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Authigenic low-Mg calcite in Lake Turkana, Kenya
0899-5362/89 $3.00 + 0.00 (~) 1989 Pergamon Press plc
Journal of African Earth Sciences. Vol. 8, Nos. 2/3/4, pp. 533-540, 1989 Printed in Great Britain
Authigenic Low-Mg Calcite in Lake Turkana, Kenya John D. HALFMAN,* Thomas C. JOHNSON,* William J. SHOWERS ** and Guy S. LISTER *** Department of Earth Sciences, * University of Notre Dame, Notre Dame, IN 46556 Department of Marine, Earth and Atmospheric Sciences ** North Carolina State University Raleigh, NC 27695 Geologisches Institut *** ETH - Zentrum CH-8092 Zurich, Switzerland
Abstract- LakeTurkanais an importantmodemanalogto ancientrift environmentsin east Africaand elsewhere.Carbonate is secondin abundanceafter the detrital silicate fractionof the sedimentsin the lake. It consistsof ostracod tests and a siltsizedmicrite.ScanningelectronmicroscopyandX-raydiffractionanalysisrevealedthemicriteto be euhedralcrystalsoflowMg calciteabout 10 microns in length.Carbon isotopevaluesof the micriteindicatethat it forms in the upper watercolumn rather thanin the sedimentsafterburial.The oxygenisotopeanalysesindicatethatthe micritecouldbe at isotopicequilibrium with the modemlake waters at temperaturesof 36°C. This suggeststhat the micriteformseitherin the surfacewatersof the open lake or in isolatedshallowbays, and is then transportedto the deep lake. INTRODUCTION
Lake T u r k a n a (formerly Lake Rudolf) is t h e largest c l o s e d - b a s i n lake in t h e E a s t African Rift S y s t e m . The b a s i n is located in t h e desolate Nort h e m F r o n t i e r District of Kenya, y e t h a s b e e n the site of m u c h r e s e a r c h activity in r e c e n t y e a r s . The incentive for t h e s e s t u d i e s h a v e b e e n derived prim a r i l y from t h e tectonic i m p o r t a n c e of the rift valley s e t t i n g (Baker et al. 1972), paleontological a n d a n t h r o p o l o g i c a l discoveries of s o m e of the oldest fossil h o m i n i d s (Brown et al. 1985), t h e longt e r m a n d h l g h - r e s o l u t i o n record of climatic c h a n g e (Owen et al. 1982, H a K m a n a n d J o h n s o n in press), a n d economic i m p o r t a n c e for p e t r o l e u m a n d minerals (Robbins 1983). F u r t h e r m o r e , t h e lake occupies a very h o t a n d arid s e t t i n g w i t h rainfall less t h a n 200 m m / y r a n d a n average a n n u a l t e m p e r a t u r e of 30°C. It c a n be c o n s i d e r e d as the ,arid region e n d m e m b e r , of large rift valley l a k e s a n d a n i m p o r t a n t m o d e m a n a l o g for a n c i e n t rift environm e n t s in Africa a n d elsewhere. Within t h e p a s t d e c a d e several p a p e r s have b e e n w r i t t e n on Lake T u r k a n a t h a t i n c l u d e t h e lillmology (Ferguson a n d H a r b o t t 1982), r e c e n t s e d i m e n t o l o g y fYuretich
1979, 1986), s t r u c t u r e a n d s e i s m l c - s t r a t i g r a p h y of the s e d i m e n t s (up to 4 k m thick) u n d e r l y i n g the lake ( D u n k e l m a n 1986, J o h n s o n et al. 1987). An i m p o r t a n t c o m p o n e n t of the s e d i m e n t s in arid lakes, b e s i d e s detrital clastic a n d biogenic material, is inorganic precipitates, especially carb o n a t e s . C a r b o n a t e s e d i m e n t s are deposited in lakes in a variety of climatic s e t t i n g s i n c l u d i n g h y p e r s a l i n e lakes of arid regions (e.g., E u g s t e r a n d Hardie 1978). alkaline l a k e s in t e m p e r a t e regions (e.g., Kelts a n d H s u 1978) a n d in t h e tropics (e.g., C o h e n a n d T h o u i n 1987). M a n y a n c i e n t rift sedim e n t s in e a s t Africa a n d elsewhere were deposited u n d e r tropical c o n d i t i o n s (e.g.. V a n H o u t e n 1964) a n d c o n t a i n a b u n d a n t c a r b o n a t e . The origin of the c a r b o n a t e s (e.g., biogenic or authigenic) in l a c u s t r i n e s t r a t a is n o t always a p p a r e n t so the s t u d y of it i n m o d e m lakes c a n provide u s e f u l clues for the i n t e r p r e t a t i o n of t h e a n c i e n t deposits. C a r b o n a t e p h a s e s in l a c u s t r i n e s y s t e m s comm o n l y c o n t a i n key e n v i r o n m e n t a l information. For example, the mineralogical p r o g r e s s i o n from calcite, to m a g n e s i a n calcite, t h e n aragonite a n d finally dolomite is c o n s i d e r e d a reliable indicator of i n c r e a s i n g Mg2"/Ca 2÷, t h u s i n c r e a s i n g salinity of
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the lake water (Muller et al. 1972). Information indicate t h a t faulting is active today (Johnson et al. concerning past productivity, t e m p e r a t u r e and 1987). Lake level h a s fluctuated t h r o u g h o u t the salinity m a y be elicited from carbon and oxygen range of the basin. Lowstands at about 60 m below stable isotope ratios of carbonates (Abell 1982, the present lake's surface were interpreted from McKenzie 1985). Down-core fluctuations in high resolution seismic profiles and were estimacarbonate content from Lake T u r k a n a are inter- ted to have occurred prior to 10,000 years ago preted to reflect cyclic climatic change on a time (Johnson et al. 1987). Early Holocene deposits scale of decades to centuries (Halfman and record h i g h s t a n d s from 50 to 80 m above the J o h n s o n in press). Valid interpretations require present level (Owen et al 1982). These lake level t h a t the carbonate p h a s e s precipitate from the fluctuations are s y n c h r o n o u s with other basins water column or of biogenic origin and not of t h r o u g h o u t the tropics on a millennial time scale detrital or diagenetic origin. The purpose of this for the past 30,000 years (Butzer et al. 1972, Streetpaper is to determine the mode of formation for Perrott and Harrison 19841. They are interpreted to reflect global climatic variability, and are predicted the carbonates in Lake Turkana. by general- circulation models (Kutzbach and Street Perrott 1985). HYDROLOGY AND RECENT SEDIMENTS The m o d e r n sediments of Lake T u r k a n a are Lake T u r k a n a occupies the ,Turkana Depres- primarily detrital silicates (Yuretich 1979, 1986). sion, and receives r u n o f f a n d sediment from a wide Coarse grained n e a r s h o r e deposits are influenced geographical area (Fig. 1). It is approximately 260 by high wave energy and the presence of volcanic k m long with a m e a n width of 30 km, and h a s a n h e a d l a n d s (Cohen et al. 1986). The d o m i n a n t average and m a x i m u m depth of approximately 35 pattern of fine-grained sedimentation offshore, as m and 115 m, respectively. The hydrology of the revealed by high-resolution (1- and 28-kHz) basin is dominated by the perennial Omo River, seismic profiles, is one of simple, ponded infilling which drains the Ethiopian Plateau to the north, (Johnson et al. 1987). Sedimentation rates based and provides about 90% of the fresh water flowing on radiocarbon dates of two of our piston cores are into the lake (Ferguson and Harbott 1982). The about 0.3 and 0.5 c m / y r (Halfman a n d J o h n s o n in seasonal Turkwel and Kerio Rivers contribute most press). The composition of the sediments varies of the remaining fluvial input. Other streams, systematically from n o r t h to south. The relative direct rainfall and s u b t e r r a n e a n flow are conside- a b u n d a n c e ofbiogenic c o m p o n e n t s increases with red insignificant in the water budget (Yuretich and increasing distance from the Omo River, i.e., from Cerling 1983). The lake is moderately saline (2.4°/ rare and poorly preserved diatom clays in the north oo and alkaline (pH 9.2) (Yuretich and Cerling basin to well-preserved ostracod-diatom silty clays 1983). The principle ions are Na ÷ (32 mmoles/l), in the s o u t h basin. This trend results from decreaHC03- (19 meq/l) and Cl-(13 mmoles/l) with rela- sed dilution of the biogenic c o m p o n e n t s by detrital tively low concentrations of Ca 2÷, Mg2÷and S042°. silicates with increased distance from the Omo The hydrogeochemical budget indicates t h a t Ca 2÷ delta (Halfman 1987). The relative composition of the silicate minerals is s u p e r - s a t u r a t e d with respect to calcite, Mg2+is s u p e r - s a t u r a t e d with respect to Mg-smectites, and is primarily controlled by the source rock compoS042- m a y be r e d u c e d to a sulfide (Yuretich and sition and weathering intensity (Yuretich 1979, Cerling 1983). The water column is well mixed by 1986). The north basin is rich in kaolinite and strong diurnal winds, and has a relatively uniform resistates t h a t reflects the weathering of mafic water t e m p e r a t u r e of 25 to 26°C and dissolved volcanics in the humid, tropical setting of the Omo oxygen concentrations that range from about 70 to basin. In contrast, the sediments adjacent to the Turkwel-Kerio drainage system are coarser, con100% saturation (Ferguson and Harbott 1982). Both tectonics and climate are important exter- tain quartz, feldspar, illite and smectite, and reflect nal factors t h a t control sedimentation in Lake the arid climate and exposures of Precambrian Turkana. The formation of the rift has provided a n gneiss and schist. Water-sediment reactions are effective trap for water, water-laid sediments, vol- very important in the a c c u m u l a t i o n of smectite. caniclastics and volcanic flows since the Pliocene The sediments are laminated, except in the and possibly the Miocene. Half-graben units with nearshore deposits, despite well-oxygenated botaltemating polarities occur along strike of the lake tom waters (Yuretich 1979, Cohen 1984). The (Dunkelman 1986, Rosendahl 1987). Four half- benthic standing crop, which could potentially graben units underlie the n o r t h e r n depositional homogenize the sediments consists primarily of basin suggesting t h a t clastic deposition from the epibenthic detritovores (e.g., ostracods). Their Omo River m a s k s earlier structural controls on a b u n d a n c e declines from n e a r s h o r e to offshore deposition. High-angle growth faults are common- environments, presumably due to a parallel decline ly observed in high-resolution seismic profiles and in edible detritus (Cohen 1984). The light-dark
Authigenic low-Mg calcite in Lake Turkana, Kenya
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couplets are differentiated by carbonate content and are interpreted to reflect a n interannualvariation (about 4 years) in the dilution of the carbonates by detrital silicates (Halfman and J o h n s o n in press). Carbonate is the next most a b u n d a n t compon e n t to the detrital silicate fraction in the recent sediments of Lake T u r k a n a (Yuretich 1979, Halfm a n 1987). Carbonate concentration of the sedim e n t increases with increasing distance from the Omo River (Halfman 1987). Bulk carbonate concentrations from the recent sediments average 10% (CaC03) of the dry weight, and range from I to 28% (Halfman 1987). The average composition of the calcite is about 3 mole % Mg, or low-Mg calcite, as revealed by X-ray diffraction analyses (Yuretich 1986). The carbonate fraction in the offshore sedim e n t s h a s two m a i n components, ostracod carapaces, and micron-slzed crystals of carbonate (Halfman 1987). Other c o m p o n e n t s including gastropod shells and possibly dolomite were rarely detected in the surface sediments. Beach rock occurs along portions of the lake's margin. The m o d e m ostracod population in the offshore sedim e n t s is dominated by Sclerocypris clavata with lesser a m o u n t s ofHemicypris Intermedia, and is limited to between 10 and 100 t e s t s / m 2, whereas, the ostracod population in nearshore deposits is dominated by Hemicypris klele, Cypridies torosa, and Gomphocythere a n g u l a t a and contain between 100 and 1,000 tests/m2along most of the nearshore zone except at isolated vegetated shorelines where the population increases to about 100,000 t e s t s / m 2 (Cohen 1982). In this paper, we focus on the micrite fraction of the sediment. Analyses were designed to determine the mode of origin of this previously u n s t u d i e d c o m p o n e n t of the sediment.
and s o u t h basins. The ostracod and micrite fractions were separated from each organic-free, sediment subsample by successive sieving. The organic m a t t e r was digested with dilute hydrogen peroxide. A 150 micron sieve isolated the ostracod fraction, from which Juvenile and adult forms of Sclerocyprls clavata were picked, and approximately 15 tests were lightly c r u s h e d before each analysis. The micrite fraction constituted the carbonate that passed t h r o u g h a 44 micron sieve. Isotopic analysis was performed on the evolved carbon dioxide after reacting the sample with 100% orthophosphoric acid at 50°C. A Finnegan MAT 251 m a s s spectrometer at the NCSU Stable Isotope Laboratory was u s e d for isotopic analysis and the results reported relative to the PDB standard. The s t a n d a r d deviation for replicate ostracod analyses is O. 14°/oo for 81sC and 0.08°/00 for 6 ]a0. The s t a n d a r d deviation for replicate micrite analyses is 0.050/00 for 8~3C and 0.26°/00 for 8 ~ . Details of all the m e t h o d s are included in Halfman (1987).
AUTHIGENIC MICRITE
Lacustrine carbonates are derived from a combination of four possible processes: biogenic precipitation of calcareous tests, clastic input, primary inorganic precipitation, and post-depositional diagenesis. Comparison between the micrite 8~3C values and the corresponding values of the dissolved inorganic bicarbonate (DIC) i n t h e sediment pore waters reveals significant differences. The 8~zC values for the micrite average 0.76°/00 a n d -O. 14°/oo for cores LT84-2P and 7P, respectively (Fig. 2). The 8 ~3Cvalues for the DIC yield increasing values down both cores from 0°/oo to +12°/oo (PDB) (T.Cerling personal c o m m u n i c a t i o n 1985). This indicates t h a t the micrite is not a diagenetic METHODS precipitate, b e c a u s e if it were, its carbon isotope Ten piston cores, each approximately 11 meters values would be heavier (Irwin et al. 1977, Talbot long, were collected from the lake with an ETH and Kelts 1986). corer provided by the Geologisches Institut, ETHThe micrite was previously a s s u m e d to be bioZentrum, Zurich (Kelts et al. 1986, Fig. 1). The genic debris (Yuretich 1979). yet only the cocores were collected in November, 1984, as part of existence of both forms supported the hypothesis. Duke University's Project PROBE. Analyses of isolated micrite and species specific The micrite was analyzed by three principle ostracod samples reveal significantly different methods. A s c a n n i n g electron microscope (SEM) results. Only samples from core LT84-2P had was u s e d to identify the constituents of the fine- sufficient ostracod tests to be compared geograined sediments. The mineralogy of the fine chemically with the micrite. The 6 ~aC values are fraction was identified by X-ray diffraction. Stable approximately 1°/oo heavier and the ~180 values isotope composition was determined from isolated are approximately 2°/00 lighter in the micrite ostracod and micrite fractions of the sediment. fraction t h a n in the ostracod tests (Fig. 2). This Approximately 15 g of wet sediment was sub- suggests that the micrite is not derived from sampled at about 1 m intervals down two cores, fragmented tests of Sclerocypris clavata. The LT84-2P and LT84-7P. These cores were selected micrite could be fragments ofother species because for analysis as being representative of the n o r t h isotopic vital effects m a y be different between
Authigenic low-Mg calcite in Lake Turkana, Kenya
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Fig. 2. Stable isotope ratios versus depth down cores, LT84-2P and 7P. Only core LT84-2P had sufficient ostracod tests for analysis. The micrite fraction (see text) was analyzed fi-om both cores. Juvenile (open symbol) and adult (closed symbol) forms of,~ler~yprlm elavata were used in this study. The oxygen (circle) and carbon (square) isotope values are reported relative to the PDB standard. The mean value for each proflleis indicated al~er the X. species, althoughvital effects for ostracods are not clearly d o c u m e n t e d (P. DeDeckker personal comm u n i c a t i o n 1987). The range of the oxygen isotope values for the ostracods reported here are equal to and slightly lighter t h a n different species of o s t r a c o d s from core LT84-1OP analyzed separately b y G. Lister. SEM analysis showed m u c h of the micrite to be euhedral, semi-acicular crystals a b o u t 10 microns in size (Fig. 3). The crystals were observed always to be individual s p e c i m e n s rather t h a n aggregates of several crystals. The micrite is morphologically different t h a n platy ostracod fragments. Platy material with s u b - m i c r o n sized pores is revealed b y SEM photos (Fig. 3) t h a t m a y be ostracod debris. Smear slide analysis of the b u l k sediment and sieved fractions rarely revealed fragmented ostracods. The contribution to the bulk carbonate content b y the ostracod tests in the profundal sediments is calculated to be less t h a n 1% of the dry sediment, a s s u m i n g that: (I) the tests are p u r e calcite, (2) the animals live in the u p p e r cm of sediment and molt 100 times, and (3) the sediments have an average porosity of 90% a n d density of 2.6 g / c m a. This value is significantly less t h a n the m e a s u r e d b u l k carbonate content of the sediments. The euhedral c h a r a c t e r and lack of an appreciable influx of detrital c a r b o n a t e s to the lake from the drainage b a s i n (T. Ceding personal communication 1985) suggests t h a t the micrite is not detrital,
i.e., not transported t h r o u g h fluvial systems. The increased carbonate concentrations away from the Omo River also s u p p o r t s the non-detrital origin (Halfman 1987). The morphology of the crystals is similar to primary inorganic precipitates of calcite in temperate lakes; these typically are not perfect r h o m b s or needles, b u t are stubby, e q u a n t or blocky polyhedra t h a t reveal crystal faces, and are a b o u t 10 microns in length (Bihmskill 1969, Kelts and H s u 1978). Sizes up to 40 microns have b e e n reported (Lee et al. 1987). X-ray diffraction of the fine fraction confirms Yuretich's (1986) findings t h a t the micrite is a IowMg calcite. Low-Mg calcite is the composition that is predicted for c a r b o n a t e s precipitating in a lake with T u r k a n a ' s chemistry (Muller e t a/1972). The evidence presented so far suggests that the euhedral crystals are primary precipitates. The evidence p r e s e n t e d so far suggests that the euhedral crystals are primary precipitates. Interpretation of the stable isotope data confirms this hypothesis. The carbon isotope values of the micrlte are heavier (about 1°/oo) t h a n the ostracod tests. This difference p a y be explained by a photosynthetic fractionation of the c a r b o n isotopes, where carbon- 12 is preferentially transferred from the surface waters to the deeper waters b y a photosynthetic-respiration pathway. Photosynthetically produced organic m a t t e r is enriched in carbon-12
538
JOHN D. HALFMANet al.
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p
Flg. 3. SEM photographs of the fine fraction of the sediment from 29 cm down core LT84-2P. The arrow-shaped particles, most visible in the lower right photo, are euhedral calcite. The platy, porous particles, visible in the upper right photo may be an ostracod fragment. The other components include detrital grains and diatoms. relative to c a r b o n - 13, a n d as it sinks, it oxidizes, - 1°/oo ( m e a s u r e d twice in 1979, SMOW, T, Cerling releasing c a r b o n - 1 2 enriched C02to t h e b o t t o m p e r s o n a l c o m m u n i c a t i o n 1985). The micrite with a waters. As c a r b o n a t e p r e c i p i t a t e s are basically in m e a n 61s0 v a l u e of 1°/oo (PDB). e q u i v a l e n t to isotopic equilibrium with the dissolved inorganic 31.9°/oo (SMOW), w o u l d be in isotopic equilibrium c a r b o n , t h e c a r b o n a t e c o n t a i n s a c a r b o n - i s o t o p e with t h e lake w a t e r f l i t s t e m p e r a t u r e of f o r m a t i o n c o m p o s i t i o n characteristic of t h e w a t e r d e p t h w a s 36°C or t h e isotopic c o m p o s i t i o n of t h e w a t e r (McKenzie 1985, Lee et al. 1987). The o s t r a c o d s are w a s lowered b y 1°/oo. A f o r m a t i o n t e m p e r a t u r e of epibenthic detritovores (Cohen 1984), a n d t h u s it 36°C is higher t h a n 25.5°C, the m e a n t e m p e r a t u r e of t h e w a t e r c o l u m n ( F e r g u s o n a n d H a r b o t t 1982). m a y explain the lighter c a r b o n isotope values. W a t e r t e m p e r a t u r e s above 30°C were r e p o r t e d for Authigenic c a r b o n a t e s s h o u l d precipitate at t h e s u r f a c e w a t e r (at 1 m depths) in t h e o p e n lake isotopic equilibrium with the water. The equilia n d isolated, shallow b a y s ( F e r g u s o n a n d H a r b o t t b r i u m isotopic fractionation factor for t h e calcite1982). This m a y indicate t h a t t h e c r y s t a l s precipiw a t e r s y s t e m is: tate in s u p e r - h e a t e d s u r f a c e w a t e r s , p e r h a p s in 1000 I n (0<) = 2.78 (108) • T -2 - 2.89 t h e u p p e r m o s t millimeter on c a l m days. or in assuming: shoreline pools with s u b s e q u e n t t r a n s p o r t to t h e i 0 0 0 I n (0c)= 180 ~u~- la0water (SMOW) w h e r e (c<) is t h e isotopic fractionation factor, T is deep basin. A s e c o n d h y p o t h e s i s c o n c e r n s a less saline, the t e m p e r a t u r e of the reaction (°K) a n d the isotope ratios are relative to the SMOW s t a n d a r d t u r b i d w a t e r m a s s g e n e r a t e d d u r i n g t h e flood (Friedman a n d O'Neil 1977). The ~ s O of Lake s e a s o n of t h e O m o River. The w a t e r m a s s e x t e n d s T u r k a n a h a s b e e n m e a s u r e d at 5.6°/00 ( m e a s u r e d from t h e O m o River to t h e central basin. The in 1975 a n d 1980) a n d t h e O m o River effluent at c o n d u c t i v i t y of the lake w a t e r d r o p s b y a b o u t 60%
Authigenic Iow-Mg calcite in Lake Turkana, Kenya n o r t h of North Island, a n d b y 10% t h r o u g h t h e r e m a i n d e r of t h e n o r t h b a s i n d u r i n g p e a k flood s e a s o n ( F e r g u s o n a n d H a r b o t t 1982). The m i x i n g of t h e two w a t e r m a s s e s of different isotopic composition w o u l d be sufficient to precipitate micrite in e q u i l i b r i u m in t h e n o r t h b a s i n at p r e s e n t w a t e r c o l u m n t e m p e r a t u r e s , a l t h o u g h slightly h i g h e r t e m p e r a t u r e s (28°C) t h a n t h e m e a n lake t e m p e r a t u r e s are r e q u i r e d for t h e a r e a s o u t h of North Island. The m e a n 6~s0 v a l u e for LT84-7P is a b o u t 1°/oo lower t h a n 2P, possibly reflecting t h e mixing p h e n o m e n a . T h i s m e c h a n i s m r e q u i r e s t h e CaC03 a c c u m u l a t i o n rate to be f a s t e r n e a r t h e Omo River t h a n f a r t h e r away. Relative s e d i m e n t a t i o n r a t e s for t h e n o r t h b a s i n cores c a n be inferred from a s t r a t i g r a p h y b a s e d on b u l k c a r b o n a t e profiles (Halfman 1987). T h e s e r a t e s w i t h t h e b u l k carbon a t e c o n c e n t r a t i o n s reveal h i g h e r m a s s a c c u m u l a tion r a t e s for CaC03 closer to the Omo River t h a n f a r t h e r away. A t h i r d possibility is t h a t the micrite w a s f o r m e d d u r i n g a time w h e n t h e lake w a s less saline t h a n today. S e a s o n a l t e m p e r a t u r e , p h o t o s y n t h e t i c a n d salin i t y v a r i a t i o n s are t h e maj or f a c t o r s t h a t influence s u p e r s a t u r a t i o n w i t h r e s p e c t to inorganic precipit a t i o n of calcite (Kelts a n d H s u 1978). W a r m i n g of s e a s o n a l t e m p e r a t e lakes, a s s i m i l a t i o n of COs or p h o t o s y n t h e t i c evaporitic c o n c e n t r a t i o n of t h e ions are in s o m e c a s e s sufficient for s u p e r s a t u r a t i o n . A t h r e e y e a r limnologic s t u d y of Lake T u r k a n a indic a t e s t h a t t h e r e is a m i n i m a l (1°C) s e a s o n a l variation in t h e w a t e r t e m p e r a t u r e (Ferguson a n d Harb o t t 1982). The s a m e s t u d y s h o w e d t h a t lake level d r o p s a b o u t i m d u r i n g the non-flood s e a s o n of the Omo River. A s s u m i n g t h e only factor to influence t h e c o n c e n t r a t i o n of t h e principle ions is t h i s loss of water, t h e evaporitic e n r i c h m e n t of the concent r a t i o n s is m i n i m a l . For example, t h e c o n c e n t r a tion of Ca 2÷ is only i n c r e a s e d b y 1%. Pro-ductivity a n d p l a n k t o n i c b i o m a s s m e a s u r e m e n t s indicate a s e a s o n a l b l o o m of b l u e - g r e e n algae d u r i n g the flooding of t h e Omo River (Harbott 1982). Presum a b l y , t h e Omo River provides a limiting n u t r i e n t . This s u g g e s t s t h a t the micrite precipitation is biogenically m o d u l a t e d a n d c o n c u r r e n t with the flooding of t h e Omo River. CONCLUSIONS The c a r b o n a t e f r a c t i o n of the r e c e n t s e d i m e n t s in Lake T u r k a n a were a n a l y z e d to d o c u m e n t the m o d e of origin for t h e micrite. The micrite is e u h e d r a l , semi-acicular, low-Mg calcite. Previous w o r k s u g g e s t e d t h a t t h e micrite w a s either biogenic debris or a n a u t h i g e n i c precipitate, b u t t h e c o n c l u s i o n s were b a s e d on limited data. A geoc h e m i c a l a n d morphological c o m p a r i s o n b e t w e e n t h e micrite a n d o s t r a c o d t e s t s reveals t h a t the
539
micrite is n o t biogenic debris. The ~I 3C v a l u e s s u g g e s t t h a t t h e micrite precipit a t e s inorganically in t h e s u r f a c e w a t e r s of the lake. Its f o r m a t i o n m a y be e n h a n c e d b y p h o t o s y n theUc activity. The micrite is in isotopic equilib r i u m w i t h the lake w a t e r if its t e m p e r a t u r e of f o r m a t i o n is h i g h e r t h a n t h e m e a n w a t e r t e m p e r a t u r e of t h e lake. This s u g g e s t s t h a t t h e micrite precipitates in the s u p e r - h e a t e d s u r f a c e waters, p e r h a p s on c a l m d a y s , or in shallow shoreline pools. Isotopic equ111brium c o u l d also be achieved if t h e isotopic c o m p o s i t i o n of t h e lake were 1°/oo lower. This is h y p o t h e t i c a l l y possible in t h e n o r t h e r n p a r t of t h e n o r t h b a s i n d u r i n g t h e flood s e a s o n of t h e Omo River. Acknoledgements - We thank the captain and crew of the M/V Halcyon, operated and owned by the Kenyan Department of Fisheries for their valuable assistance in the field, at times under stressful conditions, and R. Leakey, National Museum of Kenya for IogisUcal assistance. K. Ghilardi was invaluable in direcUng the operaUon of the ETH piston corer. T. Ceiling kindly allowed us to use his isotope data of sediment porewater and lake water. D. Van Nieuwenhuise kindly provided ostracod identifications. We thank C. Paull for his careful review o fan earlier draft of this manuscript. Financial support was provided by Project PROBE and two grants to T.C.J. numbered PRF- 17428-AC2 from the American Chemical Society and DPE-S542-G-SS-OO from the Agency for International Development. At the time of the Turkana field program, Project PROBE was funded by Amoco, Arco, Conoco, Esso, Marathon, Mobil, Pecten, Penzofl, and Shell IntemaUonal Oil Companies, and by the World Bank. REFERENCES Abell, P.I. 1982. Paleoclimates at Lake Turkana, Kenya, from oxygen isotope raUos of gastropod shells. N a t u r e 297.321-323. Baker, B.H., Mohr, P.A. and Williams, L.A.J. 1972. C-eology of the Eastem Rift System of Africa. Geol. Soc. Amer. Special Paper. 136, 67pp. Brown, F.H., Harris, J., Leakey, R. and Walker, A. 1985. Early Homo erectus skeletons from west Lake Turkana, Kenya. N a t u r e 316, 788-792. Brunskill, G.J. 1969. FayettviUe Green Lake, NewYork. II. PrecipitaUon and sedimentation of calcite in a meromlctic lake with laminated sediments. L/mno/. O c e a n o g r . 14, 830-847. Butzer, K.W., Isaac, G.L., Richardson, J.L. and Washburn-Kamau, C. 1972. Radiocarbon dating of East African lake levels. Sclence. 175, 1069-1076. Cohen, A.S. 1982. Ecological and paleoecological aspects of the rift valley lakes of East Africa. Ph.D. DissertaUon, Univ. California, Davis. Cohen, A.S. 1984. Effect ofzoobenthic standing crop on laminae preservaUon in tropical lake sediment, Lake Turkana, East Africa. J. P a l e o n t . 58, 499-510. Cohen, A.S., Ferguson, D.S., Gram, P.M., Hubler, S.L. and Sims, K.W. 1986. The distribution of coarsegrained sediments in modem Lake Turkana, Kenya:
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Implications for clasUc sedimentation models in lakes. In: Frostich, L.E. etal. (editors) Sedlmentation in the African Rifts. Geol. Soc. Lond. Spec. Pub. 25, Blackwell, London. Cohen, A.S. a n d Thouin, C. 1987. Nearshore carbonate depostion in Lake Tanganyika. Geo/ogy. 15, 414-418. Dunkelman, T.J. 1986. The structural and stratigraphic evolution of Lake T u r k a n a , Kenya, as deduced from multichannel reflection data. M.S. Thesis, Duke Univ., D u r h a m . Eugster, H.P. and Hardie, L.A. 1978. Saline lakes. In: Lerman,.~ (editor Lakes: Geology, Chemistry, Physics. Springer Verlag, New York. Ferguson, A.J.D. and Harbott, B.J. 1982. Geographical, physical and chemical aspects of Lake Turkana. In: H o p s o n , / L J . (editor) Lake Turkana: A report o n the findings of the Lake T u r k a n a Project' 1972 - 1975. Overseas Development Administration, London. Freidman, I. and O2qeil, J.R. 1977. CompilaUon of stable isotope fractionation factors of geochemical interest. In: Data of Geochemistry, 6th ed., U.S. Geol. Surv. Prof. Paper. KKI-KK12. Halfman, J.D. 1987. High-resolution Sedimentology and Paleoclimatology of Lake Turkana, Kenya.Ph. D. Dissertation, Duke Univ., Durham. Halfman, J.D. and J o h n s o n , T.C. 1988. High-resolution record ofclimaUc change during thepast 4 Ka from Lake Turkana, Kenya. Geo/ogy. 16, 496-500. Harbott, B.J. 1982. Studies on algal dynamics and prim a r y productivity in LakeTurkana. In:Hopson, A.J. (editor) Lake Turkana: A report o n the findings of the Lake T u r k a n a Project, 1972- 1975. Overseas Development Administration, London. Irwin, H., Curtis, C. and Coleman, M. 1977. Isotopic evidence for source of diagenetlc carbonates formed during burial of organic-rich sediments. Nature 269, 209-213. J o h n s o n , T.C. , Halfman, J.D. , Rosendahl, B.R. and Lister, G.S. 1987. Climatic and tectonics effects on sedimentation in a rift valley lake: Evidence from Lake Turkana, Kenya. B u l l Geol. Soc. Am. 98, 439-447. Kelts, K. and Hsu, K.J. 1978. Freshwater carbonate sedimentation. In: Lerman, A. (editor) Lakes: Geology, Chemistry, Physics. Springer Verlag, New York. Kelts, K., Briegel, U., Ghilardi, K. and Hsu, K. 1986. The limnogeology-ETH coring system. Schwetz. Z. Hydrol. 48, 104-115. Kutzbach, J.E. a n d Street-Perrott, F.A. 1985. Milankovitch forcing of fluctuations in the level of
tropical lakes from 18 to 0 kyrs BP. N a t u r e 3 1 7 , 130134. Lee, C., McKenzie, J.A. and S t u r m , M. 1987. Carbon isotope fractlonation and changes in the flux and composiUon of particulate m a t t e r resulting from biological activity during a s e d i m e n t trap experiment in Lake Greifen, Switzerland. L/mnoL Oceanogr. 32, 8396. McKenzie, J.A. 1985. C a r b o n isotopes and productivity in the lacustrine a n d m a r i n e environment. In: W. S t u m m (editor) Chemical processes in lakes. Wiley, New York. Muller, G., Irion, G., and Forstner, V. 1972. Formation and diagenesis of inorganic Ca-Mg c a r b o n a t e s in the lucastrine environment. N a t u r w l s e n s c h a f t e n 25,158164 Owen, R.B. , Barthelme, J.W. , Renaut, R.W. and Vincens, A. 1982. Palaeolimnologyand archaeology of Holocene deposits n o r t h - e a s t o fLake T u r k a n a , Kenya. Nature. 298, 523-529. Robbins, E.I. 1983. Accumulation of fossil fuels and metallic minerals in active and ancient ~ systems. Tectonophysics 94, 633-658. Rosendahl, B.R. 1987. Architecture of continental riffs with special reference to E a s t Africa. Ann. Rev. Earth Planet. ScL 15, 445-503. Talbot, M.R. a n d Kelts, K. 1986. P r l m a r y a n d diagenetic carbonates in the anoxlc sediments of Lake Bosumtwi, Ghana. Geo/ogy 14, 912-916. Street-Perrott, F.A. and Harrison, S.P. 1984. Temporal variations in lake levels since 30,000 yr BP - a n index of the global hydrological cycle. In: Hansen, J.E. a n d Takahashi, T. (editors) Climate processes a n d climate sensitivity. AGU Monograph 29. Van Houten, F.B. 1964. Cyclic lacustrine sedimentation, UpperTriassic Lockatong Formation, central New J e r s e y and adjacent Pennsylvania. B u l l GeoL Surv. Kans. 169, 497-531. Yuretich, R.F. 1979. M o d e m sediments and sedimentary processes in Lake Rudolf (Lake Turkana), E a s t e r n Rift Valley, Kenya. S e d i m e n t o l o g y 2 6 , 313-331. Yuretich, R.F. 1986. Controls on the composition of m o d e m sediments, l.a~eTurkana, Kenya. In: Frostich, L.E. et aL (editors) Sedimentation in the African Rifts. Geol. Soc. Lond. Spec. Pub. 25, Blackwell, London. Yuretich, R.F. and Cerllng, T.E. 1983. Hydrogeochemistry of Lake T u r k a n a , Kenya: Mass balance and mineral reactions in a n alkaline lake. Geochimlc. Cosmochlrru Acta 47, 1199-1109
Journal of African Earth Sciences. Vol. 8, Nos. 2/3/4, pp. 533-540, 1989 Printed in Great Britain
Authigenic Low-Mg Calcite in Lake Turkana, Kenya John D. HALFMAN,* Thomas C. JOHNSON,* William J. SHOWERS ** and Guy S. LISTER *** Department of Earth Sciences, * University of Notre Dame, Notre Dame, IN 46556 Department of Marine, Earth and Atmospheric Sciences ** North Carolina State University Raleigh, NC 27695 Geologisches Institut *** ETH - Zentrum CH-8092 Zurich, Switzerland
Abstract- LakeTurkanais an importantmodemanalogto ancientrift environmentsin east Africaand elsewhere.Carbonate is secondin abundanceafter the detrital silicate fractionof the sedimentsin the lake. It consistsof ostracod tests and a siltsizedmicrite.ScanningelectronmicroscopyandX-raydiffractionanalysisrevealedthemicriteto be euhedralcrystalsoflowMg calciteabout 10 microns in length.Carbon isotopevaluesof the micriteindicatethat it forms in the upper watercolumn rather thanin the sedimentsafterburial.The oxygenisotopeanalysesindicatethatthe micritecouldbe at isotopicequilibrium with the modemlake waters at temperaturesof 36°C. This suggeststhat the micriteformseitherin the surfacewatersof the open lake or in isolatedshallowbays, and is then transportedto the deep lake. INTRODUCTION
Lake T u r k a n a (formerly Lake Rudolf) is t h e largest c l o s e d - b a s i n lake in t h e E a s t African Rift S y s t e m . The b a s i n is located in t h e desolate Nort h e m F r o n t i e r District of Kenya, y e t h a s b e e n the site of m u c h r e s e a r c h activity in r e c e n t y e a r s . The incentive for t h e s e s t u d i e s h a v e b e e n derived prim a r i l y from t h e tectonic i m p o r t a n c e of the rift valley s e t t i n g (Baker et al. 1972), paleontological a n d a n t h r o p o l o g i c a l discoveries of s o m e of the oldest fossil h o m i n i d s (Brown et al. 1985), t h e longt e r m a n d h l g h - r e s o l u t i o n record of climatic c h a n g e (Owen et al. 1982, H a K m a n a n d J o h n s o n in press), a n d economic i m p o r t a n c e for p e t r o l e u m a n d minerals (Robbins 1983). F u r t h e r m o r e , t h e lake occupies a very h o t a n d arid s e t t i n g w i t h rainfall less t h a n 200 m m / y r a n d a n average a n n u a l t e m p e r a t u r e of 30°C. It c a n be c o n s i d e r e d as the ,arid region e n d m e m b e r , of large rift valley l a k e s a n d a n i m p o r t a n t m o d e m a n a l o g for a n c i e n t rift environm e n t s in Africa a n d elsewhere. Within t h e p a s t d e c a d e several p a p e r s have b e e n w r i t t e n on Lake T u r k a n a t h a t i n c l u d e t h e lillmology (Ferguson a n d H a r b o t t 1982), r e c e n t s e d i m e n t o l o g y fYuretich
1979, 1986), s t r u c t u r e a n d s e i s m l c - s t r a t i g r a p h y of the s e d i m e n t s (up to 4 k m thick) u n d e r l y i n g the lake ( D u n k e l m a n 1986, J o h n s o n et al. 1987). An i m p o r t a n t c o m p o n e n t of the s e d i m e n t s in arid lakes, b e s i d e s detrital clastic a n d biogenic material, is inorganic precipitates, especially carb o n a t e s . C a r b o n a t e s e d i m e n t s are deposited in lakes in a variety of climatic s e t t i n g s i n c l u d i n g h y p e r s a l i n e lakes of arid regions (e.g., E u g s t e r a n d Hardie 1978). alkaline l a k e s in t e m p e r a t e regions (e.g., Kelts a n d H s u 1978) a n d in t h e tropics (e.g., C o h e n a n d T h o u i n 1987). M a n y a n c i e n t rift sedim e n t s in e a s t Africa a n d elsewhere were deposited u n d e r tropical c o n d i t i o n s (e.g.. V a n H o u t e n 1964) a n d c o n t a i n a b u n d a n t c a r b o n a t e . The origin of the c a r b o n a t e s (e.g., biogenic or authigenic) in l a c u s t r i n e s t r a t a is n o t always a p p a r e n t so the s t u d y of it i n m o d e m lakes c a n provide u s e f u l clues for the i n t e r p r e t a t i o n of t h e a n c i e n t deposits. C a r b o n a t e p h a s e s in l a c u s t r i n e s y s t e m s comm o n l y c o n t a i n key e n v i r o n m e n t a l information. For example, the mineralogical p r o g r e s s i o n from calcite, to m a g n e s i a n calcite, t h e n aragonite a n d finally dolomite is c o n s i d e r e d a reliable indicator of i n c r e a s i n g Mg2"/Ca 2÷, t h u s i n c r e a s i n g salinity of
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the lake water (Muller et al. 1972). Information indicate t h a t faulting is active today (Johnson et al. concerning past productivity, t e m p e r a t u r e and 1987). Lake level h a s fluctuated t h r o u g h o u t the salinity m a y be elicited from carbon and oxygen range of the basin. Lowstands at about 60 m below stable isotope ratios of carbonates (Abell 1982, the present lake's surface were interpreted from McKenzie 1985). Down-core fluctuations in high resolution seismic profiles and were estimacarbonate content from Lake T u r k a n a are inter- ted to have occurred prior to 10,000 years ago preted to reflect cyclic climatic change on a time (Johnson et al. 1987). Early Holocene deposits scale of decades to centuries (Halfman and record h i g h s t a n d s from 50 to 80 m above the J o h n s o n in press). Valid interpretations require present level (Owen et al 1982). These lake level t h a t the carbonate p h a s e s precipitate from the fluctuations are s y n c h r o n o u s with other basins water column or of biogenic origin and not of t h r o u g h o u t the tropics on a millennial time scale detrital or diagenetic origin. The purpose of this for the past 30,000 years (Butzer et al. 1972, Streetpaper is to determine the mode of formation for Perrott and Harrison 19841. They are interpreted to reflect global climatic variability, and are predicted the carbonates in Lake Turkana. by general- circulation models (Kutzbach and Street Perrott 1985). HYDROLOGY AND RECENT SEDIMENTS The m o d e r n sediments of Lake T u r k a n a are Lake T u r k a n a occupies the ,Turkana Depres- primarily detrital silicates (Yuretich 1979, 1986). sion, and receives r u n o f f a n d sediment from a wide Coarse grained n e a r s h o r e deposits are influenced geographical area (Fig. 1). It is approximately 260 by high wave energy and the presence of volcanic k m long with a m e a n width of 30 km, and h a s a n h e a d l a n d s (Cohen et al. 1986). The d o m i n a n t average and m a x i m u m depth of approximately 35 pattern of fine-grained sedimentation offshore, as m and 115 m, respectively. The hydrology of the revealed by high-resolution (1- and 28-kHz) basin is dominated by the perennial Omo River, seismic profiles, is one of simple, ponded infilling which drains the Ethiopian Plateau to the north, (Johnson et al. 1987). Sedimentation rates based and provides about 90% of the fresh water flowing on radiocarbon dates of two of our piston cores are into the lake (Ferguson and Harbott 1982). The about 0.3 and 0.5 c m / y r (Halfman a n d J o h n s o n in seasonal Turkwel and Kerio Rivers contribute most press). The composition of the sediments varies of the remaining fluvial input. Other streams, systematically from n o r t h to south. The relative direct rainfall and s u b t e r r a n e a n flow are conside- a b u n d a n c e ofbiogenic c o m p o n e n t s increases with red insignificant in the water budget (Yuretich and increasing distance from the Omo River, i.e., from Cerling 1983). The lake is moderately saline (2.4°/ rare and poorly preserved diatom clays in the north oo and alkaline (pH 9.2) (Yuretich and Cerling basin to well-preserved ostracod-diatom silty clays 1983). The principle ions are Na ÷ (32 mmoles/l), in the s o u t h basin. This trend results from decreaHC03- (19 meq/l) and Cl-(13 mmoles/l) with rela- sed dilution of the biogenic c o m p o n e n t s by detrital tively low concentrations of Ca 2÷, Mg2÷and S042°. silicates with increased distance from the Omo The hydrogeochemical budget indicates t h a t Ca 2÷ delta (Halfman 1987). The relative composition of the silicate minerals is s u p e r - s a t u r a t e d with respect to calcite, Mg2+is s u p e r - s a t u r a t e d with respect to Mg-smectites, and is primarily controlled by the source rock compoS042- m a y be r e d u c e d to a sulfide (Yuretich and sition and weathering intensity (Yuretich 1979, Cerling 1983). The water column is well mixed by 1986). The north basin is rich in kaolinite and strong diurnal winds, and has a relatively uniform resistates t h a t reflects the weathering of mafic water t e m p e r a t u r e of 25 to 26°C and dissolved volcanics in the humid, tropical setting of the Omo oxygen concentrations that range from about 70 to basin. In contrast, the sediments adjacent to the Turkwel-Kerio drainage system are coarser, con100% saturation (Ferguson and Harbott 1982). Both tectonics and climate are important exter- tain quartz, feldspar, illite and smectite, and reflect nal factors t h a t control sedimentation in Lake the arid climate and exposures of Precambrian Turkana. The formation of the rift has provided a n gneiss and schist. Water-sediment reactions are effective trap for water, water-laid sediments, vol- very important in the a c c u m u l a t i o n of smectite. caniclastics and volcanic flows since the Pliocene The sediments are laminated, except in the and possibly the Miocene. Half-graben units with nearshore deposits, despite well-oxygenated botaltemating polarities occur along strike of the lake tom waters (Yuretich 1979, Cohen 1984). The (Dunkelman 1986, Rosendahl 1987). Four half- benthic standing crop, which could potentially graben units underlie the n o r t h e r n depositional homogenize the sediments consists primarily of basin suggesting t h a t clastic deposition from the epibenthic detritovores (e.g., ostracods). Their Omo River m a s k s earlier structural controls on a b u n d a n c e declines from n e a r s h o r e to offshore deposition. High-angle growth faults are common- environments, presumably due to a parallel decline ly observed in high-resolution seismic profiles and in edible detritus (Cohen 1984). The light-dark
Authigenic low-Mg calcite in Lake Turkana, Kenya
535
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4o30 '
LAKE TURKANA
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Contour I ntervol IOm
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.
Fig. 1. Bathymetric map of Lake Turkana showing the location of the piston cores collected from the lake. The North basin extends from Eliye-Springs to the Omo River. The South basin is approximated by the 40 m contour surrounding South Island. The outline of Africa shows the approximate location for Lake Turkana.
536
JOHN D. HALFMANet al.
couplets are differentiated by carbonate content and are interpreted to reflect a n interannualvariation (about 4 years) in the dilution of the carbonates by detrital silicates (Halfman and J o h n s o n in press). Carbonate is the next most a b u n d a n t compon e n t to the detrital silicate fraction in the recent sediments of Lake T u r k a n a (Yuretich 1979, Halfm a n 1987). Carbonate concentration of the sedim e n t increases with increasing distance from the Omo River (Halfman 1987). Bulk carbonate concentrations from the recent sediments average 10% (CaC03) of the dry weight, and range from I to 28% (Halfman 1987). The average composition of the calcite is about 3 mole % Mg, or low-Mg calcite, as revealed by X-ray diffraction analyses (Yuretich 1986). The carbonate fraction in the offshore sedim e n t s h a s two m a i n components, ostracod carapaces, and micron-slzed crystals of carbonate (Halfman 1987). Other c o m p o n e n t s including gastropod shells and possibly dolomite were rarely detected in the surface sediments. Beach rock occurs along portions of the lake's margin. The m o d e m ostracod population in the offshore sedim e n t s is dominated by Sclerocypris clavata with lesser a m o u n t s ofHemicypris Intermedia, and is limited to between 10 and 100 t e s t s / m 2, whereas, the ostracod population in nearshore deposits is dominated by Hemicypris klele, Cypridies torosa, and Gomphocythere a n g u l a t a and contain between 100 and 1,000 tests/m2along most of the nearshore zone except at isolated vegetated shorelines where the population increases to about 100,000 t e s t s / m 2 (Cohen 1982). In this paper, we focus on the micrite fraction of the sediment. Analyses were designed to determine the mode of origin of this previously u n s t u d i e d c o m p o n e n t of the sediment.
and s o u t h basins. The ostracod and micrite fractions were separated from each organic-free, sediment subsample by successive sieving. The organic m a t t e r was digested with dilute hydrogen peroxide. A 150 micron sieve isolated the ostracod fraction, from which Juvenile and adult forms of Sclerocyprls clavata were picked, and approximately 15 tests were lightly c r u s h e d before each analysis. The micrite fraction constituted the carbonate that passed t h r o u g h a 44 micron sieve. Isotopic analysis was performed on the evolved carbon dioxide after reacting the sample with 100% orthophosphoric acid at 50°C. A Finnegan MAT 251 m a s s spectrometer at the NCSU Stable Isotope Laboratory was u s e d for isotopic analysis and the results reported relative to the PDB standard. The s t a n d a r d deviation for replicate ostracod analyses is O. 14°/oo for 81sC and 0.08°/00 for 6 ]a0. The s t a n d a r d deviation for replicate micrite analyses is 0.050/00 for 8~3C and 0.26°/00 for 8 ~ . Details of all the m e t h o d s are included in Halfman (1987).
AUTHIGENIC MICRITE
Lacustrine carbonates are derived from a combination of four possible processes: biogenic precipitation of calcareous tests, clastic input, primary inorganic precipitation, and post-depositional diagenesis. Comparison between the micrite 8~3C values and the corresponding values of the dissolved inorganic bicarbonate (DIC) i n t h e sediment pore waters reveals significant differences. The 8~zC values for the micrite average 0.76°/00 a n d -O. 14°/oo for cores LT84-2P and 7P, respectively (Fig. 2). The 8 ~3Cvalues for the DIC yield increasing values down both cores from 0°/oo to +12°/oo (PDB) (T.Cerling personal c o m m u n i c a t i o n 1985). This indicates t h a t the micrite is not a diagenetic METHODS precipitate, b e c a u s e if it were, its carbon isotope Ten piston cores, each approximately 11 meters values would be heavier (Irwin et al. 1977, Talbot long, were collected from the lake with an ETH and Kelts 1986). corer provided by the Geologisches Institut, ETHThe micrite was previously a s s u m e d to be bioZentrum, Zurich (Kelts et al. 1986, Fig. 1). The genic debris (Yuretich 1979). yet only the cocores were collected in November, 1984, as part of existence of both forms supported the hypothesis. Duke University's Project PROBE. Analyses of isolated micrite and species specific The micrite was analyzed by three principle ostracod samples reveal significantly different methods. A s c a n n i n g electron microscope (SEM) results. Only samples from core LT84-2P had was u s e d to identify the constituents of the fine- sufficient ostracod tests to be compared geograined sediments. The mineralogy of the fine chemically with the micrite. The 6 ~aC values are fraction was identified by X-ray diffraction. Stable approximately 1°/oo heavier and the ~180 values isotope composition was determined from isolated are approximately 2°/00 lighter in the micrite ostracod and micrite fractions of the sediment. fraction t h a n in the ostracod tests (Fig. 2). This Approximately 15 g of wet sediment was sub- suggests that the micrite is not derived from sampled at about 1 m intervals down two cores, fragmented tests of Sclerocypris clavata. The LT84-2P and LT84-7P. These cores were selected micrite could be fragments ofother species because for analysis as being representative of the n o r t h isotopic vital effects m a y be different between
Authigenic low-Mg calcite in Lake Turkana, Kenya
Isotopic 2P Ostracod 4 0 v
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Value (%0, PDB)
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7P Micrite
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Fig. 2. Stable isotope ratios versus depth down cores, LT84-2P and 7P. Only core LT84-2P had sufficient ostracod tests for analysis. The micrite fraction (see text) was analyzed fi-om both cores. Juvenile (open symbol) and adult (closed symbol) forms of,~ler~yprlm elavata were used in this study. The oxygen (circle) and carbon (square) isotope values are reported relative to the PDB standard. The mean value for each proflleis indicated al~er the X. species, althoughvital effects for ostracods are not clearly d o c u m e n t e d (P. DeDeckker personal comm u n i c a t i o n 1987). The range of the oxygen isotope values for the ostracods reported here are equal to and slightly lighter t h a n different species of o s t r a c o d s from core LT84-1OP analyzed separately b y G. Lister. SEM analysis showed m u c h of the micrite to be euhedral, semi-acicular crystals a b o u t 10 microns in size (Fig. 3). The crystals were observed always to be individual s p e c i m e n s rather t h a n aggregates of several crystals. The micrite is morphologically different t h a n platy ostracod fragments. Platy material with s u b - m i c r o n sized pores is revealed b y SEM photos (Fig. 3) t h a t m a y be ostracod debris. Smear slide analysis of the b u l k sediment and sieved fractions rarely revealed fragmented ostracods. The contribution to the bulk carbonate content b y the ostracod tests in the profundal sediments is calculated to be less t h a n 1% of the dry sediment, a s s u m i n g that: (I) the tests are p u r e calcite, (2) the animals live in the u p p e r cm of sediment and molt 100 times, and (3) the sediments have an average porosity of 90% a n d density of 2.6 g / c m a. This value is significantly less t h a n the m e a s u r e d b u l k carbonate content of the sediments. The euhedral c h a r a c t e r and lack of an appreciable influx of detrital c a r b o n a t e s to the lake from the drainage b a s i n (T. Ceding personal communication 1985) suggests t h a t the micrite is not detrital,
i.e., not transported t h r o u g h fluvial systems. The increased carbonate concentrations away from the Omo River also s u p p o r t s the non-detrital origin (Halfman 1987). The morphology of the crystals is similar to primary inorganic precipitates of calcite in temperate lakes; these typically are not perfect r h o m b s or needles, b u t are stubby, e q u a n t or blocky polyhedra t h a t reveal crystal faces, and are a b o u t 10 microns in length (Bihmskill 1969, Kelts and H s u 1978). Sizes up to 40 microns have b e e n reported (Lee et al. 1987). X-ray diffraction of the fine fraction confirms Yuretich's (1986) findings t h a t the micrite is a IowMg calcite. Low-Mg calcite is the composition that is predicted for c a r b o n a t e s precipitating in a lake with T u r k a n a ' s chemistry (Muller e t a/1972). The evidence presented so far suggests that the euhedral crystals are primary precipitates. The evidence p r e s e n t e d so far suggests that the euhedral crystals are primary precipitates. Interpretation of the stable isotope data confirms this hypothesis. The carbon isotope values of the micrlte are heavier (about 1°/oo) t h a n the ostracod tests. This difference p a y be explained by a photosynthetic fractionation of the c a r b o n isotopes, where carbon- 12 is preferentially transferred from the surface waters to the deeper waters b y a photosynthetic-respiration pathway. Photosynthetically produced organic m a t t e r is enriched in carbon-12
538
JOHN D. HALFMANet al.
Q
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Flg. 3. SEM photographs of the fine fraction of the sediment from 29 cm down core LT84-2P. The arrow-shaped particles, most visible in the lower right photo, are euhedral calcite. The platy, porous particles, visible in the upper right photo may be an ostracod fragment. The other components include detrital grains and diatoms. relative to c a r b o n - 13, a n d as it sinks, it oxidizes, - 1°/oo ( m e a s u r e d twice in 1979, SMOW, T, Cerling releasing c a r b o n - 1 2 enriched C02to t h e b o t t o m p e r s o n a l c o m m u n i c a t i o n 1985). The micrite with a waters. As c a r b o n a t e p r e c i p i t a t e s are basically in m e a n 61s0 v a l u e of 1°/oo (PDB). e q u i v a l e n t to isotopic equilibrium with the dissolved inorganic 31.9°/oo (SMOW), w o u l d be in isotopic equilibrium c a r b o n , t h e c a r b o n a t e c o n t a i n s a c a r b o n - i s o t o p e with t h e lake w a t e r f l i t s t e m p e r a t u r e of f o r m a t i o n c o m p o s i t i o n characteristic of t h e w a t e r d e p t h w a s 36°C or t h e isotopic c o m p o s i t i o n of t h e w a t e r (McKenzie 1985, Lee et al. 1987). The o s t r a c o d s are w a s lowered b y 1°/oo. A f o r m a t i o n t e m p e r a t u r e of epibenthic detritovores (Cohen 1984), a n d t h u s it 36°C is higher t h a n 25.5°C, the m e a n t e m p e r a t u r e of t h e w a t e r c o l u m n ( F e r g u s o n a n d H a r b o t t 1982). m a y explain the lighter c a r b o n isotope values. W a t e r t e m p e r a t u r e s above 30°C were r e p o r t e d for Authigenic c a r b o n a t e s s h o u l d precipitate at t h e s u r f a c e w a t e r (at 1 m depths) in t h e o p e n lake isotopic equilibrium with the water. The equilia n d isolated, shallow b a y s ( F e r g u s o n a n d H a r b o t t b r i u m isotopic fractionation factor for t h e calcite1982). This m a y indicate t h a t t h e c r y s t a l s precipiw a t e r s y s t e m is: tate in s u p e r - h e a t e d s u r f a c e w a t e r s , p e r h a p s in 1000 I n (0<) = 2.78 (108) • T -2 - 2.89 t h e u p p e r m o s t millimeter on c a l m days. or in assuming: shoreline pools with s u b s e q u e n t t r a n s p o r t to t h e i 0 0 0 I n (0c)= 180 ~u~- la0water (SMOW) w h e r e (c<) is t h e isotopic fractionation factor, T is deep basin. A s e c o n d h y p o t h e s i s c o n c e r n s a less saline, the t e m p e r a t u r e of the reaction (°K) a n d the isotope ratios are relative to the SMOW s t a n d a r d t u r b i d w a t e r m a s s g e n e r a t e d d u r i n g t h e flood (Friedman a n d O'Neil 1977). The ~ s O of Lake s e a s o n of t h e O m o River. The w a t e r m a s s e x t e n d s T u r k a n a h a s b e e n m e a s u r e d at 5.6°/00 ( m e a s u r e d from t h e O m o River to t h e central basin. The in 1975 a n d 1980) a n d t h e O m o River effluent at c o n d u c t i v i t y of the lake w a t e r d r o p s b y a b o u t 60%
Authigenic Iow-Mg calcite in Lake Turkana, Kenya n o r t h of North Island, a n d b y 10% t h r o u g h t h e r e m a i n d e r of t h e n o r t h b a s i n d u r i n g p e a k flood s e a s o n ( F e r g u s o n a n d H a r b o t t 1982). The m i x i n g of t h e two w a t e r m a s s e s of different isotopic composition w o u l d be sufficient to precipitate micrite in e q u i l i b r i u m in t h e n o r t h b a s i n at p r e s e n t w a t e r c o l u m n t e m p e r a t u r e s , a l t h o u g h slightly h i g h e r t e m p e r a t u r e s (28°C) t h a n t h e m e a n lake t e m p e r a t u r e s are r e q u i r e d for t h e a r e a s o u t h of North Island. The m e a n 6~s0 v a l u e for LT84-7P is a b o u t 1°/oo lower t h a n 2P, possibly reflecting t h e mixing p h e n o m e n a . T h i s m e c h a n i s m r e q u i r e s t h e CaC03 a c c u m u l a t i o n rate to be f a s t e r n e a r t h e Omo River t h a n f a r t h e r away. Relative s e d i m e n t a t i o n r a t e s for t h e n o r t h b a s i n cores c a n be inferred from a s t r a t i g r a p h y b a s e d on b u l k c a r b o n a t e profiles (Halfman 1987). T h e s e r a t e s w i t h t h e b u l k carbon a t e c o n c e n t r a t i o n s reveal h i g h e r m a s s a c c u m u l a tion r a t e s for CaC03 closer to the Omo River t h a n f a r t h e r away. A t h i r d possibility is t h a t the micrite w a s f o r m e d d u r i n g a time w h e n t h e lake w a s less saline t h a n today. S e a s o n a l t e m p e r a t u r e , p h o t o s y n t h e t i c a n d salin i t y v a r i a t i o n s are t h e maj or f a c t o r s t h a t influence s u p e r s a t u r a t i o n w i t h r e s p e c t to inorganic precipit a t i o n of calcite (Kelts a n d H s u 1978). W a r m i n g of s e a s o n a l t e m p e r a t e lakes, a s s i m i l a t i o n of COs or p h o t o s y n t h e t i c evaporitic c o n c e n t r a t i o n of t h e ions are in s o m e c a s e s sufficient for s u p e r s a t u r a t i o n . A t h r e e y e a r limnologic s t u d y of Lake T u r k a n a indic a t e s t h a t t h e r e is a m i n i m a l (1°C) s e a s o n a l variation in t h e w a t e r t e m p e r a t u r e (Ferguson a n d Harb o t t 1982). The s a m e s t u d y s h o w e d t h a t lake level d r o p s a b o u t i m d u r i n g the non-flood s e a s o n of the Omo River. A s s u m i n g t h e only factor to influence t h e c o n c e n t r a t i o n of t h e principle ions is t h i s loss of water, t h e evaporitic e n r i c h m e n t of the concent r a t i o n s is m i n i m a l . For example, t h e c o n c e n t r a tion of Ca 2÷ is only i n c r e a s e d b y 1%. Pro-ductivity a n d p l a n k t o n i c b i o m a s s m e a s u r e m e n t s indicate a s e a s o n a l b l o o m of b l u e - g r e e n algae d u r i n g the flooding of t h e Omo River (Harbott 1982). Presum a b l y , t h e Omo River provides a limiting n u t r i e n t . This s u g g e s t s t h a t the micrite precipitation is biogenically m o d u l a t e d a n d c o n c u r r e n t with the flooding of t h e Omo River. CONCLUSIONS The c a r b o n a t e f r a c t i o n of the r e c e n t s e d i m e n t s in Lake T u r k a n a were a n a l y z e d to d o c u m e n t the m o d e of origin for t h e micrite. The micrite is e u h e d r a l , semi-acicular, low-Mg calcite. Previous w o r k s u g g e s t e d t h a t t h e micrite w a s either biogenic debris or a n a u t h i g e n i c precipitate, b u t t h e c o n c l u s i o n s were b a s e d on limited data. A geoc h e m i c a l a n d morphological c o m p a r i s o n b e t w e e n t h e micrite a n d o s t r a c o d t e s t s reveals t h a t the
539
micrite is n o t biogenic debris. The ~I 3C v a l u e s s u g g e s t t h a t t h e micrite precipit a t e s inorganically in t h e s u r f a c e w a t e r s of the lake. Its f o r m a t i o n m a y be e n h a n c e d b y p h o t o s y n theUc activity. The micrite is in isotopic equilib r i u m w i t h the lake w a t e r if its t e m p e r a t u r e of f o r m a t i o n is h i g h e r t h a n t h e m e a n w a t e r t e m p e r a t u r e of t h e lake. This s u g g e s t s t h a t t h e micrite precipitates in the s u p e r - h e a t e d s u r f a c e waters, p e r h a p s on c a l m d a y s , or in shallow shoreline pools. Isotopic equ111brium c o u l d also be achieved if t h e isotopic c o m p o s i t i o n of t h e lake were 1°/oo lower. This is h y p o t h e t i c a l l y possible in t h e n o r t h e r n p a r t of t h e n o r t h b a s i n d u r i n g t h e flood s e a s o n of t h e Omo River. Acknoledgements - We thank the captain and crew of the M/V Halcyon, operated and owned by the Kenyan Department of Fisheries for their valuable assistance in the field, at times under stressful conditions, and R. Leakey, National Museum of Kenya for IogisUcal assistance. K. Ghilardi was invaluable in direcUng the operaUon of the ETH piston corer. T. Ceiling kindly allowed us to use his isotope data of sediment porewater and lake water. D. Van Nieuwenhuise kindly provided ostracod identifications. We thank C. Paull for his careful review o fan earlier draft of this manuscript. Financial support was provided by Project PROBE and two grants to T.C.J. numbered PRF- 17428-AC2 from the American Chemical Society and DPE-S542-G-SS-OO from the Agency for International Development. At the time of the Turkana field program, Project PROBE was funded by Amoco, Arco, Conoco, Esso, Marathon, Mobil, Pecten, Penzofl, and Shell IntemaUonal Oil Companies, and by the World Bank. REFERENCES Abell, P.I. 1982. Paleoclimates at Lake Turkana, Kenya, from oxygen isotope raUos of gastropod shells. N a t u r e 297.321-323. Baker, B.H., Mohr, P.A. and Williams, L.A.J. 1972. C-eology of the Eastem Rift System of Africa. Geol. Soc. Amer. Special Paper. 136, 67pp. Brown, F.H., Harris, J., Leakey, R. and Walker, A. 1985. Early Homo erectus skeletons from west Lake Turkana, Kenya. N a t u r e 316, 788-792. Brunskill, G.J. 1969. FayettviUe Green Lake, NewYork. II. PrecipitaUon and sedimentation of calcite in a meromlctic lake with laminated sediments. L/mno/. O c e a n o g r . 14, 830-847. Butzer, K.W., Isaac, G.L., Richardson, J.L. and Washburn-Kamau, C. 1972. Radiocarbon dating of East African lake levels. Sclence. 175, 1069-1076. Cohen, A.S. 1982. Ecological and paleoecological aspects of the rift valley lakes of East Africa. Ph.D. DissertaUon, Univ. California, Davis. Cohen, A.S. 1984. Effect ofzoobenthic standing crop on laminae preservaUon in tropical lake sediment, Lake Turkana, East Africa. J. P a l e o n t . 58, 499-510. Cohen, A.S., Ferguson, D.S., Gram, P.M., Hubler, S.L. and Sims, K.W. 1986. The distribution of coarsegrained sediments in modem Lake Turkana, Kenya:
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