Carbonate Stromatolites from a Messinian

Sedimentology (2010) 57, 142–161 doi: 10.1111/j.1365-3091.2009.01094.x

Carbonate stromatolites from a Messinian hypersaline setting in the Caltanissetta Basin, Sicily: petrographic evidence of microbial activity and related stable isotope and rare earth element signatures

ELVIRA OLIVERI*, RODOLFO NERI*, ADRIANA BELLANCA* and ROBERT RIDING *Dipartimento di Chimica e Fisica della Terra (CFTA), Universita` di Palermo, Via Archiraﬁ 36, 90123 Palermo, Italy (E-mail: [email protected]) Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA

Associate Editor: Tracy Frank

ABSTRACT Lower Messinian stromatolites of the Calcare di Base Formation at Sutera in Sicily record periods of low sea-level, strong evaporation and elevated salinity, thought to be associated with the onset of the Messinian Salinity Crisis. Overlying aragonitic limestones were precipitated in normal to slightly evaporative conditions, occasionally influenced by an influx of meteoric water. Evidence of bacterial involvement in carbonate formation is recorded in three dolomite-rich stromatolite beds in the lower portion of the section that contain low domes with irregular crinkly millimetre-scale lamination and small fenestrae. The dominant microfabrics are: (i) peloidal and clotted dolomicrite with calcite-filled fenestrae; (ii) dolomicrite with bacterium-like filaments and pores partially filled by calcite or black amorphous matter; and (iii) micrite in which fenestrae alternate with dark thin wispy micrite. The filaments resemble Beggiatoa-like sulphur bacteria. Under scanning electron microscopy, the filaments consist of spherical aggregates of dolomite, interpreted to result from calcification of bacterial microcolonies. The dolomite crystals are commonly arranged as rounded grains that appear to be incorporated or absorbed into developing crystal faces. Biofilm-like remains occur in voids between the filaments. The dolomite consistently shows negative d13C values (down to )11Æ3&) and very positive d18O (mean value 7Æ9&) that suggest formation as primary

precipitate with a substantial contribution of organic CO2. Very negative d13C values (down to )31Æ6&) of early diagenetic calcite associated with the

dolomite suggest contribution of CO2 originating by anaerobic methane oxidation. The shale-normalized rare earth element patterns of Sutera stromatolites show features similar to those in present-day microbial mats with enrichment in light rare earth elements, and M-type tetrad effects (enrichment around Pr coupled to a decline around Nd and a peak around Sm and Eu). Taken together, the petrography and geochemistry of the Sutera stromatolites provide diverse and compelling evidence for microbial inﬂuence on carbonate precipitation. Keywords Calcare di Base, Messinian, microbial carbonate, petrography, rare earth elements, Sicily, stable isotopes, stromatolites.

142 2009 The Authors. Journal compilation 2009 International Association of Sedimentologists Carbonate stromatolites from a Messinian hypersaline setting 143

INTRODUCTION This study centres on dolomite-rich stromatolites from the Calcare di Base Formation in the Microbes can play important roles in the pro- Caltanissetta Basin, near Sutera town, Sicily duction, accumulation and diagenesis of sedi- (Fig. 1). The focus was: (i) investigation of the ments (Nealson, 1997; Riding & Awramik, 2000; processes promoting carbonate precipitation in Tucker, 2001; and references therein), even these stromatolite beds that formed in a Messinian though their action and impact may differ greatly hypersaline setting; and (ii) relating the petro- according to the types of sediments and envi- graphic evidence of microbial activity to geo- ronments. Castanier et al. (2000) emphasized the chemical (O and C isotopes, Sr, REE) signatures. relationship between carbonate precipitation and Rare earth element patterns are used here for the organic matter enrichment in heterotrophic bac- first time to highlight and discuss the imprint of terial communities. These microbes can orches- bacterial activity in stromatolites from the Calcare trate CaCO3 precipitation and stromatolite di Base. formation, lithifying organosedimentary structures that show a layered structure (Riding, 2000; Dupraz & Visscher, 2005). In particular, GEOLOGICAL SETTING sulphate-reducing bacteria (SRB) have been suggested to mediate anaerobic carbonate precipita- During the Miocene, the Mediterranean region tion in microbial mats (Jørgensen & Cohen, 1977; was affected by the MSC (Hsu¨ et al., 1973). This Lyons et al., 1984; Canfield & Des Marais, 1991; important evaporitic event resulted from isolation Visscher et al., 1998, 2000; Reid et al., 2000; of the Mediterranean Sea from the world ocean. Sprachta et al., 2001; Dupraz et al., 2004). Many Large volumes of Messinian evaporites were researchers have highlighted the role that SRB deposited in a series of sub-basins, delimited by can play in primary dolomite formation (Comp- local barriers, within a large pre-Messinian basin ton, 1988; Middleburg et al., 1990; Vasconcelos (Cavazza & Wezel, 2003). Evaporite deposition et al., 1995; Vasconcelos & McKenzie, 1997; was preceded in many Mediterranean peripheral Wright, 1999; Van Lith et al., 2003; Wright & basins by formation of cyclic, thinly bedded Wacey, 2005; Meister et al., 2007). Carbon isoto- diatom-rich deposits (e.g. the Tripoli Formation pic values can indicate bacterial involvement in in Sicily) (Rouchy, 1982; Bellanca et al., 1986; carbonate precipitation (Murata et al., 1969; Martıń & Braga, 1994; Clauzon et al., 1996; Riding Claypool & Kaplan, 1974; Compton, 1988). The et al., 1998, 2000). A similar sequence was distribution of geochemical elements, particu- deposited in some deep-sea areas (Ocean Drilling larly rare earth elements (REE) that are essen- Program (ODP) Site 372, Cita et al., 1978; ODP tially immobile during diagenesis, can also Site 654, Pierre & Rouchy, 1990). The increase in provide useful information to identify bacterial biosiliceous productivity has been linked to a contributions in sedimentary deposits (Takah- variety of factors, e.g. upwelling of marine deep ashi et al., 2005). waters (McKenzie et al., 1979–1980; Rouchy, In the Italy area, during the Messinian Salinity 1986; Mu¨ ller & Hsu¨ , 1987) and increase in Crisis (MSC), the transition from marine to terrestrial nutrient supply (Van der Zwaan, evaporitic conditions may be reflected in a 1979; Hodell et al., 1994; Rouchy et al., 1998), variety of different sedimentary deposits related to either increasing isolation of the Med- (Tripoli, Calcare di Base and Gessoso-Solfifera iterranean (McKenzie et al., 1979–1980) or global Formations), which contain a record of marked eustatic control (Suc et al., 1995), or to both of environmental changes. An example of this is these (Rouchy, 1982; Rouchy & Saint-Martin, the Calcare di Base Formation of Southern Italy, 1992). the significance of which is still debated The onset of MSC evaporite deposition has (Rouchy & Caruso, 2006; Guido et al., 2007; been suggested to have been synchronous over Roveri et al., 2008). On the basis of very negative the entire Mediterranean Basin and to have carbon isotope signatures, McKenzie et al. occurred between 5Æ70 to 5Æ96 Ma (Gautier et al., (1979–1980) and Bellanca et al. (1983, 1986) 1994; Krijgsman et al., 1999, p. 654; Cavazza & ascribed precipitation of early diagenetic Wezel, 2003). In contrast, on the basis of magneto- carbonate, in diatomites of the Messinian Tripoli stratigraphic and sequence-stratigraphic data Formation in Sicily, to the action of SRB; from evaporitic deposits in Sicily, Butler et al. however, at that time, no petrographic evidence (1995, 1999) suggested that evaporite onset was of bacterial biomineralization was provided. diachronous, over a period of at least 800 ka, in

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Fig. 1. General geological setting of the Caltanissetta Basin, Sicily (simplified from Butler et al., 1995) showing the location of the Sutera section. the different sub-basins of the Caltanissetta Basin. Calcare di Base Formation (evaporitic limestone), Riding et al. (1998) also proposed that in shallow Lower Gypsum and Salt. According to the classic marginal Mediterranean basins evaporite deposi- view, the Calcare di Base was deposited in the tion tended to either pre-date or post-date the shallow portion of the Caltanissetta Basin and evaporites of the deeper Mediterranean and that represents the stratigraphic equivalent of the these marginal evaporites are not completely Lower Gypsum that occurs in the deepest portion contemporaneous with those of the deeper basins. of the basin. The upper unit is formed by the The Caltanissetta Basin (Fig. 1) contains one of Upper Gypsum overlain by the Arenazzolo For- the best exposed records of Messinian strata mation, siliciclastic lacustrine and fluvial/allu- (Krijgsman & Zachariasse, 1994; Hilgen et al., vial facies (Decima & Wezel, 1973; Cita & 1995), deposited within a series of thrust-related Colombo, 1979), in turn capped by the pelagic synclines (Butler et al., 1995); these form the Early Pliocene Trubi Formation, which marks the outer part of the Maghrebian Chain in a zone re-establishment of normal marine conditions as a between the Northern Sicilian Mountains (Euro- consequence of flooding from the Atlantic (Cita, pean plate) and the Hyblean Plateau (African 1975; Di Stefano et al., 1999; Rouchy et al., 2001). foreland). In this area, the classic stratigraphic According to Roveri et al. (2001) and Manzi model recognizes two main evaporitic units sep- et al. (2007), in Sicily as well as in the Northern arated by an intra-Messinian tectonic unconfor- Apennines, the Lower Gypsum includes both mity (Decima & Wezel, 1971; Butler et al., 1995; primary gypsum (shallow water facies) and resedi- Rouchy & Caruso, 2006). From bottom to top, the mented gypsum (deep water facies). In Sicily, the lower unit consists of the Tripoli Formation, primary Lower Gypsum was deposited only in the

2009 The Authors. Journal compilation 2009 International Association of Sedimentologists, Sedimentology, 57, 142–161 Carbonate stromatolites from a Messinian hypersaline setting 145 innermost wedge-top basins and in the Hyblean (Fig. 2E) and alternate with dolomitic marls (of foreland ramp basins, and is not associated with variable thickness), laminated dolostones (up to the Salt and/or the Calcare di Base (Roveri et al., 1 m thick) and millimetre-thick claystone beds 2008). (Fig. 2A). Higher in the section, claystone beds up In many outcrops, the Calcare di Base Forma- to 5 m thick alternate with aragonitic limestones tion consists of massive pale cream limestone, up to 4 m thick. commonly brecciated and intercalated with dolostones, grey clays and marls (Ogniben, 1957, Stromatolites 1963; Decima & Wezel, 1973; Decima et al., 1988). At Sutera, the lower portion of the strati- The lowest stromatolitic bed (I in Fig. 2A) is graphic sequence includes three stromatolite 37 cm thick, grey to brown in colour, and exhibits beds. The lowest of these is in stratigraphic very low domes, irregular laminae (1Æ5 to 3 mm), contact with marls of the underlying Tripoli and intense bioturbation (Fig. 3A). The middle Formation. The Calcare di Base is commonly stromatolitic bed (II in Fig. 2A) is 44 cm thick and characterized by the presence of halite moulds, consists of very thin alternating brown and grey calcite pseudomorphs after gypsum, and peloidal laminae, with fine open pores and domes up to microfabrics that have been linked to bacterial 4Æ5 cm wide and 1Æ5 cm high (Fig. 3B). Higher in activity (Tamajo, 1961; Ogniben, 1963; Decima the same bed, the alternation of laminae is et al., 1988). It also contains sulphur-bearing interrupted by grey to dark-grey crinkly layers limestone derived from bacterial sulphate reduc- (Fig. 3C). The uppermost stromatolitic bed is tion (BSR). 62 cm thick (III in Fig. 2A) with thick to thin For many authors, the Calcare di Base is layers (1 to 5Æ5 mm), very low domes, small laterally equivalent to the Lower Gypsum and fenestral pores and vertical burrows (Fig. 3D). marks the onset of the MSC, preceding deposition of the sulphate and salt evaporites of the Gessoso- Solfifera Formation (McKenzie, 1985; Bellanca & ANALYTICAL METHODS Neri, 1986; Decima et al., 1988; Caruso et al., 1997; Bellanca et al., 2001; Rouchy & Caruso, A total of 72 samples was collected from the 2006). In contrast, taking the view that the Calcare Sutera section. In the laboratory, sub-samples di Base is mostly a non in situ brecciated were obtained from the stromatolite layers. Each limestone, Roveri et al. (2008) argue that its base sample was crushed in an agate mortar to obtain represents a regional-scale unconformity and that fine uncontaminated powders. it does not mark the onset of the MSC. Roveri Bulk sample mineralogy was determined by et al. (2008) support Hilgen & Krijgsman (1999) in powder X-ray diffraction using a Philips PW14 recognizing the dolostones above the Tripoli 1373 (Koninklijke Philips Electronics NV, Am- Formation as the only part of the Calcare di Base sterdam, The Netherlands) with Cu-Ka radiation that is the lateral equivalent of the primary Lower filtered by a monochromator crystal and a scan- Gypsum. ning speed of 22h/min. The relative proportions of minerals were determined using the methods and data of Schultz (1964) and Barahona et al. Sutera stratigraphy (1982). Petrographic thin sections of selected The section studied here is within the Caltanis- carbonate samples were observed using polarized setta Basin (1343¢44¢¢ E, 3731¢44¢¢ N) and is light microscopy. Scanning electron microscope located about 2 km north-west of Sutera town (SEM) investigations were performed on fresh (Fig. 1). At Sutera, the Calcare di Base overlies the broken surfaces and on polished, carbon-coated Lower Messinian Tripoli Formation which, in thin sections, using a LEO 440 with EDS system turn, overlies Tortonian clays of the Terravecchia OXFORD ISIS 300 Link and a Si (Li) PENTAFET Formation. The sequence studied comprises the detector (Leo Electron Microscopy Ltd., Cam- uppermost 3 m of the Tripoli Formation and the bridge, UK). lower ca 16 m of the Calcare di Base Formation Carbonate oxygen and carbon isotopic compo- (Fig. 2A). The passage between these two forma- sitions were obtained from bulk-rock samples. To tions is marked by the three dolomite-rich stro- remove organic matter, samples were roasted for matolite beds that are the focus of this study 40 min at 370 C in high-vacuum glass tubes, (Fig. 2B to E). These beds display millimetre- then dissolved in 100% phosphoric acid at 25 C scale stratiform layering (flat stromatolites) under high vacuum for 12 h for calcite and

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Fig. 2. Sutera section. (A) Lithological column showing sample horizons (S 2 to S 73) and the three stromatolite beds (I, II and III). (B) to (E) Progressively closer views of the stratigraphic succession and stromatolite beds. Person for scale in (B) is ca 1.8 m tall. In (C) the red arrow points to the stromatolite unit which is also shown by the red bar. Scale size on (C) = 44 cm; hammer size on (D) = 31Æ5 cm; coin size on (E) = 2Æ5 cm. 2009 The Authors. Journal compilation 2009 International Association of Sedimentologists, Sedimentology, 57, 142–161 Carbonate stromatolites from a Messinian hypersaline setting 147

A B

1 cm 1 cm

C D

1 cm 1 cm Fig. 3. Stromatolites. (A) Bottom bed showing irregular laminae (1Æ5 to 3 mm thick), grey–brown colour, local very low domes and evidence of intense bioturbation. (B) Middle bed with very thin brown and grey laminae, small open pores and domes up to 4Æ5 cm wide and 1Æ5 cm high. (C) Upper part of the middle bed, showing alternation of laminae interrupted by grey to dark–grey crinkly layers. (D) Top bed, showing laminae of variable thickness (1 to 5Æ5 mm), local very low domes, small fenestrae and vertical burrows. aragonite and 72 h for dolomite. After reaction, described by Franzini et al. (1975). Analytical CO2 was cryogenically separated from the other errors were below 1% for Si and Al, below 3% for gases and measured with a Finnigan Delta S Fe and below 10% for Sr. Analyses for REE were mass spectrometer (Thermo Scientific, Dreieich, carried out on 26 selected samples, via induced Germany). When both calcite/aragonite and dolo- coupled mass spectrometry (ICP-MS), at Activa- mite were present, the techniques of Epstein et al. tion Laboratories Ltd (ActLabs; Ancaster, ON, (1964) and Becker & Clayton (1972) were used. To Canada). For sample preparation, 0Æ5 g of bulk allow for the different reaction rates of the sample was acid-digested in the following stages: carbonates, the CO2 gas was collected at time (i) hydrofluoric acid; (ii) a mixture of nitric and intervals of 20 min for more reactive calcite and perchloric acids; (iii) dried using precise pro- after 72 h for dolomite. The isotopic data are grammer controlled heating in several ramping expressed in d& units and reported against the and holding cycles; and (iv) brought back into Pee Dee Belemnite (PDB-1) standard. The repro- solution using hydrochloric acid. The ICP-MS ducibility for the isotopic determinations was used was a Perkin Elmer Elan 6000 (Weltech ±0Æ1& for d18O and ±0Æ05& for d13C. Enterprises, Inc., Capitol Heights, MD, USA). The Major and minor element concentrations detection limit for REE (0Æ1 ppm) was that given were determined by X-ray fluorescence using a by ActLabs. The abundance of REE was normal- RIGAKU ZSX PRIMUS spectrometer (Rigaku ized to the average of North American, European International Corp., Osaka, Japan) on pressed, and Russian shale composites adopted in many boric-acid backed pellets of bulk samples. Data previous studies (Piper, 1974; Gromet et al., 1984; reduction was achieved using the method Sholkovitz, 1988; Murray et al., 1991).

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Table 1. Mean values and variation ranges of carbonate mineralogy (%), carbonate stable isotopes (d& versus PDB) and Sr concentrations (ppm) for lithologies from the Sutera section; n = sample number.

Stromatolites Dolostones Aragonitic lime- Claystones Marls (n = 20) (n = 16) (n = 19) stones (n = 11) (n = 13)

Mean Min Max Mean Min Max Mean Min Max Mean Min Max Mean Min Max Dolomite 28 2 64 78 43 99 85 43 99 3 3 3 4 2 6 Calcite 7 2 14 38 11 53 12 3 38 4 2 6 9 6 13 Aragonite 41 32 57 – – – – – – 89 75 98 – – – 18 d Odolomite 4Æ0 )0Æ87Æ66Æ64Æ17Æ95Æ74Æ17Æ9–––– –– 13 d Cdolomite )6Æ0 )12Æ20Æ3 )5Æ7 )11Æ3 )1Æ8 )7Æ7 )13Æ7 )1Æ3–––– –– 18 d OCa-carb )0Æ2 )0Æ2 )0Æ22Æ00Æ83Æ7– – – 1Æ2 )0Æ22Æ5– – – 13 d CCa-carb 1Æ81Æ81Æ8 )5Æ2 )31Æ6 )17Æ4– – – 1Æ0 )0Æ11Æ9– – – Sr 819 128 5061 394 283 535 285 169 437 5795 5166 6922 414 89 1163

RESULTS AND INTERPRETATIONS (Fig. 4A); and (ii) pelmicrite/pelsparite including small peloids and disseminated pyrite. The stromatolites from the middle bed (Figs 2A, 3B and Mineralogy 3C) exhibit lamination resembling a dense felt X-ray diffraction analysis of bulk samples from preserved as dolomicrite that includes voids the lower and middle part of the Sutera section partially filled by calcite and, more rarely, by shows dolomite as the predominant carbonate black amorphous matter (Fig. 4B and C). The phase (up to 99%; Table 1). The marls contain third stromatolite bed (Figs 2A and 3D) consists between 2% and 64% dolomite, although in of micrite in which fenestrae (generally 0Æ2to marly layers in the upper part of the section 2 mm in size) alternate with irregular thin layers aragonite is the most abundant carbonate mineral. of dark wispy micrite (Fig. 4D). Both the lower Stromatolite beds I and II (Fig. 2A) display dual and middle beds contain filaments ca 30 to 70 lm carbonate mineralogy with dolomite ranging from in width. In places the filaments appear fragmen- 43% to 99% and calcite up to 53%. In stro- tary, but can be elongate and locally are distinc- matolite bed III (Fig. 2A), dolomite (mean value tively recurved (Fig. 4B). Internally the filaments 97%) is the only carbonate present. The upper generally consist of dense micrite but can be part of the section shows aragonite contents traversed by light-coloured narrow transverse ranging from 32% to 98% (higher values in the gaps (Fig. 4C). aragonitic limestones), together with small per- Under the SEM untreated samples from the centages of calcite, dolomite (mean values of 4% middle stromatolite bed show that the filaments and 3%, respectively) and celestite. are preserved in dolomicrite as a felted mass Clay minerals are present in variable propor- (Fig. 5A) in which individual filaments consist of tions throughout the section, naturally being rounded aggregates, each ca 40 lm in size, of concentrated in the claystone levels (up to 85%) dolomite crystals (Fig. 5B). The crystal faces and generally being absent in stromatolites and locally show micron-sized rounded bodies par- aragonitic limestones. All samples contain detri- tially incorporated or absorbed into the rhombo- tal quartz with percentages being low in the hedral crystals (Fig. 5C). Biofilm-like remains are carbonate layers (e.g. 0% to 4% in stromatolites) present in voids between filaments (Fig. 5D). and higher (up to 60%) in the claystones. Euhedral calcite crystals filling fenestral pores (Fig. 4A) locally have filamentous strands of branched material that may be organic, attached Petrography to and extending from the crystal faces (Fig. 6A). Textural features of stromatolites and Thin sections viewed by SEM show that the aragonitic limestones clotted dolomicrite exhibits surfaces marked Carbonate in the first stromatolite bed (Figs 2A with scars and black spots (Fig. 6B and C). The and 3A) occurs as millimetre-scale alternations of black spots appear as clusters enclosed within dense micrite and peloidal laminae. The peloidal dolomicrite (Fig. 6C). In some cases, these spots laminae show: (i) fenestral pelsparite with irreg- are arranged as rings around dark rounded ular, locally dendritic, clumps of clotted micrite bodies (Fig. 6B). An enlarged view of the

2009 The Authors. Journal compilation 2009 International Association of Sedimentologists, Sedimentology, 57, 142–161 Carbonate stromatolites from a Messinian hypersaline setting 149

A B

150 µm 150 µm

C D

150 µm 75 µm

Fig. 4. Stromatolites in thin section. (A) Bottom bed, showing peloidal laminae and fenestral pelsparite with irregular, locally dendritic, clumps of clotted micrite (plane polarized light). (B) Middle bed, showing peloidal- clotted dolomicrite with fenestrae (partially filled by calcite) and recurved filaments (plane polarized light). (C) Middle bed, showing dolomicrite and filaments, with voids partially filled by black amorphous matter (plane polarized light). (D) Top bed with layers of dark thin wispy micrite and open fenestrae (crossed polars). rounded bodies shows structures with a net-like polymeric substances) within microbial mats, fabric (Fig. 6D). promoting calcification (Riding & Toma´s, 2006). In SEM, the aragonitic limestones from the The filaments present in Sutera stromatolites thick bed in the upper part of the section are seen resemble the large motile thiophilic bacterium to consist of a mass of needle-like aragonite Beggiatoa. Typically, Beggiatoa spp. use oxygen crystals (Fig. 7A) of variable length. Generally, to oxidize sulphide to sulphate in sediments rich the needles are loosely packed and some of them in hydrogen sulphide (Jørgensen & Revsbech, form dumb-bell aggregates (Fig. 7B) similar to 1983). Since this reaction proceeds rapidly and those reported in cold seep sites. The needles spontaneously where oxygen is abundant, Beggi- show preferential growth along their c-axis atoa only proliferates at the anoxic–oxic transi- (Fig. 7B), typical of primary precipitation of tion (Møller et al., 1985). Beggiatoa uses its aragonite (Gabrielli et al., 1999). gliding ability to follow this chemical zone as it migrates during daily cycles in microbial mat Interpretation microenvironments (Garcia-Pichel et al., 1994). The clotted–peloidal microfabrics in the stromat- Beggiatoa mats can be unusually thick (>1 cm) in olite beds (Fig. 4) are typical of microbial carbon- coastal-upwelling zones (Møller et al., 1985) and ates (Riding, 2000) in which heterotrophic at hydrothermal (Gundersen et al., 1992) and bacteria such as sulphate reducers degrade methane seeps (Boetius & Suess, 2004). Filamen- organic material (bacterial cells, extracellular tous cyanobacteria such as Scytonema (see

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A B

1000 µm 100 µm

C D

15 µm 30 µm

Fig. 5. SEM images of stromatolites (middle bed). (A) and (B) Dolomicrite preserving a felt-like mass of elongate filaments. (C) Spherical aggregates of dolomite crystals with micron-sized rounded shapes incorporated or absorbed into rhombohedral crystals. (D) Biofilm-like remains in voids between filaments.

Monty, 1965) have been reported in hypersaline precipitation under sedimentary conditions settings (Vai & Ricci Lucchi, 1977; Rouchy & (Warthmann et al., 2000; Van Lith et al., 2003; Monty, 1981). However, in size (diameter up to Wright & Wacey, 2005). The SRB degrade organic 70 lm) and dense felted arrangement the Sutera matter consuming sulphate and producing sul- filaments (Figs 4 and 5) more closely resemble phide (H2S), which can favour pyrite precipita- Beggiatoa or similar thiophilic bacteria. Similar tion (FeS2) when iron is available (Herbert et al., filaments from a Tortonian methane seep in 1998). The presence of disseminated pyrite in the Central Italy have also been compared with dolomicrite could reflect SRB activity in Sutera Beggiatoa (Peckmann et al., 2004; Fig. 2). In stromatolites. addition, the distinctively recurved shape of Biofilm-like alveolar structures in the dolomi- some of the Sutera filaments (Fig. 4B) recall the crite (Fig. 6D) could suggest mineralization at the typical looping behaviour of present-day Beggia- expense of organic material, such as extracellular toa as it reverses direction to avoid zones of polymeric substances (EPS), which contain elevated oxygen concentration (Møller et al., organic templates that can act as carbonate min- 1985; Figs 4 and 8). Such abrupt chemotactic eral nucleation sites (Dupraz & Visscher, 2005). responses suggest relatively steep oxygen gradi- Also, the clusters of black spots floating in the ents (Møller et al., 1985). This suggestion would dolomicrite mass (Fig. 5C) and the spherical be consistent with the very shallow-water envi- aggregates of dolomite crystals (Fig. 6C) could ronment inferred for the basal Calcare di Base at reflect microbial carbonate precipitation. On the Sutera. whole, the microtextural investigations suggest Recently, culture experiments have demon- that the dolomite of Sutera stromatolites may strated that SRB are key players in dolomite have been strongly influenced by the metabolic

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A B

2 µm 400 µm

C D

6 µm 9 µm

Fig. 6. SEM images of stromatolites (middle bed). (A) Euhedral calcite crystals ﬁlling fenestral pores and dis- playing strands of material that may be organic, attached to and extending from the crystal faces. (B) Dolomicrite surface marked by scars and black spots. (C) Black spot material enclosed within dolomicrite. (D) Detail of net-like fabric.

A B

20 µm 8 µm

Fig. 7. SEM images of aragonitic limestone (thick bed). (A) Mass of needle-like aragonite crystals; (B) some of which form dumb-bell shaped aggregates.

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Fig. 8. Profiles of d18O &, Sr ppm and d13C & for the Sutera section (lithological key as in Fig. 2), with sample mineralogy shown by colour: dolomite (blue), calcite (red) and aragonite (green). activity of SRB and that EPS may have provided Interpretation of isotope results nucleation sites for dolomite precipitation. The relatively heavy d18O values of the dolomite (Fig. 8) reflect marine waters exposed to moderate to strong evaporation. The sudden decrease in Geochemistry d18O in the marly intercalation between the Stable oxygen and carbon isotopes stromatolite beds is interpreted as evidence of a Dolomite from the Sutera section is characterized brief dilution event. The negative d13C values of by high d18O values, generally between 4Æ1 and the Sutera dolomite (down to )13Æ7&) suggest 7Æ9&, and by a wide range of mostly negative d13C large availability of organic, isotopically light values from )13Æ7& to 0Æ3& (Fig. 8). A negative carbon in dolomite-forming waters, implying spike in the d18O dolomite curve (down to 0Æ8&) organic matter degradation processes probably marks the marly level intercalated between the II favoured by periods of increased stagnation. The and III stromatolite beds. Near the transition to negative shifts in both oxygen and carbon isotope the first thick claystone level, oxygen and carbon values towards the mid-part of the section (Fig. 8) isotope values of dolomite decline to )0Æ8& and suggest dilution of evaporated marine waters by )11Æ9&, respectively. Calcite associated with continental runoff. dolomite from the stromatolite beds displays Although previous studies give conflicting oxygen isotope values from 0Æ8& to 3Æ7& and is results regarding differential oxygen isotope frac- strongly depleted in 13C, with values of )31Æ6& to tionation during the precipitation of carbonate )17Æ4&. minerals (Land, 1980; McKenzie, 1981; Schmidt In the upper portion of the section, essentially et al., 2005), the ca 5& lower d18O values of aragonitic carbonates exhibit isotopic composi- calcites from the Sutera section in comparison to tion in the ranges )0Æ2& < d18O<2Æ5& and coexisting dolomites suggest that the two miner- )0Æ1& < d13C<1Æ9& (Fig. 8). Similar isotopic als were not strictly cogenetic and reflect differing values were reported by Decima et al. (1988) for temperature and more dilute water during crys- samples of aragonitic limestones from the same tallization. This is consistent with petrographic section. observations indicating an early diagenetic origin

2009 The Authors. Journal compilation 2009 International Association of Sedimentologists, Sedimentology, 57, 142–161 Carbonate stromatolites from a Messinian hypersaline setting 153 for the calcite. The extremely negative d13C values exhibit a mean Sr concentration of 340 ppm. This of the same calcites (from )17Æ4 down to )31Æ6&; is lower than the Sr content of marine to hypers- Fig. 8, Table 1) might indicate a methane carbon aline dolomites (ca 600 ppm, Behrens & Land, source for this carbonate. 1972; 500 to 800 ppm, Land, 1980) but higher The isotopic compositions of the aragonitic than that reported for ancient marine or marine– limestones are in the range of carbonates formed meteoric dolomites (generally < 200 ppm, Machel from normal to slightly evaporated marine waters. & Anderson, 1989). As a whole, these Sr concen- Even taking into account possible effects of arago- tration data support the interpretation of the nite–dolomite isotope fractionation, the shift in the stable isotope results in terms of a transition of d18O curve near the beginning of aragonite precip- the Sutera depositional environment from itation is large enough to indicate a change in strongly evaporated to normal or slightly evapo- depositional conditions in the Sutera basin. rated conditions, with dolomites generally Overall, the isotopic data support the view that recording evaporative episodes. the variation in carbonate mineralogy through the Sutera section reflects evolution from Rare earth elements strongly evaporated marine waters (in which The main compositional data of REE + Y in the the stromatolites formed) to normal or slightly stromatolite samples and, for comparison, in the evaporated marine waters (in which the arago- dolostones, aragonitic limestones and claystones/ nitic limestones formed). Deposition probably marls from the Sutera section are shown in occurred in a semi-closed marine environment Table 2. Stromatolites have the lowest total REE subject to wide variations in salinity related to (RREE) concentrations, whereas the aragonitic episodes of meteoric water influx (see Guido limestones and claystones/marls display higher et al., 2007). RREE values probably due to the presence of more abundant REE-rich terrigenous components Sr distribution (Table 2). Dolostones overlying the stromatolites Mean values and ranges of Sr concentrations in exhibit intermediate values. Lan/Ybn ratios of the Sutera lithologies are listed in Table 1. The stromatolites are > 1, indicating moderate enrich- vertical log of Sr data shows two clearly distinct ment in light REE (LREE) compared to heavy REE parts: low values in the lower half of the section, (HREE) in contrast to marine carbonate that is followed by an abrupt increase that is sustained typically LREE-depleted. In addition, the stro- through the upper half of the section (Fig. 8). The matolites are slightly enriched in LREE relative to rise in concentrations (up to 6922 ppm; Table 1) middle REE (MREE) (Lan/Smn =1Æ1to1Æ5, Prn/ commences at the transition to the aragonitic Smn =1Æ0to1Æ3; Table 2) and, at least for some limestones in the upper portion of the section. samples, slightly depleted in HREE relative to Dolomite-rich carbonates (i.e. stromatolites and MREE (Dyn/Ybn =upto1Æ6; Table 2). dolostones) from the lower part of the section have a mean Sr concentration of 340 ppm. The REE interpretations stromatolites fall within the range from 283 to The estimation of sea water chemistry in the 535 ppm (Table 1). geological past is dependent upon sedimentary proxies (e.g. directly precipitated evaporites) Interpretation of Sr results thought to reliably reflect contemporaneous sea High Sr/Ca ratios in carbonate sediments are water composition, based on studies of elements inferred to correspond to aragonite enrichment with known and predictable behaviour in modern (Azzaro et al., 1993; Thomson et al., 2004) and Sr sea water and their fractionation in modern sea concentrations in the Sutera section reflect the water precipitates. Sedimentary records of REE abundance of aragonite (Fig. 8). Incorporation of have been used widely to derive palaeo-sea water Sr2+ into aragonite appears to be independent of REE patterns, although the stability of these salinity (Gaetani & Cohen, 2006), and aragonitic elements during burial and diagenesis is still a limestones from the Sutera section show Sr matter of discussion (Banner et al., 1988; McLen- concentration values (up to 6922 ppm; Table 1) nan, 1989; Holser, 1997; Webb & Kamber, 2000; that are similar to those commonly reported for Shields & Webb, 2004) and REE fractionation may aragonite precipitated from normal marine waters occur during marine mineral precipitation. How- (8300 ppm, Kinsman, 1969; 9000 ppm, Bathurst, ever, owing to their tendency to incorporate REE 1975). Dolomite-rich carbonates (i.e. stromatolites from sea water with uniform and high partition and dolostones) from the lower part of the section coefficients, microbial carbonates are currently

2009 The Authors. Journal compilation 2009 International Association of Sedimentologists, Sedimentology, 57, 142–161 154 09TeAtos ora compilation Journal Authors. The 2009 .Oieie al. et Oliveri E.

Table 2. REE + Y composition (ppm) and shale-normalized REE index ratios of samples from the Sutera section grouped according to lithology; nd = not detected.

Sample La Ce Pr Nd Sm Eu Gd Tb Dy Y Ho Er Tm Yb RREE Lan/Ybn Lan/Smn Prn/Smn Dyn/Ybn Prn/Ndn Stromatolites S24 2Æ43Æ60Æ620Æ40Æ10 0Æ4nd0Æ32Æ0nd0Æ2 nd 0.2 10Æ21Æ03 1Æ10 1Æ11 0Æ96 1Æ13 S23 1Æ82Æ70Æ41Æ50Æ30Æ07 0Æ3nd0Æ31Æ6nd0Æ1ndnd 7Æ6nd1Æ10 0Æ99 nd 1Æ00 S21 3Æ56Æ30Æ82Æ90Æ60Æ11 0Æ5nd0Æ52Æ6nd0Æ3nd0Æ215Æ71Æ50 1Æ07 0Æ99 1Æ60 1Æ04 S16 4Æ17Æ81Æ03Æ30Æ70Æ13 0Æ5nd0Æ42Æ6nd0Æ3nd0Æ218Æ41Æ75 1Æ07 1Æ06 1Æ28 1Æ14 S 15d 2Æ74Æ20Æ62Æ10Æ40Æ09 0Æ4nd0Æ31Æ7nd0Æ2nd0Æ211Æ21Æ16 1Æ23 1Æ11 0Æ96 1Æ07 S 15c 3Æ66Æ30Æ92Æ70Æ60Æ13 0Æ4nd0Æ42Æ00Æ10Æ2ndnd15Æ4nd1Æ10 1Æ11 nd 1Æ25

S 15b 2Æ94Æ80Æ72Æ20Æ40Æ08 0Æ4nd0Æ32Æ0nd0Æ2nd0Æ212Æ21Æ24 1Æ33 1Æ30 0Æ96 1Æ20

09ItrainlAscaino Sedimentologists, of Association International 2009 S 15a 1Æ93Æ10Æ41Æ50Æ30Æ07 0Æ3nd0Æ31Æ6nd0Æ1ndnd 8Æ1nd1Æ16 0Æ99 nd 1Æ00 S14 2Æ43Æ70Æ51Æ70Æ30Æ07 0Æ3nd0Æ31Æ8nd0Æ1ndnd 9Æ5nd1Æ46 1Æ24 nd 1Æ11 S08 4Æ37Æ11Æ03Æ60Æ70Æ15 0Æ7nd0Æ63Æ70Æ10Æ3nd0Æ318Æ91Æ23 1Æ12 1Æ06 1Æ28 1Æ04 Dolostones S44 4Æ88Æ81Æ13Æ70Æ80Æ14 0Æ7nd0Æ63Æ30Æ10Æ3nd0Æ321Æ31Æ37 1Æ10 1Æ02 1Æ28 1Æ12 S42 13Æ420Æ83Æ011Æ52Æ40Æ51 2Æ20Æ31Æ711Æ30Æ41Æ10Æ21Æ058Æ71Æ15 1Æ02 0Æ93 1Æ09 0Æ98 S38 4Æ77Æ51Æ14Æ21Æ00Æ25 1Æ10Æ21Æ06Æ50Æ20Æ6nd0Æ422Æ21Æ01 0Æ86 0Æ82 1Æ60 0Æ98 S36 7Æ915Æ41Æ86Æ91Æ60Æ37 1Æ60Æ21Æ47Æ80Æ30Æ7nd0Æ638Æ61Æ13 0Æ90 0Æ83 1Æ49 0Æ98 S29 11Æ418Æ82Æ89Æ92Æ10Æ43 1Æ80Æ31Æ58Æ20Æ30Æ90Æ10Æ851Æ21Æ22 0Æ99 0Æ99 1Æ20 1Æ06 S27 3Æ56Æ10Æ83Æ10Æ60Æ13 0Æ6nd0Æ53Æ3nd0Æ3nd0Æ215Æ81Æ50 1Æ07 0Æ99 1Æ60 0Æ97 Aragonitic limestones S63 16Æ833Æ64Æ015Æ02Æ90Æ61 2Æ60Æ32Æ114Æ40Æ41Æ30Æ21Æ281Æ11Æ20 1Æ06 1Æ02 1Æ12 1Æ00 S61 14Æ522Æ63Æ412Æ72Æ80Æ60 2Æ50Æ42Æ313Æ80Æ51Æ40Æ21Æ365Æ40Æ95 0Æ95 0Æ90 1Æ13 1Æ01 S57 11Æ117Æ82Æ79Æ92Æ40Æ57 2Æ50Æ42Æ212Æ40Æ41Æ20Æ20Æ952Æ41Æ06 0Æ85 0Æ83 1Æ56 1Æ03 S54 33Æ367Æ17Æ729Æ65Æ61Æ18 4Æ60Æ63Æ015Æ70Æ51Æ40Æ21Æ0 155Æ82Æ85 1Æ09 1Æ02 1Æ92 0Æ98 Claystones/marls S70 28Æ747Æ37Æ527Æ75Æ91Æ19 4Æ90Æ74Æ120Æ40Æ82Æ30Æ32Æ1 133Æ81Æ17 0Æ89 0Æ94 1Æ25 1Æ02 S64 30Æ455Æ87Æ627Æ55Æ21Æ10 4Æ50Æ63Æ518Æ30Æ720Æ31Æ8 141Æ21Æ45 1Æ07 1Æ08 1Æ24 1Æ04 Æ Æ Æ Æ Æ Æ Æ Æ Æ Æ Æ Æ Æ Æ Æ Æ Æ Æ Æ Æ Sedimentology S49 1512774416034068 2 804249605150214767092 0 81 0 96 1 09 1 03 S34 24Æ739Æ95Æ520Æ54Æ10Æ82 3Æ40Æ52Æ615Æ80Æ51Æ50Æ21Æ3 105Æ71Æ63 1Æ10 1Æ00 1Æ28 1Æ01 S18 36Æ557Æ89Æ133Æ97Æ11Æ47 5Æ70Æ84Æ723Æ60Æ92Æ70Æ42Æ3 163Æ71Æ36 0Æ94 0Æ95 1Æ31 1Æ01 S04 29Æ747Æ16Æ825Æ35Æ21Æ10 4Æ60Æ63Æ822Æ90Æ82Æ20Æ31Æ9 129Æ71Æ34 1Æ04 0Æ97 1Æ28 1Æ01 , 57 142–161 , Carbonate stromatolites from a Messinian hypersaline setting 155

Fig. 10. Cross-plots of Al (%) versus Prn/Smn and Prn/ Ndn ratios for Sutera stromatolites, dolostones, aragonitic limestones and claystones/marls (key as in Fig. 9).

Fig. 9. Cross-plots of Al, Si and Fe (%) versus RREE REE, carried by clay minerals or Fe oxyhydrox- (ppm) for Sutera stromatolites, dolostones, aragonitic ides, progressively and significantly decreases limestones and claystones/marls. from claystones/marls to aragonitic limestones, dolostones and stromatolites. The stromatolites considered to be reliable proxies for reconstruction seem to have been only slightly affected by of ancient sea water REE geochemical signatures detrital contamination. Moreover, it is to be noted (Webb & Kamber, 2000; Olivier & Boyet, 2006). that parameters expressing relative REE concen- Fractionation patterns of dissolved REEs in pres- trations, such as Prn/Smn and Prn/Ndn, do not ent-day oceans typically show low REE concen- correlate with Al (Fig. 10), Fe and RREE (the last trations, with HREEs >LREEs, and a distinct two are not shown in the figure). This suggests ‘Ce-anomaly’ (German & Ederfield, 1990). Negative that factors other than detrital contamination Ce anomalies reflect oxidation of Ce3+ to Ce4+, largely determine the Sutera REE distribution. reducing its solubility (Moffett, 1990, 1994; Sho- lkovitz & Schneider, 1991; DeCarlo et al., 1998). In interpreting REE data, it is necessary to DISCUSSION consider detrital contamination that could effec- tively mask a sea water signature in the carbon- Microbial activity can account for many of the ates due to the much larger REE concentration microfabrics observed in the Sutera stromatolites, and shale-like REE signature of terrigenous mate- in which dolomite is the dominant carbonate. rials (‘flat pattern’; German & Ederfield, 1990; Bau Based on petrographic observations, this dolo- & Dulski, 1996; Byrne et al., 1996; Webb & mite is not replacive and is inferred to be a Kamber, 2000). Carbonate samples with < 1 to primary precipitate. It is proposed that dolomite 2% shale contamination may be representative of which preserves the microbial filaments and the water composition from which they precipi- which consists of rounded crystal aggregates tated (Nothdurft et al., 2004). (Fig. 5B and C) formed, at least in the nucleation For the Sutera samples, RREE correlates well phase, through bacterial mediation, which helped with elements representative of fine-grained and to overcome kinetic inhibitors. Based on results of sand-sized detritus, such as Al and Si (Fig. 9). A laboratory experiments and sediment investiga- significant correlation also occurs between RREE tions, various authors have suggested that and Fe concentrations. Point distributions in BSR overcomes kinetic barriers to dolomite Fig. 9 suggest that the detrital contribution of precipitation by removing sulphate, reducing

2009 The Authors. Journal compilation 2009 International Association of Sedimentologists, Sedimentology, 57, 142–161 156 E. Oliveri et al. the solubility of Mg2+ and Ca2+ ions, increasing pH and carbonate alkalinity and providing nucleation sites for carbonate precipitation (Compton, 1988; Vasconcelos & McKenzie, 1997; Warth- mann et al., 2000; Van Lith et al., 2003; Wright & Wacey, 2005). Sulphate-reducing bacteria can remove sulphate and release Mg2+ in the solution 0 from MgSO4 , according to:

0 3 2þ 2CH2O þ MgSO4 ! H2S þ 2HCO þ Mg

This reaction is accompanied by pyrite precipitation (‘disseminated pyrite in dolomicrite’, see Petrography section above) and NH3 produced bacterially (Berner, 1980). The ammonia, which is Fig. 11. d13C versus d18O cross-plot for stromatolite highly soluble, increases the pH of the microenvi- carbonates. The rectangle at the top indicates isotope ronment via generation of hydroxide and produces compositions of dolomites from ephemeral lakes of the higher levels of alkalinity that favour dolomite Coorong Region (data from Rosen et al., 1989). precipitation. This process requires the presence of organic matter utilized by the bacteria as an energy water values (Walter et al., 1993), variations in source. The organic matter can be derived from diffusive fluxes (Boehme et al., 1996) and differ- cellular material and cell products, such as EPS. ences in metabolic pathways (Goevert & Conrad, Progressive consumption of organic matter by 2008). Even under hypersaline conditions, Sutera heterotrophic bacteria, including SRB, can result carbon isotope values of ca )5& (Fig. 11) strongly in localized carbonate precipitation. In the Sutera suggest organically derived carbon. This interpre- samples, this could account for the clotted–peloi- tation is reinforced by the presence of even more dal dolomicrite (Fig. 4A) and partially preserved negative d13C values (down to )11Æ3&)ofthe biofilm-like structure in dolomicrite (Fig. 5D). dolomites. It is therefore inferred that these dolo- Consistent with this view, primary precipita- mite precipitates reflect strong organic activity tion may have formed micron-sized rounded consistent with bacterial mediation. shapes (see Fig. 5B and C) that were subsequently With regard to the very negative carbon isotopic incorporated or absorbed into rhombohedral signatures of calcite, inferred here to possibly dolomite. Similar structures in dolomites from reflect a methane carbon source (see above), the Coorong region have been interpreted as numerous studies have documented the geochem- diagenetic self-organisation of mineral grains ical evidence for microbial production and oxida- driven by neomorphism (Wright, 1999). In the tion of methane in marine sediments (citations in Sutera stromatolites, bacterial-mediated precipi- Megonigal et al., 2003). Kinetic effects on carbon tation produced clotted dolomicrite with micro- during CH4 production and oxidation tend to peloids and fenestral pores (Fig. 4B and C) that concentrate the lighter 12C isotope in the resulting opened between the micrite and micropeloids, CO2 (Barker & Fritz, 1981; Whiticar et al., 1986) due to shrinkage of degrading organic matter (cf. and in the carbonates that can form. On the other Riding & Toma´s, 2006). During a subsequent, but hand, the very negative d13C values of calcite still early, diagenetic phase, sparry calcite cement (down to )31Æ6&; see Fig. 8) suggest that microbial filled fenestral pores (Fig. 4A). communities may have promoted calcite precipi- Plotting d18O versus d13C data of carbonates from tation through methane oxidation, using sulphate the stromatolite beds shows that the dolomite and as the electron acceptor according to the following associated calcite occupy two distinct fields net equation (Barnes & Goldberg, 1976; Reeburgh, (Fig. 11). High d18O dolomite values (up to 7Æ9&), 1976; Iversen & Jørgensen, 1985): similar to those of well-known modern dolomites 2 from ephemeral lakes of the Coorong region in CH4 þ SO4 ! HCO3 þ HS þ H2O Australia (Rosen et al., 1989), are consistent with precipitation under hypersaline environmental As reported before, contemporaneous occurrence conditions. Under these conditions, and those at of methanogenesis and anaerobic oxidation of Sutera, d13C values are liable to be subject to a methane has been observed in modern sediments variety of influences, such as buffering of pore (Orcutt et al., 2005). A distinctive feature of the

2009 The Authors. Journal compilation 2009 International Association of Sedimentologists, Sedimentology, 57, 142–161 Carbonate stromatolites from a Messinian hypersaline setting 157

first tetrad is poorly defined, due to a negative anomaly for redox-sensitive Ce. However, there is enrichment in Pr accompanied by a decline in Nd (Prn/Ndn from 1 to 1Æ25; Fig. 10), which is typical of the first tetrad (Takahashi et al., 2005). These observations of Sutera stromatolites, consistent with their petrography and isotope geochemistry, strengthen the view that distinctive LREE enrichment together with the tetrads observed in the REE patterns can provide geochemical signatures of past bacterial activity. Fig. 12. Shale-normalized REE + Y patterns for Sutera stromatolites. Note the convex curved segments (tetrad effect, dotted curves) and the peaks around Sm and Eu (MREE). CONCLUSION

REEn patterns for the Sutera section is that the During deposition of the basal part of the Prn/Smn ratios are > 1 for most stromatolite Messinian Calcare di Base at Sutera, shallow samples, and generally < 1 for the other litholo- hypersaline marine conditions favoured the estab- gies (Fig. 10), emphasizing LREE enrichment in lishment of microbial mat communities with the stromatolites. Similar enrichments have been Beggiatoa-like filaments and heterotrophs such as reported by Tlig & M’Rabet (1985) for dolomites sulphate-reducing bacteria. The dolomite appears from sabkha environments and ascribed to the to be persistently linked to bacterial sulphate influence of inorganic complexes that increase reduction (BSR). Co-occurrence of early dolomi- solubility of the heavy REE in basic solutions, tized bacterial filaments and negative d13C values with respect to the lighter elements of the REE could indicate BSR that promoted carbonate pre- group. However, evaporitic dolomites from non- cipitation and provided H2S for thiophilic bacte- stromatolite strata in the Sutera section display ria. In this view, the carbonate composition, Prn/Smn £1. Enrichment of LREE is observed in dolomite mineralogy, microfabrics (clotted, peloi- present-day microbialites (Webb & Kamber, 2000) dal) and microfossils (Beggiatoa-like filaments) of and Upper Jurassic (Olivier & Boyet, 2006) micro- the Sutera stromatolites are intimately linked. bial carbonates, and in Miocene organogenic Consistent with the petrography and geochemical dolomites (Miller & Leybourne, 2003). signatures of the stromatolites, including a dis- In a study of REE adsorption onto bacterial cell tinctive light rare earth element (LREE) enrichment walls, Takahashi et al. (2005) attributed enrich- and tetrads in the REE patterns, it is suggested that ment around Pr to the tetrad effect, resulting in bacterial biomineralization processes had a major REEn patterns that no longer increase or decrease role in carbonate precipitation within these depos- smoothly with ionic radius but instead show its. On this basis, stable (C and O) isotopes and REE internal irregularities. The tetrad effect refers to patterns from microbial carbonates can provide a convex curves (M-type tetrad effect; Masuda reliable source of information on the chemistry of et al., 1987) that appear in four regions of the ancient marine sedimentary environments. REEn pattern: La–Ce–Pr–Nd, (Pm)–Sm–Eu–Gd, Gd–Tb–Dy–Ho and Er–Tm–Yb–Lu (Peppard et al., 1969; Jørgensen, 1970; Nugent, 1970; Ma- ACKNOWLEDGEMENTS suda & Ikeuchi, 1979; Kawabe, 1992; Bau, 1996). Takahashi et al. (2005) recognized the tetrad We thank J.P. Calvo for his helpful comments on a effect particularly in the first two tetrads and preliminary version of the manuscript. Sedimen- ascribed it to REE forming inner sphere com- tology reviewers C.B. Schreiber and D.T. Wright plexes during their adsorption onto bacteria. critically read the manuscript and made helpful In many of the stromatolite samples from Sutera, suggestions. Thanks are also due to E. Curcuruto the second tetrad is prominent and readily recog- for assistance during a preliminary sampling of nizable as a convex curved segment (M-type tetrad the Sutera section, and to G. Capasso and effect; Fig. 12). A similar trend, with a peak F. Grassa (INGV, Palermo) for facilitating stable around Sm and Eu (MREE), is reported by Takah- isotope analysis. The International Association of ashi et al. (2007) for distribution patterns related Sedimentologists (IAS) sponsored EO’s participa- to complexing sites at the bacterial surface. The tion in EGU 2006. EO’s PhD research was

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2009 The Authors. Journal compilation 2009 International Association of Sedimentologists, Sedimentology, 57, 142–161