inhabitable to molluscs; alkalinity never rose to the point of mollusc extinction in view of the fact that the same radiative endemics occur throughout this member. This imposes an upper limit of 16 meq/1 during Lower Member time. No diatoms have been observed in these sediments, and the fish fauna is quite normal throughout. Ratios of exchangeable sodium to exchangeable calcium indicate an alkalinity range from approximately 8 to 13 meq/1. The only important authigenic minerals in the Lower Member are calcite and montmoriUonite. The ubiquitous occurrence of calcite indicates that the alkalinity exceeded 3 or 4 meq/1. The Lower Member lake was slightly more alkaline than the Kubi Algi lake, probably with an alkalinity range of 7 to 14 meq/1. Deltas were built out from the Bakate Gap during this time and the shoreline shifted to the west (Findlater, 1976).

Upper Member, Koobi Fora Formation, Guomde Formation, and Galana Boi Beds: unstable lake situation, fluctuating between fresh or brackish and moderately or highly alkaline Molluscs are a good starting point for determining water characteristics in this third phase of the lake's history. The Upper Member fauna is completely cosmopolitan, indicating that the endemics had been extinguished and that the molluscan population was never able to redevelop endemic forms. The mollusc population had almost certainly been through phases of extinction and renewal from the common East African stock. A clear example of this happened during the past 10,000 years. During Galana Boi times, approximately 10,000 years B.P. (Vondra et al., 1971), a fully cosmopolitan mollusc population occupied the basin (WiUiamson, 1978). Today the mollusc population is virtually extinct. Possible evidence for fish dwarfism is found in the "Fish Beds" of Ileret and a sandstone above the Koobi Fora Tuff near Koobi Fora. Faunal evidence suggests that the lake exceeded an alkalinity of 16 meq/1 a number of times; rarely it exceeded 40 meq/1; however, much of the time it was fresh enough for molluscs to survive.

278

The ratio of exchangeable sodium to exchangeable calcium of clays suggests an alkalinity range of at least 8--22 meq/1. Unfortunately, this technique is limited to pure montmorillonite clays and cannot be applied either to analcimerich horizons which may have the highest alkalinities or to silty horizons. The authigenic mineral assemblage also indicates high alkalinities. Analcime occurs in several horizons, indicating an alkalinity of at least 100 meq/1. Chabazite and dolomite also occur in the Upper Member and Guomde Formation. The paleoenvironmental significance of these minerals is not clear. These three lines of evidence suggest that the Upper Member and Guomde Lake had an alkalinity that usually ranged from about 8 to 25 meq/1; rarely it was as high as 200 meq/1. Evidence for more alkaline lake waters may be found by drilling the present lake sediments. Fig.17 shows paleoalkalinity estimates for the three time periods mentioned using each of the three methods for determining paleoalkalinity and Table V member

Method

Formation

1 I

k,lkalinity (meq/~ ] 10 I t l IJr~ll I

100 "I

Fauna and f l o r a upper

KOOBI FORk,

Exchangeable cations Mineral

equilibria I

I

I

,

I

IIIIIII

I

~ lllll[

I

;

i;

....

II

I

Fauna and f l o r a l oweP

Exchangeable cations KOOBI FORk`

Mineral

equilibria Fauna and f l o r a KUBI ALGI

Exchangeable cations Mineral

equilibria

i

I

I iJ,l,I

Fig. 17. Comparison of alkalinity estimates using biological considerations, cation exchange equilibria and authigenic mineral assemblages.

shows estimates of the composition of paleo-lake waters during the three phases. This chemical history has important implications on the food and water sources of early hominids: Alkalinity less than 16 meq/1 greater than 16 meq/1 less than 40 meq/1

molluscs abundant, possible food supply molluscs extinct in lake but could survive in perennial streams fish "normal" in species distribution and size; possible food supply

279 greater than 40 meq/1

only cichlid fish can adapt to these conditions; they become dwarfed at successively higher alkalinities

less than 16 meq/l

good drinking water calcium level low; may require additional water supplies water undrinkable for many mammals; water holes may be important fluoride high enough to build strong teeth

16 to 25 meq/1 greater than 25 meq/1 greater than 1 meq/l

TABLE V Calculated water compositions of Plio-Pleistocene Lake Turkana (in ppm, except alkalinity which is in meq/1)

Upper Member, Koobi Fora Formation b Lower Member, Koobi Fora Formation Kubi Algi Formation

x y z

Na

K

Ca

Mg

Alk

8000 800 250

Ci

SO4 F

250 55 25

2 5 7

6 2.5 3.5

200 1500 25 150 10 55

250

25

7

3.5

10

55

20

8

10

65

11

12

8

4

20

10

3

10

300 45 20

SiO2 a

200 150 20 20 8 10

a SiO2 values are from comparisons with other large lakes in the Eastern Rift from Table I and have not been calculated. b The lake had a composition similar to z during times when molluscs thrived. Elevated alkalinities and depressed calcium levels caused molluscan extinction, indicating that concentrations similar to y were not uncommon, although the transition between z and y may have been over several tens of thousands of years. Analcime and dwarfed fish are indicative of higher alkalinities, probably similar to composition x; these highly alkaline conditions were rare.

Evidence for a major climatic change about 1.7--1.9 m.y.B.P. A v a i l a b l e e v i d e n c e suggests t h a t L a k e T u r k a n a w a s a f r e s h w a t e r l a k e f r o m m i d - K u b i A l g i t i m e to t h e e n d o f L o w e r M e m b e r t i m e . D u r i n g this p e r i o d t h e l a k e n e v e r b e c a m e v e r y a l k a l i n e ; h o w e v e r an a b r u p t c h a n g e in t h e c h e m i s t r y o f t h e l a k e o c c u r r e d a t t h e t o p o f t h e L o w e r M e m b e r . I t is m a r k e d b y a s m a l l d i s c o n f o r m i t y w h i c h has l o w r e l i e f n e a r K o o b i F o r a a n d several m e t e r s o f r e l i e f t o t h e w e s t a l o n g t h e K a r a r i e s c a r p m e n t . I t is also m a r k e d b y t h e e x t i n c t i o n of the p h y l e t i c e n d e m i c species t h a t characterize the L o w e r M e m b e r , so t h a t o n l y a c o s m o p o l i t a n m o l l u s c a n f a u n a r e m a i n s ( W i l l i a m s o n , 1978). C h a n g e s in a u t h i g e n i c m i n e r a l s o c c u r a t a b o u t t h i s level. T h e K B S T u f f near the top of the Lower Member contains the first zeolite (clinoptilolite)

280

encountered in the sediments. Dolomite first appears 16 m above the KBS Tuff. Chabazite occurs in many of the ruffs in the Upper Member and Guomde Formation. Thus, the first moderately alkaline lake to occupy the basin, causing molluscan extinction and new authigenic minerals, occurred just prior to the deposition of the KBS Tuff. Possibly this represents the first time since at least mid-Kubi Algi time that the lake fell below overflow level and evaporation from the lake surface exceeded net input by rain and runoff. Four factors could accomplish this: increasing total evaporation by increasing the lake surface (Fig. 18); increasing total evaporation by increasing evaporation rate; decreasing input by decreasing drainage area (stream capture); and decreasing input by decreasing rainfall. These four possibilities, or any combination of them, could all explain the changes observed in the sediments. The second and fourth possibilities are climatic; the first and third are a result of tectonic and geomorphologic processes. Unfortunately, it may not be possible to evaluate the first and third possibilities until more studies throughout the basin have been made. Throughout Koobi Fora Formation times, a major input into the basin came through Bakate Gap which contributes nothing today (Bowen, 1974; Findlater, 1976). Other major sources of supply cannot be evaluated without more information. e Isotope studies of carbonates give evidence for a climatic change taking place about this time (Cerling et al., 1977; Cerling, 1977). It is particularly compelling evidence because a similar change to more arid conditions occurred

,'-',

KERIO - OMO TROUGH

I

I

I / I / /

/

/

/ /

/

i / / /

( J SUGUTA TROUGH

Fig.18. Tectonic troughs controlling the configuration of Lake Turkana. Total evaporation from lake surface is dependent on how much of these two troughs is occupied. The south end of the Suguta trough was cut off by a volcanic barrier in the late Pleistocene (Truckle, 1976). This probably caused a dramatic rise in the level of the main lake and a dramatic fall in the level of the isolated lake.

281

at the same time in Olduvai Gorge, Tanzania. Lacustrine carbonates from below the KBS Tuff (1.8 m.y.) in the Koobi Fora Formation show low values of lSO. Calcretes and groundwater nodules below the Lemuta Member (1.65 m.y:) are the most strongly depleted carbonates in the Olduvai sequence (Cerling et al., 1977; Cerling, 1977). These results suggest that the isotopic composition of the water that precipitated the older carbonates was significantly lighter than the later waters; this could happen in a more humid climate where waters were not partially evaporated. Good evidence for other climatic events at about 1.2 and 0.5 m.y.B.P, is found at Olduvai Gorge in Tanzania. No midPleistocene sediments of similar age are found around Lake Turkana. This may indicate that since the end of Koobi Fora Formation times the lake has been almost exclusively closed and usually at quite low levels. Faunal evidence from the Lake Turkana Basin and from Olduvai Gorge support these conclusions. Tuff H2 from the Plio-Pleistocene Shungura Formation 100 km north of the Koobi Fora area may be correlated with the KBS Tuff of the Koobi Fora Formation (Cerling, 1977). Studies of bovids from the Shungura Formation suggest that a major faunal change took place in or above member G. Earlier fauna is characterized by animals that require water and luxuriant growth; it was replaced by a fauna adapted to drier conditions (Gentry, 1975). A similar change in bovids occurs at about the KBS Tuff level in the Koobi Fora Formation (Harris, 1976). Fauna below the Lemuta Member at Olduvai Gorge (1.9 to 1.65 m.y.B.P.) is primarily swampdwelling; above the Lemuta Member a dry-plains fauna predominates (Hay, 1976). CONCLUSIONS

(1) Modern East African lakes follow a simple evaporation trend and can be used as an analogue for Plio-Pleistocene East African lakes. (2) Gypsum is not an evaporite phase and therefore is not indicative of arid conditions in this system. Calcite, fluorite, montmoriUonite, analcime, gaylussite, natron and trona are useful paleoalkalinity indicators. (3) Fauna, flora, exchangeable cations, and mineral assemblages can be used to estimate paleoalkalinities of ancient East African lakes. (4) Plio-Pleistocene Lake Turka'na was a freshwater lake from late Kubi Algi time until about 1.8 m.y. ago. Since then it has fluctuated between a freshwater lake (alkalinity about 10 meq/1) and a moderately alkaline lake (200 meq/1). (5) This transition was due to climatic rather than tectonic reasons. ACKNOWLEDGMENTS

This work was supported by grants from the National Science Foundation, The National Geographic Society and the Penrose Foundation. Richard E. Leakey and The National Museum of Kenya provided generous field support.

282 I t h a n k G l y n n L. Isaac, R i c h a r d L. H a y , B e t t i n a W. Cerling, J a m e s R. O'Neil, M a r c C. M o n a g h a n , William F. M c K e n z i e , D e n n i s W. P o w e r s a n d P e t e r J. Williamson f o r m a n y s t i m u l a t i n g discussions o n t h e v a r i o u s a s p e c t s of this p r o j e c t . B r u c e E. B o w e n , J. W. K. Harris, Paul Abell, I a n F i n d l a t e r , K a m o y a K i m e u , J. A. Stevens, P e t e r R o b i n s a n d m e m b e r s o f t h e E a s t T u r k a n a R e s e a r c h P r o j e c t w e r e v e r y h e l p f u l in t h e field. L a b o r a t o r y w o r k c o u l d n o t h a v e b e e n c o m p l e t e d w i t h o u t t h e aid o f R o n a l d K. Stoessell, M a r k R e e d , R i c h a r d J. R e e d e r a n d Brian Viani. J o n a t h a n R i c h a r d s o n k i n d l y m a d e d i a t o m i d e n t i f i c a t i o n s . T h i s p a p e r was s u b s t a n t i a l l y i m p r o v e d b y t h e r e v i e w s o f D. A. L i v i n g s t o n e , B. F. J o n e s , A. M. S t u e b e r , a n d R. R. T u r n e r .

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