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Oxfordian microbial laminites from La Manga Formation, Neuquen Basin, Argentina: remarkable nanobacteria preservation.

1. Introduction

A renovated interest in the study of stromatolites rose from the new interdisciplinary approaches of geobiology dealing with biomineralization and microbe-mineral interactions (e.g., Reid et al., 2000; Weiner and Dove, 2003; Franke and Bazylinski, 2003; Visscher and Stolz, 2005; Dupraz et al., 2009; Spadafora et al., 2010). These frontier studies, which are based in modern settings including open marine, freshwater, alkaline lake, hypersaline, and hot spring environments, should give some keys for interpreting the past since the origins of life in Earth. In fact, these microbial carbonates are widespread throughout the geological record (e.g., Riding, 2000; Flugel, 2004), and have been easily recognized both in the field and in thin sections by generations of geologists because their distinctive morphology and defining lamination. However, these fossil stromatolites rarely preserve evidences of the microbial communities responsible of their formation and growth, an aspect which hampered their genetic interpretation and, in particular, the characterization of the microbial activity. In this paper, we present a case study of ancient peritidal stromatolites showing exceptional preservation of the forming microbial communities, which should help in building bridges between the new findings in modern stromatolites and their fossil counterparts. These have been found in the peritidal carbonates of the Callovian-Oxfordian La Manga Formation, in the Neuquen Basin (Argentina), which correspond to the shallowest facies of a wide carbonate ramp (Palma et al, 2007, 2009, 2010, 2013). The remarkable stromatolite preservation observed in different sedimentary sections allows for the recognition of macro and microfeatures, as well as the reconstruction of growth fabrics, laminae variations, the exceptional preservation of microbial communities including rod-like bacteria.

[FIGURE 1 OMITTED]

2. Geological setting

Located at the west margin of the South American platform, the Neuquen Basin is limited by a magmatic arc to the west and a tectonic foreland to the east (Fig. 1). The foreland consists of the Sierra Pintada belt to the northeast and the North Patagonian massif to the south. It is a typical retro-arc basin developed to the east of the Cordillera Principal between 36[degrees]S and 39[degrees]S. Mitchum and Uliana (1985), Legarreta and Gulisano (1989), and Vergani et al. (1995) provide extensive information on the geologic setting of the region.

The carbonate platform of the La Manga Fm. constitutes most of the sedimentary record of the Callovian-Oxfordian in the Neuquen Basin. It represents the middle part of the Lotena Mesosequence, which mainly consists of carbonates with interbedded shales, marls, and occasional sandstones (Fig. 2).

Based on detailed stratigraphic work in the stratotype section (La Manga Creek, Mendoza), the depositional environment of La Manga Fm led to identification of two informal units. Unit 1 was deposited in an outer ramp setting, while deposition of unit 2 took place in intertidal-supratidal environments.

The sharp contact between the outer ramp facies of unit 1 and the overlying intertidal-supratidal facies of unit 2 can be interpreted as the result of an abrupt fall (forced regression) of the relative sea level during the end of Middle Oxfordian or Upper Oxfordian.

[FIGURE 2 OMITTED]

Detailed sedimentological and biostratigraphic studies (Palma et al, 2007, 2009, 2010, 2012, 2013) allow to interpret facies and sedimentary environments not previously described for this unit. In the present study, the stromatolites were sampled from the upper part of the sequence, in the socalled Unit 2 by Palma et al. (2010). They are best exposed and best preserved at the Arroyo Los Blancos, La Manga and Yeseras Grandes sections (Fig. 1a-b), where the Unit 2 has 23m, 12m, and 12m in thickness respectively.

The age of the La Manga Fm. in the studied localities has been determined on the basis of ammonites and coccolithophorids. The former include Mirosphinctes sp. that could appear in the Middle Oxfordian as well as in the Upper Oxfordian (Palma et al, 2012). The presence of the coccolithophorid Cyclagelosphaera sp. indicates a Middle Oxfordian age (Concheyro, pers. comm.).

3. Methods

Standard thin sections of the stromatolites for conventional microscopic analysis were taken from the indicated sections. In order to recognize the mineralogy, X-ray diffraction patterns were carried out on a Philips 3020 goniometer (Ni-filtered CuK[alpha], 35 Kv, 40 Ma, without secondary monochromator). Focusing on carbonates mineral phases present in the samples, non-oriented step-scan data were taken from 20 to 70[degrees] 2[theta], with a step width of 0.04[degrees] and a counting time of 2s/ step.

Carbonates mineral and other crystalline phases identified on the powder diffraction patterns were analyzed with the FULLPROF program, which is a multipurpose profile-fitting program including Rietveld refinement (Rodriguez Caravajal, 2001). The Rietveld method (Rietveld, 1969) was used for quantitative determination of the mineral phases present in the samples. The starting crystallographic data used for each mineral were extracted from the literature.

Scanning electron microscope (SEM) investigations were performed on fresh broken surfaces and on polished carbon coated thin sections, using a Zeiss (Supra 40) scanning electronic microscope with an Oxford Instruments EDS energy dispersive spectroscopy detector (INCA) for elemental analysis. These instruments are housed at the Centro de Microscopia Electronica in the Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina.

4. Oxfordian microbial laminites

4.1. Macro and mesostructure

For examining the stromatolitic microbial fabric at different scales (macro-meso- and microstructural) we followed Shapiro (2000). The stromatolitic beds are 2 to 80 cm thick and have been recognized in all the studied sections, however, they are particularly abundant in Los Blancos section. The typical macrostructure is tabular, while the mesostructure ranges from planar to crinkle laminated (Fig. 3a-c). It is important to have in mind that remarkable domal forms are absent and that no preferred orientation of the stromatolites were observed.

Two main facies have been defined in the stromatolites: light grey microbial stromatolite with generally planar lamination, and crinkle laminated stromatolite beds (Fig. 3a-c). The stromatolites are closely related to mudstone and wackestones facies, as well as to flat-pebbles conglomerates and intraclastic breccias (Palma et al, 2012, 2013). In both types of stromatolite facies, laminae appear to have been partially turn-up, while still in a soft nonlithified state. Similar characteristics were observed on modern cyanobacterial flats in the intertidal to supatidal zone (Friedman et al, 1985).

Planar types (Fig. 3a-b): They are well represented in all the studied sections. They occur as 2 to 80 cm thick beds separated by 8 to 15 cm thick mudstone-wackestones or flatpebble conglomerates. On top of the stromatolites, bedding planes with some irregular crinkled relieve are present, as well as some bioturbation assigned to Scolicia isp? and frequent fenestrae and desiccation cracks (Fig. 3b).

Crinkle type (Fig. 3c): This type of stromatolite is also well represented in all sedimentary sections. They occur as approximately 7 to 12 cm thick beds. Mud-cracks, sheetcracks, fenestrae and tepee structures are common (Fig. 3c). Tepees are characterized by small antiform deformation structures and reach 5 to 10 cm in thickness. They are associated with planar stromatolites, flat-pebble conglomerates and/or intraclastic breccia associated facies.

Flat-pebble conglomerates are the most common associated facies. It consists of rip-up clasts derived from underlying planar to crinkle stromatolite beds. Textures range from clast- to matrix-supported, and its thickness varies between 4 and 30 cm. They are commonly associated with desiccation mud-cracks and tepees. Subangular to angular planar stromatolite intraclasts associated facies appear parallel, discontinuous or continuous to bedding. Nevertheless, in many cases, these intraclasts are randomly distributed or may have a slight imbrication.

4.2. Microstructure

The internal architecture of the stromatolites is rather simple. In terms of lamination patterns the stromatolites are classified into two groups: stromatolites with dominant planar lamination ("planar laminites") (Fig. 3b), and stromatolites with crinkle lamination ("crinkle laminites") (Fig. 3d).

Planar laminites (Fig. 3b) are composed of sheets of continuous laminae characterized by the alternation of dark submillimeter to up to 2 mm in thickness micritic laminae, with intercalated millimeter microsparitic laminae ranging in thickness between 3 to 13 mm. Micrite appears as clotted or peloidal or as homogenous micrite. Occasionally, the laminae built up into millimeter convex-up dome (up to 15 mm high) growing on the planar or wavy surface (Fig. 3e). Changes in thickness from lamina to lamina are evident; nevertheless, no specific trend in internal architecture of the stromatolites was distinguished. EDS and X-ray data showed that the microbial laminae are constituted of low-Mg calcite crystals ranging in sizes from 5 to 30 [micro]m (Figs. 4, 5, 6a-b). Changes in the laminae composition are not evident, except for the stromatolites from the Yeseras Grandes where thin gypsum laminae were observed. Subhorizontal to cylindrical fenestrae, horizontal sheet-cracks (Fig. 3b) as well as polygonal cracks were observed. Fenestral voids are filled with small crystals of granular calcite. Small gastropods and coccolithophorids have been found among the laminae. Additionally, a relatively small proportion of quartz and feldspar grains has been observed.

Crinkle laminites (Fig. 3c) are composed of sheets of continuous to discontinuous small crinkles (Fig. 3d) pronounced by the alternation of dark submillimeter to millimeter micrite laminae up to 2 mm in thickness, with millimetric microsparitic laminae between 3 to 33 mm in thickness. These laminae have a heterogeneous fabric composed of cloudy microsparitic calcite crystals and irregular masses of homogenous micrite. Although at macroscopic scale the laminae exhibit an apparent lateral continuity, when observed under the microscope it becomes clear that the laminae are discontinuous showing frequent disruption of the lamina couplets (Fig. 3d). A complex internal microfabric has been observed, where evidence of erosion and irregularly distributed micritic rip-up intraclasts appear (Fig. 3f). Desiccation features of the crinkle laminate facies included micro-tepees, fenestrae, mud-cracks, and sheet-cracks. Tepees are very small, 10 to 20 cm in size. They appear commonly associated with a suite of features related to subaerial exposure.

4.3. SEM and xRD analysis

Extensive SEM examinations of polished and etched stromatolite samples were carried out. SEM observations revealed the presence of sub-pm scale filaments and nanoglobules in the studied samples. The size and shape of the microfossils are determined by bacterial morphology, which includes rodlike bacteria, and isolated subspherical structures. Rod-shape bacterium is sometimes called bacillus meanwhile the spherical to subspherical shapes are called coccoids.

The rod-like bacteria and shaped-like filaments consist of a dense meshwork of small irregular distributed filaments, which range from 150 nm and 640 nm in length. (Fig.5a-b) whereas diameters ranging from 54 nm and 105 nm show much smaller variability flattened ends and smooths surfaces. Although many rods occur isolated, sometime others show and incipient branching. Filaments appear straight or smoothly curved. In figure 5e, these filaments would seem to wrap micritic crystals. The result of the EDS analysis reveals that the mineralogical composition of the rod-like bacteria is mostly calcite as well as for the matrix (Fig. 5c-d).

Subspherical bodies (coccoid forms) range in size between 70 and 89 nm approximately (Fig.5f). These structures could represent the relicts of nanobacterial cells. The Oxfordian nanobacteria reported in this paper are comparable in size with published examples of nanobacteria bodies mentioned by Schrieber and Arnott (2003).

Scanning electron microscopy reveals that the stromatolites are micritic characterized by abundant subhedral crystals and measure from 2 and 4 pm in size. However, the distribution of crystal grain sizes is heterogeneous and locally microspar crystals (>4 mm) with dominant anhedral or subhedral crystals are also common. Among network rod-shaped nanobacteria is very common the presence of framboidal pyrite. Based on SEM observations of the micro-textures, disseminated spheroidal to sub-spheroidal aggregates, 4[micro]m in diameter composed of individual euhedral crystallites of size less than 0.5 micron have been identified (Fig. 6c-e). EDS spectrum analyzes of these bodies are showing in figure 6d-f. Their presence seems to be a quite common feature in modern and ancient microbialites (Ezaki et al. 2008; Spadafora et al., 2010).

[FIGURE 3 OMITTED]

4.4. Mineralogy

Four microbialite samples were selected and prepared for X-ray diffraction (XRD) analysis (Fig. 4). In each one, three to four sets of laminae were recognized, but a no substantial mineralogical variation between successive stromatolite laminae has been determined.

[FIGURE 4 OMITTED]

X-ray diffraction analysis shows calcite as the predominant carbonate phase (up to 98.11%). All samples contain detrital quartz and feldspar grains with percentages being low as < 1% to 5% and <1% to 4.91% respectively. Detrital grains probably occur between or among stromatolitic laminae. Two samples show low percentages of siderite (<1%). The origin of siderite may be considered authigenic. In fact, it may be originated in a variety of sedimentary environments (Huggett et al., 2000), but its formation in very shallow intertidal settings is related to large amounts of decaying organic matter and high bacterial activity, like sulphate reduction (Pye, 1984). XRD analysis demonstrates that all types of stromatolitic laminae have the same mineralogical composition (Table 1).

5. Discussion

Stromatolites are the result of a combination of processes that can include sediment trapping, binding, microbial calcification and carbonate precipitation induced by cyanobacteria and also associated bacteria (Riding, 2000). Microbial processes, such as photosynthesis, cyanobacterial calcification, and decomposition of organic matter, directly or indirectly influenced and controlled the carbonate precipitation by repeatedly accretive growth of microbial communities (Flugel, 2004; Kazmierczak et al., 2004).

In La Manga Formation, planar to crinkle stromatolites with continuous to discontinuous laminae are regarded as stromatolites in which microbial influences were dominant. Stromatolitic laminae, either planar or crinkle, show microscopic features such as micritic laminae, clotted to peloidal micrite and fenestrae which are similar to examples de<05_TB001> scribed from Proterozoic and Early Phanerozoic nearshore settings (e.g., Pratt and James, 1982; Batten et al., 2004) to Recent in age (Friedman et al., 1985; Riding, 2000).

[FIGURE 5 OMITTED]

The lacking of significant changes in the thickness of laminae suggests that the influence of wave action or tidal currents regime was probably minimal, with the sediment input as limiting factor. The presence of convex-up laminae growing on planar or wavy laminae could be related to the sediment cohesion; nonetheless this kind of growth could be suggestive of changes in the style of calcification (Reid et al., 2000). Furthermore the frequent microspar laminae are related to recrystallized original micritic laminae.

Only at the Yeseras Grandes locality (Fig. 1a) stromatolites show alternations of submillimeter- to millimeter-thick beds of gypsum and calcite. This could be indicating more restricted conditions and active evaporation as well as different chemical conditions of the water caused by salinity fluctuations in the upper intertidal to supratidal setting. Similar facies have been previously described in modern microbial mats in the coastal sabkha at Abu Dhabi, United Arab Emirates (Handford et al., 1982).

Planar and crinkle laminations are often vertical- and horizontally disrupted by subaereal exposition processes (sheetcracks, mud-cracks, and fenestral pores). The crinkle irregu lar surfaces on top of the stromatolites beds were interpreted as redeposited fragments of desiccated mats (e.g. Schieber, 1998).

[FIGURE 6 OMITTED]

The origin of fenestrae is attributed to dessication and shrinkage as well as to gas bubbles, which were produced by the degradation of organic matter. The abundance of fenestral structures suggests a shallow water realm ranging from intertidal to supratidal depositional facies (e.g. Beukes, 1987; Altermann, 2008). The cavities are filled with sparry calcite, which is attributed to early diagenesis (Flugel, 2004).

According to Riding (2000), the clotted to peloidal microfabrics in stromatolite beds are typical of microbial carbonates in which sulphate reducing bacteria degrade organic matter within the microbial mats, promoting calcification (Riding and Tomas, 2006). These processes may have built stromatolites since the Early Archean (Hofmann, 2000; Schopf, 2006).

Peloids, frequent within the stromatolitic laminae, are irregularly spherical. Chafetz (1986) and Riding (2002) proposed that peloids could be attributed to calcified bacteria. It has been reported that CaCO3 precipitation takes place as a result of degradation of organic matter by heterotrophic bacteria, which plays a significant role in the lithification process of microbial mats (Riding and Tomas, 2006).

Studies made on modern microbial mats suggest that the formation of peloidal carbonate precipitates is associated with bacterial metabolic activities (Dupraz et al., 2004). Nonetheless, the peloidal microfabrics can be formed in situ during very early diagenesis, closely linked with the degradation and calcification of organic matter (EPS) driven by heterotrophic bacteria, mainly sulfate-reducing bacteria (Riding and Tomas, 2006; Dupraz et al., 2009). Actually, from a sedimentological point of view, cyanobacteria are particularly important because of their ability to precipitate minerals and also for trapping and binding existing sediments. On the other hand, cyanobacteria can also be regarded as almost exclusively responsible for the micritization of carbonate grains in shallow tropical seas (Golubic et al., 2000). Actually, micritization of allochems particles is a common feature in the La Manga Formation.

The origin of the flat-pebble conglomerates (FPB) facies is related to rework from lithified or partially lithified microbial mats and re-deposition by storms during the marine flooding on top of the microbial laminated facies. In fact, the high profusion of FPC levels is a direct expression of the frequency of storm events (Palma et al., 2013).

Although storm activity probably fluctuated significantly (eventually becoming enhanced), the microbial communities recovered soon after the storm had ceased. In fact, planar to crinkle stromatolites evidence the trapping by microbial mats of fine scarce grained sediments settled down from the water after a storm in upper intertidal to lower supratidal environments. The uncommon detrital materials suggest that stromatolites may have formed mostly by in situ precipitation of carbonate rather than by trapping and bidding the sediments (cf. Riding, 1999). Studies in recent stromatolites show that planar to crinkle microbial mats grow frequently above mean low water level or tidal flats that are periodically flooded and exposed (Reid et al., 2011).The presence of abundant ammonites such asMirosphynctes sp., regular echinoids and reworked intraclasts on the top of stromatolite beds is straight evidence of storm action (Palma et al., 2012, 2013) as well as the presence of coccolithophorids within the microbial laminae.

The remarkable preservation of nanobacteria in the stromatolites from the peritidal deposits from the La Manga Fm. suggests that diagenesis did not destroy any rod-like bacteria, so that they probably maintained their primary carbonate mineralogy. This is extremely surprising because stromatolites from peritidal settings are usually very prone to dolomitization (Mancini and Parcell, 2001). In the case herein presented, rod-like bacteria were not affected by dolomitization.

Studies carried out by Visscher et al. (2005) show that modern stromatolites from the Bahamas Islands are the result of different microbial communities. Although cyanobacteria are always associated to stromatolites, they are not the only microbes. Recent studies in stromatolites from Shark Bay, Australia, show that diversity is represented mainly by bacteria and archeas (Burns et al., 2004). The co-existence of rod-like bacteria and nanobacteria has been recognized in modern stromatolites from Exuma Island, Bahamas by Reid et al. (2011).

Astonishingly, putative nanobacteria observed in the stromatolites from the La Manga Fm. exhibit rod-like or filamentous morphology contrasting with the usually described spheroidal morphology of nanobacteria (e.g. Folk, 1993; Folk and Chafetz, 2000). Nanometer-scale objects (20-200 nm wide) were described as "nanobacteria" by Folk (1993). These are basically objects found in sedimentary rocks that have typical morphology of bacteria by much smaller size, usually an order of magnitude below typical bacteria, as suggested by Schieber and Arnott (2003). The existence of nanobacteria has been challenged by the geological and microbiological communities because it has been difficult to extract genetic material due to their microscopic size. Nevertheless, Berner (1998) suggested that "if microorganisms use a catalytic biopolymer instead of a genetic biopolymer in the construction of cell walls, it may be possible to obtain smaller structures".

Modern bacteria (1-2 [micro]m) display morphologies ranging from spherical (coccoid), rod-shaped (bacillus), curved (vibriod), to spiral (spirochete or spirilli). Those of certain biogenicity have filamentous or rod-shaped morphology have been described from the Early Archean (Brock and Madigan, 1991). Extremes include nanobacteria which can be as small as 0.1-0.2 [micro]m (Brock and Madigan, 1991). Of all the above mentioned morphologies, the rod- and worm-like calcified bacterial cells are those which may best serve to identify bacterially induced carbonate precipitation (Castanier et al., 1999; Warren et al., 2001).

The mineralogical and environmental implications of bacterially induced carbonate precipitation have been mentioned by several authors such as Folk (1993), Vasconcelos et al. (1995), Riding (2000), and Braissant et al. (2003, and references therein). Nanobacteria take place in different environments ranging from rocks to present-day sediments and living organisms, where they play a key role in precipitation in bacteria (e.g. Bontognali et al., 2008). In addition, nanoform microorganisms have been observed in modern natural environments as well as in laboratory culture experiments (van Lith et al., 2013). They have been considered as extremely small bacteria by Pacton and Gorin (2011).

Kazmierczak et al. (1996, 2004) and Dupraz et al. (2004) studied fossil and modern cyanobacterial mats finding similar coccoid shapes which they were interpreted as result of the selective degradation and mineralization of an organic EPS matrix. Riding (2000) suggested that in spite of bacterial type (autotrophic or heterotrophic; marine or non-marine), most morphologies of carbonates formed in the presence of bacteria are distinctive and could offer insights into this biomineralization process.

Experiments carried out by Schieber and Arnott (2003) showed spheroidal objects analogous to the nanobacteria size range (minor 50 nm). These structures could be considered as product of decay and mineralization of organic tissues (e.g. Kirkland et al., 1999; Schieber and Arnott, 2003; Perri and Tucker, 2007). A biological origin is proposed for spherulitic structures for the Mesoproterozoic Gaoyuzhuang Fm., China (1.4-1.5 Ga) (Lee and Golubic, 1999).

There are many reports on the presence of nanometer-scale spheroids, 30-150 nm wide, which appear associated with organic debris in samples from the Holocene Tunisian microbial mat, and the Miocene Monterey Formation (California, USA) (in Pacton et al., 2010). Likewise Fratesi et al. (2004) recognized aggregations of bacteria together with microspheres (nanobacteria) in samples associated with biofilms from the Carter Sandstone of Alabama USA.

Schopf et al. (1965) have mentioned the presence of well-preserved rod-shaped and coccoid bacteria in the cherts of the Precambrian Gunflint Iron Formation (1.9 x 109 years old) and consider that these organisms are morphologically comparable to certain modern iron bacteria. In fact, new research carried out by Sanchez-Roman et al. (2014) demonstrated the presence of rod-shaped bacteria embedded in the surface of siderite nanocrystals in samples from Rio Tinto (Spain). They suggest that precipitation of carbonates could be related to the microbial reduction of ferric iron coupled with the oxidation of organic compounds.

In order to explore their potential role in degradation and preservation of organic matter (OM) in rocks, Pacton et al. (2010) have investigated the presence of microbial recent activity and in the fossil record showing nanometer-scale spheroids with extracellular polymeric substances (EPS) and bacterial cell walls.

Numerous articles have addressed to the problem of the microbial diversity and their mineralogy and the role of some microorganisms on mineral precipitation (Reid et al., 2000; Lopez Garcia et al., 2005). In our case, we suggest that intertidal to supratidal settings could have been supersaturated with carbonates minerals so that accretion, lamination and lithification of stromatolites occurred, similarly to the process occurring in modern stromatolites (Reid et al., 2000). Peritidal environments from the La Manga Formation were affected by regular seawater inundation and subaerial exposure (including episodic dessication) and episodic storm influences (Palma et al., 2013).

Microbes, including cyanobacteria, fungi, microalgae and others have a significant influence on stromatolite formation (Nealson, 1997; Riding, 2000; Riding and Awramik, 2000) which together contribute to the biostabilization of the sedimentary peritidal areas. Although photosynthesis facilitates the precipitation of calcium carbonates such as aragonite (Dupraz and Visscher, 2005; Kremer et al., 2008), decom position of cyanobacterial extracelular organic matter by microheterotrophs could also participate in the precipitation of calcium carbonate (Paerl et al., 2001). It is noteworthy that during SEM observations of the analyzed samples of the La Manga Fm., the presence of cyanobacteria was not observed, which could probably be related to decaying colonies by bacterial activity and physicochemical degradation as suggested Dupraz and Visscher (2005) and Braissant et al. (2007).

The occurrence of framboidal pyrites is often related to the metabolic activity of sulfate-reducing bacteria and the decay of organic matter. In fact, some authors consider the presence of framboidal pyrite as an indicator of the involvement of sulfate-reducing bacteria in the precipitation of the microbialites (Westphal et al., 2010). The formation of pyrite framboids in sediments requires an anaerobic environment, either within the sediment or inside a local microenvironment. It is worth mentioning that sulfate reduction occurs in Bahamian stromatolites in an anoxic layer several millimeters below the photosynthetic zone (Visscher et al., 1998). Isolated framboidal aggregates of pyrite also indicate microbial activity meanwhile framboidal shape is also considered a morphological biomarker (Popa et al., 2004). Likewise, Raiswell et al. (1993) and Taylor and Macquarker (2000) suggested that framboidal pyrites formed during early diagenesis from ironrich porewaters at sites of sulfide presence (i.e., near organic matter, due to bacterial sulfate reduction).

6. Conclusions

Oxfordian stromatolites from the La Manga Fm have planar and crinkle forms with internally similar textures. The laminations are interpreted as the periodical response of the microbial community to the microbial growth and its surrounded environment. Extensive SEM examinations show a network of rod-shaped nanobacteria and small subspherical aggregates that strongly resemble coccoid bacteria. Because of their abundance, they probably play a critical role in the precipitation of carbonate minerals. The Oxfordian stromatolites from the La Manga Formation can be consider as biologically induced formations, where chemical variations (pH, alkalinity) and metabolic activity induced by microbes or sulphate-reducing bacteria could have increased the alkalinity and stimulate carbonate precipitation.

http://dx.doi.org/10.5209/rev_JIGE.2015.v41.n3.51568

Acknowledgments

This research was supported by Consejo Nacional de Investigaciones Cientificas (CONICET) PIP-0546, and by FONCYT (2012-2542). We are grateful to Dr. Maria Claudia Marchi for help during the work with the SEM microscope and the EDS system (CMA, Centro de Microscopias Avanzadas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires). The authors also thank A. Riccardi and A. Concheyro for paleontological determinations. We tank to G. Herrero (Universidad Complutense, Madrid, Spain) for his help with the preparation of the samples and J.C. Poblete for his assistance in the field. Thanks to A. Gandolfo for helpful comments on earlier versions of the manuscript. Coments of anonymous reviewers are kindly acknowledged. This is the contribution R-182 of the Instituto de Estudios Andinos Don Pablo Groeber (IDEAN).

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R.M. Palma (1) *, D.A. Kietzmann (1), M. Comerio (1,2), J. Martin-Chivelet (3,4), J. Lopez-Gomez (4), G.S. Bressan (1)

(1) Universidad, de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Dept. de Ciencias Geologicas -CONICET- IDEAN. Argentina.

(2) CETMIC, C.C. 49, Manuel B. Gonnet, La Plata. Argentina.

(3) Departamento de Estratigrafia, Facultad de Ciencias Geologicas, Universidad Complutense de Madrid, 28040 Madrid, Spain

(4) Instituto de Geociencias (CSIC, UCM). C/ Jose Antonio Novais '2, 28040 Madrid, Spain

e-mail addresses: palma@gl.fcen.uba.ar (RMP, Corresponding author); diegokietzmann@gl.fcen.uba.ar (DAK); comerio@gl.fcen.uba.ar (C.C.); martinch@ucm. es (JMC); jlopez@geo. ucm. es(JLG)

Received: 22 September 2015 / Accepted: 20 November 2015 / Available online: 20 December 2015
Table 1.--Results from X-ray diffraction analysis.
Quantitative mineral-phase-analysis of study samples,
based in the Rietvetd method

                                Sample

           A-1    A-2    A-3     B-1    B-2    B-3    B-4     C-1

Calcite   98.54   98.4   98.2   93.98   97.4   97.5   94.2   94.99
          0.38    0.54   0.48   0.35    0.33   0.35   0.34

Siderite   < 1    < 1    < 1                                  < 1

Quartz     < 1    < 1    < 1    2.37    1.22   1.37   < 1    3.32
                                0.03    0.03   0.03          0.06

Feldspar   < 1    < 1    1.13   3.64    1.38   1.17   4.91   1.33
                         0.06   0.07    0.05   0.05   0.11   0.14

                           Sample

          C-2   C-3   C-4   D-1     D-2     D-3

Calcite   97    94    95    95.9   98.82   98.91
          0.5   0.7   0.3   0.36   0.35    0.41

Siderite  < 1   < 1

Quartz    1.9   2.7   5.4   2.32   1.18    1.09
           0    0.1    0    0.03   0.03    0.03

Feldspar  < 1   3.2         1.76
                0.2         0.08

The modal proportions, with standard errors, are quoted in wt%.
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Author:Palma, R.M.; Kietzmann, D.A.; Comerio, M.; Martin-Chivelet, J.; Lopez-Gomez, J.; Bressan, G.S.
Publication:Journal of Iberian Geology
Date:Dec 1, 2015
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