Environmental significance of Upper Miocene phosphorites at hominid sites in the Lukeino Formation (Tugen Hills, Kenya)

    Environmental significance of Upper Miocene phosphorites at hominid sites in the Lukeino Formation (Tugen Hills, Kenya) Perrine Dericquebourg, Alain Person, Lo¨ıc S´egalen, Martin Pickford, Brigitte Senut, Nathalie Fagel PII: DOI: Reference:

S0037-0738(15)00155-4 doi: 10.1016/j.sedgeo.2015.07.005 SEDGEO 4883

To appear in:

Sedimentary Geology

Received date: Revised date: Accepted date:

28 May 2015 17 July 2015 18 July 2015

Please cite this article as: Dericquebourg, Perrine, Person, Alain, S´egalen, Lo¨ıc, Pickford, Martin, Senut, Brigitte, Fagel, Nathalie, Environmental significance of Upper Miocene phosphorites at hominid sites in the Lukeino Formation (Tugen Hills, Kenya), Sedimentary Geology (2015), doi: 10.1016/j.sedgeo.2015.07.005

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ACCEPTED MANUSCRIPT Environmental significance of Upper Miocene phosphorites at hominid sites in the

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Lukeino Formation (Tugen Hills, Kenya)

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Perrine Dericquebourg (1), Alain Person (2), Loïc Ségalen (2), Martin Pickford (3), Brigitte

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Senut (3), Nathalie Fagel (1)

(1) Université de Liège. UR AGEs (Argiles, Géochimie et Environnements sédimentaires).

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B18. Département de Géologie, paléontologie et minéralogie. Quartier Agora. Allée du six

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Août 14, B-4000 Liège, Belgique.

(2) Sorbonne Université, UPMC Univ Paris 06, CNRS, UMR 7193 ISTeP, F-75005, Paris, France 05.

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(3) Sorbonne Université, MNHN, UPMC Univ Paris 06, CNRS, UMR 7207 CR2P, F-75005, Paris, France.

Abstract

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Corresponding author: [email protected]

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The Lukeino Formation contains an important sedimentary and fossiliferous record of the late Miocene (6.09-5.68 Ma), which has in particular yielded the fossil remains of the oldest East African bipedal hominid called Orrorin tugenensis. This fluvio-lacustrine sedimentary succession crops out in the Kenyan part of the East African Rift. It is mainly composed of clay to sandy clay deposits intercalated with volcanic ash horizons, and localized layers of carbonates and diatomites. A detailed sedimentological and mineralogical study of the Lukeino Formation was conducted to throw light on the environmental conditions in which the hominids lived. Several centimetric, relatively continuous and indurated phosphatic horizons, of sedimentary origin, were identified at two sites (Sunbarua and Kapcheberek). Mineralogical

ACCEPTED MANUSCRIPT (XRD) and geochemical analyses as well as observations by SEM, which was coupled with an energy dispersive spectroscopy (EDS) microprobe, indicate that the autochthonous

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phosphate layers are composed of a micritic matrix of francolite (38-93%), with incorporation

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of silicates in variable proportions from one layer to another. The phosphate matrix contains

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very well preserved and abundant diatom frustules in the basal phosphate layer. These diatoms are identified as Aulacoseira granulata, implying a pH of 7.8-8.2 for freshwaters of the Palaeolake Lukeino. Calcitic tubular structures, linked to a possible bacterial origin, are

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also observed locally.

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Phosphate layers occur abruptly within a thick clay-sandy series, associated with an intense runoff phase during the deposition of this interval of the Lukeino Formation. The massive and cyclic input of phosphorus to the lake promoted productivity to the stage where it

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caused a diatom bloom. The establishment of several phosphate horizons testifies to successive phases of eutrophication of Palaeolake Lukeino. The diatom cells provided some

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of the organic matter, which was decomposed by bacterial activity at the bottom of the lake in suboxic conditions, but in insufficient quantities to fully form the phosphatic materials. The

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rest of the organic matter needed for phosphogenesis came from terrigenous supply (plant debris), suggesting the presence of dense vegetation in the catchment of the Palaeolake Lukeino, during this well-drained interval of deposition of the Lukeino Formation.

Keywords: Lacustrine sediment, mineralogy, diatoms, phosphogenesis, palaeoenvironment, Upper Miocene Lukeino Formation.

1. Introduction Phosphate deposits have a wide geographic distribution, deposits of variable importance being reported from every continent (Orris and Chernoff, 2004), including

ACCEPTED MANUSCRIPT Antarctica (Cathcart and Schmidt, 1977). Their stratigraphic distribution extends from the Precambrian to the Present. Phosphate deposits can be classified into three main categories

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according to their origin: 1) igneous, 2) resulting from an alteration of guano, and 3)

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sedimentary. Over geological time most sedimentary phosphate deposits are of marine origin,

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but lacustrine phosphorites also represent an important phosphorus sink (Föllmi, 1996; Mackey and Paytan, 2009), although the known occurrences are much rarer. Initially, phosphorus comes from the weathering of igneous and sedimentary rocks and

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the leaching of soils in continental settings, but organic matter also represents an important

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source (Föllmi, 1996; Filippelli, 2008; Ruttenberg, 2014). Phosphorus is an essential element for life, necessary for the production of DNA and certain biomineralizations including teeth, skeletons, and should be a limiting nutrient, like iron or nitrate, in certain ecosystems.

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Conversely, the massive input of phosphorus can promote an excess in primary production, causing the eutrophication of lakes. Phosphogenesis takes place under specific

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physicochemical and biological conditions that make phosphorites a reliable indicator of environmental conditions at the time of accumulation. Several major global phosphate

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episodes were identified by Cook and McElhinny (1979) on the basis of their abundance in the geological record. These periods, affected by an acceleration of the phosphorus cycle, are in the Cambrian, Ordovician, Permian, Jurassic, late Cretaceous-Palaeocene and Miocene. Particularly important Miocene phosphate deposits are related, amongst other factors, to uplift of the Tibetan Plateau and the establishment of an intensified Indian-Asian Monsoon at ca 8 Ma, which led to increased chemical weathering of continental rocks, and the release of abundant amounts of phosphorus into marine ecosystems (Filippelli, 2008). In East Africa, most of the numerous phosphate deposits are of igneous origin (especially in Tanzania, Uganda and southern Kenya) and are mainly related to carbonatite and carbonatite-nephelinite volcanism (Notholt, 1991; Van Kauwenbergh, 1991). Several

ACCEPTED MANUSCRIPT phosphate deposits, linked to the transformation of guano, are located in the south of Kenya (Forti et al., 2003, for volcanic cave phosphates of Mt Suswa and the Nyambeni Range) and

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in Tanzania (Schlüter and Hampton, 1997, for the Lake Manyara area). Previously only a few

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sedimentary phosphate deposits have been observed in Ethiopia, Somalia, Mozambique and

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southern Tanzania (Van Kauwenbergh, 1991). The only significant sedimentary phosphate deposits large enough to be mined are located in Tanzania, in Plio-Pleistocene lacustrine deposits from Minjingu (Lake Manyara Basin). These deposits have been linked to a

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transformation of guano (Van Kauwenbergh, 1991; Szilas et al., 2008).

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We identified several phosphate layers in the fluvio-lacustrine Upper Miocene Lukeino Formation in Kenya. This formation yielded fossils of Orrorin tugenensis, the oldest obligate bipedal hominid in eastern Africa (Pickford and Senut, 2001; Senut et al., 2001;

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Pickford et al., 2002), whose remains were found, among other sites, at Kapcheberek. In this study, we carried out scanning electron microscope (SEM) observations, as

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well as mineralogical and geochemical analyses in order to determine the nature of the phosphorites and to throw light on palaeoenvironmental conditions at the time of deposition,

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at a crucial time in hominid evolution.

2. Geological context

The study area is located in the central part of the East African Rift in Kenya (Gregory Rift) (Fig. 1). The upper Miocene sediments of the Lukeino Formation crop out in the eastern foothills of the Tugen Hills, northwest of Lake Baringo (Pickford, 1974, 1978). The area corresponds to a large tilted block within the rift (Pickford, 1974; Chapman et al., 1978; Hill et al., 1986; Pickford et al., 2009). It is located between the Elgeyo Escarpment, which forms the western flank of the rift and the axial rift depression in which Lake Baringo and Lake Bogoria are located.

ACCEPTED MANUSCRIPT This part of the Gregory Rift has been subjected to intense volcanic and tectonic activity since the early Miocene. The formation of half-grabens has structured the region and led to the

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uplift of the nascent Tugen Hills during the Middle Miocene, favouring the development of a

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series of graben lakes (Pickford, 1974; Hill et al., 1986; Williams and Chapman, 1986;

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Tiercelin et al., 1987; Tiercelin and Lezzar, 2002). Between about 9 and 7 Ma, a renewal in volcanic activity occurred, characterized by phonolite flows that formed the Ewalel Formation, which almost completely infilled the graben, burying the deposits of the Middle

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Miocene Palaeolake Ngorora. This activity continued until the eruption of extensive lavas of

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the Kabarnet Trachyte Formation between 7 and 6.1 Ma. At the same time, faults located in the Tugen Hills were reactivated, leading to the creation of a new graben about 6.5 Ma, called the Lukeino Depression in which Palaeolake Lukeino and perilacustrine flats developed.

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Fluvio-lacustrine sediments (~ 100 m thick) accumulated in this depression from about 6.1 to 5.7 Ma ago.

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The base of this formation overlies a trachyte flow of the Kabarnet Trachyte Formation (Fig. 2). The composition of the clayey-sandy sediments is alternately dominated by siliciclastic

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minerals derived from runoff during wet periods, by neoformed clay minerals when the environment became more confined, and by pyroclastics during phases of volcanic activity. In addition, locally, there are carbonates, diatomites and phosphates (Fig. 2). Clastic sediment deposition was interrupted by several volcanic eruptions, leading to the accumulation of ash layers, a basaltic flow (Kapsomin Basalt) and a dolerite sill (Rormuch Sill). The Lukeino Formation is sealed by the Kaparaina Basalts that infilled the depression and obliterated the palaeolake. Radiometric dates on the lava flows (measured on groundmass) that bracket the Lukeino Formation, indicate an age between 6.09 ± 0.14 Ma for the underlying Kabarnet Trachyte, and 5.68 ± 0.18 Ma for the overlying Kaparaina Basalts at the top of the series (Sawada et al., 2002). Other studies performed on these lava flows indicated similar ages:

ACCEPTED MANUSCRIPT between 6.06 ± 0.13 Ma (sanidine; Hill et al., 1985) and 6.37 ± 0.05 Ma (anorthoclase; Kingston et al., 2002) for the Kabarnet Trachyte, and between 5.65 ± 0.13 Ma (feldspar; Hill

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et al., 1985) and 5.72 ± 0.05 Ma (anorthoclase; Deino et al., 2002) for the Kaparaina Basalt.

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3. Materials

Four lacustrine phosphate layers have been observed at Sunbarua and two at Kapcheberek (see Google Earth image in Fig. 2). Samples were collected in August 2010 and

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2011 as part of the Kenya Palaeontology Expedition led by two of us (Brigitte Senut and

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Martin Pickford). The sedimentary sections sampled at these two sites, separated from each other by about 1 km, are situated just above the Rormuch Sill and are considered to be the same age (Fig. 2). Intercalated between the Rormuch Sill and the Kaparaina Basalts, they

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occur in the upper half of the Lukeino Formation. The sampling intervals ranged between 0.11 m, following lithological variations, in order to obtain a detailed sedimentological and

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mineralogical record. The phosphatic layers and enclosing clay layers were sampled at relatively close intervals.

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The total sedimentary succession (~ 50 m thick), which crops out at Sunbarua, consists predominantly of clayey-silt to sandy deposits, with a more important pyroclastic component towards the top of the series. The basal part of the site, where the phosphate layers are located, is characterized by clayey to sandy clays, with locally a component of volcanic ashes (Fig. 2). The succession contains several ferruginous layers (1-5 cm thick). Three phosphate layers (Fig. 2) were sampled at Sunbarua: samples SBA02 (phosphate horizon #1), SBA23 (phosphate horizon #2) and SBA36 (phosphate horizon #3). The succession at Sunbarua is cut by a post-depositional listric fault, explaining the dip of the sedimentary layers, with the sampled phosphate layers located in the lower block (Fig. 3a, c). These same phosphate layers are also observed in the upper sedimentary block. Complementary sampling of sample SB02

ACCEPTED MANUSCRIPT (Fig. 3a) was also carried out about 75 m north of the main axis of the sedimentary series of Sunbarua (Sunbarua Section 2), and can be considered as the lateral equivalent of sample

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SBA02 (phosphate horizon #1). This demonstrates the continuity of the phosphate layers.

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The phosphatic layers are thin and well indurated and occur within an important clay

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series, thus marking a break in clastic sedimentation (Fig. 3a, c). The samples SBA02 and SB02 are from the basal, 2 cm thick, phosphate bed (phosphate horizon #1), which is extremely indurated, white, laterally-continuous and shows a deformation by rhombic jointing

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at the outcrop scale (Fig. 3d). It shows no trace of alteration or development of oxides and

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appears abruptly in the thick clayey-silty succession. The second phosphatic layer (SBA23, 2 cm thick) is slightly less indurated, beige in colour and is again without oxidation. Sample SBA36 was extracted from the uppermost phosphate layer (phosphate horizon #3). This

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brown to black layer is richer in oxides and is characterized by an alteration by nodular jointing at the outcrop scale. Silty-clays are intercalated between these indurated nodules

previous ones.

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which range from five to ten cm in diameter. This layer is overall less continuous than the

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The sedimentological section of Kapcheberek (Fig. 2) was collected at the base of the site, just above the Rormuch Sill. The sediments in the lower two thirds of the section are clayey with a substantial silty component, whereas the upper part is dominated by clayeysand. This succession includes numerous ferruginous layers a few centimeters thick, composed of goethite and hematite. Two phosphatic layers about 2 cm thick have been identified and analyzed: samples KBK24 and KBK26. They are located in the lower part of the section (Fig. 2) and are beige to light brown in colour with traces of oxidation. They also appear to be rather more weathered, less continuous and less indurated than those at Sunbarua, especially specimen KBK26 (Fig. 3b).

ACCEPTED MANUSCRIPT In all, three stratigraphic horizons of lacustrine phosphate were identified. Samples SBA02 (phosphate horizon #1), KBK26 (phosphate horizon #2) and KBK24 (phosphate

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horizon #3) correlate with SB02, SBA23 and SBA36, respectively (Fig. 2), although the

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nodular jointing of sample SBA36 in the Sunbarua sedimentary section is not observed at

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Kapcheberek.

4. Methods

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SEM observations were made using a ZEISS supra V5 instrument provided with a

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BRUKER detector in energy dispersive spectroscopy (EDS) at the Petrology, Geochemistry, Magmatic Mineralogy Laboratory (PG2M) - ISTeP of the University Pierre and Marie Curie (Paris). The EDS analyses were performed to determine the semi-qualitative chemical

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composition of phosphate using the Esprit software. The error in the measures is ± 20%. The observations were made on polished thin sections and fragments of samples. The phosphate

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thin sections were also studied by optical microscopy at the Biomineralizations and Sedimentary Environments Laboratory (BES) - ISTeP (UPMC, Paris). Mineralogical and

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element geochemical analyses were carried out at the same laboratory. All clay and phosphate samples were analyzed by X-ray diffraction (XRD) using the powder method. Samples were manually crushed in an agate mortar to obtain a fine (10 microns) powder. The analyses were conducted with a BRUKER AXS D2-phaser diffractometer. The apparatus is equipped with a theta/2theta device, a copper anticathode tube (30 k, 10 mA) to a wavelength of λCuKα = 1.5418 Ǻ and a rapid detector LynxEye. All samples were studied following the same protocol. The acquisition range of the spectrum is between 5 and 75° 2θ with a step of 0.020231° 2θ and a time per step of 0.1 s. Interpretations of the spectra were performed using the MacDiff software. The semi-quantitative estimation of the relative proportions of mineral species (Fig. 2, circular charts) was derived from the

ACCEPTED MANUSCRIPT calculation of the corrected surfaces of the major peak for each mineral identified (Moore and Reynolds, 1997).

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Elementary geochemical analyses were performed on the phosphorite samples. The

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phosphorus percentage was determined by a UV Visible absorption spectrophotometer

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Spectronic 301 - MILTON ROY; fluorine dosing was carried out using a combined selective electrode of fluoride ions (DC219-F model of METTLER TOLEDO); barium, strontium, iron and manganese contents determined by ICP-AES with a HORIBA JOBIN YVON JY2000

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instrument; calcium and magnesium were measured with flame atomic absorption

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spectrophotometer using a HITACHI Z-8100 device. The uncertainty of measurements is ± 5%.

5.1. SEM observations

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5. Results

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Optical microscopy (Fig. 4a) shows that samples SBA02 and SB02 (phosphate horizon #1) are comprised of a matrix that cements an abundance of diatoms. The SEM observations

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also reveal that all the samples from Sunbarua (SB02, SBA02, SBA23, SBA36) and Kapcheberek (KBK24, KBK26) are composed of micrometric crystals of apatite constituting the matrix (Fig. 4b). The EDS analyses, performed at eight spots on the SB02 thin section, indicate varying contents of different chemical constituents (Table 1a). Other elements were also incorporated in the phosphate matrix in smaller proportions. The CaO/P2O5 average ratio (n = 8 analyses) obtained is 1.5, compatible with that of the carbonate-fluorapatite reference (specimen from Wheal Franco, Devon, England; Palache et al., 1951) (Table 1d). Figures 4a and 4b show the abundant presence of diatoms in the phosphate matrix of the basal phosphate layer (SB02, SBA02) (Fig. 2). There is only one species, Aulacoseira granulata with well-preserved frustules (Fig. 4c). In thin sections, mineralization was also

ACCEPTED MANUSCRIPT observed inside the diatom frustules. Fine needle shaped crystals of apatite (1-3 microns long) (Fig. 4d), as well as clays appear to develop from the walls of diatoms, which form a support,

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until they fill the interior of the frustule (Fig. 4e).

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In contrast, in the upper phosphate layers from Sunbarua (SBA23, SBA36) and

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Kapcheberek (phosphate horizon #2 and #3), diatoms are not clearly visible in the phosphate matrix. Only small fragments of diatoms are observed in association with external molds, which suggest their prior dissolution. Sample SBA36 shows frustules being dissolved in the

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phosphate matrix (Fig. 4f).

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Nodular clays, K-feldspars and quartz are also incorporated into the matrix (Fig. 4g). Silicates are present in much higher proportions in samples SBA23, KBK26 and KBK24, giving the matrix a different appearance compared with SBA02 and SB02.

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The SB02 sample (phosphate horizon #1) is characterized by the localised presence of tubular structures enclosed in the phosphate matrix, with an inner diameter of 2-5 μm and a

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variable length (20-80 μm), and sometimes branched (Fig. 5a). The EDS analyses reveal that their walls are composed only of CaCO3 crystals (Fig. 5b). Some tubes are constituted of

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much smaller calcitic crystals, giving a smoother appearance to the structure. Filaments, passing through the tubular structures and partially covered with calcite crystals, are also observed (Fig. 5b). Apart from these calcitic forms, the structure and composition of the SB02 sample are identical with those of SBA02. The sandy clays that enclose the phosphate layers consist of clayey and volcano-clastic minerals, in association with a few fragments of siliceous sponge spicules and A. granulata (Fig. 4h). The clay layer beneath the basal phosphate layer contains well-preserved diatoms that are present in large numbers, sometimes in clusters, but are not as abundant as in the phosphate layer (SB02, SBA02). In contrast, the clayey sediments above the basal phosphate layer contain fewer altered and fractured A. granulata frustules. Small diatom fragments

ACCEPTED MANUSCRIPT presumably belonging to another unidentifiable diatom species are also observed in this clay layer. Moreover, no trace of phosphate is detected by EDS analysis in these samples, which

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are composed only of kaolinite, smectite, quartz and feldspars (see pie charts in Fig. 2 and

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mineralogical results below).

5.2. X-Ray Diffraction analyses

All of the phosphorites are composed of between 35 and 95% phosphate (Table 2 and

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pie charts on Fig. 2). The phosphate mineral constituting these layers is carbonate-fluorapatite

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(francolite), characterized by XRD (Fig. 6) and confirmed by EDS and elemental geochemistry analyses. The other minerals observed are quartz (0-10%), feldspar (sanidine; ~5-25%), kaolinite (~5-20%) and smectite (~0-25%). Sample SB02 (phosphate horizon #1)

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contains ~20% calcite, which was not observed in other samples and which is related to the presence of the calcitic tubular structures noted above (Fig. 7).

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For Sunbarua, the phosphatic fraction ranges between ~60-95%, with the basal phosphorite layer (SBA02) containing ~95% apatite (Figs. 6, 7). Its lateral equivalent SB02

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has a much lower relative proportion of apatite (~60%) explained by the abundance of calcite. The second layer SBA23 shows a decrease of apatite to ~65%, and a relatively increased proportion of smectite and sanidine compared to the other phosphate samples. The last layer, SBA36 (phosphate horizon #3), is composed of 80% apatite, similar to SBA02. The two phosphate layers (phosphate horizon #2 and #3) at Kapcheberek (KBK24, KBK26; Fig. 6) have the same proportion of apatite (~40%), which is much lower than for the Sunbarua samples (Fig. 7). The detrital fraction is much higher in the samples from Kapcheberek compared to those from Sunbarua (Fig. 7). The sandy clays enclosing the phosphate layers at the sites of Sunbarua and Kapcheberek present an assemblage of kaolinite (25-45%), smectite (5-50%), quartz (10-

ACCEPTED MANUSCRIPT 20%), sanidine (15-45%) and hematite (0-5%) in variable proportions (Table 2, Fig. 2). Apatite is not observed in the clay layers enclosing phosphate layers, neither in the XRD

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spectra nor in SEM observations.

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5.3. Geochemical analyses

The P2O5 content in the four samples of lacustrine phosphate from Sunbarua is between 30% and 34% (Table 1b). The SBA02 sample, located at the base of the Sunbarua

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sedimentary succession and having the highest proportion of apatite (Table 2, Fig. 2), is the

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richest in P2O5. The value measured on the KBK26 sample, from Kapcheberek (phosphate horizon #2), is lower (~25%), compatible with the mineralogical analysis indicating an apatite proportion of only 38% (Table 2, Fig. 2).

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The CaO values are similar (~43%) for SBA02, SBA23 and SBA36, but lower for KBK26 (~33%). The CaO/P2O5 ratio remains constant (1.3-1.4) for these four samples

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(Table 1). Only the sample SB02 (phosphate horizon #1) shows an abnormally high value of CaO (~60%), thus causing an increase of the CaO/P2O5 ratio (2.0), which reflects the presence

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of calcitic tubular structures.

The phosphate samples KBK26, SBA02, SBA23 and SBA36 contain low contents of BaO (0.005% to 0.03%), SrO (0.05%) and MnO (0.20% to 0.56%). Only sample SB02, which is composed of 20% calcite shows higher values (0.12% BaO; 0.09% SrO; 1.19% MnO) (Fig. 8). The four samples from Sunbarua show similar fluorine contents (2.2% to 2.4%) with a constant F/P2O5 ratio of 0.1. Sample KBK26, from Kapcheberek (phosphate horizon #2), is depleted in fluorine (1.1%) and has an F/P2O5 ratio significantly lower, equal to 0.04 (Table 1b, Fig. 8).

ACCEPTED MANUSCRIPT The contents of Fe2O3 (2.4-4.3%) and MgO (0.18-0.92%) are high in phosphatic layers compared to the carbonate-fluorapatite reference standard (specimen from Wheal Franco,

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Devon, England; Palache et al., 1951), particularly for KBK26 (Table 1d, Fig. 8).

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6. Discussion 6.1. Characterization of phosphorite layers

The basal layer (phosphate horizon #1), observed only at Sunbarua, is composed

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almost entirely of phosphate. The phosphatic fraction decreases in the upper layers (phosphate

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horizon #2 and #3) but remains high in these samples (Table 2, Fig. 7), suggesting input of phosphorus into the lake from the surrounding catchment. These three phosphate horizons correspond to a clear break in the clayey sedimentation and appear to have resulted from

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abrupt and repetitive events. This may reflect a recurrence in the environmental conditions associated with their genesis.

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The second and third phosphate horizons at Kapcheberek have relative proportions of apatite significantly lower than that at Sunbarua (Table 2, Fig. 7). The contents of CaO and

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P2O5 are also lower in the sample analysed from Kapcheberek. However, the CaO/P2O5 ratio remains consistent with the samples from Sunbarua (Table 1b). SEM observations confirm that the samples show the same characteristics (micritic matrix of apatite with a more or less important incorporation of detrital elements) (Fig. 4). This decrease in the proportion of apatite results from a dilution of phosphate in the detrital fraction, which is much higher at Kapcheberek due to its more proximal location (Fig. 7). The sample SB02 (phosphate horizon #1) shows a higher CaO content (Table 1a, b), probably related to the presence of calcitic tubular structures described above (Fig. 5). Several hypotheses have been considered concerning the identification of these calcitic forms, such as fungal hyphae or bacterial or algal encrustations (Verrecchia and Verrecchia, 1994). Several

ACCEPTED MANUSCRIPT studies of phosphorites, of various ages, show phosphate crusts that are very similar in morphologies and sizes (Zhegallo et al., 2000; Rozanov, 2005), sometimes associated with

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filaments (Rao et al., 1992), and interpreted as fossil cyanobacterial mats. Furthermore,

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bacterial filaments passing through silicified cyanobacterial sheaths are observed in hot

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springs at Lake Bogoria (Renaut et al., 1998). These structures are almost identical to those observed in the present study. However, here, they comprise tubular structures composed of calcite, sometimes branched, which is compatible with the sheath descriptions of some

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species of calcified cyanobacteria made by Riding (2011). These tubular forms, observed very

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locally in one phosphate horizon in the Lukeino Formation, probably correspond to fossil mats of filamentous cyanobacteria whose sheaths were mineralized. The four phosphatic samples from Sunbarua (phosphate horizon #1, #2 and #3) have

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the same F/P2O5 ratio, but the sample from Kapcheberek (phosphate horizon #2) shows a high deficit in fluorine (Table 1b, Fig. 8). Chemical analyses on water from Lakes Baringo and

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Bogoria, which are the current graben lakes closest to the Lukeino Formation deposits, show that fluorine concentrations are higher in hydrothermal springs and lacustrine waters in their

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immediate vicinity (Tiercelin, 1981; Tiercelin and Vincens, 1987). The fluorine contents also seem to be more important near hot springs (mean of 90°C) than warm springs; and with concentrations sufficient to precipitate fluorite, and possibly fluorapatite (Owen et al., 2008). These results suggest that fluorine constituting the phosphorites of the Lukeino Formation may reflect hot spring sources. In addition, the stromatolitic carbonates observed at several sites in the Lukeino Formation testify to the presence of hydrothermal springs in Palaeolake Lukeino (Pickford et al., 2009). The lower fluorine content at Kapcheberek could result from the site being located at a greater distance from former hydrothermal inputs. The variations observed in the degree of preservation of diatom frustules between samples (especially the sample SBA36 showing frustules being dissolved) is likely due to a

ACCEPTED MANUSCRIPT local increased of pH in the lake, as observed in extant Lake Bogoria (Tiercelin, 1981; Tiercelin and Vincens, 1987), leading to silica dissolution, and which may reflect a more

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confined environment.

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Apart from this fluorine depletion and the calcium enrichment in the SB02 sample,

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probably due to calcified cyanobacteria, the three phosphate horizons of the Lukeino Formation are similar in terms of mineralogy, chemical composition and structure. This likely indicates the same mechanism of phosphogenesis.

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Results of this study can be compared to geochemical analyses performed by different

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authors on marine phosphorites (Rao et al., 1992, 2008; Brookfield et al., 2009), lacustrine phosphorites (Swirydsczuk et al., 1981; Ogihara and Ishiwatari, 1998) and phosphorites linked to guano (Van Kauwenbergh, 1991), of various ages and from diverse geographic

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locations (Table 1d). The contents of CaO and P2O5 as well as CaO/P2O5 ratios obtained in the majority of studies are similar to our results. The fluorine content is overall a little higher

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in these studies, but the F/P2O5 ratios of several phosphorites and the francolite used as a reference (specimen from Wheal Franco, Devon, England; Palache et al., 1951) are consistent

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with the Lukeino Formation phosphorites. The most significant difference is the high content of Al2O3 and especially of Fe2O3 in the phosphorites from Lukeino (Table 1a, b), which is related to the detrital components characteristic of the formation.

6.2. Environmental conditions and deposition mechanisms of the Lukeino lacustrine phosphates The deposition of phosphate layers in lacustrine basins is potentially the result of a set of several processes that occur under specific physicochemical and environmental conditions. In the case of the Lukeino Formation, the combination of three major phenomena likely contributed to phosphogenesis:

ACCEPTED MANUSCRIPT 1) Intense runoff Phosphorus in lakes is ultimately derived from the catchment and reflects weathering of

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continental rocks and soils, with biological processes playing an important role in the

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phosphorus cycle. The phosphate layers are included in a thick series of sandy-clay deposits

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dominated by detrital minerals (kaolinite, sanidine, quartz), suggesting a high runoff and detrital sediment input during the deposition of this interval of the Lukeino Formation. Dissolution of minerals from volcanic activity (Ca-plagioclases and ferromagnesian minerals)

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releases Ca, Fe and Mn as oxide-hydroxides into the sedimentary environment. Oxidizing

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conditions in the lake promote precipitation of FeOOH (goethite) and MnOOH (groutite), which will likely play a role in phosphogenesis (Cosmidis et al., 2014). Subsequently, a change in climatic conditions may induce the development of a vegetal cover. The intense

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runoff on a soil fixed by the vegetal cover promoted the massive inflow of organic matter, thus phosphorus, into Palaeolake Lukeino with no or few particular materials. This induces a

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decrease of sedimentation rates.

2) Extreme abundance of diatoms

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Phosphorus in lakes acts as a limiting nutrient (Guildford and Hecky, 2000; Ruttenberg, 2014); its abundance causes an intense increase in primary production. The extremely large quantities of monospecific diatoms observed in the phosphorites may thus reflect a diatom bloom of the species A. granulata. This species is an extant freshwater planktonic taxon that develops optimally under a pH of 7.8-8.2 (Gasse, 1986). This coincides with the pH necessary for phosphogenesis (Knudsen and Gunter, 2002). In addition, this diatom species has already been observed in association with phosphate minerals in lacustrine deposits (Stamatakis and Koukouzas, 2001) but not, to our knowledge, in similar proportions (~60-80%) as those observed in Palaeolake Lukeino (Fig. 4a). 3) Development of anoxic to suboxic conditions in lake bottom waters

ACCEPTED MANUSCRIPT The diatom blooms should induce a decrease in oxygen concentrations of the lake, which reflects general eutrophication, with the establishment of stagnant anoxic to suboxic deep

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waters, would provide essential conditions for phosphogenesis (Mackey and Paytan, 2009;

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Ruttenberg, 2014). The cells of diatoms, deposited on the bottom of the lake, provide organic

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matter that is degraded by bacterial activity (de las Heras et al., 1989). The oxidation of organic matter releases CO2, H2O, NH3 and H3PO4 close to the redoxcline (Ruttenberg, 2014). Secondly, orthophosphate is combined with FeOOH to form an unstable iron-phosphate

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(Jilbert and Slomp, 2013; Cosmidis et al., 2014). These particles are entrained into the

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suboxic zone, where they are subjected to redox cycling. Fe is reduced to Fe(II) and returns in solution to the oxic zone, where it is transformed into iron oxide. The released PO4 is associated with CO3, calcium and fluorine in the environment to form francolite. Francolite

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crystals, developed from the walls of diatoms, which form a support and filling the diatom fossil frustules (Fig. 4e), indicate that sedimentation of phosphate layers, clearly

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autochthonous, was initiated inside diatoms. The amount of phosphorus contained in the organic matter from diatoms is insufficient

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to explain the phosphate layers observed at Lukeino. A high additional input of detrital organic matter from the surrounding land (leaves, wood, plants, etc) is thus likely. This suggests the probable presence of a forest cover in the catchment at the time of deposition, which may also correspond to riparian vegetation, with rivers, which could transfer organic matter to the lake from farther away. This hypothesis is consistent with other studies performed on the Lukeino Formation. According to Bamford et al. (2013), studies carried out on fossil leaves contained in diatomaceous layers suggest that “the vegetation was a deciduous forest or woodland with open areas nearby”. Moreover, the analysis performed by Roche et al. (2013) on the teeth of large herbivores also testifies to open areas with the presence of patches of woodland. Dung of these large vertebrates, whose fossils are found in

ACCEPTED MANUSCRIPT abundance in the Lukeino Formation, might also have been an additional input of phosphorus into the lake (Grey and Harper, 2002).

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It is likely that phosphogenesis process stopped abruptly due to the resumption of

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intense runoff, associated with massive inputs of detrital particles (resumption of clay

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sedimentation). This may be related to a modification of vegetal cover, which may be linked to a variation of the annual rainfalls distribution. This change of rainfall regime caused movements in the water column, possibly in association with temperature changes, and

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therefore a re-oxygenation of the lake deep waters and the end of degradation of organic

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matter by bacteria. The restoration of the lake after this eutrophication phase allowed the development of a higher biodiversity, with sponge spicules and different diatom species observed in the clay layers above the phosphate deposits.

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Therefore, the succession of these three phosphate horizons, marking a break in clastic sedimentation, likely testifies to several variations of the vegetal cover, contemporary with

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7. Conclusions

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Orrorin tugenensis, in the catchment of the Lake Lukeino at the time of deposition

The identification of three well defined, narrow phosphate horizons in the late Miocene Lukeino Formation, Gregory Rift Valley, Kenya, is important for four main reasons. 1. The quantity of lacustrine phosphorites known in the world is not high and phosphorites of sedimentary origin are rare in East Africa. 2. The phosphorites of the Lukeino Formation show that diatoms played a major role in phosphogenesis mechanisms. The abundance of diatoms indicates an optimum pH of 7.8-8.2 in the lake at the time of deposition. 3. The establishment of several phosphate horizons testifies to successive eutrophication events during the existence of Palaeolake Lukeino. Furthermore, studies show evidence

ACCEPTED MANUSCRIPT of the role of orbital parameters on climate and environment change in Africa (deMenocal, 2004; Maslin et al., 2014). It would be interesting to study whether these

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successive phosphate events, associated with variations in environmental conditions,

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may be linked with changes in orbital parameters and thus climate.

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4. The three phosphate events were related to periods of intense runoff associated with increased inputs of organic matter into the lake, suggesting the presence of a forest cover in at least part of the catchment. These deposits thus provide important

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palaeoenvironmental data concerning the middle part of the Lukeino Formation and thus

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indicating the environment in which the emergence of bipedalism in the earliest recorded East African hominids probably occurred.

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Acknowledgements

Authorization to carry out research in the Tugen Hills was provided by the

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Government of Kenya (The Ministry of Research and Technology, at the time the Ministry of Higher Education, Science and Technology). Local affiliation is with the Orrorin Community

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Organisation. The field work was financed by the CNRS (GDRI 193, UMR 7207). The authors acknowledge the French Ministry of Foreign Affairs (Advisory Commission on Archaeological Research Abroad), Egerton University, Kenya, as well as the colleagues of the Kenya Palaeontology Expedition for their input on the field. We wish greatly to thank Omar Boudouma (ISTeP-UPMC, France) for the acquisition of SEM images; Marylène Person (ISTeP-UPMC, France) for her help regarding geochemical analyses; Joël Laval (ULg, Belgium) for producing the thin sections. We are grateful to Emmanuelle Javaux and Philippe Gerrienne (ULg, Belgium) for their advice regarding calcitic tubular structures. The authors wish to thank K. Föllmi and an anonymous reviewer for their constructive comments on the manuscript.

ACCEPTED MANUSCRIPT

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References

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Bamford, M.K., Senut, B., Pickford, M., 2013. Fossil leaves from Lukeino, a 6-million-year-

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old Formation in the Baringo Basin, Kenya. Geobios 46, 253-272.

Brookfield, M.E., Hemmings, D.P., Straaten, P.V., 2009. Paleoenvironments and origin of the sedimentary phosphorites of the Napo Formation (Late Cretaceous, Oriente Basin, Ecuador).

NU

Journal of South American Earth Sciences 28, 180-192.

MA

Cathcart, J.B., Schmidt, D.L., 1977. Middle Paleozoic sedimentary phosphate in the Pensacola Mountains, Antarctica. Geological Survey Professional Paper 456, E1-E18. Chapman, G.R., Lippard, S.J., Martyn, J.E., 1978. The stratigraphy and structure of the

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D

Kamasia Range, Kenya Rift Valley. Journal of the Geological Society of London 135, 265281.

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Cook, P.J., McElhinny, M.W., 1979. A reevaluation of the spatial and temporal distribution of sedimentary phosphate deposits in the light of plate tectonics. Economic Geology 74, 315-

AC

330.

Cosmidis, J., Benzerara, K., Morin, G., Busigny, V., Lebeau, O., Jézéquel, D., Noël, V., Dublet, G., Othmane, G., 2014. Biomineralization of iron-phosphates in the water column of Lake Pavin (Massif Central, France). Geochimica and Cosmochimica Acta 126, 78-96. De las Heras, X., Grimalt, J.O., Albaigés, J., Julia, R., Anadon, P., 1989. Origin and diagenesis of the organic matter in Miocene freshwater lacustrine phosphates (Cerdanya Basin, Eastern Pyrénées). Organic Geochemistry 14, 667-677. Deino, A.L., Tauxe, L., Monaghan, M., Hill, A., 2002.

40

Ar/39Ar geochronology and

paleomagnetic stratigraphy of the Lukeino and lower Chemeron Formations at Tabarin and Kapcheberek, Tugen Hills, Kenya. Journal of Human Evolution 42, 117-140.

ACCEPTED MANUSCRIPT deMenocal, P.B., 2004. African climate change and faunal evolution during the PliocenePleistocene. Earth and Planetary Science Letters 220, 3-24.

T

Filippelli, G.M., 2008. The global phosphorus cycle: past, present, and future. Elements 4, 89-

IP

95.

SC R

Föllmi, K.B., 1996. The phosphorus cycle, phosphogenesis and marine phosphate-rich deposits. Earth-Science Reviews 40, 55-124.

Forti, P., Galli, E., Rossi, A., 2003. Minerogenesis of volcanic caves of Kenya. International

NU

Journal of Speleology 32, 3-18.

MA

Gasse, F., 1986. East African diatoms. Taxonomy, ecological distribution. Bibliotheca diatomologica, 11. J. Cramer, Berlin, 202pp.

Grey, J., Harper, D.M., 2002. Using stable isotope analyses to identify allochthonous inputs to

Studies 38, 245-250.

TE

D

Lake Naivasha mediated via the hippopotamus gut. Isotopes in Environmental and Health

CE P

Guildford, S.J., Hecky, R.E., 2000. Total nitrogen, total phosphorus, and nutrient limitation in lakes and oceans: Is there a common relationship? Limnology and Oceanography 45, 1213-

AC

1223.

Hill, A., Drake, R., Tauxe, L., Monaghan, M., Barry, J.C., Behrensmeyer, A.K., Curtis, G., Jacobs, B.F., Jacobs, L.L., Johnson, N., Pilbeam, D., 1985. Neogene palaeontology and geochronology of the Baringo Basin, Kenya. Journal of Human Evolution 14, 759–773. Hill, A., Curtis, G., Drake, R., 1986. Sedimentary stratigraphy of the Tugen Hills, Baringo, Kenya. In: Frostick, L.E., Renaut, R.W., Reid, I., Tiercelin, J.-J. (Eds.), Sedimentation in the African Rifts. Geological Society of London Special Publication 25, pp. 285-295. Jilbert, T., Slomp, C.P., 2013. Iron and manganese shuttles control the formation of authigenic phosphorus minerals in the euxinic basins of the Baltic Sea. Geochimica and Cosmochimica Acta 107, 155-169.

ACCEPTED MANUSCRIPT Kingston, J.D., Jacobs, B.F., Hill, A., Deino, A., 2002. Stratigraphy, age and environments of the late Miocene Mpesida Beds, Tugen Hills, Kenya. Journal of Human Evolution 42, 95-116.

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Knudsen, A.C., Gunter, M.E., 2002. Sedimentary Phosphorites – An Example: Phosphoria

IP

Formation, Southeastern Idaho, USA. In: Kohn, M.J., Rakovan, J., Hughes, J.M. (Eds.),

SC R

Phosphates – Geochemical, Geobiological, and Materials Importance. Reviews in Mineralogy and Geochemistry, 48. Mineralogical Society of America, Washington, DC, pp. 363-389. Mackey, K.R.M., Paytan, A., 2009. Phosphorus Cycle. In: Schaechter, M. (Ed.),

NU

Encyclopedia of Microbiology (Third Edition). Academic Press, Oxford, pp. 322-334.

MA

Maslin, M.A., Brierley, C.M., Milner, A.M., Schultz, S., Trauth, M.H., Wilson, K.E., 2014. East African climate pulses and early human evolution. Quaternary Science Reviews 101, 117.

TE

D

Moore, D.M., Reynolds, R.C., 1997. X-Ray Diffraction and the Identification and Analysis of Clay Mineral (Second Edition). Oxford University Press, Oxford, 378pp.

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Notholt, A.J.G., 1991. African phosphate geology and resources: a bibliography, 1979-1988. Journal of African Earth Sciences 13, 543-552.

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Ogihara, S., Ishiwatari, R., 1998. Unusual distribution of hydrocarbons in a hydrothermally altered phosphorite nodule from Kusu Basin, northern Kyushu, Japan. Organic Geochemistry 29, 155-161.

Orris, G.J., Chernoff, C.B., 2004. Chapter 20 Review of world sedimentary phosphate deposits and occurrences. In: Hein, J.R. (Ed.), Handbook of Exploration and Environmental Geochemistry, 8. Elsevier Science, Amsterdam, pp. 559-573. Owen, R.A., Owen, R.B., Renaut, R.W., Scott, J.J., Jones, B., Ashley, G.M., 2008. Mineralogy and origin of rhizoliths on the margins of saline, alkaline Lake Bogoria, Kenya Rift Valley. Sedimentary Geology 203, 143-163.

ACCEPTED MANUSCRIPT Palache, C., Berman, H., Frondel, C., 1951. Dana’s System of Mineralogy (Seventh Edition). John Wiley and Sons, INC, New York, 1124pp.

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Pickford, M., 1974. Stratigraphy and Palaeoecology of five late Cainozoic formations in the

IP

Kenya Rift Valley. Unpublished PhD Thesis, University of London.

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Pickford, M., 1978. Stratigraphy and mammalian palaeontology of the late-Miocene Lukeino Formation, Kenya. In: Bishop, W.W. (Ed.), Geological Background to Fossil Man. Scottish Academic Press, Edinburgh, pp. 263-278.

NU

Pickford, M., Senut, B., 2001. The geological and faunal context of Late Miocene hominid

MA

remains from Lukeino, Kenya. Comptes Rendus de l’Académie des Sciences de Paris-Series IIA, Earth and Planetary Sciences 332, 145-152.

Pickford, M., Senut, B., Gommery, D., Treil, J., 2002. Bipedalism in Orrorin tugenensis

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D

revealed by its femora. Comptes Rendus Palevol 1, 191-203. Pickford, M., Senut, B., Cheboi, K., 2009. The Geology and Palaeobiology of the Tugen

1, 4-133.

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Hills, Kenya: Rift Tectonics, Basin Formation, Volcanics and Sedimentation. Geo-Pal Kenya

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Rao, P.V., Lamboy, M., Natarajan, R., 1992. Possible microbial origin of phosphorites on Error Seamount, northwestern Arabian Sea. Marine Geology 106, 149-164. Rao, P.V., Hegner, E., Naqvi, S.W.A., Kessarkar, P.M., Ahmad, S.M., Raju, D.S., 2008. Miocene phosphorites from the Murray Ridge, northwestern Arabian Sea. Palaeogeography, Palaeoclimatology, Palaeoecology 260, 347-358. Renaut, R.W., Jones, B., Tiercelin, J.-J., 1998. Rapid in situ silicification of microbes at Loburu hot springs, Lake Bogoria, Kenya Rift Valley. Sedimentology 45, 1083-1103. Riding, R., 2011. Calcified cyanobacteria. In: Reitner, J., Thiel, V. (Eds.), Encyclopedia of Geobiology. Encyclopedia of Earth Science Series. Springer, Heidelberg, pp. 211–223.

ACCEPTED MANUSCRIPT Roche, D., Ségalen, L., Senut, B., Pickford, M., 2013. Stable isotope analyses of tooth enamel carbonate of large herbivores from the Tugen Hills deposits: Palaeoenvironmental context of

T

the earliest Kenyan hominids. Earth and Planetary Science Letters 381, 39-51.

IP

Rozanov, A.Y., 2005. Bacterial paleontology. In: Hoover, R.B., Rozanov, A.Y., Paepe, R.R.

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(Eds.), Perspectives in Astrobiology. IOS Press, Amsterdam, pp. 132-145. Ruttenberg, K.C., 2014. 10.13 - The global phosphorus cycle. In: Holland, H.D., Turekian, K.K. (Eds.), Treatise on Geochemistry (Second Edition). Elsevier, Oxford, pp. 499-558.

NU

Sawada, Y., Pickford, M., Senut, B., Itaya, T., Hyodo, M., Miura, T., Kashine, C., Chujo, T.,

MA

Fujii, H., 2002. The age of Orrorin tugenensis, an early hominid from the Tugen Hills, Kenya. Comptes Rendus Palevol 1, 293-303.

Schlüter, T., Hampton, C., 1997. Geology of East Africa. Beiträge zur regionalen Geologie

TE

D

der Erde, 27. Berlin, Borntaeger, 484pp.

Senut, B., Pickford, M., Gommery, D., Mein, P., Cheboi, K., Coppens, Y., 2001. First

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hominid from the Miocene (Lukeino Formation, Kenya). Comptes Rendus de l’Académie des Sciences de Paris-Series IIA, Earth and Planetary Sciences 332, 137-144.

AC

Stamatakis, M.G., Koukouzas, N.K., 2001. The occurrence of phosphate minerals in lacustrine clayey diatomite deposits, Thessaly, Central Greece. Sedimentary Geology 139, 3347.

Swirydczuk, K., Wilkinson, B.H., Smith, G.R., 1981. Synsedimentary lacustrine phosphorites from the Pliocene Glenns Ferry Formation of southwestern Idaho. Journal of Sedimentary Petrology 51, 1205-1214. Szilas, C., Bender Koch, C., Msolla, M.M., Borggaard, O.K., 2008. The reactivity of Tanzanian Minjingu phosphate rock can be assessed from the chemical and mineralogical composition. Geoderma 147, 172-177.

ACCEPTED MANUSCRIPT Tiercelin, J.J., 1981. Rifts continentaux: tectonique, climats, sédiments. Exemples: la sédimentation dans le nord du Rift Gregory (Kenya) et dans le Rift de l’Afar (Ethiople)

T

depuis le Miocène. Thèse Doct. Etat, Université Aix-Marseille II, 260pp.

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Tiercelin, J.J., Vincens, A. (Eds.) 1987. Le Demi-graben de Baringo-Bogoria, Rift Gregory,

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Kenya. Bulletin du Centre de Recherche et d’Exploration, Elf Aquitaine Production, Pau, France 11, 249-540.

Tiercelin, J.-J., Lezzar, K.-E., 2002. A 300 million years history of rift lakes in Central and

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East Africa: an updated broad review. In: Odada, E.O., Olago, D.O. (Eds.), The East African

MA

Great Lakes: Limnology, Paleolimnology and Biodiversity. Advances in Global Change Research 12, pp. 3-60.

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Fertilizer Research 30, 127-150.

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Van Kauwenbergh, S.J., 1991. Overview of phosphate deposits in East and Southeast Africa.

Verrecchia, E.P., Verrecchia, K.E., 1994. Needle-fiber calcite: A critical review and a

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proposed classification. Journal of Sedimentary Research A64, 650-664. Williams, L.A.J., Chapman, G.R., 1986. Relationships between major structures, salic

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volcanism and sedimentation in the Kenya Rift from the equator northwards to Lake Turkana. In: Frostick, L.E., Renaut, R.W., Reid, I., Tiercelin, J.-J. (Eds.), Sedimentation in the African Rifts. Geological Society of London Special Publication 25, pp. 59-74. Zhegallo, E.A., Rozanov, A.Yu., Ushatinskaya, G.T., Hoover, R.B., Gerasimenko, L.M., Ragozina, A.L., 2000. Atlas of Microorganisms from Ancient Phosphorites of Khubsugul (Mongolia). Huntsville, Alabama, USA, NASA, 167pp.

Table captions

ACCEPTED MANUSCRIPT Table 1. Chemical composition of phosphate deposits. The location of the samples is in Figure 2. KBK – Kapcheberek; SBA and SB – Sunbarua. All values are in %. na = not

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analyzed. * Analyses performed at UPMC – Paris. *2 Calculated in this paper from the

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bibliographic data. (a) EDS microprobe analyses realized on several spots of the sample SB02

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(thin section). (b) Geochemical analyses of studied phosphate beds from Kapcheberek and Sunbarua. (c) SRM values used for geochemical analyses (bone ash: hydroxylapatite). (d) For comparison, chemical composition by EDS analyses of other sedimentary phosphorites

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(composed of francolite) from different studies: 1) Reference carbonate-fluorapatite,

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specimen from Wheal Franco, Devon, England (Palache et al., 1951). 2) Massive slab phosphorites, Error Seamount phosphorites, Arabian Sea, Oligocene-Lower Miocene (Rao et al., 1992). 3) Phosphorite nodules, sample “MRP1 nodules”, Murray Ridge, Arabian Sea,

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Miocene (Rao et al., 2008). 4) Phosphate Layer (average), marine phosphorites, Napo Formation, Ecuador, Late Cretaceous. EDS analyses except F and CaO/P2O5 and F/P2O5

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ratios (Brookfield et al., 2009). 5) “Phosphorite nodule non-altered”, lacustrine, freshwater, Kusu Formation, Japan, 0.5-0.2 Ma (Ogihara and Ishiwatari, 1998). 6) “Phosphate cements”,

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lacustrine, freshwater, Glenns Ferry Formation, Idaho, USA, Pliocene (Swirydczuk et al., 1981). 7) Lacustrine phosphate deposits linked to a transformation of guano, sample “Concentrate”, Minjingu (Lake Manyara Basin), Tanzania, Pleistocene (Van Kauwenbergh, 1991).

Table 2. Mineralogical composition of phosphate layers and enclosing sandy-clay sediments. All values are in percent; the uncertainty is ± 5%. See Figure 2 for the stratigraphic position of samples. KBK – Kapcheberek; SBA - Sunbarua. tr. = trace

Figures captions

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Figure 1. Location map showing the position of the Tugen Hills, northwest of Lake Baringo

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(Kenya). The star indicates the location of the studied sites (Sunbarua and Kapcheberek)

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within the Lukeino Formation. The fault network represented on the map is after Pickford and

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Senut (2001).

Figure 2. Synthesis of sedimentological and mineralogical data associated with the

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phosphatic layers at Kapcheberek and Sunbarua. The brown area on the synthetic lithological

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column of the Lukeino Formation corresponds to the stratigraphic position of the Kapcheberek and Sunbarua sedimentary sections. The lithology of the Formation is elaborated from works carried out on several sedimentary successions (Dericquebourg, in

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prep.); the stratigraphy and ages of the Lukeino Formation are after Sawada et al. (2002). Dotted lines are used to correlate the phosphate layers of Kapcheberek and those of Sunbarua.

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Pie charts correspond to mineralogical results from phosphatic layers and enclosing clays, obtained by X-ray diffraction. The apatite proportion in phosphate horizons is shown in light

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green on these charts.

Figure 3. Field photos of lacustrine phosphates from Kapcheberek and Sunbarua. (a) View of the SB02 phosphate layer at the base of Sunbarua. Arrows indicate the location of the phosphatic layer. The layer is about 2 cm thick included in a thick brown silty clay unit. (b) View of the base of the KBK sedimentary section (Kapcheberek), arrows indicate the two phosphatic layers KBK24 and KBK26 observed at the site. (c) Overview of Sunbarua, arrows indicate the position of two phosphate layers (SBA23 and SBA36) and their lateral extent. (d) Detailed view of the base phosphate layer at Sunbarua (SBA02); this layer included in the clayey sedimentary series is indurated and has rhombic jointing.

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Figure 4. Optical microscopy observation (a) and SEM images of phosphate layers (b-g) and

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clay layers (h). (a) Well preserved diatom frustules, enclosed in the beige matrix, and

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constituting the phosphate layer located at the base of Sunbarua (SBA02 sample – thin

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section). (b) Phosphatic horizon components, with the phosphate matrix forming a cement around abundant diatoms (SB02 sample – thin section). (c) Aulacoseira granulata (SBA02 sample - fragment). (d) Apatite crystals in the form of needles and clay minerals inside the

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diatom frustules (SB02 sample – thin section). (e) Francolite matrix which includes feldspars

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and clays. The diatom frustules are also filled with apatite and clays (SB02 sample – thin section). (f) Diatom frustules in the process of dissolution (SBA36 sample - fragment). (g) Nodular clays, quartz and feldspars included in the phosphate matrix (KBK26 sample -

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fragment). (h) Clayey sediments, enclosing phosphate layers, with numerous well-preserved diatoms. The clay layers are also composed of quartz and potassic feldspars (sanidine), and

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contain siliceous fragments of sponge spicules (SB01 sample - fragment).

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Figure 5. SEM images showing the calcitic tubular structures observed only in the sedimentary sample SB02. (a) Tubular structures in the phosphate matrix. (b) Tubular structure. The wall of the tube is covered with calcite crystals.

Figure 6. XRD spectra of two phosphatic samples (SBA02 – Sunbarua and KBK26 – Kapcheberek). Minerals are: A – apatite (carbonate-fluorapatite); K – kaolinite; S – smectite; Q – quartz; F – feldspar (sanidine). The hkl indices of carbonate-fluorapatite are also indicated.

ACCEPTED MANUSCRIPT Figure 7. Ternary diagram showing variations in the mineralogical composition of the phosphate samples between calcite, apatite and KSQ (sum of the Kaolinite, Sanidine and

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Quartz contents). KBK – Kapcheberek; SBA and SB - Sunbarua.

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Figure 8. Ternary diagram showing variations in the chemical composition of phosphate samples. The location of the samples is provided in Fig. 2. KBK – Kapcheberek; SBA and SB

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– Sunbarua.

ACCEPTED MANUSCRIPT Table 1

Mg Sr K2 CaO F Sampl P2O CaO F Fe2O SiO Al2O BaO Mn TiO Na2 total O O O P2O P2O e 5% % % 3% 2% 3% % O%2% O% % % % % 5 5

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Reference

' ' ' ' '

0.9

38.1 55.9 2.5 1.0

0.7

0.3

34.4 55.1 2.6 3.5

2.4

1.1

28.5 46.3 4.5 0.4 17.9 1.0 33.6 37.3 1.4 1.5 15.4 3.8 33.3 56.1 2.5 1.9

5.2

0.1

35.5 53.0 3.3 0.6

5.7

0.9

8.8

2.9

7.3

1.4

30.2 52.8 3.6 0.8

Mean 33.6 51.5 3.1 1.3

'

min.

'

max. 38.1 56.1 4.5 3.5 17.9 3.8

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b)

28.5 37.3 1.4 0.4

This study KBK26 24.8 32.6 1.1 4.1

0.7

0.1

na

na

0.9 2 0.1 8 0.3 3 0.1 8 0.2 7

0.39

0.00 0.41 0.07 0.00 0.12 0.04 0.08 0.00 0.41

0.67

0.0 5 0.0 5 0.0 5 0.0 5 0.0 9

0.42 0.16 0.71 0.60 0.08 0.37 0.43 0.08 0.71

0.07 0.10 0.07 0.13 0.05 0.05 0.21 0.06 0.09 0.05 0.21

0.0 0 0.0 1 0.0 6 0.0 0 0.0 0 0.0 4 0.0 6 0.0 5 0.0 3 0.0 0 0.0 6

100. 0 100. 0 100. 0 100. 0 100. 0 100. 0 100. 0 100. 0 100. 0

1.6 0.1 1.5 0.1 1.6 0.1 1.6 0.2 1.1 0.04 1.7 0.1 1.5 0.1 1.7 0.1 1.5 0.1

-

1.1 0.04

-

1.7 0.2

0.51 na

na

na 79.5 1.4 0.1

0.20 na

na

na 81.1 1.3 0.1

0.56 na

na

na 85.6 1.4 0.1

1.19 na

na

na 97.3 2.0 0.1

na

1.0 0.0 0.00 0.03 na 4 3 3

na

na 93.8 1.3

0.1

1.1 4

-

0.0 0.00 3 2

-

0.81

-

-

0.03

0.0 0.00 3 2

-

-

na

'

SBA23 31.9 42.2 2.2 4.3

na

na

SBA02 33.9 45.9 2.4 2.4

na

na

SB02 30.5 59.8 2.2 3.1

na

na

0.01

0.0 0 0.3 0 0.0 0 0.2 0 5.3 6 0.0 4 0.0 0 0.0 8 0.7 5 0.0 0 5.3 6

na 63.9 1.3 0.04

na

AC

0.00

0.0 0 0.3 9 0.2 7 0.4 4 0.6 4 0.1 4 0.4 9 0.1 5 0.3 2 0.0 0 0.6 4

na

SBA36 31.2 42.4 2.2 3.0

'

0.00

0.24 na

'

'

0.0 5 0.1 6 0.0 5 0.0 0 0.0 4 0.0 5 0.0 3 0.1 5 0.0 7 0.0 0 0.1 6

SC R

'

2.6

NU

'

35.2 55.7 4.2 0.9

MA

'

SB0208 SB0209 SB0210 SB0212 SB0213 SB0214 SB0215 SB0216

D

This study

IP

a)

0.01 0.00 5 0.03 0.12

c) SRM (bone 40.8 51.7 0.1 0.04 na ash) SRM Manufacture (bone 41.1 53.5 0.1 0.09 0.3 r data ash) SRM Mean Laboratory * 42.3 52.8 0.1 0.04 (longterm) number of 3 10 3 5 analys es d) This study

11

8

7

0.00 3

0.0 0.00 97.2 1.3 2 3

- 95.4 1.2

0.00 3

ACCEPTED MANUSCRIPT

2)

Rao, 2008

3) 4) 5) 6) 7)

3.7 1 1.2 0 2.7 3 4.8 6 -

0.34

-

-

2.53 0.25 0.19 0.49 0.05 0.09 14.6 0.78 1 21.6 1.32 1.01 5 0.63

3.7 1.73 0.58 6

-

0.1 0 0.4 9 0.9 4 0.8 2 0.1 1 0.3 2

-

-

-

-

-

MA D TE CE P AC

-

-

-

-

0.1 3 0.0 0.0 - 0.01 0.98 2 6 0.1 - 0.03 - 0.19 1 0.0 0.0 - 4.60 0.01 5 1 -

- 1.07

29.0 41.7 3.1 3.2 1.4 0.89 9.40 1.20 0.20 0 0 0 0 0

NU

Brookfield, 2009 Ogihara, 1998 Swirydsczuk , 1981 Van Kauwenber gh, 1991

53.9 4 51.9 7 47.5 0 46.2 0 34.2 8 52.1 6

-

0.83

T

Rao, 1992

38.1 3 26.4 6 31.2 0 23.3 5 26.9 9 36.2 1

IP

1)

SC R

Palache, 1951

-

-

0.19

-

0.1 0.7 1.30 3 8

-

1,41 0,10 *2 *2 0,05 1.96 *2

-

1.52 0.09

-

1.47 0.13

-

1,27 *2

-

1.44 0.10

-

1,44 0,11 *2 *2

-

ACCEPTED MANUSCRIPT Table 2

Kaolinite %

Sample

Smectite Sanidine Quartz % % %

Hematite Calcite % Apatite % %

min. max.

Phosphate layers 17 20 10 7 4 9 4 20

23 7 13 7 0 23

min. max.

AC

Variation

CE P

TE

KBK24 KBK26 SBA36 SBA23 SBA02 SB02

6 10 tr. tr. tr. 0 10

16 25 7 12 3 3 3 25

4 4 4 3 4 0 4

-

-

-

22 -

38 38 81 66 93 58 38 93

IP

24 23 33 40 35 16 37 44 42 27 22 31 16 44

SC R

14 19 20 11 12 11 14 14 11 10 11 13 10 20

NU

23 18 15 7 17 48 22 5 8 16 27 19 5 48

MA

Variation

35 40 32 42 32 25 27 37 39 43 37 35 25 43

D

KBK23 KBK25 KBK27 SBA37 SBA35 SBA24 SBA22 SBA03 SBA01 SB03 SB01 Mean

T

Clay layers

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

Figure 1

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

Figure 2

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

Figure 3

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

Figure 4

AC

Figure 5

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

Figure 6

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

Figure 7

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

Figure 8

ACCEPTED MANUSCRIPT

Highlights : We identify three horizons of lacustrine phosphorites in Miocene Lukeino Formation



The abundance of Aulacoseira granulata indicates periodic blooms in Palaeolake



The phosphate horizons indicate eutrophication events



The phosphate events are related to intense runoff periods



The deposits suggest a forest cover for the Orrorin hominids environment

AC

CE P

TE

D

MA

NU

SC R

IP

T

