Mineralogy and origin of rhizoliths on the margins of saline, alkaline Lake Bogoria, Kenya Rift Valley

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Sedimentary Geology 203 (2008) 143 – 163 www.elsevier.com/locate/sedgeo

Mineralogy and origin of rhizoliths on the margins of saline, alkaline Lake Bogoria, Kenya Rift Valley Richard Alastair Owen a , Richard Bernhart Owen a,⁎, Robin W. Renaut b , Jennifer J. Scott b , Brian Jones c , Gail M. Ashley d a Department of Geography, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China Department of Geological Sciences, University of Saskatchewan, Saskatoon, Canada SK S7N 5E2 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Canada AB T6G 2E3 d Department of Geological Sciences, Rutgers University, Piscataway, NJ 08854, United States b

c

Received 10 March 2007; received in revised form 16 November 2007; accepted 22 November 2007

Abstract A wide range of rhizoliths occurs around the margins of Lake Bogoria, Kenya. These include root casts, moulds, tubules, rhizocretions, and permineralised root systems. These rhizoliths are variably composed of opaline silica, calcite, zeolites (mainly analcime), fluorite, and possibly fluorapatite, either alone or in combinations. Some rhizoliths are infilled moulds with detrital silicate grains. Most rhizoliths are in situ, showing both vertical and horizontal orientations. Reworked rhizoliths have been concentrated locally to form dense rhizolites. Hot-spring fluids, concentrated by evapotranspiration and capillary evaporation, have provided most of the silica for the permineralisation of the plant tissues. Precipitation involved the growth of silica nanospheres and microspheres that coalesced into homogeneous masses. Calcite rhizoliths formed following evaporative concentration, evapotranspiration, and (or) CO2 degassing of Ca-bearing runoff water that infiltrated the sediment, or by mixing of runoff with saline, alkaline groundwater. Fluorite precipitated in areas where mixing of hot-spring and meteoric waters occurred, or possibly where hot-spring fluids came into contact with pre-existing calcite. Zeolitic rhizoliths formed during a prolonged period of aridity, when capillary rise and evaporative pumping brought saline, alkaline waters into contact with detrital silicate minerals around roots. © 2007 Elsevier B.V. All rights reserved. Keywords: Rhizoliths; Silica; Fluorite; Calcite; Zeolite; Lake Bogoria

1. Introduction Fossilised roots and root traces form in a variety of sediments and have been used to infer past continental environments, depth of the palaeowater table, and palaeoclimatic conditions (e.g., Mount and Cohen, 1984; Hembree and Hasiotis, 2007). Much of the research into these organo-sedimentary structures has been directed toward their macromorphology, orientation and geochemistry, and has focused principally on calcareous materials (e.g., James, 1972; Ward, 1975; Klappa, 1979; Jones and Ng, 1988; Wright, 1992; Wright et al., 1995; Driese et al., 1997; Kleinert and Strecker, 2001; Candy, 2002; Kosir, 2004; Liutkus et al., 2005). Few investigations have reported on the nature and ⁎ Corresponding author. Tel.: +852 26880412; fax: +852 34115990. E-mail address: [email protected] (R.B. Owen). 0037-0738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2007.11.007

formation of non-calcareous fossil root systems. Examples include research into silicified plant roots (e.g., Hendry, 1987; Jones et al., 1998; Nelson et al., 2001; Channing and Edwards, 2003; Channing et al., 2004; Gutiérrez-Castorena et al., 2005, 2006), mixed silica-calcite rhizoliths (e.g., Hendry, 1987; Hill et al., 1995), magnesite rhizoliths (Sanz-Rubio et al., 1999), and iron mineral precipitation around root systems (e.g., Elick et al., 1998; Sundby et al., 1998; Kraus and Hasiotis, 2006). In this paper, we describe and interpret fossil root systems composed of a wide range of minerals that are well preserved in the marginal sediments of Lake Bogoria, a saline, alkaline lake partly fed by hot springs in the central Kenya Rift Valley. Several terms have been used for fossilised roots and root traces. Northrop (1890) used the word “rhizomorph” to describe root-like structures in the Bahamas. Klappa (1980) noted that ‘rhizocretion’ had been widely used for such structures, but then

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proposed the general term ‘rhizolith’ to include all petrified and concretionary accumulations in and around roots. Among others, Cohen (1982) and Mount and Cohen (1984) adopted this term in their studies of fossil roots in the Lake Turkana basin of northern Kenya, and the terminology is now widely used. Klappa (1980) identified five types of rhizoliths: 1) root moulds (tubular voids left after roots have decayed); 2) root casts (former moulds filled by cement or lithified detrital sediment); 3) root tubules (cemented sediment cylinders around root moulds); 4) rhizocretions (pedogenic accumulations of mineral matter around roots); and 5) root petrifactions (replaced organic materials or impregnations of cellular tissues). This classification has been adopted in this study. The rhizoliths at Lake Bogoria are unusual for their diverse mineralogy and morphological variety in a relatively small area. The rhizoliths are variably composed of calcite, opaline silica, fluorite and analcime, reflecting the wide range of fluid compositions that have existed at different times in the lake-marginal soils. The main aims of this paper are therefore to: 1) describe the macro- and micro-morphological characteristics of the rhizoliths; 2) examine their compositional variety; 3) suggest the processes that might have contributed to their formation; and

4) develop models that might be used to interpret similar fossilised roots in ancient sedimentary sequences. The evidence presented shows that rhizoliths in closed-basin lakes are not only a useful tool in reconstructing former lake level fluctuations of variable duration, but are also potentially useful in recognising former hydrothermal activity. 2. The Lake Bogoria Basin Perennial, saline, alkaline Lake Bogoria (∼ 35 km2) lies in a semi-arid region at an elevation of ∼ 990 m, about 30 km north of the equator (Fig. 1), and occupies a half-graben in the central Kenya Rift Valley. The basin is bounded to the east and south by a major border fault that cuts through Miocene and Pliocene basalts and trachytes, and forms the 700 m-high Bogoria and Emsos escarpments (Renaut and Tiercelin, 1994). Grid-faulted Pleistocene trachyphonolites and basalts, tilted to the north and east, form the western margins of the basin. The northern boundary consists of a watershed divide (∼ 999 m) at the northern end of the alluvial-deltaic Sandai Plain. Lake Bogoria lies about 22 m higher than the level of Lake Baringo, a freshwater lake only 22 km to the north.

Fig. 1. Location of study areas at Lake Bogoria, Kenya. (A) General setting of Lake Bogoria. Left insets show general location. (B) Loburu Delta and rhizolith sampling sites. (C) Simplified geology on Sandai Plain (after Renaut, 1993) and sampling area location.

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Organic-rich muds and sodium carbonate evaporites underlie the deepest parts of the lake (∼ 10 m maximum depth). These deposits are surrounded laterally by siliciclastic sediments laid down in lake-marginal alluvial fans and deltas, including the prominent Loburu Delta (Fig. 1) where many types of rhizoliths are preserved (Renaut, 1982; Renaut and Tiercelin, 1994). Coarse sands and gravels that form regressive terraces around the shoreline developed during former periods of higher lake level (Renaut and Owen, 1991). Stromatolitic limestones that coat bedrock outcrops, boulders and gravels around the lake margins are another visible record of former higher lake levels and fresher waters. The major water inputs derived are from the Sandai River to the north and Emsos River to the south, and from about 100 small ephemeral streams, and three groups of hot springs (Renaut and Tiercelin, 1994). Recharge from the Sandai River has declined recently following water abstraction for irrigation. Acacia spp., Euphorbia spp., and Commiphora spp. dominate vegetation in the adjacent uplands, with Acacia also occurring locally on lowland plains, particularly near streams (Vincens, 1986). The modern lake-marginal plains are variably covered with grasses such as Sporobolus spicatus and small shrubs. Halophytes such as Cyperus laevigatus are common near the shore, with areas of Cyperus papyrus and Typha domingensis in swamps on parts of the Loboi Plain (Onkware, 2000; Ashley et al., 2004; Owen et al., 2004).

from several outcrops of the Loboi Silts on the Sandai Plain north of the lake (Fig. 1). The mineralogy of the rhizoliths and their host sediments was determined by standard petrographic methods and by X-ray diffraction using a Rigaku R200 X-ray diffractometer. Diatoms in unconsolidated samples from Loburu were mounted in Naphrax and identified on smear slides. The rhizoliths and host sediments were examined in thin section using standard petrographic techniques. Rhizoliths and adhering sediment were mounted in epoxy glue and sectioned prior to microscope studies. Rhizoliths were also picked from sediment substrates, washed clean of unconsolidated materials, and photographed. Rhizolith fragments were mounted on stubs with carbon tape for SEM investigations. Silver paint was run from the upper surface down the side of the sample and onto the stub to reduce charging effects. Some images were photographed at 5 kV on a LEO 1530 field emission scanning electron microscope (FE-SEM). Elemental spot and linescan data were obtained using an Oxford Instruments energy dispersive X-ray (EDX) system operated at 8 kV. Other images were obtained by examining small fractured pieces of each sample on a JEOL 6301FE field emission scanning electron microscope, with an accelerating voltage of 5 kV.

3. Methods

The Loburu Delta, which rests on tilted fault platforms of trachyphonolites, consists of two lobes separated by a marshy embayment (Figs. 1B, 2A; Owen et al., 2004). About 60 hot springs discharge on the delta plain in two main groups aligned

Rhizolith and sediment samples were collected from five sites on the Loburu Delta on the western shores of Lake Bogoria, and

4. Field locations and sampling sites

Fig. 2. Field setting of rhizolith-bearing sites. (A) Loburu Delta viewed from its western boundary escarpment showing a broad embayment with marshes lying downstream from hot springs. Rhizolith sampling sites shown by white arrows. (B) Siliceous clustered rhizoliths recently exposed and surrounded by Cyperus laevigatus marsh. Note two rocky mounds in upper left, which mark active hot-spring locations. (C) Sandai Plain, with small outcrops of Loboi Silts emerging on flat, gravel-strewn surface.

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along north–south trending faults. About ten of these springs have subfossil travertine deposits, with minor modern silica precipitation around their margins and along their outflow channels (Renaut et al., 1998). The modern vegetation includes small stands of Acacia spp., with sparse grasses and shrubs near the lake. Areas of dense C. laevigatus marsh (Fig. 2A, B) occur downstream from several of the hot springs and near the shoreline. Distally thinning spreads of colluvial breccias and gravel lie below the fault scarp that forms the western edge of the delta (Fig. 3, Section A). Channel orthoconglomerates and stratified sands and silts occur in mid-delta sites, with bedded clayey silts, silts and fine to medium sands dominant in more distal and shoreline areas (Fig. 3). The rhizoliths are mainly preserved in these pale brown muds, silts and fine sands, which range from weakly consolidated to moderately indurated. Their distribution is patchy and most rhizoliths are present in the central marshy parts of the delta near the active hot springs. The densest concentrations of rhizoliths are visible on bedding planes near the shoreline when the lake level is low, as it was in summer 2006 (Fig. 4A–C). The bedding planes slope very gently (1–2°) lakewards, and many display desiccation cracks, with polygons ranging from b10 cm to N 50 cm in diameter. Others rhizoliths are preserved in small, mesa-like residual mounds (Fig. 4F) up to 50 cm high by a few metres long that lie above the general delta plain surface. Minor exposures occur in walls of shallow (b30 cm deep) ephemeral channels and small erosional beachscarps. Digging test pits and natural exposures showed that

rhizoliths also form laterally discontinuous beds in some areas of modern marsh (Fig. 2B), and extend below littoral beach sands. Locally, modern (unmineralised) plants are directly rooted in the older rhizolith-bearing units. In all cases, the rhizoliths are evidently fossil features and were not observed forming around the roots of modern plants. Most are preserved in old exhumed deltaic sediments that have been buried repeatedly by younger unconsolidated sediments that were later eroded, following periodic changes in lake level. At any location, there is usually a single rhizolith-bearing unit rather than stacked sequences separated by intervening sediments. The rhizoliths are not, however, restricted to a single depositional unit or bedding plane surface, but are present in sediments that have a vertical range of up to ∼ 30 cm. The age of the rhizolith-bearing sediments at Loburu is poorly constrained. In a few places they underlie eroded fossil travertine deposits that may be several thousand years old. Stromatolitic limestone crusts also locally rest on similar exhumed deltaic deposits that may be laterally equivalent to or older than the rhizolith-bearing units. These limestone coatings have been dated elsewhere at ∼ 5,000 y BP (Casanova and Renaut, 1987), suggesting that the rhizolith-bearing sediments are of mid-Holocene age or older. To extend the range of rhizolith types, some examples from the Sandai Plain along the northern shore of Lake Bogoria were also studied (Figs. 1C, 2C). In this area, the late Pleistocene Loboi Silts are disconformably overlain by the Holocene Bogoria Silts (Farrand et al., 1976; Renaut, 1982, 1993; Scott et al., in

Fig. 3. Sediment logs from the two sampling sites. Sections A–D show sequences present on Loburu Delta. Sections X–Z indicate lithologies present on Sandai Plain. Siliceous rhizoliths occur only at Loburu. Analcimic root mats only occur on Sandai Plain, where calcite rhizoliths are also common.

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Fig. 4. Field photos of study sites on Loburu Delta. Penknife is 9 cm long; lens cap = 5 cm. (A) Siliceous vertical rhizoliths in delta plain silts showing regular spacing, Site R1. (B) Densely clustered siliceous rhizoliths showing vertical and horizontal (bottom right) orientations, Site R5. (C) Dense group of siliceous rhizoliths projecting up to 1 cm above less-resistant sediments, Site R1 (pen top = 6 cm) (D) Evenly-spaced bulbous, fluorite-rich, rhizoliths at Site R2. (E) Clustered siliceous rhizoliths at Site R5. Rhizoliths have a thin pale bluish grey coating of fluorite. (F) Silica-calcite rhizoliths eroding from a small mound at Site R3 (hammer = 25 cm). (G) Horizontal silicified mats, moulds and siliciclastic casts at Site R4. Note younger silts resting on top of the mat, suggesting that the mat is not a contemporary feature (pen = 12 cm). (H) Silicified mats with root moulds and scattered vertically oriented silica rhizoliths, Site R5. Mat surface has thin coating of fluorite. (I) Thin section of silicified mat (m) with horizontally oriented silica layers and open laminar fenestrae. Also note vertical cast (f) on left filled with siliciclastic sediments. (J) Dense network of horizontal silicified rhizoliths at Site R5. Note variable diameter of overlap of some rhizoliths. (K) Modern microbial mat (m) penetrated by vertical stems of sedges (Cyperus laevigatus) near Site R5. This may be a modern analogue for fossil silicified mat shown in (H). Site locations are shown in Fig. 1B.

press-a; Tiercelin and Vincens, 1987). The former are composed of siltstones (Fig. 3) deposited in lake-marginal and wetland environments. The degree of cementation of these sediments increases towards the south as well as upwards within the profiles, which also have an increasingly reddish colour towards the surface. 5. Hydrochemistry of Bogoria basin waters The Bogoria Basin lies in a semi-arid zone, with ∼ 900 mm precipitation per year and an annual potential evaporation of

∼2500 mm. The climate is monsoonal, with most rainfall in April, followed by a smaller peak in November (Harper et al., 2003). Lake Bogoria is hydrologically closed and very sensitive to climate variations. El Niño events, for example, appear to result in 5–7 year cycles of rainfall variability (Ashley et al., 2004) that bring about lake level changes of one to 3 m. The water is of Na-HCO3-CO3-Cl composition, and reported salinities for surface waters range from 55–135 g l− 1 TDS (Total Dissolved Solids) (Cioni et al., 1992; Renaut and Tiercelin, 1994). The surface water pH varies from 9.3–10.6 and is usually lowest at the north end of the lake (Renaut et al., 1987).

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Fresh stream waters and hot springs form the main inputs to the lake. The salinity and composition of the lake brines reflect ion fractionation processes such as evaporative concentration, mineral precipitation, sorption and degassing that are typical of other saline, alkaline lakes (cf. Eugster, 1980). About 200 hot springs discharge at Bogoria, of which ∼60 are present on the Loburu Delta. These range from seeps and small pools (b 1 m diameter) to larger perpetually spouting springs, ebullient pools N 6 m in diameter, and several geysers. Spring temperatures are 65–99 °C and the waters are chemically uniform (Table 1), except where mixing with lake water takes place. The Na-HCO3 spring waters have salinities of 4–6 g l− 1 TDS and contain high silica concentrations (80–110 mg l− 1), but only very minor silica crusts are actively forming (Renaut et al., 1998). Although the calcium concentration is low (b2 mg l− 1) in the spring fluids, fossil travertines are present at several springs. Fluoride concentrations are high (62–74 mg l− 1). Allen et al. (1989) and Cioni et al. (1992) suggested that the spring waters derive mainly from local meteoric sources that have undergone geothermal heating and have mixed with small amounts of lake water. 6. Results 6.1. Rhizoliths of the Loburu Delta The structures were identified as rhizoliths based on many criteria including their general form, size, orientation, lateral relationships, spatial density, horizontal rhizomous arrangement, bulbous protrusions, and preserved cellular features. The modern delta plain muds locally have vertical burrows, produced mainly by tiger beetles (Coleoptera: Cicindelidae) (Scott et al., in press, b). These burrows range in diameter from 1–3 mm, 4–6 mm, and ∼ 10 mm, and are up to ∼ 20 cm deep. This raises the possibility that some vertical structures could be burrow casts rather than rhizoliths. However, this is considered unlikely because many of the vertical casts show textures that indicate replacement and (or) encrustation rather then pore-filling.

The distribution of rhizoliths at Loburu Delta is patchy. Rhizoliths are restricted to a few specific areas, mainly near the sites of present and former hydrothermal activity. Pits dug into the delta sediments both north and south of the areas examined did not expose in situ mineralised rhizoliths, although brown root markings and invertebrate traces are locally common across the delta plain (Scott, 2005; Scott et al., in press-b). Regular spatial variations in the rhizolith type (e.g. root tubule, root cast, etc.) with respect to shoreline were not found, so the rhizoliths were examined at those sites where they are abundant and accessible, rather than using a systematic grid. Boiling waters and locally soft ground would, in any case, have made such a systematic approach hazardous. 6.1.1. Macromorphology and substrates Despite an extensive search, no conclusive evidence for recent rhizolith formation could be found in the root systems of the modern littoral plants, either above or below the present water table. White (1953) reported calcite (“limestone”) tubules that still contained “remains of a twig” from Lake Bogoria. Although their location was unspecified, these twigs were dated as “thirty generations ago” based on tribal tradition, which implies a rough age of about 1000–1500 years before present. We also found wood fibres inside a horizontal botryoidal calcite tubule at ∼ 40 cm depth 10 m southeast of spring KL6 at central Loburu (for spring locations, see Renaut, 1982), but it is unclear if those plant fibres were of the same age as the enclosing calcite tubule or younger and simply occupying the open porosity. The host sediments that contain the rhizoliths are composed mainly of massive clayey silts, silts, and fine sands comprised mainly of detrital K-feldspar (sanidine), together with volcanic glass, sodic pyroxenes, and other minor mafic minerals. Based on XRD analyses, the clay minerals are mostly smectites, with minor illite. Most deltaic sediments are slightly to moderately indurated, with patchy, discontinuous vadose micritic calcite being the most common cement. Thin (10–30 µm), pendant and meniscus cements of opaline silica locally coat detrital feldspar surfaces but are uncommon except near the active hot springs. Efflorescent

Table 1 Representative chemical analyses of waters in the Lake Bogoria Basin Sample Rivers Sandai R. Parkirichai (Loburu) Groundwater Central Loburu Delta Hot springs Loburu Loburu Loburu Loburu Lake (surface) Lake Lake

Number

Date

Temp (°C)

pH

Na (mg l− 1)

K (mg l− 1)

Ca (mg l− 1)

Mg (mg l− 1)

Cl (mg l− 1)

Alk. (meq l− 1)

SO4 (mg l− 1)

F (mg l− 1)

SiO2 (mg l− 1)

BW1 KL1

10–91 8–77

17.0 19.0

7.20 7.50

80 101

4.5 6.0

27 26

6.5 8.0

42 46

3.93 5.02

4.7 3.0

2.3 2.6

60 45

L-3

8–94

47.0

8.35

1005

13.5

2.7

1.2

190

35.16

47

–

76

KL14a-91 KL14a3-95 KL6a-94 KL8a-94

11–91 06–95 02–94 02–94

94.0 82.0 98.0 98.5

8.15 8.35 8.30 8.55

1295 1480 1310 1395

18.0 19.5 20.2 22.0

1.5 0.8 1.6 1.3

0.5 0.4 0.8 0.5

220 238 222 235

44.36 50.37 46.40 47.07

65 71 65 64

72 74 62 70

100 92 108 110

K1 BW2

01–70 10–91

– 26.5

10.6 9.95

24400 24600

387 435

0 12.0

2.2 b1.0

6390 5680

965.00 683.81

216 330

1064 110

260 225

After Renaut and Tiercelin, 1994. Tr: trace. –: not determined.

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salts, composed of trona (NaHCO3 · Na2CO3 · 2H2O) with less abundant thermonatrite (Na2CO3 · H2O) and halite (NaCl), locally encrust the subaerial surfaces of the rhizolith-bearing sediments. The diatom floras (mainly Rhopalodia gibberula, Epithemia argus, Anomoeoneis sphaerophora) imply that the host sediments were deposited in saline, very shallow water, either along the open lacustrine littoral zone or in ponded (e.g., behind beach ridges) lake-marginal wetlands. At most sites examined, a single rhizolith-bearing unit is present in the upper 50–80 cm of sediment rather than a vertical succession of rhizolith-bearing palaeosols. This does not exclude the possibility of multiple phases of rhizolith formation— younger rhizolith-bearing horizons could have been eroded, for example—but the evidence favours one main phase of rhizolith formation. The upper surfaces of individual rhizoliths lie either at, or a few mm above or below the exhumed sediment surfaces. Those same bedding planes, where traced laterally on the delta plain, are covered in places by stromatolitic calcite crusts that formed during former period(s) of higher lake level and more dilute lake water. These microbial limestones have been dated at ∼ 4740–5290 y BP (Casanova and Renaut, 1987), although these apparent radiometric ages may be too old because of recycling of isotopically “old” carbon during the geothermal

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fluid circulation. The absolute age of the rhizoliths is therefore uncertain, but they are probably several thousand years old. The Loburu sediments contain a range of rhizolith morphologies and mineral compositions that reflect the many chemical processes (cementation, replacement, dissolution) that occurred both in and adjacent to living and recently dead plants. The presence of both horizontal and vertical fossil rhizoliths in the Loburu sediments suggests the preservation of more than one plant taxon, different parts of one rhizomous species, or the preservation of plants that produced both vertical and horizontal root components. Several minerals are present in the Loburu rhizoliths, including opaline silica (opal-A), fluorite (and possibly fluorapatite), and calcite. Pyrite is a minor accessory in some rhizoliths. Based on their dominant morphology and mineralogy and mode of occurrence the main rhizolith types are: 1) siliceous vertical rhizoliths; 2) clustered siliceous rhizoliths; 3) horizontal mats, moulds, and siliciclastic casts; 4) silica-calcite rhizoliths; and 5) bulbous, fluorite-rich, rhizoliths (Figs. 4 and 5). 6.1.1.1. Type 1: Siliceous vertical rhizoliths. The siliceous vertical rhizoliths are located on the non-vegetated delta-marginal plain at sites R1 and R2 (Figs. 1B, 4A–C). They are typically 2– 4 mm in diameter and up to ∼20 cm long. Rhizoliths of this type

Fig. 5. Cleaned rhizoliths. (A) Siliceous vertical rhizoliths. (B) Bulbous, fluorite-rich, rhizoliths. Fluorite-rich areas form darker, upper, parts of rhizoliths. Right-hand block shows siliceous vertical rhizoliths rising through clayey silts. Surface contains darker fluorite patches and protruding siliceous rhizoliths (arrows). (C) Silicacalcite rhizoliths. Note vertical light coloured rhizoliths joined by calcite-cemented rhizocretions. (D) Siliceous horizontal mat (top) overlying siliceous vertical rhizoliths (bottom). Thin root mat shown in lower photo by arrows. Horizontal bars = 2 cm.

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are straight, vertically oriented and generally unbranched, although some specimens bifurcate downwards (Fig. 5D). Most are isodiametric along their length, but many commonly taper towards their base (Fig. 4A). Cleaned siliceous rhizoliths are generally white to pale beige, and are fibrous parallel to their long axes. At the sediment surface these rhizoliths can protrude up to 1 cm above the less-resistant clayey silts in which they are rooted, and many show rounded upper terminations (Fig. 4C). Rhizolith spacing appears to be broadly regular at the surface and in the sediment profile (Fig. 4A), but denser groups are present locally (Fig. 4C). Linear transects measured across the sediment surface showed an average spacing of about eight rhizoliths per 10 cm. These silica rhizoliths have been locally eroded and reworked in the swash zone. Many are found in coarse sand and granule beach deposits and in regressive littoral terrace sediments, where locally they can form N 50% of the sediment mass. 6.1.1.2. Type 2: Clustered siliceous rhizoliths. Dense clusters of siliceous rhizoliths are present near several hot springs at Site R5 in a damp area with a discontinuous cover of living sedges (C. laevigatus). These rhizoliths occur in tight bundles that in places form semi-continuous beds with an irregular hummocky and knobby surface (Fig. 2B). The outcrop area is 20 by 15 m, but shallow digging showed that it extends westwards below the vegetated sediments of the present marsh. Individual rhizoliths, 1–4 mm in diameter and up to ∼5 cm long, show both subhorizontal and vertical orientations (Fig. 4B). At the sediment surface, many rhizoliths form bundles (N50 cm long) that radiate subhorizontally from a central focus (Figs. 2B, 4E) and appear to be fossilised rhizomes. At and just below the sediment surface, the clustered rhizoliths commonly have a thin (1–2 mm) bluish grey coating that externally has a fine (mmscale) mammillary surface. This coating, which petrographic and EDX analyses confirm is composed mainly of fluorite and silica with minor calcite, provides the cement that has locally preserved the rhizoliths as a semi-continuous bed. The coatings cover the uppermost external surfaces of the rhizoliths and envelop the upper parts of individual roots and rhizomes. 6.1.1.3. Type 3: Horizontal mats, moulds, and siliciclastic casts. These rhizoliths are characterised by horizontally laminated microbial mats composed of opaline silica, with vertical siliceous rhizoliths both below and penetrating through the silicified mats (Figs. 4H and 5D). Numerous circular to ovoid root and stem moulds (5–10 mm in diameter) penetrate the silicified mat (Fig. 5D), some of which are filled with detrital siliciclastic sediments (Fig. 4I). The type 3 rhizoliths are present at Site R4 (Fig. 4G), where they rest upon an exhumed sediment surface and have been partially eroded. Where preserved, individual fossilised mats are typically 2–5 mm thick and are overlain locally by at least several decimetres of silty clays. These white to pale brown rhizoliths are 2–5 mm in diameter and up to ∼ 15 cm long; some show root bifurcation (Fig. 5D). Also present in the same areas as the silicified mats are horizontal siliceous rhizoliths that have a reticulate pattern (Fig. 4J). These rhizoliths, which before fossilization appear to

have been rhizomes, are 3–10 mm in diameter. Those with the largest diameter tend to be the most elongate and might have been the dominant rhizomes. Some of these rhizoliths form a single layer, but in places, horizontal rhizoliths are superimposed or intermeshed with one another over a vertical range of at least 5 cm. Unlike the vertical rhizoliths, which tend to be circular in cross section, these rhizomous forms are commonly flatter with more irregular diameters. 6.1.1.4. Type 4: Silica-calcite rhizoliths. Type 4 silica-calcite rhizoliths occur at Site R3 (Fig. 1B) in an eroding mound that stands about 60 cm above the surrounding delta plain (Fig. 4F). They consist of calcite-cemented clayey siltstone nodules, up to 1.5 cm across, that form asymmetrically around, or adjacent to, siliceous rhizoliths similar to those of Type 1. These rhizoliths are vertically oriented with the siliceous parts 2–4 mm in diameter and 2–5 cm long. Lateral spacing between the siliceous components of the rhizoliths is 1–3 cm. Many rhizoliths have a small knob projecting from one side. Calcite commonly cements two or more silica rhizoliths together (Fig. 5C). 6.1.1.5. Type 5: Bulbous fluorite rhizoliths. Fluorite is present in the silica-calcite rhizoliths at Site R3, but is most abundant at Site R2 (Fig. 1B). There, an exhumed sediment surface is covered with fluorite-rich knobs (Fig. 4D) that have an irregular spheroidal or ovoid shape and are ∼2 cm long (Fig. 5B) by ∼1 cm in diameter. Ovoid types tend to be vertically oriented, loose, and are easily plucked from the surface. Horizontal spacing between the knobs is typically 1–4 cm. Commonly, two or more of these rhizoliths appear to be linked horizontally just below the exhumed surface. These bulbous rhizoliths are mainly restricted to the sediment surface and most do not extend N 1 cm below it. 6.1.2. Micromorphology and EDX data Silica is present in varying amounts in each of the five rhizolith types, but is most abundant in Types 1, 2 and 3. EDX spot analyses of these rhizoliths confirmed the predominance of silicon and oxygen, with minor sodium and carbon in several samples. XRD analysis of the siliceous vertical rhizoliths confirmed that the silica is non-crystalline opal-A. Some examples also contain dense networks of blade-like and acicular Na-salts, probably trona and (or) thermonatrite (Fig. 6H). Cellular structures (20–50 µm long; 10–20 µm wide) are variably preserved (Fig. 6A–D) in the silica. In some examples, thin (b1 µm), sheetlike layers of silica form smooth linings that mimic original cells (Fig. 6B, G). In other examples, nanospheres and microspheres of silica appear to have formed in cells and have preserved the general cellular shape (Fig. 6C–E). These spherical particles are ∼ 250 nm to 3 µm in diameter, with most being ∼2–3 µm (Fig. 6E). They vary in density from isolated spheres, through open networks with few contact points, to dense aggregates that coalesce and have few interparticle pores. The spheres may be arranged in seemingly random threedimensional networks (Fig. 6C) or lie parallel to one another and to former cell walls (Fig. 6D). Fractured microspheres possess a homogeneous internal structure, and in some cases have nucleated around plant tissues that are also silicified (Fig. 6F).

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Fig. 6. SEM images of siliceous rhizoliths. (A–B) Low magnifications show preservation of rectilinear cellular structures. (C–F) Details of broadly uniform (1–3 µm diameter) silica nano- and microspheres. (F) Spheres merge to produce uniform larger homogeneous masses. Note growth on plant debris. (G–H) Blades of trona (?) occur in association with some siliceous rhizoliths.

Homogeneous silica deposits also form planar sheets in Type 3 horizontal mats, moulds, and siliciclastic casts at Site R4 (Fig. 7A, B). The mat laminae are 5–100 µm thick and are separated locally by open fenestrae (mostly b 50 µm wide and b 150 µm long). Bladed and acicular trona crystals are common in these pores (Fig. 7B). The central parts of several Type 4 (silica-calcite) rhizoliths at Site R3 consist of concentric laminae (10–20 µm thick) that are also formed of homogeneous silica (Fig. 7C, D). These laminae usually surround a central void, or a space with a loose network of silica microspheres. Where present, the concentric silica laminae are bounded on each side by parallel spaces of similar thickness, with the rings being connected by perpendicular silica walls that appear to reflect a former cellular structure. The presence of calcite in these samples is indicated by a broad correlation between Ca and C in the line scan data (Fig. 8), and by the reaction of the rhizoliths with dilute HCl. The samples also show strong correlation between Ca, F, and P that probably

reflects the presence of fluorite and possibly fluorapatite. Silicon is associated with oxygen as expected for opaline silica. The elemental distributions show a distinct asymmetry across the samples (Fig. 8), with opaline silica and fluorite and (or) calcite each occurring separately. Type 5 rhizoliths (Fig. 9A) contain a central fissure or void, or multiple small spaces. Higher magnification images (Fig. 9B) show that these openings are commonly lined by fluorite crystals that spot EDX data confirm are composed predominantly of Ca and F, which is also present in the surrounding matrix. The presence of C in some regions that contain Ca may indicate calcite. Significant amounts of Al, K, and Na are probably due to the presence of detrital feldspars (Fig. 9A). The central cores of Type 5 rhizoliths from Site R5 are formed of very fine silica and fluorite laminae (Fig. 9C–E). The fluorite laminae are laterally discontinuous (Fig. 9C) and thinner than most of the silica laminae (Fig. 9E). They may represent cements that have filled pores produced by dissolution of organic matter.

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Fig. 7. SEM images of rhizoliths. (A–B) Siliceous horizontal mats, with blades of trona (?) in “B”. Note open fenestrae between laminae. (C–D) Silica-calcite rhizoliths showing silica walls separated by voids. These features appear to reflect cellular structures.

Fig. 8. EDX data for silica-calcite rhizolith at Site R3. (A) Overall SEM image of rhizolith. (B) Detailed SEM of central part of rhizolith shown by white box in “A”. (C–E) Details of central part of rhizolith in “B” where locations are marked by white boxes. Possible cellular structures marked by white arrows in “E”. Linescan data show presence of fluorine, which correlates well with calcium and phosphorus (fluorite and/or fluorapatite?). Calcium also correlates with carbon, probably reflecting presence of calcite. Silicon occurs in different parts of the rhizolith to those where Ca, F, and P are abundant. (F) Thin section of a silica-calcite rhizolith showing a sharp boundary between silica and Ca-rich areas, and asymmetry of the calcareous portion.

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Fig. 9. SEM photomicrograph of silica-fluorite relationships in rhizoliths. (A) Type 5, bulbous, fluorite-rich, rhizoliths. Note central void lined with fluorite. Black arrows indicate feldspars. (B) Detail of central pore lining in “A”, showing euhedral fluorite. (C) Backscattered electron image of mixed silica-fluorite rhizolith showing uneven distribution of the two phases, possibly due to replacement. (D) Rhizolith composed of silica and fluorite (Si & F) encased by calcite (Ca). (E) Intercalated silica and fluorite in core of rhizolith. Fluorite laminae are laterally discontinuous, contorted, and thinner than silica laminae. (F) Opaline silica spheres encased in fluorite laminae. Some spheres (e.g., centre) showing possible evidence of binary fusion, implying that they could be bacteria; others lack clear evidence of abiotic vs. biotic origin. (G) Ovoid tube-like mouldic pore in silica core of rhizolith that is partly filled by fluorite crystals. Vaguely concentric structure suggests that fluorite may have replaced organic matter of a rootlet or root hair. (H) Silicified filamentous microbe from interior rhizolith. (I) Fragmented pennate diatoms on exterior surface of rhizolith.

In places, the fluorite laminae contain small silica microspheres (b1 µm) that clearly predate the fluorite (Fig. 9F). It is unclear if these spheres are abiotic or silicified bacterial cells. Parts of the silica cores of the rhizoliths also contain tubular structures ≤ 500 µm in diameter that are partly filled by fluorite cements. Some show a broadly concentric structure (Fig. 9G) that may represent cementation or replacement of cellular material. The fluorite is generally pore-filling, euhedral, and can be zoned, implying gradual growth. These rhizoliths also include moulds of filamentous microbes, probably bacteria (Fig. 9H). Some rhizoliths at Site R5 contain an external calcite envelope that shows little internal structure, and lacks silica or fluorite (Fig. 9D). Adhering to the external walls of the rhizoliths are

siliciclastic grains, broken diatoms, and skeletal crystals of NaCO3 salts (Fig. 9I). 6.2. Rhizoliths on the Sandai Plain The Loboi Silts are comprised of poorly sorted, zeolitic, silty and sandy claystones, and patchy calcretes. These lithologies and the presence of polygonal desiccation cracks suggest an ancient pedogenic setting (Hay, 1970; Renaut, 1993). Several types of fossil root indicators occur, including: 1) zeolitic root mats; 2) root marks; and 3) calcite rhizoliths. Zeolitic root mats occur locally in the upper 10–20 cm of the Loboi Silts. These reddened sediments include a dense network

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of hollow tubes, b3 mm in diameter and several centimetres long (Fig. 10A). In thin section, these root moulds are commonly lined with illuvial clay (Fig. 10B), which may also form geopetal fills. In some cases, authigenic clays are also present (Renaut, 1993). Sparite, microsparite, and less commonly micrite cements, either line or fill the openings (Fig. 10C–E). Matrix materials close to moulds are commonly enriched with Fe-oxihydroxides. Submicroscopic analcime (NaAlSi2O6·H2O), confirmed by XRD, lines pores and forms a cement in the detrital clayey silts (Fig. 10D–E). The analcime consists of euhedral and subhedral icosotetrahedra, mostly 2–3 µm in diameter, that fill and encrust rhizoliths and cavities in the sediments (Fig. 10G–H). Clay cutans commonly coat the analcime and detrital components. Etched feldspars are present in several samples (Fig. 10H). In places, blackish brown to reddish brown root marks are present. These occur as either,

closely spaced, near-vertical forms (tens of centimetres long, 2–5 mm wide), or as less dense vertical and subhorizontal types, that reach depths of 20 cm and are 4–10 mm wide (Renaut, 1993). Micrite and pore-filling microsparite (Fig. 10D) occur in horizontal and vertical calcite root casts that are present locally in the upper parts of the Loboi Silt sections (Sections X and Y, Fig. 3). Most are stubby or elongate, 1–5 cm long and 0.3– 1.5 cm in diameter. Some have been eroded and reworked by sheetfloods and in ephemeral washes, and form surficial lag deposits. Locally, redeposited rhizoliths have been buried by fine sands and then cemented by calcite to form thin (2–5 cm) rhizolitic limestones that are a few metres in lateral extent. Calcite rootcasts are present in small outcrops of pebbly or massive calcrete, 2–20 cm thick, that cap the Loboi Silts at several locations (Section X, Fig. 3). These have both horizontal

Fig. 10. Root mats and rhizoliths in Loboi Silts, Sandai Plain. (A) Zeolitic fossil root mat showing predominantly horizontal rhizoliths. (B) Thin section of root mould lined with clays. (C) Backscattered electron image showing isopachous phreatic calcite in partly collapsed rhizolith tubule. Matrix is analcime-cemented. “C” is calcite; “A” is analcime. (D) Thin section of root tubule with pore-lining sparite. (E) Thin section showing porosity in Loboi Silts due to either dissolution, root hairs, or possibly burrows. (F) Thin section of well-developed, isopachous pore-lining analcime cement. (G) SEM of root tubule, note location of detail shown in “H”. (H) Analcime encrusting outer surface of tubule in “G”. Black arrow points to etched feldspar. White arrows point to clay coated analcime.

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and vertical orientations and locally penetrate several centimetres into the underlying zeolitic Loboi Silts. The calcretes locally extend downwards as tapering vertical plates, up to 5 cm long, that may represent calcite precipitation in former desiccation cracks. 7. Rhizolith formation 7.1. Silicification of plant roots Silica is the dominant mineral in most of the Loburu rhizoliths. At Bogoria, siliceous rhizoliths have only been found near the sites of active hot springs, which implies that thermal fluids are the most likely source for most of the silica. This assumption is supported by the close association of the opaline silica with fluorite and the presence of silica in hot-spring deposits elsewhere at Lake Bogoria (Renaut and Owen, 1988; Renaut and Jones, 1997). High concentrations of silica (N250 mg l− 1 SiO2) and fluoride (N1,000 mg l− 1 F) are also present in the lake waters. If, however, lake waters were the primary source of F− and SiO2 (e.g., following submergence of the littoral marsh), then similar rhizoliths would also be expected elsewhere around the shoreline. Other possible sources of silica include surface runoff waters (which contain 30–65 mg l− 1 SiO2) and the corrosion of silicate minerals, volcanic glass, phytoliths, and diatoms by alkaline fluids. Several mechanisms can be proposed to explain the root silicification. Hot springs and geysers in New Zealand, Iceland, Yellowstone, and elsewhere commonly have extensive deposits of sinter near their vents that precipitate upon cooling of silicarich (N250 mg l− 1 SiO2) waters (e.g., Jones and Renaut, 2003). Plants of all types and their roots are commonly silicified around high-temperature springs (e.g., Trewin et al., 2003). In contrast, the Loburu hot springs contain much less silica (92–110 mg l− 1; Table 1) and lack extensive sinter aprons. Modern sinters at Loburu are limited to thin surficial crusts that form in microbial mats along the subaerial margins of hot-spring pools and their outflow channels (Renaut et al., 1998). The modern hot-spring fluids are probably in equilibrium with chalcedony (Fig. 11; Cioni et al., 1992), but are undersaturated with respect to opal-A at the vents. The modern waters must evaporate to attain saturation with respect to opal-A because cooling alone should not induce opal-A precipitation (cf. Rimstidt and Cole, 1983). Evaporative concentration can occur during runoff and by capillary evaporation from shallow groundwaters. The fluids do not require much water loss to become saturated (120–130 mg l− 1 SiO2 at pH 7), but opal cements are not widely disseminated in the host sediments at Loburu, suggesting that evaporation alone is not responsible for the plant silicification. The close association of opal with the roots and stems suggests that the plants themselves might have contributed to their own silicification through evapotranspiration, which would locally increase the silica concentration. All plants accumulate silica in their tissues, particularly the cell walls (Sommer et al., 2006). Silica provides mechanical strength, resistance to toxicity and fluid loss, and prevention of disease (e.g., Epstein, 1994,

Fig. 11. Saturation index data for 25 Loburu spring waters determined by SOLMINEQ88 (Kharaka et al., 1988). Calcite and fluorite range from marginally undersaturated to supersaturated at the vent. Of the silica species chalcedony is the closest to equilibrium, but opal-A is undersaturated in all samples. Dolomite is supersaturated, but is probably kinetically inhibited.

1999). Silica is taken up through roots as monosilicic acid (H4SiO4) and absorbed, commonly precipitating as opal-A in tissues, including the leaves and roots. While the plants are alive, the silica is transported in the transpiration stream. In some living plants, silica is concentrated by passive evapotranspiration in the leaves (e.g., Jones et al., 1963), whereas in others silica accumulates at the base of the plant and in the roots (e.g., McNaughton et al., 1985). Absorption of silica is likely to continue so long as the silica concentrations do not inhibit growth. If dissolved silica concentrations become very high, however, the silica and associated salts might then have an adverse effect on plant metabolism, potentially killing the plant. After death, abiotic capillary evaporation might succeed evapotranspiration and continue to provide silica to the plant remains. Cellular structures and broad morphological features, such as growth rings, are well preserved in some samples, which supports the inference of early silicification. Biogenic silica that formed within the root during life might also have served as a template for post-mortem silicification (cf. Hendry, 1987). Silicification might also involve the common affinity of silica for plant matter. Leo and Barghoorn (1976) suggested that the mechanism for silica nucleation on plant materials involves hydrogen bonding between the hydroxyl in the silicic acid and lignin and cellulose. They proposed that hydrogen bonding occurs between hydroxyl and carboxyl groups in cell tissues and monosilicic and polysilicic acids. As time passes, the hydroxyl and carboxyl bonds change to stronger siloxane bonds. Substrates lacking these negatively charged hydroxyl and carboxyl groups tend to remain unsilicified. SEM studies of the Loburu rhizoliths show that silica occurs mainly as nanospheres and microspheres in both extracellular and intracellular sites. Commonly, the spheres have merged and

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become cemented to form homogeneous masses. The silicification process at Loburu appears to have developed mainly through permeation and void filling processes (permineralisation). Opal spheres have been reported from many other studies. Schultze-Lam et al. (1996) and Konhauser and Ferris (1996), for example, noted that silica spheres that formed in microbial cells in hot springs gradually coalesce and displace cytoplasm, which may also gradually decay. Channing and Edwards (2003) described the results of experimental immersion of plants in hot springs at Yellowstone National Park that produced opaline silica spheres similar to those at Loburu. They noted that amorphous opaline silica (opal-A) films were deposited on plant tissues within 30 days of immersion in hot-spring water. At the same time, nanospheres and microspheres formed colloidal suspensions within plant cells. By 330 days, a strong supporting framework of silica had developed that prevented collapse of the plant structure. Eventually, the microspheres grew larger and coalesced to form homogeneous masses of silica. The Loburu silicified rhizoliths are either straight and ≥20 cm long, or formed mainly by horizontal rhizomes with their associated roots. The rhizomous plants probably lived in areas where the water table lay almost at or very close to the delta surface for most of the time. Many of the silicified rhizomes likely originated from the sedge C. laevigatus, which is dominant in the wet marshy parts of Loburu today (Onkware, 2000; Fig. 2A, B). The root systems of modern C. laevigatus are similar to those inferred from Type 2 rhizoliths, and have similar diameters and spacing to those rhizoliths. In contrast, salt-tolerant plants with longer roots, such as the grass S. spicatus, are found in drier areas of the delta where the water table was normally deeper (Onkware, 2000). Type 3 rhizoliths consist of moulds and casts formed by siliciclastic fills in horizontal silicified mats. These are laterally continuous and characterised by fenestral pores which indicate formation by silicification of a microbial mat. Elsewhere, the silicified mats are largely unbroken and form thin (1–3 mm) crusts upon the substrate. Modern thermophilic microbial mats at Loburu tend to be present only in water hotter than ∼40°C, which is too hot for most macrophytes. However, blackish mats that are present today on cooler damp parts of the delta plain in sites where C. laevigatus is actively growing may be a modern analogy (Fig. 4K). These cyanobacterial mats are ∼0.5–1 cm thick and broken in places by plants that are rooted through the mats into the underlying soil. Fig. 12A shows a general model for plant silicification on the Loburu Delta. Silica can be delivered to the plant roots by infiltration of surface runoff of thermal and non-thermal origin, immersion of root systems in groundwater, or periodic flooding by lake water. The situation is complicated by the location of the delta on the margin of a closed-basin lake that fluctuates frequently in level and salinity. In this setting, the source of fluids supplied to the plants can change rapidly and unpredictably. However, the fluid supply must be maintained for sufficient time to allow silicification to occur, although this process can be rapid in geothermal settings (Channing et al., 2004). Today, thermal runoff flows through the littoral wetlands in narrow low-sinuosity channels and bypasses most of the macrovegetation. Locally, Cyperus and Sporobolus grow b 10 cm

from flowing hot water in the spring outflow channels but show no visible evidence for silicification. The high density of rhizoliths in a restricted area of the Loburu Delta suggests that silicification might have affected all the plants at about the same time. One possible scenario is for diversion of thermal waters across the littoral marsh, which would rapidly kill the plants and simultaneously provide silica. Lateral migration of spring outflow occurs periodically following lake level rise as the thermal channels adjust to the changing base level (regression normally leads to channel incision). Simple flooding, however, would be unlikely to cause silicification over a broad area because the channels become re-established rapidly, refocusing thermal outflow across a relatively small zone. Silicification by thermal outflow over a broad area might be achieved if the spring fluids were ponded and concentrated by evaporation. Evaporative wicking of fluids upwards through the dead plants by capillary processes (cf. Hinman and Lindstrom, 1996) could potentially do this—sodium carbonate salts, for example, have been observed crystallizing in dead sedges by this method—but maximum solute concentration would probably occur above ground rather than in the root systems unless the latter became exposed by erosion. If, alternatively, the silica concentration of the spring fluids was formerly higher than today or saturated with respect to opal-A, subaerial sinters would probably have formed, but instead they are absent. An alternative mechanism for precipitation would be for silica to be supplied from shallow groundwater. The groundwater would have to have been silica-rich but cool enough, at least initially, for plants to take root. Shallow hot (47 °C) groundwater that seeped into a pit dug on the central delta plain a few metres from a hot spring contained 76 mg l− 1 SiO2 (Table 1). The silica concentration is above that of other shallow groundwaters in the region (typically ∼45–60 mg l− 1 SiO2) because it has mixed locally with thermal waters. To attain saturation with respect to amorphous silica, such groundwater would only need to be concentrated by a factor of two to three. Evapotranspiration and capillary evaporation of cool shallow groundwater that has partly mixed with thermal fluids is a possible mechanism to explain the siliceous rhizoliths. Evapotranspiration by plants rooted in shallow groundwater might have led to silica being drawn up through plant stems while alive, leading to partial silicification. Soon after death, water would be drawn upward through the plant fibres by capillary evaporation forming opal-A, both in the roots and stem, but only as far as capillary pressures would permit. Consequently, only the roots and basal parts of the stem would have become silicified, with the poorly mineralised upper stem breaking off or decaying in situ (Fig. 12B). If thermal groundwaters invaded the root systems, the plants would die and silica would become readily available. Similar processes were inferred in fossil spring deposits west of Lake Turkana, Kenya (Renaut et al., 2002), and in recently silicified plants at Reporoa in New Zealand (Jones et al., 1998). Such processes imply that an “event” occurred that affected all plants in the area at about the same time. In addition to surface or shallow subsurface flooding by hot waters, other possible explanations include rising hot groundwater followed

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Fig. 12. Summary models for rhizolith formation. (A) General situation on the Loburu Delta. Na-Ca-HCO3 runoff provides a source of Ca2+ for calcite precipitation in sediments. Fluorite forms where fluoride-rich spring waters mix with Ca-rich fluids. Fluorite may also form where fluoride solutions come into contact with preexisting calcite. Spring fluids supply elevated SiO2 to voids in plant tissues in the adjacent marshy settings. (B) Evapotranspiration and evaporative concentration of silica-rich fluids in pools and marshes concentrate silica in plants. Post mortem capillary rise continues to draw silica-rich fluids upwards. (C) Rhizolith formation on Sandai Plain. Saline, alkaline groundwater is subject to capillary rise and evaporative pumping. In upper profile waters react with siliciclastics to produce zeolites. Infiltrating Ca-bearing surface waters precipitate calcite upon mixing with saline alkaline waters, or following evaporative concentration and CO2 degassing.

by silicification by capillary evaporation, or a falling water table linked to falling lake level and increased aridity. The latter process would kill many plants, but be accompanied by channel incision, which would make flooding by thermal waters less likely. 7.2. Calcite precipitation and rhizocretions Calcite is a subsidiary mineral in the Loburu rhizoliths. At Loburu (site R3, Fig. 1), subfossil silicified roots are partially

surrounded by hardened mud that reacts with HCl, suggesting that the main cementing agent is calcite (confirmed by SEM examination and EDX analyses). These calcite rhizocretions are arranged asymmetrically around silica rhizoliths and resemble wing-like protrusions on the sides of the silicified roots. Elsewhere at Loburu (site R5), calcite forms rims around earlier silica and fluorite rhizoliths (Figs. 8F, 9D). At both sites, the calcite postdates roots that had already been silicified, implying that is unrelated to activity of the host plant (e.g., evapotranspiration). The potential sources of the Ca2+ (HCO3-CO3 being

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abundant in all the fluids) include thermal spring waters, surface runoff, shallow groundwater, and lake water. At different times, all these Ca-bearing fluids can come in contact with living plant roots and older rhizoliths, making it difficult to determine which fluid is mainly responsible for calcite precipitation. At least ten hot springs at Loburu have subfossil hydrothermal travertines, but only minor carbonate is precipitating today. The modern spring fluids contain little calcium (0.8– 1.6 mg l− 1 Ca) (Table 1) because much of the Ca has been depleted by early carbonate precipitation in soils and in the subsurface plumbing system of the springs (Renaut and Jones, 1997). Spring waters are unlikely to be the main source of the calcium. Modern surface runoff and shallow dilute groundwaters (including local warm springs fed by dilute groundwaters) have the highest concentrations of calcium in the basin (up to ∼ 35 mg l− 1 Ca). Many of these waters are saturated or slightly undersaturated with respect to calcite. Calcite is a common cement in soils and shallow sediments on the delta plains, typically forming patchy vadose micritic and microsparitic cements. Much of this precipitation is attributed to evaporation and CO2 degassing of runoff waters at the surface and in the shallow vadose zone. Renaut (1993) noted that water percolating through the sediments on the Sandai Plain contained 2.7–27 mg l− 1 Ca. In contrast, local alkaline groundwater contained only up to 6.5 mg l− 1, suggesting Ca loss through calcite precipitation. Groundwater salinity and pH tend to decrease with distance from the shoreline, possibly due to a weak Ghyben–Herzberg effect (cf. Yechieli and Wood, 2002), but the groundwater salinity is unstable and increases periodically when rising lake water floods the delta. The lake-marginal groundwater is generally brackish, reflecting the mixing of dilute, rain-derived groundwater with more saline waters of lacustrine or thermal origin. Groundwater at the lakeshore today is saline and alkaline and consequently Ca-poor. If the main Ca-source for calcareous rhizoliths was shallow groundwater, then that groundwater would probably need to have been fairly dilute to retain the calcium ions. Brief periods of wetter climatic conditions could lead to dilution and introduction of Ca2+ to shallow groundwater, and a slight elevation (few dm) of the water table. Under such conditions, Ca2+ might have become available to form calcite in the rhizosphere and form local cements. Prolonged heavy rains, however, would have led to a simultaneous rise in lake level, widespread submergence of the Loburu Delta, and dilution of the lake water. Such periods in the past (Renaut, 1982; Casanova and Renaut, 1987) have led to stromatolitic calcite encrustations around the stems of littoral macrophytes, but are unlikely to explain the Loburu rhizoliths. The model in Fig. 12A suggests that the calcareous rhizocretions on the Loburu Delta formed, at least in part, from surface runoff and meteoric rainwater that was percolating through the sediments. Precipitation would likely have occurred during relatively dry periods after dilute water was supplied to the rhizosphere. The R3 site lies close to an ephemeral stream that feeds a small northern lobe of the Loburu Delta. That stream, in the past, has been partly fed by Ca-bearing warm springs because fossil travertine deposits of unknown age lie

∼100 m upstream from the western edge of the delta. It is possible, therefore, that Ca-bearing waters (both runoff and shallow groundwaters) could have been locally supplied from the shifting ephemeral stream waters. Deocampo and Ashley (1999) suggested that calcite is unlikely to precipitate in wetlands because plant decay tends to lower the pH to a point where calcite crystallisation is prevented. Calcite around the silicified rhizoliths might, therefore, indicate a change from wetter to drier conditions at sites R3 and R5. This could have been achieved either climatically or by streams shifting their flow paths, as suggested above. For example, a fall in lake level linked to increased aridity could have induced a fall of the water table and lakeward migration of any freshwater lens. Plants that had been silicified when the water table was relatively high might then have been exposed to percolating fresher waters due to occasional Ca-bearing runoff on the drier delta plain. Kleinert and Strecker (2001), for example, described silicified rhizoliths from Neogene palaeosols in Argentina that were encased in younger calcite nodules, and attributed alternating calcite and silica laminae in rhizoliths to variations in seasonal moisture supply. The origin of unusual wing-shaped projections on some of the rhizoliths (Fig. 5C) is uncertain. It could reflect the structure of the original plant, such as the site where a horizontal rhizome joins the main stem (as in C. laevigatus), or be related to preferential cementation on one side of the rhizolith, which implies a specific direction of water flow. In contrast to the Loburu rhizoliths, calcite is the dominant mineral forming rhizoliths in the zeolitic palaeosols of the Sandai Plain, where there is no geothermal activity. Dilute Cabearing surface runoff that infiltrated the soils (Fig. 12C) might have mixed with saline, alkaline pore water in the capillary fringe above the lake-marginal water table. This could have induced subsurface calcite precipitation in root moulds and other pores as a result of the increase in pH and CO32− activity (Renaut, 1993; Nelson et al., 2001, p. 669). Alternatively, these rhizoliths might have formed after the main phase of zeolite formation. Calcite might have formed from dilute Ca-bearing groundwater associated with higher lake levels or rain-fed recharge, which would also probably raise the water table and induce groundwater dilution. Evapotranspiration and capillary rise processes could then have induced calcite precipitation in the upper soil profile. 7.3. Fluorite rhizoliths Fluorite-bearing rhizoliths are very unusual although minor fluorite has been reported from Ca-poor alkaline soils (Barbiéro and Van Vliet-Lanoe, 1998). Fluorite (CaF2) was confirmed as a component of the rhizoliths by X-ray diffraction at all the study sites on the Loburu Delta, but it is most abundant at sites R2, R3 and R5. The petrographic evidence shows that fluorite was commonly precipitated after opaline silica in siliceous rhizoliths, forming euhedral crystals in pores (Fig. 9B). Many of the textural relationships, however, are less clear. In some rhizoliths the fluorite appears to have replaced opal-A (Fig. 9C); in others, the two minerals are intercalated, which implies that fluorite

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precipitation alternated periodically with silica precipitation (Fig. 9E). High fluoride concentrations are common in Kenya Rift waters (e.g., Kilham and Hecky, 1973; Nair et al., 1984). The fluoride content of the Loburu hot-spring waters is high (62–74 mg l− 1), especially compared with runoff waters (b6 mg l− 1). Fluoride levels in Lake Bogoria are significantly higher (110–1,350 mg l− 1), mainly due to evaporative concentration, partial hydrothermal recharge, and the low concentration of alkaline earths. In waters where the Ca2+ concentration is low, fluoride will increase in the solution during evaporation (e.g., Gueddari, 1984; Barbiéro and Van Vliet-Lanoe, 1998). In common with the silica rhizoliths, fluorite-bearing rhizoliths are only found near hot springs, which implies that thermal fluids rather than the F-rich lake waters are the primary source of fluoride for the rhizoliths. The fluorite can be explained as a replacement of a precursor, or by direct precipitation from solution. Surdam and Eugster (1976) suggested that fluorite in the sediments of Lake Magadi formed by the reaction of highly alkaline, calcium-free, fluoride-rich interstitial waters directly with pre-existing calcite: CaCO3 þ F2 ¼ CaF2 þ CO2− 3 Textural evidence for the partial replacement of calcite or aragonite by fluorite has been found in travertines from Lake Bogoria (Renaut, 1982), and has been inferred at other localities (e.g., Icole et al., 1990). In some Loburu rhizoliths, fluorite is associated with calcite (Fig. 8), whereas in others the two minerals occur separately (Fig. 9D). Clear evidence for carbonate replacement by fluorite was not found, but the process is feasible where F-rich thermal fluids are in contact with alkaline earth carbonates. Minor calcite co-precipitates with silica in some of the modern spring deposits (Renaut et al., 1998), so any calcite that was formerly present with silica in the rhizoliths could potentially have reacted with later Ca-poor, F-bearing fluids to form fluorite. Replacement of earlier silica by fluorite might have been feasible if the pore fluids became highly alkaline (pH N 9). Some of the fabrics indicate that fluorite could be replacing former plant tissues (Fig. 9G). Unlike replacement of organic matter by non-crystalline silica that permeated cell walls and organic structures (permineralisation), growth of fluorite crystals appears to have destroyed original plant fabrics. Unequivocal evidence for replacement of plant tissues by fluorite was not found; some fluorite, however, could be filling porosity produced by earlier plant decay. Euhedral pore-filling fluorite cements, however, provide clear evidence for precipitation directly from solution (Fig. 9B). The modern Loburu spring waters are saturated with respect to both calcite and fluorite (Fig. 11). Although the calcium concentration of the spring fluids is low (b 2 mg l− 1 Ca2+), fluoride is a strongly electronegative element. Minor calcite is precipitating near some modern spring vents where rapid CO2 degassing increases the pH and the availability of CO32− for calcite precipitation (Jones and Renaut, 1995). At sites several metres from the vents, recent travertine is absent. With distance from the vent, the outflow

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waters undergo cooling which should increase the solubility of CaCO3. In a former analogous setting, any residual Ca2+ might have combined with the highly reactive F− to precipitate fluorite directly from solution if its ion activity product exceeded that of calcite. Renaut et al. (1999) reported recent fluorite precipitation at Lorusio hot springs in the northern Kenya Rift, where thermal fluids with a similar composition to those at Loburu underwent evaporative concentration more than 10 m from the active vent. An alternative method to direct precipitation from thermal fluids or replacement of older calcite would be mixing of Cabearing dilute water with F-rich thermal fluids (Fig. 12A). Where these two fluids are mixed in the rhizosphere, fluids supersaturated with respect to fluorite could be formed. Precipitation might occur on favourable substrates, perhaps enhanced by evaporation or evapotranspiration. The most likely situation might be where dilute runoff infiltrates the soil and mixes with F-enriched groundwater, although other mixing scenarios are also feasible. The fluorite that postdates silica precipitation in many rhizoliths implies that the fluid composition changed over time. The cause(s) of this change is unclear, but the fluids that precipitated fluorite might have been undersaturated with respect to amorphous silica, perhaps implying dilution of groundwaters. Cyclic alternation of fluorite and opaline silica laminae in the rhizoliths could indicate seasonal or longer-term changes in fluid composition. If seasonal, then rhizolith growth would have been rapid, as implied by the plant preservation. The bulbous fluorite rhizoliths (Type 5) are silica poor, showing that some silica and fluorite precipitation was spatially, as well as temporally, separated. The stubby nature and morphology of these rhizoliths suggests that precipitation might have been focused at the base of the stem near the sediment/air interface. The EDX scans detected small amounts phosphorus in some of the rhizoliths (Fig. 8). Authigenic fluorapatite has been reported in some hot-spring deposits (Stauffer, 1982; Gaciri and Davies, 1993) so it may be present although this is unconfirmed. Bone debris is locally common in the deltaic sediments, derived both from mammals and birds, so fine bone debris could also account for locally high P concentrations. 7.4. Zeolite rhizoliths The sediments at the northern end of Lake Bogoria contain a variety of rhizoliths of two basic compositions: calcite and zeolite. The zeolitic types are dominated by analcime with euhedral crystals, which suggests direct precipitation of analcime from solutions that flowed through open spaces such as root moulds and tubules. Clays coating the analcime and root moulds indicate that there was a later stage of illuviation. Renaut (1993) suggested that the Loboi Silts, in which the zeolitic rhizoliths occur, formed in a low-lying arid delta plain, similar to that present today. This surface lay near the margin of a saline lake and was subjected to soil forming processes. The locally abundant mudcracks and calcrete that are observed in the area were formed in this setting. An environmental model, suggested by Renaut (1993) for the formation of the zeolites, also serves as a basis for interpreting the rhizoliths (Fig. 12C).

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The plain was perhaps occupied by salt-tolerant grasses and scrub, and characterised locally by bare patches of soil with efflorescent trona, thermonatrite, and halite crusts. Shallow, saline, alkaline waters, rich in Na+ and CO32−, would have been drawn upwards following capillary evaporation, evapotranspiration and evaporative pumping. These pore fluids would then have reacted with detrital clays, volcanic glass and silicate minerals over a protracted length of time, perhaps several thousand years, to form the analcime. The occurrence of the zeolitic root mats suggests that, at times, the vegetation cover was relatively dense over parts of the surface, perhaps in areas of shallow groundwater near the lake or other water sources. Elsewhere, scattered plants gave rise to isolated root tubules and moulds. 8. Discussion Rhizoliths are common features in the Neogene sedimentary record of the East African Rift and are indicators of proximity to ancient land surfaces. However, apart from detailed studies at Lake Turkana (Cohen 1982; Mount and Cohen 1984) and at Olduvai in Tanzania (Liutkus et al., 2005), they have received little attention beyond a general recognition that they signify former soil profiles and can indicate the position of the palaeowater table at the time of formation. Given the number of plants associated with modern soils and land surfaces it might be expected that they should be observed more often in the fossil record. However, they are commonly restricted to particular horizons or sedimentary units, implying that specific conditions are required for them to form. The examples from Lake Bogoria demonstrate that rhizoliths are spatially and temporally restricted to a few beds and also show that they have good potential for providing even greater amounts of palaeoenvironmental information, especially where there are unusual mineralogies that can only form in specific settings. The models developed for Lake Bogoria (Fig. 12) have implications for both sedimentation and palaeoenvironmental interpretations in other areas. Calcareous rhizoliths of the type found on the Sandai Plain, which are the most common type precipitated from near surface waters in the Kenya Rift (e.g., Cohen, 1982; Mount and Cohen, 1984), form in a wide range of fluvial and lake-marginal settings both in Kenya and worldwide (e.g., Klappa, 1980; Melchor et al., 2002; Joseph and Thrivikramaji, 2005). In some locations, however, rhizoliths have a different mineralogy or may show stratigraphic variations in composition that could reflect environmental or other changes. For example, while much of the mid-Pleistocene Olorgesailie Formation in the southern Kenya Rift contains pedogenic horizons with calcareous rhizoliths, a significant change occurs in the uppermost Member 14 and overlying Olkesiteti Formation, where both silicified and calcareous rhizoliths are present, though in stratigraphically different beds. This raises significant questions as to what might have caused silica rhizoliths to develop. The information from Bogoria suggests that a consideration of factors such as fluid sources, the role of rainfall and groundwater variability, the influence of faulting on thermal fluids, and early diagenesis might be important in explaining

rhizolith changes at Olorgesailie and potentially in other continental basins. At Loburu, silicification of roots and lower stems has taken place (Fig. 12A). In contrast, although silica mobility is high, precipitation can sometimes be inhibited in rift sediments in East Africa, because of the very high pH environments that prevail. The presence of silica (diatoms, phytoliths, silicified plants) in wetlands has also been reported from the Ngorongoro Crater in Tanzania (Deocampo and Ashley, 1999), where preservation is related to relatively low pH conditions caused by plant respiration and the presence of organic acids from decay. Deocampo and Ashley (1999) also noted that in drier areas away from such marshy settings, calcite is usually the first mineral to exceed its solubility and precipitate. Plant decay does not seem to have been a major factor at Loburu, where neutral to high pH conditions prevail across most of the delta plain. Onkware (2000) noted that high salinity can inhibit the decay of halophytic plants. As a consequence, thick root mats can develop. This may partly explain the dense bundles of rhizoliths commonly present at Loburu. Excavation and the presence of minor pyrite, however, confirm that reducing conditions develop in the shallow waterlogged soil zone, implying that some organic decay does occur. Silica and fluorite-bearing rhizoliths have several possible interpretations. High concentrations of both fluoride and silica are associated with saline, alkaline lakes, but they also characterise thermal fluids. The restriction of fluorite-silica rhizoliths to the hot-spring sites at Loburu implies that the fluid source for their formation was mainly hydrothermal. However, immersion of root systems in saline, alkaline lake-marginal fluids (surface waters or shallow groundwaters) could potentially produce silica and (or) fluorite-bearing rhizoliths. This would mostly affect littoral and aquatic macrophytes with shallow root systems. With high solute concentrations in both geothermal and alkaline lake fluids, mineralization could be rapid and enhance the possibility for tissue and cellular replacement (cf. Trewin et al., 2003). The high solubility of silica with high pH and high temperature make rift basins favourable settings for plant root silicification, especially where geothermal activity is present. Nonetheless, not all silica rhizoliths necessarily formed from highly alkaline and hot fluids. Most dilute runoff and groundwaters in the Kenya Rift have high silica concentrations that reflect the predominant volcanic bedrock, abundant soluble volcanic glass in the rift sediments, and silicate hydrolysis during weathering. Moderate levels of evaporative concentration and reduced pH in areas of organic decay, such as wetlands where the pH is less well buffered, may permit plant root silicification. Plant and root silicification has been reported from settings without high alkalinity and thermal fluids (e.g., Knoll, 1985; Schopf, 1971; Hendry, 1987). The processes, timing and rates of plant silicification under these conditions merit further study. The zeolitic rhizoliths and rootmats to the north of Lake Bogoria are highly unusual and are probably limited to saline, alkaline soils in dry environments. They require highly saline, alkaline fluids to form (cf. Hay, 1970; Renaut, 1993; Mees et al., 2005) and probably a period of prolonged environmental stability

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because soil zeolites do not seem to form rapidly. They are commonly associated with salt-tolerant plants, but could also affect other plants following salinization of the rhizosphere, which would tend to slow down the rate of organic decay. The Bogoria rhizoliths suggest that closer examinations of fossilised roots could yield additional valuable data in palaeoenvironmental studies. Rhizoliths have been used previously to infer former water table depths, but they also have implications for the water chemistry at the time of their formation. Some of the unusual mineralogies reported here might facilitate the recognition of spring systems in ancient deposits. Zeolitic rhizoliths yield information on likely groundwater conditions, as do calcite rhizoliths. In some cases, compositional changes (e.g. silica-fluorite laminae, calcite encrusting silica) in individual rhizoliths indicate fluid variations, and perhaps changing rainfall or groundwater supplies. 9. Conclusions A wide range of rhizolith morphologies and compositions is present around the margins of Lake Bogoria. The main minerals forming the rhizoliths, either singly or in combination, are opaline silica (opal-A), fluorite, and calcite, with minor pyrite and possibly fluorapatite. The rhizoliths include root casts, rhizocretions, moulds, tubules, permineralised roots, and root marks. In part, this range of rhizolith types reflects the variety of plant species and plant tissues that were fossilised. It also reflects a range of processes and the diverse fluid compositions that brought about the preservation of roots and root traces. Hot-spring fed waters on the Loburu Delta have relatively high levels of silica and fluoride, but low levels of calcium. The present silica levels seem insufficient to induce plant silicification and formation of silica rhizoliths, but further concentration through evapotranspiration while the plants are alive and capillary evaporation after death, can induce conditions suitable for silicification. Runoff and dilute meteoric groundwaters have higher levels of calcium than spring fluids and can precipitate calcite in and adjacent to roots structures following evaporative concentration or CO2 degassing. Fluorite might have formed by mixing of meteoric and spring waters, or it could have developed where fluoride in spring waters reacted with pre-existing calcite. Zeolites (mainly analcime), associated with rhizoliths on the Sandai Plain, probably formed through the alteration of siliciclastic minerals by highly alkaline sodium-rich waters, following evapotranspiration of shallow, lake-marginal groundwater. A distinctive feature of the rhizoliths around the margins of Lake Bogoria is their spatial variability, with contrasting rhizoliths occurring within short distances of each other. This reflects the rapid changes in the environmental settings at the time of their formation, with wetlands coexisting with drier areas that also lie close to a large saline alkaline water body. Each of these habitats has contrasting physical and chemical conditions. Rhizoliths reflect these settings and, given their close relationships with environments, should perhaps be given greater attention in palaeoenvironmental investigations.

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Acknowledgements Research in Kenya was undertaken under research permits issued by the Ministry of Science and Technology, Republic of Kenya (13/001/31C 103/9), which we grateful acknowledge. Funding for this study was provided by the Hong Kong Baptist University (FRG/05-05/II-50 to RBO), the Hong Kong Research Grants Council (HKBU 2013/06P to RBO), the Natural Sciences and Engineering Research Council of Canada (Research Grant RG 629-03 to RWR and A6090 to BJ), and the National Science Foundation (NSF-EAR 0207705 to Ashley and Hover). This research follows on from an undergraduate dissertation by RAO, supervised by Dr. P. Barker, at Lancaster University. We thank William Kimosop, Chief Warden of the Lake Bogoria National Reserve and John Ego (National Oil Corporation of Kenya) for their support and assistance with this research. We thank two anonymous reviewers and the journal editor for their comments, which helped to improve an earlier version of the manuscript. References Allen, D.J., Darling, W.G., Burgess, W.G., 1989. Geothermics and hydrogeology of the southern part of the Kenya Rift Valley with emphasis on the Magadi–Nakuru area. Br. Geol. Surv. Res. Rep. SD/89/1, 68 pp. Ashley, G.M., Mworia, J.M., Muasya, A.M., Owen, R.B., Driese, S.G., Hover, V.C., Renaut, R.W., Goman, M.F., Mathai, S., Blatt, S.H., 2004. Sedimentation and recent history of a freshwater wetland in a semi-arid environment: Loboi Swamp, Kenya, East Africa. Sedimentology 51, 1301–1321. Barbiéro, L., Van Vliet-Lanoe, B., 1998. The alkali soils of the middle Niger valley: origins, formation and present evolution. Geoderma 84, 323–343. Candy, I., 2002. Formation of a rhizogenic calcrete during a glacial stage (Oxygen Isotope Stage 12): its palaeoenvironmental stratigraphic significance. Proc. Geol. Assoc. Lond. 113, 259–270. Casanova, J., Renaut, R.W., 1987. Stromatolites du milieu lacustre. In: Tiercelin, J.-J., Vincens, A. (Eds.), Le Demi-graben de Baringo-Bogoria Rift Gregory, Kenya. Bull. Centres Rech. Explor.-Prod. Elf Aquitaine, pp. 490–498. Channing, A., Edwards, D., 2003. Experimental taphonomy: silicification of plants in Yellowstone hot-spring environments. Trans. R. Soc. Edinb. Earth Sci. 94, 503–521. Channing, A., Edwards, D., Sturtevant, S., 2004. A geothermally influenced wetland containing unconsolidated geochemical sediments. Can. J. Earth Sci. 41, 809–827. Cioni, R., Fanelli, G., Kinyariro, J.K., Marini, L., 1992. Lake Bogoria hot springs (Kenya): geochemical features and geothermal implications. J. Volcanol. Geotherm. Res. 50, 231–246. Cohen, A.S., 1982. Paleoenvironments of root casts from the Koobi Fora Formation, Kenya. J. Sediment. Petrol. 52, 401–414. Deocampo, D.M., Ashley, G.M., 1999. Siliceous islands in a carbonate sea: modern and Pleistocene spring-fed wetlands in Ngorongora Crater and Oldupai Gorge, Tanzania. J. Sediment. Petrol. 69, 974–979. Driese, S.G., More, C.I., Elick, J.M., 1997. Morphology and taphonomy of root and stump casts of the earliest trees (Middle to Late Devonian), Pennsylvania and New York, USA. Palaios 12, 524–537. Elick, J.M., Driese, S.G., Mora, C.I., 1998. Very large plant and root traces from the Early to Middle Devonian: implications for early terrestrial ecosystems and atmospheric p(CO2). Geology 26, 143–146. Epstein, E., 1994. The anomaly of silicon in plant biology. Proc. Nat. Acad. Sci. U. S. A. 91, 11–17. Epstein, E., 1999. Silicon. Ann. Rev. Plant Physiol. Plan Mol. Biol. 50, 641–664. Eugster, H.P., 1980. Geochemistry of evaporitic lacustrine deposits. Annu. Rev. Earth Planet. Sci. 8, 35–63.

162

R.A. Owen et al. / Sedimentary Geology 203 (2008) 143–163

Farrand, W.R., Redding, R.W., Wolpoff, M.H., Wright, H.T., 1976. An archeological investigation on the Loboi Plain, Baringo District, Kenya. Tech. Rep. Mus. Anthropol. Univ. Mich. 4. Gaciri, S.J., Davies, T.C., 1993. The occurrence and geochemistry of fluoride in some natural waters of Kenya. J. Hydrol. 143, 395–412. Gueddari, M., 1984. Géochimie et themodynamique des evaporites continentals. Etude du lac Natron en tanzanie et du Chott el Jerid en tunisie. Mém. Sci. Géol. 76. Gutiérrez-Castorena, M.C., Stoops, G., Ortoz Solario, C.A., Avila, G.L., 2005. Amorphous silica materials in soils and sediments of the Ex-Lago de Texcoco, Mexico: an explanation for its subsidence. Catena 60, 205–266. Gutiérrez-Castorena, M.C., Stoops, G., Ortoz Solario, C.A., Sánchez-Guzmán, P., 2006. Micromorphology of opaline features in soils on the sediments of the ex-Lago de Texcoco, México. Geoderma 132, 89–104. Harper, D.M., Brooks Childress, R., Harper, M.M., Boar, R.R., Hickley, P., Mills, S.C., Otieno, N., Drane, T., Vareschi, E., Nasirwa, O., Mwatha, W.E., Darlington, J.P.E.C., Escuté-Gasulla, X., 2003. Aquatic biodiversity and saline lakes: Lake Bogoria National Reserve, Kenya. Hydrobiologia 500, 259–276. Hay, R.L., 1970. Silicate reactions in three lithofacies of a semi-arid basin, Olduvai Gorge, Tanzania. In: Morgan, B.A. (Ed.), Fiftieth Anniversary Symposia: Mineralogy and Petrology of the Upper Mantle, Sulfides, and Geochemistry of Non-marine Evaporites. Mineral. Soc. Am. Spec. Pap., vol. 3, pp. 237–255. Hembree, D.I., Hasiotis, T.S., 2007. Paleosols and ichnofossils of the White River Formation of Colorado: insight into soil ecosystems of the North American midcontinent during the Eocene-Oligocene transition. Palaios 22, 123–142. Hendry, D.A., 1987. Silica and calcium carbonate replacement of plant roots in tropical dune sands, SE India. In: Frostick, L.E., Reid, I. (Eds.), Desert Sediments: Ancient and Modern. Geol. Soc. Lond. Spec. Publ., vol. 35, pp. 309–319. Hill, C.A., Dublyansky, Y.V., Harmon, R.S., Schluter, C.M., 1995. Overview of calcite/opal deposits at or near the proposed high-level nuclear waste site, Yucca Mountain, Nevada, USA: Pedogenic, hypogene, or both? Environ. Geol. 26, 69–88. Hinman, N.W., Lindstrom, R.F., 1996. Seasonal changes in silica deposition in hot spring systems. Environ. Geol. 132, 237–246. Icole, M., Masse, J.-P., Perinet, G., Taieb, M., 1990. Pleistocene lacustrine stromatolites, composed of calcium carbonate, fluorite, and dolomite, from Lake Natron, Tanzania: depositional and diagenetic processes and their paleoenvironmental significance. Sediment. Geol. 69, 139–155. James, N.P., 1972. Holocene and Pleistocene calcareous crust (caliche) profiles: criteria for subaerial exposure. J. Sediment. Petrol. 42, 817–836. Jones, B., Ng, K.-C., 1988. The structure and diagenesis of rhizoliths from Cayman Brac, British West Indies. J. Sediment. Petrol. 58, 457–467. Jones, B., Renaut, R.W., 1995. Noncrystallographic calcite dendrites from hotspring deposits at Lake Bogoria, Kenya. J. Sediment. Res. A65, 154–169. Jones, B., Renaut, R.W., Rosen, M.R., Klyen, L., 1998. Primary siliceous rhizoliths from Loop Road hot-springs, North Island, New Zealand. J. Sediment. Res. 68, 115–123. Jones, B., Renaut, R.W. (Eds.), 2003. Sedimentology of Hot Spring Systems (Special Issue). Can. J. Earth. Sci., 40, pp. 1439–1738. Jones, L.H.P., Milne, A.A., Wadham, S.M., 1963. Studies of silica in the oat plant, III. Distribution of silica in the plant. Plant Soil 18, 358–371. Joseph, S., Thrivikramaji, K.P., 2005. Rhizolithic calcrete in Teris, southern Tamil Nadu; origin and paleoenvironmental implications. J. Geol. Soc. India 65, 158–168. Kharaka, Y., Gunter, W.D., Aggarwal, P.K., Perkins, E.H., DeBraal, J.D., 1988. SOLMINEQ.88: a computer program for the modeling of water-rock interactions. U. S. Geol. Surv. Water Res. Invest. Rep. 88–4227. Kilham, P., Hecky, R.E., 1973. Fluoride: geochemical and ecological significance in East African waters and sediments. Limnol. Oceanogr. 18, 932–945. Klappa, C.F., 1979. Calcified filaments in Quaternary calcretes: organo-mineral interactions in the subaerial vadose environment. J. Sediment. Petrol. 49, 955–968. Klappa, C.F., 1980. Rhizoliths in terrestrial carbonates: classification, recognition, genesis and significance. Sedimentology 24, 657–674.

Kleinert, K., Strecker, M.R., 2001. Climate change in response to orographic barrier uplift: paleosol and stable isotope evidence from the late Neogene Santa Maria Basin, northwest Argentina. Geol. Soc. Amer. Bull. 113, 728–742. Knoll, A.H., 1985. Exceptional preservation of photosynthetic organisms in silicified carbonates and peats. Phil. Trans. R. Soc. B 311, 111–122. Konhauser, K.O., Ferris, F.G., 1996. Diversity of iron and silica precipitation by microbial mats in hydrothermal waters, Iceland: implications for PreCambrian iron formations. Geology 24, 323–326. Kosir, A., 2004. Microcodium revisited: root calcification products of terrestrial plants on carbonate-rich substrates. J. Sediment. Res. 74, 845–857. Kraus, M.J., Hasiotis, S.T., 2006. Significance of different modes of rhizolith preservation to interpreting paleoenvironmental and paleohydrologic settings: examples from Paleogene paleosols, Bighorn Basin, Wyoming, U.S.A. J. Sediment. Res. 76, 633–646. Leo, R.F., Barghoorn, E.S., 1976. Silicification of wood. Harvard Univ. Bot. Mus. Leaflets 25, 1–47. Liutkus, C.M., Wright, J.D., Ashley, G.M., Sikes, N., 2005. Paleoenvironmental interpretation of lake-margin deposits using δ13C and δ18O results from Pleistocene carbonate rhizoliths, Olduvai Gorge, Tanzania. Geology 33, 377–380. McNaughton, S.J., Tarrants, J.L., McNaughton, M.M., Davis, R.H., 1985. Silica as a defense against herbivory and a growth promotor in African grasses. Ecology 66, 528–535. Mees, F., Stoops, G., Van Ranst, E., Paepe, R., Van Overloop, E., 2005. The nature of zeolite occurrences in deposits of the Olduvai Basin, northern Tanzania. Clays Clay Miner. 53, 659–673. Melchor, R.N., Genise, J.F., Miquel, S.E., 2002. Ichnology, sedimentology and paleontology of Eocene calcareous paleosols from a palustrine sequence, Argentina. Palaios 17, 16–35. Mount, J.F., Cohen, A.S., 1984. Petrology and geochemistry of rhizoliths from Plio-Pleistocene fluvial and marginal lacustrine deposits, east Lake Turkana, Kenya. J. Sediment. Petrol. 54, 263–275. Nair, K.R., Manji, F., Gitonga, J.N., 1984. The occurrence and distribution of fluoride in groundwaters of Kenya. Int. Assoc. Hydrol. Sci. Publ. 44, 75–86. Northrop, J.I., 1890. Notes on the geology of the Bahamas. Trans. N.Y. Acad. Sci. 10, 4–22. Nelson, S.T., Karlsson, H.R., Paces, J.B., Tingey, D.G., Ward, S., Peters, M.T., 2001. Paleohydrologic record of spring deposits in and around Pleistocene pluvial Lake Tecopa, southeastern California. Geol. Soc. Am. Bull. 113, 659–670. Onkware, A.O., 2000. Effect of soil salinity on plant distribution and production at Loburu Delta, Lake Bogoria National Reserve, Kenya. Austral Ecol. 25, 140–149. Owen, R.B., Renaut, R.W., Hover, V.C., Ashley, G.M., Muasya, A.M., 2004. Swamps, springs, and diatoms: wetlands of the semi-arid Bogoria–Baringo Rift, Kenya. Hydrobiologia 518, 59–78. Renaut, R.W., 1982. Late Quaternary Geology of the Lake Bogoria FaultTrough, Kenya Rift Valley, Ph.D. thesis, University of London, 498 pp. Renaut, R.W., 1993. Zeolitic diagenesis of late Quaternary fluviolacustrine sediments and associated calcrete formation in the Lake Bogoria Basin, Kenya Rift Valley. Sedimentology 40, 271–301. Renaut, R.W., Jones, B., 1997. Controls on aragonite and calcite precipitation in hot spring travertines at Chemurkeu, Lake Bogoria, Kenya. Can. J. Earth Sci. 34, 801–818. Renaut, R.W., Jones, B., Le Turdu, C., 1999. Calcite lilypads and ledges at Lorusio Hot Springs, Kenya Rift Valley: travertine precipitation at the airwater interface. Can. J. Earth Sci. 36, 649–666. 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. Renaut, R.W., Morley, C.K., Jones, B., 2002. Fossil hot-spring travertine in the Turkana Basin, northern Kenya: structure, facies, and genesis. In: Renaut, R.W., Ashley, G.M. (Eds.), Sedimentation in Continental Rifts, vol. 73. SEPM Spec. Publ., pp. 123–141. Renaut, R.W., Owen, R.B., 1988. Opaline cherts associated with sublacustrine hydrothermal springs at Lake Bogoria, Kenya Rift Valley. Geology 16, 699–702. Renaut, R.W., Owen, R.B., 1991. Shore-zone sedimentation and facies in a closed rift lake: the Holocene beach deposits of Lake Bogoria, Kenya. In:

R.A. Owen et al. / Sedimentary Geology 203 (2008) 143–163 Anadón, P., Cabrera, L., Kelts, K. (Eds.), Lacustrine Facies Analysis (Spec. Publ. Int. Assoc. Sedimentol. 13). Blackwell, Oxford, pp. 175–195. Renaut, R.W., Tiercelin, J.-J., 1994. Lake Bogoria, Kenya Rift Valley: a sedimentological overview. In: Renaut, R.W., Last, W.M. (Eds.), Sedimentology and Geochemistry of Modern and Ancient Saline Lakes, vol. 50. SEPM Spec. Publ., pp. 101–123. Renaut, R.W., Tiercelin, J.-J., Pagé, P., 1987. Alimentation, hydrologie. In: Tiercelin, J.J., Vincens, A. (Eds.), Le demi-graben de Baringo-Bogoria, Rift Gregory, Kenya. 30000 ans d'histoire hydrologique et sédimentaire. Bull. Centre Rech. Explor.-Prod., vol. 11. Elf-Aquitaine, pp. 284–309. Rimstidt, J.D., Cole, D.R., 1983. Geothermal mineralization I: the mechanism of formation of the Beowawe, Nevada, siliceous sinter deposit. Am. J. Sci. 283, 861–875. Sanz-Rubio, E., Hoyos, M., Calvo, J.P., Rouchy, J.M., 1999. Nodular anhydrite growth controlled by pedogenic structures in evaporite lake formations. Sediment. Geol. 125, 195–203. Schopf, J.M., 1971. Notes on plant tissue preservation and mineralization in a Permian deposit of peat from Antarctica. Am. J. Sci. 271, 522–543. Schultze-Lam, S., Fortin, D., Davis, B.S., Beveridge, T.J., 1996. Mineralization of bacterial surfaces. Chem. Geol. 132, 171–181. Scott, J.J., 2005. Taphonomy of Modern and Ancient Vertebrate Traces in the Marginal Sediments of Saline, Alkaline and Freshwater Lakes, Baringo– Bogoria Basin, Kenya Rift Valley. M.Sc. thesis, University of Saskatchewan, 418 pp. Scott, J.J., Renaut, R.W., Owen, R.B., in press-a. Preservation and paleoenvironmental significance of a footprinted surface on the Sandai Plain, Lake Bogoria, Kenya Rift Valley. Ichnos. Scott, J.J., Renaut, R.W., Owen, R.B., Sarjeant, W.A.S., in press-b. Biogenic activity, trace formation, and trace taphonomy in the marginal sediments of saline, alkaline Lake Bogoria, Kenya Rift Valley. In: Bromley, R., Buatois, L.A. (Eds.), Sediment–Organism Interactions: a Multifaceted Ichnology. SEPM Special Publication 88, pp. 309–330.

163

Sommer, M., Kaczorek, D., Kuztakov, Y., Breuer, J., 2006. Silicon pools and fluxes in soils and landscapes—a review. J. Plant Nutr. Soil Sci. 169, 310–329. Stauffer, R.E., 1982. Fluorapatite and fluorite solubility controls on geothermal waters in Yellowstone National Park. Geochim. Cosmochim. Acta 46, 465–474. Sundby, B., Vale, C., Caçador, I., Catarino, F., Madureira, M.-J., Caetano, M., 1998. Metal-rich concretions on the roots of salt-marsh plants: mechanism and rate of formation. Limnol. Oceanogr. 43, 245–252. Surdam, R.C., Eugster, H.P., 1976. Mineral reactions in the sedimentary deposits of the Lake Magadi region, Kenya. Geol. Soc. Amer. Bull. 87, 1739–1752. Tiercelin, J.J., Vincens, A. (Eds.), 1987. Le demi-graben de Baringo-Bogoria, Rift Gregory, Kenya. 30000 ans d'histoire hydrologique et sédimentaire. Bull. Centre Rech. Explor.-Prod Elf-Aquitaine, vol. 11, pp. 249–540. Trewin, N.H., Fayers, S.R., Kelman, R., 2003. Subaqueous silicification of the contents of small ponds in an Early Devonian hot-spring complex, Rhynie, Scotland. Can. J. Earth Sci. 40, 1697–1712. Vincens, A., 1986. Diagramme pollinique d'un sondage pléistocène supérieurholocène du lac Bogoria (Kenya). Rev. Palaeobot. Palynol. 47, 169–192. Ward, W.C., 1975. Petrology and diagenesis of carbonate eolianites of northeastern Yucutan Peninsula, Mexico–Belize Shelf–Carbonate sediments, clastic sediments, and ecology. Am. Assoc. Petrol. Geol. Stud. Geol. (2). White, T.H., 1953. Some speculations on the sudden occurrence of floods in the history of Lake Magadi. J. East. Afr. Nat. Hist. Soc. 22, 69–71. Wright, V.P., 1992. Paleosol recognition: a guide to early diagenesis in terrestrial settings. In: Wolf, K.H., Chilingarian, G.V. (Eds.), Diagenesis, III. Elsevier, Amsterdam, pp. 591–619. Wright, V.P., Platt, N.H., Marriot, S.B., Beck, V.H., 1995. A classification of rhizogenic (root-formed) calcretes, with examples from the Upper Jurassic– Lower Cretaceous of Spain and Upper Cretaceous of southern France. Sediment. Geol. 100, 143–158. Yechieli, Y., Wood, W.W., 2002. Hydrogeologic processes in saline systems: playas, sabkhas, and saline lakes. Earth-Sci. Rev. 58, 343–365.