Qualitative fatty acid and n-alkane stratigraphy of the Lake Turkana Basin, Kenya

Orff. Geochem. Vol. 4. pp. 37 to 50. 1982

0146-6380/82/010037-14S03.00/0 Pergamon Press Ltd

Printed in Great Britain

Qualitative fatty acid and n-aikane stratigraphy of the Lake Turkana Basin, Kenya A. DALLASWAIT* and PAUL I. ABELL Department of Chemistry, University of Rhode Island, Kingston, RI 02881, U.S.A. (Received 12 November 1981; accepted in revised form 19 February 1982) Abstract--In order to improve understanding of the stratigraphy of the Lake Turkana Basin, one of the important sites in the evolution of early man, this study evaluates the usefulness of organic biological marker compounds, n-alkanes and fatty acids, for correlation of isolated sedimentary strata. Eighty-five paleosol samples were collected from well-defined sedimentary horizons in two regions (Area 103 and Area 130) of the Koobi Fora area of Lake Turkana. Results indicate that most of the organic matter present was derived from terrestrial plant waxes. In sediments where extensive diagenesis has occurred, microbial input of organic matter may have been substantial. Algae were either not an important source of organic matter, or their marker compounds have been removed or altered by degradative processes. The fate of the original paleosol organic matter has been governed to some extent by weathering processes, especially in Area 130. Weathering decreased the amount of extractable lipids, particularly fatty acids and the low molecular weight alkanes (C1~-C20); produced or retained relatively large amounts of alkanes greater than C2 t within a unimodal distribution; and lowered CPI values. Consequently, stratigraphic correlation by unique alkane and fatty acid distributions has been confined to short distances (many meters). Both n-alkanes and fatty acids have been retained better by association with clay minerals than by sand matrices. The alkane distribution of sandstones differs from that of clay organics in having a narrower carbon chain length distribution and lower CPI values. In Area 103, where weathering was less severe, compositional variations with stratigraphic position indicate that lipid material has been retained within each of the facies examined.

(Albrecht and Ourisson, 1971; Philp et al., 1976; Didyk et al., 1978; Eglinton et al., 1979; Volkman et al., 1981). The tenet that these recognizable organic remnants could be used as a stratigraphic correlation parameter has been often mentioned by organic geochemists but rarely demonstrated (Reed and Mankiewicz, 1975). The Lake Turkana Basin seems an ideal area in which to test this belief. Saturated fatty acids and n-alkanes were chosen as appropriate marker compounds to study because of their ubiquity and their resistance to the harsh degradative forces (e.g. high surface temperature, oxidative processes) inherent in an arid alkaline environment. In fact, Wait (1979) found very little lipid material other than alkanes and fatty acids in certain East Turkana paleosols. The purpose of this study is to evaluate the potential of using biological marker compounds (n-alkanes and fatty acids) as stratigraphic correlators by: (a) qualitative distribution; (b) comparing vertical distribution: (c) inferring paleoenvironmental conditions during and after sedimentation. The study will also provide a much needed study of the fate of specific organic compounds in arid alkaline environments.

INTRODUCTION THE LAKETURKANABASINhas been shown to be one of the most important sites in the evolution of early man (Coppens et al., 1976: Walker and Leakey, 1978; Johanson and White, 1979: Cronin et al., 1981). East Turkana, site of many archeological finds (Leakey and Leakey, 1978), is characterized by extensive sedimentary beds in the form of escarpments and outcrops extending back 20-30 km from the present lake margin. Contained in the stratigraphic column are a discontinuous series of volcanic tufts which provide radiogenic dates for a time framework for the sediment beds, thus proving valuable for paleontology, paleoanthropology and geochemical studies. It would be extremely useful to the many scientific disciplines to be able to correlate isolated sedimentary strata as well. It is well recognized that some organic compounds which were the complex biochemical products of ancient organisms have survived in sedimentary formations for relatively long periods of time (biological markers), often with little or no transformation of their molecular skeleton (Blumer, 1973; Eglinton, 1973). Detection of biological markers may provide information on the origin of the organic material and the paleoenvironmental conditions of sedimentation

STUDY AREA Geolo.qic setting

* Present address: Energy Resources Co. Inc., Environmental Sciences Division, 185 Alewife Brook Parkway. Cambridge, MA 02138, U.S.A.

The geology of East Africa has been largely determined by the tectonic activity of the African plate. 37

38

A DALLASWAIT and PAUL 1 ABELL

The area is dominated by an extensive rift valley system and associated geologic structures such as domes and volcanoes (Oxburgh, 1978: Bahat, 1979). Also present throughout the rift valley are sizable lakes, including Lake Turkana (formerly Lake Rudolf). Situated in Northwest Kenya at a latitude of 2.5-4.5 N, a longitude of 36 E and an altitude of 375 m, the lake is now 250 km long and 15-30 km wide and has a maximum depth of 125 m" average depth is 35 m (Yuretich, 1979). The study area for this project is situated in the Koobi Fora area of the Turkana Basin• The Koobl Fora formation has been described by Bowen and Vondra (19731 as a series of laminated claystones, siltstones and fine-grained sandstones that are overlain by lenticular conglomerates, mudstones, thin beds of algal stromatolites, fossiliferous limestones and volcanic tufts. The facies are complexly interbedded and

lntertongued, with their relationship at any one time related to the level of the lake, which in turn is controlled by variations in climate and tectonic activity (Vondra and Bowen, 1978; Hallam, 1981)• The present-day lake is about 80 m below overflow level and numerous escarpments of lacustrine and fluvial sediments are exposed. Paleosol samples examined in this study were collected from outcrops in Area 103 and Area 130, as depicted in Fig. 1. Their relationship to the stratigraphy of the East Turkana Basin is shown in Fig. 2 The volcamc tufts interspersed within the Koobl Fora stratigraphic column provide a time framework for the geochemistry discussed here. The Kararl. Okote and KBS Tufts define the relative age of paleosols collected in Area 103 and Area 130. The Kararl Tuff has a generally accepted age of 1.3 Myr and the Okote Tuff of 1.57 Myr (Fitch and Miller, 1976}. The

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age of the KBS Tuff has been a source of controversy, but recent agreement between K-Ar dates (McDougall et al., 1980: Drake et al., 1980), 4°Ar-agAr dates (McDougall, 1981) and fission-track ages (Gleadow, 1980) seems to place the age around 1.8 Myr.

39

explain paleogeochemical trends. Although not well understood, climatic conditions have been inferred by oxygen isotope analysis of calcrete and carbonate nodules (Ceding et al., 1977) and 61so and 613C analyses of algal stromatolites (Abell et al., 1982). It is believed that a cool, wet climate existed up to about 2 Myr before present, when a much drier and warmer climate developed abruptly prevailing until the record is obscured at 1.4 Myr. A cooler climate followed at the end of the Pleistocene.

Paleontology The fossil evidence indicates that ancient Lake Turkana had a climate and environment not very different from modern conditions (Harris, 1978: Bonneville, 1976). Present fauna and flora may, therefore, resemble an ancient ecological situation. On the land surface, the present groundcover is patchy. Acacias (several species) and Commiphora africans are the most obvious large woody plants, but various aloes, euphorbias, several grasses and reed beds along the lake are also contributors. The lake itself was host to a substantial population of algae, as evidenced by the several beds of stromatolites (Johnson, 1974). EXPERIMENTAL

Sample collection Regional geochemistry Ancient lake chemistry influenced the character of the initial organic content to the lake sediments. Cerling (1977) has suggested that the earliest Lake Turkana water chemistry was fresh until the end of the Kubi Algi Formation (ca. 4 Myr). The Lower Member of the Koobi Fora Formation was probably derived in the presence of a fresh-to-brackish lake, whereas the Upper Member, from its beginning to the present, is associated with a brackish lake. The pH of the present lake is 9.2 (Yuretich, 1979), while alkalinity analyses by Tailing and Talling (1965) and Ceding (1979) indicate a high sodium carbonate and chloride content ( N a + = 767 mg/l, CO~-2 = 660mg/l, C I - = 440 mg/l). Inorganic characterization of the paleosols in the area is lacking, although some preliminary studies of major and trace elements in regional tufts have been provided (Luedtke, 1975; Ceding, 1977). In the Koobi Fora area of the lake (North Basin), Yuretich (1979) has noted that the very fine-grained sediments are characterized by a montmorillonite/kaolinite ratio of 1.5 to 2.0; the silt fraction quartz/feldspar ratio ranges from 1.5 to 2.5 with pyroxenes and amphiboles constituting 5-7% of the total, while sand grains comprise mostly quartz, plagioclase feldspars and blue-green amphiboles. The sand grains are believed to be derived from metamorphic escarpments south and east of the lake. The Omo River, which flows into Lake Turkana from the north, contributes most of the terrigenous material in the North Basin (Yuretich, 1979). Understanding the paleoclimatic situation may help

During the summers of 1973 and 1974, 85 paleosol samples were collected by Abell in Areas 103 and 130 of the Koobi Fora region of East Turkana (Fig. 1). The sample sites were from recognized stratigraphic sections with fully documented tuffaceous marker horizons. Sample sites in Area 103 (60 samples) and Area 130 (25 samples) are depicted in Fig. 3. The stratigraphic relationships and a physical description of each sample are fully documented elsewhere (Wait, 1979). Samples were collected from freshly exposed sediment beds to minimize any potential contamination or alterations by weathering processes. Samples (150-300g) were collected with an all-metal trowel, placed into acid-dichromate and solvent-washed glass bottles with aluminum foil-lined caps, and stored at ambient temperature until extraction.

Analytical methodology The details of the analytical procedure have been described elsewhere (Wait, 1979). Briefly, a 50g sample was first vacuum dried for about 20hr at ambient temperature, and then ground to 200 mesh in a 8500 Shatterbox (Spex Industries). The sediment was then ultrasonically extracted with 65 ml of a freshly distilled benzene-formic acid (69:31) saturated azeotropic mixture (16hr standing, 15rain sonication), followed by two additional benzene extractions (65 and 70ml, 15 rain sonication). The formic acid was added in excess to completely digest the carbonate matrix. Sterile Ottawa sand spiked with C~6, 24, 28, 32, 36 alkanes and C12, 14, 16, ~s, 20, 22, 24 fatty

40

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acids showed extract]on recoveries of about 95°0. The combined extracts were then concentrated on a rotary evaporator with subsequent nitrogen blowdown to near dryness. All extracts were stored at 0°C. Fatty acids and n-alkanes were isolated from the lipid extract by thin-layer chromatography (TLC) on Brinkman 25 #m UV-254 Silica Gel G plates. The extract residue was developed on an activated TLC plate with a 95:5:1 mixture of hexane:diethyl ether: acetic acid after predevelopment with chloroform: methanol (1:1). R s values for n-alkanes and fatty acids were 0.81 and 0.14 respectively. Visualization of the plates in an iodine chamber showed that about 60% of the plates had one to three slightly colored zones between the n-alkane and fatty acid spots. These spots represent moderately polar compounds but have not been further characterized. Considering the inherent oxidative environment, fatty alcohols, alkyl ketones and oxygenated terpenoids and aromatic hydrocarbons are all possible constituents. However, their infrequent and random occurrence on the TLC plates precluded their use as correlating agents. For analysis, appropriate zones were quantitatively removed and extracted three times with 2 ml of benzene and the extracts concentrated under nitrogen.

Fatty acids were esterified with diazomethane (Levitt, 1973). Extracts were analyzed on a Varian 1400 gas chromatograph equipped with a flame ionization detector, a Varian 20 single-pen strip chart recorder, and a Spectra Physics Minigrator digital integrator Both n-alkane and FAME extracts were chromatographed by splitless injection (Schomberg et al., 1977; Yang et aL, 1978) onto a 25m x 0.25mm OV-I SCOT glass capillary column, using a temperature program of 100 to 265°C at 8cC/min and held for 15 min. Injector temperature was 230°C and detector temperature 310°C; nitrogen carrier gas was 3 ml/min and a 2 pl injection of hexane residue extract was used. Typical n-alkane and FAME chromatograms are shown in Fig. 4. Preliminary analysis of Koobi Fora paleosots (Margolis, 1976; Abell and Margolis, 1982) indicated that very low concentrations of alkanes were present. Wait (1979), using an external standard method, found concentration levels of single n-alkane and FAME compounds to be about 1-10 ng/g. These concentrations are too close to the detection limit of the method to be a useful parameter in this study. Appropriate precautions for glassware and solvent preparation were used as detailed elsewhere (Wait, 1979). Procedural blanks, consisting of sterilized

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The Lake Turkana Basin, Kenya Ottawa sand (baked at 1000°C for 72 hr), were processed with every batch of 5 to 7 samples and always found to be clean ( < 1 ng/g/component). Duplicate analyses were performed on about 10So of the samples, with precision of the relative weight percent of each compound within +7.6~o. Replicate analyses of a FAME calibration standard (NIH-F Mixture, the Hormel Institute, University of Minnesota) found instrumental precision of the relative weight percent of each compound within +6.7%. Identification was made by comparison of retention time indices with appropriate standards. Dual column confirmation was used on selected samples using a 42 in. x ~ in. stainless steel column, packed with 5% Dexsil 300 on 80/100 Chromosorb W (acid washed and dimethylchlorosilated). Also, GC/MS confirmation of the X-2 alkane and FAME components was provided by analysis on a Hewlett-Packard 5985 GC/MS system equipped with a 30m SE-52 fused silica capillary column. CORRELATION STUDIES The idea that organic compounds in sediments may be viewed in much the same way as the morphological remains of organisms was first proposed by Eglinton and Calvin (1967). Reed and Mankiewicz (1975) have suggested that bioorganic molecules incorporated in sediments are retained by biostratigraphic principles. Using alkanes, isoalkanes, aikenes and steroids, Reed (1977b) has attempted to correlate the sediment beds of Mono Lake, California. He concluded that within one depositional environment, the lipid composition of a sediment bed is constant laterally. When different depositional environments were noticeable, variations in sediment lipid composition were evident (Reed, 1977b), presumably because of changes in benthic communities and geochemical microenvironments, along with variations in source material, lithology and transport pathways. Meyers and Takeuchi (1979) found this same situation in Lake Huron surficial sediments. Reed's (1977b) study indicates that the best probability of correlating sediments is with samples taken from one stratum formed in the same depositional environment. Most previous correlation studies by organic geochemists have focused on petroleum hydrocarbon chemistry and are concerned with correlation of petroleum hydrocarbons with source rocks. Two features of petroleum hydrocarbon chemistry differ from the hydrocarbon chemistry of Recent sediments; (a) loss of unique hydrocarbon structures; (b) mobility of petroleum hydrocarbons. Early petroleum correlation studies consisted of bulk geochemical analyses, GLC methods and ~3C/~2C ratios (e.g. Williams, 1974). More recent attempts focus on molecular characterization. By analyzing for biological marker compounds, in particular alkanes, isoprenoids, steranes and terpanes, attempts are now being made to fingerprint crude oils and

41

source rocks for correlation purposes (Pym et al., 1975; Leythaeuser et al., 1977; Seifert and Moidowan, 1979, 1981; Ekweozor et al., 1979; Samman et al., 1981). Correlation of sedimentary horizons can be complicated by many factors. Physical processes that may complicate potential correlation include turbidity flows, bottom currents, advective currents, sediment slumps and bioturbation. A recent study of the fate of benzanthracene in an enclosed marine ecosystem showed that a combination of metabolic processes and physical and chemical mixing produced a nonuniform distribution of the parent compound and metabolites in the sediments (Hinga et al., 1980). Another complicating factor is the alteration of sedimentary organic matter in outcrops by weathering. Reduction of organic content by weathering may be as high as 60°,o overall (Clayton and Swetland, 1978) and up to 50~o in the lipoidal fraction (Leythaeuser, 1973; Clayton and Swetland, 1978). Aromatics seem to be more susceptible to weathering processes than are saturates and n-aikanes show a larger loss relative to branched and cyclic hydrocarbons (Clayton and Swetland, 1978). At Mono Lake Reed (1977b) demonstrated that weathered sediments exhibit a relative decrease in low molecular weight alkanes and a relative increase in n-C22. (Aerobic microbial activity is known to assist in the selective removal of lower molecular weight hydrocarbons [e.g. Hankin and Kolattukudy, 19681.) Reed (1977a) also notes that cycloalkane structures seem to resist weathering although methyl, ethyl and propyl ring substituents may be degraded. The severity with which sedimentary organic matter may be altered depends on a number of factors. Clayton and Swetland (1978) have summarized the likeliest physical and chemical parameters as "the mineralogy, the type and volume of porosity of the rock, the type of organic matter and its stage of thermal maturation, the climate of the area, the amount and character of biological activity in the rock, the tectonic history of the area and the situation of the bed". RESULTS AND DISCUSSION Analytical data for n-alkanes and fatty acids (including C17-C3s CPIp, C25-C31 CPIp, C16-C34 CPIA, relative percent composition of each constituent and concentration ranges) have been given elsewhere (Wait, 1979). For most samples, a majority of the n-alkanes were found from C2s to C33, with a C25 to C3~ CPI distribution between 2 and 4. This range is comparable to distributions found for higher plant waxes (Caidicott and Eglinton, 1973: Nishlmoto. 1974a,c; Tulloch, 1976). Most of the fatty acids ranged from C26 to C34 with about the same CPI values as the n-alkanes. In sediments, the presence of n-C.,2 to n-C34 fatty acids is believed to indicate a terrigenous higher plant origin (Brooks et al., 19761. Frequently.

42

A DALLASWAITand PAUL 1. ABELL

terrestrial alkanes are preserved in sediments from C2~ to C35, with C29 and C31 as the dominant components (Brassell et al., 1978). The appearance of n-alkane and fatty acid distributions essentially identical to those of contemporary terrestrial plant materials suggests that the Koobi Fora sediments accumulated In environments dominated by high terrigenous input such as terrestrial basins and deltaic and lake margin deposits (e.g. Giger et al., 1980). Yuretich (1979) claims that most of the detrital material contained in the present lake sediments of the Koobl Fora area (North Basin) comes from the Omo River. Examination of the Omo River detntal load shows an abundance of terrestrial plant debris (Yuretich, 1979). Presumably, lipid matter associated with aeolian particulates, another contributor to Lake Turkana detritus (Yuretich, 1979), is also of terrestrial origin (Simoneit et al., 1977). Although microorgamsms may have an alkane distribution similar to that of terrestrial plant waxes, they are devoid of any odd-even predominance, and their peak maxima vary widely (Jones, 1969). This type of distribution was not observed with the n-alkane components above C26. Conversely, a number of weathered samples (especially in Area 130l have a narrow intermediate carbon chain length range, a unimodal distribution and low CPI values, all of which may indicate microbial activity (Jones, 1969). The general absence of low molecular weight alkanes and fatty acids may be due to lack of initial input or subsequent removal by weathering processes (Reed, 1977a,b; Clayton and Swetland, 1978). When present, the occurrence of low molecular weight sedimentary fatty acids (C15.17,19) or alkanes (C16,18,2o) may be the product of algal or microbial input (Brooks et al., 1976), or a diagenetic by-product. The fate of the original organic matter may have been governed to some extent by weathering processes, especially in Area 130. Many of the samples showed no detectable amounts of n-alkanes below C21; also, unusually large amounts of n-C22 alkanes were found in many samples, and there was a general lack of fatty acids. To elaborate, 48°0 of the n-alkane analyses detected no components below C2~. Reed (1977b) noted similar trends in some of the weathered sediment horizons of Mono Lake. The lack of components below C2~ can be explained by the physical and chemical factors which caused the weathering-induced compositional changes observed in the sedimentary lipids. Unusually high concentrations of n-C22 and n-C21 alkanes were found in most of the samples from Area 130 and a few of the samples from Area 103. Schenck (1968) and Reed (1977b) noticed increased concentrations of rt-C22 alkanes with increased sediment weathering. The reasons for this are presently unclear, although Schenck (1968) suggests that n-C22 predominance may result from deposition of a specific ancient organism containing r1-C22 alkanes or structurally

related precursors (e.g. r/-C22 fatty acids). For example, the prominent n-alkane in the fungus Debaryomyces hansenii is n-C22 (Merdinger and Devine, 1965). In Recent sediments from the Southern California Bight, Simoneit and Kaplan (1980) found alkane maxima in some sediments at /1-C22 and r/-C23. They suggest that this occurrence may represent microbially altered algal detritus, yet their conclusion cannot be substantiated m this study because of the general lack of the C~5 to C2o alkanes that would normally be expected. Contamination of sediment extracts with C22 alkanes has been suggested by Douglas and Grantham (1973), but does not seem applicable because of procedural blank results. Approximately 60,°o of the samples analyzed contained no fatty acids. Sandstone and clay-sand samples contained no detectable amounts of fatty acids, whereas 52°,, of the clay samples retained discernible concentrations of fatty acids. It may be assumed that fatty acids were once present in the sandstone samples since other lipoidal material--hydrocarbons-still remains. However, the original composition of these two environments may have been different due to varied biological input. In general, finer particles are usually richer in adsorbed organic matter (Rashid and Reinson, 1979; Wade and Quinn, 1979). Although increased permeability and decreased surface area would deter preservation of organic matter in sand, the apparent affinity of fatty acids for mineral particles (Meyers and Quinn, 1973: Morns and Calvert, 1975: Hedges, 1977) and organic matter (Schnitzer and Neyround, 1975: Harrison, 1978: Thompson and Eglinton, 1978) may lengthen their geochemical lifetime. Several geochemists have studied the molecular association of fatty acids with mineral particles and organic matter, and factors affecting that relationship (Nishimura, 1977; Cranwell, '1978, 1981: Van Vleet and Quinn, 1979a,b; Farrington et al., 1977; Miller et al., 1977: Almon and Johns, 1977: Suess, 1970, 1973: Barcelona and Atwood, 1979: Lahann and Campbell, 1980; Kawamura and Ishlwatari, 1981). Area 103

Area 103, bordering the eastern shore of Lake Turkana (Fig. 1), is characterized by numerous gullies cutting down through the ancient sediments to near the present lake level. A clay horizon is visible throughout these gullies. Formation of the clay horizon occurred prior to the formation of the Okote Tuff (1.57 Myr). Twelve sites in five outcrops were chosen for sampling (Fig. 3a). Samples were taken at the top iX-series) and bottom iT-series) of the clay horizon at each site. Samples were also taken from the sandstone horizon that resides directly underneath the clay bed (U-series). Overlying the clay horizon is a series of interbedded .clays, sands and indurated sand stones from which intermittent samples were also taken (W and V series).

The Lake Turkana Basin, Kenya AIkanes. The abundance of chemotaxonomic (Caldicott and Eglinton, 1973; Tulloch, 1976) and geochemical data (Brassell et al., 1978) for n-alkanes, as well as alkane ubiquity and resistance to degradation (Albrecht and Ourisson, 1971), suggests that these compounds may be useful as stratigraphic correlators. The X-series samples all contained n-C2t to n-Cat alkanes, with four samples containing alkanes as low as Ct~ and two samples with alkanes as high as Cas. The major homoiogue varied widely between samples, but was generally either C27, C29 or Ca i. Since C25 to Cat alkanes were consistently present in all X-series samples, CPI may be a useful correlation parameter. lsoprenoid alkanes were not detected in any significant quantities. Most terrestrial plants have CPI values greater than 4 (Cooper and Bray, 1963), while aquatic organisms range from 0.4 to 1.5 (Han and Calvin, 1969a). Once incorporated into sedimentary material, CPI approaches unity with increasing age (Bray and Evans, 1961, 1965). The overall CPI values for X-series samples ranged from 1.23 to 2.52 and averaged 1.72, with no apparent trends. The qualitative distribution of the alkanes indicates that most of these compounds originated from terrestrial sources. This conclusion is also suggested by the presence in the X-series samples of alkanes from C27 to C3s (with C29 and Cat prevalent in many samples) and a CPI approaching 2. Algal hydrocarbons usually lie between Ct 5 and C2o, with CI 5 or C17 considered the distinctive compound (Han et al., 1968; Blumer et al., 1971; Nishimoto, 1974b). A lack of CI7 may indicate that the lighter hydrocarbons in these samples did not originate predominantly from algal sources but were rather the result of microbial input and/or the byproducts of the diagenesis of higher molecular weight n-alkanes and fatty acids. Unfortunately, no overall trends seemed evident throughout the horizon, although some samples in close proximity to one another did exhibit similar distributions. Possibly the porous sand layer directly overlying the X-series samples has allowed sporadic diagenetic processes to occur at the top of the clay horizon. To examine the vertical homogeneity within the clay horizon, a sample (T-series) was removed just below each X-series sample. Alkane distributions among the T-series samples were noticeably more consistent than those in the X-series (e.g. Fig. 5). Most T-series samples showed hydrocarbon distributions ranging from C21 to Ca3, with maxima at C25, C27, C29 or C31, again suggesting a terrestrial plant source for a majority of the alkanes. The relative increase of C25 may indicate a larger contribution of microbial alkanes in the T-series than in the X-series. CPI values for the T-series ranged from 12.5 to 2.56 and averaged 1.78--almost identical to the CPI range and average of the X-series. However, diagenesis of C2s to C31 alkanes in T-series samples seemed more advanced than in the X-series (C25-C31 CPI values were 1.62 and 2.03 respectively). The higher C25-Ca~

43

CPI of the X-series is balanced by the presence of low molecular weight alkanes that were not present in the T-series, and which had virtually no odd-even trends, thus lowering the overall CPI. The carbon chain length distributions in the T-series were almost identical throughout the twelve sites; and peak maxima and C25-C3t CPI values correlated well within each outcrop, although not throughout the entire area. A sizeable sandstone horizon lies directly beneath the paleosol bed from which the X and T samples were collected. Sandstone samples (U-series) were collected from this horizon, in positions corresponding to the location of the X and T samples, in order to examine the fate of sand organics versus clay organics and to investigate whether the organic composition existing in both these facies has been retained. The ages of the X, T and U series samples are not very different (months or a few years), as they probably represent a transgressive phase of the lake. Alkanes were detected in all twelve samples within a range of C20 to C3t. All samples contained C21 to C29 alkanes in a unimodal distribution, with a maximum at C2s in seven of the twelve samples. Johnson and Calder (1973) have suggested that a maximum at C23 to C25 might inicate microbially altered algal detritus. CPI's ranged from 0.94 to 2.08, averaging 1.29. This range is significantly lower than the clay horizon, indicating that more extensive diagenetic activity has occurred or that a different biological source produced the alkanes. This comparison shows that aikane characteristics are unique to each facies (e.g. Fig. 5). CPI values correlate closely within the sandstone horizon, except at sites VII and XI, which have significantly higher values. This result should be expected for sediments that have experienced a high degree of diagenesis, since CPI values ultimately approach unity. Correlation of alkane chain length and maxima in clay and sandstone samples is similar, but not quite as obvious. Compared to the overlying clay horizon, continuity of these parameters in the sandstone bed is more reasonable. This uniform distribution is not surprising since diagenesis causes a loss in unique lipid distributions, producing more uniform characteristics. Overlying the clay horizon is a series of interbedded clays, sands and indurated sandstones. These strata probably represent littoral margins with relatively short and stable periods of existence. Suites of clay paleosols from sites II, IV and VII (V-series) along with sites IX, X, XI and XII (W-series) were collected to examine the vertical variations of lipoidal material from one horizon to another. Because of the incongruity of these paleosols over any distance, the use of these analyses for correlation is not feasible. In fact, at some sites (I, III, V, VI and VIII), so few well-defined paleosols existed that no samples were taken. The n-alkane composition of the V and W samples differs from those of the X-, T- and U-series in that 67°,~,of the samples contain alkanes in the Ct7 to C2o carbon chain length range. This range usually indicates an algal or possibly bacterial source. How-

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i 33

35

Fig. 5 Alkane distributions at Sites VII, VIII and IX of Area 103 (X-axis = Carbon chain length: Y-axis = Relative percent composition)

ever, distribution trends noted in these samples (low C17 levels) were not typical of an algal input. Han et al. 0968) noted similar trends in recent algal ooze (Mud Lake, Florida) and attributed it to bacterial activity. Also present in these paleosols were substantial amounts of high molecular weight alkanes, indicating that terrestrial higher plants may have been a partial source of the lipid material. This is a logical explanation if these sediments accumulated during vacillating coastal environments where both allochthonous and autochthonous sources existed. Except at site II, vertical comparison of the samples shows that alkane distributions differ significantly. This is best demonstrated at site IV, where the following alkane distributions were noted: V-5 (44.6% CI~-CIg), V-6 (42.2°,o C21-C23), V-7 (59.5% C2v, C29, C31), V-8 (59.4°,0 C22-C2s). V-5, V-6, and V-8 showed little odd-even

dominance, while V-7 has a very high CPI value of 9.5. These distinctly different distributions indicate retention of the alkanes in each horizon. If migration of lipoidal matter was occurring, it should encourage homogenization of alkane distributions, thus removing unique compositions. Fatty acids. In this particular environment fatty acids seem more susceptible to degradation than alkanes. Cranwell (1981) has found fatty acids less stable than n-alkanes in Recent lake sediments. However, fatty acids are known to be preserved in ancient sediments (Seifert, 1975), some as old as 400 Myr (Kvenvolden, 1970). One of the oldest sedimentary rocks discovered on earth (3.4 Byr) contained fatty acids, with n-C16 the prominent component (Han and Clavin, 1969b). Whereas CPI values of fatty acids in ancient sediments approach unity {1-4), Recent sedi-

The Lake Turkana Basin, Kenya

45

In the T-series, fatty acids were detected in only five of the twelve samples. C18 to C3,, fatty acids were found, but both distributions and CPI values varied widely between samples. The presence of fatty acids in only five samples suggests that these compounds are useless for correlating paleosols in this environment; however, their presence or absence may be a useful correlation parameter in other stratigraphic situations. Fatty acids were not detected in any of the U-series sandstone samples. This finding may indicate low

ment CPI values normally range from 5 to 9 (Kvenvolden, 1970). Fatty acids were detected in eight of the 12 X-series samples analyzed. Variations in the carbon chain length are evident between C16 and C3,,, with CPI values varying from 1.2 to 4.2. The relationship of the fatty acid distribution and overall CPI values has been compared with corresponding alkane samples in Table 1. No direct relationship seems evident, nor are any correlation trends between the fatty acids apparent.

Table 1. Comparison of distributions and CPI values of X-, T- and U-series samples for n-alkanes and fatty acids Site

n-alkane Sample distribution

Fatty acid distribution

CPI*

CPIp*

CPI~

I

X T U

C21-C33 C21-C33 C21-C3:

CIg-Ca2 ND ND

1.76 1.97 1.23

1.56 2.18 NA

1.90 NA NA

I1

X T U

C21-C33 C2rC3a

C22-C32

C21-C31

ND

1.75 2.56 1.11

1.43 2.25 NA

2.89 2.44 NA

X T U

Ct.,-C31

CI8-C32

C21-C33

C19-C26

C2o-C3o

ND

1.53 1.85 1.17

2.08 1.71 NA

1.85 0.81 NA

IV

X T U

Cr7-C3t C21-Caa C21-C31

ND ND ND

1.23 1.54 1.22

1.40 1.77 NA

NA NA NA

V

X T U

C21-C33

C1s-C28 ND ND

1.81 1.95 1.34

2.26 1.31 NA

1.18 NA NA

ND Cla-C32 ND

1.32 1.65 0.94

2.27 1.72 NA

NA 2.42 NA

CI6-C32 ND ND

1.75 1.25 2.08

2.43 1.32 NA

4.20 NA NA

Cla-C3,t ND

1.41 1.80 1.23

3.00 !.53 NA

2.34 1.34 NA

C,s-Ca, C22-C34 ND

2.52 1.39 1.14

2.22 1.25 NA

2.27 2.71 NA

C22-C32 ND ND ND ND NS

1.46 1.59 1.21 2.21 1.67 1.78

1.70 1.43 NA 2.02 1.21 NA

3.33 NA NA NA NA NA

ND ND ND

1.89 2.09 1.01

1.88 1.78 NA

NA NA NA

III

X T U

VI

VII

VIII

IX

X1

XII

C23-C31 C2t-C31 Cr7-C31

C18-C33 C2o-C29

X T U

C2o-Cat C21-Ca3

X T U

CIa-C31 C21-Ca3

X T

C21-C3a C2~-Caa

U

C2o-C29

X T U X T U

CI 8-C33

X T U

Ci a-Ca5

C2 i--C29

C26-C32

C21-C26

C2o-C3t

C21-C32 C2o-C31 C,~-C35 C2rC33

C21-C29 C21-C33 C21-C29

*CPIp range from C17 to C3s. tCPlr range from C2~ to C3t. ~.CPIA range from C,~ to C34. ND = none detected. N A = not applicable.

46

A DALLASWAIT and PAUL 1. ABELL

original concentrations, selectwe susceptibility to weathering processes, or migration. As with the V- and W-series alkanes, fatty acids detected in these samples each had distintive distributions. Sixty-three percent of the samples contained fatty acids that exhibited CPI values ranging from 1.7 to 7.4. Most of the fatty acid distributions indicated significant terrestrial plant input. Three V-series samples contained significantly large amounts of C, s, which could have algal, microbial or higher plant origins (Volkman et al., 1980). The relative absence of C~6 fatty acid, a prominent component of many ancient sediments [Kvenvolden, 1970), may be a peculiar characteristic of this type of environmental situation. Comparison of fatty acid molecular weight distributions In) of each sample and the respective (n - 1) alkanes fits for some samples, yet varies considerably in others. Obvious differences in fatty acid distributions throughout the continuous straugraphlc successions substantiate the theory that lipid materials are retained within these faoes Area 130

Area 130 is located about 30 km north-northeast of Area 103 (Fig. 1). Instead of the prevalent but widely dispersed outcrops in Area 103, Area 130 is characterlzed by a continuous convoluted escarpment rising about 150ft above the flood plain (Fig. 3b) Along with the continuous Karari Tuff that caps the escarpment, a second tuff (Okotet can be found about 4 m above the paleosol horizon of interest, providing a convenient marker bed (Fig. 2). During formation, the paleosol probably resembled modern-day Serozens or Solonetzes (Fitzpatrick, 1972). This paleosol consists of an A and a B horizon. The A horizon of a paleosol is usually an organic-rich soil. Below it, where organisms are probably anaerobic, is a heavily leached B horizon. Usually horizon A contains detntal organic matter, while horizon B is a light-colored leached material that may still contain bound orgamcs. Due to age and environmental stresses (e.g. high temperature, oxidaUve processes), most of these characteristics have been diminished. Twenty-five samples from seven samphng sites were collected from the paleosol horizons (A and BJ. Horizon A was usually about 2 m. thick and horizon B was usually about 4in. thick, but thicknesses varied over long distances. In some places A remained constant while B became nonexistent. In other areas, a calcareous horizon overlying A extended down into B. Two samples from the calcareous horizon were analyzed to determine the fate of lipids in this type of lithology. No alkanes or fatty acids were detected in either sample ( < 1 ng/g}. In general, fatty acid and hydrocarbon analyses of the paleosol indicated that a greater degree of weathering had occurred m Area 130 than m Area 103. Only 26%~, of the samples contained fatty acids, and two samples contained no detectable alkanes. Narrow carbon chain length ranges, low CPI values, and max-

~ma from C2~ to C25 characterized most of the alkane results. Alkane distributions in a section of the escarpment are depicted in Fig. 6. U-series samples from Area 103 showed similar trends, but not to the diagenetic extent exhibited m Area 130. Their profiles indicated more microbial-like composition rather than the terrestrial plant atkanes seen in most of the other paleosol samples. In Lake Biwa selective loss of short-chain (
The Lake Turkana Basin, Kenya

47

30"

30-

30

~0"

30-

25"

25"

25

25"

25-

20"

20'

20

I

le-

20-

!L!I,

13

!l!!lh

I0

I0"

5"

5'

1111!

i

192~ 2 3 2 3 2 ? 2 9

19 2123 25 27 29

19 21 23 2 5 2 7 29

R-6

R--5

R - 11

12 21 2 3 2 3 27 29

1921 2 3 2 3 2 7 2 ~ 3 1

S--4

S--5

Karart T u f f

0"~ ~ ' b ~>";,''~"

~'~

"

~- ~

, ~ . ' ~ - : : . ; * , , - , ° : ;. . . . . . ~ , %

"rO~"

~ ~..:

:,-.-~" ',;"4;% ~",;"~'2:Y'"

/

"." -':_~d~:'~" ~.~ "

,;::.:""*'.,~';'e:-::~'.:'~'~',*.::'%2;e:";~e:.Q"~e'-

\

R-7

"~.,I~ :~:~.2o","~;"

\

R--8

30-

30-

30

25"

25

25

20'

20

20

~"

'~.~..

°*'',°,

' "~

~

"~:',=*-~':;~:~;~

II

I

R--12

S--6

(None Detected)

15,

10-

5,

!l ! !!L! !l!!!l ill 15

15

I0

I0

5

5

i

1921 23 25 2 7 2 e

Ig 2 1 2 3 2 5 27 29

r

1921 23 252~" 29

Fig. 6. Alkane distributions in a section of the escarpment in Area 130 (X-axis = Carbon chain length: Y-axis = Relative percent composition).

usually quite different, indicating that the material has been retained within each facies. This constraint does not hold for Area 130, where weathering has affected the lipid composition. Both n-alkanes and fatty acids were found to be preserved better by clay minerals than by sand matrices. In Area 103, the alkane distribution of clay organics differs significantly from those found in sandstones (no sandstones were analyzed in Area 130). Most alkanes detected in the sandstone samples had a narrower carbon chain length distribution (C21-C29) than did those in the clay paleosol samples. Other characteristics of sandstone alkanes include unimodal distribution with low CP! values and a maximum around C25. No fatty acids were detected in any sand samples. It appears that the fate of the original paleosol organic matter has been governed to some extent by

weathering processes, especially in Area 130; many of the samples showed no detectable amounts of the low molecular weight n-alkanes (C17-C2o) but had unusually large amounts of n-C21 and n-C22 (and to a lesser extent CEs, C24 and C25), low CPI values, low quantities of extractable lipids and a general lack of fatty acid compounds. The composition of paleosol samples formed in one depositional environment was expected to be comparable over extended distances, but weathering effects of the arid alkaline environments at Koobi Fora have relegated any correlation to short distances. The best alkane correlations were found with Area 103 T-series samples. The sporadic presence of fatty acids allowed no correlation of these compounds. Direct parallels in the distribution of n-alkanes and fatty acids within a particular sample are not evident, if not because of differences in initial input, then

48

A. DALLAS WAIT and PAUL 1. ABELL

Brooks, P., Eghnton, G., Gaskell, S., McHugh, D., Maxwell, J, and Philp, R_ 1976, Llplds of recent sediments, Part !: Straight-chain hydrocarbons and carboxylic acids of some temperature lacustrine and sub-tropical lagoonal/tidal flat sediments: Chem. Geol., v 18, p 21-38. Caldicott, A. B., and Eghnton, G , 1973, Surface waxes, in Miller, L P, ed., Phytochemistry, Vol. III, Van Nostrand, p. 162-194. Ceding, T., 1977, Paleochemlstry of Plio-Pleistocene Lake Acknowledgements--We would like to thank the Petroleum Turkana and diagenesis of its sediments: Ph.D. &ssertaResearch Fund of the American Chemical Society tion, Umv. of California, Berkeley. (PRF-5676-AC2) and the American Hoechst Corporation Ceding, T E., 1979, Paleochemistry of Plio-Pleistocene for financial support, and Mr RICHARD LEAKEY and the Lake Turkana, Kenya: Paleogeoyr. Paleochmatol National Museum of Kenya for field support. Dr MARPaleoecol., v. 27, p. 247-285. SHALL MARGOLIS initiated many of the organic geochemi- Cerhng, T., Hay, R., and O'Neill, J., 1977, Isotopic evidence for dramatic climatic changes in East Africa during the cal techniques used here. MtKE McGREGOR provided valuable suggestions on the manuscript. NElL MOSESMAN Pleistocene: Nature, v. 267, p. 137-138. (Energy Resources Co., Inc.) provided confirmatory Clayton, J., and Swetland, P., 1978, Subaerial weathering of GC/MS analyses. We would also like to thank Dr STEVEN sedimentary organic matter: Geochim. Cosmochim. Acta, THOMPSON and Professor GEO~WEY EGLINTON (University v 42, p. 305-312. of Bristol) for training Wait in the 'art" of preparing and Cooper, J., and Bray, E. 1963, A postulated role ol fatt) installing glass capillary GC columns. acids m petroleum formation: Geochim. Cosmochim Acta, v. 27, p. 1113-1127. Coppens, Y., Howell, F., Isaac, G., and Leakey, R., 1976, Earliest Man and Environments m the Lake Rudolf REFERENCES Basin, Univ. of Chicago Press. 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because of selective weathering of certain classes of compounds. Finally, if a paleosol capped by a porous sedimentary rock is to be studied, samples collected near the b o t t o m of the horizon may give a more representative portrait of the organic paleosol than material bordering the upper facies boundary.

The Lake Turkana Basin, Kenya and olefinic hydrocarbons in Recent sediments of Greifensee, Switzerland: Geochim. Cosmochim. Acta, v. 44, p. 119-129.

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49

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50

A. DALLASWAIT and PAUL I. ABELL

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