Micromorphology and genesis of Quaternary stromatolitic tufa crust within the exposed Cretaceous Rayda Formation in Al-Jabal Al-Akhdar, Oman

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Micromorphology and genesis of Quaternary stromatolitic tufa crust within the exposed Cretaceous Rayda Formation in Al-Jabal Al-Akhdar, Oman Fikry I. Khalaf Geology Department, Faculty of Science, Port Said University, 23 December Street, Qism Al-Zahoor, Port-Said Governorate, Egypt

A R T I C LE I N FO

A B S T R A C T

Keywords: Stromatolitic tufa Lamination Stable isotopes Microbialite Oman

This study aimed to improve the understanding of the causes of lamination in terrestrial tufa, using samples of terrestrial Quaternary stromatolitic tufa deposits occurring within narrow karst grooves carved along the bedding planes of the exposed Cretaceous Rayda Formation in Al-Jabal Al-Akhdar, Oman. These stromatolitic tufa deposits consist wholly of low-Mg calcite and are formed of vertically stacked laminated planar and digitate-like columnar structures. Lamination is exhibited as vertically stacked couplets of dark micritic and light sparitic laminae. Significant variations in the abundance and thickness of the micritic laminae, as well as progressive changes in the size and fabric of calcite in the sparitic laminae were recognised. Thorough microscopic and nanoscopic examinations revealed that these laminations initially developed as stacked, indistinct micritic laminae formed of calcitic microbialite that had initially precipitated within a biologically active microbial biofilm which thrived on the wet bedrock surface. This was followed by the growth of microglobules at certain horizons within the microbialitic laminae, forming incipient, discontinuous thin sparitic laminae consisting of laterally stacked biogenic skeletal disphenoidal calcite microcrystals. The latter were further growing via abiogenic syntaxial precipitation of calcite, forming coarsely crystalline dagger-shape bladed calcite crystals. These processes resulted in the formation of well-developed lamination. It is therefore suggested that the stromatolitic lamination has been generated through five consecutive stages of biogenic and abiogenic precipitation of calcite within initially developed microbial biofilms. Stable isotope values (δ13C and δ18O) suggest that these tufa deposits had precipitated from continuously renewed freshwater enriched with bicarbonate dissolved from the marine limestone bedrock during a cold pluvial period.

1. Introduction Tufa, travertine, and speleothem are terrestrial freshwater carbonates that are commonly precipitated on limestone terrain (Viles, 2004). The terms tufa and travertine had been used interchangeably for an extensive period of time. At present, there is a general consensus among scientists that the tufa refers to meteogene deposits produced from ambient-temperature waters, while the travertine is used for thermogene deposits produced from hydrothermal waters (Pedley, 1990; Ford and Pedley, 1996). A typical feature of tufas is the presence of macrophytes; however, microphytes (blue green algae) and prokaryotes may dominate some types, such as stromatolitic tufa (Pedley, 2009). Several attempts at classification of tufa deposits have been made. Pedley (1990) and Ford and Pedley (1996) distinguished five major models for tufa deposits, namely perched springline, cascade, fluvial, lacustrine, and paludal. Tufa is genetically divided into two main groups, namely autochthonous and allochthonous (Koşun, 2012). The studied stromatolitic tufa herein may be described as a type of autochthonous tufa. Stromatolites are biologically developed laminated sediments,

primarily carbonates, the formation of which are most significantly influenced by cyanobacteria (Winsborough and Golubic, 1987; Freytet and Verrecchia, 1998). They occur in a wide variety of settings, including marine, lacustrine, riverine, and terrestrial (as freshwater tufa stromatolites or stromatolitic tufa) environments. The study of stromatolites in marine environments has attracted the attention of many scientists since the early sixties (Logan, 1961; Arp et al., 2003; Decho et al., 2005; Dupraz and Visscher, 2005). However, throughout the last two decades, there has been more interest in those precipitated in freshwater environments (i.e., stromatolitic tufa) (Pedley, 1994; Pentecost, 2005; Turner and Jones, 2005; Bissett et al., 2008; Rogerson et al., 2008; Pedley et al., 2009; Arenas et al., 2010; Arp et al., 2010; Gradzinski, 2010; Jones and Peng, 2012; Capezzuoli et al., 2014). These prior studies have revealed that tufa stromatolites are usually initiated by a mat of interwoven cyanobacterial filaments (microbial biofilm), in which photosynthesis commonly induces precipitation of calcium carbonates. Field monitoring of naturally occurring freshwater microbial biofilms, as well as laboratory examinations of biofilms cultured in a mesocosm, have provided valuable information about such microbially biomediated calcium carbonate precipitation. Results of these studies

https://doi.org/10.1016/j.catena.2019.104314 Received 29 March 2019; Received in revised form 3 October 2019; Accepted 7 October 2019 0341-8162/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Fikry I. Khalaf, Catena, https://doi.org/10.1016/j.catena.2019.104314

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2009; Al-Busaidi, 2012). Using stable isotope data of the Quaternary travertine deposits, Clark and Fontes (1990), Neff et al. (2001) and Fleitmann et al. (2007) reconstructed the principal Late Pleistocene and Holocene climatic episodes of northern Oman. They concluded that the region witnessed a long pluvial episode during the late Pleistocene (from > 33,000 to 19,000 yr BP). This was followed by a phase of climatic deterioration dominated by hyperaridity, from ca. 16,300 to 13,000 yr BP, which coincided with the end of the northern hemisphere glacial maximum. During this period, the Arabian Gulf was dry. A shorter pluvial period prevailed during the Early Holocene, from 12,500 to ca. 6500 yr BP, which was attributed to the northward shift of the Southwest Indian Monsoon, causing an increase in precipitation (Glennie, 1998; Fleitmann et al., 2003). The climate then shifted back to hyperaridity, and the region was subjected to intermittent wet and dry periods, leading up to the current climate.

have explained the roles of cyanobacteria and their extrapolymeric substances (EPS) in the precipitation of calcium carbonate. The studies have also discussed how the initially precipitated nanoglobules of amorphous calcium carbonate (ACC) clustered around cyanobacterial trichomes and organised themselves into skeletal crystals (Turner and Jones, 2005; Pedley, 2014; Khalaf, 2017). Tufa stromatolites are mostly laminated due to vertically stacked couplets of dark micritic and lighter sparitic laminae (Pedley, 1994). The genesis of this laminated structure has been a topic of contention. It has been ascribed by some authors to climate change and variations in environmental energy (Pedley, 1994; Arp et al., 2010; Gradzinski, 2010; Arenas et al., 2010; Dabkowski et al., 2013). Others have attributed such lamination to biotic and abiotic activities, and the effects of EPS (Pedley, 2009; Pedley and Rogerson, 2010; Pedley, 2014). Therefore, the ultimate cause of lamination in tufa stromatolites remains unresolved (Pedley, 2013). Stable isotope (oxygen and carbon) signatures in tufas have been successfully used to elucidate climatic and environmental conditions that were influential in their formation (Andrews et al., 1997; Henning et al., 1983; Andrews et al., 1994; Andrews and Brasier, 2005; Andrews, 2006; Soriano et al., 2017). The Sultanate of Oman is characterised by an abundance of Early to Late Holocene tufa, travertine, and calcite veins. These deposits are produced via calcite precipitation from hyperalkaline springs scattered within northern Oman (Clark and Fontes, 1990). The research published on these deposits has mostly focused on determination of the palaeoclimatic conditions which prevailed during the Late Quaternary period (Burns et al., 2001; Fleitmann et al., 2004, 2007; Kelemen and Matter, 2008). Research concerned with the petrography and genesis of the tufa deposits in Oman is very limited (Khalaf, 2017). Therefore, in the present study, the petrographic and nanoscopic characteristics of some Late Quaternary stromatolitic tufa deposits within the exposed limestone of the Cretaceous Rayda Formation in the northern mountains of Oman were examined. Further, an attempt was made to understand the cause of the tufa stromatolitic laminations. The genesis of the various calcite morphotypes are also discussed.

4. Materials and methods Twenty samples were collected from the tufa crust. The collected samples were slabbed and thin sectioned. Hand specimens and slabs were macroscopically examined and photographed. Thin sections were petrographically examined, and the structures and textures were described. Fifteen representative tufa samples were selected and examined using a Supra 50 LEO-141 variable pressure scanning electron microscopy (VPSEM) system equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. Seven samples were analysed by X-ray powder diffraction (XRD) and X-ray fluorescence (XRF) at the Department of Earth and Environmental Sciences laboratories of Kuwait University. A set of 7 samples was selected for bulk analysis of mineral constituents, concentrations of major oxides, and stable carbon and oxygen isotopes. The mineralogical compositions of the samples were determined by XRD using Cu Kα1 radiation (λ = 1.5406 Å) and a Ni filter, at a voltage of 40 kV and a current of 40 mA. Semi-quantitative analysis of the major oxides was carried out using XRF. Fragments of millimetre size were selected from pristine parts of the columnar tufa stromatolites for stable isotope analysis, which was performed at the stable isotope laboratory of the University of Arizona (USA). The isotopes were measured using an automated carbonate preparation device (KIEL-III) coupled to a gas-ratio mass spectrometer (Finnigan MAT 252). Powdered samples were reacted with dehydrated phosphoric acid under vacuum at 70 °C. The isotope ratio measurement system was calibrated and had precisions of ± 0.10 for δ18O and ± 0.08‰ for δ13C (1 sigma). The results are presented in δ‰ units relative to Vienna Pee Dee Belemnite (VPDB) standards (Table 1).

2. Field occurrence Al-Jabal Al-Akhdar is located in the central part of the Oman Mountains. It is an eroded antiform with an approximately 2500 mthick succession of Precambrian to Cretaceous carbonates at its core (Haan et al., 1990). The study area is located in the village of Al-Hamra within Al-Jabal Al-Akhdar (Fig. 1), where a sequence of the Early Cretaceous latest Jurassic Rayda Formation is well exposed along a roadcut (Fig. 2A). It occurs as thin bedded, light grey hard porcellaneous limestone characterised by a frequent occurrence of cephalopod fossils, primarily belemnites. Beds range in thickness from 15 to 40 cm, and vary in dip from nearly horizontal to less than 15° to the southeast. The exposed surface of the Rayda Formation displays a low step terrace morphology, where the surface is cut across in the dip direction. The thin bedding is clearly marked by reddish deposits along the bedding planes. Close inspection of these deposits revealed that they are calcareous tufa crusting narrow dissolution grooves along the bedding planes, which reach 30 cm in thickness and extend laterally along the bedding planes for several meters (Fig. 2A and B).

5. Results 5.1. Macromorphology The calcareous crust material was reddish and coherent. It was mostly precipitated on the ceilings of the dissolution grooves, commonly in evenly distributed contact with the bedrock, although corrugated, uneven contacts were also recognised. Vertical slabs of the crust material exhibited different architectural patterns, the most common of which was a vertically stacked sequence of more than five alternating thinner planar and thicker columnar bands (Fig. 2C and D). Occasionally, the sequence was terminated by a slightly porous, light grey coliform crust. The planar bands were lighter in colour, mostly impervious, and varied in thickness between 2 and 8 mm. They were commonly wavy and laterally discontinuous. The columnar bands were commonly reddish, very porous, and consisted of laterally arranged microstalactitic columns. The latter were generally cylindrical with uneven surfaces, varying in length from a few millimetres to several centimetres, and reaching 8 mm in diameter. Some stalactitic columns

3. Climate The Sultanate of Oman is characterised by a subtropical dry, hot desert climate with low annual rainfall. Unlike the surrounding desert area, the climate of the Al-Jabal Al-Akhdar region tends to resemble a Mediterranean climate. It is characterised by wet, cold winters which account for most of the annual rainfall of 115–413 mm, and with average monthly temperatures ranging between 1 and 21.5 °C, occasionally dropping below −3 °C. It has hot summers with average temperatures of 31–34 °C and an average humidity of 22% (Kwarteng et al., 2

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Fig. 1. Map of the study area.

couplet consisted of dark micritic and lighter sparitic laminae. The stromatolitic columns had corrugated surfaces, and some were branched. They were usually coated with a reddish, organic-rich micrite that reached 200 µm in thickness. The stromatolitic columns are separated by elongated cavities. Significant variations in the textural and architectural characteristics of the stromatolitic lamination were noted. The variations were manifested as changes in the thicknesses of the micritic and sparitic laminae, as well as in the fabric of the sparry calcite. Such changes were exhibited by several types of laminations. Indistinct, blurred lamination consisted of relatively thick, dark micritic laminae and very thin, lighter spiritic laminae is mostly observed near the contact with the bedrock (Fig. 3D). The micritic laminae reached 200 µm in thickness and enclosed thin, dark-brown algal mats, while the sparitic laminae were mostly just a few microns thick. Clear lamination was observed at the lower part of the tufa columns, where the sparitic laminae were thicker and the micritic laminae were significantly thinner (Fig. 3E). However, thick micritic patches interrupting the well-developed lamination were commonly observed; these may represent remnants of the initially precipitated thick micritic laminae. It was observed that the micrite in the micritic laminae had been replaced by sparry calcite crystals, while the thin algal mats coated with micrite had not been replaced (Fig. 3F and H). A SEM image shows that the sparitic laminae consisted of laterally or obliquely stacked bladed calcite microcrystals reaching 20 µm in length, and that they had been growing at the expense of the micritic material (Fig. 3G). More developed lamination was observed where the sparitic laminae were thicker and the micritic laminae had mostly disappeared. The sparitic laminae consisted of laterally stacked dagger-like and calcite crystals. They reached 300 µm in length and 100 µm in width, and were arranged perpendicularly to the pre-existing micritic laminae. The latter were marked by thin brownish bands crossing the dagger-like

reached 9 cm in length and 5 cm in width. They displayed thin lamination and abundant dissolution pores of a few millimetres in diameter (Fig. 2F). The microstalactitic columns were separated by syndepositional elongated intercolumnar cavities that were mostly empty, although some were entirely or partially filled with reddish, structureless sediment. The thicknesses of the columnar bands corresponded to the lengths of the microstalactitic columns. Crust materials precipitated on both sides of wedges of bedrock within the dissolution grooves were also recognised (Fig. 2E). This material occurred on the tops of the wedges as a porous band of widely spaced microstalactitic columns. A 4 cm-thick planar, laminated band consisting of alternating 5 mm-thick porous, reddish laminae and 3 mmthick massive, lighter laminae, was precipitated on the bottom side of the wedge. This band was succeeded by a 2 cm-thick porous columnar band.

5.2. Microscopic description The host rock was generally formed of biosparite mostly consisting of echinoidal fragments tightly cemented by coarsely crystalline sparry calcite crystals. The latter were mostly syntaxially overgrown on the monocrystalline echinoidal calcitic fragments (Fig. 3A). Some beds were formed of biomicrite exhibiting a wackestone texture, wherein echinoid fragments and subordinate amounts of detrital quartz grains floated in a micritic groundmass. This limestone was partially silicified, where both allochem grains and cement within some bands had been replaced by microcrystalline quartz. The laminated and columnar crusts displayed stromatolitic structures consisting of vertically stacked, wavy laminated couplets. In the columnar crust, the laminated couplets laterally extended from one side of the column to the other in a smoothly curved manner. Occasionally, they were contorted and exhibited as linked domes (Fig. 3B). Each 3

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Fig. 2. (A) general view of the Rayda Formation exposure; the black arrow points to a 30 cmthick tufa crust deposit; (B) detached bedrock, where the red arrow points to the tufa crust. (C to D) Polished vertical slab of tufa crust: (C) contact of the crust with the host rock (HR), where arrows point to planar bands; (D) vertically stacked sequence of alternating thinner planar bands (black arrow) and thicker columnar bands (red arrow). (E) Porous band of widely spaced microstalactitic columns on the upper side of a bedrock wedge within the karst grooves (black arrow) and planar laminated band (red arrow) precipitated on the bottom side of the wedge. (F) Relatively large microstalactitic columns separated by an intercolumnar cavity, displaying thin lamination and abundant dissolution pores. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

crystals were mostly incomplete, rich in micritic inclusions, and exhibited various shapes and sizes (Fig. 4C). However, they were commonly prismatic with pyramidal terminations, and could reach 200 µm in length and 80 µm in width (Fig. 4D). Patches of calcite cement were scattered within the cavity-filling micrite and exhibited two fabrics. The most common was a xenotopic mosaic of well-sorted, subhedral, finely crystalline calcite crystals. The other was represented by scattered, fan-shaped clusters of radiating coarse calcite crystals reaching 300 µm in length and 100 µm in width. They were commonly tinted brown due to an abundance of tiny brownish inclusions (Fig. 4E). Some vugs within the intercolumnar cavity filling were partially

calcite crystals. Further magnification revealed that these bands consisted of clusters of brownish clots which displayed irregular shapes and were contained within the calcite crystals (Fig. 4A). Some of the columnar crust consisted entirely of a vertically stacked, banded mosaic of laterally arranged, elongated, and coarse dagger-like calcite crystals that in some cases exceeded 400 µm in length and 150 µm in width. However, lamination could still be observed due to the occurrence of barely visible, extremely thin dark marks (Fig. 4B). The intercolumnar cavities were mostly filled with reddish, structureless sediments primarily consisting of micrite, small quantities of detrital quartz and calcite grains, and traces of clay minerals. It also included a few scattered authigenic quartz crystals (Fig. 3C). These Table 1 Concentrations of major oxides, and stable isotopes ratios. Sample

R1 R2 R4 R5 R8 R17 R18 Average

Oxides wt.%

δ13C (‰ PDB)

δ18O (‰ PDB)

CaO

SiO2

MgO

Al2O3

SO3

K2 O

Na2O

Fe2O3

MnO

TiO2

P2O5

SrO

LOI

Mean

Std. Dev.

Mean

Std. Dev.

50.47 48.13 50.77 49.09 50.90 50.70 49.41 49.92

3.32 2.16 2.39 2.87 2.75 2.16 3.84 2.78

1.10 0.88 0.83 0.64 0.66 0.49 0.77 0.77

3.42 2.27 2.39 1.22 2.14 0.89 1.51 1.98

0.14 0.12 0.12 0.12 0.05 0.07 0.17 0.11

0.29 0.21 0.19 0.12 0.32 0.13 0.15 0.20

0.66 1.56 0.69 1.07 0.57 0.37 0.44 0.77

1.38 1.06 1.00 0.59 0.73 0.37 0.66 0.83

0.02 0.01 0.00 0.01 0.02 0.01 0.01 0.01

0.16 0.12 0.12 0.07 0.13 0.07 0.09 0.11

0.04 0.03 0.03 0.11 0.05 0.04 0.03 0.05

0.05 0.04 0.03 0.03 0.02 0.02 0.05 0.03

38.83 43.20 41.32 43.92 41.53 44.52 42.79 42.30

−3.34 −2.73 −3.50 −6.57 −3.63 −4.44 −2.46 −3.81

0.016 0.019 0.021 0.018 0.025 0.009 0.023 0.019

−2.83 −2.92 −3.03 −3.79 −4.16 −3.48 −2.61 −3.26

0.059 0.035 0.044 0.085 0.024 0.054 0.021 0.046

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Fig. 3. Photomicrographs of the laminated tufa crust: (A) echinoidal biosparite host rock mainly consisting of echinoidal fragments (ec) cemented by sparry calcite syntaxially overgrown on the monocrystalline echinoidal calcitic fragments (s); (B) columnar crusts displaying laminated structure; (C) reddish structureless sediments filling the intercolumnar cavities, where arrows point to authigenic quartz crystals; (D) incipient stromatolitic lamination exhibited as indistinct blurred lamination consisting of relatively thick dark micritic and thin lighter spiritic laminae; (E) clear lamination demonstrated by the thicker sparitic laminae (arrow); (F) remnants of the initially precipitated thick micritic laminae (arrow) within the well-developed lamination; (G) SEM image showing sparitic laminae consisting of stacked bladed calcite microcrystals (sp) replacing micritic laminae (m); (H) red arrow points to the replacement front where sparitic laminae developed at the expense of micritic laminae, and black arrows point to unreplaced thin algal mats. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

cemented by clustered needle fibre calcite (NFC) and also included calcified tissues. The latter occurred as scattered fragments ranging in size from 0.5 to 8 mm clearly displaying well-preserved tissue cells which may be of algal origin (Fig. 4F). Cells had been replaced by single or multiple sparry calcite crystals (cytomorphic), but the cell walls were clearly recognisable. NFC occurred as a mesh of clustered, randomly oriented calcite needles. It was commonly associated with calcified algal tissues. Individual calcite needles reached 100 µm in length (Fig. 4G). Algal globules of a few millimetres in size were recognised within the intercolumnar cavity filling. They were very dark and fringed by discontinuous couplets of dark micritic and light sparitic laminae (Fig. 4H).

5.3. Nanoscopic examination SEM investigation of the stromatolitic crust revealed that the micritic laminae were mostly composed of calcareous microglobules and fossilised microorganisms. The abundance and variability of the fossilised microorganisms suggest that these micritic laminae had initiated on pre-existing microbial mats. The latter were mostly comprised of cyanobacteria including a variety of filamentous and unicellular species (Fig. 5A). Several cyanobacteria morphotypes were recognised, although the most common were similar to Schizothrix filaments and Streptococcus bacteria (Fig. 5A and B). They were commonly agglutinated with their EPS. The unicellular cyanobacteria were mainly represented by their coccoidal forms (like Pleurocapsales cyanobacteria) which appeared as smooth, hollow spheres of 1 µm in diameter, with some exhibiting knobby surfaces (Fig. 5C). Cohesive platelets tens of

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Fig. 4. Photomicrographs of the laminated tufa crust (A) well-developed sparitic laminae consisting of laterally stacked daggerlike coarse calcite crystals growing perpendicular to pre-existing micritic laminae (black arrows); (B) banded mosaic of laterally arranged elongated dagger-like coarse calcite crystals, where the lamination can be observed by extremely thin dark marks (arrow); (C) incomplete authigenic quartz crystal; (D) SEM image of euhedral prismatic quartz crystal with pyramidal terminations, where the arrow points to triangular cavity that may indicate the incompleteness of the crystal; (E) scattered fan-shaped clusters of radiating coarse calcite crystals (rc); (F) scattered fragments of well-preserved tissue cells replaced by cytomorphic calcite; (G) clustered NFC in some vugs within the intercolumnar cavity filling; (H) algal globules within the intercolumnar cavity fill fringed by discontinuous couplets of dark micritic and light sparitic laminae (arrows).

stromatolitic lamination. The stacked nanoglobules tended to form two morphotypes of microglobules, namely rhombohedral and disphenoidal. The initial rhombohedral microglobules were formed of clusters of spherical nanoglobules. They reached 4 µm in size and exhibited rough rhombic faces built of coalesced flattened spherical nanoglobules (Fig. 5F). The surface roughness was attributed to an abundance of occluded EPS areas. These embryonic rhombs were further developed to form near-complete calcite rhombs displaying welldeveloped interfacial angles and smooth faces. The latter still exhibited agglutinated nanoglobules and a few inter-nanoglobular pores of the degraded EPS (Fig. 5G). This morphotype was commonly developed further to form a mosaic of equant microcrystalline calcite crystals that reached 10 µm in length. Some of these developed crystals still exhibited the clustered nanoglobules on some faces as well as microbial

microns in diameter and a micron thick displaying a peculiar fabric were frequently recognised within the micritic laminae (Fig. 5D). These were formed of clusters of unicellular and filamentous cyanobacteria agglutinated with their EPS. The microglobules ranged from 2 to 4 µm in size and consisted of a clustered mixture of fossilised microbes and nanoglobules < 0.2 µm in size (Fig. 5E). The latter were similar to those described by Pedley (2013) as amorphous calcium carbonate (ACC). They were agglutinated by the microbial EPS, commonly stacked on the degraded EPS sheath, and also clustered on the cyanobacteria filaments and coccoids. Two forms of nanoglobules were recognised, namely smooth spheres and arrowhead-shaped. Smooth spherical nanoglobules mostly occurred within the intercolumnar cavity-filling micrite, while the arrowheadshaped nanoglobules were abundant in the micritic laminae within the 6

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Fig. 5. SEM images of the stromatolitic tufa microbialite: (A) filamentous cyanobacteria agglutinated by their EPS; (B) cyanobacteria morphotypes similar to Schizothrix filaments and Streptococcus bacteria; (C) coccoidal cyanobacteria; (D) cohesive platelets formed of clusters of unicellular and filamentous cyanobacteria agglutinated with their EPS; (E) microglobules consisting of clustered mixtures of fossilised microbes (arrow) and nanoglobules (cn); (F) arrows point to initial rhombohedral microglobules formed of clusters of spherical nanoglobules; (G and H) complete euhedral calcite rhombs (IR) displaying well-developed interfacial angles, and smooth faces still exhibiting agglutinated nanoglobules and a few inter-nanoglobular pores of the degraded EPS, which are associated with fossiliferous coccoidal bacteria (CB); (I) the arrow points to disphenoidal microglobules scattered within the micritic laminae; (J) the arrow points to laterally stacked disphenoidal microglobules forming spiky incomplete and cavernous microcrystals initiating the sparitic laminae; (K and L) arrows point to growing laterally stacked dagger-like crystals penetrating the fossilised microbial mat and forming well-developed sparitic laminae.

were growing vertically upwards, pushing up remnants of the fossilised microbial mat or enclosing them to form the well-developed sparitic laminae. SEM observations revealed the occurrence of two associated forms of fibrous calcite, namely NFC and calcite nanofibres (CNF), which differ in both size and morphology. The dimensions and shape of the CNF were close to those described by Bindschedler et al. (2014). They were mostly smooth rods with hemispherical tips, with lengths between 300 nm and 1.5 µm and diameter of < 100 nm. They were commonly clustered, forming agglomerates and meshed sheets (Fig. 6A, B, and C). They were also associated with coccoidal and filamentous cyanobacteria. Clusters of CNF and ACC nanoglobules forming microglobules were also recognised (Fig. 6D). Occasionally, individual, as well as bundles of two or more, nanofibres were agglutinated or embedded in

fossils (Fig. 5H). The disphenoidal microglobules were frequently scattered within the micritic laminae. They were commonly formed of the arrowheadshaped nanoglobules and were mostly elongated, although some were spiky. They were laterally and vertically stacked parallel to their elongation direction, forming initial sphenoidal microglobules reaching 3 µm in length and 1 µm in width (Fig. 5I). These microglobules were laterally stacked, forming spiky, incomplete cavernous microcrystals that reached 10 µm in length and were less than 2 µm wide (Fig. 5J). They were similar to those described as embryonic crystals by Pedley (2013), who suggested that this crystal fabric is typical of EPS-hosted bioprecipitates. These embryonic crystallites were laterally stacked, forming larger dagger-like crystals penetrating the fossilised microbial mat and forming incipient sparitic laminae (Fig. 5K and L). The latter

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Fig. 6. SEM images of the stromatolitic tufa microbialite: (A) meshed sheets of interwoven CNF; (B) agglomerates of CNF agglutinated with the EPS of coccoidal bacteria (arrow); (C) bundled CNF (arrow) agglutinated with EPS; (D) microglobules formed of CNF agglutinated with EPS, where some are segmented and display hemispherical tips (arrows); (E) the arrow points to bundled CNF; (F) interwoven mesh of smooth, straight NFC filling voids within the intercolumnar cavity micritic filling; (G and H) individual NFC crusted by mosaics of flat nanoglobules and coccoidal bacteria (arrow); (I and J) compound forms of NFC displaying peculiar shapes; (K) dense mat of randomly clustered NFC within the micritic laminae; (L) complex NFC exhibiting irregular outgrowths on their sides (arrow); (M) complex NFC displaying the shape of rhomb chains (arrow).

generally scattered within the micritic laminae and exhibited several shapes, the most common of which was a cylindrical shape > 20 µm in length and 2 µm in diameter. They occurred both as individual needles and as bundles of two or more needles juxtaposed along their long axes. These needles were commonly crusted by mosaics of flat nanoglobules and coccoidal bacteria (Fig. 6G and H). Some compound forms were also recognised, wherein two or more needles were attached perpendicularly, displaying peculiar shapes (Fig. 6I and J). Complex needles exhibited irregular outgrowths on their sides and in some cases exceeded 4 µm in width. Such outgrowths were commonly developed along a plane on one side, transforming the needle into an elongated platelet shape (Fig. 6L). Further magnification showed that these outgrowths consisted of clustered nanoglobules. They were commonly clustered, forming a dense mat of randomly oriented NFC within the micritic laminae (Fig. 6K). Some complex needles displayed the shape of rhomb chains (Fig. 6M).

microbial EPS (Fig. 6E). These nanofibres were similar in size and shape to fossilised filamentous bacteria (Khalaf, 2017). However, they differed in their modes of occurrence. NFC is needle-like, but significantly differs from the CNF in both dimensions and shape. NFC has larger dimensions than those of the CNF, varying in width between 1 and 3 µm and sometimes reaching 30 µm in length. It exhibited differences in occurrence, morphology, and shape. It occurred as individual needles, bundles of two or three coalesced needles, interwoven meshes of smooth needles, and clusters of complex needles. Three main morphotypes of NFC were recognised, namely smooth straight needles, crusted needles, and complex needles. These morphotypes were mostly monocrystalline, and some displayed central grooves. Smooth straight needles commonly occurred as an interwoven mesh filling voids within the intercolumnar cavity micritic filling. They crossed each other, and individual needles reached 25 µm in length and 1–2 µm in diameter (Fig. 6F). Crusted needles were 8

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processes progressed along with further accumulation of the cyanobacteria biofilms. Photosynthesis resulted in an increase in alkalinity due to the uptake of CO2, which induced the precipitation of calcium carbonate from the meteoric water rich in dissolved calcium bicarbonate (Shiraishi et al., 2008; Arp et al., 2010; Perri et al., 2012a). The release of Ca2+ ions from degraded EPS may have contributed to the precipitation of this calcium carbonate (Dupraz et al., 2009; Arp et al., 2010; Perri and Spadafora 2011; Perri et al., 2012b). Shiraishi et al. (2008) and Rogerson et al. (2010) concluded that the daylight enhances the precipitation of calcium carbonate as indicated by strong increases in pH and Ca2+ consumption at the biofilm surface. Rogerson et al. (2008) and Pedley (2013) highlighted the role of a selective intra-EPS diffusion process in conveying Ca2+ to specific extracellular precipitation sites within the EPS, from where it can later be expelled to the surrounding water. Therefore, it is suggested that both illumination and EPS significantly controlled the rate of precipitation of calcium carbonate within the cyanobacteria biofilms. The calcium carbonate may have initially precipitated as amorphous gel within intra-EPS sites, where the local alkalinity thus increased significantly (Decho et al., 2009). ACC nanoglobules were then developed from this amorphous gel, and nucleated on the cyanobacteria and the remnants of their degraded EPS. Spadafora et al. (2010) and Perri et al. (2012b) recognised similar nanoglobules in modern microbialites. Further, Bontognali et al. (2008) and Pedley (2013) reported analogous nanoglobules in microbially induced precipitates during culturing experiments. The rare presence of EPS was attributed to its progressive degradation and occlusion as the ACC nanoglobules clustered to form microglobules (Pedley, 2013). The ACC nanoglobules which precipitated within the biofilm attained an arrowhead shape, while those precipitated within the intercolumnar cavity fill ended up with a spherical shape. Such variation in the morphology of the ACC nanoglobules may be attributed to the variability of microorganisms and/or variations in the EPS composition (Kawaguchi and Decho, 2002). The arrowhead-shaped nanoglobules were clustered around microbial cells, forming microglobules representing the building units of the micritic laminae (Tang et al., 2012). Therefore, the older cyanobacteria biofilms were significantly calcified and consisted of associations between the calcified cyanobacteria and the calcite biomineral grains (microglobules). This developed thrombolitic micrite laminae, and their vertical stacking resulted in the indistinct micritic laminar structure. Development of incipient sparitic laminae: At a certain horizon within the initially calcified biofilm, some of the microglobules were enlarged and tended to organise in disphenoidal form by further clustering of nanoglobules along their c-axes, reaching a few microns in length. They were laterally stacked and some were fused, initiating embryonic disphenoidal skeletal porous calcite microcrystals (Jones and Renaut, 1996; Jones and Peng, 2014). Some of these skeletal microcrystals were laterally arranged, creating incipient discontinuous thin sparitic laminae within the stacked biogenic micritic laminae. Formation of well-developed rhythmic lamination: This stage consisted of a transition from biologically mediated to physiochemical calcite precipitation. This transition involved the completion of the skeletal calcite microcrystals via cementation of the inter-microglobular pores with physiochemically precipitated calcite (Turner and Jones, 2005; Pedley et al., 2009). As suggested by Addadi et al. (2003), Tang et al. (2009), and Rodriguez-Blanco et al. (2011), the decay of EPS facilitated the physiochemical precipitation of calcite, wherein it was syntaxially precipitated on the biogenic disphenoidal calcite microcrystals, forming coarsely crystalline dagger-shaped, bladed calcite crystals reaching ~10 µm in length. During this stage, a rhythmic lamination of alternating dark micritic and lighter sparitic laminae of equal thickness was developed. Obliteration of micritic laminae: Physiochemical precipitation of calcite continued, leading to enlargement of the laterally stacked bladed calcite crystals within the sparitic laminae by further syntaxial overgrowth. The latter process progressed, penetrating the micritic laminae

5.4. Mineralogy and geochemistry XRD analysis revealed that the studied crust consisted almost wholly of low-Mg calcite (between 98% and 100%) and traces (< 2%) of quartz. The relative weight percentages of the major oxides present reflect the calcareous nature of the studied crust, with CaO concentrations varying from 48.13% to 50.9% (average 49.92%) (Table 1). The traces of quartz are reflected in the average concentration of SiO2, which was 2.73%. Traces of MgO were also recorded, ranging between 0.49% and 0.77%. Concentrations of Al2O3, K2O, and Na2O were very low, with averages of 1.98%, 0.2%, and 0.77%, respectively. The presence of these compounds may correspond to trace amounts of clays that mostly occurred within the intercolumnar cavity filling. Concentrations of SrO ranged between 0.02% and 0.05%, with an average of 0.03%. The concentration of Fe2O3, ranging between 0.37% and 1.38% (average 0.83%), may reflect the ferruginous nature of the intercolumnar cavity-filling micrite. The crust samples were characterised by narrow ranges of δ13C and δ18O values. The δ13C values ranged between −6.75 and −2.46‰ with an average of −3.81‰ and the δ18O values ranged between −4.16 and −2.61‰ with an average of −3.26‰ (Table 1). 6. Discussion The studied tufa crust has not been dated. However, it may be suggested that the deposits had precipitated concurrently with the other Late Pleistocene-Early Holocene tufa deposits in the region (Clark and Fontes, 1990). This period witnessed two pluvial episodes. During these wet episodes, the meteoric water falling on the exposed carbonate sequence of the Al-Jabal Al-Akhdar highlands may have initially had near-neutral pH, high partial CO2 pressure, and consequently low saturation with respect to calcite. This water would slowly dissolve carbonate and gradually lose some of its CO2 during its cascading flow over these highlands. Eventually, this water would have become supersaturated with calcite due to increases in Ca2+ concentration and alkalinity. This water was therefore considered to have been the main source of calcium carbonate for the precipitated tufa crust. 6.1. Cause, genesis, and diagenesis of lamination The tufa crust exhibited alternating planar and digitate-like columnar stromatolitic structures. These stromatolites were thinly laminated, with lamination exhibited by vertically stacked couplets each consisting of relatively thin and dark micritic laminae and thicker and lighter sparitic laminae. Variations in the relative abundance and thickness of these laminae, as well as in the size and fabric of the calcite within the sparitic laminae, were recognised. These varied from indistinct micritic lamination to well-developed, coarsely crystalline sparitic laminated structures. Detailed microscopic and nanoscopic examinations indicated that these laminations had developed through successive stages of biogenic and abiogenic precipitation of calcite. The following is a suggested paragenetic scheme for the development of the stromatolitic lamination, which may help in understanding the cause of such lamination. Formation of microbial biofilm: Initially, the wet bedrock surface within the karst grooves facilitated the accumulation of cyanobacteria, and a relatively thick biofilm developed (Pedley, 2013). This biofilm grew unevenly on the bedrock as adjacent kinks, forming the bases of the stromatolitic columns. The cyanobacteria were growing and flourishing, and continued their biological activities of photosynthesis and respiration. Development of indistinct micritic laminar structure: During this stage, the microbial biofilms calcified and were changed to calcareous microbialite, forming indistinct laminated micrite. Calcium carbonate precipitated as a result of interactions between the microorganisms and their EPS, and water percolated slowly through the biofilm. These 9

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Fig. 7. δ13C and δ18O bivariate plot for the tufa crust.

by calcification of cyanobacteria trichomes (Verrecchia and Verrecchia, 1994; Verrecchia, 2000; Cailleau et al., 2009). It is further suggested that its outgrowths developed via precipitation and clustering of nanoglobules on the cyanobacteria EPS. The occurrence of incomplete authigenic quartz may suggest that it developed at a late stage of the tufa diagenesis. The detrital quartz grains may have been partially dissolved during the high pH conditions that facilitated the precipitation of calcite. The biogenic and abiogenic precipitation of calcite resulted in the depletion of the carbonate, and the residual fluids became supersaturated with respect to silica. The depletion of carbonate may have led to a significant reduction in pH, and therefore quartz started to precipitate within the micritic groundmass (Siever, 1962; Hesse, 1989; Gorrepati et al., 2010).

and enclosing remnant microglobular micrite, thereby forming welldeveloped dagger-shaped sparry calcite crystals. By this point, the sparitic laminae had become thicker and dominated the laminar structure. The above described development stages are illustrated in Fig. 8. 6.2. Diagenesis of intercolumnar cavity filling The formation of the stromatolitic columns was followed by accumulation of residual sediments (terra rossa) within the intercolumnar cavities. These sediments are most likely a washed-out residuum of the dissolution of the calcareous bedrock, and were enriched with microorganisms during their wet phases. The microorganisms may have also played an essential role in the precipitation of spherical ACC nanoglobules that clustered to form rhombic microglobules. These further developed to form mosaics of rhombohedral calcite biominerals, leading to lithification of the cavity-filling sediments. The cavity-filling micrite also includes scattered fan-shaped clusters of radiating coarse calcite crystals. These are tinted brown due to abundant inclusions of tiny organic particles that may be remnants of bacteria. These are similar in shape to what have been described as ray-crystal shrubs by Chafetz and Guidry (1999). It is suggested that this type of calcite crystals may have been initiated by bacterially induced precipitation and developed further via abiogenic precipitation (Pentecost, 1990). The CNF and NFC were commonly associated, and mostly occurred in the intercolumnar cavity filling. Although they significantly differed in both size and mode of occurrence, their close association suggest similar origins. The mode of occurrence and morphology of the CNF may suggest that they had been developed by calcification of nanosized algal hyphae (Bindschedler et al., 2012) by the initially precipitated amorphous calcium carbonate gel. It is debatable whether the NFC is of physiochemical origin (Jones and Ng, 1988; Jones and Khale, 1993; Borsato et al., 2000), indirect biogenic origin (Harrison, 1977; Borsato et al., 2000), or direct biogenic origin (Callot et al., 1985; Jones, 1988; Verrecchia and Verrecchia, 1994; Loisy et al., 1999; Verrecchia, 2000). The morphology of the NFC suggests that it may have been generated

6.3. Stable isotopes and palaeoenvironment Stable isotopes of carbon and oxygen in Quaternary tufa deposits have been used as palaeoenvironmental and palaeoclimatic indicators (Andrews et al., 1993, 1997; Andrews et al., 1994, 2000; Arenas et al., 2000; Horvatincic et al., 2000). The studied tufa is characterised by a relatively wide range of δ13C values (between −6.75 and −2.46‰ with an average of −3.81‰) and a narrow range of δ18O values (between −4.16 and −2.61‰ with an average of −3.26‰). These isotope values are within the ranges typical of Quaternary tufas (Andrews et al., 1993, 1997, 2000; Arenas et al., 2000; Horvatincic et al., 2000). Andrews et al (1997) concluded that oxygen and carbon isotope values do not vary significantly between different types of microbial precipitate at a given site. They assumed that microorganisms within a biofilm only provide sites for nucleation of the calcium carbonates, while precipitation is mainly controlled by environmental factors. However, the present study shows that the biological activities of these microorganisms played significant roles in the precipitation of calcium carbonate. The low negative values of δ13C may suggest a composite source of carbon. This may include inorganic carbon derived from dissolved carbonate from the bedrock (Cruz et al., 2006; Arenas et al., 2007), and 10

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Fig. 8. Schematic diagram illustrates the paragenetic stages of the development of the tufa stromatolite.

incipient sparitic laminae. In the last stage the micritic laminae were mostly obliterated due to further syntaxial overgrowth of calcite, leading to the formation of large, laterally stacked, well-developed dagger-shaped sparry calcite crystals at the expense of the micritic laminae. Eventually, the sparitic laminae became thicker and dominated the laminar structure. The formation of the stromatolitic columns was followed by accumulation of residual sediments (terra rossa) within the intercolumnar cavities. These sediments displayed various textures and fabrics of biologically mediated calcite, including various forms of CNF and NFC. The last diagenetic stage of these sediments was represented by the precipitation of incomplete authigenic quartz. Stable isotope (δ13C and δ18O) values indicate that the present tufa precipitated from flowing freshwater enriched in calcium bicarbonate due to dissolution of the limestone bedrock during an Early Holocene cold pluvial period.

organic carbon derived from the decay of organic matter and from CO2 released during respiration of cyanobacteria. It may also be suggested that the δ13C values were influenced by the biogenic precipitation of calcite from a microenvironment preferentially enriched with δ13C (Merz, 1992; Andrews et al., 1997). The narrow range of oxygen isotope values and the poor covariance of δ13C and δ18O values (r2 = 0.402) (Fig. 7) may suggest that the studied tufa precipitated from freshwater that was continuously renewed without significant fractionation by evaporation (Talbot, 1990; Leng and Marshall, 2004). The relatively low negative values of δ18O may indicate the influence of dissolved bicarbonate from the marine limestone bedrock. They may also be related to low-temperature conditions (Andrews et al., 1994; Andrews, 2006), which may suggest that the studied tufa precipitated during a cold pluvial period.

Declaration of Competing Interest 7. Conclusions We declare that we have no conflict of interest. The studied stromatolitic tufa were crusting narrow karst groves carved along the bedding planes of the Rayda Formation limestone which is exposed within Al-Jabal Al-Akhdar in northern Oman. It exhibits alternating planar and digitate-like columnar structures. The latter are thinly laminated and mainly composed of low-Mg calcite. The lamination is exhibited as vertically stacked couplets of dark micritic and light sparitic laminae. These laminae vary in their abundance and thickness, and the calcite crystals exhibit progressive changes in both their size and fabric. Detailed microscopic and nanoscopic examinations indicated that this lamination had developed through five successive biogenic and abiogenic stages: the first involved accumulation and flourishing of cyanobacteria on the wet bedrock and formation of microbial biofilms. Next, indistinct thick micritic laminar structures developed by biogenic precipitation of calcium carbonate and eventual calcification of the microbial biofilms consisting of an accumulation of biomineralised disphenoidal calcite microglobules. In the third stage incipient sparitic laminae were developed by lateral stacking of embryonic bladed calcite microcrystals within the micritic laminae. This was followed by the formation of well-developed rhythmic lamination as a result of syntaxial overgrowth of physiochemically precipitated calcite on the embryonic bladed calcite microcrystals within the

Acknowledgements This research was supported by the Earth and Environmental Sciences Department, Faculty of Science, Kuwait University. The XRD analysis was performed by the Research Facility Unit (GS01/01) of the Faculty of Science, Kuwait University. The Nanoscopy Unit of the Faculty of Science, Kuwait University is also acknowledged. The isotope analysis was carried out by the Environmental Isotope Laboratory of the University of Arizona. The authors are grateful to Mr. N. Basilli for his assistance in the laboratory work, and to Mr. M. Saber for performing the XRF analysis. Mr. Y. Abdullah and Mr. H. Mahmoud are also acknowledged for their help in the preparation of thin sections and sample grinding. References Addadi, L., Raz, S., Weiner, S., 2003. Taking advantage of disorder: amorphous calcium carbonate and its role in biomineralization. Adv. Mater. 15, 959–970. Al-Busaidi, M., 2012. The struggle between nature and development: linking local knowledge with sustainable natural resources management in Al-Jabal Al-Akhdar

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