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Characterization of environmental conditions during microbial Mg-carbonate precipitation and early diagenetic dolomite crust formation: Brejo do Espinho, Rio de Janeiro, Brazil ANELIZE BAHNIUK1,2*, JUDITH A. MCKENZIE1, EDOARDO PERRI3, ¨ GELI4, CARLOS EDUARDO REZENDE5, TOMASO R. R. BONTOGNALI1, NATALIE VO THIAGO PESSANHA RANGEL5 & CRISOGONO VASCONCELOS1 1
Geological Institute, ETHZ, 8092 Zurich, Switzerland
2
Universidade Federal do Parana´, UFPR/DGEOL/LAMIR, 81651 – 980 Curitiba, Brazil 3
Dipartimento di Biologia, Ecologia, e Scienze Della Terra, Universita` della Calabria, Rende (CS), Italy
4
Institut des Sciences de la Terre, Universite´ Joseph Fourier, BP53, 38041 Grenoble Cedex, France
5
Universidade Estadual do Norte Fluminense, Centro de Biocieˆncias e Biotecnologia, Laborato´rio de Cieˆncias Ambientais, UENF, 28013-602 Campos dos Goytacazes, Rio de Janeiro State, Brazil *Corresponding author (e-mail:
[email protected]) Abstract: For many years, sedimentary dolomite rocks have been considered to be primarily a replacement product of the calcium carbonate components comprising the original limestone, a process known as secondary replacement dolomitization. Although numerous dolomite formations in the geological record are composed of fine-grained crystals of micritic dolomite, an alternative process, that is, direct precipitation, is often excluded because of the absence of visible or geochemical indicators supporting primary precipitation. In this research, we present a study of a modern coastal hypersaline lagoon, Brejo do Espinho, Rio de Janeiro State, Brazil, which is located in a special climatic regime where a well-defined seasonal cycle of wet and dry conditions occur. The direct precipitation of modern high-Mg calcite and Ca-dolomite mud from the lagoonal waters under low-temperature hypersaline conditions is associated with the activity of microbial organisms living in this restricted environment. The mud undergoes an early diagenetic transformation into a 100% dolomite crust on the margins of the lagoon. The biomineralization process, characterized by the variations of the physico-chemical conditions in this environment during the annual hydrological cycle, is integrated with isotopic analysis to define the early diagenetic processes responsible for the formation of both dolomitic mud and crust. The carbon isotope values indicate a contribution of respired organic carbon, which is greater for the crust (d13C ¼ 2 9.5‰ Vienna Pee Dee Belemnite (VPDB)) than mud (d13C ¼ 21.2‰ VPDB). The oxygen isotope values reflect a moderate degree of evaporation during mud formation (d18O ¼ 1.1‰ VPDB), whereas it is greatly enhanced during early diagenetic crust formation (d18O ¼ 4.2‰ VPDB). The clumped isotope formation temperature derived for the Brejo do Espinho mud is 34 8C, whereas it is 32 8C for the crust. These temperatures are consistent with the upper range of measured values during the dry season when the lagoon experiences the most hypersaline conditions.
There are only a few modern environments where the mineral dolomite precipitates under Earth’s surface conditions. Thus, studies in these settings are important because they may have been more prevalent in ancient marine or lacustrine ecosystems. Although dolomite is common in the geological record and widespread in time and space, we continue to have a limited understanding of the environmental variables that promoted its growth
and diagenesis. Based on the study of select modern restricted environments, the microbial dolomite model is put forward to explain dolomite precipitation under low-temperature conditions promoted by microbial organisms living under relatively extreme conditions (Vasconcelos 1994; Vasconcelos & McKenzie 1997; van Lith et al. 2003; Moreira et al. 2004; Vasconcelos et al. 2006; Sa´nchez-Roma´n et al. 2009; Bontognali et al. 2012; Delfino et al.
From: Bosence, D. W. J., Gibbons, K. A., Le Heron, D. P., Morgan, W. A., Pritchard, T. & Vining, B. A. (eds) Microbial Carbonates in Space and Time: Implications for Global Exploration and Production. Geological Society, London, Special Publications, 418, http://dx.doi.org/10.1144/SP418.11 # 2015 The Geological Society of London. For permissions: http://www.geolsoc.org.uk/permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics
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2012). If this hypothesis is correct, the distribution of microbial dolomite in ancient sedimentary rocks may provide a window into the microbial ecology of the past. One way to advance our understanding of the biomineralization process is to characterize the basic environmental conditions and annual water cycle of modern environments where dolomite forms. This study aims to furnish new insights to enhance the understanding of diagenetic processes in the modern hypersaline coastal environment of Brejo do Espinho lagoon, located in Rio de Janeiro State, Brazil.
Geological setting Brejo do Espinho is a shallow (,0.5 m) hypersaline, dolomite-forming lagoon, located c. 100 km east of Rio de Janeiro city on the Atlantic coast (Fig. 1). The lagoon is situated within a Pleistocene dune system, allowing water seepage from the hypersaline lagoon Lagoa Araruama on the continental side and seawater from the Atlantic Ocean to influence its hydrology. The average annual precipitation and evaporative water losses in the region are c. 830 and 1400 mm, respectively (Barbie´re & Coe Neto 1999), giving the region a semi-arid climate and promoting hypersaline conditions (Barbie´re 1985). An overview of Brejo do Espinho is shown in Figure 2, together with details of the specific sampling locations. The physical conditions, water chemistry and mineralogy of Brejo do Espinho lagoon have been studied previously (van Lith et al. 2003; Moreira et al. 2004; Vasconcelos et al. 2006; Sa´nchezRoma´n et al. 2009, Bahniuk 2013). Monthly
average water temperature varies seasonally between 278 and 32 8C, and salinity varies over an extraordinarily wide range (from a minimum of 20‰ in the rainy season to a maximum of 100‰ near the end of the dry season). The pH averages c. 7.7, and authigenic mineral precipitation is dominated by high-Mg calcite and Ca-dolomite. The amount of the latter mineral increases with depth below the surface, reaching 100% Ca-dolomite at c. 25 cm (Moreira et al. 2004; Vo¨geli 2012). The age of the first Holocene carbonate sediments in the lagoon is less than 3600 yr BP based on 14C dating of the underlying organic carbon-rich marine sediments in Lagoa Vermelha, which is in a similar setting along the coast c. 15 km west of Brejo do Espinho. Carbonate precipitation began from this date when the lagoons became isolated from the ocean and the closed lagoonal environment began to be influenced by terrestrial inflow (Vasconcelos & McKenzie 1997). The present study acquired additional temperature and geochemical data during an annual cycle in 2011 to explore the near subsurface sedimentary conditions and better understand the formation of dolomite mud and very early diagenetic dolomite crusts.
Sampling and methods Water sampling Surface water samples were collected monthly from the centre of the Brejo do Espinho lagoon in c. 50 cm water depth from January to December 2011 (Fig. 2c). The water column has a maximum
Fig. 1. Location of Brejo do Espinho lagoon study area on the Atlantic coast, c. 100 km east of Rio de Janeiro, Brazil. Modified from Google Earth.
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Fig. 2. (a) Aerial view of Brejo do Espinho, highlighting the sampling site in the boxed area. (b) Detail of the sample collection site indicating the different locations of the crust and core samples. (c) Schematic sampling profile of the crust and mud. Note the absence of horizontal and vertical scale. (d) Core used during the sampling campaign showing the surface microbial mat overlying the carbonate mud. (e) Close-up of the microbial mat covering the mud. (f ) Hand sample of the dolomite crust. HMC, high-Mg calcite.
depth of 50 cm and is well mixed, making it unnecessary to sample at different depths. Conductivity and pH were measured in the field using a
portable device model ‘Multi 3430’, manufactured by Wissenschaftlich-Technische Werksta¨tten GmbH. Standard solutions, included in the kit,
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Table 1. Physico-chemical parameters of Brejo do Espinho 2011
Precipitation* (mm)
Conductivity† (mS cm21)
pH†
d18O (VSMOW, ‰)‡
Jan Feb Mar Apr May Jun July Aug Sep Oct Nov Dec
13 0 168 136 158 25 7 18 7 140 129 151
41.4 73.5 60.7 38.1 42.4 45.9 53.1 57.4 85.4 59.5 137.6 71.4
8.3 8.3 8 8.3 8.1 7.7 8.2 8.2 7.6 7.4 7.8 8.1
2.52 4.84 1.06 20.89 0.76 1.7 2.1 3.19 2.52 2.8
*National Institute of Meteorology and Hydrology (INMET Brazel). † Data from Vo¨geli (2012). ‡ Vienna Standard Mean Ocean Water.
were used to calibrate the instrument before the analyses were made. The monthly data for 2011 are compiled in Table 1. Corresponding monthly data for the regional precipitation pattern are from the National Institute of Meteorology and Hydrology (INMET Brazil) and are likewise included in Table 1.
Oxygen isotope analyses of water The oxygen isotopic values of the monthly water samples were determined by the CO2-equilibration method at 25 8C for c. 18 h (Hindshaw et al. 2011). The measurements were made using a Gas Bench II (Thermo Scientific) coupled to a Thermo Scientific Delta V Plus isotope ratio mass spectrometer housed in the Stable Isotope Laboratory at the ETH Zurich. The isotopic results are reported in the conventional per mil notation with respect to Vienna Standard Mean Ocean Water (VSMOW) and are listed in Table 1. The sample standard deviation (2sSD) was less than 0.1‰. It was not possible to make the oxygen isotopic measurements on waters collected during November and December 2011 because the samples were contaminated during sample preparation.
Water temperature monitoring Two Tinytag Aquatic 2 data loggers were installed in Brejo do Espinho in May 2010 to record lagoon water/sediment interface and pore-water temperatures. One data logger was located at the water/ sediment interface and the other was buried in the sediment at 20 cm depth. The data loggers measured temperatures every 10 min and remained in place for 2 years (2010 and 2011). The data were
downloaded in situ during February and August of each year (Bahniuk 2013). The data are included in Table 2. There is a gap in the data collection for the water/sediment interface during June and July 2011 when the loggers did not record any temperatures. There may have been a technical problem with the logger or alternatively the water in the lagoon might have been so low due to evaporation during these months that it was not possible for the data loggers to measure the water/sediment interface temperature.
Sediment sampling Sediment samples were collected at Brejo do Espinho lagoon in January 2010, August 2011 and January 2012. Samples of dolomite crust were collected from the borders of the lagoon and 50 cm cores were taken from the middle of the lagoon, which were then subsampled for microbial mats and mud (Fig. 2c).
Mineralogy studies Sediment mineralogy was determined by X-ray diffraction (XRD) of bulk sediment samples using a Bruker X-ray diffractometer model AXS D8 Advance with a LynxEye detector. Scans were run from 2u angles of 58 to 908 at a scan rate of 0.8 s per 0.028, using divergence slit V12. Minerals were identified using the XRD Wizard software at ETH Zurich. The mol% MgCO3 in the dolomite and calcite was calculated from the d-spacing values in the XRD spectra, following Zhang et al. (2010). Thin sections of dolomite crust samples were examined under transmitted light using the Nikon Optiphot Microscope at the ETH Zurich.
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Table 2. Summary of water/sediment interface and pore-water temperatures obtained in Brejo do Espinho lagoon in 2010 and 2011 Jan (8C)
Feb (8C)
Mar (8C)
Apr (8C)
Water/sed 2010 Average Minimum Maximum Water/sed 2011 Average Minimum Maximum Pore water 2011 Average 28.36 Minimum 27.94 Maximum 29.08
27.89 27.14 28.86
26.46 25.04 28.36
May (8C)
Jun (8C)
July (8C)
Aug (8C)
Sep (8C)
Oct (8C)
Nov (8C)
Dec (8C)
24.64 22.55 28.22
22.21 19.63 24.26
23.46 21.59 24.96
22.19 18.34 24.26
22.62 20.46 25.07
24.34 20.61 29.82
26.51 22.19 31.56
29.36 25.29 32.62
22.22 17.56 27.35
22.34 16.23 30.72
24.64 18.28 33.33
24.45 18.20 32.89
22.75 20.81 29.47
22.36 19.99 24.77
24.48 21.79 27.03
24.37 21.89 26.80
26.67 22.85 31.14
24.21 21.07 29.33
26.67 25.24 27.96
24.62 21.58 27.29
Scanning electron microscope (SEM) studies of the nanofacies were carried out on freshly broken surfaces of the crust, using an FEI-Philips ESEMFEG Quanta 200F at the Universita´ della Calabria, Italy, operating in a range of 5–20 kV with working distance between 6 and 15 mm. Samples were carbon- or gold-coated, depending on whether they were prepared for microanalysis or textural study with the SEM, respectively. Semi-quantitative element analyses of micron-sized spots were obtained using an EDAX energy dispersive X-ray spectrometer (EDS), operating at 20 kV with a working distance of 12 mm, during SEM observations.
Combined clumped and stable isotope measurements of carbonates Recently, a technique called ‘carbonate clumped isotope thermometry’ was developed (Wang et al. 2004; Eiler 2006a, b; Ghosh et al. 2006a, b; Schauble et al. 2006). It is based on grouping together or ‘clumping’ 13C and 18O in the carbonate mineral lattice into bonds with each other. That is, not only are the 13C/12C and 18O/16O ratios of carbonates examined, but also the fraction of 13C and 18O atoms that are joined together into the same carbonate ion group (13C18O16O2), expressed as D47. This represents an advantage with respect to the classic ‘carbonate–water thermometer’, in which the oxygen isotope compositions of both carbonates and the waters from where they grew must be known in order to determine the temperature of formation. Using the clumped isotope method, palaeotemperatures can be obtained directly from the carbonates and, by combining these temperatures with known d18O values of those carbonates, it is also possible to constrain the interpretation of isotopic composition of the waters from which
22.57 21.39 23.99
22.11 19.53 23.79
carbonate precipitated. To date, the clumped isotope application has been used in studies of palaeoclimate (Came et al. 2007), palaeoaltimetry (e.g. Ghosh et al. 2006b), palaeobiology (Eagle et al. 2011), atmospheric chemistry (Affek et al. 2008) and diagenesis (Ferry et al. 2011). This study applied the method of clumped isotope geochemistry to a mud and crust sample (one of each) from Brejo do Espinho lagoon, both to gain further insight into carbonate deposition in this modern environment and to establish whether this new isotope tool can be applied to ancient carbonates that formed in similar environments. The isotope measurements reported in this study were performed in the Laboratory for Stable Isotope Geochemistry at Caltech, California, in January/ February 2010 and in January 2012. The samples were reacted with phosphoric acid in a common acid bath at 90 8C and product CO2 purified by cryogenic and gas chromatographic methods using the automated device described by Passey et al. (2010). All purified gases were analysed for d13C, d18O and D47 using a Thermo IRMS 253 configured for clumped isotope measurements. Analyses of d13C and d18O were standardized based on accepted values for NBS-19 reacted at 25 8C and accepted temperature dependence of the acid digestion fractionation. The acid digestion fractionation factor assumed for dolomite at 90 8C was 1.0093 (Rosenbaum & Sheppard 1986). The isotopic composition of the water (d18OVSMOW) in equilibrium with the dolomite samples was determined using the equation 1000 ln adolomite-water ¼ 2.73 × 106T 22 + 0.26 (Vasconcelos et al. 2005), where a is the fractionation factor between dolomite and water and T is the absolute temperature (8K). The isotopic results are reported in the conventional per mil notation
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with respect to Vienna Pee Dee Belemnite (VPDB) for both d13C and d18O of carbonates and standard mean ocean water (VSMOW) for d18O of water. Measurements of D47 were standardized by comparison with CO2 heated for at least 2 h at 1000 8C, using methods described by Huntington et al. (2009). These data were not directly standardized relative to the absolute reference frame of Dennis et al. (2011). However, we did analyse a number of secondary carbonate standards that were considered in the Dennis et al. (2011) study. Thus, our results can be converted into the absolute reference frame using the ‘secondary reference frame’ method (i.e. through a plot of measured v. accepted values of carbonate standards). Repeated measurements of the two Brejo do Espinho mud and crust samples were accompanied by periodic analyses of intralab consistency standards, Carmel Chalk, Carrera Marble and 102GC-AZ01. The average standard error of each analysis based on mass spectrometric reproducibility alone was +0.010 (1SE). These average errors are generally consistent with expected counting statistics limits and suggest no significant additional experimental errors. The average standard error for the average of each unknown sample is equivalent to a temperature error of c. +1.6 8C. Measured D47 values in the Caltech intralab reference frame of Huntington et al. (2009), were converted into equivalent temperatures using the function described by Ghosh et al. (2006a).
Results Temperature, conductivity, pH and d18O of water Figure 3 shows a plot of the measured water/ sediment interface and pore-water temperatures recorded in the Brejo do Espinho lagoon v. the average monthly precipitation during 2011. The data given in Table 2 demonstrate that seasonal variations in temperature and precipitation in this region are closely correlated (Fig. 3a, b). Because of the malfunction of the logger at the water/sediment interface, data for June and July 2011 were not collected, but temperature data obtained for June and July 2010 were inserted into the graph to replace the missing values (Fig. 3a). The water/sediment interface temperatures and precipitation data indicate that June to September is a relatively cooler and drier period, whereas March to May and October to December are relatively warmer and wetter (Fig. 3a). In contrast, January and February are much drier months. The pore-water temperature data for the annual cycle follow a similar trend to those for the water/sediment interface but show less
variability during a single month (Fig. 3b). Note that the temperature at the depth of the pore-water temperature measurements (20 cm) is relatively stable during any given month, varying about 5 8C between maximum and minimum values (Fig. 3b), whereas water/sediment interface temperatures can vary in specific months by up to 15 8C (Fig. 3a). The temperature difference between water/sediment interface and pore-water is c. 3 8C. Conductivity is a good measure of the water salinity and has been associated with carbonate precipitation in lagoon environments (Vasconcelos & McKenzie 1997). We measured this parameter in our monthly surface water samples to better evaluate the relationship between conductivity and seasonal variations in rainfall. Figure 4 shows that conductivity does not vary seasonally with the amount of rainfall in any recognizable pattern, indicating that the budgets of water and/or salt in the lagoon depend on factors other than the balance between rainfall and evaporation. There is some indication that conductivity steadily increases during dry seasons: that is, during June, July and August 2011, precipitation values drop below the average precipitation line (APL) as conductivity increases through this period (Fig. 4). However, at the beginning of the rainy season (September to December), conductivity first decreases and then increases again. These data suggest that the decrease in conductivity is due to dilution by rainwater followed by an increase probably due to the dissolution of evaporite minerals that precipitated during the previous dry season as the areal extent of the lagoon shrank. Figure 4 also includes d18O values of the monthly water samples collected in 2011 plotted against conductivity and precipitation. The data indicate a connection between the d18O value of the water and amount of precipitation. that is, the three lowest d18O values occur during the rainy season, March to May. The data also suggest that conductivity and d18O may co-vary with the lowest conductivity associated with the lowest d18O value in April 2011. We propose that the d18O record best tracks the water budget of the lagoon: that is, the values rise due to evaporation and fall with introduction of fresh rainfall or runoff, and possibly ground water influxes from the sea and/or continental side. The conductivity values also follow variations in evaporation and rainfall, but this relationship is more complex because it reflects the dissolution of previously precipitated evaporite minerals at the lagoon margin.
Carbonate mineralogy and SEM Carbonate minerals, predominantly high-Mg calcite, Ca-dolomite and/or dolomite with some calcite,
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Fig. 3. Annual water/sediment interface and pore-water temperatures in Brejo do Espinho measured during 2011 compared with the regional precipitation pattern. Precipitation data from the National Institute of Meteorology and Hydrology (INMET Brazil). (a) Average, minimum and maximum surface water/sediment interface temperatures. Because the logger did not record temperatures during June and July 2011, data for the same period in 2010 were inserted, as indicated by points enclosed in parentheses. (b) Average, minimum and maximum pore-water temperatures.
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Fig. 4. Graph showing the relationships among conductivity (mS cm21), precipitation (mm) and d18O of the water (VSMOW)), based on monthly water sampling during 2011. Precipitation data are from the National Institute of Meteorology and Hydrology (INMET Brazil). APL, average precipitation line or mean annual precipitation; VSMOW, Vienna Standard Mean Ocean Water.
compose the mineralogy of the core samples. Typically, the uppermost part of the core contains a microbial mat that is very soft, reddish and waterrich with high-Mg calcite being the main carbonate phase (Fig. 2d). As observed in short cores, 8– 20 cm depth (Fig. 5), the sediment is composed of a dark-grey, homogeneous mixture of fine-grained high-Mg calcite and Ca-dolomite, termed aphanitic or leiolitic facies. The carbonate mineralogy of typical mud samples from below the microbial mat consists of an approximately 30:60:10 mixture of calcite, high-Mg calcite (c. 25 mol% MgCO3) and Ca-dolomite (c. 47 mol% MgCO3). The XRD spectrum for a typical mud sample taken at 2 cm below the microbial mat illustrates the mixed carbonate mineralogy of the in situ precipitate (Fig. 6). At c. 20 cm depth, the sediment transitions into light-greyish laminae facies comprising 100% dolomite (Moreira et al. 2004; Vo¨geli 2012). Below 40 cm, the core contains black, organic carbon-rich, weakly laminated, sandy sediments deposited when Brejo do Espinho was connected to the open ocean (Fig. 5). The early diagenetic dolomite crust sampled along the margins of the Brejo do Espinho is composed exclusively of 100% dolomite (c. 48 mol% MgCO3), although samples often retain some high-Mg calcite that has not been diagenetically altered. An XRD spectrum of a typical crust sample indicates that it comprises a nearly stoichiometric dolomite resulting from early diagenesis of a former mixed carbonate mud (Fig. 6). We
follow the classification of Tucker (1988) to distinguish two categories of calcite: low Mg-calcite with 0–4 mol% MgCO3 and high-Mg calcite with more than 4 mol% MgCO3, where 11–19 mol% MgCO3 is the most common range. We use the classification of Vasconcelos & McKenzie (1997) for dolomite, where Ca-dolomite ranges from 30 to 45 mol% MgCO3 and dolomite from 45 to 50 mol% MgCO3. The SEM observations of the most recently deposited mud samples, all of which comprise unconsolidated material, indicate that the high-Mg calcite occurs as needle-like crystals and locally forms spherical aggregates (Fig. 7a). There are abundant extracellular polymeric substances (EPS) near the centres of these aggregates, suggesting that EPS may promote crystal nucleation. The elemental chemical composition of the spherical crystals investigated with EDX spot analyses (areas ,1 mm) shows that the Mg and Ca peaks are nearly equivalent suggesting that the mol% MgCO3 content of the carbonate is very high (Fig. 7b). Further random EDX spot analyses of the peloids and acicular crystals in the field of view indicate that only a slight difference exists between the Mg:Ca ratios of the two different crystal structures. The crust samples collected along the lagoon border are light grey in colour and are composed of 100% dolomite (Fig. 8a). Because the crust is aphanitic and relatively structureless, a thin section was prepared for petrographic examination. The
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intercalation of micrite and microsparite (Fig. 8d). The observed fenestral porosity is associated with the microsparitic cement (Fig. 8e). Based on examples from the late Miocene of SE Spain (Braga et al. 1995), the features observed in the Brejo do Espinho dolomite crusts have been termed leiolitic fabrics. SEM observations of fresh surfaces of dolomite crust reveal a peloidal texture, with non-carbonate grains (silica, presumably detrital quartz) associated with dolomitic peloids (Fig. 9a). Isophachous cement composed of acicular crystals (,10 mm in length) surrounds the dolomitic peloids (which are 50 –100 mm in size) (Fig. 9b). Aggregates of very fine micrite constitute both peloids and fringing cements. SEM observations reveal that the micrite, whether forming the peloids or occurring in the cements, is formed of apparently anhedral mineral units, less than 1 mm in diameter (Fig. 9d). The majority of these nanostructures consist of 80 –300 nm nanospheres or nanoglobules, which locally coalesce to form small rods or polyhedrons with an apparent hexagonal geometry (Fig. 9d). A simple bacteria-like fossil biota was found in the crusts (Fig. 9c, d). This consists of empty moulds and mineralized bodies of subspherical coccoid forms, varying in dimension between 1 and 5 mm, characterizing both peloids and cement. SEM analyses revealed the diffuse presence of organic matter remains in both the mineral microstructure of peloids and acicular cements. Organic matter remains are composed of a carbon-enriched substance, dehydrated as the structure is resistant to the high vacuum in the SEM. They appear as planar or sheet-like membranes, ,1 mm thick and tens of microns wide, or as filaments, with features indicating an original mucus-like behaviour, possibly representing EPS remains (Fig. 9c). Organic membranes and filaments may envelop mineralized bacterial fossils (Fig. 9d). Submicron organic structures frequently occur in strict association with the nanospherical mineral units, or with the small polyhedrons, indicating a close association with these nanomineral phases (Fig. 9d). Moreover, at the nanoscale, it is common to observe a replacement of such organic substrates by nanocrystalline dolomite (Fig. 9c, d). Fig. 5. Photographic image of the Brejo do Espinho core showing the downward transition from dark-grey aphanitic to lighter grey laminated carbonate facies followed by the underlying deposit of black, organic carbon-rich, weakly laminated, sandy sediments of marine origin.
sample displays a peloidal microfabric, with aggregates of dark micrite surrounded by microsparite (Fig. 8b, c). In some parts of the thin section, it is possible to observe laminae formed by the
Clumped and stable isotope studies of mud and crust samples Here we present the results of stable isotope analyses of the analysed mud and crust samples from Brejo do Espinho and incorporate clumped isotope temperature data to refine our interpretation. The bulk stable isotope composition of the highMg calcite and Ca-dolomite mud from Brejo do Espinho is characterized by an average d13C value of 21.2‰ VPDB and average d18O value of 1.1‰
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Fig. 6. X-ray defraction (XRD) spectra of the studied carbonate samples from Brejo do Espinho. The red XRD spectrum is for a mixed carbonate mud sample from 2 cm below microbial mat shown in Figure 2e. Note the occurrence of three carbonate phases: calcite (C); high-Mg calcite (MC) and Ca-dolomite (D); quartz (Q). The blue XRD spectrum for the sample of early diagenetic dolomite crust indicates that it is 100% well-ordered dolomite (D). The location of the (104) reflects a 50:50 stoichiometry for the dolomite. Note the presence of the (015) and (021) superstructure ordering reflections, indicated by arrows. The inset is an enlargement of the spectra between 29 and 38 2-theta, which illustrates the diagenetic transformation of the mixed carbonate mud to dolomite crust.
VPDB, whereas the dolomite crust has a relatively depleted d13C value of 29.5‰ VPDB and enriched d18O value of 4.2‰ VPDB. The relatively lower oxygen isotope value for the mud implies that it may have precipitated in equilibrium with less evaporated water than the relatively more evaporated water involved in the formation of the lithified crust as indicated by its more positive value. Furthermore, the carbon isotope data indicate a greater input of the isotopically lighter 12C derived from respired organic matter during the crust formation than during the precipitation of the mud. Based on clumped isotope measurements (D47), the formation temperature for the Brejo do Espinho mud is 34 8C, whereas it is 32 8C for the crust. With these temperatures, together with the average d18O value of the mud and crust, it is possible to calculate the average d18O value of the formation waters using the calibrated d18O palaeothermometer for dolomite (Vasconcelos et al. 2005). In applying this formula, it is assumed that the high-Mg calcite and Ca-dolomite have similar oxygen isotope equilibrium with the formation water as does the 100% Ca-dolomite. Therefore, the mud would have precipitated in isotopic equilibrium with less evaporated water (d18OVSMOW ¼ 2.4‰) than the highly evaporated water (d18OVSMOW ¼ 5.1‰) associated with the crust formation.
Discussion The physico-chemical data obtained during the 2010–2011 period can be compared with previous studies from the region. Vasconcelos & McKenzie (1997) collected samples of rainwater, water from surrounding lagoons, well water and Atlantic Ocean water and determined the possible water sources recharging Lagoa Vermelha, Brazil; van Lith et al. (2003) studied the Brejo do Espinho water cycle in detail and, in comparing their data with data from Lagoa Vermelha, concluded that the two hydrological systems are very similar. In other words, the geographic settings are the same, the lagoons have similar morphologies, and climate controls on the hydrological cycle are identical. Because Brejo do Espinho belongs to the same hydrological regime as Lagoa Vermelha, its recharge system is undoubtedly the same. Hence, during the dry season, the major recharge into Brejo do Espinho comes through the porous barrier dunes from both the Atlantic Ocean and Araruama Lagoon (Fig. 2a). However, even during the wet season, saline water from these two external water bodies continues to seep into the lagoon. The isotopic composition of lagoon waters represents a balance between the input of meteoric and saline water with a significant decrease in d18O occurring during the wet season.
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Fig. 7. (a) Scanning electron microscrope (SEM) image of recently deposited high-Mg calcite/Ca-dolomite mud displaying round spherical crystal aggregates and needle-like structures. (b) EDX spot measurement of spherical crystal (+) shows equally high Mg and Ca peaks, suggesting a very high mol% MgCO3 content of the carbonate.
Carbonate, including dolomite, consisting of an aggregation of mineral nanoparticles formed by non-classical crystallization mechanisms Niederberger & Co¨lfen 2006) has been recognized as a common biosignature of modern and ancient microbial carbonate (Lopez-Garcia et al. 2005; Benzerara et al. 2006, 2010; Perri & Tucker 2007; Sa´nchezRoma´n et al. 2007; Bontognali et al. 2008; Manzo et al. 2012; Perri et al. 2012a, b, 2013). Previous studies of Brejo do Espinho sediments described diverse mechanisms responsible for carbonate precipitation. For example, van Lith et al. (2002, 2003) showed that high salinity is an important factor for the precipitation of Mg-carbonate, and Moreira et al. (2004) demonstrated that sulphide oxidation induces the organomineralization of dolomite within the microbial mat. Based on aerobic culture experiments with moderately halophilic aerobic bacteria, Sa´nchez-Roma´n et al. (2009)
demonstrated that Mg-carbonate could be precipitated under aerobic conditions and described the homogeneous and fine laminae facies, which occur respectively above and below 10 cm depth. All of these previous studies characterize processes involved in mineral formation. However, they do not describe the environmental conditions related to the formation of two different sedimentary facies, that is, the crust and mud found, respectively, at or just below the surface under the living microbial mat. The present study considers only the mud and crust samples collected at the present-day surface in order to better link sample analyses to the actual meteorological/hydrological data from the nearby meteorological station (INMET Brazil). Brejo do Espinho exhibits well-stratified mats composed of microbial communities (Delfino et al. 2012). Within the bio-organic layers are discrete white patches of microcrystalline carbonate
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Fig. 8. (a) Dolomite crust hand sample. Inset shows location where the thin section was cut. (b) and (c) Photomicrographs of dolomite crust from Brejo do Espinho in which peloidal microfabric can be observed (crossed polars). Yellow arrows indicate dark dolomite micritic peloids, surrounded by lighter microsparitic crystals (pink arrows). (d) and (e) Photomicrographs of dolomite crust from Brejo do Espinho showing peloidal microfabric with ostracods and details of the high fenestral porosity associated with the dissolution of microsparite (crossed polars).
precipitates (micrite) consisting of high-Mg calcite and Ca-dolomite. Microbial mats develop most intensively when seawater influx is strong. High evaporation and increasing salinity prevent the growth of eukaryotic organisms, which minimizes grazing during this period. Carbonate precipitates in association with the microbial mat as a consequence of increased pH, resulting from active
microbial metabolisms. The slightly negative average d13C value (21.2‰ VPDB) of the mud is consistent with a contribution of carbonate ions derived from organic matter. The microbial mats are not preserved in the modern Brejo do Espinho environment during the wet season (Fig. 10a) because the decrease of salinity allows grazing organisms, such as gastropods and crustaceans
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Fig. 9. (a) Scanning electron microscrope (SEM) photomicrograph showing an overview of crust sample with some silica (Si) grains together with dolomitic peloids. Large cavities lined by acicular crystals are visible. (b) Close-up view of a peloid and the surrounding isophachous cement. Note that both are composed of the same dolomite crystals. (c) Mineralized coccoid bacteria; note remains of organic matter (blue arrow) partially replaced by dolomite nanocrystals (red arrow). (d) Dolomite crystals commonly form small-scale tetrahedrons comprising nanocrystals, which coalesce into subspherical units. Note the remains of organic matter and possible bacterial moulds.
(e.g. Artemia sp.), commonly found in these saline environments, to invade and feed on the organic matter in the mats. During the following dry period (Fig. 10b), the remaining organic matter is further degraded with exposure, whereas the highMg calcite and Ca-dolomite mud remains in place to form the accumulating homogeneous aphanitic sediment. The dolomite crusts develop along the borders of the Brejo do Espinho lagoon during the dry period (Fig. 2b). They comprise 100% Ca-dolomite and show discrete internal lamination. During desiccation, two different mechanisms might explain the formation of the crust as illustrated in Figure 10c:
(1) surface water evaporation leading to carbonate supersaturation and/or (2) ‘evaporative pumping’ resulting from an upward ‘Darcy-flow’ of subsurface waters bringing supersaturated solutions to the site of crust formation (Hsu & Siegenthaler 1969). Both mechanisms could result in the precipitation of micritic dolomite cement around the peloids (Figs 8c, e & 9a). The presence of fossil EPS and bacterial bodies replaced by the nanospheres, successively forming mineral polyhedrons, implies that organic matter played a fundamental role in the precipitation of the mineral that forms the peloids and cements of the dolomite crusts (Bontognali et al. 2010, 2014). This is confirmed by the
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Fig. 10. Photographs of Brejo do Espinho lagoon during (a) the wet season and (b) the dry season. (c) Schematic model for crust formation on the margins of Brejo do Espinho during the dry season. The crust may form by two different mechanisms: (1) surface water evaporation leading to carbonate supersaturation (orange arrows) and/or (2) evaporative pumping (yellow arrows) bringing supersaturated solutions to the site of crust formation.
negative value of d13C ¼ 29.5 VPDB, that indicates a source of carbon from decaying organic matter involved in the formation of the mineral. Variations in the stable oxygen isotopic compositions (d18O values) of carbonates have been used to estimate palaeoenvironmental conditions at the time of carbonate precipitation. One of the challenges of such work is that the d18O of carbonate depends both on the temperature of precipitation and the d18O of waters from which it precipitates; both are important palaeoenvironmental properties, but neither can be resolved from the d18O of the carbonate without some additional constraints. Thus, palaeotemperatures derived from the clumped isotope method can help resolve this dilemma. For this study, the clumped isotope formation temperatures for the measured mud and crust samples, were 348 and 32 8C, respectively. These temperatures fall in the upper temperature range of values measured with the in situ data loggers (Table 2, Fig. 3). Furthermore, the calculated d18OVSMOW
values of the formation waters (2.4‰ and 5.1‰ for the mud and crust, respectively) are in agreement with the isotopically heavier lagoon waters sampled during the 2011 dry season months (January to February and July to October; Table 1, Fig. 4). The d18OVSMOW values of the lagoon waters range between 2.1‰ and 4.8‰ during these dry periods. Thus, this correlation of more positive d18OVSMOW values for both the mud and crust samples with the dry season suggests that both the initial carbonate precipitation and the early diagenesis occur under the more evaporative conditions during the seasonal cycle. A key result of this study is the demonstration that carbonate clumped isotope thermometry, using published calibrations, combined with published estimates of the oxygen isotope fractionation between dolomite and water, appear to yield environmental conditions within the ranges documented by the temperatures and d18O values of Brejo do Espinho waters measured during 2011. Our results
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suggest that combined clumped and traditional isotope methods can be useful to accurately reconstruct environmental conditions of ancient dolomite formation when well-preserved samples of similar materials (e.g. lagoonal dolomitic crusts) from the geological record are analysed. Finally, we note that carbonate clumped isotope thermometry measures a lattice-scale process that can be reset by recrystallization or diffusion without destroying larger scale fabrics. Thus, it is possible for a rock to appear to have well-preserved primary depositional fabric (e.g. peloids or laminae), but record a diagenetic temperature resulting from micritization or other secondary processes. In this study, petrographic controls of the crust samples (Fig. 8) showed where primary structures are maintained and may plausibly preserve the original isotopic signal.
Conclusions Studies in modern environments where microbial carbonate precipitates offer an excellent opportunity to understand early diagenetic processes in association with microbial processes and environmental conditions. Brejo do Espinho is located in a special climate regime, where a well-defined seasonal cycle of wet and dry conditions occurs. The variation of measured physico-chemical conditions in this environment was evaluated to add data to the inventory of isotopic studies associated with microbial metabolisms. Furthermore, based on the recorded isotopic signals, it was possible to distinguish the environmental conditions under which two remarkable dolomite facies, mud and crust, formed. The crust displays the more positive d18O value, which is interpreted as reflecting a period when the lagoon is nearly or completely desiccated and the 18O content of the saline water is highly enriched. The mineralogical studies of the crust show that the dolomite peloids are surrounded by dolomitic microspar cement, which is characteristic of exposure episodes, and most likely is responsible for the formation of the leiolite facies that are found to be widespread in the geological record. These observations support the hypothesis that the crust formed under more extreme arid conditions than the mud, which precipitates in equilibrium with less 18O-enriched lagoon water. However, based on clumped isotope thermometry, both mud and crust formed at relatively warm Earth surface temperatures, greater than 30 8C. In addition, the core from Brejo do Espinho shows an upward transition from a lamina to aphanitic or leiolitic facies, which possibly indicates that the evaporation–precipitation hydrological balance has been modified in recent times. This change in environmental conditions could have had an impact on the equilibrium
balance between producers and consumers of the microbial mats. These stochastic factors may have exerted an important control on the preservation of sedimentary structures. On the one hand, for example, events associated with an increase in microbial mat growth rate could result in the preservation of the lamination structures. On the other hand, environmental conditions, which promote a population increase of grazing organisms in the system, would decrease the degree of microbial structure preservation. We would like to sincerely thank Art Saller and an anonymous reviewer for their valuable comments and suggestions, which greatly improved the original manuscript. The encouragement and contributions of the corresponding editor, Dan Bosence, are likewise recognized. Furthermore, we would like to express our appreciation to Dr John Eiler for providing generous access to his laboratory at Caltech and assistance with the interpretation of the data. We acknowledge the support of the Swiss National Science Foundation Grant N8200020127327. This study is part of the research collaboration (PETHROS) between Petrobras and ETH Zurich.
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